A unified mechanism for the stoichiometric reduction of H+ and C2H2 by [Fe4S4(SPh)4]3− in MeCN

Abstract

The kinetics and mechanisms of the conversions of H+ into H2 and C2H2 into C2H4 by [Fe4S4(SPh)4]3–, using [Hlut]+ (lut = 2,6-dimethylpyridine) as the proton source, have been investigated in MeCN. At high concentrations of [Hlut]+, [Fe4S4(SPh)4]3– rapidly binds three protons to give [Fe4S2(SH)2(SPh)3(SHPh)], and it is only in this protonation state that the cluster is capable of transforming the substrates. Kinetic studies indicated that subsequent dissociation of the thiol from [Fe4S2(SH)2(SPh)3(SHPh)] to generate [Fe4S2(SH)2(SPh)3] is also essential for H2 and C2H4 production. It is proposed that the vacant site on one of the Fe atoms allows protonation of this Fe by [Hlut]+ to form [Fe4HS2(SH)2(SPh)3]+. Reduction of this species by another molecule of reduced cluster {probably [Fe4S2(SH)2(SPh)3(SHPh)]} gives the “super-reduced” cluster [Fe4HS2(SH)2(SPh)3] {and [Fe4S2(SH)2(SPh)3(SHPh)]+}. Subsequently the “super-reduced” cluster releases H2 and produces [Fe4S2(SH)2(SPh)3(SHPh)]+. In the presence of C2H2, [Fe4HS2(SH)2(SPh)3]+ binds the alkyne to form [Fe4HS2(SH)2(SPh)3(C2H2)]+. Subsequent reduction (as above) produces the “super-reduced” [Fe4HS2(SH)2(SPh)3(C2H2)], then C2H4. However, binding C2H2 does not completely suppress H2 formation and [Fe4HS2(SH)2(SPh)3(C2H2)] produces H2ca. 30% of the time. The results of earlier studies on the reduction of H+ and C2H2 by structurally analogous Fe–S-based clusters are discussed and shown to be consistent with this mechanism.