Ethylene and Styrene Carbon Monoxide Copolymerization Catalyzed by Pyrazolyl Palladium(II) Complexes

The pyrazolyl pyridylimine ligands [2-(3,5-dimethylpyrazol-1 -yl)ethyl]pyridin-2-ylmethyleneimine (L1) and [2-(3,5-di-tert-butylpyrazol-1-yl)ethyl]pyridin-2-ylmethyleneimine (L2) and pyrazolyl thienylimine ligands [2-(3,5-dimethylpyrazol-1-yl)ethyl]thiophen-2-ylmethyleneimine (L3), [2- (3,-di-tert-butyl-pyrazol-1-yl) ethyl] thiophen-2-ylmethyleneimine (L4), [2-(3,5-dimethylpyrazol-1-yl)ethyl]-2-bromo-thiophen-2-ylmethyleneimine (L5), and [2-(3,5-di-tert-butylpyrazol-1-ypethyl]-2-bromothiophen-2-ylmethyleneimine (L6) were synthesized by condensation of the appropriate pyrazolyl-amine and the corresponding aldehyde. Reactions of L1-L6 with [PdCl(Me)(cod)] gave the corresponding palladium(II) complexes [PdCl(Me)(L)] (where L = L1 (la), L2 (2a), L3 (3a), IA (4a), L5 (5a), L6 (6a)) in very good yields. The pyridylimine ligands L1 and L2 were found to coordinate via the imine and pyridine nitrogen atoms, while the thienylimine ligands L3 L6 coordinate via imine and pyrazolyl nitrogen atoms to the palladium. The cationic complexes [Pd(Me)(A)]* (where A = L1 (1b), L2 (2b), L3 (3b), L4 (4b), L5 (5b), L6 (6b)) were synthesized from 1a-6a, respectively, using Na[BAr4] (Ar = 3,5-(CF3)(2)C6H3) in a 1:1 ratio. The cationic palladium complexes 1b and 2b were stabilized by the pyrazolyl nitrogen atom of ligands L1 and L2, respectively, while complexes 3b-66 were stabilized by NCMe. Attempts to activate 1a-6a with Na[BAr4] to produce active catalysts for ethylene/CO and styrene/CO led to the formation of palladium black Using the cationic complexes 1b-6b, only 2b and 3b were active in the copolymerization of ethylene/CO and styrene/CO.