An EPR and ENDOR investigation of a series of diazabutadiene-group 13 complexes

Paramagnetic diazabutadienegallium(II or III) complexes, (Ar-DAB)2Ga] and {(Ar-DAB*)GaX}2] (X = Br or I; Ar-DAB = {N(Ar)C(H)}2, Ar = 2,6-diisopropylphenyl), have been prepared by reactions of an anionic gallium N-heterocyclic carbene analogue, K(tmeda)]:Ga(Ar-DAB)], with either "GaI" or MoBr2(CO)2(PPh3)2]. A related InIII complex, (Ar-DAB*)InCl2(thf)], has also been prepared. These compounds were characterised by X-ray crystallography and EPR/ENDOR spectroscopy. The EPR spectra of all metal(III) complexes incorporating the Ar-DAB ligand, (Ar-DAB(.))MX(2)(thf)(n)] (M = Al, Ga or In; X = Cl or I; n = 0 or 1) and (Ar-DAB)2Ga], confirmed that the unpaired spin density is primarily ligand centred, with weak hyperfine couplings to Al (a = 2.85 G), Ga (a = 17-25 G) or In (a = 26.1 G) nuclei. Changing the N substituents of the diazabutadiene ligand to tert-butyl groups in the gallium complex, (tBu-DAB*)GaI2] (tBu-DAB={N(tBu)C(H)}2), changes the unpaired electron spin distribution producing 1H and 14N couplings of 1.4 G and 8.62 G, while the aryl-substituted complex, (Ar-DAB*)GaI2], produces couplings of about 5.0 G. These variations were also manifested in the gallium couplings, namely aGa approximately 1.4 G for (tBu-DAB*)GaI2] and aGa approximately 25 G for (Ar-DAB*)GaI2]. The EPR spectra of the gallium(II) and indium(II) diradical complexes, {(Ar-DAB*)GaBr}2], {(Ar-DAB*)GaI}2], {(tBu-DAB*)GaI}2] and {(Ar-DAB*)InCl}2], revealed doublet ground states, indicating that the Ga-Ga and In-In bonds prevent dipole-dipole coupling of the two unpaired electrons. The EPR spectrum of the previously reported complex, (Ar-BIAN*)GaI2] (Ar-BIAN = bis(2,6-diisopropylphenylimino)acenaphthene) is also described. The hyperfine tensors for the imine protons, and the aryl and tert-butyl protons were obtained by ENDOR spectroscopy. In (Ar-DAB*)GaI2], gallium hyperfine and quadrupolar couplings were detected for the first time.