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 monoterphenyl 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-Pr3C6H2-), 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 hydrolyzed 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 partial 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 spectroscopy. The major conclusions from the experimental data are that a single ortho terphenyl substituent of the kind reported here are not as effective as the ligand (Mes* (Mes* = 2,4,6-t-Bu(3)C(6)H(2)-) in preventing further coordination and/ or aggregation involving the aluminum centers. in effect, one terphenyl ligand is not as successful as a Mes* substituent in masking the metal through agostic and/or steric effects.