The structures of the dicationic tetranitrosyl iron complex with cysteamine [Fe2S2(CH2CH2NH3)2(NO)4]2+ and its decomposition products in protic media: an experimental and theoretical study

Abstract

Decomposition products of [Fe2S2(CH2CH2NH3)2(NO)4]SO4·2.5H2O (1′) were studied by electrochemistry and mass spectrometry. The structures of the dicationic tetranitrosyl iron complex with cysteamine of the composition [Fe2S2(CH2CH2NH3)2(NO)4]2+ (1) and possible products of its decomposition and NO replacement by an aqua ligand were studied by quantum chemical methods at the density functional theory level. Taking into account the solvation effects, the replacement of the nitrosyl ligand in dication 1 by an aqua ligand was found to be less favorable in aqueous solution than in the gas phase. The pK value was calculated for the proton abstraction from the NH3 group of compound 1 (7.2), and the removal of NO from the deprotonated form of the complex was found to be much easier. This result is consistent with the experimental data on an increase in the rate of NO formation in aqueous solutions of 1 with increasing pH from 6 to 8 assuming that the increase in pH is accompanied by an increase in the percentage of the less stable deprotonated form of the complex and that OH− does not participate in the elementary step of NO formation. The kinetic curves of NO formation are well described by a two-step scheme of consecutive first-order reactions of the NO formation and consumption. In the gas phase, dication 1 was found to be unstable to decomposition into two mononuclear cationic dinitrosyl iron complexes with cysteamine. This result is consistent with the fact that these cations are observed in the electrospray ionization mass spectrometric experiment. The major peak in the mass spectra is associated with the [Fe2S2(CH2CH2NH3)2(NO)4 − H]+ ion. As follows from the calculations, this is due to the deprotonation of the dication as it gets rid of the hydration shell, because even the dimer of water molecules is more basic than dication 1.