Organometallic Reaction Mechanisms: Olefin Polymerization Catalysis and C-H Bond Activation by Early Transition Metal Bisphenolate Complexes and Protonolysis of Bipyrimidine Platinum Methyl Complexes

Abstract

Mechanistic aspects of organometallic transformations such as polymerization, C H activation, and protonolysis have been examined. Relationships between catalyst geometry and polymer microstructure were defined. The mechanism of an intramolecular C H activation process was found to involve two competing pathways. The protonolysis of platinum methyl complexes was investigated for kinetic isotope effects and observation of intermediates. Bisphenolate ligands with pyridine and benzene diyl linkers were synthesized and metalated with group 4 and 5 transition metals. The solid state structures of some of the group 4 complexes were solved. The titanium, zirconium, hafnium, and vanadium complexes were tested for propylene, 1 hexene, and ethylene/1 octene polymerization activities with methylaluminoxane as co catalyst. Titanium and zirconium (IV) precatalysts with pyridine diyl linkers provided mixtures of isotactic and atactic polypropylene while titanium (IV) precatalysts with benzene diyl linkers gave atactic polypropylene only. The hafnium (IV) precatalyst with a pyridine diyl linker generated moderately isotactic polypropylene. A titanium dibenzyl complex featuring a ligand with two phenolates linked by a benzene diyl was found to undergo thermal decomposition to give toluene and a cyclometalated dimeric complex. The thermal decomposition followed first order kinetics and was studied at a number of temperatures to determine the activation parameters. Deuterated isotopologs were synthesized to measure the kinetic isotope effects. The complexes with deuterium in the benzyl methylene positions decomposed slower than the protio analogs. Isotopologs of toluene with multiple deuteration positions were observed in the product mixtures. These data are consistent with competing α-abstraction and σ-bond metathesis. The protonolysis of bipyrimidine ligated platinum (II) complexes was explored. The bipyrimidine platinum dimethyl complex (bpm)PtMe₂ was shown to undergo protonation at the metal upon addition of trifluoroacetic acid (tfa) to give a platinum (IV) hydride intermediate which reductively eliminated methane to give (bpm)PtMe(tfa). Using a mixture of deutero and protio acid, all isotopologs of methane were observed. The protonation of (bpm)PtMe(tfa) was less straightforward as no intermediates were found, and CH₄, CH₃D, and CH₂D₂ were observed upon addition of a mixture of deutero and protio acid. The protonation of a nitrogen of the bpm ligand was also examined and determined improbable under the present conditions.