A Phosphanyl‐Phosphagallene that Functions as a Frustrated Lewis Pair

Abstract Phosphagallenes (1 a/1 b) featuring double bonds between phosphorus and gallium were synthesized by reaction of (phosphanyl)phosphaketenes with the gallium carbenoid Ga(Nacnac) (Nacnac=HC[C(Me)N(2,6‐i‐Pr2C6H3)]2). The stability of these species is dependent on the saturation of the phosphanyl moiety. 1 a, which bears an unsaturated phosphanyl ring, rearranges in solution to yield a spirocyclic compound (2) which contains a P=P bond. The saturated variant 1 b is stable even at elevated temperatures. 1 b behaves as a frustrated Lewis pair capable of activation of H2 and forms a 1:1 adduct with CO2.

Complete consumption of both starting materials was confirmed by 1 H and 31 P NMR spectroscopy and conversion can be seen to be quantitative. Isolation of a pure sample of 1a was not possible due to a subsequent rearrangement in solution over the course of a several days. However, it was possible to obtain partial NMR spectroscopic characterisation from the reaction mixture. Crystallisation by cooling a concentrated hexane solution to -35°C allowed for confirmation of the identity of 1a, which crystallises alongside the rearranged product 2. 1    After 48 hours at -35°C rearranged compound 2 was observed by NMR spectroscopy: Figure S3. 31 P{ 1 H} NMR spectrum of reaction mixture containing 1a, 2 (inset) and an unidentified species after 48 hours (C 6 D 6 ).
The solution was heated to 40°C for 6 days, by which time complete consumption of 1a could be observed by NMR spectroscopy. The solvent was removed under reduced pressure.
Addition of hexane (1 mL) to the resulting orange oil resulted in formation of a pale-yellow precipitate. The suspension was cooled to -35°C for 3 hours and the hexane decanted. The yellow solid was dried under reduced pressure yielding 2 as a yellow powder. The decanted hexane was placed in the freezer to yield a second crop of 2 as pale-yellow crystals (combined yield 18mg, 46%) 1 H NMR (500 MHz, C6D6) δ = 7.86 (d, 3     (40.4 mg, 0.086 mmol; 1.2 eq) were dissolved in toluene (2 mL). Immediate effervescence was observed, accompanied by a darkening of the solution from yellow to red. Complete consumption of both starting materials was confirmed by 1 H and 31 P NMR spectroscopy and conversion to 1b can be seen to be quantitative. 1b is stable in solution for at least 1 week and is highly sensitive to oxygen and moisture. Removal of the solvent and recrystallisation from hexane resulted in analytically pure red crystals suitable for X-ray diffraction (33 mg, 50 %).
The thermal stability was assessed by heating a solution of 1b in toluene to 80°C overnight, no change to the 31 P NMR spectrum was observed. The NMR sample used for the collection of spectra shown in Figures       S.I.14

Synthesis of [SP]P(H)Ga(H)(NacNac) (3)
A toluene (0.5 mL) solution of 1b (40 mg, 0.043 mmol) was prepared in situ as described in section 1.4. The solution was added to a gas-tight NMR tube and degassed using the freezepump-thaw method. The headspace was replaced by 2 bar of H2. The solution immediately lightened in colour, turning from red to orange. Conversion to a new product was quantitative by 1 H and 31 P NMR spectroscopy. Removal of the solvent produced a light-yellow solid which was insoluble in hexane or pentane. Slow evaporation of a toluene solution at room temperature yielded colourless crystals suitable for X-ray diffraction. Placing the crystallised mixture into the freezer induced precipitation of 3 as a microcrystalline powder (27 mg, 67 %).     A toluene (0.5 mL) solution of 1b (40 mg, 0.043 mmol) was prepared in situ as described in section 1.4. The solution was added to an air-tight NMR tube and degassed using the freezepump-thaw method. The headspace was replaced by 2 bar of CO2. The solution immediately lightened in colour, turning from red to orange. Conversion to a new product was quantitative by 1 H and 31 P NMR spectroscopy. Removal of the solvent resulted in an orange oil, which upon washing with small amounts of hexane (3 × 0.5 mL) precipitated out an analytically pure white solid (31 mg, 74%). The hexane washings were filtered and allowed to sit at room temperature for two days, forming large colourless crystals of 4 suitable for X-ray diffraction (ca. 5 mg).   S.I.22

