Fusing Triphenylphosphine with Tetraphenylborate: Introducing the 9-Phosphatriptycene-10-phenylborate (PTB) Anion

Abstract

In a fusion of two ubiquitous organometallic reagents, triphenylphosphine (PPh₃) and tetraphenylborate (BPh₄⁻), the 9-phosphatriptycene-10-phenylborate (PTB) anion has been prepared for the first time. This borato species has been fully characterized by a suite of spectroscopic methods, and initial reactivity studies introduce it as a competent ligand for transition metals, including Co(II) and Fe(II).

Graphical abstract

Owing to low-cost, high availability, and ease of synthesis and handling, triphenylphosphine (PPh₃) and tetraphenylborate (BPh₄⁻) have evolved to become prevailing reagents in the synthetic chemist’s toolbox. Congruent with interest in developing zwitterionic organometallic complexes for catalysis,¹ including examples from our group featuring P,B- and N,B-containing ligands,² we envisioned a tethering strategy between these two partners in the form of a triptycene. Some years ago, we explored the use of conceptually related 3- and 4-substituted acyclic diphenylphosphino(tetraphenyl)borates as electron-releasing ligands for coordination to platinum-group metals, along with some preliminary C-C cross coupling investigations (Chart 1A).^2a,b It is noteworthy that a host of other phosphine-containing ligands incorporating group 13 elements have also been reported.³

Chart 1.

Representative borate ligands

Since the early report of parent 9-phosphatriptycene in 1974 by Bickelhaupt et al. (Chart 1B),⁴ several research groups have reported related geometrically constrained compounds containing group 14 or 15 elements, though the synthesis of anionic group 13 analogues has not been achieved.⁵ In this work, we present the formal joining of PPh₃ and BPh₄⁻ in the form of the 9-phosphatriptycene-10-phenylborate anion. To fully characterize this phosphino(borate), several derivatives, including cis-dimethylplatinum(II) and pentakis(carbonyl)tungsten(0) PTB complexes, have been prepared. We also show that the PTB anion can be employed for the synthesis of paramagnetic, Co(II) and Fe(II) complexes, serving roles as both σ-donating ligand and halide abstracting equivalent.

The route used to prepare the 9-phosphatriptycene-10-phenylborate (PTB) anion [3]⁻ is shown in Scheme 1. Treatment of tris(o-bromophenyl)phosphine^5a with ^tBuLi followed by slow addition of PhBCl₂ at −78 °C and careful warming to room temperature furnishes tetrakis(tetrahydrofuran) lithium 9-phosphatriptycene-10-phenylborate, [3][Li(THF)₄] as a white solid in 64% yield after work-up. The countercation of [3]⁻ is straightforwardly tailored by treatment of a CH₃OH solution of [3][Li(THF)₄] with [Y]Br (Y = ASN; ASN = 5-azoniaspiro[4.4]nonane or NEt₄) giving [3][ASN] or [3][NEt₄] as white solids in 97 and 87% yield, respectively.

Scheme 1.

Preparation of 9-phosphatriptycene-10-phenylborate and Mercury depiction of the solid-state molecular structure of [3][ASN] (displacement ellipsoids are shown at the 50% probability, hydrogens and ASN⁺ counterion omitted for clarity). Selected bond lengths [Å] and angles (°). P(1)-C(1) 1.827(2), P(1)-C(3) 1.839(2), P(1)-C(5) 1.837(2), B(1)-C(2) 1.649(2), B(1)-C(4) 1.655(2), B(1)-C(6) 1.647(2), B(1)-P(1) 3.020(2), <C-P(1)-C 96.5 (avg.), <C-B(1)-C 104.4 (avg.), <(C-P-C) 289.4.

