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 (n Pr)2Al(μ-t Bu2pz)(μ-n Pr)Al(n Pr)2 (63%) and the dimeric complex [(n Pr)2Al(μ-t Bu2pz)]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 [(n Pr)2Al(μ-t Bu2pz)]2 (68%) or [(i Bu)2Al(μ-t Bu2pz)]2 (96%), respectively, as colorless crystalline solids. The solid state structures of Et2Al(μ-Ph2pz)(μ-Et)AlEt2 and (n Pr)2Al(μ-t Bu2pz)(μ-n Pr)Al(n Pr)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 sourceΔG(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 View the MathML sourceΔG(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.