Photoinduced NO and HNO Production from Mononuclear {FeNO}⁶ Complex Bearing a Pendant Thiol

Light triggers the formation of HNO from a metal-nitrosyl species, facilitated by an intramolecular pendant thiol proton. Two {FeNO}⁶ complexes (the Enemark-Felthan notation), [Fe(NO)(^TMSPS2)(^TMSPS2H)] (1, ^TMSPS2H₂ = 2,2'-dimercapto-3,3'-bis(trimethylsilyl)diphenyl)phenylphosphine; H is a dissociable proton) with a pendant thiol and [Fe(NO)(^TMSPS2)(^TMSPS2CH₃)] (2) bearing a pendant thioether, are spectroscopically and structurally characterized. Both complexes are highly sensitive to visible light. Upon photolysis, complex 2 undergoes NO dissociation to yield a mononuclear Fe(III) complex, [Fe(^TMSPS2)(^TMSPS2CH₃)] (3). In contrast, the pendant SH of 1 can act as a trap for the departing NO radical upon irradiation, resulting in the formation of an intermediate A with an intramolecular [SH···ON-Fe] interaction. As suggested by computational results (density functional theory), the NO stretching frequency (ν_NO) is sensitive to the intramolecular interaction between the pendant ligand and the iron-bound NO, and a shift of ν_NO from 1833 (1) to 1823 cm^-1 (A) is observed experimentally. Subsequent photolysis of the intermediate A results in HNO production and a thiyl group that then coordinates to the Fe center for the formation of [Fe(^TMSPS2)₂] (4). In contrast with the common acid-base coupling pathway, the HNO is not voluntarily yielded from 1 but rather is generated by the photopromoted pathway. The photogenerated HNO can further react with [Mn^III(^TMSPS3)(DABCO)] (^TMSPS3H₃ = (2,2'2''-trimercapto-3,3',3''-tris(trimethylsilyl)triphenylphosphine; DABCO = 1,4-diazabicyclo[2.2.2]octane) in organic media to yield anionic [Mn(NO)(^TMSPS3)]^- (5^-) with a {MnNO}⁶ electronic configuration, whereas [Mn^III(^TMSPS3)(DABCO)] reacts with NO gas for the formation of a {MnNO}⁵ species, [Mn(NO)(^TMSPS3)] (6). Effective differentiation of the formation of HNO from complex 1 with the pendant SH versus NO from 2 with the pendant SMe is achieved by the employment of [Mn^III(^TMSPS3)(DABCO)].