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

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.