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

Abstract

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, Et₄N[Ni(nmp)(SPh-o-NH₂-p-CF₃)] (1; nmp²⁻ = dianion of N-(2-mercaptoethyl)picolinamide; ⁻SPh-o-NH₂-p-CF₃ = 2-amino-4-(trifluoromethyl)benzenethiolate) and its oxidized analogue 1^ox, respectively. The ultimate products of these reactions are the disulfide of ⁻SPh-o-NH₂-p-CF₃ and the S,S-bridged tetrameric complex [Ni₄(nmp)₄], a result of S-based redox activity. However, introduction of NO to 1 affords the green dimeric {NiNO}¹⁰ complex (Et₄N)₂[{Ni(κ²-SPh-o-NNO-p-CF₃)(NO)}₂] (2) via NO-induced loss of nmp²⁻ 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-(Et₄N)₂[Ni(SPh-o-NNO-p-CF₃)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.