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-phenanthrolines (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 in 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 terms 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.