Inter-ligand intramolecular through-space anisotropic shielding in a series of manganese carbonyl phosphorous compounds

Eight novel manganese carbonyl complexes of the type [Mn(bpy-Bu-t)(CO)(3)PR3](+) (bpy-Bu-t = 4,4 '-di-tert-butyl-2,2 '-bipyridine; R = Cy, Bu-n, Me, p-tol, Ph, p-F-Ph, OEt, and OMe), have been synthesized and characterized by H-1 NMR, FTIR, UV/Vis, HRMS and CV. X-ray crystallographic structures of [Mn(bpy-Bu-t)(CO)(3)(PCy3)](+) and [Mn(bpy-Bu-t)(CO)(3)(PPh3)](+) were obtained. The short Mn-P bond length allows for close proximity of the bipyridine ligand and the phosphine R groups, resulting in strong anisotropic shielding of certain bipyridine protons by aryl R groups (reordering the bipyridine H-1 NMR pattern in the most extreme case). Electrochemical analysis of the compound series reveals that while each is a competent precatalyst for electrochemical carbon dioxide reduction (to carbon monoxide), the lability of the PR3 ligand results in similar catalytic performance amongst the series.