Chiral amine ligands at rhenium(V) pentahydride complexes allow for characterization of an energetically accessible and reversible steric inversion of diastereotopic phosphorus atoms

Abstract

Three new rhenium(V) pentahydride complexes stabilized by a pair of triphenylphosphine ligands and a chiral amine ligand were prepared. Dynamic 31P-{1H} NMR spectroscopy of the chiral complexes demonstrates an unusual steric inversion for the diastereotopic pair of phosphorus atoms in each chiral complex. One chiral ReH5(PPh3)2(amine) complex is supported by an aromatic amine ligand and demonstrates unambiguously that such ligands participate in a steric inversion dynamic process rather than a simple rotation about the Re-N bond. The activation parameters and rate constants for the steric inversion can be determined directly or indirectly for non-hydride atom dynamic processes in two of the three chiral complexes from simulations of dynamic NMR spectra. Characterization of the steric inversion step by atoms other than hydride ligands allows for a means to test whether any hydride ligand exchange processes are manifestations of the steric inversion process. Under the experimental conditions in this report, a pairwise exchange of hydride ligands that reside in pseudododecahedral A sites appears to be a separate manifestation of the steric inversion dynamic process. A change in the nominal coordination number seems likely to cause the steric inversion that is reported.