Enhanced Catalytic Activity of Nickel Complexes of an Adaptive Diphosphine–Benzophenone Ligand in Alkyne Cyclotrimerization

Abstract

Adaptive ligands, which can adapt their coordination mode to the electronic structure of various catalytic intermediates, offer the potential to develop improved homogeneous catalysts in terms of activity and selectivity. 2,2′-Diphosphinobenzophenones have previously been shown to act as adaptive ligands, the central ketone moiety preferentially coordinating reduced metal centers. Herein, the utility of this scaffold in nickel-catalyzed alkyne cyclotrimerization is investigated. The complex [(^p-tolL1)Ni(BPI)] (^p-tolL1 = 2,2′-bis(di(para-tolyl)phosphino)-benzophenone; BPI = benzophenone imine) is an active catalyst in the [2 + 2 + 2] cyclotrimerization of terminal alkynes, selectively affording 1,2,4-substituted benzenes from terminal alkynes. In particular, this catalyst outperforms closely related bi- and tridentate phosphine-based Ni catalysts. This suggests a reaction pathway involving a hemilabile interaction of the C=O unit with the nickel center. This is further borne out by a comparative study of the observed resting states and DFT calculations.

Introduction

The search for improved activity and selectivity in transition-metal based homogeneous catalysts strongly relies on the development of steering ligands to tune the steric and electronic properties of the metal center to the requirements of particular transformations. Recent years have seen the development of more sophisticated adaptive or cooperative ligands that can adjust their binding modes or engage in chemical transformations along a catalytic cycle.¹⁻⁴ The use of such ligands also plays an important role in the development of catalysts based on inexpensive and nontoxic first-row transition metals that can compete with, or even surpass, their counterparts based on precious metals, contributing to a more sustainable chemistry.⁵ Perhaps one of the simplest implementations of the concept of adaptive ligands are hemilabile ligands, which are polydentate ligands featuring a weakly binding moiety that can reversibly (de)coordinate the metal center.⁶ Since their introduction in synthetic chemistry by Jeffrey and Rauchfuss,⁷ hemilabile ligands have played a significant role in the current transition from noble to base metals in homogeneous catalysis. The hemilabile interaction is important not only for opening up a masked coordination site but also to stabilize catalytic intermediates, by allowing the supporting ligand to adapt its binding mode throughout the reaction coordinate. Most commonly, weak donor groups, such as ether (OR₂),^7,8 amine (NR₃),⁹ or imine derivatives (R₂C=NR),^8b,10 are employed as the labile unit, often resulting in enhanced catalytic activity for different kinds of reactions. Examples include olefin oligomerizations and polymerizations,^8c,11 (cyclo)trimerization,^9c,11 carbonylation of methanol and methyl acetate,¹² (transfer) hydrogenation of ketones or dehydrogenation of alcohols,^9b,13,14 hydroacylation of alkenes and alkynes,^8d,15 and coupling reactions.^9c,11−19

In addition to weak σ-donor moieties, σ-acceptor groups²⁰ (e.g., triarylborane)²¹ or π-ligands² (e.g., C=C double bond)²² can also act as hemilabile fragments. Our group recently reported the hemilabile character of the ligand 2,2′-bis(diphenylphosphino)benzophenone²³ (^PhL1) bound to nickel²⁴ and other first-row transition metals.^25PhL1 was shown to function as a hemilabile π-acceptor ligand, with the central C=O bond coordinating to the metal center in its more reduced forms (Figure 1). Ru,^23,26 Rh,²⁷⁻²⁹ Os,³⁰ and Ir³¹ complexes of ^PhL1 were also studied, more specifically in the catalytic hydrogenation of ketones,^23,26,28,30 in which the formation of a hydroxyalkyl species was accessible via the hydrogenation of the ketone moiety of the ligand.²⁸

Figure 1.

Previous work.²⁴ Coordination of a diphosphine benzophenone ligand, ^PhL1, to nickel(0), nickel(I), and nickel(II).

In this context, we set out to assess the potential utility of an η²-bound hemilabile π-acceptor moiety incorporated in a pincer type architecture for catalysis, using the cyclotrimerization of alkynes as a benchmark reaction. Since its discovery by Reppe et al.³² using a [Ni(CO)₂(PCl₃)₂] catalyst, intermolecular [2 + 2 + 2] cyclotrimerization of alkynes has been widely studied.³³ Metal catalyzed cyclotrimerization is an elegant method for the formation of cyclic frameworks, particularly convenient when the desired product is not accessible via traditional aromatic substitution reactions. Many transition metal systems have been developed (e.g., Ti,³⁴ Fe,³⁵ Co,³⁶ Ni,³⁷ Mo,³⁸ Ru,³⁹ Rh,⁴⁰ Pd,⁴¹ Ir⁴²). Literature precedents for the nickel-catalyzed conversion of alkynes to substituted benzene regio-isomers are known, both from an intramolecular^{36c,37g−37l,37n,37p,37r,37s} and intermolecular approach.^{9c,32,36a,37a−37f,37m,37o,37q,37t−37z} For the [2 + 2 + 2] intermolecular alkyne cyclotrimerization, it starts from the early example by Reppe et al.³² to the combination of divalent nickel halide salts and reductants^36a,37f or even activated Ni-particles.^37b Phosphine,^{9c,36a,37j,37m,37t,37v} NHC,^37v,37x mixed phosphine/imino (P,N),^9c or 1,4-diazadiene^{37c,37e,37u,37w,37y,37z} ligands, have been introduced to Ni systems in order to improve selectivity and catalytic activity. Phosphine complexes generally favor the formation of benzene derivatives while 1,4-diazadiene complexes yield cyclooctatetraenes.

Besides their synthetic utility, alkyne condensations are also useful probes for ligand effects in catalysis.⁴³ The possible formation of different products and isomers, such as cyclotrimers, cyclotetramers, linear oligomers, polymers, and other compounds, provides useful information about the reactivity of a particular catalyst. For example, Uyeda^37w,44 and Ess⁴⁴ recently applied this approach to the evaluation of the effect of catalyst nuclearity, and Jones^9c applied it to assessing differences in catalytic performance between bidentate PP and PN ligands (Figure 2). In this study, we assess the effect of using hemilabile π-acceptor tridentate ligand (^p-tolL1; Figure 3) in alkyne cyclotrimerization through comparison with diverse nickel systems. Ni(0) adopting a strong tridentate architecture (ligand = ^p-tolL2; Figure 3) and Ni complexes supported by a bidentate ligand (^PhL3; Figure 3) were selected for comparison with ^p-tolL1.

Figure 2.

Examples of Ni(0) complexes used to evaluate the catalytic properties of a ligand–metal system by their reactivity with alkynes.^9c,37w Blue bold font = property studied. R = Ph, CF₃, tert-butyl.

Figure 3.

Tridentate and bidentate ligands of comparison in this study.

Here we report the synthesis of nickel(0) complexes of ^p-tolL1, ^p-tolL2, and ^PhL3 featuring benzophenone imine (BPI) as a labile protecting ligand. Their stoichiometric reactivity with terminal alkynes is investigated, and the resulting Ni–alkyne complexes are thoroughly characterized. Then, a comparative study of these bi- and tridentate Ni(0)-catalysts in the cyclotrimerization of terminal alkynes is described, aiming to study the effect of a hemilabile π-acceptor system. For all tested alkyne substrates, ^p-tolL1 outperforms ^p-tolL2 and ^PhL3, as well as the structurally more different diphosphine rac-BINAP (^PhL4), in terms of selectivity and activity. Reactivity studies combined with DFT calculations provide insight into the increased catalytic activity, suggesting that ^p-tolL1 can adapt its binding mode throughout the reaction pathway, by the labile (de)coordination of its ketone unit.

Results and Discussion

Ni(0)-Benzophenone Imine Complexes

The diphosphine–benzophenone pincer-type ligand²³ 2,2′-bis(di(para-tolyl)phosphino)-benzophenone²⁵ (^p-tolL1; Figure 3) and the triphosphine pincer-type ligand⁴⁵ bis(2-(di-(para-tolyl)phosphino)phenyl)phenylphosphine (^p-tolL2; Figure 3) were synthesized according to (adapted) literature procedures,^24,25,46 by lithiation of o-bromo(diarylphosphino)benzene⁴⁷ with n-BuLi followed by reaction with the appropriate electrophile (Scheme 1). para-Tolyl substituents on the phosphine were introduced both to improve the solubility of the complexes and to provide a convenient ¹H NMR handle for characterization. The ³¹P NMR spectrum of ^p-tolL2 indicates the presence of an AB₂ system (Δν ≈ J_A,B)⁴⁸ with a coupling constant of ³J_A,B = 155 Hz, as previously observed for its phenyl-substituted analogue.⁴⁶ The diphosphine ether ligand ^PhL3(49) was obtained from commercial sources and used as received.

Scheme 1. Synthesis of ^p-tolL1 and ^p-tolL2 from o-Bromo(di-(para-tolyl)phosphino)benzene.

Coordination of the chelating phosphine ligands to a nickel(0) center was achieved by reaction with Ni(cod)₂ (cod = cyclooctatetraene) in the presence of benzophenone imine (BPI), in a stoichiometric ratio, affording respectively complexes (i) [(^p-tolL1)Ni(BPI)] (^p-tol1; Scheme 2), (ii) [(^p-tolL2)Ni(BPI)] (^p-tol2; Scheme 2), and (iii) [(^PhL3)Ni(BPI)] (^Ph3; Scheme 2). BPI was chosen as coligand since it binds strongly enough to Ni(0) to prevent the formation of dimeric species²⁴ but weakly enough to be exchanged with different types of potential substrates such as alkenes, nitriles, and alkynes (see below; Scheme 4).

Scheme 2. Ni⁰–Benzophenone Imine Complexes.

(A) General synthetic route. (B) Isolated complexes. BPI = benzophenone imine.

Scheme 4. Ligand Exchange Reaction between the Benzophenone Imine from ^p-tol1 and Different Kinds of Substrates and Coligands.

NMR characterization of ^p-tol1 indicates that the BPI ligand is bound in an η¹ fashion. The ¹³C NMR signal corresponding to the imine appears as a triplet (169.8 ppm,³J_C,P = 6.1 Hz) close to that of the free imine at 177.3 ppm, consistent with retention of C=N double bond character. In the ¹H NMR spectra, slight deshielding of the C=N–H proton by 0.24 ppm with respect to free BPI is also in accordance with η¹(N) binding. Moreover, the ATR-IR spectrum displays a signal at 3152 cm^–1 in the region corresponding the N–H stretch. In contrast, the backbone C=O moiety is bound side-on: the corresponding ¹³C NMR signal appears at 119.0 ppm (t, ³J_C,P = 5.1 Hz), considerably shifted from the value of 197.4 ppm found in the free ligand.²⁵ This large shift is useful as a diagnostic tool to determine whether the CO moiety is coordinated to the Ni center. The ³¹P NMR spectra of ^p-tol1 consist of a single singlet signal at 15.5 ppm, indicating that the two phosphorus atoms are equivalent on the NMR time scale. However, from ¹H and ¹³C NMR analysis, it appears that complex ^p-tol1 contains two chemically different methyl groups from the para-tolyl substituents (2.01 and 1.91 ppm in ¹H NMR, 21.3 and 21.1 ppm in ¹³C NMR). Therefore, the para-tolyl substituents belonging to a single P-donor site are not equivalent on the NMR time scale, which can be used as a secondary indication for ketone coordination.

In a similar fashion, NMR data support η¹ binding of the BPI ligand in the triphosphine complex ^p-tol2. The ¹³C NMR signal corresponding to the N=C bond is a doublet of triplets at 168.9 ppm (³J_C,P = 8.1 Hz, ³J_C,P = 7.4 Hz). The imine proton appears at 10.25 ppm in the ¹H NMR spectrum (deshielding of 0.48 ppm compared to the free BPI ligand) and couples with the phosphorus nuclei (dt, ³J_H,P = 3.2 Hz, ³J_H,P = 2.7 Hz). The ³¹P NMR spectrum is consistent with an AK₂ system (Δν ≈ 5J_A,K)⁴⁸ with a coupling constant ²J_A,K = 85 Hz. From ¹H and ¹³C NMR analysis, it appears that the complex contains two chemically different para-tolyl substituents (2.14 and 1.95 ppm in ¹H NMR, 21.4 and 21.1 ppm in ¹³C NMR), as also observed with complex ^p-tol1.

In the ¹H NMR of ^Ph3, the N–H signal from the coordinated imine appears as a broad singlet at δ_H = 9.71 in the expected 1:38 ratio compared to the aromatic region of the spectrum. The ³¹P NMR spectrum exhibits a single resonance at 32.4 ppm. No satisfactory ¹³C NMR data could be obtained due to the low solubility of the complex and its progressive decomposition, making the assignment of the binding mode of BPI in solution uncertain. The side-on binding of BPI observed in the solid state (see below) may be preserved in solution, in which case fast rotation of BPI ligand on the ¹H NMR time scale would render the two phosphorus atoms equivalent. The N–H stretch from the bound imine appears as a broad weak band in the ATR-IR spectrum, located at 3200 cm^–1.

More insights into the structural and electronic properties of nickel complexes ^p-tol1 and ^Ph3 were obtained by X-ray crystal structure determination (Figure 4). Crystals suitable for X-ray diffraction were not accessible with ^p-tol1 but with the structurally related complex ^Ph1, in which the para-tolyl ligand ^p-tolL1 is replaced by the phenyl analogue 2,2′-bis(di(phenyl)phosphino)-benzophenone (^PhL1).

Figure 4.

Molecular structures of [(^PhL1)Ni(BPI)] (^Ph1) and [(^PhL3)Ni(BPI)] (^Ph3) in the crystal (50% probability level). Hexane solvent molecules (^Ph1) and C−H hydrogen atoms are omitted for clarity.

