Tuning Reactivity of Bioinspired [NiFe]-Hydrogenase Models by Ligand Design and Modeling the CO Inhibition Process

Despite the report
of several structural and functional models
of the [NiFe]-hydrogenases, it is still unclear how the succession
of electron and proton transfers during H2 production catalysis
are controlled in terms of both sequence (order of the chemical or
redox steps) and sites (metal and/or ligand). To address this issue,
the structure of the previously described bioinspired [NiFe]-hydrogenase
complex [LN2S2NiIIFeIICp­(CO)]+ (LNiIIFeIICp, with LN2S2 = 2,2′-(2,2′-bipyridine-6,6′-diyl)­bis­(1,1′-diphenylethanethiolate)
and Cp = cyclopentadienyl) has been fine-tuned by modifying exclusively
the Fe site. In [LN2S2NiIIFeIICp*­(CO)]+ (LNiIIFeIICp*, with Cp*
= pentamethylcyclopentadienyl), the Cp– ligand has
been replaced by Cp*– to change both the redox and
structural properties of the overall complex as a consequence of the
steric hindrance of Cp*–. The LNiIIFeIICp* complex acts as an efficient electrocatalyst
to produce H2. Density functional theory (DFT) calculations
support a CEEC cycle, following an initial reduction. The initial
protonation leads to the cleavage of one thiolate–iron bond
and the next reduction to the generation of a bridging Fe-based hydride
moiety. Interestingly, the second protonation step generates a species
containing a terminal Ni-based thiol and a bridging hydride. In the
presence of CO, the electrocatalytic activity of LNiIIFeIICp* for H2 production is markedly
inhibited (about 90% of loss), while only a partial inhibition (about
30% of loss) is observed in the case of LNiIIFeIICp. DFT calculations rationalized this effect by predicting
that interactions of the one- and two-electron-reduced species for LNiIIFeIICp* with CO are thermodynamically
more favorable in comparison to those for LNiIIFeIICp.