Neutral and cationic dendritic palladium(II) complexes containing N,N′-iminopyridine chelating ligands.: Synthesis and their use for the syndiospecific copolymerization of CO/4-tert-butylstyrene

Series of neutral and cationic palladium carbosilane dendritic compounds of the general formula Gn-ONNMem[Pd(X)Y] (X, Y = Cl; X = Me, Y = Cl; X = Me, Y = MeCN), containing 4 (n = 1), 8 (n = 2) or 16 (n = 3) terminal pyridylimine complexes, substituted with m methyl groups (m = 0, 2, 3), along with the corresponding monometallic counterparts (n = 0), have been synthesized. Monometallic or dendritic chloro methyl or cationic methyl palladium compounds consist of a cis/trans mixture of diastereoisomers, with compositions according to their electronic and steric features. The cationic compounds Gn-ONNMem[PdMe(MeCN)](+) are found to be active catalysts for the alternating copolymerization of CO and 4-tert-butylstyrene, producing mainly syndiotactic polyketones due to a chain-end stereocontrolled mechanism. Modification of the pyridylimine ligand framework by methyl substituents has a decisive influence on the activities of the palladium compounds. Some of them are found to retain some activity after several days of catalytic reaction. Also, the size (i.e., generation n) of the dendritic precursor affects the catalyst performance and the microstructure of the copolymerization products. Thus, higher generation catalysts show superior activities and produce shorter and less stereoregular copolymer chains.