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.

Abstract

The stepwise formation of bridging (mu-) 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 mu-hydrides. Protonation of Fe(2)(edt)(CO)(4)(dppv) (1) gave a single mu-hydride with dppv spanning apical and basal sites, which isomerized at higher temperatures to place the dppv into a dibasal position. Protonation of Fe(2)(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 Fe(2)(edt)(CO)(3)(PMe(3))(dppv) (3) was a single mu-hydride wherein PMe(3) 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 Fe(2)(pdt)(CO)(3)(PMe(3))(dppv) (4) proceeded similarly to the edt analogue 3, however a terminal hydride was observed, albeit only briefly and at very low temperatures (-90 degrees C). Low-temperature protonation of the bis-chelates Fe(2)(xdt)(CO)(2)(dppv)(2) produced exclusively the terminal hydrides [HFe(2)(xdt)(mu-CO)(CO)(dppv)(2)](+) (xdt = edt and pdt), which subsequently isomerized to a pair of mu-hydrides. At room temperature these (dppv)(2) derivatives convert to an equilibrium of two isomers, one C(2)-symmetric and the other C(s)-symmetric. The stability of the terminal hydrides correlates with the (C(2)-isomer)/(C(s)-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 D(2)O. This isomerization mechanism is proposed to be intramolecular, involving a 120 degrees rotation of the HFeL(3) 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 mu-hydride isomers occurs on alternating FeL(3) via 120 degree rotations without generating D(2)O-exchangeable intermediates.