Structure and charge analysis of a cyclic aluminium hydride: cyclo-1,5-bis-mu-dimethylamino-3,7-di-mu-hydrido-2,4,6,8-tetrakis(dimeth ylaluminium)

The title compound, [Al-4(CH3)(8)(C2H7N)(2)H-2], crystallizes as eight-membered rings with -(CH3)(2)Al-(CH3)(2)N-(CH3)(2)Al- moieties connected by single hydride bridges. In the X-ray structure, the ring has a chair conformation, with the hydride H atoms being close to the plane through the four Al atoms. An optimized structure was also calculated by all-electron density functional theory (DFT) methods, which agrees with the X-ray structure but gives a somewhat different geometry for the hydride bridge. Charges on the individual atoms were determined by valence shell occupancy refinements using MoPro and also by DFT calculations analyzed by several different methods. All methods agree in assigning a positive charge to the Al atoms, negative charges to the C, N, and hydride H atoms, and small positive charges to the methyl H atoms.