Cationic Charges Leading to an Inverse Free-Energy Relationship for N-N Bond Formation by MnVI Nitrides.

MnV N 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(C5 H5 )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(C5 H5 )2 +/0 ). One-electron oxidation of the MnV N 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 MnVI N 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.