The ³¹P{¹H} NMR spectrum of [3][Li(THF)₄] (CD₃CN, 298 K) displays a singlet at δ = − 43.7 ppm (d, ³J_P-B = 3.8 Hz), while ¹¹B{¹H} NMR spectroscopy provides a sharp resonance at δ = − 8.75 ppm (d, ³J_P-B = 3.8 Hz); c.f. δ_B = − 6.7 ppm for Li[BPh₄].⁶ Notably, this is a rare case where three-bond scalar coupling between phosphorus and boron (I = 3/2) is experimentally observed. Analogous NMR spectroscopic parameters are observed for [3][ASN] and [3][NEt₄] (see ESI) -all of which are significantly upfield-shifted compared to PPh₃ (δ_P = −3.1 ppm), but in the same range as 1 (δ_P = −64.8 ppm) and 2 (δ_P = −44.4 ppm). Additionally, the ⁷Li{¹H} NMR spectrum of [3][Li(THF)₄]provides a signal at δ_Li = − 1.2 ppm.

Crystalline material of [3][Li(THF)₄] was obtained from a saturated THF solution at −35 °C, while crystals of [3][ASN] or [3][NEt₄] could be grown from a saturated CH₃OH solution at −35 °C. The crystal structure of [3][ASN] is shown in Scheme 1 and features a constrained phosphorus atom having an average <C-P-C bond angle of 96.5° c.f. 106.3° for PPh₃ (Table 1) and an average <C-B-C bond angle of 104.4°. The <C-P-C bond angle for [3][ASN] is the smallest reported among the known 9-phosphatriptycenes (<C-P-C = 98.0° for 1 and 99.3° for 2; Table 1).

Table 1.

Characterizing the 9-phosphatriptycene-10-phenylborate (PTB) anion.

	Eox/V	Lone pair orbitalc	1JP,Se (Hz) Ar3P=Se	1JPt,C (Hz) cis-PtMe2L2	ν(CO) (cm−1) W(CO)5L	<C-P-C (°) by XRDh
PPh3	1.05a	HOMO	736d	616d	1943f	106.3
1	1.68a	HMMO-5	828d	--	1946f	99.3
2	--	--	795d	607d	--	98.0
[3]−	0.97b	HMMO-4	746e	597e	1943g	96.5

^aIrreversible peak potential vs. Ag/Ag⁺ in CH₂Cl₂ with 0.1 M [NⁿBu₄][ClO₄].

^bIrreversible peak potential vs. Fc/Fc⁺ in CH₂Cl₂ with 0.1 M [NⁿBu₄][ClO₄] for NEt₄⁺ counterion.

^cSingle-point calculations and geometry optimization were performed using DFT: B3LYP/6-311G(d) for P, and 6-31G(d) for all other atoms.

^din CDCl₃.

^ein MeCN-d₃ for ASN⁺ counterion.

^fin hexane.

^gsolid-state for Li(THF)₄⁺ counterion.

^hAverage of three bonds.

Note: Data for PPh₃ and 1 are sourced from ref. 5c; data for 2 is sourced from ref. 5a.

Having prepared PTBs [3]⁻, we next assessed phosphine s-character and σ-donating ability (Table 1 & Scheme 2). In agreement with incorporation of an insulated borate, the cyclic voltammogram of [3][NEt₄] shows an irreversible oxidative feature at 0.97 V that is cathodically-shifted compared to related phosphatriptycene 1 and PPh₃ (Figure 1A). Compound [3]⁻ was further evaluated based on the following characteristics: a) ¹J_P,Se = 746 Hz for the phosphine selenide, [4][ASN] (X-ray, Figure S30);⁷ b) ¹J_Pt,C = 597 Hz for the Pt(II) complex, cis-[Pt(CH₃)₂(PTB)₂][NEt₄]₂ ([5][NEt₄]₂; Figure 1B), and c) ν(CO) = 1943 cm⁻¹ for the W(0) complex, [W(CO)₅(PTB)][Li(THF)₄] ([6][Li(THF)₄]). Despite having the smallest <C-P-C bond angle of the series, these data point toward [3]⁻ as having lower s-character and similar σ-donating ability when compared with 1 or 2. Of note, oxidation of PTB to the phosphine selenide, [4][ASN], also results in increased pyramidalization at phosphorus (<(C-P-C) = 301.8° (c.f. 289.4°)) and a decreased P(1)-B(1) distance of 2.911(5) Å (c.f. 3.020(2) Å) corresponding to a five-fold increase in ³J_P-B to 19.3 Hz.

