Crystal Engineering Using the Unconventional Hydrogen Bond. Synthesis, Structure, and Theoretical Investigation of Cyclotrigallazane

Cyclotrigallazane, [H 2 GaNH 2 ] 3, was prepared by condensing liquid ammonia onto solid trimethylamine gallane, GaH 3 (NMe 3 ), 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.