Simultaneous nitrosylation and N-nitrosation of a Ni-thiolate model complex of Ni-containing SOD

Nitric oxide (NO) is used as a substrate analogue/spectroscopic probe of metal sites that bind and activate oxygen and its derivatives. To assess the interaction of superoxide with the Ni center in Ni-containing superoxide dismutase (NiSOD), we studied the reaction of NO(+) and NO with the model complex, Et(4)N[Ni(nmp)(SPh-o-NH(2)-p-CF(3))] (1; nmp(2–) = dianion of N-(2-mercaptoethyl)picolinamide; (–)SPh-o-NH(2)-p-CF(3) = 2-amino-4-(trifluoromethyl)benzenethiolate) and its oxidized analogue 1(ox), respectively. The ultimate products of these reactions are the disulfide of (–)SPh-o-NH(2)-p-CF(3) and the S,S-bridged tetrameric complex [Ni(4)(nmp)(4)], a result of S-based redox activity. However, introduction of NO to 1 affords the green dimeric {NiNO}(10) complex (Et(4)N)(2)[{Ni(κ(2)-SPh-o-NNO-p-CF(3))(NO)}(2)] (2) via NO-induced loss of nmp(2–) as the disulfide and N-nitrosation of the aromatic thiolate. Complex 2 was characterized by X-ray crystallography and several spectroscopies. These measurements are in-line with other tetrahedral complexes in the {NiNO}(10) classification. In contrast to the established stability of this metal-nitrosyl class, the Ni–NO bond of 2 is labile and release of NO from this unit was quantified by trapping the NO with a Co(II)–porphyrin (70–80% yield). In the process, the Ni ends up coordinated by two o-nitrosaminobenzenethiolato ligands to result in the structurally characterized trans-(Et(4)N)(2)[Ni(SPh-o-NNO-p-CF(3))(2)] (3), likely by a disproportionation mechanism. The isolation and characterization of 2 and 3 suggest that: (i) the strongly donating thiolates dominate the electronic structure of Ni-nitrosyls that result in less covalent Ni–NO bonds, and (ii) superoxide undergoes disproportionation via an outer-sphere mechanism in NiSOD as complexes in the {NiNO}(9/8) state have yet to be isolated.