SYNTHESIS AND CHARACTERIZATION OF STERICALLY ENCUMBERED DERIVATIVES OF ALUMINUM HYDRIDES AND HALIDES - ASSESSMENT OF STERIC PROPERTIES OF BULKY TERPHENYL LIGANDS

The synthesis and characterization of several sterically encumbered mo noterphenyl derivatives of aluminum halides and aluminum hydrides are described. These compounds are [2,6-Mes(2)C(6)H(3)AlH(3)LiOEt(2)](n) ( 1), (Mes = 2,4,6-Me(3)C(6)H(2)-), 2,6-Me(2)C(6)H(3)AlH(2)OEt(2) (2), [ 2,6-Mes(2)C(6)H(3)AlH(2)](2) (3), 2,6-Mes(2)C(6)H(3)-AlCl(3)LiOEt(2)]( n) (4), [2,6-Mes(2)C(6)H(3)-AlCl(3)LiOEt(2)](n)], (5), [2,6-Mes(2)C(6) H(3)AlCl(2)](2) (6), TriphAlBr(2)OEt(2) (7), (Triph = 2,4,6-Ph(3)C(6)H (2)-), [2,6-Trip(2)C(6)H(3)-AlH(3)LiOEt(2)](2) (8) (Trip = 2,4,6-i-Pr3 C6H2-), 2,6-Trip(2)C(6)H(3)AlH(2)OEt(2) (9), [2,6-Trip(2)C(6)H(3)AlH(2 )](2) (10), 2,6-Trip(2)C(6)H(3)AlCl(2)OEt(2) (11), and the partially h ydrolyzed derivative 2,6-Trip(2)C(6)H(3)Al(Cl)(0.68)(H)(0.32)(mu-OH)(2 ) . 2C(6)H(6) (12). The structures of 2, 3a, 4, 6, 7, 9a, 10a, 10b, 11 , and 12 were determined by X-ny crystallography. The structures of 3a , 9a, 10a, and 10b, are related to 3, 9, and 10, respectively, by part ial occupation of chloride or hydride by hydroxide. The compounds were also characterized by H-1, C-13, Li-7, and Al-27 NMR and IR spectrosc opy. The major conclusions from the experimental data are that a singl e ortho terphenyl substituent of the kind reported here are not as eff ective as the ligand (Mes (Mes* = 2,4,6-t-Bu(3)C(6)H(2)-) in preventi ng further coordination and/ or aggregation involving the aluminum cen ters. in effect, one terphenyl ligand is not as successful as a Mes s ubstituent in masking the metal through agostic and/or steric effects.