Insights into tungsten catalyzed ring expansion polymerization

The pincer tungsten (formal IV) alkylidene complex depicted in Figure 1 is a potent catalyst for ring-expansion polymerization of alkynes, e.g. reaching TONs of > 17,000 and activities in excess of 5,000 kg mol-1 h-1 (with phenylactylene) and degrees of polymerization, Pn, of up to 1000-2000 (with propene and 1-decene).1-4 Monomer dependent polymer dispersity and the known sensitivity of the system to changes in the alkylidene R-group hint at the complexity of the polymerization mechanism. Several mechanistic pathways are possible, including an initial insertion of the monomer into the catalyst backbone tungsten-aryl bond, which for example has been noted to lead to the active species in Hf(IV) pyridylamido catalyzed olefin polymerization.5 The key question is how the catalyst avoids unproductive benzene formation. In this regard, the flexibility of the alkylidene ligand to change from being part of the backbone to being part of the growing ring appears crucial. Using a combination of DFT mechanism exploration and experimental insight we will highlight recent developments

Isolation of an Elusive Phosphametallacyclobutadiene and Its Role in Reversible Carbon-Carbon Bond Cleavage

The reactivity of phosphaalkynes, the isolobal and isoelectronic congeners to alkynes, with metal alkylidyne complexes is explored in this work. Treating the tungsten alkylidyne [t BuOCO]W≡Ct Bu(THF)2 (1) with phosphaalkyne (10) results in the formation of [O2 C(t BuC=)W{η2 -(P,C)-P≡C-Ad}(THF)] (13-t BuTHF ) and [O2 C(AdC=)W{η2 -(P,C)-P≡C-t Bu}(THF)] (13-AdTHF ); derived from the formal reductive migratory insertion of the alkylidyne moiety into a W-Carene bond. Analogous to alkyne metathesis, a stable phosphametallacyclobutadiene complex [t BuOCO]W[κ2 -C(t Bu)PC(Ad)] (14) forms upon loss of THF from the coordination sphere of either 13-t BuTHF or 13-AdTHF . Remarkably, the C-C bonds reversibly form/cleave with the addition or removal of THF from the coordination sphere of the formal tungsten(VI) metal center, permitting unprecedented control over the transformation of a tetraanionic pincer to a trianionic pincer and back. Computational analysis offers thermodynamic and electronic reasoning for the reversible equilibrium between 13-t Bu/AdTHF and 14

A New ONO^3‑ Trianionic Pincer-Type Ligand for Generating Highly Nucleophilic Metal–Carbon Multiple Bonds

Appending an amine to a CC double bond drastically increases the nucleophilicity of the β-carbon atom of the alkene to form an enamine. In this report, we present the synthesis and characterization of a novel CF₃–ONO^3‑ trianionic pincer-type ligand, rationally designed to mimic enamines within a metal coordination sphere. Presented is a synthetic strategy to create enhanced nucleophilic tungsten–alkylidene and −alkylidyne complexes. Specifically, we present the synthesis and characterization of the new CF₃–ONO^3‑ trianionic pincer tungsten–alkylidene [CF₃–ONO]­WCH­(Et)­(O^<i>t</i>Bu) (<b>2</b>) and −alkylidyne {MePPh₃}­{[CF₃–ONO]­WC­(Et)­(O^<i>t</i>Bu)} (<b>3</b>) complexes. Characterization involves a combination of multinuclear NMR spectroscopy, combustion analysis, DFT computations, and single crystal X-ray analysis for complexes <b>2</b> and <b>3</b>. Exhibiting unique nucleophilic reactivity, <b>3</b> reacts with MeOTf to yield [CF₃–ONO]­WC­(Me)­(Et)­(O^<i>t</i>Bu) (<b>4</b>), but the bulkier Me₃SiOTf silylates the <i>tert</i>-butoxide, which subsequently undergoes isobutylene expulsion to form [CF₃–ONO]­WCH­(Et)­(OSiMe₃) (<b>5</b>). A DFT calculation performed on a model complex of <b>3</b>, namely, [CF₃–ONO]­WC­(Et)­(O^<i>t</i>Bu) (<b>3</b>′), reveals the amide participates in an enamine-type bonding combination. For complex <b>2</b>, the Lewis acids MeOTf, Me₃SiOTf, and B­(C₆F₅)₃ catalyze isobutylene expulsion to yield the tungsten–oxo complex [CF₃–ONO]­W­(O)­(^<i>n</i>Pr) (<b>6</b>)

