An electrospray ionization mass spectrometry study of the nitroprusside–cation–thiolate system

Abstract

The complex anion [Fe(CN)₅(NO)]²⁻ (nitroprusside) has been defined intact using electrospray ionization mass spectrometry (ESIMS), with minor daughter ions predictable fragments of the parent [Fe(CN)₅NO]²⁻. Direct evidence for ion pairing of [Fe(CN)₅NO]²⁻ with monovalent alkali metal and some alkylammonium cations was found independently using ESIMS and ¹³C NMR techniques. The results are relevant to the significant influence of the concentration and nature of cations on the solution equilibrium between nitroprusside with thiolates and their nitrosothiol adducts that presumably arises through charge reduction in tight ion pairs facilitating adduct formation. Ion-paired species {M⁺[Fe(CN)₅NO]²⁻}⁻, {M₃[Fe(CN)₅NO]}⁺ and more complex clusters of the type {M_x[Fe(CN)₅NO]_y}^z−/z+ (up to x = 8 and y = 5) were observed by ESIMS. The trend in the ion pair formation seen by ESIMS follows the series Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺ and Me₄N⁺ > Et₄N⁺ > Pr₄N⁺ > Bu₄N⁺ and is consistent with purely electrostatic expectations for unsolvated ions. Whereas the poorly solvated alkylammonium series bulk solution behaviour resembles that found by the ESI technique, the activity of alkali metal cations proceeds conversely, i.e. Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺, verified by a ¹³C NMR study, because the hydrated cation radii play a crucial role in the bulk solution in those cases. Differences in changes of the chemical shift upon addition of various cations points to the axial CN⁻ position as especially important for binding to these cations. Further, short-lived adducts with thiolates of the type {M⁺₃[Fe(CN)₅NO(C₄H₄O₄S)]⁴⁻}⁻ have been defined by their observation in ESIMS directly and through their decomposition products, providing further support for reversible nitrosothiol complex formation. Mercaptosuccinate ion pairing with cations, which would make formation of the proposed nitrosothiol complex easier, is also observed by ESIMS. The dithiolato complex {K⁺[Fe(CN)₅NO(C₄H₆O₄S)₂]²⁻}⁻, as well as the disulfide anion radical were detected; both species are suggested intermediates in the spontaneous and autocatalyzed redox decomposition of the [Fe(CN)₅N(O)SR]^{(n + 2)−} complexes. The ESIMS study has thus provided information relevant to both thermodynamic and kinetic processes in a reaction where species of limited stability are involved.