In accord with NMR data, the structure of ^Ph1 (Figure 4; left) exhibits a side-on bound ketone moiety with Ni1–O11 and Ni1–C71 bond lengths of 2.0217(14) and 1.9760(19) Å, respectively. The C71–O11 bond length (1.320(2) Å) is found between those of unbound ^PhL1 (1.213(3) Å)²³ and a C–O single bond (1.43 Å in ethanol⁵⁰). The ketone is slightly less activated than in [(^PhL1)Ni(PPh₃)],²⁴ which displays a C–O bond length of 1.310(2) Å, consistent with less pronounced π-back-donation arising from the weaker donor character of BPI with respect to PPh₃. The sum of bond angles (351.3(3)°) around the carbon atom lies between those expected for sp² (328.5°) and sp³ (360°) hybridization at the carbon, in accordance with the Dewar–Chatt–Duncanson model. The N12–C12 bond length of 1.289(2) Å is close to the value of a C–N double bond (ca. 1.28 Å)⁵⁰ and comparable to the value found by Zhao et al. (1.294(3) Å)^51a for η¹-coordination mode in their synthesized NHC-based nickel(0) complex, [(IPr)Ni(η²-BPI)(η¹-BPI)] (IPr = 1,3-bis(diisopropylphenyl)-imidazolium), showing that the π-back-donation to the BPI ligand is small. Accordingly, the valence angles around the imine carbon add up to 360.0(3)°. Finally, the torsion angle (∠Ni1–N12–C12–CPh) of 20.0(3)° shows that the nickel atom is slightly out of the plane of BPI, which is probably sterically driven.

In contrast to that of ^Ph1, the crystal structure of ^Ph3 (Figure 4, right) reveals a trigonal planar complex in which the imine coligand is coordinated to the nickel in an η² fashion (Ni1–C37 = 2.018(2) Å, Ni1–N1 = 1.8985(19) Å). The C–N bond axis is parallel to the metal coordination plane, which is thought to maximize π-back-donation from the high-lying in-plane d orbital with significant σ*(P–Ni) character. Accordingly, strong π-back-donation into the π*(N–C) orbital is evidenced by an elongated C37–N1 bond (1.373(3) Å vs 1.28 Å⁵⁰ in the free imine) and the pyramidalization of the C37 atom, which displays a sum of valence angles of 355.3(3)°, in line with known η²(C,N)-Ni(0) complexes.⁵¹ As expected, the central oxygen atom from the ligand is not binding to the metal center (Ni1–O1 = 3.3668(16) Å), as also confirmed by a Wiberg bond index lower than 0.01 calculated by NBO analysis at a B3LYP/def2TZVP level of theory. The distance between the nickel and oxygen atoms is similar to the values reported from Ni(0) complexes bearing this ligand.⁵² Finally, the measured P1–Ni1–P2 bite angle of 108.59(2)° and the Ni1–O1 distance of 3.3668(16) Å are in the same range as those found for [(^PhL1)Ni(Cl₂)],²⁴ in which the C=O unit is not bound to the Ni ion (P1–Ni–P2 112.996(13)°; Ni–C = 3.4031(12) Å; Ni–O = 3.1012(10) Å), supporting the use of ^Ph3 as a model complex for the unbound state of ^p-tolL1.

In summary, complexes ^p-tol1 and ^p-tol2 are geometrically similar and mainly differ by the central coordinating unit the ligand (C=O vs P). Complex ^Ph3 differs in that the central oxygen atom is not coordinated and can be seen as a structural analogue of the unbound state of ligand ^RL1 (R = p-tolyl or Ph). Interestingly, moving from tridentate to bidentate mode is coupled to a change in binding mode of the BPI ligand from end-on to side-on, which can be understood from enhanced π-back-donation from a d¹⁰ L₂Ni fragment (see Supporting Information for more insight by NBO analysis into the different coordination modes of the imine coligand; Table S8).

Ni(0)–Alkyne Complexes

With complexes ^p-tol1–^Ph3 in hand, their reactivity toward terminal alkynes was investigated, leading to the generation of Ni–alkyne complexes [(^p-tolL1)Ni(HC≡CPh)] (^p-tol4-Ph), [(^p-tolL1)Ni(HC≡CCH₂OMe)] (^p-tol4-CH₂OMe), [(^p-tolL2)Ni(HC≡CPh)] (^p-tol5-Ph), and [(^PhL3)Ni(HC≡CPh)] (^Ph6-Ph) (Scheme 3). Ni-complexes ^p-tol5-Ph and ^Ph6-Ph, bearing phenylacetylene as coligand, are sufficiently stable to be isolated and are discussed first.

Scheme 3. Ni⁰-Terminal Alkyne Complexes.

(A) General synthetic route. (B) Generated complexes. ^p-tol4-Ph and ^p-tol4-CH₂OMe were characterized in-situ in solution, while ^p-tol5-Ph and ^Ph6-Ph were isolated. BPI = benzophenone imine.

The ³¹P NMR spectrum of ^p-tol5-Ph displays two signals at 78.7 ppm (t, ²J_P,P = 35.4 Hz) and at 28.1 ppm (d, ²J_P,P = 35.4 Hz) in a ratio of 2:1, indicating that the ligand binds in a tridentate fashion. Two ¹H NMR singlets of the same intensity at 2.12 and 1.91 ppm correspond to the methyl group from diastereotopic para-tolyl substituents bound to the same phosphorus atom. A single alkyne is bound to Ni as indicated by the terminal proton signal at 6.30 ppm, which appears as a doublet of triplets (³J_H,P = 25.6 Hz, ³J_H,P = 7.2 Hz) and couples with a carbon nucleus at 92.9 ppm in HMQC (¹H–¹³C). Further confirmation of this assignment is provided by treatment of ^p-tol2 with d-phenylacetylene, resulting in the disappearance of the signal at 6.30 ppm in an otherwise identical ¹H NMR spectrum (Supporting Information; Figure S68 and S76). In agreement with the spectroscopic characterization, the solid-state structure of ^p-tol5-Ph (Figure 5, left) reveals a pseudo-tetrahedral geometry in which the alkyne moiety is coordinated in η² fashion.

Figure 5.

Molecular structure of [(^p-tolL2)Ni(HC≡CPh)] (^p-tol5-Ph) and [(^PhL3)Ni(HC≡CPH)] (^Ph6-Ph) in the crystal (50% probability level). Toluene solvent molecules (^Ph6-Ph) and hydrogen atoms (except alkyne) have been omitted for clarity.

Compound ^Ph6-Ph, supported by the bidentate diphosphine ether ligand ^PhL3, displays two signals at 29.2 ppm and at 27.3 ppm coupling to each other (²J_P,P = 14.0 Hz) and in a ratio of 1:1 in the ³¹P NMR spectrum. This is consistent with a trigonal planar geometry in which rotation of the alkyne ligand is slow on the ¹H NMR time scale; warming up the sample up to 100 °C does not result in coalescence of the two ³¹P NMR signals. This interpretation is supported by reaction of ^Ph3 with 1 equiv of the symmetric alkyne diphenylacetylene, resulting in a symmetrical analogue of ^Ph6-Ph displaying a single ³¹P NMR singlet at 28.1 ppm (Supporting Information; Figure S98). The proton from the alkyne gives a ¹H NMR signal at 6.9 ppm, as revealed by HMQC (¹H–¹³C) and comparison with deuterated analogue. The solid state structure (Figure 5, right) of ^Ph6-Ph is in accord with the NMR data and reveals a trigonal planar geometry with a P1–Ni–P2 bite angle of 104.94(2)°, where the oxygen atom is not bound to the nickel center (Ni1–O1 = 3.3317(15) Å). The alkyne is bound in an η²-coordination mode, the C37–C38 axis being in the Ni coordination plane.

While stable tricoordinate Ni(0) alkyne complexes with formally 16 valence electrons analogous to ^Ph6-Ph are well documented,^53,54 compound ^p-tol5-Ph represents an unusual example of tetracoordinate, 18 VE Ni(0)–alkyne complex, to the best of our knowledge the first to be structurally characterized. Spectroscopic and structural data point toward a considerably weaker activation of the C≡C triple bond in this geometry (Table 1). First, the ¹³C NMR chemical shifts of the two alkyne carbons (δ_C = 92.9 and 101.0) in ^p-tol5-Ph are moderately deshielded in comparison with the free alkyne (δ_C = 77.5 and 83.3), indicating relatively weak rehybridization from sp to sp². This contrasts with a stronger activation of the C≡C triple bond of phenylacetylene in ^Ph6-Ph evidenced by the ¹³C NMR signals from the coordinated phenylacetylene at 125.7 ppm (C≡C–H, dd, ²J_C,P = 40.1 Hz, ²J_C,P = 7.1 Hz) and 136.4 ppm (C≡C–Ph, dd, ²J_C,P = 38.9 Hz, ²J_C,P = 8.4 Hz), which were assigned with the assistance of APT ¹³C NMR and by comparison with the d-phenylacetylene complex. Second, the acetylic hydrogen signal of ^p-tol5-Ph (δ_H = 6.30) is less shifted than that of ^Ph6-Ph (δ_H = 6.9), consistent with a stronger rehybridization toward sp². Third, the C≡C stretch vibration from the alkyne in ^p-tol5-Ph is found at 1823 cm^–1 in the FT-IR spectrum versus 2126 cm^–1 for free phenylacetylene and versus 1749 cm^–1 for ^Ph6-Ph. Finally, the slightly elongated C47–C48 distance at 1.231(8) Å (vs 1.182–1.190 Å in free phenylacetylene)⁵⁵ and the rather large C47–C48–C49 angle at 152.0(6)° are also consistent with a relatively weak activation of the C–C triple bond, contrasting with the longer C37–C38 bond length (1.269(3) Å) and the more acute the C37–C38–C39 angle (144.07(19)°) found for ^Ph6-Ph. The C47–C48 bond length in ^p-tol5-Ph is the shortest we are aware of for nickel(0)–alkyne complexes. Together, these data suggest that the π-back-donation from the nickel to phenylacetylene is more pronounced in ^Ph6-Ph, resulting in a stronger rehybridization toward sp².

Table 1. Selected Spectroscopic and X-ray Crystal Structure Values of Phenylacetylene, ^p-tol5-Ph and ^Ph6-Ph^a.

	phenylacetylene	p-tol5-Ph	Ph6-Ph
δC(C≡C–H) [ppm]	77.5	92.9	125.7
δC(C≡C–Ph) [ppm]	83.3	101.1	136.4
δH(C≡C–H) [ppm]	2.73	6.30	6.9
ν(C≡C) [cm–1]	2126	1823	1749
C≡C [Å]	1.182–1.19055	1.231(8)	1.269(3)
∠C≡C–CPh [deg]	177.39–179.4955	152.0(6)	144.07(19)

^aVibrational frequencies are measured with ATR-IR (neat). NMR chemical shifts are given in C₆D₆.

The observations made for complex ^Ph6-Ph parallel those reported for mononuclear, 16 valence electron, tricoordinate Ni–alkyne, in which the C–C bond is significantly more elongated than in ^p-tol5-Ph, regardless of substituent groups on the alkyne. For example, strong activation of the alkyne is observed for bidentate diimine and mixed P,N supported species (up to 1.296(6) Å),^{9c,53a,53b,53i,53l,53n,53r} while bidentate phosphorus ligands afford C–C bond lengths from 1.260(4) to 1.283(3) Å.^{52b,53d−53f,53h,53j,53m,53o−53q} The substantially weaker rehybridization observed in ^p-tol5-Ph can be assigned to its unique coordination geometry (i.e., tetracoordinate complex; 3 donor ligands; pseudo-T_d geometry) compared to ^Ph6-Ph and the reported Ni–alkyne complexes (i.e., tricoordinate complexes; 2 donor ligands; trigonal planar geometry). The weak activation of phenylacetylene in ^p-tol5-Ph is presumably a consequence of the smaller splitting of the d-orbitals in its pseudo-tetrahedral geometry (3 donor ligands), resulting in less pronounced π-back-donation from Ni to the antibonding orbitals of the alkyne. Similar considerations were used by Lee and co-workers to explain the rather weak activation of CO₂ in the tetracoordinate pseudo-tetrahedral nickel complex [(PP^MeP)Ni(η²-CO₂)] (PP^MeP = PMe[2-PⁱPr₂–C₆H₄]₂)⁵⁶ with respect to tricoordinate Ni-η²-CO₂ adducts supported by bidentate phosphines such as [(dtbpe)Ni(η²-CO₂)] (dtbpe = 1,2-bis(di-tert-butylphophino)-ethane).⁵⁷

Finally, reaction of the diphosphine-ketone complex ^p-tol1 with terminal alkynes generally led to a mixture containing the alkyne complex, [(^p-tolL1)Ni(HC≡CR¹)] (^p-tol4-R¹; R¹ = substituent on the alkyne), the starting material ^p-tol1, BPI, the substrate, and cyclotrimerization products. This indicates that cyclotrimerization occurs concomitantly with ligand exchange, precluding the isolation of pure alkyne complexes. Nonetheless, in situ characterization of alkyne complexes was possible. Interestingly, ¹³C NMR of ^p-tol4-Ph indicates that the ketone moiety is not bound to the metal center: addition of phenylacetylene to ^p-tol1 causes the disappearance of the triplet at 119.0 ppm, characteristic of the η²(C,O) coordination to nickel and the appearance of a triplet at 202.7 ppm (³J_C,P = 4.9 Hz). In addition, all the para-tolyl groups in ^p-tol4-Ph are equivalent at room temperature, with singlets appearing at 2.00 and at 21.2 ppm in ¹H and ¹³C NMR, respectively. This is consistent with an unbound ketone allowing fast ring inversion of the chelate macrocycle. A single ³¹P NMR singlet at 33.0 ppm down to −80 °C indicates fast rotation of the alkyne, exchanging the two phosphorus atoms on the NMR time scale even at low temperature (Supporting Information; Figure S59).

