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

Complexes with cations of the type
[Ga­(L)2]n+ where L = bis­(4-methyl-2-(1H-pyrazol-1-yl)­phenyl)­amido
and n = 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, Hab of about 200 cm–1, and a large rate constant for
electron transfer, ket, on the order of
1010 s–1 between redox active ligands.
The dioxidized complex [Ga­(L0)2]3+ shows a half-field ΔMs = 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.