Metal–Ligand Cooperation as Key in Formation of Dearomatized Ni^II–H Pincer Complexes and in Their Reactivity toward CO and CO₂

Abstract

The unique synthesis and reactivity of [(^RPNP*)NiH] complexes (1a,b), based on metal–ligand cooperation (MLC), are presented (^RPNP* = deprotonated PNP ligand, R = ⁱPr, ^tBu). Unexpectedly, the dearomatized complexes 1a,b were obtained by reduction of the dicationic complexes [(^RPNP)Ni(MeCN)](BF₄)₂ with sodium amalgam or by reaction of the free ligand with Ni⁰(COD)₂. Complex 1b reacts with CO via MLC, to give a rare case of a distorted-octahedral PNP-based pincer complex, the Ni(0) complex 3b. Complexes 1a,b also react with CO₂ via MLC to form a rare example of η¹ binding of CO₂ to nickel, complexes 4a,b. An unusual CO₂ cleavage process by complex 4b, involving C–O and C–P cleavage and C–C bond formation, led to the Ni–CO complex 3b and to the new complex [(PⁱPr₂NC₂O₂)Ni(P(O)ⁱPr₂)] (5b). All complexes have been fully characterized by NMR and X-ray crystallography.

Late-transition-metal complexes of electron-donating and bulky “pincer” ligands have found important applications in synthesis, bond activation, and catalysis.¹⁻⁶ The lutidine-based pincer ligands (Scheme 1) are highly electron donating ligands with a relatively low trans influence of the pyridinic nitrogen and benzylic “arms” amenable to deprotonation.

Scheme 1. H–X Bond Activation by Aromatization/Dearomatization MLC Reactivity.

In recent years our group has developed a novel mode of metal–ligand cooperation (MLC), involving aromatization/dearomatization of lutidine-based pincer complexes (Scheme 1).⁷⁻¹² This mode of reactivity enabled the activation of various substrates such as alcohols,¹³⁻¹⁷ amines,¹⁸⁻²¹ nitriles,^22,23 boranes,²⁴ dihydrogen,²⁵⁻²⁷ and dioxygen,²⁸ as well as activation of C_sp2–H²⁹ and C_sp3–H³⁰ bonds, and it is a key step in the design of several environmentally benign catalytic reactions.⁷⁻¹¹

Due to the importance of CO₂ as a potential C1 building block,^31,32 we have explored the reactivity of dearomatized pincer complexes toward CO₂. It was found that MLC is also involved in the activation of CO₂ by dearomatized lutidine-based complexes of Fe,³³ Ru,^34,35 Re,³⁶ Ni,³⁷ and Ir,³⁸ reversibly forming a new C–C bond between the ligand backbone and CO₂. Recently, we reported the reductive cleavage of CO₂ by dearomatized (^tBu-PNP)Ir–H and (^tBu-PNP)Rh–H complexes via MLC (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine),^38,39 leading to the design of a cycle of photocarbonylation of benzene.³⁹

Herein we report a rare case of Ni(η¹-CO₂-κC) complexes, obtained by direct coordination of free CO₂ to lutidine-based Ni–hydride complexes [(R-PNP*)NiH] (R-PNP* = dearomatized PNP ligand, R = ^tBu, ⁱPr) involving metal–ligand cooperation.

CO₂ coordination to Ni complexes is well known,⁴⁰ and Ni complexes serve as efficient catalysts in CO₂ activation and utilization as a C1 building block,^31,41 mainly in CO₂ hydrogenation⁴²⁻⁴⁵ and carboxylation.⁴⁶⁻⁵⁶ The first structurally characterized metal–CO₂ complex was (PCy₃)₂Ni(η²-CO₂), reported by Aresta et al. in 1975 (Scheme 2).⁵⁷ Since then, several similar Ni(η²-CO₂) complexes have been reported,^58,59 including a unique five-coordinated Ni(η²-CO₂) complex.⁶⁰ A unique coordination mode of μ-η²,η²-CO₂ in a dinuclear Ni complex was reported by Sadighi and co-workers,⁶¹ and a rare case of Ni^III(η¹-(CO₂)^•-κO) was reported by Chiou, Liaw, and co-workers.⁶² Recently a rare case of Ni^II(η¹-CO₂-κC) was reported by Lee⁶³ (Scheme 2).

Scheme 2. Mononuclear Ni CO₂ Complexes.

