Synthesis and characterization of aluminum complexes of 2-pyrazol-1-yl-ethenolate ligands

The synthesis and the characterization of some new aluminum complexes with bidentate 2-pyrazol-1-yl-ethenolate ligands are described. 2-(3,5-Disubstituted pyrazol-1-yl)-1-phenylethanones, 1-PhC(O)CH2-3,5-R2C3HN2 (1a, R = Me; 1b, R = But), were prepared by solventless reaction of 3,5-dimethyl pyrazole or 3,5-di-tert-butyl pyrazole with PhC(O)CH2Br. Reaction of 1a or 1b with AlR31 (R1 = Me, Et) yielded N,O  -chelate alkylaluminum complexes [R21AlOC(Ph)CH{(3,5-R2C3HN2)-1}] (2a, R = R1 = Me; 2b, R = But, R1 = Me; 2c, R = Me, R1 = Et). Compound 1a was readily lithiated with LiBun in thf or toluene to give lithiated species 3. Treatment of 3 with 0.5 equiv of MeAlCl2 or AlCl3 yielded five-coordinated aluminum complexes [XAl(OC(Ph)CH{(3,5-Me2C3HN2)-1})2] (4, X = Me; 5, X = Cl). Reaction of 5 with an equiv of LiHBEt3 generated [Al(OC(Ph)CH{(3,5-Me2C3HN2)-1})3] (6). Complex 6 was also obtained by reaction of 3 with 1/3 equiv of AlCl3. Treatment of 5 with 2 equiv of AlMe3 yielded complex 2a, whereas with an equiv of AlMe3 afforded a mixture of 2a and [Me(Cl)AlOC(Ph)CH{(3,5-Me2C3HN2)-1}] (7). Compounds 1a, 1b, 2a–2c and 4–6 were characterized by elemental analyses, NMR and IR (for 1a and 1b) spectroscopy. The structures of complexes 2a and 5 were determined by single crystal X-ray diffraction techniques. Both 2a and 5 are monomeric in the solid state. The coordination geometries of the aluminum atoms are a distorted tetrahedron for 2a or a distorted trigonal bipyramid for 5.

Graphical abstractReaction of 2-(3,5-disubstituted pyrazol-1-yl)-1-phenylethanones with AlR3 (R = Me, Et) affords four-coordinated N,O-chelate aluminum complexes. Lithiated 2-(3,5-dimethylpyrazol-1-yl)-1-phenylethanone reacts with MeAlCl2 or AlCl3 to give five- or six-coordinated N,O-chelate aluminum complexes.Figure optionsDownload full-size imageDownload as PowerPoint slide