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 (Mn = 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.