Diiron chelate complexes relevant to the active site of the iron-only hydrogenase

A novel unsymmetrically disubstituted propanedithiolate compound [Fe-2(CO)(4)(kappa(2)-dmpe)(mu-pdt)] (1) (pdt = SCH2CH2CH2S, dmpe = Me2PCH2CH2PMe2) was synthesized by treatment of [Fe-2(CO)(6)(mu-pdt)] with dmpe in refluxing THE Compound 1 was characterized by single-crystal X-ray diffraction analysis. Protonation of 1 with HBF4 center dot Et2O in CH2Cl2 gave at room temperature the mu-hydrido derivative [Fe-2(CO)(4)(kappa(2)-dmpe)(mu-pdt)(mu-H)](BF4)] (2). At low temperature, H-1 and P-31-{H-1} NMR monitoring revealed the formation of a terminal hydride intermediate 3. Comparison of these results with those of a VT NMR study of the protonation of symmetrical compounds [Fe-2(CO)(4)L-2(mu-pdt)] [L = PMe3, P(OMe)(3)] suggests that in disubstituted bimetallic complexes [Fe-2(CO)(4)L-2(mu-pdt)], dissymmetry of the complex is required to observe terminal hydride species. Attempts to extend the series of chelate compounds [Fe-2(CO)(4)(kappa(2)-L-2)(mu-pdt)] by using arphos (arphos = Ph2AsCH2CH2PPh2) were unsuccessful. Only mono- and disubstituted derivatives [Fe-2(CO)(6-n)(Ph2AsCH2CH2PPh2)(n)(mu-pdt)] (n = 1, 4a; n = 2, 4b), featuring dangling arphos, were isolated under the same reaction conditions of formation of 1. Compound 4b was structurally characterized.