Scheme 2.

Assessing the s-character and σ-donating properties of [3]⁻.

Figure 1.

A) The CV for [3]⁻ with the oxidative feature highlighted (using an internal Fc/Fc⁺ reference). B) Mercury depiction of the solid-state molecular structure for [5][NEt₄]₂ (50% ellipsoid probability, hydrogens and counterions omitted for clarity).

Protonation of [3][NEt₄] was also shown to be facile. Treatment of an MeCN solution of [3][NEt₄] with [H-OEt₂][BAr^F₄] results in formation of the neutral zwitterion, PTB-H (3-H) along with [NEt₄][BAr^F₄]. Protonation is evidenced by ³¹P NMR spectroscopy that shows a broad doublet at δ = −25.3 ppm (¹J_P,H = 512 Hz), which collapses to a singlet on ¹H decoupling. Notably this value of ¹J_P,H is close to that observed for [H-PPh₃]⁺ (¹J_P,H = 510 Hz) and lower than that observed for the [H-PH₃]⁺ cation (¹J_P,H = 548 Hz), which is near ideally sp³⁻hybridized.⁸

Next, we studied the coordination chemistry and spectroscopic features of [3]⁻ with Co and Fe. Combination of two equiv. of [3][NEt₄] with CoBr₂ in CH₃CN at ambient temperature produced a dark turquoise solution (Scheme 3). Metalation of [3][NEt₄] was confirmed by ¹H NMR spectroscopy, which displayed paramagnetically shifted resonances, while no discernable signal was obtained by ³¹P NMR spectroscopy (CD₃CN, 298 K). In an effort to provide material suitable for single crystal X-ray diffraction, cooling a CH₃CN solution of this reaction mixture to −35 °C resulted in a color change from turquoise to forest green, while cooling a saturated THF solution caused no change in color, indicative of a solvent-dependent coordination equilibrium. This process was later studied by variable temperature UV-Visible spectroscopy (vide infra).

Scheme 3.

Generation of Co(II) and Fe(II) PTB complexes

Ultimately, the identity of the constituent Co(II) species was obtained by layering the aforementioned CH₃CN solution with THF at −35 °C, which gave three different crystal morphologies (ESI, Figure S26): i) thin blue sheets, ii) turquoise blocks, and iii) orange blocks (Figure 2). Analysis by single crystal X-ray diffraction confirmed the identity of each of these species to be: the mono(PTB) complex [Co(PTB)Br₃][NEt₄]₂ ([7][NEt₄]₂) and bis(PTB) complexes [Co(PTB)₂Br₂][NEt₄]₂ ([8][NEt₄]₂) and [Co(PTB)₂(NCCH₃)₂Br][NEt₄] ([9][NEt₄]). The Co(1)-P(1) bond length for [7][NEt₄]₂ [2.358(4) Å] and [8][NEt₄]₂ [2.386(4)/2.383(5) Å] agree with those reported for Co(PPh₃)₂Br₂ [2.349(2) Å],⁹ while for the Co(II) zwitterion, [9][NEt₄], a shorter distance of 2.248(4) Å is noted. For the pseudo-tetrahedral complex [8][NEt₄], the angles, <Br(1)-Co(1)-Br(2) (117.54(9)°) and <P(1)-Co(1)-P(2) (121.1(2)°) are also in line with those reported for Co(PPh₃)₂Br₂ (117.27(3)° and 115.88(2)°). Finally, for the trigonal bipyramidal complex [9][NEt₄], the P(1)-Co(1)-P(2) bond angle (177.58°) is close to 180°, while the sum of angles in the equatorial plane is equal to 360°.

Figure 2.

Mercury depiction of the solid-state molecular structure of the anions: [7]²⁻, [8]²⁻, and [9]¹⁻ (displacement ellipsoids are shown at the 50% probability, hydrogens and counterions omitted for clarity).

