Activation and Deactivation of Neutral Palladium(II) Phosphinesulfonato Polymerization Catalysts

Abstract

13C-Labeled ethylene polymerization (pre)catalysts [κ2-(anisyl)2P,O]Pd(13CH3)(L) (1-13CH3-L) (L = pyridine, dmso) based on di(2-anisyl)phosphine benzenesulfonate were used to assess the degree of incorporation of 13CH3 groups into the formed polyethylenes. Polymerizations of variable reaction time reveal that ca. 60–85% of the 13C-label is found in the polymer after already 1 min polymerization time, which provides evidence that the pre-equilibration between the catalyst precursor 1-13CH3-L and the active species 1-13CH3-(ethylene) is fast with respect to chain growth. The fraction of 1-13CH3-L that initiates chain growth is likely higher than the 60–85% determined from the 13C-labeled polymer chain ends since (a) chain walking results in in-chain incorporation of the 13C-label, (b) irreversible catalyst deactivation by formation of saturated (and partially volatile) alkanes diminishes the amount of 13CH3 groups incorporated into the polymer, and (c) palladium-bound 13CH3 groups, and more general palladium-bound alkyl(polymeryl) chains, partially transfer to phosphorus by reductive elimination. NMR and ESI-MS analyses of thermolysis reactions of 1-13CH3-L provide evidence that a mixture of phosphonium salts (13CH3)xP+(aryl)4–x (2–7) is formed in the absence of ethylene. In addition, isolation and characterization of the mixed bis(chelate) palladium complex [κ2-(anisyl)2P,O]Pd[κ2-(anisyl)(13CH3)P,O] (11) by NMR and X-ray diffraction analyses from these mixtures indicate that oxidative addition of phosphonium salts to palladium(0) species is also operative. The scrambling of palladium-bound carbyls and phosphorus-bound aryls is also relevant under NMR, as well as preparative reactor polymerization conditions exemplified by the X-ray diffraction analysis of [κ2-(anisyl)2P,O]Pd[κ2-(anisyl)(CH2CH3)P,O] (12) and [κ2-(anisyl)2P,O]Pd[κ2-(anisyl)((CH2)3CH3)P,O] (13) isolated from pressure reactor polymerization experiments. In addition, ESI-MS analyses of reactor polymerization filtrates indicate the presence of (odd- and even-numbered alkyl)(anisyl)phosphine sulfonates (14) and their respective phosphine oxides (15). Furthermore, 2-(vinyl)anisole was detected in NMR tube and reactor polymerizations, which results from ethylene insertion into a palladium–anisyl bond and concomitant β-hydride elimination. In addition to these scrambling reactions, formation of alkanes or fully saturated polymer chains, bis(chelate)palladium complexes [κ2-P,O]2Pd, and palladium black was identified as an irreversible catalyst deactivation pathway. This deactivation proceeds by reaction of palladium alkyl complexes with palladium hydride complexes [κ2-P,O]Pd(H)(L) or by reaction with the free ligand H[P,O] generated by reductive elimination from [κ2-P,O]Pd(H)(L). The model hydride complex 1-H-PtBu3 has been synthesized in order to establish whether 1-H-PtBu3 or H[P,O] is responsible for the irreversible catalyst deactivation. However, upon reaction with 1-(13)CH3-L or 1-CH2CH3-PPh3, both 1-H-PtBu3 and H[P,O] result in formation of methane or ethane, even though H[P,O] reacts faster than 1-H-PtBu3. DFT calculations show that reductive elimination to form H[P,O] and (alkyl)[P,O] from 1-H/(alkyl)-PtBu3 is kinetically accessible, as is the oxidative readdition of the P–H bond of H[P,O] and the P–anisyl bond of (alkyl)[P,O] to [Pd(PtBu3)2]. These calculations also indicate that for a reaction sequence comprising reductive elimination of H[P,O] from 1-H-PtBu3 and reaction of H[P,O] with 1-CH3-PtBu3, 1-CH3-dmso, or 1-CH2CH3-PPh3 to form methane or ethane, the rate-limiting step is reductive elimination of H[P,O] with a barrier of 124 kJ mol–1. However, a second reaction coordinate was found for the reaction of 1-H-PtBu3 with 1-CH3-PtBu3 or 1-CH3-dmso, which evolves into bimetallic transition-state geometries with a nearly linear H-(CH3)-Pd alignment and which exhibits a barrier of 131 or 95 kJ mol–1 for the formation of methane.