Article pubs.acs.org/Organometallics
Lewis Acid Modification and Ethylene Oligomerization Behavior of Palladium Catalysts That Contain a Phosphine-Sulfonate-Diethyl Phosphonate Ancillary Ligand Nathan D. Contrella and Richard F. Jordan* Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637, United States S Supporting Information *
ABSTRACT: The multifunctional phosphine-sulfonate-diethyl phosphonate ligand [1-(P(4-tBu-Ph)(2-PO3Et2-5-Me-Ph)-2-SO3-5-MePh]− ([OP-P-SO]−) was used to form complexes of type [κ2-(OPP-SO)]PdMe(L) (L = 2,6-lutidine, 2b; L = pyridine, 2c). B(C6F5)3 abstracts the Pd-bound sulfonate group of 2b and induces a switch to a phosphine-diethyl phosphonate coordination mode, affording [κ2(OP-P-SO-B(C6F5)3)]PdMe(2,6-lutidine) (3). In contrast, MgCl2 binds to the sulfonate and diethyl phosphonate units of 2b, generating the dipalladium species [{κ2-(OP-P-SO)PdMe}2(μ-Cl)][MgCl] (4) by simple self-assembly. AgB(C6F5)4 reacts with 4 in the presence of THF to selectively abstract the Mg-Cl to form [{κ2-(OP-P-SO)PdMe}2(μ-Cl)Mg(THF)][B(C6F5)4] (5). The ethylene polymerization behaviors of 2b, 2c, 3, and 5 are quite similar (Mn: 120−1170 Da, activity: 60−290 kg (mol Pd)−1 h−1). All of these catalysts produce low-molecular-weight polyethylene with predominantly internal unsaturation, but little branching. The reaction of 4 with 2 equiv of AgB(C6F5)4 to abstract both chloride ions generates an active ethylene polymerization catalyst that produces linear polyethylene with a bimodal molecular weight distribution.
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INTRODUCTION Palladium(II) complexes that contain ortho-phosphino-arenesulfonate (P-SO−) ligands (Chart 1, A) are versatile catalysts for the homopolymerization of ethylene to highly linear polyethylene (PE)1 and the copolymerization of ethylene with a variety of polar vinyl monomers.2 The combination of strong (phosphine) and weak (sulfonate) donor groups in the P-SO− ligand is believed to inhibit β-hydride elimination, which leads to chain walking and polymer branching.1,3 The polymerization activity and the molecular weight (MW) of the polymers produced by these catalysts may be tuned by incorporating different substituents (R) on the P-SO scaffold4 and by variation of the labile ligand (L),5 but are generally low compared to other catalysts.6 Several classes of cationic Pd complexes that are structurally similar to (P-SO)PdMe(L) catalysts have been generated by variation of the weak donor group.7 These species typically react with ethylene to afford oligomeric or low-MW PE. For example, (P-SO)PdMe(L)-derived complexes with a boranecoordinated sulfonate unit (B) exhibit higher activity, but lower MW, compared to A.8 Similarly, phosphine-trifluoroborate Pd complexes (C) catalyze ethylene dimerization,9 and phosphinesulfonamide complexes (D) catalyze ethylene di- and trimerization.10 Phosphine-diethyl phosphonate complexes (PPO)PdMe(L) (E) and phosphine-bis(diethyl phosphonate) complexes (P-(PO)2)PdMe(L) (F) exhibit moderate polymerization activities and generate PE with low (
