Self-Assembled Tetranuclear Palladium Catalysts That Produce High Molecular Weight Linear Polyethylene

Abstract

The phosphine-bis-arenesulfonate ligand PPh(2-SO(3)Li-4-Me-Ph)(2) (Li(2)[OPO]) coordinates as a kappa(2)-P,O chelator in Li[(Li-OPO)PdMe(Cl)] (2a) and (Li-OPO)PdMe(L) (L = pyridine (2b); MeOH (2d); 4-(5-nonyl)pyridine) (py', 3)). 2a reacts with AgPF(6) to form {(Li-OPO)PdMe}(n) (2c). Photolysis of 2d yields {(OPO)Pd}(2) (5) in which the [OPO](2-) ligand coordinates as a kappa(3)-O,P,O pincer. 3 self-assembles into a tetramer in which four (Li-OPO)PdMe(py') units are linked by Li-O bonds that form a central Li(4)S(4)O(12) cage. The Pd centers are equivalent but are spatially separated into two identical pairs. The Pd-Pd distance within each pair is 6.04 A. IR data (upsilon(ArSO(3)(-)) region) suggest that the solid state structures of 2a-c are similar to that of 3. 3 reacts with the cryptand Krypt211 to form [Li(Krypt211)][(OPO)PdMe(py')] (4). 3 is in equilibrium with a monomeric (Li-OPO)PdMe(py') species (3') in solution. 2a-c and 3 produce polyethylene (PE) with high molecular weight and a broad molecular weight distribution, characteristic of multisite catalysis. Under conditions where the tetrameric structure remains substantially intact, the PE contains a substantial high molecular weight fraction, while, under conditions where fragmentation is more extensive, the PE contains a large low molecular weight fraction. These results suggest that the tetrameric assembly gives rise to the high molecular weight polymer. In contrast, the monomeric complex 4, which contains a free pendant sulfonate group that can bind to Pd, oligomerizes ethylene to a Schultz-Flory distribution of C(4)-C(18) oligomers.