Synthesis of Co^II–NO^– Complexes and Their Reactivity as a Source of Nitroxyl

Metal-nitroxyl (M–HNO/M–NO^–) coordination units are found in denitrification enzymes of the global nitrogen cycle, and free HNO exhibits pharmacological properties related to cardiovascular physiology that are distinct from nitric oxide (NO). To elucidate the properties that control the binding and release of coordinated nitroxyl or its anion at these biological metal sites, we synthesized {CoNO}⁸ (1, 2) and {CoNO}⁹ (3, 4) complexes that contain diimine–dipyrrolide supporting ligands. Experimental (NMR, IR, MS, EPR, XAS, XRD) and computational data (DFT) support an oxidation state assignment for 3 and 4 of high spin Co^II (S_Co = 3/2) coordinated to ³NO^– (S_NO = 1) for S_tot = 1/2. As suggested by DFT, upon protonation, a spin transition occurs to generate a putative low spin Co^II–¹HNO (S_Co = S_tot = 1/2); the Co–NO bond is ∼0.2 Å longer, more labile, and facilitates the release of HNO. This property was confirmed experimentally through the detection and quantification of N₂O (∼70% yield), a byproduct of the established HNO self-reaction (2HNO → N₂O + H₂O). Additionally, 3 and 4 function as HNO donors in aqueous media at pH 7.4 and react with known HNO targets, such as a water-soluble Mn^III-porphyrin ([Mn^III(TPPS)]^3–; TPPS = meso-tetrakis­(4-sulfonatophenyl)­porphyrinate) and ferric myoglobin (metMb) to quantitatively yield [Mn­(TPPS)­(NO)]^4– and MbNO, respectively.