The dearomatized complex [(^tBu-PNP*)NiH] (1a, Scheme 3), reported by van der Vlugt et al., was obtained by reaction of LiAlH₄ with the dearomatized complex [(^tBu-PNP*)NiCl].⁶⁴ Surprisingly, we obtained the Ni(II) complex 1a by attempting to reduce the dicationic Ni(II) complex [(^tBu-PNP)Ni(MeCN)](BF₄)₂⁶⁴ with sodium amalgam (Scheme 3). Single crystals of complex 1a were obtained by slow evaporation of its benzene solution. The unreported X-ray structure of 1a is shown in Figure 1. The new complex [(ⁱPr-PNP)Ni(MeCN)](BF₄)₂ was also prepared, and upon similar treatment with sodium amalgam the corresponding dearomatized hydrido complex 1b was obtained (Scheme 3). Crystals suitable for X-ray diffraction of 1b (Figure 1) were obtained by evaporation of its pentane solution. The X-ray structures of 1a,b reveal the expected square-planar geometry.

Scheme 3. Synthesis of Complexes 1a,b.

Figure 1.

Single-crystal X-ray structures with thermal ellipsoids shown at the 50% probability level of complex 1a (left) and complex 1b (right). Hydrogen atoms, except Ni–H and side arm protons, are omitted for clarity. See the Supporting Information for a full description of the structures.

Complex 1b exhibits in the ¹H NMR spectrum a hydride signal at −18.13 ppm (²J_PH = 65 Hz), and the ³¹P{¹H} NMR spectrum exhibits an AB pattern centered at 54.72 ppm (²J_PP = 225.1 Hz), similar to the spectra reported for 1a.⁶⁴

We believe that the reduction of complexes 2a,b leads initially to Ni(0) intermediates forced into a square-planar geometry by the relatively rigid PNP pincer ligand. As a d¹⁰ ML₄ complex, Ni(0) complexes prefer to adopt a tetrahedral geometry, and Ni(0) square-planar complexes are as yet unknown. Only one d¹⁰ square-planar complex is known, a Pt(0) complex.⁶⁵ The postulated unstable Ni(0) intermediates rearrange by metal–ligand cooperation, in which proton transfer from the benzylic position of the pincer ligand to the metal center takes place, yielding the Ni(II) dearomatized hydride complexes 1a,b. In support of this mechanism, reaction of the Ni⁰(COD)₂ complex with the R-PNP ligands (R = ^tBu, ⁱPr) resulted in the dearomatized complexes 1a,b exclusively (Scheme 3).

In order to explore the possibility of reverting the metal–ligand cooperative proton transfer from the arm to the metal center, thus gaining a Ni(0) complex, complexes 1a,b were reacted with CO. Upon treatment of complex 1a with 1 equiv of CO no reaction was observed, likely as a result of steric congestion imposed by the bulky ^tBu-PNP ligand. However, upon addition of 1 equiv of CO to the ⁱPr-PNP complex 1b, a rapid color change occurred, yielding the unusual neutral pincer Ni(0) carbonyl complex 3b (Scheme 4). Complex 3b exhibits a singlet peak at 68.36 ppm in the ³¹P{¹H} NMR, indicating a metal–ligand cooperative transfer of a proton from the metal back to the unsaturated arm, yielding a symmetric complex. The carbonyl ligand gives rise to a triplet peak at 204.02 ppm in the ¹³C{¹H} NMR spectrum, at a slightly higher field in comparison to that of the recently reported nickel monocarbonyl anionic complex {Na(12-C-4)₂}{(^acriPNP)Ni(CO)}.⁶⁶ Slow evaporation of the benzene solution resulted in formation of single crystals suitable for X-ray diffraction (Scheme 4), and these adopt a distorted-tetrahedral geometry. The P–Ni–CO angle is the only characteristic tetrahedral angle (111.3(2)°), while the N–Ni–P and the N–Ni–CO angles are 85.0(1) and 125.8(2)°, respectively. The Ni–CO and C–O bond distances are 1.862(6) and 1.030(6) Å, respectively, and the CO IR band appears at 1888 cm^–1 in benzene solution. In comparison, the recently reported monocarbonyl Ni(0) anionic complex [Na(12-C-4)2][(^acriPNP)Ni(CO)], which also adopts a distorted-tetrahedral geometry due to the rigid PNP ligand, exhibits Ni–CO and C–O bond lengths of 1.77(1) and 1.18(1) Å, respectively, and an IR CO band appears at 1828 cm^–1, indicating a higher degree of π back-donation from the low-valent nickel center in comparison with 3b.⁶⁶

Scheme 4. Formation of the Ni(0) Complex 3b and Its Single-Crystal X-ray Structure with Thermal Ellipsoids Shown at the 50% Probability Level.

Hydrogen atoms are omitted for clarity. See the Supporting Information for a full structural description.

The dearomatized hydride complexes 1a,b react with CO₂ to give the aromatized complexes 4a,b with a rare η¹ coordination mode of carbon dioxide (Scheme 5). While complex 1b reacts readily with CO₂ at ambient temperature with only 0.5 bar of CO₂, forming complex 4b in 82% yield (according to ³¹P NMR) after 8 h, the conversion of 1a to 4a requires the more forcing conditions of 8 bar, 80 °C, and 72 h, yielding 60% of the product.