The C≡C–H¹H NMR signal of the coordinating phenylacetylene could not be located, presumably hidden in the crowded aromatic region of the spectrum. In contrast, the assignment of the acetylic proton of the bound alkyne was possible in the ¹H NMR spectrum of ^p-tol4-CH₂OMe where it appears as a triplet of triplets at 6.34 ppm (³J_H,P = 15.6, ⁴J_H,H = 1.6 Hz) and integrates in a ratio of 1:12 compared to the four CH₃ groups of the ligand, confirming that a single alkyne molecule is bound. The integrals, coupling constants, and multiplicity of CH₂ (4.63 ppm, d, ⁴J_H,H = 1.6 Hz) and OCH₃ (3.32 ppm, s) of the bound alkyne are in agreement with the proposed structure. Similar to ^p-tol4-Ph, both ¹H and ¹³C NMR of ^p-tol4-CH₂OMe reveal that the ketone moiety is not bound to the metal center: the ketone resonance appears as a triplet at 203.1 ppm (³J_C,P = 4.7 Hz) and the methyl groups from the para-tolyl are all equivalent, with a singlet at 2.03 and a doublet 22.2 ppm (⁴J_C,P = 2.3 Hz) in ¹H and ¹³C NMR, respectively. A singlet ³¹P NMR signal at 32.7 ppm at room temperature is consistent with fast rotation of the methyl propargyl ether similar to ^p-tol4-Ph. However, gradually cooling the sample down to −85 °C first shows the appearance of a new unsymmetrical species at ca. −10 °C, as characterized by two doublets (²J_P,P = 23.3 Hz) at 32.1 and 34.1 ppm (Supporting Information; Figure S67). The central signal corresponding to ^p-tol4-CH₂OMe does split into two new doublets (²J_P,P = 23.3 Hz) with a coalescence temperature between −50 and −60 °C. This indicates that the rotation of methyl propargyl ether can be frozen on the NMR time scale, but also that two distinct species are formed at low temperature. This is tentatively attributed to a secondary interaction of the oxygen atom from H–C≡C–CH₂OMe with Ni. In addition, the second species observed at low temperature may be either a diastereomeric conformer or an isomeric structure in which the central ketone fragment binds to the nickel center.

The fast rotation of the alkyne fragment in complex ^p-tol4-Ph contrasts with the observed behavior of the tricoordinate ^Ph6-Ph. DFT calculations on the acetylene complex ^p-tol4-H reveal that the rotation of acetylene is assisted by transient coordination of the ketone moiety to the Ni center, resulting in a low overall rotation energy barrier of 5.5 kcal/mol (Figure 6). The first transition state, ^p-tol4-H(TS1), from the unbound ketone state ^p-tol4-H to the bound state ^p-tol4-H(I1), is characterized by the coordination of the ketone moiety (ν = −113 cm^–1; ΔG^TS1 = 4.3 kcal/mol) to generate a four-coordinate intermediate ^p-tol4-H(I1). The second transition state, ^p-tol4-H(TS2), corresponds to the rotation of acetylene (ν = −95 cm^–1; ΔG^TS2 = 4.1 kcal/mol) accompanied by partial decoordination of one of the phosphine arms. The low overall energy barrier of ΔG^TS,overall = 5.5 kcal/mol is in qualitative agreement with the observed fast rotation of alkynes at room temperature. For comparison, a rotation energy barrier of 25.0 kcal/mol was calculated for the rotation of acetylene in the diphosphine ether complex ^Ph6-H (Supporting Information; Figure S112), in qualitative agreement with the observation of two ³¹P NMR signals for ^Ph6-Ph up to 100 °C.

Figure 6.

(top) Energy diagram for the rotation of acetylene around the Ni-coordination plane (∠C1–C2–Ni–P1) of ^p-tol4-H through two different transition states, ^p-tol4-H(TS1) = ^p-tol4-H(TS1′) and ^p-tol4-H(TS2), and one intermediate, ^p-tol4-H(I1) = ^p-tol4-H(1′). Numbers in parentheses are G° values given in kcal/mol. ΔG^TS1 = 4.3 kcal/mol. ΔG^TS1′ = 2.9 kcal/mol. ΔG^TS2 = 4.1 kcal/mol. ΔG^TS,overall = 5.5 kcal/mol. (bottom) Optimized geometry of transition states ^p-tol4-H(TS1) and ^p-tol4-H(TS2) as well as intermediate ^p-tol4-H(I1) at a B3LYP/6-31g(d,p) level of theory under vacuum. The imaginary frequency (−113 cm^–1) of ^p-tol4-H(TS1) shows the coordination of the ketone (C3–O) to Ni, while the imaginary frequency (−95 cm^–1) of ^p-tol4-H(TS2) is the rotation of acetylene around the C1–C2–Ni–P1 torsion angle. Selected bond lengths [Å] for ^p-tol4-H(TS1): C3–O = 1.25; Ni–C3 = 2.35; Ni–O = 2.55; Ni–P1 = 2.17; Ni–P2 = 2.19; C1–C2 = 1.27. Selected bond lengths [Å] for ^p-tol4-H(I1): C3–O = 1.30; Ni–C3 = 2.01; Ni–O = 2.06; Ni–P1 = 2.18; Ni–P2 = 2.24; C1–C2 = 1.25. Selected bond lengths [Å] for ^p-tol4-H(TS2): C3–O = 1.32; Ni–C3 = 1.96; Ni–O = 1.92; Ni–P1 = 2.14; Ni–P2 = 2.45; C1–C2 = 1.24. Hydrogen atoms have been omitted for clarity.

The differences in electronic and structural properties of Ni–alkyne complexes ^p-tol4-Ph–^Ph6-Ph were further investigated by DFT calculations (geometry optimization and NBO analysis), as summarized in Table 2. The optimized geometries of ^p-tol4-Ph, ^p-tol5-Ph, and ^Ph6-Ph (Table 2, entries 1–4) are consistent with the obtained crystal structures in which the C1–C2 bond length, C1–C2–C3 angle and Ni–C1 and Ni–C2 distances calculated for ^p-tol5-Ph indicate weaker activation of the HC≡CPh substrate. Accordingly, Wiberg bond indexes (WBIs) calculated in the NBO basis at a B3LYP/def2TZVP level of theory show that the binding of phenylacetylene induces a decrease of the C1–C2 WBI (Table 2, entry 5) from 2.82 to 2.30 in ^p-tol5-Ph versus 2.20 in ^p-tol4-Ph and 2.17 in ^Ph6-Ph. The smaller Ni–C1 and Ni–C2 WBIs (Table 2, entries 6 and 7) of 0.31 in ^p-tol5-Ph corroborate a weaker orbital interaction between the HC≡CPh ligand and the Ni-center in the triphosphine complex ^p-tol5-Ph compared ^p-tol4-Ph (WBIs of 0.43 and 0.39) and ^Ph6-Ph (WBIs of 0.42 and 0.39).

Table 2. Selected DFT Bond Distances and Angles and Wiberg Bond Indexes (WBI)^a.

entry	property	HC≡CPh	p-tol4-Ph	p-tol5-Ph	Ph6-Ph
DFT Bond Length and Angle [in Å and deg]
1	C1–C2	1.21	1.29	1.26	1.30
2	∠C1–C2–C3	180	141	150	141
3	Ni–C1		1.84	1.94	1.84
4	Ni–C2		1.89	1.97	1.89
Wiberg Bond Index (WBI)
5	C1–C2	2.82	2.20	2.30	2.17
6	Ni–C1		0.43	0.31	0.42
7	Ni–C2		0.39	0.31	0.39

^aGeometry optimizations of [(^p-tolL1)Ni(HC≡CCPh)] (^p-tol4-Ph), [(^p-tolL2)Ni(HC≡CPh)] (^p-tol5-Ph), and [(^PhL3)Ni(HC≡CPh)] (^Ph6-Ph) were performed at a B3LYP/6-31g(d,p) level of theory. WBIs were calculated by NBO analysis at a B3LYP/def2TZVP level of theory from the optimized geometries. Hydrogen atoms have been omitted for clarity.

In summary, we showed in this section that ligands ^p-tolL1–^PhL3 all form Ni(0)–alkyne complexes with phenylacetylene. The triphosphine ^p-tolL2 binds in a tridentate mode in ^p-tol5-R¹, resulting in unusually weak activation of the alkyne, while ^PhL3 acts as a bidentate ligand in the tricoordinate ^Ph6-R¹, in which a strong Ni–alkyne interaction causes a high rotation barrier around the Ni–alkyne axis. The hemilabile ligand ^p-tolL1 is able to sample both binding modes: it binds as a bidentate ligand in ^p-tol4-R¹, resulting in a similar geometry as that of ^Ph6-R¹, but the observed low rotation barrier around the Ni–alkyne axis is indicative of the ability of ^p-tolL1 to transiently adopt a tridentate mode.

Ligand Exchange Reactions on [(^p-tolL1)Ni(BPI)]

Having observed that the ketone moiety decoordinates upon binding of an alkyne to the (^p-tolL1)Ni fragment, we sought to probe the generality of this hemilabile behavior. NMR-tube experiments showed that the BPI coligand can be reversibly exchanged with several types of ligands, such as benzonitrile (K_eq ≈ 2.7 × 10^–3; ΔG_298.15 ≈ 3.5 kcal/mol), styrene (K_eq ≈ 1.5 × 10^–2; ΔG_298.15 ≈ 2.4 kcal/mol), and diphenylacetylene (K_eq ≈ 6.1 × 10^–3; ΔG_298.15 ≈ 3.1 kcal/mol), as depicted in Scheme 4 (see Supporting Information for more details). The proposed mode of coordination of the benzonitrile ligand in ^p-tol7-PhCN relies on DFT calculations, which predict the η²-(C,N) binding to be 9.4 kcal/mol energetically less stable than the η¹-(N) coordination mode (Supporting Information; Table S11). With triphenylphosphine, the exchange is irreversible. In every case, with the exception of diphenylacetylene, ¹³C NMR demonstrates that the ketone moiety is still bound to nickel (no peaks between 190 and 210 ppm). The coordinated ketone appears at 120.4 ppm (dt, ²J_C,P = 13.8 Hz, ²J_C,P = 4.4 Hz) for the PPh₃ complex ^p-tol7-PPh₃ and shows two chemically different methyl groups from the para-tolyl substituents at 1.97 ppm (s, 6H) and 1.96 ppm (s, 6H) in ¹H NMR and at 21.2 and 21.1 ppm, respectively, in ¹³C NMR. Unfortunately for the PhCN complex ^p-tol7-PhCN and for the styrene complex ^p-tol7-C₂H₃Ph, the carbon resonance of the bound ketone could not be assigned, as the weak signal is covered by those of the substrate and the starting complex ^p-tol1. However, for all of these complexes, ¹H NMR shows that the two methyl groups from the para-tolyl substituents on the phosphine are not equivalent (δ_H = 2.04 and 2.03 for ^p-tol7-PhCN, 2.05 and 2.01 for ^p-tol7-C₂H₃Ph), which is also indicative of the pincer-type binding mode of ^p-tolL1 to nickel, with the central ketone moiety bound, as observed with ^p-tol1 and ^p-tol7-PPh₃ and in opposition to ^p-tol4-R¹. With the diphenylacetylene complex ^p-tol7-C₂Ph₂, the absence of interaction of the ketone is demonstrated by the presence of a triplet at 198.3 of ³J_C,P = 4.7 Hz and with all four C–H₃ being chemically equivalent (δ_H = 1.95; δ_C = 21.2), similar to the closely related terminal alkyne complex ^p-tol4-CH₂OMe. The difference in coordination between olefin and alkyne complexes is consistent with alkynes being stronger π-acceptors than olefins and illustrates the ability of the diphosphine ketone framework to adapt its binding mode to the electronic properties of a substrate.

Catalytic Comparison

Having shown that the hemilabile diphosphine-ketone ligand ^p-tolL1 is able to adapt its coordination mode to a substrate bound to Ni(0), we turn to its performance as a supporting ligand in the cyclotrimerization of alkynes. We compare hemilabile ^p-tol1 with the strong tridentate ^p-tol2 and the bidentate ^Ph3 to delineate specific effects of the hemilabile behavior. In addition, the smaller bite-angle rac-BINAP (^PhL4) system was also tested.⁵⁸ Terminal alkyne substrate with diverse electronic properties were selected for comparison: first, phenylacetylene (8), as a standard aryl-substituted substrate; second, methyl propiolate (9), as electron poor alkyne and since its additional oligomerization into cyclooctatetraene (COT) regio-isomers is commonly observed;⁵⁹ and finally, methyl propargyl ether (10) as an electron rich substrate. The catalytic outcome of the reaction of these alkynes with 0.5 mol % of isolated BPI-catalysts ^p-tol1, ^p-tol2, and ^Ph3 and in situ generated ^PhL4 catalyst at room temperature was analyzed. The organic products were separated from the reaction mixture as a mixture of 1,2,4- (a) and 1,3,5- (b) substituted arenes, in addition to cyclotetramerization products (when applicable), COTs (c), in which their relative ratio was determined by ¹H NMR, as described in Table 3. To exclude a particular role of the BPI or cod coligands, the active catalysts were also generated in situ by mixing ^p-tolL1–^PhL3 with 1 equiv of Ni(cod)₂ and 200 equiv of the corresponding alkyne. The reactions display similar activity as the values reported in Table 3 (Supporting Information; Table S4) and form the same resting states (respectively, ^p-tol4-R¹, ^p-tol5-R¹, ^Ph6-R¹) under these conditions.

