Reactions of Hydroborating Reagents with Phosphinorhodium Hydride Complexes: Molecular Structures of a Rh2B3 Metallaborane Cluster, an L2Rh(η2-H2BR2) Complex and a Mixed Valence Rh Dimer Containing a Semi-Bridging Bcat (Cat=1,2-O2c6h4) Group

The formation of products derived from competing reactions in Rh-catalyzed alkene hydroborations prompted us to study in situ interactions of several hydroborating reagents with unsaturated phosphinorhodium complexes. The reaction of [Rh(μ-H)(DiPPE)]2 (1) with borane–dimethylsulfide gave several rhodium–boron containing products including the structurally characterized metallaborane [RhH(DiPPE)]2B3H7 (2) [DiPPE=1,2-bis(diisopropylphosphino)ethane]. Addition of thexylborane and 9-H-BBN dimers, containing alkyl groups, to 1 gave the corresponding mononuclear substituted borohydride complexes Rh(η2-H2BRR′)(DiPPE) (4: R=H, R′=C(CH3)2CH(CH3)2; 5: R,R′=bicyclo-[3.3.1]-nonane [C8H14]). The analogous monodentate phosphine complex Rh(η2-H2BC8H14)(PPri3)2 (6) was isolated and structurally characterized. Addition of catecholborane (HBcat; cat=1,2-O2C6H4) to 1 gave the unusual dinuclear species Rh(DiPPE)(μ-H)2(μ-Bcat)RhH(DiPPE) (7) containing a semi-bridging Bcat group, confirmed by single-crystal X-ray diffraction, and zwitterionic Rh(η6-catBcat)(DiPPE) (8). Implications for rhodium-catalyzed hydroborations are addressed.