Protonation at metal-metal bonds is of fundamental interest in the context of the function of the active sites of hydrogenases and nitrogenases. In diiron dithiolate complexes bearing carbonyl and electron-donating ligands, the metal-metal bond is the highest occupied molecular orbital (HOMO) with a "bent" geometry. Here we show that the experimentally measured rates of protonation (kH) of this bond and the energy of the HOMO as measured by the oxidation potential of the complexes (E1/2(ox)) correlate in a linear free energy relationship: ln kH = ((F(c - βE1/2(ox)))/(RT)), where c is a constant and β is the dimensionless Brønsted coefficient. The value of β of 0.68 is indicative of a strong dependence upon energy of the HOMO: measured rates of protonation vary over 6 orders of magnitude for a change in E1/2(ox) of ca. 0.55 V (ca. 11 orders of magnitude/V). This relationship allows prediction of protonation rates of systems that are either too fast to measure experimentally or that possess additional protonation sites. It is further suggested that the nature of the bridgehead in the dithiolate ligand can exert a stereoelectronic influence: bulky substituents destabilize the HOMO, thereby increasing the rate of protonation.

The formation of transient metal hydride(s) at the metallo-sulfur active sites of [FeFe]-hydrogenase is implicated in both hydrogen evolution and uptake reactions. Stopped-flow spectroscopic techniques can provide insight into the reactivity patterns of model {2Fe2S} sub-sites towards protons, and this information contributes to understanding the nature of the biological systems. In this study we have focussed on the influence of the nature of the bridging dithiolate ligand in influencing the kinetics and activation energy parameters for protonation in synthetic sub-sites including Fe2{micro-[S(CH2)(n)S]}(CO)4(PMe3)2 [n = 2, ethane-1,2-dithiolate (edt) or n = 3, propane-1,3-dithiolate (pdt)], Fe2[(micro-SCH2)2NH](CO)4(PMe3)2 and (NEt4)2{Fe2[(micro-SCH2)2NH](CO)4(CN)2}. Notably we find that (i) the presence of a nitrogen in the dithiolate bridge does not accelerate metal-metal bond protonation, and that (ii) immobilisation of (NEt4)2[Fe2(micro-pdt)(CO)4(CN)2] in a polymer matrix stabilises otherwise short-lifetime protonation products.

The synthesis and crystallographic characterization of a complex possessing a well-defined {2Fe3S(μ-H)} core gives access to a paramagnetic bridging hydride with retention of the core geometry. Chemistry of this 35-electron species within the confines of a thin-layer FTIR spectro-electrochemistry cell provides evidence for a unprecedented super-reduced FeI(μ-H)FeI intermediate.

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