Proton affinity of [Fe4S4{SCH2CH(OH)Me}4]2− in methanol: relevance to hydrogen bonding of Fe–S clusters in proteins

Abstract

The reaction between PhS⁻ and [Fe₄S₄{SCH₂CH(OH)Me}₄]²⁻ to form [Fe₄S₄(SPh)₄]²⁻ has been studied in methanol, in the presence of the weak acid, [NHEt₃]⁺. The kinetics are similar to those observed earlier for a variety of Fe–S-based clusters studied in MeCN. A major difference between the studies in the two solvents concerns the identity of the solution species. In MeCN, [NHEt₃]⁺ is a sufficiently strong acid to convert all PhS⁻ to PhSH. However, in MeOH, PhSH is a comparatively stronger acid and consequently PhS⁻ is little protonated by [NHEt₃]⁺. The rate law for the reaction in MeOH is consistent with a mechanism in which initial protonation of a thiolate ligand is followed by protonation of the cluster core (presumably a μ₃-S) which labilises the terminal thiol ligand. Subsequent attack of PhS⁻ at the vacant site thus created on one of the Fe atoms completes the first act of substitution. Analysis of the data yields pK_a = 8.5 for [Fe₄S₃(μ-SH){SCH₂CH(OH)Me}₄]⁻. The relevance of this result to hydrogen bonding interactions of Fe–S-based clusters in proteins is discussed. The X-ray crystal structure of [NMe₄]₂[Fe₄S₄{SCH₂CH(OH)Me}₄] is also reported, and the arrangement of the ligands is consistent with an extensive hydrogen bonding network between some of the hydroxyl groups.