Hydrogen sulfide (H2S) is the most recently discovered endogenously-produced gasotransmitter, playing important roles in inflammatory response, metabolic regulation, and neurotransmission. H2S is more challenging to study than its fellow gasotransmitters, nitric oxide and carbon monoxide, because it is a reductant, a potent nucleophile, and an acid with two biologically relevant pKa's. Additionally, H2S is metallophilic, often forming intractable and insoluble metal salts. Although significant work has been devoted to the detection and quantification of H2S in biological systems, the fundamental reaction chemistry of H2S with bioinorganic targets remains underexplored. Phthalocyanines (Pc) are a useful class of compounds for understanding the redox activity and binding of H2S to transition metal centers. Reaction of ZnPc and CoPc with the diprotic (H2S(g)) and monoanionic (SH�) forms of H2S was studied. Different interactions were observed depending on the redox activity of the metal and the protonation state of H2S. Monoanionic SH� reacts stoichiometrically with both ZnPc and CoPc, whereas H2S(g) shows no reactivity toward either complex. Redox inactive ZnPc ligates SH� as demonstrated by UV-Vis and 1H NMR experiments. This binding can be cycled by protonation of the bound [ZnPc-SH]� complex. Redox active CoPc undergoes a one-electron reduction at the metal center by SH� to form [CoPc]� as demonstrated by control experiments with [NBu4][BH4]. The resultant complex is readily reoxidized to the parent CoPc upon exposure to the atmosphere in a cyclable manner. These studies demonstrate the importance of redox activity and H2S protonation state in controlling the reactivity of H2S with transition metal complexes.

Reversible reactivity of hydrogen sulfide with zinc and cobalt phthalocyanines

248th National Meeting (2014)

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