Probing the sample by optical spectroscopy allowed for the study of complex speciation in CH₃CN as a function of temperature. At 25 °C, the UV-Vis spectrum showed the presence of several bands between 600 and 700 nm [617, 642, 668, 681, and 696 nm] (Figure 3A). Related absorptions have been documented for the PPh₃ analogues, [Co(PPh₃)Br₃][NEt₄] and Co(PPh₃)₂Br₂.¹⁰ Cooling the sample to −40 °C, however, gradually revealed the formation of a new species, exhibiting clear isosbestic behavior with growth of bands at 310 and 376 nm. We posit that these data result from a low-temperature metathesis equilibrium of [8][NEt₄]₂ (turquoise) to give the bis(acetonitrile) adduct [9][NEt₄] (orange) and [NEt₄]Br; these species account for the forest green color observed at low temperature. This result highlights the ability of [3]⁻ to undergo metathesis with an M-X (X = halide) bond, providing an entryway into coordinatively unsaturated metal complexes that can react with L-type donor ligands (e.g., NCMe).

Figure 3.

A) Variable temperature UV-Visible absorbance data collected on a mixture of [7]²⁻, [8]²⁻, and [9]¹⁻ in CH₃CN. Inset shows expansion of 550 to 750 nm window and the frozen cuvette at −40 °C. Temperature increments are in 5 °C. B) Zero-field ⁵⁷Fe Mössbauer spectra of a mixture of [10]²⁻ (blue, 43%), [11]²⁻ (green, 47%), and an unknown species (yellow, 10%) as a frozen CH₃CN solution collected at 80 K.

A related set of Fe(II) PTB complexes were also obtained with coordination being substantiated by NMR spectroscopy (Scheme 3). Layering of a CH₃CN solution with THF and cooling to −35 °C gave colorless blocks – one of which was analyzed by X-ray diffraction as [Fe(PTB)Br₃][NEt₄]₂, [10][NEt₄]₂ (X-ray, Figure S32). The Fe(1)-P(1) bond length for [10][NEt₄]₂ [2.4320(6) Å] is similar to that noted for [Fe(PPh₃)Br₃][HIPr] (HIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolium) [2.463(2) Å] and is otherwise comparable to the Co derivative [7][NEt₄].¹¹

The ⁵⁷Fe Mössbauer spectrum of a frozen CH₃CN solution revealed the presence of three Fe-containing species with parameters suggesting the presence of two four- and one higher-coordinate Fe(II) centers (Figure 3B).¹² For the two major species, isomer shifts of δ = 0.87 and 0.79 mm/s, with quadrupole splittings of ΔE_Q = 2.34 and 2.87 mm/s, respectively were observed. These doublets are assigned to the four-coordinate species [Fe(PTB)Br₃][NEt₄]₂ (43%, [10][NEt₄]₂) and [Fe(PTB)₂Br₂][NEt₄]₂ (47%, [11][NEt₄]₂). For comparison, values of δ = 0.86 and 0.83 mm/s and ΔE_Q = 2.10 and 2.71 mm/s were reported for [Fe(quinoline)Br₃][NEt₄] and Fe(quinoline)₂Br₂.¹³ A third species, accounting for the remaining 10% of the overall fit, displayed an unusually large isomer shift, δ = 1.72 mm/s with ΔE_Q = 2.52 mm/s. We presume this species to be a higher-coordinate Fe(PTB)_xBr_y(NCCH₃)_z complex, such as [Fe(PTB)₂(NCCH₃)₂Br][NEt₄], akin to [9][NEt₄]. Given the low known isomer shift for [Fe(PEt₃)₂(NCCH₃)₄]²⁺ (δ = 0.37 mm/s), it is unlikely that this doublet results from the neutral zwitterion, Fe(PTB)₂(NCCH₃)₄.¹⁴

Inspired by the success of PPh₃ and BPh₄⁻ as reagents of broad utility, we have provided here the first synthesis of the 9-phosphatriptycene-10-phenylborate anion. The s-character and σ-donating capability of this ligand have been assessed, and preliminary forays into coordination chemistry with Co and Fe demonstrate that this anion can be used to advantage as both a ligand and halide-abstracting agent. Future work will focus on the small-molecule reactivity of these and other PTB-ligated transition metal complexes.