Synthesis and Structure of Mono- and Bis(amidinate) Complexes of Aluminum

The synthesis and structures of mono- and bis(amidinate) aluminum complexes are described. The reaction of AlMe3 and 1 equiv of carbodiimide, R'N=C=NR', affords {MeC(NR')2}AlMe2 (1a, R' = iPr; 1b, R' = Cy = cyclohexyl). The reaction of R'N=C=NR' with MeLi or tBuLi generates Li[RC(NR')2] (2a, R = Me, R' = iPr; 3a, R = tBu, R' = iPr; 3b, R = tBu, R' = Cy; 3c, R = tBu, R' = SiMe3). 2a, 3a, and 3b may be isolated or reacted in situ, while attempted isolation of 3c gave [Li(tBuCN){-N(SiMe3)2}]2 (3d). The reaction of 1 equiv of AlCl3 with 2a or 3a-c affords {RC(NR')2}AlCl2 (4a, R = Me; 5a-c, R = tBu), and the reaction of AlMe2Cl with 3a-c affords {tBuC(NR')2}AlMe2 (6a-c). Alkylation of 5a,b with 2 equiv of PhCH2MgCl or Me3CCH2Li yields {tBuC(NR')2}Al(CH2Ph)2 (7a,b) or {tBuC(NR')2}Al(CH2CMe3)2 (8a,b). The reaction of 0.5 equiv of AlCl3 with 2a or 3a,b yields {RC(NR')2}2AlCl (9a, R = Me; 10a,b, R = tBu). Complexes 1b, 3d, 4a, 5a,b, 6b, 8b, 9a, and 10a,b have been characterized by X-ray crystallography. The crystallographic results establish that steric interactions between the R and R' groups influence the R'-N-Al angle and, hence, the steric environment at aluminum.