Self-Assembled Cage Structures and Ethylene Polymerization Behavior of Palladium Alkyl Complexes That Contain Phosphine-Bis(arenesulfonate) Ligands

The phosphine-bis-arenesulfonate ligands PR2(2-SO3Li-5-R1-Ph)­(2-SO3–-5-R1-Ph) (Li­[OPO–]; Li­[1a–e], a: R1 = Me, R2 = Ph, b: R1 = iPr, R2 = Ph, c: R1 = Cy, R2 = Ph, d: R1 = tBu, R2 = Ph, e: R1 = R2 = tBu, f: R1 = R2 = Cy) coordinate as κ2-P,O chelators in (Li-OPO)­PdMeL complexes (4a–c,f: L = 4-(5-nonyl)­pyridine = py′; 5b–d: L = py; 2e: L = py′). 4a–c,f and 5b–d self-assemble into tetrameric structures in which four (Li-OPO)­PdMeL units are arranged around a cubic Li4S4O12 cage formed from the four non-Pd-bound ArSO3Li units and one oxygen from each of the Pd-bound ArSO3– units. The {(Li-OPO)­PdMeL}4 assemblies are in equilibrium with monomeric (Li-OPO)­PdMeL species in CD2Cl2 and CDCl2CDCl2 solution. Crystallization of 2e from a toluene/pentane mixture at −40 °C yields a trimeric C1-symmetric assembly (6e) in which three (Li-1e)­PdMe­(py′) units are held together by a Li3S3O9 core. Crystallization of 2e from toluene/pentane at room temperature results in partial loss of py′ and formation of a Pd6 assembly, {(Li-1e)­PdMe­(py′)}4{(Li-1e)­PdMe}2 (7e). 4a–c,f, 5b–d, and 2e exist as monomeric (Li-OPO)­PdMeL complexes in CD3OD solution, and under these conditions exchange of the Pd-bound and non-Pd-bound ArSO3– units is much faster for 4a,c (ΔG⧧ = 12.3, 12.2 kcal/mol at −10 °C) than for 2e (ΔG⧧ = 19.2 kcal/mol at 24 °C). This difference in molecular rigidity may contribute to the differences in the self-assembly behavior of these compounds. 4a,f and 5b catalyze the polymerization of ethylene to linear polyethylene. The molecular weight and molecular weight distribution of the polymer are strongly influenced by the extent of dissociation of the Pd4 cage under the polymerization conditions.