Electrochemical Properties and Reactivity Study of [MnV(O)(μ-OR–Lewis Acid)] Cores

A mononuclear manganese(V) oxo complex of a bis(amidate)bis(alkoxide) ligand, (NMe 4 )[Mn V (HMPAB)(O)] [ 2; H 4 HMPAB = 1,2-bis(2-hydroxy-2-methylpropanamido)benzene], was synthesized and structurally characterized. A Mn–O term distance of 1.566(4) Å was observed in the solid-state structure of 2, consistent with the Mn≡O formulation. The reaction of redox-inactive metal ions (M n + = Li +, Ca 2+, Mg 2+, Y 3+, and Sc 3+ ) with 2 resulted in the formation of 2 -M n + species, which were characterized by UV–vis, 1 H NMR, cyclic voltammetry, and in situ IR spectroscopy. Theoretical calculations suggested that the alkoxide oxygen atoms of the ligand scaffold are energetically most favorable for coordinating the M n + ions in 2 . Complex 2 revealed one-electron-reduction potential at −0.01 V versus ferrocenium/ferrocene, which shifted anodically upon coordination of M n + ions to 2, and such a shift became more prominent with stronger Lewis acids. The oxygen-atom transfer (OAT) reactivities of 2 and 2 -M n + species with triphenylphosphine were compared, which exhibited a systematic increase of the reaction rate with increasing Lewis acidity of M n + ions, and a plot of log k 2 versus Lewis acidity of M n + ions (Δ E ) followed a linear relationship. It was observed that 2 -Sc 3+ was ca. 3200 times more reactive toward the OAT reaction compared to 2 . Hammett analysis of 2 exhibited a V-shaped plot, indicating a change of the reaction mechanism upon going from electron-rich to electron-deficient Ar 3 P substrates. In contrast, 2 -Ca 2+ and 2 -Sc 3+ showed an electrophilic nature toward the OAT reaction, thus demonstrating the role of the Lewis acid in controlling the OAT mechanism. The hydrogen-atom abstraction reaction of 2 and 2 -M n + adducts with 1-benzyl-1,4-dihydronicotinamide was investigated, and it was observed that the rate of reaction did not vary considerably with the Lewis acidity of M n + ions. On the basis of Eyring analysis of 2 and 2 -M n + adducts, we hypothesized an entropy-controlled hydrogen-atom-transfer reaction for 2 -Sc 3+, which is different from the reaction mechanism of 2 and 2 -Ca 2+ .