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

Abstract

Treatment of triethylaluminum with 3,5-diphenylpyrazole in a 2:1 stoichiometry afforded the ethyl-bridged complex Et 2Al(μ-Ph 2pz)(μ-Et)AlEt 2 (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- tBu 2pz)(μ- nPr)Al( nPr) 2 (63%) and the dimeric complex [( nPr) 2Al(μ- tBu 2pz)] 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- tBu 2pz)] 2 (68%) or [( iBu) 2Al(μ- tBu 2pz)] 2 (96%), respectively, as colorless crystalline solids. The solid state structures of Et 2Al(μ-Ph 2pz)(μ-Et)AlEt 2 and ( nPr) 2Al(μ- tBu 2pz)(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 Δ H ‡ = 1.5 ± 0.1 kcal/mol, Δ S ‡ = −46.8 ± 39.0 cal/K mol, and Δ G ( 298 K ) ‡ = 15.4 ± 11.7 kcal / mol for Et 2Al(μ-Ph 2pz)(μ-Et)AlEt 2 and Δ H ‡ = 1.7 ± 0.1 kcal/mol, Δ S ‡ = −46.6 ± 43.4 cal/K mol, and Δ G ( 298 K ) ‡ = 15.6 ± 11.7 kcal / mol for ( nPr) 2Al(μ- tBu 2pz)(μ- nPr)Al( nPr) 2. The negative values of Δ S ‡ imply ordered transition states relative to the ground states, and rotation along the N-AlR 3 vector without aluminum–nitrogen bond cleavage is proposed.