INTERACTIONS OF 1,4-DIAZABICYCLO[2.2.2]OCTANE WITH GROUP-III METAL TRIMETHYLS - STRUCTURES OF ME3M.N(C2H4)3N.MME3 (M = AL, GA)

The reactions of (Me3Al)2 and Me3Ga.OEt2 with 1,4-diazabicyclo[2.2.2]octane (dabco) gave the Lewis acid-base complexes Me3M.N(C2H4)3N.MMe3 (M = Al (1), Ga (2)). These complexes are discrete 2:1 adducts of the metal trialkyl to dabco. H-1 NMR spectra revealed an upfield shift of both the dabco methylene protons and the metal alkyl protons on coordination of the Lewis base. Methyl proton resonances move downfield as the metal center is varied from Al to Ga to In (comparisons being made with the previously reported complex trimethylindium 1,4-diazabicyclo[2.2.2]octane).1 Complexes 1 and 2 were also characterized by IR spectroscopy, where it was found that the vibrational frequency for the M-C asymmetric stretching mode is lower for the adduct species than for the parent metal trimethyls. The molecular structures of 1 and 2, which are isostructural, were determined by X-ray crystallographic analysis. Crystal data for 1: trigonal, R3mBAR, a = b = 11.223 (2) angstrom, c = 22.757 (8) angstrom, alpha = beta = 90-degrees, gamma = 120-degrees, Z = 6, R = 0.073. Crystal data for 2: trigonal, R3mBAR, a = b = 11.231 (4) angstrom, c = 22.693 (13) angstrom, alpha = beta = 90-degrees, gamma = 120-degrees, Z = 6, R = 0.043. The molecules have 3m symmetry, and although the Al-C and Ga-C bond lengths were almost equal, the Al-N bond at 2.066 (8) angstrom was significantly shorter than the Ga-N bond at 2.159 (9) angstrom. Notable differences in volatility, solubility, and reactivity between 1 and 2 are discussed.