As understanding of the significance of nitrosylated iron in human physiology continues to expand, synthetic chemists are prompted to create molecular mimics of potential to define fundamental structure and electronic properties. Thus a large literature exists regarding the mono-nitrosyliron unit in a heme environment where two redox levels {Fe(NO)}7/8 are known. The di-nitrosyliron unit is a likely NO storage and transport agent that exists as cysteine, glutathione or histidine stabilized protein bound low molecular weight forms. They also exhibit one-electron redox chemistry, {Fe(NO)2}9/10. Interestingly, the non-heme active site of nitrile hydratase, shows the presence of iron or cobalt in a planar Cys-Ser-Cys (N2S2) tripeptide motif. In the as-isolated inactive forms, cysteinyl sulfur and NO occupy as axial ligands, reminiscent of octahedral {Fe(NO)}7/8. Our studies of MN2S2 as metalloligands, led to the combination of mono-nitrosyliron and di-nitrosyliron units, {Fe(NO)}7-{Fe(NO)2}9, which showed promise in the development as a proton reduction electrocatalyst. The cationic tri-nitrosyldiiron complex, [(NO)Fe(N2S2).Fe(NO)2]+ have shown impressive thermodynamic stability by its isolation from six synthetic routes. Electrochemical studies of the cationic complex showed two, reversible, one-electron reduction events, which were assigned for the existence of three redox states, viz., {Fe(NO)}7-{Fe(NO)2}9, {Fe(NO)}7-{Fe(NO)2}10, and {Fe(NO)}8-{Fe(NO)2}10. The oxidized complex was systematically reduced to its neutral and anionic forms by sequential addition of 1 and 2 equivalents of KEt3BH, respectively. All the three redox-states were structurally characterized, see graphic. The anionic tri-nitrosyldiiron complex, {Fe(NO)}8-{Fe(NO)2}10, shows an impressive linear {Fe(NO)}8. Computational studies support the experimental results showing a triplet, high spin (HS) {Fe(NO)}8 that undergoes an impressive spin-polarization to achieve linearity. Further studies are being carried out to understand the electronic distribution in three redox states.

Triplet, high-spin, linear {Fe(NO)}8: Redox feature of a stable diiron trinitrosyl complex

255th National Meeting (2018)

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