Aluminum Alkyl Complexes Containing Guanidinate Ligands

The synthesis and structures of new aluminum complexes incorporating guanidinate ligands (R2NC(NR‘)2-) are described. The reaction of iPrNCNiPr with LiNR2 reagents yields Li[R2NC(NiPr)2] guanidinate salts, which are reacted in situ with AlCl3 or AlMe2Cl to afford {R2NC(NiPr)2}AlCl2 (1a, R = Me; 1b, R = Et; 1c, R = iPr; 1d, R = SiMe3) or {R2NC(NiPr)2}AlMe2 (2a, R = Me; 2b, R = Et; 2c, R = iPr), respectively. The reaction of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) with AlMe3 generates {(μ-hpp)AlMe2}2 (3). Complexes 1a, 1d, and 3 have been characterized by X-ray crystallography. 1a and 1d adopt monomeric structures with symmetric chelated bidentate guanidinate ligands. Delocalization of the −NR2 lone pair into the chelate ring is important for 1a but not for 1d, due to N−Si π-bonding and steric crowding. The bicyclic structure of the hpp- ligand enforces a dimeric μ-hpp- structure for 3.