Table 3. Catalytic Comparison for the Cyclotrimerization of Phenylacetylene (8), Methyl Propiolate (9), and Methyl Propargyl Ether (10), Using Catalysts ^p-tol1 to ^Ph3, and the ^PhL4 System^a.

entry	substrate (-R1)	catalyst	yield 1,2,4- (a) [%]	yield 1,3,5- (b) [%]	yield COTs (c) [%]	ratio a/b/c
1	phenyl acetylene, 8 (-Ph)	p-tol1	86.9	3.2	0	97:3:0
		p-tol2	3.1	1.9	0	62:38:0
		Ph3	2.8	0.2	0	94:6:0
		PhL4 + Ni(cod)2	4.6	1.9	0	70:30:0
2	methyl propiolate, 9 (−CO2Me)	p-tol1	90.2	6.3	2.5	91:7:2
		p-tol2	24.5	2.1	7.5	72:6:22
		Ph3	65.0	12.3	6.5	77:15:8
		PhL4 + Ni(cod)2	14.0	3.0	32	29:6:65
3	methyl propargyl ether, 10 (−CH2OMe)	p-tol1	71.6	6.9	0	90:10:0
		p-tol2	<1	<1	0	b
		Ph3	<1	<1	0	b
		PhL4 + Ni(cod)2	<1	<1	0	b

^aSimilar results were obtained when the active catalysts were generated in situ using ^p-tolL1 to ^PhL3, 1 equiv of Ni(cod)₂, and the corresponding terminal alkyne (Supporting Information; Table S4). Yields and ratios were averaged over two runs (Supporting Information; Tables S2 and S3). Yields are isolated yields. Ratios were determined by ¹H NMR.

^bFor systems ^p-tol2, ^Ph3, and ^PhL4, only trace amounts of products 10a and 10b were detected in which an accurate determination of the ratio between the two regio-isomers was not possible.

Mixing phenylacetylene (8) with 0.5 mol % of ^p-tol1 in toluene at room temperature for 16 h yields 86.9% of 1,2,4-triphenylbenzene (8a) and 3.2% of 1,3,5-triphenylbenzene (8b), in an isomeric ratio of 97:3. Under the same conditions, the complexes ^p-tol2 and ^Ph3 and the ^PhL4-Ni system afforded only low yields of 8a: 3.1%, 2.8%, and 4.6%, respectively. In addition, a lower regioselectivity toward the 1,2,4 isomers of 62:38 and 70:30 compared to the 1,3,5 isomer was observed with ^p-tol2 and ^PhL4 + Ni(cod)₂, respectively. With the more electron poor alkyne methyl propiolate (9), the reaction catalyzed by ^p-tol1 led to nearly exclusive cyclotrimerization with the formation of the 1,2,4-isomer 9a in a ratio of 91:7 compared to its 1,3,5 analogue 9b and with a yield of 90.2%. Only small amounts of the cyclotetramers, tetramethyl-cyclooctatetraene-tetracarboxylates (9c) are observed, in a ratio of 2:98 compared to the trimerization products. In contrast, the yields obtained for 9a with the other catalysts in the series range from 14.0% to 65.0%. More importantly, catalysts ^p-tol2 and ^Ph3 and the ^PhL4 system display lower selectivity. The tridentate triphosphine catalyst ^p-tol2 exhibits a regioselectivity of 92:8 (1,2,4-/1,3,5-isomer) and produces cyclotetramers in a 22:78 ratio compared to the cyclotrimerization products. The bidentate systems ^Ph3 and ^PhL4 are also less selective and lead to the formation of 9a in 84:16 and 82:18, respectively, compared to 1,3,5-benzene product 9b. The bidentate catalyst ^Ph3 displays good chemoselectivity (92:8) toward cyclotrimerization, whereas the other diphosphine system ^PhL4 appears to even favor cyclotetramerization products 9c (35:65). Finally, methyl propargyl ether (10), bearing an electron rich substituent was also tested and afforded lower yields than 8 and 9. With the ketone-based catalyst ^p-tol1, 10 is converted into 1,2,4-tris(methoxymethyl)benzene (10a) in a yield of 71.6%, and the regioselectivity toward 10a remains high (90:10). Under the same conditions, the other complexes showed poor activity (less than 1% total yield). Overall, for the three examined substrates, catalyst ^p-tol1, bearing the hemilabile diphosphine benzophenone ligand ^p-tolL1, was shown to be more active and more selective toward the formation of the 1,2,4-trisubstituted benzene product.

Additional Substrates

In addition to the substrates described in Table 3, catalyst ^p-tol1 was applied to the cyclotrimerization of terminal alkynes 11a to 13a with high selectivity for 1,2,4-substituted benzenes and in good isolated yields (>65%, Table 4). Ethyl propiolate (11) (entry 1) affords triethylbenzene-1,2,4-tricarboxylate (11a) at room temperature. At 50 °C, 4-ethynylanisole (13) produces the desired 1,2,4-cyctrotrimerization product in a lower yield than the weakly electron-withdrawing analogue 1-ethynyl-4-fluorobenzene (12), supporting a preference for electron-withdrawing substrate (entries 2 and 3). The preference for electron poor alkynes is commonly observed in Ni-catalyzed cyclotrimerization of terminal alkynes.^{37m,37v−37y} In comparison with the Ni-catalysts reported in literature,³⁷ complex ^p-tol1 is a competitive catalyst in terms of activity and regioselectivity (from 90:10 to 97:3). A catalyst loading of ^p-tol1 of 0.05 mol % mediated the cyclotrimerization of ethyl propiolate (11) into triethylbenzene-1,2,4-tricarboxylate (11a) in 87% isolated yield, reaching a TON of 1740 after 16 h.

Table 4. Additional Substrates for Alkyne Cyclotrimerization Catalyzed by ^p-tol1^c.

^aReaction temperature = room temperature.

^bReaction temperature = 50 °C.

^cReported yields are isolated yield for the 1,2,4-trisubstituted benzene isomer. Isomeric ratios between the 1,2,4 and 1,3,5 isomer and between the trimers and tetramers were determined by ¹H NMR. Yields and ratios were averaged over two runs (Supporting Information; Table S5). Catalytic method A was applied (see experimental part).

Mechanistic Considerations

The comparison study between ^p-tol1 and the other Ni-systems (^p-tol2, ^Ph3, and ^PhL4) strongly suggests that the hemilabile π-acceptor moiety contributes to the high catalytic performance. In this section, the role of the ketone fragment in the improved catalytic performance of ^p-tol1 is studied and supported by computational modeling of some targeted compounds. To address the question, an overview of the general mechanism in metal-catalyzed [2 + 2 + 2] alkyne cyclotrimerization is discussed first (Scheme 5), followed by differences in reactivity among the three catalysts tested in the comparison study.

Scheme 5. Proposed Intermediates for the Transition-Metal-Catalyzed [2 + 2 + 2] Cyclotrimerization of Acetylene, as Commonly Reported in Literature³³.

Scheme 5 shows a commonly proposed mechanism for the cyclotrimerization of acetylene catalyzed by transition metals.³³ For base metals, most mechanistic studies have been carried out using cobalt complexes as catalysts.⁶⁰ From the acetylene metal complex 14-C₂H₂, the association of a second molecule of acetylene generates complex 14-(C₂H₂)₂, which requires an open coordination site in 14-C₂H₂. Next, the oxidative coupling of the two coordinated alkyne fragments from 14-(C₂H₂)₂ gives rise to the key metallacyclopentadiene intermediate. This step is generally thought to be rate determining. The metallacyclopentadiene intermediate can be best described as one of two resonance structures, 14-MCP or 14-MCP′, depending on the nature of metal complex used. For example Saá et al. calculated that CpRuCl (Cp = cyclopentadiene)-catalyzed alkyne cyclotrimerization proceeds via 14-MCP′, while the cobalt system CpCo proceeds via 14-MCP.^60d Late transitions metals tend to favor the concerted-oxidative cyclization between two alkynes and the low-valent metal center.^60a However, a stepwise zwitterionic diradical pathway, involving the formation of a σ(C–C) bond and one Ni–C bond cannot be excluded. When acetylene is substituted, the formation of the substituted metallacyclopentadiene is of importance as different 14-MCP regio-isomers can be generated. The substituted 14-MCP will thus dictate the overall selectivity toward the product formation of the 1,2,4- or 1,3,5-trisubstituted benzene regio-isomer (more precisely the transition state TS1 between the bisalkyne and metallacyclopentadiene intermediate).⁶¹ In the last step (insertion of the third alkyne), different mechanisms have been proposed.⁶² The insertion of a third alkyne could either proceed via a Diels–Alder type [4 + 2] cycloaddition, in which the metal does not participate in the bond formation (Scheme 5, pathway a),^60e or with the assistance of the metal, prior to the new C–C bond formation (Scheme 5; pathway b). 7-Metallanorbornadiene (14-[4 + 2])⁶³ is frequently presented as an unstable species and immediately collapses to arene, forming a metal–arene adduct. As regards pathway b, the coordination of the third alkyne is followed by either migratory insertion (metallacycloheptatriene 14-I) or [2 + 2] cycloaddition (cycloadduct 14-[2 + 2]). These complexes can also be formed in a concerted way, without involving the direct formation of an acetylene metallacyclopentadiene complex 14-AMCP, but still with participation of the metal center. It has also been suggested that an additional pathway could occur via an intermolecular [4 + 2] cycloaddition forming the η⁴-bound intermediate, 14-[4 + 2]′.^60d Pathway a is likely to happen for strong donor ligands or solvents.^60d The insertion of a fourth alkyne (especially if the intermediate formed is 14-I) leads to formation of COT side products.^61a Eventually, the formation of the benzene product happens by either reductive elimination or ligand exchange of benzene with a molecule of acetylene (cycloadduct 14-[2 + 2] rearranges itself prior to the reductive elimination).

In this overall picture, several effects of the hemilabile π-acceptor ketone moiety ion complex ^p-tol1 can be envisioned. First, the hemilabile interaction of the ketone unit can be thought to facilitate the alkyne uptake, especially in comparison with the tridentate phosphine system ^p-tol2. In situ NMR spectroscopy experiments with the three complexes ^p-tol1, ^p-tol2, and ^Ph3 showed that the corresponding monoalkyne complexes ^p-tol4-R¹–^Ph6-R¹ are the resting states of the catalyst in all cases. For the ketone catalyst ^p-tol1 and the bidentate catalysts ^Ph3, these resting states (^p-tol4-R¹ and ^Ph6-R¹) are 16 VE species that can readily accept an incoming alkyne molecule, while the corresponding triphosphine complex ^p-tol5-R¹ is saturated (18 VE). Most probably, coordination of a second equivalent of alkyne to ^p-tol5-R¹ requires decoordination of one of the phosphorus atoms, which is likely to raise the overall reaction barrier and result in lower activity under the same conditions (Supporting Information; Scheme S4). The unique feature of the ketone unit is that the tricoordinate alkyne complex can be accessed directly from the tetracoordinate precursor ^p-tol1. A similar process can be thought to happen at the end of the catalytic cycle: the ketone may accelerate product release by transient coordination to Ni(0), which would explain the lower propensity of ^p-tol1 to form cyclooctatetraenes.

Second, the interaction of the ketone with the nickel center could help to stabilize transient intermediates, such as the key metallacycle 14-MCP species. In order to assess structural differences between the metallacyclopentadiene supported by the different ligands, ^p-tolL1, ^p-tolL2, and ^PhL3, we made use of geometry optimization by DFT calculation at a B3LYP/6-31g(d,p) level of theory using acetylene as a model substrate.

Unsurprisingly, the ketone moiety is not bound in the bisalkyne diphosphine benzophenone complex ^p-tol15-(C₂H₂)₂. However, geometry optimization of the metallacyclopentadiene intermediate formed by oxidative coupling reveals a pentacoordinate geometry in which the ketone is bound to the metal (^p-tol16-MCP, Scheme 6), suggesting hemilabile behavior of ^p-tolL1 in the key oxidative coupling step. Respective Ni–O and Ni–C distances of 1.93 and 1.99 Å, as well as an elongated C–O bond (1.35 Å), indicate a strong interaction with the metal center. In addition, a P1–Ni–P2 angle of 171° results in an approximate trigonal-bipyramidal geometry with apical P atoms. The fact that the C=O unit binds side-on to a formal Ni(II) center can be surprising at first sight in view of its low propensity to bind to divalent metal halides.^24,25 This can be understood by a synergistic interaction between a strongly σ-donating bidentate hydrocarbyl ligand and the strongly π-accepting ketone ligand in the equatorial plane of the trigonal bipyramid.

Scheme 6. C–C Coupling Reaction Step, as Modeled by DFT Calculations.

R = para-tolyl.

For comparison, the geometry of the metallacycles supported by the triphosphine ^p-tolL2 and the diphosphine ether ^PhL3 ligands were also optimized (Figure 7). The triphosphine-supported [(^p-tolL2)Ni(C₄H₄)] (^p-tol17-MCP) adopts a similar trigonal bipyramid geometry to ^p-tol16-MCP. This binding mode may contribute to explain the differences in reactivity with the rac-BINAP system, for which this mode is inaccessible. In contrast, [(^PhL3)Ni(C₄H₄)] (^Ph18-MCP) exhibits a distorted square planar geometry, in which the ^PhL3 is bound in bidentate manner (N–O = 3.20 Å; WBI(Ni–O) < 0.01). Hence, the metallacycle in ^Ph18-MCP is not stabilized by its central ether donor group,⁶⁴ which may partly explain the lower regioselectivity of ^Ph3 is toward the 1,2,4-trisubstituted arene cyclotrimerization product. Furthermore, no significant modifications of the P1–O–P2 bite angle from phenylacetylene analogue ^Ph6-Ph are visible.

Figure 7.

C–C coupling reaction step. Comparison of metallacyclopentadiene molecular structures bearing the ^p-tolL1, ^p-tolL2, and ^PhL3 ligands, optimized at a B3LYP/6-31g(d,p) level of theory. Hydrogen atoms have been omitted for clarity.

