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

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.