Palladium complexes with 3-alkyl-substituted-1,10-phenanthrolines: effect of the remote alkyl substituent on the CO/olefin copolymerization reactions

The coordination chemistry of a series of 3-alkyl-substituted-1,10-phenanthroline (3-R-phen) to palladium as well as the catalytic behavior of the corresponding bischelated derivatives, [Pd(3-R-phen)2][PF6]2, in the CO/vinyl arenes copolymerization reaction has been investigated in detail. The alkyl substituents differ for length and steric hindrance. The crystal structure characterization reveals that the two molecules of 3-R-phen are bound to palladium in a syn arrangement with the alkyl groups on the same side of the square planar geometry. In solution a dynamic process involving the equilibrium between syn and anti isomers is evidenced by NMR spectroscopic analysis. This is in agreement with the results of DFT calculations, which indicate similar stabilities for the two isomers. The severe distortions from the ideal square planar coordination geometry observed in the solid state are rationalized, through the DFT analysis, in term of the HOMO orbitals responsible for the Pd-N bonds. The [Pd(3-R-phen)2][PF6]2 complexes efficiently promote the CO/styrene and CO/p-Me-styrene copolymerizations to the corresponding syndiotactic polyketones. Yields and molecular weights show an increasing trend on increasing the steric demand of the R substituent and the values recorded are the best ones ever reported for copolymerization reactions of this kind in the absence of the oxidant. From the TON numbers this result seems related to an increase of the olefin insertion rate, which proceeds faster when 3-R-phen are used as ligands. A change of the physical nature of the active species, from homogeneous to heterogeneous, occurs during the polymerization process and the time at which this variation takes place depends on the nature of the olefin. The positive effect of the alkyl substitution is less evident in the CO/ethylene copolymerization