Dehydrocoupling of phosphine–boranes using the [RhCp*Me(PMe3)(CH2Cl2)][BArF4] precatalyst: stoichiometric and catalytic studies

Detailed experimental and computational studies are reported on the fundamental B–H and P–H bond activation steps involved in the dehydrocoupling/dehydropolymerization of primary and secondary phosphine–boranes, H3B·PPhR′H (R = Ph, H), using the [RhCp*(PMe3)Me(ClCH2Cl)][BArF4] catalyst.

°C and stirred for 72 h, with the tap open to a slow flow of argon. At the end of the reaction the solution was cooled and added by cannula to stirred hexane (100 cm 3 ) and a precipitate formed. The solvent was removed by filter cannula and the resulting polymer dried in vacuo to yield a pale yellow solid (26 mg). Signals for the [BAr F 4 ]ion are visible in the 1 H and 11 B{ 1 H} NMR spectra originating from the precatalyst. See figures S2-5 for polymer NMR spectroscopy, ESI-MS and GPC data.

H 3 B·PPhHBH 2 ·PPhH 2 (2)
A toluene solution of H 3 B·PPhH 2 (0.50 cm 3 , 1.0 M solution, 0.50 mmol) was transferred by cannula to a long, thin schlenk tube containing a small stirrer bar and [RhCp*Me(PMe 3 )(CH 2 Cl 2 )][BAr F 4 ] (1) (31.9 mg, 0.025 mmol, 5 mol%). Stirring dissolved the solid and formed a yellow solution and the portion of the tube containing solvent was placed in an oil bath preheated to 100 °C. After 1 h, the solution was cooled and the solvent removed in vacuo. Hexane (5 cm 3 ) was added with vigorous stirring and sonication, then the solvent was again evaporated to dryness to remove any traces of toluene. Hexane (20 cm 3 ) was added and the solution stirred vigorously to give a pale, almost colourless solution and a

Structure of Boryl Complex [RhCp*(H 2 B·PH t Bu 2 )(PMe 3 )][BAr F 4 ] (11)
Red block crystals were grown from a solution of 11 which was kept at -20 °C and layered with pentane and stored at -18 °C for 48 hours. Crystals were kept cold, isolated and quickly transferred to the cryostream of the diffractometer. The crystals diffracted strongly at low angle but very weakly at high angle. Analysis of the complete data set did not suggest signs of twinning or other problems so the poor quality data appears to be due to poor crystal quality. The data presented below is presented for connectivity purposes and bond length and angle data is not presented.    (right). The data quality is relatively low due to the high polydispersity of the samples.

Addition of Mercury Catalytic Dehydrocoupling of H 3 B·PPh 2 H
The reaction was repeated as in scheme S1 with NMR spectra taken to ensure the reaction had begun. Mercury metal (approx. 0.2 cm 3 ) was added to one sample after 4 hours and the reaction was stirred for a further 20 h at 100 °C and NMR spectra taken of the control and mercury added reactions. The addition of mercury had no measurable effect on the rate of dehydrocoupling.     8 This peak was not observed after degassing of the sample.

Reaction of 1 with H 3 B·P t Bu 2 H Followed by 31 P{ 1 H} NMR spectroscopy
Scheme S5: Reaction of 1 with H 3 B•P t Bu 2 H followed by NMR spectroscopy. [BAr F 4 ]anions not shown.

Phosphine Exchange through Vacant Site Formation in [RhCp*(H)(PHPh 2 )(PMe 3 )][BAr F 4 ] (5)
The formation of vacant sites (an hence an active catalytic species) in 5 was probed using a phosphine exchange experiment. 5 was mixed with PPh 3 (10 eq.) in toluene at room temperature for 2 hours (scheme S8). An ESI-MS experiment (figure S19) then showed several organometallic species resulting from phosphine exchange.
Scheme S8: Reaction of 5 with PPh 3 to form products resulting from ligand scrambling.

Breakdown of Energy Contributions
The following tables detail the evolution of the relative energies as the successive corrections to the initial SCF energy are included. Terms used are: ΔE SCF energy computed with the BP86 functional ΔH Enthalpy at 0 K ΔG Free energy at 298.15 K and 1 atm ΔG DCM Free energy corrected for dichloromethane solvent ΔG D3 Free energy corrected for dispersion effects ΔG final Free energy corrected for dichloroethane solvent and dispersion effects ΔG final is the final data used in the main article. Compound labels are given in Scheme S9.
Scheme S9. Stationary points for the reactions of 10 R to give 6 R , 9 R and 11 R , where R = Me, Cy, Ph and t Bu. The alternative reaction of Int(11''-9') R to 6 R via Int(11''-6) R is also indicated. a, b a. For R = Cy product 6 Cy is equivalent to species 7 in the main paper. b. 6' R is a higher energy conformer of 6 R S-32