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)₃H⁺ and Ru(bpy)(P(OEt)₃)₃H⁺ do not react with CO₂ in acetonitrile, but the one-electron-reduction products of Ru(bpy)(P(OEt)₃)₃H⁺ and Ru(bpy)₂(P(OEt)₃)H⁺ and the two-electron-reduction product of Fe(bpy)(P(OEt)₃)₃H⁺ 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 ∼4 × 10⁻³ M⁻¹ s⁻¹ in methanol. The structures for the hydride complexes [Fe(bpy)(P(OEt)₃)₃H]⁺ and [Ru(bpy)₂(P(OEt)₃)H]⁺ and for the (η⁵-Cp)bis- and -tris-PTA complexes (PTA = 1,3,5-triaza-7-phosphatricyclo[3.3.1.13.7]decane) of iron(II) are reported. These and the CpFe(CO)(bpy)⁺ and Fe^IIPNNP compounds have been subjected to electrochemical and UV−vis spectroscopic characterization. Fe(bpy)(P(OEt)₃)₃H⁺ exhibits a quasi-reversible oxidation at +0.42 V vs AgCl/Ag in acetonitrile; Ru(bpy)(P(OEt)₃)₃H⁺ and Ru(bpy)₂(P(OEt)₃)H⁺ are oxidized irreversibly at +0.90 and +0.55 V, respectively, vs AgCl/Ag. The reduction site for Fe(bpy)(P(OEt)₃)₃H⁺ and Fe(bpy)(P(OEt)₃)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.