Iron(II) and Ruthenium(II) Complexes Containing P, N, and H Ligands: Structure, Spectroscopy, Electrochemistry, and Reactivity

The purpose of this work was to explore the possibility of using iron(II) hydrides in CO2 reduction and to compare their reactivity to that of their ruthenium analogues. Fe(bpy)(P(OEt)(3))(3)H+ and Ru(bpy)(P(OEt)(3))(3)H+ do not react with CO2 in acetonitrile, but the one-electron-reduction products of Ru(bpy)(P(OEt3)(3)H+ and Ru(bpy)(2)(P(OEt3)H+ and the two-electron-reduction product of Fe(bPY)(P(OEt3)(3)H+ do. Ru(bpy)(2)(P(OEt)(3))H+ also reacts slowly with CO2 to give a formate complex [as reported previously by Albertin et al. (Inorg. Chem. 2004, 43, 1336)] with a second-order rate constant of similar to 4 x 10(-3) M-1 s(-1) in methanol. The structures for the hydride complexes [Fe(bpy)(P(OEt3)(3)H](+) and [Ru(bpy)(2)(P(OEt)(3))H](+) and for the (eta(5)-Cp)bis- and -tris-PTA complexes (PTA = 1,3,5-triaza-7-phosphatricyclo-[3.3.1.1.3.7]decane) of iron(II) are reported. These and the CpFe(CO)(bpy)(+) and (FePNNP)-P-II compounds have been subjected to electrochemical and UV-vis spectroscopic characterization Fe(bpy)(P(OEt3)(3)H+ exhibits a quasi-reversible oxidation at +0.42 V vs AgCl/Ag in acetonitrile, Ru(bpy)(P(OEt3)(3)H+ and Ru(bpy)(2)(P(OEt3)H+ are oxidized irreversibly at +0.90 and +0.55 V, respectively, vs AgCl/Ag. The reduction site for Fe(bpy)(P(OEt)(3))(3)H+ and Fe(bpy)(P(OEt3)(3)(CH3CN)(2+) appears to be the metal and gives rise to a two-electron process. The bpy-centered reductions are negatively shifted in the ruthenium(II) hydride complexes, compared to the acetonitrile complexes. The results of attempts to prepare other iron(II) hydrides are summarized.