The photorelease of nitrogen monoxide (NO) from pentacyanonitrosyl coordination compounds of group 8 metals

Abstract

The photodetachment of NO from [M^II(CN)₅NO]²⁻ with M = Fe, Ru, and Os, upon laser excitation at various wavelengths (355, 420, and 480 nm) was followed by various techniques. The three complexes showed a wavelength-dependent quantum yield of NO production Φ(NO), as measured with an NO-sensitive electrode, the highest values corresponding to the larger photon energies. For the same excitation wavelength the decrease of Φ(NO) at 20 °C in the order Fe > Ru ≫ Os, is explained by the increasing M–N bond strength and inertness of the heavier metals. Transient absorption data at 420 nm indicate the formation of the [M^III(CN)₅H₂O]²⁻ species in less than ca. 1 μs for M = Fe and Ru. The enthalpy content of [Fe^III(CN)₅H₂O]²⁻ with respect to the parent [Fe^II(CN)₅NO]²⁻ state is (190 ± 20) kJ mol⁻¹, as measured by laser-induced optoacoustic spectroscopy (LIOAS) upon excitation at 480 nm. The production of [Fe^III(CN)₅H₂O]²⁻ is concomitant with an expansion of (8 ± 3) ml mol⁻¹ consistent with an expansion of the water bound through hydrogen bonds to the CN ligands plus the difference between NO release into the bulk and water entrance into the first coordination sphere. The activated process, as indicated by the relatively strong temperature dependence of the Φ(NO) values and by the temperature dependence of the appearance of the [Fe^III(CN)₅H₂O]²⁻ species, as determined by LIOAS, is attributed to NO detachment in less than ca. 100 ns from the isonitrosyl (ON) ligand (MS1 state).