January issue of Acta Crystallographica Section C: Structural Chemistry
The cover of the January issue of Acta Crystallographica Section C: Structural Chemistry features work by Peter Corfield and Joshua Schrier (Fordham University, New York) on the structure and charge analysis of a cyclic aluminium hydride.
In E. P. Schram's early studies on the organometallic chemistry of aluminium, his group analyzed products of the reaction between dimethylaminoboranes and methyl aluminium hydrides. In further work, the reaction of dimethylaminoborane with trimethylaluminium led to the isolation of a solid crystalline material. Analysis of single-crystal X-ray data collected in 1970-71 characterized the molecule that had been formed as cyclo-1,5-bis-mu-dimethylamino-3,7-di-mu-hydrido-2,4,6,8-tetrakis(dimethylaluminium). The molecule consists of an eight-membered ring containing singly-bridged hydride atoms, one of the first examples of such bridging at that time. Circumstances prevented completion of the refinement, although the molecular structure without atomic parameters was described in a paper on the chemical reaction. We now present details of this X-ray study based upon refinement of the 1971 data, together with an atomic charge-density analysis, and we compare the structure and charges with those found from a theoretical study.
The title cyclic aluminium hydride crystallizes as eight-membered rings with -Me2Al-Me2N-Me2Al- 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.
Despite the variations between the various theoretical methods, the data indicate qualitative agreement of partial charges obtained from the MoPro refinements and by the DFT calculations. Both the experimental and all theoretical assignments suggest substantial positive charges on the Al atoms, negative charges of about half an electron on the N and bridging H atoms, and negative charges on the C(Al) atoms that are much more negative than on the C(N) atoms. There are small positive charges on the methyl H atoms. These results are consistent with the Al-N and Al-C electronegativity differences of 1.0 and 1.5, which would indicate a polar Al-N and a more polar Al-C bond. The negative charge found for the bridging H atom is consistent with its characterization as a hydride. The MoPro refinement distinguishes between charges on the methyl H atoms on the Al and N atoms, whereas the charges estimated from the DFT calculations do not distinguish between these H atoms, although the total charges assigned to Al-bound and N-bound methyl groups are different in each of the DFT calculations. A distinction between the Al-bound and the N-bound methyl groups might be expected on chemical grounds. The limited Cu K-alpha resolution of the data used in this study forces the MoPro results to be limited to the spherical independent atom model, and the charges are not as well defined as we would wish; use of data collected with a shorter wavelength would have allowed a more sophisticated model by the MoPro program.