At this point, we showed that the hemilabile character of the ketone ligand ^p-tolL1 contributes to higher activity and selectivity in the cyclotrimerization of terminal alkynes. The combination of the two effects presented in this section, that is, decoordination and coordination of the C=O unit to assist the substrate uptake and stabilize the key metallacyclopentadiene intermediate, can explain its superior performance. Exchanging the C=O moiety in complex ^p-tol1 with a stronger donor atom, like a phosphine group (P–Ph) or with a bidentate ligand (O as central atom) decreases either the activity or the selectivity of the overall process.

Therefore, based on the computational and experimental observations obtained from this study, a simplified catalytic cycle is proposed for the cyclotrimerization of terminal alkynes catalyzed by the nickel diphosphine benzophenone system (Scheme 7). The resting state, ^p-tol4-R¹, can be generated by ligand exchange from ^p-tol1 or in situ from the ligand and Ni(cod)₂ (1). The in situ system and ^p-tol1 operate at similar rates of product formation under the same tested conditions (similar percentage of isolated yields), showing that the dissociation of the coligand (cod vs BPI) and competitive binding with the metal center does not affect the final yields, as long as ^p-tol4-R¹ can be formed. At this step, the ketone is not bound, which facilitates coordination of the second alkyne molecule to generate Ni-bisalkyne ^p-tol15-(C₂HR¹)₂ (2). The next step (3) involves the C–C oxidative coupling, which is coupled to coordination of the ketone to nickel in η²-fashion. This interaction may favor the formation of the 2,5-disubstituted metallacyclopentadiene ^p-tol16-MCP-R¹. The R¹ substituents have been arbitrarily positioned in 2,5 positions as they promote the selective formation of the 1,2,4-trisubstituted benzene. From this intermediate, the insertion of the third alkyne to form the final benzene product could go via either [4 + 2] cycloaddition, migratory insertion, or [2 + 2] cycloaddition, followed by reductive elimination of the trisubstituted benzene and ligand exchange to regenerate ^p-tol1 for a new turnover (4). Throughout the reaction coordinate, the ligand can adapt its geometry by the labile interaction of C=O to Ni and stabilize intermediates.

Scheme 7. Proposed Simplified Catalytic Cycle for Cyclotrimerization of Terminal Alkynes Catalyzed by [(^p-tolL1)Ni(BPI)] or by in Situ Generation of the Active Intermediate with ^p-tolL1 + Ni(cod)₂.

R = para-tolyl.

Conclusions

In conclusion, we have reported here the synthesis and characterizations of Ni(0) complexes, incorporating a diphosphine ketone (^p-tolL1), trisphosphine (^p-tolL2), and diphosphine ether (^PhL3) ligand, in which the binding mode of the stabilizing imine or alkyne coligand can change according to the structural and electronic characteristics of the supporting ligand. The characterization of [(^p-tolL2)Ni(HC≡CPh)] (^p-tol5-Ph) provides a rare example of weak activation of alkynes by nickel complexes, attributed to its unique coordination geometry. We show in this study that [(^p-tolL1)Ni(BPI)] (^p-tol1) is an effective catalyst in the [2 + 2 + 2] cyclotrimerization of terminal alkynes. In contrast, related Ni(0) complexes [(^p-tolL2)Ni(BPI)] (^p-tol2) and [(^PhL3)Ni(BPI)] (^Ph3) are less active or less selective toward the 1,2,4-trisubstitued benzene cyclotrimerization product under similar conditions. We attribute the enhanced reactivity of ^p-tol1 to the hemilabile character of its diphosphine benzophenone ligand. In situ NMR spectroscopy and DFT calculations suggest that C=O hemilability may facilitate substrate uptake and assist the key oxidative coupling step. A more detailed mechanistic study of the reaction and applications of diphosphine-ketone ligands to other catalytic processes are actively investigated in our laboratories.

Experimental Section

Chemicals and Reagents

Unless otherwise noted, all reactions were carried out under an inert N₂(g) atmosphere, using standard Schlenk line or glovebox techniques, and stirred magnetically. Silica gel P₆₀ (SiliCycle) was used for column chromatography. Analytical thin layer chromatography was performed using SiliCycle 60 F₂₅₄ silica gel (precoated sheets, 0.20 mm thick) from Merck KGaA (Darmstadt, Germany). Deuterated solvents were purchased from Cambridge Isotope Laboratory Incorporation (Cambridge, USA) and were degassed by standard freeze–thaw–pump procedure⁶⁵ and subsequently stored over molecular sieves. Common solvents were purified using a MBRAUN MB SPS-80 purification system or by standard distillation techniques or both.⁶⁵ They were degassed by bubbling N₂(g) through the liquid for at least 30 min and then stored over molecular sieves. Non-halogenated solvents were tested with a standard purple solution of sodium benzophenone ketyl in tetrahydrofuran to confirm effective oxygen and moisture removal. Other solvents were checked for water content by the Karl Fischer titration or by ¹H NMR. Liquid chemicals were first degassed by standard freeze–pump–thaw procedures or purged with N₂(g) and then stored over molecular sieves prior to use. Phosphorus-containing compounds were checked for oxidation by ³¹P NMR before use. O-(Bromophenyl)-diphenylphosphine,^24,47o-(bromophenyl)-di-p-tolylphosphine,²⁵ 2,2′-bis(diphenylphosphino)benzophenone (^PhL1),²⁴ and 2,2′-bis(di(p-tolyl)phosphino)benzophenone (^p-tolL1)²⁵ were synthesized according to reported procedures. All other reagents and starting materials were purchased from commercial sources and used without further purification, except when specified.

Physical Methods

The ¹H, ¹³C, ³¹P, and ¹⁹F NMR (400, 100, 161, and 400 MHz, respectively) spectra were recorded at 297 K on an Agilent MRF 400 spectrometer. All chemical shifts are reported in the standard δ notation of parts per million, referenced to residual peak of the solvent, as determined relative to Me₄Si (δ = 0 ppm).⁶⁶ Variable-temperature (VT) NMR were recorded in d-toluene from −85 °C to room temperature. Infrared spectra were recorded using a PerkinElmer Spectrum Two FT-IR spectrometer. For air-sensitive compounds, a N₂ flow was used. Absorption spectra were recorded using a Lambda 35 UV–vis spectrometer. The UV–vis solutions were prepared in the glovebox, using degassed and dried solvent, and then stored in a cuvette sealed with a Teflon cap. The acquisition and analysis of the UV–vis data were performed with PerkinElmer UW WinLab and UV WinLab Data Processor and Viewer software. GC-MS measurements were conducted on a PerkinElmer Clarus 680 GC (column PE, Elite 5MS, 15 m × 0.25 mm ID × 0.25 μm) equipped with Clarus SQ8T MS and analyzed with TurboMass software. ESI-MS analysis was recorded with a Water LCT Premier XE spectrometer. Elemental analysis was provided by Mikroanalytisches Laboratorium Kolbe, Mülheim an der Ruhr, Germany, and Medac Ltd., Surrey, UK.

Computational Methods

DFT (density functional theory) results were obtained using the Gaussian 09 software package.⁶⁷ Restricted (R) geometry optimizations use the B3LYP (Becke, three-parameter, Lee–Yang–Parr) functional and the 6-31g(d,p) basis set on all atoms. The structures were optimized without any symmetry restraints and are either minima or transition states. Frequency analyses were performed on all calculations. The transition states search was performed using the QST3 (synchronous transit-guided quasi Newton number 3) method. For NBO (natural bond orbital) calculation, the NBO6 program,⁶⁸ up to the NLMO (natural localized molecular orbital) basis set, was used at B3LYP/def2TZVP level of theory from the optimized geometry. Pictures derived from DFT calculations have been generated using the GaussView software. The B3LYP functional was chosen as it has been shown to be accurate for geometry optimization of related metal compounds,⁶⁹ including closely related ^PhL1–metal systems.^24,25

Synthetic Methods

Ligand: Bis(2-di-p-tolyl)phosphinophenyl) Phenylphosphine (^p-tolL2)

Adapted from the procedure by Koshevoy et al.⁴⁶ To a suspension of (o-bromophenyl)di-p-tolylphosphine (5.01 g, 16.6 mmol) in dried and degassed THF (76 mL), a hexane solution of n-BuLi (1.6 M, 8.50 mL, 13.5 mmol) was added dropwise within 10 min at −78 °C under a N₂ atmosphere. The reaction mixture was stirred at −78 °C for 1 h, and PPhCl₂ (0.92 mL, 6.77 mmol) was added dropwise. The mixture was stirred at this temperature for one additional hour and then allowed to slowly warm up to room temperature. The solution was stirred at room temperature for three more hours and then quenched with MeOH (30 mL). The volatiles were removed in vacuo, and the yellow amorphous residue was washed with MeOH (5 × 15 mL) to afford ^p-tolL2 as a white solid, which was dried overnight under vacuum (3.52 g, 5.10 mmol, 76%). ¹H NMR (400 MHz, C₆D₆, 25 °C): δ_H 7.40–7.33 (ArH, m, 6H), 7.33–7.25 (ArH, m, 6H), 7.19–7.16 (ArH, m, 2H), 7.01–6.94 (ArH, m, 5H), 6.93–6.87 (ArH, m, 6H), 6.84 (ArH, d, ³J_H,H = 7.9 Hz, 4H), 2.03 (CH₃, s, 6H), 2.01 (CH₃, s, 6H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P – 14.8 (p-tolP, 2P), – 16.9 (PhP, 1P) [AB₂ system, ²J_A,B = 155 Hz]. ¹³C NMR (100 MHz, C₆D₆, 25 °C): δ_C 145.6–144.8 (m), 138.4–138.1 (m), 138.0 (d, J_C,P = 12.7 Hz), 135.2–134.9 (m), 134.7 (t, J_C,P = 4.4 Hz), 134.6–134.3 (m), 129.6, 129.4 (t, J_C,P = 3.3 Hz), 129.1, 128.9, 128.2, 129.9, 21.2 (CH₃), 21.2 (CH₃). ATR-IR: ν [cm^–1] = 3043, 1494, 1439, 1184, 1090, 806, 752, 504. HRMS (ESI, CH₃CN, AgNO₃): m/z calcd for [M + Ag]⁺ 793.1472; found 793.1605.

Nickel Complexes

[(^PhL1)Ni(BPI)] (^Ph1)

Ni(cod)₂ (249 mg, 0.91 mmol), ^PhL1 (502 mg, 0.91 mmol), and benzophenone imine (181 mg, 0.92 mmol) were dissolved in dried degassed toluene (10 mL) under an inert atmosphere. The reaction mixture was stirred at room temperature for 20 min. Dried and degassed hexane (5 mL) was added to the resulting black solution, causing the precipitation of a black solid. The precipitate was collected by filtration, washed with hexane (3 × 4 mL), and dried under vacuum to afford ^Ph1 as a black powder (597 mg, 0.76 mmol, 83%). Single crystals suitable for X-ray diffraction and elemental analysis were obtained by slow exchange of hexane into a concentrated THF solution of ^Ph1. ¹H NMR (400 MHz, C₆D₆, 25 °C): δ_H 9.88 (NH, br s, 1H), 7.90 (ArH, d, ³J_H,H = 7.2 Hz, 2H), 7.83 (ArH, d, ³J_H,H = 7.6 Hz, 2H), 7.75–7.70 (ArH, m, 4H), 7.23–7.19 (ArH, m, 2H) 7.09–6.79 (ArH, m, 26H), 6.73 (ArH, t, ³J_H,H = 7.6 Hz, 2H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P 16.4 (s, 2P). ¹³C NMR (100 MHz, d₈-THF, 25 °C): δ_C 170.7 (C=N, t, ³J_C,P = 5.4 Hz), 156.0 (t, J_C,P = 18.0 Hz), 141.1, 140.2 (t, J_C,P = 17.0 Hz), 138.7, 136.7 (t, J_C,P = 11.2 Hz), 136.4 (t, J_C,P = 13.7 Hz), 132.9 (t, J_C,P = 7.7 Hz), 132.6, 131.8 (t, J_C,P = 6.6 Hz), 129.62 (t. J_C,P = 4.0 Hz), 128.9–126.7 (m) 125.8, 125.5 (t, J_C,P = 8.1 Hz), 117.2 (C=O, t, ³J_C,P = 7.6 Hz). ATR-IR: ν [cm^–1]: 3163, 3050, 1583, 1404, 1432, 1478, 1432, 1303, 1249, 1091, 912, 778, 739, 692, 515. UV–vis (toluene): λ_max [nm] 365, 574. Elemental analysis, Anal. Calcd for C₅₀H₃₉NNiOP₂·1/2 hexane: C, 76.37; H, 5.56; N, 1.68. Found: C, 76.33; H, 5.58; N, 1.52. The crystal structure contains channels along the c-axis, which are filled with disordered hexane molecules (Supporting Information; Figure S110).