Variable temperature NMR experiments
To probe the magnitude of the π-contribution to the Ga=P bond VT-NMR was performed on a solution of 1b. The methine proton of the diisopropylphenyl groups is expected to split into two distinct resonances upon hindered bond rotation from which the energy can be calculated. [4] Cooling to -80°C resulted in broadening of the 1 H NMR resonance, however no resolution was observed implying a contribution of less than 10.2 kcal/mol. The related PhP(GaTrip2)2 (Trip = 2,4,6-iPr3C6H2) displays resolution of 79 Hz at -95°C for a calculated π-contribution of 10.2 kcal/mol for the dynamic process. [7] The authors do not report the solvent used for this experiment (presumably toluene). We were hesitant to further lower the temperature due to the possibility of the solution freezing.

Single crystal X-ray diffraction data
Single-crystal X-ray diffraction data were collected using an Oxford Diffraction Supernova dual-source diffractometer equipped with a 135 mm Atlas CCD area detector. Crystals were selected under Paratone-N oil, mounted on micromount loops and quench-cooled using an Oxford Cryosystems open flow N2 cooling device. Data were collected at 150 K using mirror monochromated Cu Kα radiation (λ = 1.5418 Å; Oxford Diffraction Supernova) and processed using the CrysAlisPro package, including unit cell parameter refinement and inter-frame scaling (which was carried out using SCALE3 ABSPACK within CrysAlisPro). [5] Equivalent reflections were merged and diffraction patterns processed with the CrysAlisPro suite.
Structures were subsequently solved using direct methods and refined on F 2 using the SHELXL package. [6] S.I.24

Computational details
Where available, optimisations were performed starting from the crystal structure geometry. Initial conformational optimisations were performed using a Hartree-Fock/STO-3G method as the number of isopropyl groups made convergence time consuming. For 1a and 1b, these structures were taken as the basis for optimisations using B3LYP/6-31g(d,p) and then finally optimisations were performed at B3LYP using Def2TZVP (Ga, P, N) and Def2SVP (C, H).
The final geometry was characterised as a true minima via harmonic frequency calculations.
All calculations were performed on the Gaussian16 software package, natural bond order analysis was performed using NBO version 6.0 and NMR calculations were performed with the keyword NMR. NICS calculations on 2 were performed using PBE/6-31g(d,p).

3.1.NICS Calculations
The isotropic shielding constants were computed using the protocol outlined in section 3.
Dummy atoms were placed at the centroid (NICS(0)) and 1 Angstrom either side (NICS(1)), the latter of which was taken as a mean of the two values (4.0512 and 3.9894). S.I.28

Wiberg bond indices
Wiberg bond index (WBI) was calculated in the natural atomic orbital basis. Su and co-workers have investigated R2E13=E15Rʹ2 multiple bonds computationally. [8a] In the case of E13 = Ga, E15 = P they found no significant π-contribution to the bond, and a resultant WBI of 0.99. The WBI of the Ga=P bond of 1b DFT is 1.49, consistent with significant π-contribution. The NBO analysis on Su and co-workers' system implies a σ-bond with polarity (27.82% Ga; 72.18% P) and occupancy (1.91e) comparable to that obtained for 1b DFT (40.4% Ga; 59.6% P and 1.97e), the discrepancy in polarity may be attributed to the π-contribution to the bonding in Su's system, which is highly polarised in the case of 1b DFT. The WBI for the Schulz group's gallaarsene Ga=As bond is 1.65. [8b] Figure S25. Excerpt displaying the WBI of 1bDFT. Ga=P bond is between atom 1 and 3. S.I.35