Intramolecular Ion−Ion Interactions in Zwitterionic Metallocene Olefin Polymerization Catalysts Derived from “Tucked-In” Catalyst Precursors and the Highly Electrophilic Boranes XB(C₆F₅)₂(X = H, C₆F₅)

Abstract

The reactions of so called “tuck-in” permethyl zirconocene compounds Cp*(η5-η1-C5Me4CH2)ZrX (X = Cl (1a), C6H5 (1b), CH3 (1c)) with the highly electrophilic boranes HB(C6F5)2 and B(C6F5)3 are described. The products are zwitterionic olefin polymerization catalysts. Reactions with 1a and 1b yielded single products cleanly, but reactions with tuck-in methyl starting material 1c gave mixtures. Spectroscopic and structural studies showed that the electrophilic zirconium center in the product zwitterions was stabilized by a variety of mechanisms. In the products of reaction between 1a and 1b with HB(C6F5)2, Cp*[η5,η1-C5Me4CH2B(C6F5)2(μ-H)]ZrX (X = Cl (2a), 74%), C6H5 (2b, 62%)), the metal is chelated by a pendant hydridoborate moiety. Chloride product 2a was characterized crystallographically. In the reaction of B(C6F5)3 with 1a, the fluxional zwitterionic product Cp*[η5-C5Me4CH2B(C6F5)3]ZrCl (3a, 84%) is stabilized by a weak donor interaction between one of the ortho fluorine atoms of the −CH2B-(C6F5)3 counterion and the zirconium center (Zr−F = 2.267(5) Å). In the product of the reaction between 1b and B(C6F5)3, Cp*[η5-C5Me4CH2B(C6F5)3]ZrC6H5 (3b, 82%), a similar ortho-fluorine interaction was found in a yellow kinetic product (y-3b), which converted upon heating gently to a thermodynamic orange polymorph (o-3b) in which the zirconium center is compensated via an agostic interaction from an ortho C−H bond of the phenyl group and an interaction between the methylene group of the −CH2B-(C6F5)3 counteranion. These compounds were both characterized by X-ray crystallography. Zwitterion o-3b reacts with H2 to form the zwitterionic hydride Cp*[η5-C5Me4CH2B(C6F5)3]ZrH (4, 77%), characterized by NMR spectroscopy and X-ray crystallography to reveal a return to the ortho-fluorine mode of stabilization. Compounds 2a, 3a, o-3b, and 4 were all found to be active ethlyene polymerization catalysts; the chloride derivatives required minimal amounts of methylaluminoxane (MAO) to alkylate the zirconium center. Polymerization data are discussed in light of the structural findings for the catalysts employed.

Zwitterionic Metallocenes

“…Although further quantitative work is required, there is evidence [16] that the interactions in the ring zwitterions are weaker than those measured for conventional metallocenes. [18] Zwitterion 9 is a highly active single-component ethylene polymerization catalyst, comparable to its closest nonzwitterionic analogue.…”

“…[16] In the solid state, two types of intramolecular ion ± ion contacts occur in tuck-in/ B(C 6 F 5 ) 3 -generated zwitterions. The factors which differentiate between the preferred mode of stabilization are subtle, as illustrated by the chemistry of the tuck-in phenyl/B(C 6 F 5 ) 3 system (Scheme 8).…”

Zwitterionic complexes arising from the reaction of tucked-in titanocenes with tris(pentafluorophenyl)borane

“…Hence, the bonding interaction of this agostic hydrogen to titanium induces the large bending of C106 toward titanium and coordinatively and electronically saturates the Ti(IV) ion so that the Ti-F interaction known for complex 1 is absent. A more electropositive zirconium, however, showed agostic interactions with both the ortho-proton of the phenyl group and proton (s) [20]. The pentafluorophenyl groups in 6 are inequivalent with respect to the methylene group as follows from different valence angles at the boron atom (see Table 1).…”

Reactions of Titanium and Zirconium Derivatives of Bis(trimethylsilyl)acetylene with Tris(pentafluorophenyl)borane: A Titanium(III) Complex of an Alkynylboranate

“…If solutions of 1 are heated to 100 °C, the alkyne is eliminated and one BC bond is cleaved with a subsequent transfer of a pentafluorophenyl group from boron to zirconium, whereupon complex 2 with one μ‐H atom between boron and the zirconium center is formed (Scheme ) Such a transfer of C 6 F 5 from boron to zirconium10b is known, as is the μ‐H atom type of structure 10c. It is possible to describe compound 2 either as Zr IV σ‐C 6 F 5 hydrido complex with an electroneutral borane ligand or alternatively as a zwitterionic betaine‐boranate.…”