Stereochemical nonrigidity of a chiral rhodium boryl hydride complex: a sigma-borane complex as transition state for isomerization.

Experimental and computational studies are reported on half-sandwich rhodium complexes that undergo B-H bond activation with pinacolborane (HBpin = HB(OCMe2CMe2O)). The photochemical reaction of [Rh(eta5-C5H5)(R,R-phospholane)(C2H4)] 3 (phospholane = PhP(CHMeCH2CH2CHMe)) with HBpin generates the boryl hydride in two distinguishable isomers [(SRh)-Rh(eta5-C5H5)(Bpin)(H)(R,R-phospholane)] 5a and [(RRh)-Rh(eta5-C5H5)(Bpin)(H)(R,R-phospholane)] 5b that undergo intramolecular exchange. The presence of a chiral phosphine allowed the determination of the interconversion rates (epimerization) by 1D 1H EXSY spectroscopy in C6D6 solution yielding DeltaH = 83.4 +/- 1.8 kJ mol-1 for conversion of 5a to 5b and 79.1 +/- 1.4 kJ mol-1 for 5b to 5a. Computational analysis yielded gas-phase energy barriers of 96.4 kJ mol-1 determined at the density functional theory (DFT, B3PW91) level for a model with PMe3 and B(OCH2CH2O) ligands; higher level calculations (MPW2PLYP) on an optimized QM/MM(ONIOM) geometry for the full system place the transition state 76.8 kJ mol-1 above the average energy of the two isomers. The calculations indicate that the exchange proceeds via a transition state with a sigma-B-H-bonded borane. The B-H bond lies in a mirror plane containing rhodium and phosphorus. No intermediate with an eta2-B-H ligand is detected either by experiment or calculation. Complex 3 has also been converted to the [Rh(eta5-C5H5)Br2(R,R-phospholane)] (characterized crystallographically) and [Rh(eta5-C5H5)(H)2(R,R-phospholane)]. The latter exhibits two inequivalent hydride resonances that undergo exchange with DeltaH = 101 +/- 2 kJ mol-1. DFT calculations indicate that the boryl hydride complex has a lower exchange barrier than the dihydride complex because of steric hindrance between the phospholane and Bpin ligands in the boryl hydride.