Electronic Communication Across Diamagnetic Metal Bridges: A Homoleptic Gallium(III) Complex of a Redox-Active Diarylamido-Based Ligand and Its Oxidized Derivatives

Abstract

Complexes with cations of the type [Ga­(L)₂]^<i>n</i>+ where L = bis­(4-methyl-2-(1H-pyrazol-1-yl)­phenyl)­amido and <i>n</i> = 1, 2, 3 have been prepared and structurally characterized. The electronic properties of each were probed by electrochemical and spectroscopic means and were interpreted with the aid of density functional theory (DFT) calculations. The dication, best described as [Ga­(L^–)­(L⁰)]²⁺, is a Robin-Day class II mixed-valence species. As such, a broad, weak, solvent-dependent intervalence charge transfer (IVCT) band was found in the NIR spectrum in the range 6390–6925 cm^–1, depending on the solvent. Band shape analyses and the use of Hush and Marcus relations revealed a modest electronic coupling, <i>H</i>_ab of about 200 cm^–1, and a large rate constant for electron transfer, <i>k</i>_et, on the order of 10¹⁰ s^–1 between redox active ligands. The dioxidized complex [Ga­(L⁰)₂]³⁺ shows a half-field Δ<i>M</i>_s = 2 transition in its solid-state X-band electron paramagnetic resonance (EPR) spectrum at 5 K, which indicates that the triplet state is thermally populated. DFT calculations (M06/Def2-SV­(P)) suggest that the singlet state is 21.7 cm^–1 lower in energy than the triplet state