Counterion Effect on CO/Styrene Copolymerization Catalyzed by Cationic Palladium(II) Organometallic Complexes: An Interionic Structural and Dynamic Investigation Based on NMR Spectroscopy

Abstract

Complexes [Pd(η1,η2-C8H12OMe)bipy]+X- (2a−f) (where X = BPh4- (a), CF3SO3- (b), BF4- (c), PF6- (d), SbF6- (e), and B(3,5-(CF3)2C6H3)4- (f); bipy = 2,2‘-bipyridine; C8H12OMe = cyclooctenylmethoxy group) were synthesized by the reaction of the dimer [Pd(η1,η2-C8H12OMe)Cl]2 (1) with the bipy ligand in methanol containing Y+X- salts. They were characterized in solution by multinuclear and multidimensional NMR spectroscopy. The solid-state structure of complex 2d was obtained by X-ray single-crystal investigation. The catalytic activity of complexes 2 toward CO/styrene copolymerization in methylene chloride was tested and related to the type of counterion. The order of the catalytic activity of complexes 2a−f is the following: BPh4- ≪ CF3SO3- < BF4- < PF6- < SbF6- < B(3,5-(CF3)2C6H3)4-. If the copolymerization reactions are carried out in the presence of an excess of the bipy ligand, the anion effect is less important and the order is the following: BPh4- ≪ CF3SO3- < BF4- ≈ B(3,5-(CF3)2C6H3)4- ≈ PF6- ≈ SbF6-. The interionic structure of all complexes was investigated in CD2Cl2 at room and low temperature by 19F{1H} HOESY and 1H NOESY NMR spectroscopies. In solution, the counterions are located above or below the bipy ligand shifted toward the pyridine ring trans to the Pd−C σ 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 contacts in the 19F{1H} HOESY or 1H NOESY NMR spectra. The dynamic process that exchanges the two pyridyl rings was investigated by variable-temperature NMR spectroscopy in CD2Cl2. The activation parameters were determined. ΔG⧧298 values range from 54 to 58 kJ/mol. The negative values of ΔS⧧ (−58/−108 J K-1 mol-1), for all compounds, with the exception of 2f, suggest an associative mechanism.