Computational Insight into Rh-103 Chemical Shift-Structure Correlations in Rhodium Bis(phosphine) Complexes

Abstract

103RhNMR chemical shifts have been computed at the GIAO-B3LYP level of density functional theory (DFT) for a number of [Rh(COD)(P∩P)]+ complexes [COD = 1,5-cyclooctadiene, P∩P = chelating bis(phosphine) including bis(dimethylphosphino)ethane (dmpe), bis(diphenylphosphino)ethane (dmpe), MeDUPHOS, DIOP, BINAP, and others]. Structures have been optimized using PBE0 and M06 functionals in the gas phase, in a continuum modeling the solvent, and with [PF6]− counteranion included explicitly. Observed trends in δ(103Rh) are well reproduced for pristine PBE0-optimized cations in the gas phase or for ion pairs optimized in a continuum with M06. While there is no overall trend between computed δ(103Rh) values and complex stabilities (evaluated through isodesmic ligand exchange reactions), there is a linear relationship between the 103Rh chemical shifts and the mean Rh–P bond distances. This relationship appears to be remarkably general, encompassing various chelating ring sizes and substituents at P, including remote electron-donating and -withdrawing substituents that are characterized through their Hammett constants. The combination of 103Rh NMR and DFT computations emerges as a useful tool for structure elucidation of Rh–phosphine complexes.