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

13 C-Labeled ethylene polymerization (pre)catalysts [κ 2 -(anisyl) 2 P,O ]Pd( 13 CH 3 )(L) ( 1- 13 CH 3 -L ) (L = pyridine, dmso) based on di(2-anisyl)phosphine benzenesulfonate were used to assess the degree of incorporation of 13 CH 3 groups into the formed polyethylenes. Polymerizations of variable reaction time reveal that ca. 60–85% of the 13 C-label is found in the polymer after already 1 min polymerization time, which provides evidence that the pre-equilibration between the catalyst precursor 1- 13 CH 3 -L and the active species 1- 13 CH 3 -(ethylene) is fast with respect to chain growth. The fraction of 1- 13 CH 3 -L that initiates chain growth is likely higher than the 60–85% determined from the 13 C-labeled polymer chain ends since (a) chain walking results in in-chain incorporation of the 13 C-label, (b) irreversible catalyst deactivation by formation of saturated (and partially volatile) alkanes diminishes the amount of 13 CH 3 groups incorporated into the polymer, and (c) palladium-bound 13 CH 3 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- 13 CH 3 -L provide evidence that a mixture of phosphonium salts ( 13 CH 3 ) x P + (aryl) 4– x ( 2 – 7 ) is formed in the absence of ethylene. In addition, isolation and characterization of the mixed bis(chelate) palladium complex [κ 2 -(anisyl) 2 P,O ]Pd[κ 2 -(anisyl)( 13 CH 3 ) 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 [κ 2 -(anisyl) 2 P, O ]Pd[κ 2 - (anisyl)(CH 2 CH 3 ) P,O ] ( 12 ) and [κ 2 -(anisyl) 2 P,O ]Pd[κ 2 -(anisyl)((CH 2 ) 3 CH 3 ) 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 [κ 2 - P,O ] 2 Pd, and palladium black was identified as an irreversible catalyst deactivation pathway. This deactivation proceeds by reaction of palladium alkyl complexes with palladium hydride complexes [κ 2 - P,O ]Pd(H)(L) or by reaction with the free ligand H[P,O] generated by reductive elimination from [κ 2 - P,O ]Pd(H)(L). The model hydride complex 1-H-P t Bu 3 has been synthesized in order to establish whether 1-H-P t Bu 3 or H[P,O] is responsible for the irreversible catalyst deactivation. However, upon reaction with 1- (13) CH 3 -L or 1-CH 2 CH 3 -PPh 3, both 1-H-P t Bu 3 and H[P,O] result in formation of methane or ethane, even though H[P,O] reacts faster than 1-H-P t Bu 3 . DFT calculations show that reductive elimination to form H[P,O] and (alkyl)[P,O] from 1-H/(alkyl)-P t Bu 3 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(P t Bu 3 ) 2 ]. These calculations also indicate that for a reaction sequence comprising reductive elimination of H[P,O] from 1-H-P t Bu 3 and reaction of H[P,O] with 1-CH 3 -P t Bu 3, 1-CH 3 -dmso, or 1-CH 2 CH 3 -PPh 3 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-P t Bu 3 with 1-CH 3 -P t Bu 3 or 1-CH 3 -dmso, which evolves into bimetallic transition-state geometries with a nearly linear H-(CH 3 )-Pd alignment and which exhibits a barrier of 131 or 95 kJ mol –1 for the formation of methane.