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

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The synthesis of the phosphine-cyclopentanesulfonate pro-ligands Li/K[2-PPh 2 -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 (κ 2 - P, O - 2a )PdMe(pyridine) ( 3a ) and (κ 2 - 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(C 6 F 5 ) 3 . The borane-coordinated base-free dimer [{ 2b· B(C 6 F 5 ) 3 }PdMe] 2 ( 5b ), in which B(C 6 F 5 ) 3 binds to a sulfonate oxygen, was prepared by addition of 1 equiv of B(C 6 F 5 ) 3 per Pd to 4b or addition of 2 equiv of B(C 6 F 5 ) 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 −CH 2 CH(CO 2 Me)CH 2 – (80%) in-chain units and −CH 2 CH(CO 2 Me)Me (8%) and −CH 2 CH═CH(CO 2 Me) (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 −CH 2 CHFCH 2 – (∼80%) in-chain units and −CH 2 CF 2 H (7%), −CH 2 CHFCH 3 (5%), and −CH 2 CH 2 F (8%) chain-end units. Complexes 3b and 4b are more stable and active in ethylene polymerization than analogous (PAr 2 - CH 2 CH 2 SO 3 )PdR catalysts, but are less active than analogous (PAr 2 - arene sulfonate)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 (PMe 2 -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 (PAr 2 -arenesulfonate)PdR catalysts results from a higher barrier for migratory insertion of the trans - P, R isomer.