Nitrogen-15 and carbon-13 NMR study of Roussin salts and esters and of pentacyanoferrate complexes

Abstract

High-frequency 15N NMR spectroscopy, using 99% isotopic enrichment for 15N, shows that the Roussin esters Fe2(SR)2(NO)4 (R = Me, i'-Pr) exist as an equimolar mixture of two conformers having and C2v symmetries respectively. Similar 15N spectroscopy of [Fe4X3(NO)7]- (X = S, Se) shows that the solid-state structure for X = S persists in aqueous solution and demonstrates that the anion having X = Se is isostructural in solution; this is confirmed by natural-abundance 77Se NMR. The cubane type Fe4S4(NO)4 retains its structure in solution, and Fe4Se4(NO)4 is isostructural. A combination of 13C and 15N NMR spectra obtained at high frequency with 90% 13C and 99% 15N enrichment reveals two mechanisms for the conversion of [Fe-(CN)5NO]2- to [Fe(CN)5N02]4-, one involving nucleophilic attack by hydroxide and the other attack by nitrite. The spectral data allow ready identification of pentacyanoferrate complexes arising from the reactions of nitroprusside with nucleophiles.