Synthesis, structure, and bridge-terminal alkyl exchange kinetics of pyrazolate-bridged dialuminum complexes containing bridging n-alkyl groups

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 ΔG(298K)‡=15.4±11.7kcal/mol for Et2Al(μ-Ph2pz)(μ-Et)AlEt2 and ΔH‡ = 1.7 ± 0.1 kcal/mol, ΔS‡ = −46.6 ± 43.4 cal/K mol, and ΔG(298K)‡=15.6±11.7kcal/mol 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.

Graphical abstractThe bridging n-alkyl complexes μ-3,5-diphenylpyrazolato-μ-ethyl-tetraethyldialuminum and μ-3,5-di-tert-butylpyrazolato-μ-n-propyl-tetra-n-propyldialuminum were obtained upon treatment of the parent pyrazole with two equivalents of triethylaluminum or tri-n-propylaluminum. The X-ray crystal structures and bridge-terminal alkyl group exchange kinetics of these complexes are described.Figure optionsDownload full-size imageDownload as PowerPoint slide