The bonding, synthesis and electrochemistry of some iron-sulphur-nitrosyl compounds

Abstract

Salts of the bls(µ-thiosulphato-S)-bis(dinitrosylferrate)(2-) anion, [Fe₂(S₂o₃)₂(NO)₄]²- can be prepared by the reaction of iron(ll)/thiosulphate mixtures with nitrite ion. The crystal structure of {(Ph₃P)₂N}₂[ Fe₂(S₂0₃)₂(NO)₄] reveals it to adopt a trans structure. 15N n.m.r. studies of this salt also show it to adopt the trans structure in solution. The anion reacts with thiolate ion, RS-, to provide good yields of Fe₂(SR)₂(NO)₄; e.p.r. studies show the mononitrosyl [Fe(N0)(SR)₃]³⁻ to be a major intermediate in this reaction.

Salts of the Black Roussin ion, [Fe₄S₃(NO)₇]- react with aryldiazonium salts, ArN₂⁺ , to produce Fe₂(SAr)₂(NO)₄. ¹³C n.m.r. studies of Fe₂(SC₆H₄F)₂(NO)₄ reveal it to exist in a 1:1 ratio of cis and trans isomers. Reaction of the Black anion with trialkylsulphonium or sulphoxonium salts leads to the metathesis product. The crystal structure of Me₃S[Fe₄S₃(NO)₇] shows no evidence for the closure of the iron-sulphur cage by the sulphur of the Me₃S cation. Reaction of the Black anion with trialkyloxonium salts provides good yields of Fe₂(SR)₂(NO)₄.

Electrochemical studies on Fe₂(SR)₂(NO)₄ by conventional cyclic voltammetry shows two chemically and electrochemically reversible one electron reductions to produce [Fe₂(SR)₂(NO)₄]⁻ and [Fe₂(SR)₂(NO)₄]²⁻. The monoanion is a paramagnetic complex with g= 1.995; coupling to four equivalent nitrogens shows the presence of a delocalised electron. The electrochemical behaviour of the dianion is dependent on the electrode material; reversible on a glassy carbon electrode but quasi-reversible on platinum. Fe₂(SR)₂(NO)₄ also exhibits a three electron oxidation; the anodic current of which increases in the presence of primary amines.

Electrochemical studies on the Black Roussin ion and its selenium analogue [Fe₄Se₃(NO)₇]⁻ show similar electrochemical responses: three reversible one electron reductions and an irreversible multielectron oxidation. Extended Huckel molecular orbital calculations on [Fe₂S₂(NO)₄]²⁻ , Fe₂(SMe)₂(NO)₄ and [Fe₂(S₂0₃)₂(N0)₄]²⁻ reveal little direct Fe-Fe interaction in these complexes. Reduction or oxidation would result in the addition or removal of electron density from orbitals of mainly Fe-S character.

An electrochemical study of Fe(NO)(S₂C N R₂)₂ by cyclic voltammetry shows a reversible one electron reduction and an Irreversible oxidation in tetrahydrofuran and dichloromethane. In acetonitrile the reduction of the complex is coupled to a chemical step making the reduction chemically irreversible at low scan rates. The observed variation of E with R is due to the inductive effect of R.