Crystal engineering using the unconventional hydrogen bond. Synthesis, structure, and theoretical investigation of cyclotrigallazane

Cyclotrigallazane, [H₂GaNH₂]₃, was prepared by condensing liquid ammonia onto solid trimethylamine gallane, GaH₃(NMe₃), at -78°C and allowing the mixture to warm to room temperature and was characterized by IR, mass spectroscopy, elemental analysis, single-crystal X-ray, and neutron powder diffraction. Single-crystal X-ray diffraction at T = -167°C established that the (GaN)₃ ring was in the chair conformation. Neutron powder diffraction data collected at 25°C on the fully deuterated analogue were analyzed with Rietveld refinement to give an average bond distance for Ga-D of 1.56(₃) Å and a N-D of 1.04(5) Å. The intermolecular interactions were dominated by four Ga-H···H-N unconventional hydrogen bonds per molecule that form a chain parallel to the crystallographic a axis. The crystallographically equivalent D D bond lengths are 1.97 Å. Calculations revealed that in the gas phase, twist-boat conformations are preferred over chairs for cyclotrigallazane and the related boron and aluminum compounds by 0.9 to 2.6 kcal/mol at correlated levels of electronic structure theory. For cyclotriborazane and cyclotrigallazane, calculations suggest that each H···H hydrogen bond contributes about 3 kcal/mol to the binding energy (relative to the chair monomer); this value is very slightly higher for cyclotrialumazane.