Stereochemical Nonrigidity of a Chiral Rhodium Boryl Hydride Complex: A σ-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(η5-C5H5)(R,R-phospholane)(C2H4)] 3 (phospholane = PhP(CHMeCH2CH2CHMe)) with HBpin
generates the boryl hydride in two distinguishable isomers [(SRh)-Rh(η5-C5H5)(Bpin)(H)(R,R-phospholane)]
5a and [(RRh)-Rh(η5-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 ΔH‡ = 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 σ-B−H-bonded borane. The
B−H bond lies in a mirror plane containing rhodium and phosphorus. No intermediate with an η2-B−H
ligand is detected either by experiment or calculation. Complex 3 has also been converted to the [Rh(η5-C5H5)Br2(R,R-phospholane)] (characterized crystallographically) and [Rh(η5-C5H5)(H)2(R,R-phospholane)].
The latter exhibits two inequivalent hydride resonances that undergo exchange with ΔH‡ = 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.