Conformational mobility and pendent base effects on electrochemistry of synthetic analogues of the [FeFe]-hydrogenase active site

Dynamic NMR (¹³C and ¹H) studies of (μ-SCH₂XCH₂S)[Fe-(CO)₃]₂ complexes (X = CR₂, NR) were utilized to examine the fluxional processes that are important in the [FeFe]-hydrogenase active site models, where an open site for proton/hydrogen binding, achieved by configurational mobility of the Fe(CO)₃ unit, is required for electrocatalysis of proton reduction. In order to interrogate the effects of fluxional mobility on electrochemical response to added acid, energy barriers for the CO site exchange in Fe(CO)₃ rotors were determined for nitrogen-and carbon-based bridgehead complexes. The effect of the methyl substituent in both the NH/NCH₃ and CH₂/C(CH₃)₂ cases is to lower the Fe(CO)₃ rotational activation barrier relative to the NH or CH₂ analogues. Although the C(CH₃)₂ case has the lowest Fe(CO)₃ rotational barrier, its performance as a proton reduction electrocatalyst is 2-fold less than that for the X = NR species, indicating the proton-directing effect of the pendent base on catalytic efficiency.