Synthesis, characterization, and hydrolysis products of (η2-tBu2pz)AlH(μ:η1, η1-tBu2pz)2AlH2 - Structural characterization of a complex containing η1-, η2-, and μ:η1,η1-Pyrazolato ligands and a complex containing a terminal hydroxo ligand

Abstract

Treatment of alane-ethyldimethylamine with 3,5-di-tert-butylpyrazole (tBu2pzH) in a 2:3 molar ratio afforded [(η2-tBu 2pz)AlH(μ:η1,η1tBu 2pz)2AlH2] in 57% yield. Hydrolysis of [(η2-tBu2pz)AlH(μ:η1. η1-tBu2pz)2AlH2] afforded variable mixtures of Al(tBu2pz)3, [(η2- tBu2pz)AlH(μ:η1,η1-tBu 2pz)2AlH(OH)], and [(η2-tBu 2pz)AlH(μ:η1,η1-tBu 2pz)2AlH(η1-tBu2pz)]. The structures of all new complexes were assigned from spectral and analytical data. In addition, the X-ray crystal structures of [(η2-tBu 2pz)AlH(μ:η1,η1-tBu 2pz)2AlH(OH)] and [(η2-tBu 2pz)AlH(μ:η1,η1-tBu 2pz)2AlH(η1-tBu2pz)] were determined. [(η2-tBu2pz)AlH(μ:η1, η1-tBu2pz)2AlH(OH)] crystallizes as a dimeric molecule, and contains two bridging pyrazolato ligands, one η2-pyrazolato ligand, as well as terminal hydrido and hydroxo ligands. The hydroxo and η2-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 η2-pyrazolato ligand. The overall structure of [(η2-tBu2pz)AlH(μ: η1,η1-tBu2pz)2AlH(η 1-tBu2pz)] is very similar to that of [(η2- tBu2pz)AlH(μ:η1,η1-tBu 2pz)2AlH(OH)], except that the η1- and η2-pyrazolato ligands have an anti-disposition within the dimer. Molecular orbital calculations were carried out on [(η2-tBu 2pz)AlH(μ:η1,η1-tBu 2pz)2AlH(OH)] to understand the hydrogen bonding and the η2-pyrazolato ligand coordination. The calculations predict that there is a 1.4 kcal/mol energy difference between η1- and η2-pyrazolato ligand coordination, which implies that the observed η2-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 η2-pyrazolato ligand is estimated to have a bond strength of 3.7 kcal/mol. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.