[(^p-tolL1)Ni(BPI)] (^p-tol1)

Ni(cod)₂ (108 mg, 0.39 mmol), ^p-tolL1 (238 mg, 0.39 mmol), and benzophenone imine (71 mg, 0.39 mmol) were dissolved in dried degassed toluene (10 mL) under inert atmosphere. The reaction mixture was stirred at room temperature for 20 min. The solvent was evaporated, and the crude mixture was subsequently dissolved in dried and degassed THF (5 mL). Dried and degassed hexane (5 mL) was added to the resulting black solution, causing the precipitation of a black solid. The precipitate was filtered, washed with hexane (3 × 4 mL), and dried under vacuum to afford ^p-tol1 as a black powder (221 mg, 0.31 mmol, 79%). Single crystals for elemental analysis were obtained by slow exchange of hexane into a concentrated toluene solution of ^p-tol1. ¹H NMR (400 MHz, C₆D₆, 25 °C): δ_H 10.01 (NH, s, 1H), 7.98 (ArH, d, ³J_H,H = 7.4 Hz, 2H), 7.88 (ArH, dd, ³J_H,H = 7.7, ⁴J_H,H = 1.3, 2H), 7.66 (ArH, dt, ³J_H,H = 7.9, ⁴J_H,P = 4.6, 4H), 7.30–7.25 (ArH, m, 2H), 7.05 (ArH, dt, ³J_H,H = 7.9 Hz, ⁴J_H,P = 4.4 Hz), 6.97–6.88 (ArH, m, 6H), 6.86–6.73 (ArH, m, 14H), 2.11 (CH₃, s, 6H), 2.09 (CH₃, s, 6H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P 15.5 (s, 2P). ¹³C NMR (100 MHz, C₆D₆, 25 °C): δ_C 169.8 (C=N, t, ³J_C,P = 6.1 Hz), 156.7 (t, J_C,P = 18.1 Hz), 141.6 (t, J_C,P = 2.3 Hz), 141.2 (t, J_C,P = 16.9 Hz), 138.6 (Ar, t, J_C,P = 2.4 Hz,), 138.1, 137.2, 134.7 (t, J_C,P = 12.1 Hz), 133.9 (t, J_C,P = 14.7 Hz), 133.6, 133.5, 133.5, 133.4, 132.7 (t, J_C,P = 6.8 Hz), 130.5 (t, J_C,P = 4.3 Hz), 129.1 (t, J_C,P = 4.4 Hz), 129.0 (t, J_C,P = 4.0 Hz), 128.8, 127.5, 126.8, 126.5 (t, J_C,P = 8.1 Hz), 119.0 (C=O, t, J_C,P = 5.1 Hz), 21.3 (CH₃), 21.1(CH₃). ATR-IR: ν [cm^–1]: 3152, 3053, 2917, 2860, 1598, 1496, 1448, 1394, 1250, 1185, 1092, 1018, 912, 803, 693, 627, 514. Elemental analysis, Anal. Calcd for C₅₄H₄₇NNiOP₂: C, 76.61; H, 5.66. Found: C, 76.55; H, 5.93.

[(^p-tolL2)Ni(BPI)] (^p-tol2)

Ni(cod)₂ (163 mg, 0.59 mmol), ^p-tolL2 (405 mg, 0.59 mmol), and benzophenone imine (108 mg, 0.59 mmol) were combined together and dissolved in dried degassed toluene (7 mL) under inert atmosphere. The reaction mixture was stirred at room temperature for 1 h, and dried degassed hexane (7 mL) was added. The resulting solution was left in the freezer at −35 °C for 16 h, during which the precipitation of a black solid was observed. The solid was filtered, washed with cold hexane (5 × 4 mL), and dried under vacuum to afford ^p-tol2 as a black powder (315 mg, 0.34 mmol, 58%). ¹H NMR (400 MHz, C₆D₆, 25 °C): δ_H 10.25 (NH, dt, ³J_H,P = 3.2 Hz, ³J_H,P = 2.7 Hz, 1H), 8.49 (ArH, d, ³J_H,H = 7.6 Hz, 2H), 8.05 (ArH, dd, ³J_H,H = 7.2 Hz, ³J_H,P = 4.4 Hz, 2H), 7.66–7.60 (ArH, m, 5H), 7.55 (ArH, t, ³J_H,H = 8.0 Hz, 2H), 7.12–6.86 (ArH, m, 18H), 6.83 (ArH, d, ³J_H,H = 8.4 Hz, 6H), 6.72 (ArH, d, ³J_H,H = 7.6 Hz, 4H), 2.14 (CH₃, s, 6H), 1.95 (CH₃, s, 6H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P 37.1 (p-tolP, 2P) 28.7 (PhP, 1P) [AK₂ system, ²J_A,K = 85 Hz]. ¹³C NMR (100 MHz, C₆D₆, 25 °C): δ_C 168.5 (C=N, dt, ³J_C,P = 8.1 Hz, ³J_C,P = 7.4 Hz), 149.6–147.8 (m), 143.2, 141.3 (dt, J_C,P = 11.1 Hz, J_C,P = 2.7 Hz), 137.8 (dt, J_C,P = 13.9 Hz, J_C,P = 4.0 Hz), 137.2, 137.1–136.4 (m), 136.3, 132.8–132.6 (m), 132.4 (d, J_C,P = 14.1 Hz), 131.8 (t, J_C,P = 6.7 Hz), 131.3 (d, J_C,P = 13.8 Hz), 130.6, 129.9–129.5 (m), 129.1 (t, J_C,P = 4.2 Hz), 128.8–128.6 (m) 127.6, 127.1, 126.3, 21.4 (CH₃), 21.1 (CH₃). ATR-IR: ν [cm^–1] 3048, 2972, 2917, 2864, 1664, 1598, 1496, 1445, 1187, 1114, 804, 695, 513. Elemental analysis, Anal. Calcd for C₅₉H₅₂NNiP₃: C, 76.47; H, 5.66; N, 1.51. Found: C 75.97 H 5.49 N 1.56.

[(^PhL3)Ni(BPI)] (^Ph3)

Under inert atmosphere, a dried and degassed toluene solution (7 mL) of benzophenone imine (169 mg, 0.93 mmol) was added to a vial containing Ni(cod)₂ (255 mg, 0.93 mmol), and ^PhL3 (500 mg, 0.93 mmol). The reaction mixture was stirred at room temperature for 16 h in which a solid spontaneously precipitated. The resulting solid was filtered, washed with cold hexane (5 × 4 mL), and dried under vacuum to afford ^Ph3 as an orange powder (582 mg, 0.75 mmol, 81%). Single crystals suitable for X-ray diffraction and elemental analysis were obtained by slow diffusion of hexane into a C₆D₆ solution of ^Ph3. ¹H NMR (400 MHz, C₆D₆, 25 °C): δ_H 9.71 (NH, s, 1H), 8.00–7.85 (ArH, br s, 2H), 7.70–7.64, (ArH, br s, 2H), 7.61–7.53 (ArH, br s, 6H), 7.13–6.90 (ArH, br s, 22H), 6.78–6.72 (ArH, br m, 4H), 6.56–6.52 (ArH, br m, 2H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P 32.4 (s, 2P). ATR-IR: ν [cm^–1] 3200, 3052, 1589, 1565, 1462, 1434, 1363, 1259, 1216, 1095, 880, 747, 693, 622, 505. Elemental analysis, Anal. Calcd for C₄₉H₂₉NNiOP₂: C, 75.60; H, 5.05; N, 1.80. Found: C, 75.22; H, 5.69; N, 2.05. Due to poor solubility and progressive decomposition of ^Ph3 in common solvents, no ¹³C NMR was recorded.

In Situ Generation of [(^p-tolL1)Ni(HC≡CR¹)] (^p-tol4-R¹)

Under an inert atmosphere, 1 equiv of [(^p-tolL1)Ni(BPI)] (^p-tol1) was mixed with 1 equiv of a terminal alkyne and dissolved in dried degassed C₆D₆ (0.6 mL). The solution was transferred into a Young-type NMR tube, and the mixture was analyzed. ³¹P and ¹H NMR show the appearance of one new single species, [(^p-tolL1)Ni(HC≡CR¹)] (^p-tol4-R¹), in addition to the partial release of BPI. High in situ yield was obtained with methyl propargyl ether (R¹ = CH₂OMe) as alkyne reactant, leading to the formation of [(^p-tolL1)Ni(HC≡CH₂OMe)] (^p-tol4-CH₂OMe). The solution contains a mixture of ^p-tol4-CH₂OMe, ^p-tol1, BPI, and methyl propargyl ether. ¹H NMR (400 MHz, C₆D₆, 25 °C): δ_H 10.01 (^p-tol1, NH, s, 1H), 9.82 (BPI, NH, br. s, 1H) 8.03 (^p-tol1, ArH, d, ³J_H,H = 7.4 Hz, 2H), 7.92 (^p-tol1, ArH, dd, ³J_H,H = 7.7, ⁴J_H,H = 1.3, 2H), 7.70 (^p-tol1, ArH, dt, ³J_H,H = 7.9, ⁴J_H,P = 4.6, 4H), 7.54 (^p-tol4-CH₂OMe, p-tol-ArH, td, ³J_H,P = 8.0 Hz, ³J_H,P = 7.2 Hz, 8H), 7.35–7.29 (^p-tol1, ArH, m, 2H), 7.10 (^p-tol1, ArH, dt, ³J_H,H = 7.9 Hz, ⁴J_H,P = 4.4 Hz), 7.02–6.92 (^p-tol1, ArH, m, 6H), 6.91–6.76 (^p-tol1, ArH, m, 14H), 6.74 (^p-tol4-CH₂OMe, ArH, t, ³J_H,H = 7.6 Hz, 2H), 6.66 (^p-tol4-CH₂OMe, ArH, td, ³J_H,H = 7.6 Hz, ⁴J_H,H = 1.6 Hz, 2H), 6.34 ppm (^p-tol4-CH₂OMe, ≡CH,³J_H,P 15.6, ⁴J_H,H = 1.6 Hz, 1H) 4.63 (^p-tol4-CH₂OMe, CH₂, d, ⁴J_H,H = 1.6 Hz, 2H), 3.70 (HC≡CCH₂OMe, CH₂,⁴J_H,H = 1.6 Hz, 2H) 3.32 (^p-tol4-CH₂OMe, OCH₃, s, 3H), 3.03 (HC≡CCH₂OMe, OCH₃, s, 1H); 2.10 (^p-tol1, CH₃, s, 6H), 2.03 (^p-tol4-CH₂OMe, CH₃, s, 12H), 1.96 (HC≡CCH₂OMe, ≡CH,⁴J_H,H = 1.6 Hz, 1H), 1.91 (^p-tol1, CH₃, s, 6H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P 33.3 (^p-tol4-CH₂OMe, s, 2P), 15.5 (^p-tol1, s, 2P).

¹³C NMR was acquired in order to locate the resonances from the carbonyl and methyl groups of the ligand of ^p-tol4-CH₂OMe. ¹³C NMR (100 MHz, C₆D₆, 25 °C): δ_C 203.1 (C=O, t, ⁴J_C,P = 4.7 Hz), 22.2 (H₃C, d, ⁴J_C,P = 2.3 Hz).

[(^p-tolL2)Ni(HC≡CPh)] (^p-tol5-Ph)

Under inert atmosphere, [(^p-tolL2)Ni(BPI)] (^p-tol2) (100 mg, 0.11 mmol), and phenylacetylene (11 mg, 0.11 mmol) were mixed together and dissolved in dried degassed toluene (5 mL). The reaction mixture was subsequently stirred at room temperature for 1 h. The solvent was removed under vacuum, and the crude residue was dissolved in dried degassed THF (3 mL). Dried degassed hexane (3 mL) was added, and the mixture was put in the freezer at −35 °C for 16 h, after which a red precipitate was observed. The solid was isolated by filtration and washed with cold hexane (5 × 2 mL) to afford ^p-tol5-Ph as a red powder (59 mg, 0.07 mmol, 66%). Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a concentrated THF solution of ^p-tol5-Ph. ¹H NMR (400 MHz, C₆D₆, 25 °C): δ_H 7.92 (ArH, dd, ³J_H,H = 8.0 Hz, ³J_H,P = 6.8 Hz, 2H), 7.63–7.53 (ArH, m, 9H), 7.45–7.39 (ArH, m, 2H), 7.10–6.95 (ArH, m, 1H), 6.88 (ArH, tt, ³J_H,H = 7.2 Hz, ⁴J_H,P 1.2 Hz, 1H), 6.83 (ArH, d, ³J_H,H = 7.6 Hz, 4H), 6.72 (ArH, d, ³J_H,H = 7.6 Hz, 4H), 6.29 (≡CH, dt, ³J_H,P = 25.6 Hz, ³J_H,P = 7.2 Hz, 1H), 2.12 (CH₃, s, 6H), 1.91 (CH₃, s, 6H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P 78.7 (PhP, t, ²J_P,P = 35.4 Hz, 1P), 28.1 (p-tolP, d, ²J_P,P = 35.4 Hz, 2P). ¹³C NMR (100 MHz, C₆D₆, 25 °C): δ_C 150.0 (dt, J_C,P = 47.1 Hz, J_C,P = 16.4 Hz), 146.9 (d, J_C,P = 36.0 Hz) 146.4 (dd, J_C,P = 61.6 Hz, J_C,P = 34.3 Hz),140.1 (dt, J_C,P = 41.6 Hz, J_C,P = 26.0 Hz), 137.9, 137.0–136.6 (m), 136.8, 133.7, 133.1 (t, J_C,P = 28.0) Hz, 132.5 (t, J_C,P = 28.0 Hz), 131.8–131.5 (m), 129.8, 129.7–128.9 (m) 101.1 (≡CPh, d, ²J_C,P = 20.0 Hz), 92.9 (CH, br s), 21.3 (CH₃, d, J_C,P = 4.0 Hz), 21.1 (CH₃, d, J_C,P = 4.0 Hz). ATR-IR: ν [cm^–1] 3042, 2918, 2861, 1823, 1589, 1479, 1440,1425, 1394, 1185, 1100, 1085, 1118, 805, 758, 690, 666, 625, 612, 539, 519. Due to the high sensitivity of ^p-tol6-Ph, no elemental analysis data was obtained. The purity and identity of the compound was established by NMR and by its X-ray diffraction structure.

Alternative Synthesis of ^p-tol5-Ph

Under inert atmosphere, Ni(cod)₂ (80 mg, 0.29 mmol), ^p-tolL2 (200 mg, 0.29 mmol), and phenylacetylene (32 mg, 0.31 mmol) were mixed together and dissolved in dried degassed toluene (5 mL). The reaction mixture was subsequently stirred at room temperature for 1 h. Precipitation of a red solid was observed after adding dried degassed hexane (10 mL) and leaving the solution to stand overnight in the freezer at −35 °C. The resulting solid was filtered, washed with cold hexane (5 × 5 mL), and dried under vacuum to afford ^p-tol5-Ph as a red powder (169 mg, 0.20 mmol, 70%).

