A UNIFIED MECHANISM FOR THE STOICHIOMETRIC REDUCTION OF H-) IN MECN( AND C2H2 BY [FE4S4(SPH)(4)](3)

The kinetics and mechanisms of the conversions of H+ into H-2 and C2H2 into C2H4 by [Fe4S4(SPh)(4)](3-), using [Hlut](+) (lut = 2,6-dimethyl pyridine) as the proton source, have been investigated in MeCN. At hig h 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 subs trates. Kinetic studies indicated that subsequent dissociation of the thiol from [Fe4S(2)(SH)(2)(SPh)(3)(SHPh)] to generate [Fe4S2(SH)(2)(SP h)(3)] is also essential for H-2 and C2H4 production. It is proposed t hat 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)(SP h)(3)] {and [Fe4S2(SH)(2)(SPh)(3)(SHPh)]}. Subsequently the ''super-re duced'' cluster releases H-2 and produces [Fe4S2(SH)(2)(SPh)(3)(SHPh)] (+). In the presence of C2H2, [Fe4HS2(SH)2(SPh)(3)](+) binds the alkyn e to form [Fe4HS2(SH)(2)(SPh)(3)(C2H2)](+). Subsequent reduction (as a bove) produces the ''super-reduced'' [ Fe4HS2(SH)(2)(SPh)(3)(C2H2)], t hen C2H4. However, binding C2H2 does not completely suppress H-2 forma tion and [Fe4HS2(SH)(2)(SPh)(3)(C2H2)] produces H-2 ca. 30% of the tim e. The results of earlier studies on the reduction of H+ and C2H2 by s tructurally analogous Fe-S-based clusters are discussed and shown to b e consistent with this mechanism.