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

Source abstract

Abstract Mn V N Schiff‐base complexes incorporating a Na + ( 1Na ), K + ( 1K ), Ba 2+ ( 1Ba ), or Sr 2+ ( 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 0.880 V vs. Fe(C 5 H 5 ) 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 =0.427 V vs. Fe(C 5 H 5 ) 2 +/0 ). One‐electron oxidation of the Mn V N complexes results in bimolecular coupling to form N 2 with rates (k 2 ) 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 Mn VI 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.