Isomerization of the hydride complexes [HFe2(SR)2(PR3)x(CO)6-x]+ (x = 2, 3, 4) relevant to the active site models for the [FeFe]-hydrogenases

The stepwise formation of bridging (μ-) hydrides of diiron dithiolates is discussed with attention on the pathway for protonation and subsequent isomerizations. Our evidence is consistent with protonations occurring at a single Fe center, followed by isomerization to a series of μ-hydrides. Protonation of Fe2(edt)(CO)4(dppv) (1) gave a singleμ-hydride with dppv spanning apical and basal sites, which isomerized at higher temperatures to place the dppv into a dibasal position. Protonation of Fe2(pdt)(CO)4(dppv) (2) followed an isomerization pathway similar to that for [1H]+, except that a pair of isomeric terminal hydrides were observed initially, resulting from protonation at the Fe(CO) 3 or Fe(CO)(dppv) site. The first observable product from low temperature protonation of the tris-phosphine Fe2(edt)(CO) 3(PMe3)(dppv) (3) was a single μ-hydride wherein PMe3 is apical and the dppv ligand spans apical and basal sites. Upon warming, this isomer converted fully but in a stepwise manner to a mixture of three other isomeric hydrides. Protonation of Fe2(pdt)(CO) 3(PMe3)(dppv) (4) proceeded similarly to the edt analogue 3, however a terminal hydride was observed, albeit only briefly and at very low temperatures (-90 °C). Low-temperature protonation of the bis-chelates Fe2(xdt)(CO)2(dppv)2 produced exclusively the terminal hydrides [HFe2(xdt)(μ-CO)(CO)(dppv)2] + (xdt = edt and pdt), which subsequently isomerized to a pair of μ-hydrides. At room temperature these (dppv)2 derivatives convert to an equilibrium of two isomers, one C2-symmetric and the other Cs-symmetric. The stability of the terminal hydrides correlates with the (C2-isomer)/(Cs-isomer) equilibrium ratio, which reflects the size of the dithiolate. The isomerization was found to be unaffected by the presence of excess acid, by solvent polarity, and the presence of D2O. This isomerization mechanism is proposed to be intramolecular, involving a 120° rotation of the HFeL3 subunit to an unobserved terminal basal hydride as the rate-determining step. The observed stability of the hydrides was supported by DFT calculations, which also highlight the instability of the basal terminal hydrides. Isomerization of the μ-hydride isomers occurs on alternating FeL3via 120° rotations without generating D2O-exchangeable intermediates. © 2010 The Royal Society of Chemistry.