Scheme 5. Synthesis of Complexes 4a,b.

Whereas the η²-CO₂ coordination mode is common, the η¹-CO₂ coordination mode is rare. It was reported for [Rh(diars)₂Cl(CO₂)],⁶⁷ [Ru(bpy)₂(CO)(CO₂)]·3H₂O,^68,69 [Ir(dmpe)₂Cl(CO₂)],⁷⁰ and [Co(salen)K(η¹-CO₂)(THF)].^71,72 The only Ni-η¹-CO₂ complex is the recently reported anionic complex [Na(12-C-4)₂][(PNP)Ni-η¹-CO₂] (PNP = N[2-PⁱPr₂-4-Me-C₆H₃]₂),⁶³ which was obtained by a reduction of the carboxylate complex [(PNP)NiCOONa] and not by direct CO₂ coordination, unlike complexes 4a,b.

Crystals suitable for X-ray diffraction of 4a,b (Figure 2) were obtained from the crude reaction mixture in a pressure flask under a CO₂ atmosphere. Both complexes exhibit a distorted-square-planar geometry with P–Ni–P angles of 171.39(3) and 173.4(1)° and N–Ni–C angles of 179.0(1) and 178.6(2)° for complexes 4a,b, respectively. The CO₂ plane is almost perpendicular to the pincer ligand plane. The Ni–CO₂ bond lengths in complexes 4a,b are 1.950(3) and 1.912(4) Å, respectively, which are in the range of reported η¹-CO₂ complexes and longer by 0.11–0.044 Å than those reported for Ni−η²-CO₂.^57,59 The two C–O bond lengths are similar (1.244(3), 1.254(3) Å in 4a and 1.240(5), 1.250(5) Å in 4b), unlike the C–O bonds in reported Ni-η²-CO₂ complexes, in which the difference in length is 0.06 Å.^59,60 In addition, the large and similar distances between the oxygen atoms and the metal center (2.737(2), 2.721(1) Å in 4a and 2.749(3), 2.637(3) Å in 4b) also indicate an η¹-CO₂ coordination mode, as the Ni–O bond lengths in η²-CO₂ complexes are shorter by 0.8–0.5 Å.^57,59 The bond lengths of the CO₂ ligand in complexes 4a,b are similar to the reported bonds of the complex [Na(12-C-4)₂][(PNP)Ni-η¹-CO₂].⁶³ In addition, the Ni–CO₂ bond length in complex 4a is longer by 0.04 Å in comparison with 4b. This elongation is probably due to the more sterically demanding ^tBu-PNP ligand in comparison to the ⁱPr analogue. Aresta et al., in his seminal work on (PR₃)₂Ni(η²-CO₂) complexes (R = (C₆H₁₁), Et, Buⁿ), also observed that steric hindrance has a great influence on the Ni–CO₂ bond strength.⁵⁸ We have previously reported that the steric difference between the ^tBu-PNP and ⁱPr analogues can lead to large differences in reactivity.⁷³ The lower steric hindrance of the ⁱPr-PNP can result in a higher degree of π back-donation from the Ni center to the CO₂ ligand,⁶³ which shortens the Ni–CO₂ bond length. This is in line with the differences in reaction conditions for the synthesis of complexes 4a,b and the differences in reactivity of complexes 1a,b toward CO. According to NMR studies, whereas complex 1b reacts with substoichiometric amounts of CO₂ at room temperature to gradually form complex 4b (Figure 3), complex 1a requires at least 5 bar of CO₂ to achieve detectable conversion. In addition, while the formation of 4b under these conditions requires several minutes, detectable formation of 4a requires at least 12 h.

Figure 2.

Single-crystal X-ray structures, with thermal ellipsoids shown at the 50% probability level, of complexes 4a (left) and 4b (right). Hydrogen atoms and cocrystallized solvents are omitted for clarity. See the Supporting Information for a full description of the structures.

Figure 3.

³¹P{¹H} NMR (121.4 MHz) spectra representing gradual injection of about 0.2 equiv of CO₂ into a toluene solution of complex 1b in an NMR tube equipped with a septum cap. Similar conditions with complex 1a yielded no detectable conversion.

