Ion Pair Effects of Zwitterionic Ni/Pd Allyl Complexes Bearing an α-Sulfonate-β-diimine Ligand by Binding of B(C6F5)3 on Norbornene Polymerization

Abstract

This paper initially reports the remote binding of B(C₆F₅)₃ to zwitterionic nickel and palladium allyl complexes bearing an α-sulfonate-β-diimine ligand and the ion pair effects of zwitterionic complexes on norbornene polymerization. The coordination reaction of α-sulfonate-β-diimine lithium salt with η³-allyl nickel/palladium chloride dimers produces zwitterionic nickel and palladium allyl complexes (Ni-1 and Pd-1) with an uncoordinated sulfonate group and tight ion pairs (M⁺--O^–SO₂ < 3 Å, M = Ni and Pd). Zwitterionic Ni-1 and Pd-1 are inactive without any additional activators for norbornene polymerization. After activation by MAO, Pd-1 shows very high activity while Ni-1 exhibits moderate activity for norbornene polymerization. Using B(C₆F₅)₃ as an activator, Ni-1 shows very high activity whereas Pd-1 exhibits moderate activity. The activation mechanism studies show that the binding of B(C₆F₅)₃ to the sulfonate oxygen of zwitterionic Ni-1 and Pd-1 complexes affords the new zwitterionic complexes Ni-2 and Pd-2 with loose ion pairs (M⁺- - -O^–B > 4.68 Å). The binding of B(C₆F₅)₃ leads to less electrophilic metal centers and looser ion pairs. Enhanced norbornene polymerization activity using B(C₆F₅)₃ as an activator is a result of the ion pair effects.