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)­CoIII(NO)]2+ and [(12-TMC)­CoIII(NO)]2+, in NO-transfer and dioxygenation reactions. The Co­(III)–nitrosyl complex bearing 14-TMC ligand, [(14-TMC)­CoIII(NO)]2+, transfers the bound nitrosyl ligand to [(12-TMC)­CoII]2+ via a dissociative pathway, {[(14-TMC)­CoIII(NO)]2+ → {(14-TMC)­Co···NO}2+}, thus affording [(12-TMC)­CoIII(NO)]2+ and [(14-TMC)­CoII]2+ as products. The dissociation of NO from the [(14-TMC)­CoIII(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)­CoIII(NO)]2+ to [(14-TMC)­CoII]2+, does not occur. In addition to the NO-transfer reaction, dioxygenation of [(14-TMC)­CoIII(NO)]2+ by O2 produces [(14-TMC)­CoII(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)­CoII(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)­CoIII(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.