Tailoring of Pyridinium Amidate Palladium Complexes for Ethylene Conversion beyond Dimerization and for the Conversion of Higher α-Olefins

Ethylene polymerization with late transition metals offers the possibility of including polar monomers for the generation of functionalized polymers. However, several palladium complexes, including those with pyridyl-functionalized pyridinium amidate (PYA) ligands [Pd(Me)(MeCN)(N,N′)]+ (with N = PYA, N′ = pyridyl), undergo rapid β-hydrogen elimination and form predominantly butene derivatives. Here, we have modified a range of elements in the catalyst design, including (i) the PYA substituents (Me, Bu, CH2OCH3), (ii) the chelating imine donor, (iii) the labile neutral ligand L, and (iv) the noncoordinating anion. These variations indicated factors that prevent (L = lutidine) or slow down ethylene conversion (imine = oxalyl, triazolyl, and pyrazolyl) and factors that accelerate it. In particular, the absence of MeCN as the coordinating ligand and the introduction of BArF as the counterion are highly beneficial and lead to efficient ethylene conversion and formation of oligomers with C20-C30 chain length. Time-dependent reaction monitoring suggests a step-growth mechanism rather than the more common chain-growth mechanism with the initial formation of butene and the subsequent conversion of butene and higher olefins. Indeed, also higher α-olefins such as 1-hexene were oligomerized with this in situ-prepared catalytic PYA palladium system.