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 (νNO) is sensitive to the intramolecular interaction between the pendant ligand and the iron-bound NO, and a shift of νNO 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 [MnIII(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 [MnIII(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 [MnIII(TMSPS3)­(DABCO)].光可介导金属亚硝酰物种生成亚硝酰氢（HNO），该过程由分子内悬挂巯基质子促进。两种采用Enemark–Felthan标记法（Enemark–Felthan notation）的{FeNO}6配合物，分别为带有悬挂巯基的[Fe(NO)(TMSPS2)(TMSPS2H)]（1，其中TMSPS2H2 = 2,2'-二巯基-3,3'-双(三甲基硅基)二苯基膦；H为可解离质子）与带有悬挂硫醚的[Fe(NO)(TMSPS2)(TMSPS2CH3)]（2），均完成了光谱与结构表征。两种配合物均对可见光高度敏感。光解条件下，配合物2发生NO解离，生成单核Fe(III)配合物[Fe(TMSPS2)(TMSPS2CH3)]（3）。与之相对，配合物1的悬挂巯基（-SH）可在光照时捕获离去的NO自由基，形成具有分子内[SH···ON–Fe]相互作用的中间体A。计算结果（密度泛函理论，density functional theory）表明，NO伸缩振动频率（νNO）对悬挂配体与铁键合NO之间的分子内相互作用十分敏感，实验中观测到νNO从1833 cm⁻¹（配合物1）偏移至1823 cm⁻¹（中间体A）。对中间体A进行后续光解，可生成亚硝酰氢与硫自由基，后者随后与铁中心配位形成[Fe(TMSPS2)2]（4）。与常见的酸碱耦合路径不同，配合物1并不会自发生成亚硝酰氢，而是通过光促进路径实现该过程。光解生成的亚硝酰氢可在有机介质中与[MnIII(TMSPS3)(DABCO)]（其中TMSPS3H3 = 2,2',2''-三巯基-3,3',3''-三(三甲基硅基)三苯基膦；DABCO = 1,4-二氮杂双环[2.2.2]辛烷）发生反应，生成具有{MnNO}6电子构型的阴离子配合物[Mn(NO)(TMSPS3)]⁻（5⁻）；而[MnIII(TMSPS3)(DABCO)]与NO气体反应则生成{MnNO}5物种[Mn(NO)(TMSPS3)]（6）。通过使用[MnIII(TMSPS3)(DABCO)]，可有效区分带悬挂巯基的配合物1生成HNO与带悬挂甲硫基（SMe）的配合物2生成NO的过程。