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 pKa = 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.