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

(MnN)-N-V Schiff-base complexes incorporating a Na+ (1Na), K+ (1K), Ba2+ (1Ba), or Sr2+ (1Sr) ion in the ligand framework are reported. The Mn-VI/V reduction potentials for 1Na, 1K, 1Ba, and 1Sr are 0.591, 0.616, 0.805, and 0.880V 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, E-1/2=0.427V vs. Fe(C5H5)(2)(+/0)). One-electron oxidation of the (MnN)-N-V complexes results in bimolecular coupling to form N-2 with rates (k(2)) at 20 degrees C of 2166, 684, 857, and 99.7, an 87m(-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 (MnN)-N-VI 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.