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 -SCH(2)NRCH(2)S- dithiolate unit that bridges two Fe(I)(CO)(2)L moieties. The sulfanilic acid group imparts even greater water solubility in the presence of ß-cyclodextrin, ß-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 ß-CyD clathrate, where 1Na = Na(+)(µ-SCH(2)N(C(6)H(4)SO(3)(-))CH(2)S-)[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 ß-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 (µ-SRS)[Fe(CO)(3)](2) parent biomimetic, that are stable in O(2)-free aqueous solutions.