[FeFe]-Hydrogenase Models: Unexpected Variation in Protonation Rate between Dithiolate Bridge Analogues

The model [FeFe]-hydrogenase subsite Fe-2(mu-odt)(CO)(4)(PMe3)(2) (odt = 2-oxapropane-1,3-dithiolate) has been crystallized for the first time, revealing an apical-basal arrangement of the two phosphane groups. Protonation of this species has been studied by a combination of stopped-flow ultraviolet and infrared techniques along with time-resolved NMR spectroscopy. The kinetics of the protonation are similar to those for Fe-2(mu-edt)(CO)(4)(PMe3)(2) (edt = ethane-1,2-dithiolate) and are much slower than those for the protonation of Fe-2(mu-pdt)(CO)(4)(PMe3)(2) (pdt = propane-1,3-dithiolate). The dithiolate bridge length is therefore not the key determinant of reactivity in these simple model systems.