Comparative studies of electron transfer in orthoquinone derivatives of gallium, indium and thallium

The interactions of gallium, indium and thallium with 3,5-di-tert-butyl-1,2-benzoquinone (TBQ) and its reduced derivatives TBSQradical dot− (=3,5-di-tert-butyl-1,2-benzosemiquinonate anion) and TBCAT2− (=3,5-di-tert-butyl-catecholate) are discussed in terms of inter- and intramolecular electron transfer processes. The gallium(III) complex Ga()(TBCAT)(Bupy)2 (Bupy=3-n-butylpyridine), which is a rare example of a metal ion coordinated by both semiquinonate and catecholate ligands, has been characterized crystallographically; monoclinic, space group P21, a=10.3718(1), b=16.0047(1), c=14.8006(1) Å, β=110.151(1)°, Z=2, Rw=0.069 for 5784 unique reflections. Electron spin resonance (ESR) spectroscopy shows that this substance undergoes intermolecular electron transfer in toluene. Gallium also forms Ga()I2, which is thermally unstable, releasing I2; the solid and solution properties of this, and related compounds, have been studied by ESR spectroscopy. In all the systems investigated, gallium is in the Ga(III) state. Attempts to prepare derivatives of In or Tl in this oxidation state were unsuccessful, and all products have the metal as M(I). For gallium, electron transfer is only detected as an inter-ligand process, while for indium and thallium, ligand↔metal transfer predominates.