Comparisons of photoinduced oxidative addition of B-H, B-B, and Si-H bonds at rhodium(η5-cyclopentadienyl)phosphine centers

Ultraviolet irradiation of [Rh(eta(5)-C5H5)(PMe3)(C2H4)] (1a), [Rh(eta(5)-C5H5)(PPh3)(C2H4)] (1b), and [Rh-(eta(5)-C5H4CF3)(PMe3)(C2H4)] (1c) (collectively abbreviated as [Rh(Cp ')(PR3)(C2H4]) in the presence of HBpin (pinacolate = pin) 1,2-O2C2Me4) results in elimination of C2H4 and B-H oxidative addition, leading to the formation of boryl hydride complexes [Rh(Cp ')(Bpin)(H)(PR3)]. Complete conversion is achieved in liquid HBpin or by photolysis in hexane at -10 degrees C. Similarly, photolysis of 1a-c in the presence of B(2)pin(2) in hexane at -10 degrees C leads to B-B oxidative addition products, [Rh(Cp ')(Bpin)(2)-(PR3)]. Irradiation at room temperature leads to formation of [Rh(Cp ')(PR3)(2)] in addition to the desired products. The rhodium boryl products were characterized by multinuclear NMR spectroscopy and, in the case of [Rh(eta(5)-C5H5)(Bpin)(H)(PPh3)], by X-ray crystallography. The structure reveals a Rh-B distance of 2.0196(15) angstrom. The H center dot center dot center dot B separation of 2.09(2) angstrom together with the bond angles at the metal suggest some residual H center dot center dot center dot B interaction. Photolysis of 1a-c in the presence of tertiary and secondary silanes (HSiEt3, (HSiPr3)-Pr-i, HSi(OMe)(3), HSiMe2Et, HSiMeEt2, and H2SiEt2) results in rhodium silyl hydride complexes [Rh(Cp ')(SiR ' R-2 '')(H)(PR3)]. The structure of [Rh(eta(5)-C5H5)((SiPr3)-Pr-i)(H)(PMe3)] was determined by single-crystal X-ray diffraction, yielding a Rh-Si bond length of 2.3617(3) angstrom and a Rh-H bond length of 1.508(17) angstrom. The H center dot center dot center dot Si distance of 2.278(17) angstrom and the very unequal H-Rh-P and H-RhSi angles suggest some residual H center dot center dot center dot Si interaction. Competition reactions were performed with 1b dissolved in hexane in the presence of HBpin and B(2)pin(2) simultaneously. P-31 NMR measurements, made after brief irradiation, showed a slight preference for B-B oxidative addition over B-B oxidative addition. Similar experiments with three- way competition among HBpin, HSiMe2Et, and HC6F5, analyzed by H-1 NMR spectroscopy, showed negligible selectivity among H-B, H-C, and H-Si oxidative addition. Molecular structures were also determined by single-crystal X-ray diffraction for 1b, 1c, [Rh(eta(5)-C5H5)-(PPh3)(2)], and [Rh(eta(5)-C5H5)Cl-2(PPh3)].