P–C-Activated Bimetallic Rhodium Xantphos Complexes: Formation and Catalytic Dehydrocoupling of Amine–Boranes**

{Rh(xantphos)}-based phosphido dimers form by P–C activation of xantphos (4,5-bis(diphenylphosphino)-9,9-dimethylxanthene) in the presence of amine–boranes. These dimers are active dehydrocoupling catalysts, forming polymeric [H2BNMeH]n from H3B⋅NMeH2 and dimeric [H2BNMe2]2 from H3B⋅NMe2H at low catalyst loadings (0.1 mol %). Mechanistic investigations support a dimeric active species, suggesting that bimetallic catalysis may be possible in amine–borane dehydropolymerization.


Figure S-1 Complex 4. [BAr F 4] − anion not shown.
In a Young's crystallisation flask containing 1 (20 mg, 0.012 mmol) dissolved in 0.5 mL 1,2-C6H4F2, H3B·NMe2H (13.8 mg, 0.234 mmol) dissolved in 0.6 mL 1,2-C6H4F2 was added. Bubbling was observed immediately upon addition and the flask was sealed. After 12 hours, 1 H, 31 P{ 1 H} and 11 B NMR spectroscopies indicated that 4 was the major metal-containing product, and complete consumption of H3B·NMe2H had occurred to yield [H2BNMe2]2 as the major product of dehydrocoupling. The total volume of solution was reduced to ~ 0.5 mL in vacuo and pentane (5 mL) was added with stirring, resulting in a cloudy brown solution. On cooling to -78 °C, a brown solid precipitated. The yellow supernatant solution was decanted and the solid washed twice with pentane (2 x 3 mL), each time with sonication. The solid was recrystallised from 1,2-C6H4F2 and pentane at 5 °C, from which orange crystals suitable for X-ray diffraction had grown, alongside brown oil. The oil showed NMR spectra suggestive of a mixture of 4 and decomposition products. Some crystals could be manually separated from the oil, yield: 5 mg (40%). These were nevertheless coated finely with oil and so were unsuitable for microanalysis. Similarly, oil-coated crystals of 4 can be formed in an analogous route using 5 (30 mg, vide infra) and 20 eq. H3B·NMe2H (yield: 8 mg, 38%). Alternatively, addition of Na[H3B·NMe2·BH3] to 5 forms 4 and Na[BAr F 4] within 24 h, although 4 co-crystallised with Na[BAr F 4]  Estimated coupling constants by gNMR [9] (3) on thawing and shaking. This mixture was degassed with three freeze-pump-thaw cycles, refilled with Ar, the flask sealed and heated to 40 °C. This initially formed a mixture of 2 and 3, and periodic sampling of the reaction mixture for 31 P{ 1 H} NMR spectroscopy indicated complete conversion to 5 within 5 days, during which a dark red solution was formed. Alternatively, formation of 4 was complete within 48 h at 55 °C, although prolonged heating at this temperature caused decomposition to unidentified products. The volatiles were removed in vacuo, yielding dark red oil, which was washed and sonicated with pentane to form a dark red/orange solid. This was recrystallised from 1,2-C6H4F2/pentane at 5 °C, affording crystals suitable for X-ray diffraction. Yield: 63 mg (67%).    Unfortunately, we were unable to simulate these signals satisfactorily. The peak at δ 134.1 was assigned on the basis of chemical shift as corresponding to bridging phosphido groups. A 1 H-31 P HMBC experiment showed a correlation between the signal at δ 3.9 and the dppe chain protons, assigning this signal as P5/P6.

Addition of MeCN to complex 4
Excess MeCN (20 eq.) was added to in situ formed 4 (via dehydrocoupling of H3B·NMe2H by 5) in a high pressure NMR tube. An intractable mixture of species was observed by 1 H and 31 P{ 1 H} NMR spectroscopies. No evidence for 6 was observed.

X-ray crystallography
Relevant details about structure refinement are given in Table S and a low temperature device; reduction and cell refinement was performed using CrysAlisPro. [11] Data for 5 were collected on a Enraf Nonious Kappa CCD difractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å) and a low temperature device; [12] data were collected using COLLECT, reduction and cell refinement was performed using DENZO/SCALEPACK. [13] All structures were solved using Sir92 [14] or Superflip. [15] All were refined using CRYSTALS. [16] Specific refinement details are given below. In addition, rotational disorder of one of the CF3 groups in a non-disordered aryl ring was treated by modelling the fluorine atoms over two sites and restraining their geometry. Owing to the extensive disorder, some planarity and bond length restraints were used to give sensible structural parameters. A molecule of disordered pentane was also located, to which restraints were also applied.
All hydrogen atoms were located on the Fourier map, except those on the disordered pentane and 1,2-C6H4F2. The hydrogen atoms on these molecules were placed in calculated positions. The hydrogen atoms were refined before RIDE restraints were added. The atoms H1/H2 and H4/H5 were placed riding upon B1 and B2, respectively.

Complex 5
The Fourier difference map indicated the presence of diffuse electron density believed to be a molecule of the pentane solvent. SQUEEZE was used, leaving a void from which the electron density was removed. Rotational disorder of six of the CF3 groups of the anion was treated by modelling the fluorine atoms over two sites and restraining their geometry. The hydrogen atoms were found on the Fourier map and refined before adding RIDE restraints. The atom H1 was placed riding upon B1.

Complex 7
Rotational disorder of some of the CF3 groups on the [BAr F 4] ⎼ anions was treated by modelling the fluorine atoms over two sites and restraining their geometry. Hydrogen atoms were found on the Fourier map and refined before RIDE restraints were added. S-23