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Stereochemical Nonrigidity of a Chiral Rhodium Boryl Hydride Complex:  A σ-Borane Complex as Transition State for Isomerization

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posted on 02.04.2008, 00:00 by Marius V. Câmpian, Eric Clot, Odile Eisenstein, Ulrike Helmstedt, Naseralla Jasim, Robin N. Perutz, Adrian C. Whitwood, David Williamson
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.