Formation of a triply bridged mu-oxo diiron(III) core stabilized by two deprotonated carboxamide groups upon photorelease of NO from a [Fe-NO]6 iron nitrosyl

The iron nitrosyl (PaPy2Q)Fe(NO)](ClO4)2 (2), derived from the quinoline-based ligand PaPy2QH (N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-quinoline-2-carboxamide, where H is dissociable proton) has been characterized by spectroscopy and X-ray diffraction techniques. The 1H NMR spectrum (S = 0 ground state) and v(NO) value of 1885 cm(-1) indicate that 2 is a Fe-NO]6 nitrosyl. Although 2 is stable in the dark, exposure of an acetonitrile solution of 2 (lambdamax = 510 nm) to light in the visible range causes rapid release of NO and formation of the solvato species (PaPy2Q)Fe(MeCN)](ClO4)2 (6). Quantum yield (Phi) measurements indicate that 2 is a more efficient NO donor (Phi = 0.258) than (PaPy3)Fe(NO)](ClO4)2 (1, Phi = 0.185), a complex derived from a similar but pyridine-based ligand. Interestingly, when the photoproduct 6 is exposed to water or a small amount of base, the triply bridged diiron(III) species (PaPy2Q)FeOFe(PaPy2Q)](ClO4)2 (3) forms in good yield. This species can be independently synthesized from aerobic oxidation of the Fe(II) species (PaPy2Q)Fe(MeCN)](ClO4) in acetonitrile. The structure of 3 reveals a unique Fe(III)-O-Fe(III) link supported by two (eta2,mu2)mu-NCO bridges derived from the carboxamido groups of the two (PaPy2Q)Fe(III) moieties.