Enhanced Fe-Centered Redox Flexibility in Fe-Ti Heterobimetallic Complexes

Previously, we reported the synthesis of Ti[N(o-(NCH2P(Pr-i)(2))C6H4)(3)] and the Fe-Ti complex, FeTi[N(o-(NCH2P(Pr-i)(2) )C6H4)(3)], abbreviated as TiL (1), and FeTiL (2), respectively. Herein, we describe the synthesis and characterization of the complete redox families of the monometallic Ti and Fe-Ti compounds. Cyclic voltammetry studies on FeTiL reveal both reduction and oxidation processes at -2.16 and -1.36 V (versus Fc/Fc(+)), respectively. Two isostructural redox members, [FeTiL](+) and [FeTiL](-) (2(ox) and 2(red), respectively) were synthesized and characterized, along with BrFeTiL (2-Br) and the monometallic [TiL](+) complex (1(ox)). The solid-state structures of the [FeTiL](+/0/-) series feature short metal-metal bonds, ranging from 1.94-2.38 angstrom, which are all shorter than the sum of the Ti and Fe single-bond metallic radii (cf. 2.49 angstrom). To elucidate the bonding and electronic structures, the complexes were characterized with a host of spectroscopic methods, including NMR, EPR, and Fe-57 Mossbauer, as well as Ti and Fe K-edge X-ray absorption spectroscopy (XAS). These studies, along with hybrid density functional theory (DFT) and time-dependent DFT calculations, suggest that the redox processes in the isostructural [FeTiL](+,0,-) series are primarily pi-based and that the polarized Fe-Ti pi-bonds play a role in delocalizing some of the additional electron density from Fe to Ti (net 13%).