Aluminum Alkyl Complexes Supported by Bidentate N,N Ligands: Synthesis, Structure, and Catalytic Activity for Guanylation of Amines

The reactions of AlMe3 or AlEt3 with 2-pyridyl- or indolyl-substituted imines were studied, leading to the formation of different organoaluminum complexes. While the reactions of the iminopyridine Cy­[NCMe-2-(C5H4N)]2 (L1) derived from 1-(pyridin-2-yl)­ethanone and trans-1,2-cyclohexanediamine with AlEt3 gave the aluminum complex Cy­[NC­(Me)­(Et)-2-(C5H4N)­AlEt2]2 (1), in which the two ketimine groups of the ligand were transformed into the amido functionality through the addition of two ethyl groups, the reaction of L1 with AlMe3 afforded the aluminum complex Cy­[NC­(CH2)-2-(C5H4N)­AlMe2]2 (2) via a sp3 C–H activation with elimination of two methane molecules. The reactions of indolyl-2-aldimines (2-(RNCH)­C8H5NH (R = tBu (L2H), C6H5 (L3H), 2,6-Me2C6H3 (L4H)) with AlMe3 or AlEt3 afforded only the deprotonated indolyl aluminum complexes [2-(RNCH)­C8H5N]­AlMe2 (R = tBu (3), C6H5 (4), 2,6-Me2C6H3 (5)) and [2-(2,6-Me2C6H3NCH)­C8H5N]­AlEt2 (6), respectively. The structures of complexes 2–6 were characterized by spectral methods and X-ray crystallographic analyses. These aluminum complexes showed a high catalytic activity in the addition of amines to carbodiimides to form guanidines. The mechanism of the catalytic process was studied by control experiments and 1H NMR monitoring. Together with the isolation of the complex [2-(2,6-Me2C6H3NCH)­C8H5N]­[CyNC­(4-MeC6H3N)­(NHCy)]­AlMe (7), a probable mechanism for the guanylation reaction was proposed.