Heterocycle-Substituted Phosphinesulfonato Palladium(II) Complexes for Insertion Copolymerization of Methyl Acrylate

Abstract

A family of heterocycle-substituted binuclear phosphinesulfonato Pd(II) complexes {[R2P(C6H4SO2O)]PdMeClLi(dmso)}2 (1a–d-LiCl-dmso: 1a-LiCl-dmso, R = 2-furyl; 1b-LiCl-dmso, R = 2-thienyl; 1c-LiCl-dmso, R = 2-(N-methyl)pyrrolyl; 1d-LiCl-dmso, R = 2-benzofuryl) was synthesized, and the solid-state structures of 1a–c-LiCl-dmso were determined, which revealed various modes of bridging between the two metal fragments. 1a–d-LiCl-dmso further generated either the mononuclear Pd(II) complexes {[κ2P,O-R2P(C6H4SO2O)]PdMe(pyr)} (1a–d-pyr) by addition of pyridine or the more labile mononuclear Pd(II) complex {[κ2P,O-(2-thienyl)2P(C6H4SO2O)]PdMe(dmso)} (1b-dmso) by chloride abstraction with AgBF4. Stoichiometric methyl acrylate (MA) insertion experiments indicated that, in comparison with the other three substituents, the thienyl-substituted Pd(II) complexes undergo faster insertion of MA in a primary 2,1-fashion, and 1b-dmso possesses the fastest insertion rate due to the relative weakly coordinating dmso molecule. All palladium complexes were employed in ethylene polymerization, affording highly linear polyethylene with relatively low molecular weights (Mn = (0.5–7.4) × 103). In addition, under these pressure reactor conditions, the thienyl motif displays the highest activity (order: 1b-dmso > 1b-pyr > 1a-pyr > 1d-pyr > 1c-pyr ≫ 1a–d-LiCl-dmso). Copolymerization reactions of ethylene and MA further revealed that MA incorporation in the obtained linear copolymers depends moderately on the heterocyclic substituents.