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.