Multinuclear Palladium Catalysts for Ethylene Polymerization

Abstract

This thesis describes the synthesis of multinuclear palladium complexes that adopt self-assembled cage structures and their performances as catalysts for ethylene polymerization and ethylene/vinyl fluoride copolymerization. Chapter One introduces tetranuclear palladium(II) alkyl catalysts that contain phosphine-bis(arenesulfonate) (Li-OPO-) ligands and adopt 3-dimensional structures in which four PdMe(py) units are arranged around the periphery of a central cubic Li4S4O12 cage. The intact catalyst produces high-MW linear PE and copolymerizes ethylene with vinyl fluoride to afford linear copolymer with high VF incorporation (up to 3.6 mol %). The origins of these unique polymerization properties are unknown because partial dissociation of Pd4 catalyst into Pd1 species occurs under polymerization conditions, which complicates mechanistic studies. Other multinuclear olefin polymerization catalysts are also discussed. Chapter Two describes a series of methoxy-substituted phosphine-arenesulfonate (PO) and phosphine-bis(arenesulfonate) (Li-OPO-) ligands, and their corresponding Pd complexes. {[κ2-P,O-PPh(2-SO3-4,5-(OMe)2-Ph)(2-SO3Li-4,5-(OMe)2-Ph)]PdMe(py’)}42LiCl (5b) self-assembles around a Li4S4O12Li2Cl2 cage with SSSS configurations at the phosphorous centers. Solution NMR data for 5b are consistent with the solid-state structure. 5b partially and reversibly dissociates into monomeric Pd1 species above room temperature, but is more robust than {[κ2-P,O-PPh(2-SO3-5-Me-Ph)(2-SO3Li-5-Me-Ph)]PdMe(py’)}4 (py’ = 4-(5-nonyl)pyridine) and other assemblies based on Li4S4O12 cores. 5b produces UHMWPE with a narrow MWD characteristic of single-site catalysis in hexanes at 80 ˚C. The ethylene polymerization behavior of monomeric (PO)PdMe(py) complexes [κ2-P,O-P(2-OMe-Ph)2(2-SO3-5-OMe-Ph)]PdMe(py) (3a) and [κ2-P,O-P(2-OMe-Ph)2(2-SO3-4,5-(OMe)2-Ph)]PdMe(py) (3b) is similar to that of the bench-mark catalyst [κ2-P,O-P(2-OMe-Ph)2(2-SO3-5-Me-Ph)]PdMe(py), which indicates that the methoxy substituents on the arenesulfonate ring do not strongly influence the reactivity. Chapter Three describes a phosphine-sulfonate-phosphonate ligand [(4-tBu-Ph)(2-P(O)(OH)2-5-Me-Ph)(2-SO3--5-Me-Ph)P+H] ([OP-P-SO], 1-H3). 1-H3 reacts with Zn(OAc)2 to form tetrameric {Zn[1-H]}4, which adopts a puckered 16-membered Zn4P4O8 ring structure. The reaction of CH3OH-free {Zn[1-H]}4 with (COD)PdMe2 and pyridine ligands (L) generates tetrameric palladium complexes 4-L. X-ray diffraction analysis of 4-(4-tBu-py) and solution NMR data for other 4-L complexes (L = py, 2,6-lutidine, quinolone, 4-tBu-py) show that 4-L are isostructural with {[κ2-P,O-PPh(2-SO3-5-Me-Ph)(2-SO3Li-5-Me-Ph)]PdMe(py’)}4, with a central Zn4P4O12 zinc phosphonate core and four (phosphine-phosphonate)PdMeL units at the periphery. 4-L is converted to a trimeric species {[P(4-tBu-Ph)(2-PO3Zn(py)-5-Me-Ph)(2-SO3-5-Me-Ph)]PdMe}3 (3-L) in the presence of CH3OH, excess pyridine ligands or Et2O. 3-py adopts a unique cage structure, which contains a 12-membered Zn3P3O6 ring stacked with a 6-membered Pd3O3 ring. The solution NMR data of 3-py are consistent with the solid-state structure. Both 4-(4-tBu-py) and 3-py produce linear PE with high MW and copolymerize ethylene with vinyl fluoride with up to 1.1 mol % incorporation. Chapter Four describes the synthesis of aryl borophosphonate cage compounds [ArPO3BAr’]n with n = 4 or 6, by condensation reactions of ArP(O)(OH)2 and Ar’B(OH)2. (3,5-tBu2-Ph)P(O)(OH)2 (1) reacts with arylboronic acids that contain electron-withdrawing substituents to form borophosphonate tetramers [Ar1PO3BAr2]4 (Ar1 = 3,5-tBu2-Ph; Ar2 = o-Br-Ph, o-CF3-Ph, p-CF3-Ph, p-CHO-Ph; 3a-d) and hexamers [Ar1PO3BAr2]6 (Ar2 = p-CF3-Ph, p-CHO-Ph; 4c-d) in 80 - 93 % NMR yield. For Ar2 = p-CF3-Ph and p-CHO-Ph, both products were observed, with the tetramer favored under dilute reaction conditions and the hexamer favored under concentrated reaction conditions. The phosphine phosphonic acid (2-PPh2-Ph)P(O)(OH)2 (6) reacts with arylboronic acids that contain electron-withdrawing substituents to form tetramers [Ar1PO3BAr2]4 (Ar1 = 2-PPh2-Ph; Ar2 = p-CF3-Ph, p-CHO-Ph; 7c-d) in 70 - 75 % NMR yield. The reactions of 1 or 6 with (p-tolyl)B(OH)2 (2f), and the reaction of 6 with (o-CF3-Ph)B(OH)2 (2b), yield only trace amounts of borophosphonate cage compounds, and instead afford the corresponding [ArBO]3 boroxines and condensation products with unknown structures. These borophosphonate compounds are stable in toluene at 110 ˚C, and therefore they are potential scaffolds for the formation of multinuclear Pd catalysts.