Complexes 4a,b exhibit symmetric ¹H and ¹³C NMR spectral patterns of the ligand backbone and a singlet signal in the ³¹P{¹H} NMR spectrum at 47.65 (4a) and 35.46 ppm (4b), as expected for aromatic square-planar complexes. The CO₂ ligand gives rise to broad peaks at 174.74 and 173.25 ppm, respectively, in the ¹³C{¹H} NMR spectra, which are at a higher field in comparison to the reported chemical shifts of the CO₂ ligands in the complexes [Ru(bpy)₂(CO)(CO₂)]·3H₂O^68,69 (203.9 ppm) and [Na(12-C-4)₂][(PNP)Ni-η¹-CO₂]⁶³ (197.25 ppm). The reported Ni−η²-CO₂ complexes give rise to a signal at 164 ppm.^59,60

Although complex 4b is stable at room temperature, when a THF solution of 4b is heated to 80 °C under 5 bar of CO₂, an unexpected reaction was observed. According to the ³¹P{¹H} NMR spectrum, full conversion to two products took place, giving rise to a singlet signal at 68.3 ppm, assigned as complex 3b, and a pair of doublets at 64.3 and 102.9 ppm (²J_PP = 90 Hz), assigned as complex 5b (Scheme 6). Complexes 3b and 5b were obtained from complex 4b also at ambient temperature under 5 bar of CO₂, although at lower conversion. Complex 4a demonstrated no such reactivity, despite the use of more forcing conditions of 10 bar of CO₂ at 353 K for 7 days.

Scheme 6. CO₂ Cleavage by Complex 4b, Involving C=O and C–P Cleavage and C–C Bond Formation.

Complex 3b was extracted from the reaction mixture with pentane, and the two products, 3b and 5b, were isolated and fully characterized. Crystals suitable for X-ray diffraction of [(P^iPr2NC₂O₂)Ni(P(O)ⁱPr₂)] (5b, Figure 4) were obtained by layering of pentane over a dichloromethane solution. Complex 5b exhibits a distorted-square-planar geometry with P(1)–Ni–O(1) angle of 167.04(6)° and N–Ni–P(2) angle of 170.51(6)°. The C=O(2) and C–O(1) bond lengths are 1.229(3) and 1.296(3) Å, respectively, both longer than the free CO₂ bond length by 0.07–0.14 Å, and the C(7)–C(100) bond length is in the range of a single C–C bond, 1.519(4) Å. The NMR data of 5b fit well with its X-ray structure, and two distinguishable CH₂ signals of the benzylic positions, in which only one is coupled to a phosphorus atom, were observed in the ¹H and ¹³C{¹H} NMR.

Figure 4.

Single-crystal X-ray structures with thermal ellipsoids shown at the 50% probability level of complex 5b. Hydrogen atoms are omitted for clarity. See the Supporting Information for a full description of the structure.

Apparently, two molecules of CO₂ are involved, one leading to C–P cleavage followed by C–C bond formation, generating a carboxylato ligand, while the second molecule oxidizes the ⁱPr₂P fragment to the ⁱPr₂P=O ligand. The resulting CO is then trapped by another molecule of the starting complex to form the carbonyl complex 3b. Similar reactivity was reported recently for the Ir(I) complex [Ir(depe)(dbuP)] (depe = 1,2-bis(diethylphosphino)ethane, dbuP = 1,8-diazabicyclo[5.4.0]undec-7-ene).⁷⁴ The Ir(I) complex reacts with two CO₂ molecules to give the Ir(III) carbonyl phosphoryl complex [Ir(depe)(CO)(Ph₂PO)(dbuCO₂)], with a carboxylate incorporation into the dbuP ligand. In both complexes, the reported Ir complex and complex 4b, four new bonds were formed, M–C, M–O, C–C, and P–O, and two bonds were cleaved, C=O and P–C. P–C bond cleavage has attracted much attention in the past⁷⁵ and also more in recent studies.^76,77 Complexes bearing phosphoryl ligands (R₂P=O)^– are not common; examples were reported for Au,⁷⁸ Ru,⁷⁹ Ir,⁸⁰ and Pd.^81,82 Ni and Ru bis(phosphinite) pincer complexes were reported to decompose to phosphoryl complexes due to P–O bond cleavage under basic or wet conditions.^83,84

In summary, unexpected synthetic and reactivity pathways involving metal–ligand cooperation (MLC) of the (PNP*)Ni^II–H pincer complexes 1a,b are presented. While complex 1a does not react with CO, the less bulky complex 1b reacts with CO via MLC to give a rare case of a distorted-tetrahedral [(ⁱPrPNP)Ni(CO)] pincer complex. Notably, complexes 1a,b react with CO₂ via MLC to form rare examples of η¹ binding of CO₂ to nickel, complexes 4a,b. Upon heating under a CO₂ atmosphere, the CO₂ complex 4b undergoes an unexpected CO₂ cleavage as well as P–C cleavage, followed by C–C bond formation by carboxylation of the PNP ligand, leading to the formation of the Ni^IIPO complex 5b and the Ni⁰CO complex 3b. Further studies are aimed at understanding of the MLC mechanisms of these unusual transformations.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00160.

• Experimental procedures and IR and NMR spectra of complexes 1a,b–5a,b (PDF)

Accession Codes

CCDC 1831348–1831353 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

The authors declare no competing financial interest.