Related Experiment Video
Updated: Jul 6, 2026

Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex
Published on: July 27, 2022
Stereochemical nonrigidity of a chiral rhodium boryl hydride complex: a sigma-borane complex as transition state for
Marius V Câmpian1, Eric Clot, Odile Eisenstein
1Department of Chemistry, University of York, York YO10 5DD, UK.
Abstract:
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.
Related Concept Videos
Regioselectivity and Stereochemistry of Hydroboration
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
Prochirality
Regioselectivity and Stereochemistry of Acid-Catalyzed Hydration
Stereoisomerism of Cyclic Compounds
Molecules with Multiple Chiral Centers
Radical Halogenation: Stereochemistry
Halogenation to form a new chiral center:

