Factors That Control the Reactivity of Cobalt(III)-Nitrosyl Complexes in Nitric Oxide Transfer and Dioxygenation Reactions: A Combined Experimental and Theoretical Investigation

Metal-nitrosyl complexes are key intermediates involved in many biological and physiological processes of nitric oxide (NO) activation by metalloproteins. In this study, we report the reactivities of mononuclear cobalt(III)-nitrosyl complexes bearing N-tetramethylated cyclam (TMC) ligands, [(14-TMC)Co^III(NO)]²⁺ and [(12-TMC)Co^III(NO)]²⁺, in NO-transfer and dioxygenation reactions. The Co(III)-nitrosyl complex bearing 14-TMC ligand, [(14-TMC)Co^III(NO)]²⁺, transfers the bound nitrosyl ligand to [(12-TMC)Co^II]²⁺ via a dissociative pathway, {[(14-TMC)Co^III(NO)]²⁺ → {(14-TMC)Co···NO}²⁺}, thus affording [(12-TMC)Co^III(NO)]²⁺ and [(14-TMC)Co^II]²⁺ as products. The dissociation of NO from the [(14-TMC)Co^III(NO)]²⁺ complex prior to NO-transfer is supported experimentally and theoretically. In contrast, the reverse reaction, which is the NO-transfer from [(12-TMC)Co^III(NO)]²⁺ to [(14-TMC)Co^II]²⁺, does not occur. In addition to the NO-transfer reaction, dioxygenation of [(14-TMC)Co^III(NO)]²⁺ by O₂ produces [(14-TMC)Co^II(NO₃)]⁺, which possesses an O,O-chelated nitrato ligand and where, based on an experiment using ¹⁸O-labeled O₂, two of the three O-atoms in the [(14-TMC)Co^II(NO₃)]⁺ product derive from O₂. The dioxygenation reaction is proposed to occur via a dissociative pathway, as proposed in the NO-transfer reaction, and via the formation of a Co(II)-peroxynitrite intermediate, based on the observation of phenol ring nitration. In contrast, [(12-TMC)Co^III(NO)]²⁺ does not react with O₂. Thus, the present results demonstrate unambiguously that the NO-transfer/dioxygenation reactivity of the cobalt(III)-nitrosyl complexes bearing TMC ligands is significantly influenced by the ring size of the TMC ligands and/or the spin state of the cobalt ion.