Electron Transfer and Geometric Conversion of Co-NO Moiety in Saddled Porphyrins: Implications for Trigger Role of Tetrapyrrole Distortion

Abstract

The electrons of NO and Co are strongly delocalized in normal {Co-NO}⁸ species. In this work, {Co-NO}⁸ complexes are induced to convert from (Co^II)^+•-NO^• to Co^III-NO^- by a core contraction of 0.06 Å in saddled cobalt(II) porphyrins. This intramolecular electron transfer mechanism indicates that nonplanarity of porphyrin is involved in driving conversion of the NO units from electrophilic NO^• as a bent geometry to nucleophilic NO^- as a linear geometry. This implies that distortion acts as a trigger in enzymes containing tetrapyrrole. The electronic behaviors of the Co^II ions and Co-NO moieties were confirmed by X-ray crystallography, EPR spectroscopy, theoretical calculation, UV-vis and IR spectroscopy, and electrochemistry.