Synthesis, Characterization, and Hydrolysis Products of (η 2 ‐ t Bu 2 pz)AlH(μ:η 1 ,η 1 ‐ t Bu 2 pz) 2 AlH 2 − Structural Characterization of a Complex Containing η 1 ‐, η 2 ‐, and μ:η 1 ,η 1 ‐Pyrazolato Ligands and a Complex Containing a Terminal Hydroxo Ligand

Treatment of alane−ethyldimethylamine with 3,5‐di‐ tert ‐butylpyrazole ( t Bu 2 pzH) in a 2:3 molar ratio afforded [(η 2 ‐ t Bu 2 pz)AlH(μ:η 1 ,η 1 ‐ t Bu 2 pz) 2 AlH 2 ] in 57% yield. Hydrolysis of [(η 2 ‐ t Bu 2 pz)AlH(μ:η 1 ,η 1 ‐ t Bu 2 pz) 2 AlH 2 ] afforded variable mixtures of Al( t Bu 2 pz) 3 , [(η 2 ‐ t Bu 2 pz)AlH(μ:η 1 ,η 1 ‐ t Bu 2 pz) 2 AlH(OH)], and [(η 2 ‐ t Bu 2 pz)AlH(μ:η 1 ,η 1 ‐ t Bu 2 pz) 2 AlH(η 1 ‐ t Bu 2 pz)]. The structures of all new complexes were assigned from spectral and analytical data. In addition, the X‐ray crystal structures of [(η 2 ‐ t Bu 2 pz)AlH(μ:η 1 ,η 1 ‐ t Bu 2 pz) 2 AlH(OH)] and [(η 2 ‐ t Bu 2 pz)AlH(μ:η 1 ,η 1 ‐ t Bu 2 pz) 2 AlH(η 1 ‐ t Bu 2 pz)] were determined. [(η 2 ‐ t Bu 2 pz)AlH(μ:η 1 ,η 1 ‐ t Bu 2 pz) 2 AlH(OH)] crystallizes as a dimeric molecule, and contains two bridging pyrazolato ligands, one η 2 ‐pyrazolato ligand, as well as terminal hydrido and hydroxo ligands. The hydroxo and η 2 ‐pyrazolato ligands possess a syn‐relationship within the dimer. The hydroxy group proton does not participate in dihydrogen bonding, and instead appears to be intramolecularly hydrogen‐bonded to the π‐cloud of the η 2 ‐pyrazolato ligand. The overall structure of [(η 2 ‐ t Bu 2 pz)AlH(μ:η 1 ,η 1 ‐ t Bu 2 pz) 2 AlH(η 1 ‐ t Bu 2 pz)] is very similar to that of [(η 2 ‐ t Bu 2 pz)AlH(μ:η 1 ,η 1 ‐ t Bu 2 pz) 2 AlH(OH)], except that the η 1 ‐ and η 2 ‐pyrazolato ligands have an anti‐disposition within the dimer. Molecular orbital calculations were carried out on [(η 2 ‐ t Bu 2 pz)AlH(μ:η 1 ,η 1 ‐ t Bu 2 pz) 2 AlH(OH)] to understand the hydrogen bonding and the η 2 ‐pyrazolato ligand coordination. The calculations predict that there is a 1.4 kcal/mol energy difference between η 1 ‐ and η 2 ‐pyrazolato ligand coordination, which implies that the observed η 2 ‐pyrazolato ligand occurs due to accommodation of the bulky tert ‐butyl groups. The intramolecular hydrogen bond between the hydroxo ligand proton and the π‐cloud of the η 2 ‐pyrazolato ligand is estimated to have a bond strength of 3.7 kcal/mol. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)