Synthesis of Deuterated Analogue of ^p-tol5-Ph, [(^p-tolL2)Ni(DC≡CPh)]

The same procedure as for the synthesis of ^p-tol5-Ph was applied from Ni(cod)₂ (30 mg, 0.10 mmol), ^p-tolL2 (70 mg, 0.10 mmol), and d-phenylacetylene (11 mg, 0.10 mmol). [(^p-tolL2)Ni(DC≡CPh)] was isolated as a red solid in a 74% yield (63 mg, 0.07 mmol). [(^p-tolL2)Ni(DC≡CPh)] can also be generated from reaction of ^p-tol2 with d-acetylene in a 1:1 stoichiometry. ¹H NMR (400 MHz, C₆D₆, 25 °C): δ_H 7.92 (ArH, dd, ³J_H,H = 8.0 Hz, ³J_H,P = 6.8 Hz, 2H), 7.63–7.53 (ArH, m, 9H), 7.45–7.39 (ArH, m, 2H), 7.10–6.95 (ArH, m, 14H), 6.88 (ArH, tt, ³J_H,H = 7.2 Hz, ⁴J_H,P 1.2 Hz, 1H), 6.83 (ArH, d, ³J_H,H = 7.6 Hz, 4H), 6.72 (ArH, d, ³J_H,H = 7.6 Hz, 4H), 2.12 (CH₃, s, 6H), 1.91 (CH₃, s, 6H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P 78.7 (PhP, t, ²J_P,P = 35.4 Hz, 1P), 28.1 (p-tolP, d, ²J_P,P = 35.4 Hz, 2P). ¹³C NMR (100 MHz, C₆D₆, 25 °C): δ_C 150.0 (dt, J_C,P = 47.1 Hz, J_C,P = 16.4 Hz), 146.9 (d, J_C,P = 36.0 Hz) 146.4 (dd, J_C,P = 61.6 Hz, J_C,P = 34.3 Hz), 140.1 (dt, J_C,P = 41.6 Hz, J_C,P = 26.0 Hz), 137.9, 137.0–136.6 (m), 136.8, 133.7, 133.1 (t, J_C,P = 28.0) Hz, 132.5 (t, J_C,P = 28.0 Hz), 131.8–131.5 (m), 129.8, 129.7–128.9 (m) 101.1 (≡CPh, br m, 21.3 (≡CH₃, d, J_C,P = 4.0 Hz), 21.1 (CH₃, d, J_C,P = 4.0 Hz). ATR-IR: ν [cm^–1] 3333, 3042, 2918, 2861, 1823, 1761, 1589, 1479, 1440,1425, 1394, 1185, 1100, 1085, 1118, 805, 758, 690, 666, 625, 612, 539, 519.

[(^PhL3)Ni(HC≡CPh)] (^Ph6-Ph)

Under inert atmosphere and at room temperature, a suspension of [(^PhL3)Ni(BPI)] (^Ph3) (200 mg, 0.26 mmol) in dried degassed toluene (3 mL) was combined with a toluene solution (3 mL) of phenylacetylene (26 mg, 0.26 mmol), resulting in a light yellow solution. The reaction mixture was subsequently stirred at room temperature for 10 min. Precipitation of a yellow solid was observed after adding dried degassed hexane (6 mL) and leaving the solution to stand overnight. The resulting solid was filtered, washed with hexane (5 × 5 mL), and dried under vacuum to afford ^Ph6-Ph as a yellow powder (112 mg, 0.16 mmol, 74%). Single crystals suitable for X-ray diffraction were obtained by slow diffusion of hexane into a concentrated toluene solution of ^Ph6-Ph. ¹H NMR (400 MHz, C₆D_6, 25 °C): δ_H 7.94–7.87 (ArH, m, 4H), 7.46–7.40 (ArH, m, 4H), 7.30 (ArH, dd, ³J_H,H = 6.8 Hz, ⁴J_H,H = 1.4 Hz, 2H), 7.05–6.80 (ArH and ≡CH, m, 18H), 6.75 (ArH, ddd, ³J_H,H = 7.6 Hz, ⁴J_H,P = 4.4 Hz, ⁴J_H,H = 1.2 Hz, 1H), 6.66–6.56 (ArH, m, 3H), 6.39 (ArH, ddd, ³J_H,H = 7.2 Hz, ⁴J_H,P = 4.4 Hz, ⁴J_H,H = 1.2 Hz, 1H), 6.32 (ArH, tt, ³J_H,H = 7.6 Hz, ⁴J_H,H = 1.0 Hz, 1H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P 29.2 (d, ²J_P,P = 22.5 Hz, 1P), 27.3 (d, ²J_P,P = 22.5 Hz, 1P). ¹³C NMR (100 MHz, C₆D₆, 25 °C) δ_C 160.1 (d, ²J_C,P = 9.1 Hz), 159.8 (d, ²J_C,P = 11.4 Hz), 136.6 (≡CPh, dd, ²J_C,P = 34.9 Hz, ²J_C,P = 4.8 Hz), 136.1, 135.5 (dd, J_C,P = 34.8 Hz, J_C,P = 4.1 Hz), 134.9 (d, J_C,P = 14.5 Hz), 133.8 (d, J_C,P = 12.7 Hz), 132.9 (d, J_C,P = 1.8 Hz), 130.7, 130.0, 129.8 (d, J_C,P = 3.8 Hz), 129.6, 129.4 (d, J_C,P = 1.7 Hz), 129.3, 129.0 (d, J_C,P = 3.1 Hz), 128.8 (d, J_C,P = 1.5 Hz), 125.3, 125.7(≡CH, dd, ²J_C,P = 34.9 Hz, ²J_C,P = 4.8 Hz) 124.9 (d, J_C,P = 4.0 Hz), 124.1 (d, J_C,P = 4.0 Hz), 122.9 (d, J_C,P = 4.9 Hz), 118.3 (d, J_C,P = 3.4 Hz). ATR-IR: ν [cm^–1] 3286, 3171, 3052, 1749, 1588, 1564, 1480, 1461, 1434, 1259, 1213, 1095, 882, 834, 745, 692, 554, 503. Due to the high sensitivity of ^Ph6-Ph, no elemental analysis data was obtained. The purity and identity of the compound was established by NMR and by its X-ray diffraction structure.

Alternative Synthesis of ^Ph6-Ph

Under inert atmosphere, Ni(cod)₂ (292 mg, 1.06 mmol), ^PhL3 (567 mg, 1.05 mmol), and phenylacetylene (108 mg, 1.06 mmol) were mixed together and dissolved in dried degassed toluene (7 mL). The reaction mixture was subsequently stirred at room temperature for 20 min. Precipitation of a yellow solid was observed after adding dried degassed hexane (10 mL) and leaving the solution to stand overnight. The resulting solid was filtered, washed with hexanes (5 × 5 mL), and dried under vacuum to afford ^Ph6-Ph as a yellow powder (387 mg, 0.55 mmol, 53%).

Synthesis of Deuterated Analogue of ^Ph6-Ph, [(^PhL3)Ni(DC≡CPh)]

The same experimental procedure as for the synthesis of ^Ph6-Ph was followed, using Ni(cod)₂ (50 mg, 0.18 mmol), ^PhL3 (99 mg, 0.18 mmol), and d-phenylacetylene (19 mg, 0.18 mmol) as starting materials. [(^PhL3)Ni(DC≡CPh)] was isolated as a yellow powder (60 mg, 0.08 mmol, 48%). [(^PhL3)Ni(DC≡CPh)] can also be generated from reaction of ^Ph3 with d-acetylene in a 1:1 stoichiometry. ¹H NMR (400 MHz, C₆D₆, 25 °C): δ_H 7.94–7.87 (ArH, m, 4H), 7.46–7.40 (ArH, m, 4H), 7.30 (ArH, dd, ³J_H,H = 6.8 Hz, ⁴J_H,H = 1.4 Hz, 2H), 7.05–6.80 (ArH, m, 17H), 6.75 (ArH, ddd, ³J_H,H = 7.6 Hz, ⁴J_H,P = 4.4 Hz, ⁴J_H,H = 1.2 Hz, 1H), 6.66–6.56 (ArH, m, 3H), 6.39 (ArH, ddd, ³J_H,H = 7.2 Hz, J_H,P = 4.4 Hz, ⁴J_H,H = 1.2 Hz, 1H), 6.32 (ArH, tt, ³J_H,H = 7.6 Hz, ⁴J_H,H = 1.0 Hz, 1H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P 29.2 (d, ²J_P,P = 22.5 Hz, 1P), 27.3 (d, ²J_P,P = 22.5 Hz, 1P). ¹³C NMR (100 MHz, C₆D₆, 25 °C): δ_C 160.1 (d, ²J_C,P = 9.1 Hz), 159.8 (d, ²J_C,P = 11.4 Hz), 136.4 (≡CPh, dd, J_C,P = 34.9 Hz, J_C,P = 4.8 Hz), 136.1, 135.5 (dd, J_C,P = 34.8 Hz, J_C,P = 4.1 Hz), 134.9 (d, J_C,P = 14.5 Hz), 133.8 (d, J_C,P = 12.7 Hz), 132.9 (d, J_C,P = 1.8 Hz), 130.7, 130.0, 129.8 (d, J_C,P = 3.8 Hz), 129.6, 129.4 (d, J_C,P = 1.7 Hz), 129.3, 129.0 (d, J_C,P = 3.1 Hz), 128.8 (d, J_C,P = 1.5 Hz), 125.3, 124.9 (d, J_C,P = 4.0 Hz), 124.1 (d, J_C,P = 4.0 Hz), 122.9 (d, J_C,P = 4.9 Hz), 118.3 (d, J_C,P = 3.4 Hz). ATR-IR: ν [cm^–1] 3052, 1588, 1564, 1461, 1434, 1259, 1213, 1095, 882, 745, 692, 503.

In Situ Generation of [(^PhL3)Ni(PhC≡CPh)] for Analytical Comparison with [(^PhL3)Ni(HC≡CPh)] (^Ph6-Ph)

Under inert atmosphere, Ni(cod)₂ (7.6 mg, 0.028 mmol), ^PhL3 (15.6 mg, 0.029 mmol), and diphenylacetylene (5.0 mg, 0.028 mmol) were combined together and diluted in C₆D₆ (0.6 mL) at room temperature, turning the solution red. The mixture was transferred into a Young-type NMR tube and measured after 30 min of reaction. NMR analysis showed full conversion of the starting reagents and the release of cod, in addition to the single generation of a new species, [(^PhL3)Ni(PhC≡CPh)]. ¹H NMR (400 MHz, C₆D₆, 25 °C): δ_H 7.71–7.61 (m, 8H), 7.18–7.13 (m, 4H), 6.97–7.88 (m, 12H), 6.86–6.82 (m, 6H), 6.73–6.66 (m, 4H), 6.55 (dd, ³J_H,H = 8.0 Hz, ⁴J_H,P = 2.8 Hz, 2H), 6.44 (t, ³J_H,H = 7.6 Hz, 2H), 5.58 (cod, s, 4H), 2.21 (cod, s, 8H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P 28.1 (s, 2P). ¹³C NMR (100 MHz, C₆D₆, 25 °C): δ_C 159.8 (t, ²J_C,P = 5.0 Hz), 136.3 (≡CPh, t, ²J_C,P = 6.8 Hz), 136.3, 136.0, 135.8 135.3, 135.2, 135.0 (d, J_C,P = 3.2 Hz), 134.8, 134.7, 134.5 (t, J_C,P = 7.0 Hz), 140.0, 130.4, 129.6, 129.5 (d, J_C,P= 3 Hz), 129.3, 129.1, 128.8, 128.7, 124.4 (cod), 123.9, 121.1, 28.4 (cod).

Ligand Exchange Reactions (Scheme 4)

In Situ Generation of [(^p-tolL1)Ni(PPh₃)] (^p-tol7-PPh₃)

In the glovebox, ^p-tol1 (5 mg, 5.9 μmol) and 1 equiv of PPh₃ (1.6 mg, 5.9 μmol) were dissolved in 0.6 mL of C₆D₆. The solution was transferred into a Young-type NMR tube, and an NMR spectrum was recorded approximately 15 min after the two reactants have been mixed together, showing full conversion of ^p-tol1. The analyzed solution contains a mixture of ^p-tol7-PPh₃ and BPI in a 1:1 ratio. [(^p-tolL1)Ni(PPh₃)] (^p-tol7-PPh₃). [(^PhL1)Ni(PPh₃)], with phenyl substituents on the phosphine ligand, has previously been reported.²⁴¹H NMR (400 MHz, C₆D₆, 25 °C): δ_H 7.86 (ArH, d, ³J_H,H = 8.0 Hz, 2H), 7.62 (ArH, t, ³J_H,P = 8.8 Hz, 6H), 7.44–7.39 (ArH, m, 2H), 7.38–7.33 (ArH, m, 4H), 7.25–7.19 (ArH, m, 4H), 6.93–7.80 (ArH, m, 9H), 6.67 (p-tolArH, d, ³J_H,H = 7.6 Hz, 2H), 6.61 (p-tolArH, d, ³J_H,H = 7.6 Hz, 2H), 1.97 (CH₃, s, 6H), 1.96 (CH₃, s, 6H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P 38.5 (Ph₃P, t, ²J_P,P = 25.7 Hz 1P), 17.1 (p-tol₂P, d, ²J_P,P = 25.7 Hz, 2P). ¹³C NMR (100 MHz, C₆D₆, 25 °C): δ_C 155.1 (dd, J_C,P = 19.1 Hz, J_C,P = 16.0 Hz), 142.8 (dt, J_C,P = 8.4 Hz, J_C,P = 16.8 Hz), 138.4, 137.6 (dt, J_C,P = 29.8 Hz, J_C,P = 4.6 Hz), 136.9, 135.2 (t, J_C,P = 11.4 Hz), 134.3–136.0 (m), 132.8 (t, J_C,P = 7.0 Hz), 132.2, 130.6, 128.7–128.5 (m), 126.8, 120.4 (C=O, dt, ²J_C,P = 13.8 Hz, ²J_C,P = 4.4 Hz) 21.2 (CH₃) 21.1 (CH₃).

