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Synthesis, structure, and bridge-terminal alkyl exchange kinetics of pyrazolate-bridged dialuminum complexes containing bridging n-alkyl groups
Authors:Chatu T. Sirimanne
Affiliation:Department of Chemistry, Wayne State University, Detroit, MI 48202, United States
Abstract:Treatment of triethylaluminum with 3,5-diphenylpyrazole in a 2:1 stoichiometry afforded the ethyl-bridged complex Et2Al(μ-Ph2pz)(μ-Et)AlEt2 (79%) as a colorless crystalline solid. Treatment of tri-n-propylaluminum with 3,5-di-tert-butylpyrazole in a 2:1 stoichiometry afforded the n-propyl-bridged complex (nPr)2Al(μ-tBu2pz)(μ-nPr)Al(nPr)2 (63%) and the dimeric complex [(nPr)2Al(μ-tBu2pz)]2 (3%), respectively, as colorless crystalline solids. Treatment of tri-n-propylaluminum (1 equiv.) or triisobutylaluminum (1 or 2 equiv.) with 3,5-di-tert-butylpyrazole afforded exclusively the dimeric complexes [(nPr)2Al(μ-tBu2pz)]2 (68%) or [(iBu)2Al(μ-tBu2pz)]2 (96%), respectively, as colorless crystalline solids. The solid state structures of Et2Al(μ-Ph2pz)(μ-Et)AlEt2 and (nPr)2Al(μ-tBu2pz)(μ-nPr)Al(nPr)2 consist of 3,5-disubstituted pyrazolato ligands with a di-n-alkylalumino group bonded to each nitrogen atom. An ethyl or n-propyl group acts as a bridge between the two aluminum atoms. The kinetics of the bridge-terminal exchange was determined for the bridging n-alkyl complexes by 13C NMR spectroscopy, and afforded ΔH = 1.5 ± 0.1 kcal/mol, ΔS = −46.8 ± 39.0 cal/K mol, and View the MathML source for Et2Al(μ-Ph2pz)(μ-Et)AlEt2 and ΔH = 1.7 ± 0.1 kcal/mol, ΔS = −46.6 ± 43.4 cal/K mol, and View the MathML source for (nPr)2Al(μ-tBu2pz)(μ-nPr)Al(nPr)2. The negative values of ΔS imply ordered transition states relative to the ground states, and rotation along the N-AlR3 vector without aluminum-nitrogen bond cleavage is proposed.
Keywords:Aluminum   Trimethylaluminum   Triethylaluminum   Tri-n-propylaluminum
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