Sulfonated Diiron Complexes as Water-Soluble Models of the [Fe-Fe]-Hydrogenase Enzyme Active Site

A series of diiron complexes developed as fundamental models of the two-iron subsite in the [FeFe]-hydrogenase enzyme active site show water-solubility by virtue of a sulfonate group incorporated into the -SCH2NRCH2S- dithiolate unit that bridges two Fe-I(CO)(2)L moieties. The sulfanilic acid group imparts even greater water solubility in the presence of beta-cyclodextrin, beta-CyD, for which NMR studies suggest aryl-sulfonate inclusion into the cyclodextrin cavity as earlier demonstrated in the X-ray crystal structure of 1Na.2 beta-CyD clathrate, where 1Na = Na+(mu-SCH2N(C6H4SO3-)CH2S-) [Fe-(CO)(3)](2), (Singleton et al., J. Am. Chem. Soc. 2010, 132, 8870). Electrochemical analysis of the complexes for potential as electrocatalysts for proton reduction to H-2 finds the presence of beta-CyD to diminish response, possibly reflecting inhibition of structural rearrangements required of the diiron unit for a facile catalytic cycle. Advantages of the aryl sulfonate approach include entry into a variety of water-soluble derivatives from the well-known (mu-SRS)[Fe(CO)(3)](2) parent biomimetic, that are stable in O-2-free aqueous solutions.