In Situ Generation of [(^p-tolL1)Ni(PhCN)] (^p-tol7-PhCN) and K_eq Determination for the Reaction ^p-tol1 + PhCN ⇔ ^p-tol7-PhCN + BPI

In the glovebox, ^p-tol1 (4.9 mg, 5.8 μmol) was dissolved in 0.6 mL of C₆D₆, and 1 equiv of PhCN was added via a microsyringe (0.6 μL, 5.8 μmol). The solution was transferred to a Young-type NMR tube, and an NMR spectrum was recorded approximately 15 min after the two reactants have been mixed together. ³¹P NMR was recorded with a relaxation time of 21 s. For ¹H NMR, the singlets at 10.01 ppm (1H), 2.10 ppm (6H), and 1.91 ppm (6H) for ^p-tol1 and the triplet at 6.63 (3H) for PhCN were selected to determine the relative concentration of the reactants. As regards the determination of the concentration of products, the two singlets at 2.04 ppm (6H) and 2.03 ppm (6H) for ^p-tol7-PhCN and the singlet at 9.82 (1H) for BPI were selected. The same procedure was repeated at different stoichiometry of PhCN (i.e., 50 equiv and 200 equiv compared to ^p-tol1). ¹H NMR (400 MHz, C₆D₆, 25 °C): δ_H 10.01 (^p-tol1, NH, s, 1H), 9.82 (BPI, NH, br. s, 1H), 8.03 (^p-tol1, ArH, d, ³J_H,H = 7.4 Hz, 2H), 7.92 (^p-tol1, ArH, dd, ³J_H,H = 7.7, ⁴J_H,H = 1.3, 2H), 7.70 (^p-tol1, ArH, dt, ³J_H,H = 7.9, ⁴J_H,P = 4.6, 4H), 7.35–7.29 (^p-tol1, ArH, m, 2H), 7.10 (^p-tol1, ArH, dt, ³J_H,H = 7.9 Hz, ⁴J_HP = 4.4 Hz), 7.02–6.92 (^p-tol1, ArH, m, 6H), 6.91–6.76 (^p-tol1, ArH, m, 14H), 6.63 (PhCN, ArH, t, ³J_H,H = 7.6, 2H) 2.10 (^p-tol1, CH₃, s, 6H), 2.04 (^p-tol7-PhCN, CH₃, s, 6H), 2.03 (^p-tol7-PhCN, CH₃, s, 6H), 1.91 (^p-tol1, CH₃, s, 6H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P 15.5 (^p-tol1, s, 2P), 12.7 (^p-tol7-PhCN, s, 2P).

¹³C NMR was acquired in order to locate the resonances from the carbonyl and methyl groups of the ligand of ^p-tol7-PhCN. ¹³C NMR (100 MHz, C₆D₆, 25 °C): δ_c 22.8 (CH₃). No carbonyl peak around 200 ppm was detected.

In Situ Generation of [(^p-tolL1)Ni(C₂H₃Ph)] (^p-tol7-C₂H₃Ph) and K_eq Determination for the Reaction ^p-tol1 + C₂H₃Ph ⇔ ^p-tol7-C₂H₃Ph + BPI

In the glovebox, ^p-tol1 (4.4 mg, 5.2 μmol) was dissolved in 0.6 mL of C₆D₆ and 1 equiv of styrene was added via a microsyringe (0.6 μL, 5.2 μmol). The solution was transferred to a Young-type NMR tube, and an NMR spectrum was recorded approximately 15 min after the two reactants have reacted. ³¹P NMR was recorded with a relaxation time of 21 s. For ¹H NMR, the singlet at 10.01 ppm (1H) for ^p-tol1 and the four sets of peaks at 7.23 ppm (2H), 6.58 ppm (1H), 5.60 ppm (1H), and 5,07 ppm (1H) for C₂H₃Ph were selected to determine the relative concentration of the reactants. As regards the determination of the concentration of products, the four sets of peaks at 6.34 ppm (2H), 5.02–4.92 ppm (1H), 4.53 ppm (1H), 3.78–3.73 (1H) for ^p-tol7-C₂H₃Ph and the singlet at 9.82 (1H) for BPI were selected. The same procedure was repeated at different stoichiometry of styrene (i.e., 50 equiv and 200 equiv compared to ^p-tol1). ¹H NMR (400 MHz, C₆D₆, 25 °C): δ_H 10.01 (^p-tol1, NH, s, 1H), 9.82 (BPI, NH, br. s, 1H), 8.03 (^p-tol1, ArH, d, ³J_H,H = 7.4 Hz, 2H), 7.92 (^p-tol1, ArH, dd, ³J_H,H = 7.7, ⁴J_H,H = 1.3, 2H), 7.70 (^p-tol1, ArH, dt, ³J_H,H = 7.9, ⁴J_H,P = 4.6, 4H), 7.35–7.29 (^p-tol1, ArH, m, 2H), 7.23 (C₂H₃Ph, PhH, d, ³J_H,H = 7.2 Hz, 2H); 7.10 (^p-tol1, ArH, dt, ³J_H,H = 7.9 Hz, ⁴J_HP = 4.4 Hz), 7.02–6.92 (^p-tol1, ArH, m, 6H), 6.91–6.76 (^p-tol1, ArH, m, 14H), 6.58 (C₂H₃Ph, =CH, dd, ³J_H,H = 17.6 Hz, ³J_H,H = 10.8 Hz, 1H), 6.34 (^p-tol7-C₂H₃Ph, PhH, d, ³J_H,H = 7.2 Hz, 2H), 5.60 (C₂H₃Ph, =CH₂, d, ³J_H,H = 17.6 Hz, 1H), 5.07 (C₂H₃Ph, =CH₂, d, ³J_H,H = 10.8 Hz, 1H), 5.02–4.92 (^p-tol7-C₂H₃Ph, =CH, m, 1H), 4.53 (^p-tol7-C₂H₃Ph, =CH₂, J = 9.6 Hz, d, 1H); 3.78–3.73 (^p-tol7-C₂H₃Ph, =CH₂, m, 1H), 2.10 (^p-tol1, CH₃, s, 6H); 2.05 (^p-tol7-C₂H₃Ph, CH₃, s, 6H), 2.01 (^p-tol7-C₂H₃Ph, CH₃, s, 6H), 1.91 (^p-tol1, CH₃, s, 6H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P 39.2 (^p-tol7-C₂H₃Ph, d, ²J_P,P = 33.8 Hz, 1P), 15.9 (^p-tol7-C₂H₃Ph, d, ²J_P,P = 33.8 Hz, 1P), 15.4 (^p-tol1, s, 2P).

¹³C NMR was acquired in order to locate the resonances from the carbonyl and methyl groups of the ligand of ^p-tol7-C₂H₃Ph. ¹³C NMR (100 MHz, C₆D₆, 25 °C): δ_C 21.9 (CH₃), 21.8 (CH₃). No carbonyl peak around 200 ppm was detected.

In Situ Generation of [(^p-tolL1)Ni(C₂Ph₂)] (^p-tol7-C₂Ph₂) and K_eq Determination for the Reaction ^p-tol1 + C₂Ph₂ ⇔ ^p-tol7-C₂Ph₂ + BPI

In the glovebox, ^p-tol1 (4.7 mg, 5.7 μmol) and 1 equiv of diphenylacetylene (1.0 mg, 5.7 μmol) was dissolved in 0.6 mL of C₆D₆. The solution was transferred to a Young-type NMR tube, and an NMR spectrum was recorded approximately 15 min after the two reactants have reacted. ³¹P NMR was recorded with a relaxation time of 21 s. For ¹H NMR, the singlets at 10.01 ppm (1H), 2.10 ppm (6H), and 1.91 ppm (6H) for ^p-tol1 and the doublet of doublets at 7.52 (2H) for C₂Ph₂ were selected to determine the relative concentration of the reactants. As regards the determination of the concentration of products, the singlets at 1.95 ppm (12H) for ^p-tol7-C₂Ph₂ and the singlet at 9.82 (1H) for BPI were selected. The same procedure was repeated at different stoichiometry of diphenylacetylene (i.e., 50 equiv and 200 equiv compared to ^p-tol1). ¹H NMR (400 MHz, C₆D₆, 25 °C): δ_H 10.01 (^p-tol1, NH, s, 1H), 9.82 (BPI, NH, br. s, 1H), 8.03 (^p-tol1, ArH, d, ³J_H,H = 7.4 Hz, 2H), 7.92 (^p-tol1, ArH, dd, ³J_H,H = 7.7, ⁴J_H,H = 1.3, 2H), 7.70 (^p-tol1, ArH, dt, ³J_H,H = 7.9, ⁴J_H,P = 4.6, 4H), 7.52 (C₂Ph₂, PhH, dd, ³J_H,H = 8.0 Hz, ⁴J_H,H = 2.4 Hz, 2H), 7.35–7.29 (^p-tol1, ArH, m, 2H), 7.23 (C₂H₃Ph, PhH, d, ³J_H,H = 7.2 Hz, 2H), 7.10 (^p-tol1, ArH, dt, ³J_H,H = 7.9 Hz, ⁴J_HP = 4.4 Hz), 7.02–6.92 (^p-tol1, ArH, m, 6H), 6.91–6.76 (^p-tol1, ArH, m, 14H), 6.70 (^p-tol7-C₂Ph₂, ArH, d, ³J_H,H = 8.0 Hz 8H), 2.10 (^p-tol1, CH₃, s, 6H), 1.95 (^p-tol7-C₂Ph₂, CH₃, s, 12H), 1.91 (^p-tol1, CH₃, s, 6H). ³¹P NMR (161 MHz, C₆D₆, 25 °C): δ_P 34.0 (^p-tol7-C₂Ph₂, s, 2P), 15.4 (^p-tol1, s, 2P).

¹³C NMR was acquired in order to locate the resonances from the carbonyl and methyl groups of the ligand of ^p-tol7-C₂Ph₂. ¹³C NMR (100 MHz, C₆D₆, 25 °C): δ_C 198.3 (C=O, t, ⁴J_C,P = 4.7 Hz); 21.2 (CH₃).

Catalysis: General procedure for alkyne cyclotrimerization (Table 3 and Table 4)

Method A

In the glovebox, a toluene solution (3 mL) of alkyne was added slowly, within 1 min, into a toluene solution (3 mL) containing 0.5 mol % of the nickel catalyst (Ni-cat. ^p-tol1 to Ni-cat. ^Ph3). The solution was stirred at room temperature (substrates 8–11) or 50 °C (substrates 12 and 13) for 16 h. The solution was then opened to air, and the organic layer was extracted with HCl 1 M (1 × 10 mL) and water (2 × 10 mL). The aqueous layer was washed with Et₂O (3 × 10 mL), and the organic portions were combined together, dried over MgSO₄, filtered, and concentrated under vacuum. The product was extracted with Et₂O and filtered through a silica plug. The final product was dried in vacuo and analyzed by GC-MS and NMR. The NMR characterization values of the organic catalytic products were compared with literature. The isomeric ratios were calculated and are reported according to ¹H NMR. The reported values (yields and isomeric ratios) presented in Table 3 and Table 4 are the average values over two runs.

Method B

In the glovebox, a toluene solution (3 mL) of alkyne was added slowly, within 1 min, into a toluene solution (3 mL) containing 0.5 mol % of the ligand (^p-tolL1 to ^PhL4) and 0.5 mol % of Ni(cod)₂. The solution was stirred at room temperature for 16 h. The solution was then opened to air, and the organic layer was extracted with HCl 1 M (1 × 10 mL) and water (2 × 10 mL). The aqueous layer was washed with Et₂O (3 × 10 mL), and the organic portions were combined together, dried over MgSO₄, filtered, and concentrated under vacuum. The product was extracted with Et₂O and filtered through a silica plug. The final product was dried under vacuum and analyzed by GC-MS and NMR. The NMR characterization values of the organic catalytic products were compared with literature. The ratios were calculated and are reported according to ¹H NMR.

TON Experiment for the Cyclotrimerization of Ethyl Propiolate Catalyzed by ^p-tol1

In the glovebox, a toluene solution (3 mL) of ethyl propiolate (11, 232 mg, 2.37 mmol) was added slowly, within 1 min, into a toluene solution (3 mL) containing 0.05 mol % of the nickel catalyst ^p-tol1 (1 mg, 1.2 μmol). The solution was stirred at room temperature for 16 h. The solution was then opened to air, and the organic layer was extracted with HCl 1 M (1 × 10 mL) and water (2 × 10 mL). The aqueous layer was washed with Et₂O (3 × 10 mL), and the organic portions were combined together, dried over MgSO₄, filtered, and concentrated under vacuum. The product was extracted with Et₂O and filtered through a silica plug. The ratios between 1,2,4- (11a) and 1,3,5- (11b) regioisomers in addition to a minor amount of tetraethyl-cyclooctatetraene-tetracarboxylates (11c) were determined by ¹H NMR in a ratio of 91:7:2. Triethylbenzene-1,2,4-tricarboxylate (11a) was obtained in a yield of 87% (202 mg, 0.69 mmol).

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b05025.

• Experimental, DFT, and crystallographic details (PDF)

• Crystal structures (CIF)

The authors declare no competing financial interest.

Notes

CCDC 1882146–1882149 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.