Cationic Charges Leading to an Inverse Free-Energy Relationship for N−N Bond Formation by Mn^VI Nitrides

Abstract

© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim MnVN Schiff-base complexes incorporating a Na+ (1Na), K+ (1K), Ba2+ (1Ba), or Sr2+ (1Sr) ion in the ligand framework are reported. The MnVI/V reduction potentials for 1Na, 1K, 1Ba, and 1Sr are 0.591, 0.616, 0.805, and 0.880 V vs. Fe(C5H5)2+/0, respectively, exhibiting significant positive shifts compared to a MnN Schiff-base complex in the same primary coordination environment but with no associated alkali or alkaline earth metal ion (A, E1/2=0.427 V vs. Fe(C5H5)2+/0). One-electron oxidation of the MnVN complexes results in bimolecular coupling to form N2 with rates (k2) at 20 °C of 2166, 684, 857, and 99.7, an 87 m−1 s−1 for A, 1Na, 1K, 1Ba, and 1Sr respectively, following an inverse linear free energy relationship. Thus, increasing charge through proximal cations results in MnVIN complexes that are both more oxidizing and more stable to bimolecular coupling, a trend diametrically opposed to when complexes were modified by ligand substituents through inductive effects.