Influence of solvent on cyclic polynorbornene tacticity

Tacticity is critical to polymer properties

Deoxygenations of (silox)₃WNO and R₃PO by (silox)₃M (M = V, Ta) and (silox)₃NbL (silox = ᵗBu₃SiO): Consequences of Electronic Effects

Article discussing deoxygenations of (silox)3 WNO and R3PO by (silox)3M (M = V, Ta) and (silox)3NbL (silox = tBu3SiO) and consequences of electronic effects

Synthesis and Characterization of Group 4 Trianionic ONO^3– Pincer-Type Ligand Complexes and a Rare Case of Through-Space ¹⁹F–¹⁹F Coupling

This report describes the synthesis and characterization of a new series of group 4 complexes supported by a trianionic ONO^3– pincer-type ligand. Treating TiCl₄ with the proligand [CF₃–ONO]­H₃ (<b>1</b>) and NEt₃ in benzene afforded {[CF₃–ONO]­TiCl₃}­{HNEt₃}₂ (<b>2</b>). By means of a lithium transmetalation route, the neutral monochloride complex [CF₃–ONO]­TiCl­(THF) (<b>3</b>) was synthesized in 91% yield. The analogous Hf­(IV) derivative could not be obtained using this method. Instead, transmetalation with thallium­(I) resulted in the formation of the seven-coordinate complex [CF₃–ONHO]­HfCl₂(THF)₂ (<b>4-(THF)</b>_<b>2</b>), which was characterized by combustion analysis and X-ray crystallography. Applying vacuum to <b>4-(THF)</b>_<b>2</b> liberated the THF ligands to provide the five-coordinate THF-free complex [CF₃–ONHO]­HfCl₂ (<b>4</b>). Alkylation of complex <b>4</b> with alkyllithium or Grignard reagents resulted in a mixture of unidentifiable products. However, access to the neutral complex <b>3</b> enabled the subsequent preparation of organotitanium complexes [CF₃–ONO]­TiR­(THF) (<b>5-R</b>; R = Me, Bn, Mes). Single-crystal X-ray analysis of <b>5-Me</b> indicated that the organotitanium complexes are mononuclear. Single-crystal X-ray diffraction and NMR studies in solution confirmed that complex <b>5-Mes</b> exhibits rare through-space ¹⁹F–¹⁹F coupling (5 Hz)

Synthesis and Characterization of Tungsten Alkylidene and Alkylidyne Complexes Supported by a New Pyrrolide-Centered Trianionic ONO^3– Pincer-Type Ligand

Synthetic protocols for a pyrrolide-centered ONO^3– trianionic pincer-type ligand are presented. Treating (^<i>t</i>BuO)₃WC^<i>t</i>Bu with the proligand [pyr-ONO]­H₃ (<b>2</b>) results in the formation of the trianionic pincer alkylidene complex [pyr-ONO]­WCH^<i>t</i>Bu­(O^<i>t</i>Bu) (<b>3</b>). Addition of a mild base to complex <b>3</b> provides the trianionic pincer alkylidyne complex {MePPh₃}­{[pyr-ONO]­WC^<i>t</i>Bu­(O^<i>t</i>Bu)} (<b>4</b>). All new compounds were characterized by NMR spectroscopy, combustion analysis, and, in the case of complex <b>4</b>, single-crystal X-ray crystallography. DFT calculations performed on <b>4</b> provide insight into its electronic structure and indicate that the HOMO is ligand-based and localized on the pyrrolide π orbitals