Electronic and geometric effects of phosphatriazaadamantane ligands on the catalytic activity of an [FeFe] hydrogenase inspired complex.

The [FeFe] hydrogenase enzyme active site inspired complexes [Fe(2)(mu-C(6)H(4)S(2))(CO)(5)PTA] (1PTA) and [Fe(2)(mu-C(6)H(4)S(2))(CO)(4)PTA(2)] (1PTA(2)) (PTA = 1,3,5-triaza-7-phosphaadamantane) were synthesized and characterized. The ability of 1PTA and 1PTA(2) to catalytically produce molecular hydrogen in solution from the weak acid acetic acid was examined electrochemically and compared to previous studies on the all carbonyl containing analogue [Fe(2)(mu-C(6)H(4)S(2))(CO)(6)] (1). Computational methods and cyclic voltammograms indicated that the substitution of CO ligands by PTA in 1 resulted in markedly different reduction chemistry. Both 1PTA and 1PTA(2) catalytically produce molecular hydrogen from acetic acid, however, the mechanism by which and 1PTA and 1PTA(2) catalyze hydrogen differ in the initial reductive processes.