Counterion effect on CO/styrene copolymerization catalyzed by cationic palladium(II) organometallic complexes: An interionic structural and dynamic investigation based on NMR spectroscopy

Complexes [Pd(eta(1),eta(2)-C8H12OMe)bipy]X-+(-) (2a-f) (where X = BPh4- (a ), CF3SO3- (b), BF4- (C), PF6- (d), SbF6- (e), and B(3,5-(CF3)(2)C6H3)(4)(- ) (f); bipy = 2,2'-bipyridine; C8H12OMe = cyclooctenylmethoxy group) were s ynthesized by the reaction of the dimer [Pd(eta(1),eta(2)-C8H12OMe)Cl](2) ( 1) with the bipy ligand in methanol containing Y+X- salts. They were charac terized in solution by multinuclear and multidimensional NMR spectroscopy. The solid-state structure of complex 2d was obtained by X-ray single-crysta l investigation. The catalytic activity of complexes 2 toward CO/styrene co polymerization in methylene chloride was tested and related to the type of counterion. The order of the catalytic activity of complexes 2a-f is the fo llowing: BPh4- << CF3SO3- < BF4- < PF6- < SbF6- < B(3,5-(CF3)(2)C6H3)(4)(-) If the copolymerization reactions are carried out in the presence of an ex cess of the bipy ligand, the anion effect is less important and the order i s the following: BPh4- << CF3SO3- < BF4- approximate to B(3,5-(CF3)(2)C6H3) (4-) approximate to PF6- approximate to SbF6-. The interionic structure of all complexes was investigated in CD2Cl2 at room and low temperature by F-1 9{H-1} HOESY and H-1 NOESY NMR spectroscopies. In solution, the counterions are located above or below the bipy ligand shifted toward the pyridine rin g trans to the Pd-C sigma bond, while in the crystal structure of 2d, they are settled sideways to the cationic moiety. The best anion in catalysis is the least strong coordinating one that shows the weakest interionic contac ts in the F-19(H-1) HOESY or 1H NOESY NMR spectra. The dynamic process that exchanges the two pyridyl rings was investigated by variable-temperature N MR spectroscopy in CD2Cl2. The activation parameters were determined. Delta G(298)(double dagger) values range from 54 to 58kJ/mol. The negative value s of Delta S-double dagger (-58/-108 J K-1 mol(-1)), for all compounds, wit h the exception of 2f, suggest an associative mechanism.