Formation of a triply bridged µ-oxo diiron(iii) core stabilized by two deprotonated carboxamide groups upon photorelease of NO from a {Fe–NO}6 iron nitrosyl

Abstract

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 ν_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 (λ_max = 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 (Φ) measurements indicate that 2 is a more efficient NO donor (Φ = 0.258) than [(PaPy₃)Fe(NO)](ClO₄)₂ (1, Φ = 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 (η²,µ₂)µ-NCO bridges derived from the carboxamido groups of the two (PaPy₂Q)Fe(III) moieties.