SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF DICYCLOPENTADIENYLALUMINUM ALKYL AND TRICYCLOPENTADIENYLALUMINUM COMPOUNDS - CRYSTAL-STRUCTURE OF A BIS(ETA-2-CYCLOPENTADIENYL)ALUMINUM ALKYL COMPOUND

The reaction of magnesocene with an alkylaluminum dichloride compound offers a convenient route to the corresponding dicyclopentadienylaluminum alkyl species. This method was used to prepare (C5H5)2AlCH3 (1) and (C5H5)2AlCH2CH3 (2). The X-ray crystal structure of 1 was determined. The compound crystallizes in the orthorhombic space group F2(1)2(1)2, with a = 6.899(2) angstrom, b = 9.152(2) angstrom, c = 16.224(3) angstrom, Z = 4, V = 1024.4(4) angstrom3, and d(calc) = 1.117 g cm-3. Full-matrix least-squares refinement converged with R = 0.0559 for 768 reflections having Absolute value of F > 3sigma. Surprisingly, 1 is monomeric in the solid state, with both cyclopentadienyl rings coordinated in an eta2 fashion to the aluminum. Treatment of 1 with ethyl ether affords the ethyl ether complex Cp2AlMe.Et2O (3) as a white, microcrystalline solid. The H-1 NMR spectrum of 3 reveals small amounts of the ligand redistribution species CpAlMe2 and Cp3Al in the sample. Minor amounts of these species are apparent whenever 1 is in the presence of a Lewis base. When 1 or 3 is combined with tert-butyl isocyanide in petroleum ether, the ligand redistribution product (C5H5)3Al(C=NC(CH3)3) 4 precipitates selectively from solution. Tricyclopentadienylaluminum complexes such as 4 and the trimethylacetonitrile adduct (C5H5)3Al(N=CC(CH3)3) (5) can be prepared more directly in higher yields from AlCl3 and magnesocene. Single crystals of 4 were obtained from a dichloromethane solution cooled to -60-degrees-C, and the X-ray crystal structure was determined. Complex 4 crystallizes in the rhombohedral space group R3 with a = 10.162(2) angstrom, c = 30.529(6) angstrom, Z = 3, V = 2730.2(9) angstrom3, and d(calc) = 1.114 g cm-3. Full-matrix least-squares refinement converged with R = 0.0497 for 586 reflections having Absolute value of F > 4sigma. The com lex is a monomer with C3 symmetry and the cyclopentadienyl rings coordinated in an eta1 fashion to the aluminum.