Neutral and Cationic Dendritic Palladium(II) Complexes ContainingN,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 G n -ONNMe m [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 G n- ONNMe m [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.