Charge and electron transfer from metal-to-carbon bonds of main group organometallics MRn (M = Al, Ga, Zn) to aromatic N-heterocycles: colored precursor compounds and radical complex formation

Abstract The coordinatively unsaturated compounds AlR 3 , GaR 3 and ZnR 2 , R = isopropyl, were reacted with linearly bridging pyrazine, tetramethylpyrazine or 4,4′-bipyridine ligands and with potentially chelating 2,2′-bipyridine. The primary reaction products are diamagnetic dinuclear complexes (‘bis-adducts’) which are characterized by intense long-wavelength ligand-to-ligand charge transfer (LLCT) transitions σ(M-R)» π * (L) and which may yield radical complexes after loss of R · . The latter reaction is assumed to have a strong single electron transfer (SET) component. An isolable ‘AI(II)’ species (bpy)Al[CH(SiMe 3 ) 2 ] 2 has been characterized unambiguously by ESR, UV-Vis and cyclic voltammetry as an Al(III) complex of the bpy radical anion. Irradiation into the fairly intense LLCT absorption bands (ϵ > 3000 M −1 cm −1 ) of the dinuclear complexes between pyrazine and Al i Pr 3 or Ga i Pr 3 resulted in bleaching and formation of a mixture of non-aromatic products. Attempts are made to correlate charge transfer properties and radical formation with the geometrical and electronic structures of the precursor complexes.