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-(3,5-di-tert-butyl-pyrazol-1-yl)ethyl]thiophen-2-ylmethyleneimine (L4), [2-(3,5-dimethylpyrazol-1-yl)ethyl]-2-bromothiophen-2-ylmethyleneimine (L5), and [2-(3,5-di-tert-butyl-pyrazol-1-yl)ethyl]-2-bromothiophen-2-ylmethyleneimine (L6) were synthesized by condensation of the appropriate pyrazolylamine and the corresponding aldehyde. Reactions of L1-L6 with [PdCl(Me)(cod)] gave the corresponding palladium(II) complexes [PdCl(Me)(L)] (where L = L1 (1a), L2 (2a), L3 (3a), L4 (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)₂C₆H₃) 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-6b 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.