Oligomeric Rods of Alkyl- and Hydridogallium Imides

Reaction of [RGa(NMe<sub>2</sub>)<sub>2</sub>]<sub>2</sub>, where R = Me, Et, Bu, and Hx, with ammonia at 150 °C in an autoclave produced insoluble white powders formulated as oligomers of [RGaNH]<i><sub>n</sub></i>. The analogous reaction between NH<sub>3</sub> and MeGa[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub> at low temperature (<25 °C) formed an isolable intermediate, [MeGa(μ-NH<sub>2</sub>)N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub>, that was characterized using single-crystal X-ray diffraction. Infrared spectroscopy and X-ray diffraction of the oligomers were consistent with a rodlike structure comprised of six-membered, [RGaNH]<sub>3</sub> rings stacked perpendicular to the long axis of the rod. The method of synthesis, formula, and diffraction results suggested a structural similarity between the alkyl, [RGaNH]<i><sub>n</sub></i>, and the previously reported hydride, [HGaNH]<i><sub>n</sub></i>. The structural and electronic properties of rods having the general formula H<sub>3</sub>[(HXYH)<sub>3</sub>]<i><sub>n</sub></i>H<sub>3</sub> (XY = GaN, GeC; <i>n</i> = 1−9) were investigated using density functional theory. Atomic electronegativity differences between the group 13/15 and 14/14 systems were found to play important roles in the geometrical structures of the two rods and also caused significant differences in the electronic structures. Energetically, it was found to be increasingly favorable to add additional cyclotrigallazane rings to the GaN rods, while for the GeC rods, there was a roughly constant energy cost associated with each additional ring. The electric dipole moments of the GaN rods increased substantially with length; in the GeC rods, charge separation occurred to a much smaller extent and had a polarization opposite to that found in GaN. In addition, increased dipole moments correlated with smaller electronic excitation energies, as predicted by time-dependent density functional theory. All of the powders exhibited luminescence in the visible spectrum at room temperature. Structure observed in the photoluminescence spectra of [HGaNH]<i><sub>n</sub></i> and [MeGaNH]<i><sub>n</sub></i> was interpreted as arising from rods of different length.