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.