Synthesis and Reactivity of Palladium(II) Alkyl Complexes that Contain Phosphine-cyclopentanesulfonate Ligands

The synthesis of the phosphine-cyclopentanesulfonate pro-ligands Li/K­[2-PPh₂-cyclopentanesulfonate] (Li/K­[2a]), Li/K­[2-P­(2-OMe-Ph)₂-cyclopentanesulfonate] (Li/K­[2b]), and H­[2b], and the corresponding Pd­(II) alkyl complexes (κ²-P,O-2a)­PdMe­(pyridine) (3a) and (κ²-P,O-2b)­PdMe­(pyridine) (3b) is described. The sulfonate-bridged base-free dimer {(2b)­PdMe}₂ (4b) was synthesized by abstraction of pyridine from 3b using B­(C₆F₅)₃. The borane-coordinated base-free dimer [{2b·B­(C₆F₅)₃}­PdMe]₂ (5b), in which B­(C₆F₅)₃ binds to a sulfonate oxygen, was prepared by addition of 1 equiv of B­(C₆F₅)₃ per Pd to 4b or addition of 2 equiv of B­(C₆F₅)₃ to 3b. Compounds 3b, 4b, and 5b polymerize ethylene with low activity (up to 210 kg mol^–1 h^–1 at 250 psi and 80 °C) to linear polyethylene (M_n = 1950–5250 Da) with predominantly internal olefin placements. 3b and 4b copolymerize ethylene with methyl acrylate to linear copolymers that contain up to 11.7 mol % methyl acrylate, which is incorporated as −CH₂CH­(CO₂Me)­CH₂– (80%) in-chain units and −CH₂CH­(CO₂Me)­Me (8%) and −CH₂CHCH­(CO₂Me) (12%) chain-end units. 3b and 4b also copolymerize ethylene with vinyl fluoride to linear copolymers that contain up to 0.41 mol % vinyl fluoride, which is incorporated as −CH₂CHFCH₂– (∼80%) in-chain units and −CH₂CF₂H (7%), −CH₂CHFCH₃ (5%), and −CH₂CH₂F (8%) chain-end units. Complexes 3b and 4b are more stable and active in ethylene polymerization than analogous (PAr₂-CH₂CH₂SO₃)­PdR catalysts, but are less active than analogous (PAr₂-arenesulfonate)­PdR catalysts. Low-temperature NMR studies show that 4b reacts with ethylene below −10 °C to form the ethylene adduct cis-P,R-(2b)­PdMe­(ethylene) (7b), which undergoes ethylene insertion at 5 °C. DFT calculations for a model (PMe₂-cyclopentanesulfonate)­Pd­(Pr)­(ethylene) species show that ethylene insertion proceeds by cis-P,R/trans-P,R isomerization followed by migratory insertion, and that the lower activity of 3b and 4b vis-à-vis analogous (PAr₂-arenesulfonate)­PdR catalysts results from a higher barrier for migratory insertion of the trans-P,R isomer.