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

The phosphine-bis-arenesulfonate ligand PPh((2)-SO3Li-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 AgPF6 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 Li4S4O12 cage. The Pd centers are equivalent but are spatially separated into two identical pairs. The Pd-Pd distance within each pair is 6.04 angstrom. IR data (nu(ArSO3)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.