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>)

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)

Fast “Wittig-Like” Reactions As a Consequence of the Inorganic Enamine Effect

The tungsten alkylidyne [CF₃–ONO]­WCC­(CH₃)₃(THF)₂ (<b>3</b>) {where CF₃–ONO = (MeC₆H₃[C­(CF₃)₂O])₂N^3–} supported by a trianionic pincer-type ligand demonstrates enhanced nucleophilicity in unusually fast “Wittig-like” reactions. Experiments are designed to provide support for an inorganic enamine effect that is the origin of the enhanced nucleophilicity. Treating complex <b>3</b> with various carbonyl-containing substrates provides tungsten-oxo-vinyl complexes upon oxygen atom transfer. The rates of reactivity of <b>3</b> are compared with the known alkylidyne (DIPP)₃WCC­(CH₃)₃ (DIPP = 2,6-diisopropylphenoxide). In all cases (except acetone), complex <b>3</b> exhibits significantly faster overall rates than (DIPP)₃WCC­(CH₃)₃. New oxo-vinyl complexes are characterized by NMR, combustion analysis and single crystal X-ray diffraction. Treating <b>3</b> with acid chlorides provides the tungsten oxo chloride species [CF₃–ONO]­W­(O)Cl (<b>4</b>) and disubstituted alkynes. In the case of acetone the oxo-vinyl complex results in two rotational isomers <b>10</b>_{<i><b>syn</b></i>} and <b>10</b>_{<i><b>anti</b></i>.} The rate of isomerization was determined for the forward and reverse directions and was complimented with DFT calculations

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

Solid State Collapse of a High-Spin Square-Planar Fe(II) Complex, Solution Phase Dynamics, and Electronic Structure Characterization of an Fe(II)₂ Dimer

Square-planar high-spin Fe­(II) molecular compounds are rare, and until recently, the only four examples of non-macrocyclic or sterically driven molecular compounds of this kind shared a common FeO₄ core. The trianionic pincer-type ligand [CF₃-ONO]­H₃ (<b>1</b>) supports the high-spin square-planar Fe­(II) complex {[CF₃-ONO]­FeCl}­{Li­(Sv)₂}₂ (<b>2</b>). In the solid state, <b>2</b> forms the dimer complex {[CF₃-ONO]­Fe}₂­{(μ-Cl)₂­(μ-LiTHF)₄} (<b>3</b>) in 96% yield by simply applying a vacuum or stirring it with pentane for 2 h. A detailed high-frequency electron paramagnetic resonance and field-dependent ⁵⁷Fe Mössbauer investigation of <b>3</b> revealed a weak antiferromagnetic exchange interaction between the local iron spins which exhibit a zero-field splitting tensor characterized by negative <i>D</i> parameter. In solution, <b>2</b> is in equilibrium with the solvento complex {[CF₃-ONO]­FeCl­(THF)}­{Li₂(Sv)₄} (<b>2·Sv</b>) and the dimer <b>3</b>. A combination of frozen solution ⁵⁷Fe Mössbauer spectroscopy and single crystal X-ray crystallography helped elucidate the solvent dependent equilibrium between these three species. The oxidation chemistry of <b>2·Sv</b> was investigated. Complex <b>2</b> reacts readily with the one-electron oxidizing agent CuCl₂ to give the Fe­(III) complex {[CF₃-ONO]­FeCl₂}­{Li­(THF)₂}₂ (<b>4</b>). Also, <b>2·Sv</b> reacts with 2 equiv of TlPF₆ to form the Fe­(III) complex [CF₃-ONO]­Fe­(THF)₃ (<b>5</b>)

Solid State Collapse of a High-Spin Square-Planar Fe(II) Complex, Solution Phase Dynamics, and Electronic Structure Characterization of an Fe(II)₂ Dimer

Square-planar high-spin Fe­(II) molecular compounds are rare, and until recently, the only four examples of non-macrocyclic or sterically driven molecular compounds of this kind shared a common FeO₄ core. The trianionic pincer-type ligand [CF₃-ONO]­H₃ (<b>1</b>) supports the high-spin square-planar Fe­(II) complex {[CF₃-ONO]­FeCl}­{Li­(Sv)₂}₂ (<b>2</b>). In the solid state, <b>2</b> forms the dimer complex {[CF₃-ONO]­Fe}₂­{(μ-Cl)₂­(μ-LiTHF)₄} (<b>3</b>) in 96% yield by simply applying a vacuum or stirring it with pentane for 2 h. A detailed high-frequency electron paramagnetic resonance and field-dependent ⁵⁷Fe Mössbauer investigation of <b>3</b> revealed a weak antiferromagnetic exchange interaction between the local iron spins which exhibit a zero-field splitting tensor characterized by negative <i>D</i> parameter. In solution, <b>2</b> is in equilibrium with the solvento complex {[CF₃-ONO]­FeCl­(THF)}­{Li₂(Sv)₄} (<b>2·Sv</b>) and the dimer <b>3</b>. A combination of frozen solution ⁵⁷Fe Mössbauer spectroscopy and single crystal X-ray crystallography helped elucidate the solvent dependent equilibrium between these three species. The oxidation chemistry of <b>2·Sv</b> was investigated. Complex <b>2</b> reacts readily with the one-electron oxidizing agent CuCl₂ to give the Fe­(III) complex {[CF₃-ONO]­FeCl₂}­{Li­(THF)₂}₂ (<b>4</b>). Also, <b>2·Sv</b> reacts with 2 equiv of TlPF₆ to form the Fe­(III) complex [CF₃-ONO]­Fe­(THF)₃ (<b>5</b>)

An OCO^3– Trianionic Pincer Tungsten(VI) Alkylidyne: Rational Design of a Highly Active Alkyne Polymerization Catalyst

Synthesis, characterization, and catalytic alkyne polymerization results for the first trianionic pincer alkylidyne complex, [^<i>t</i>BuOCO]­WCC­(CH₃)₃(THF)₂ (<b>6</b>), are described. Complex <b>6</b> is a highly active catalyst for the polymerization of acetylenes and exhibits a high turnover number (4371), activity (1.05 × 10⁶ g_PPA mol_cat^–1 h^–1), and yield (87%) for the polymerization of 1-ethynyl-4-fluorobenzene

New Alkylidyne Complexes Featuring a Flexible Trianionic ONO^3– Pincer-Type Ligand: Inorganic Enamine Effect versus Sterics in Electrophilic Additions

Metal–carbon multiple bonds exhibit enhanced nucleophilicity at the α carbon within ONO trianionic pincer alkylidyne complexes. Defined as the <i>inorganic enamine effect</i>, this phenomenon is a result of the overlap of the N atom lone pair within the ONO^3– ligand and a π bond from the metal–carbon multiple bond. Treating the proligand [O^CH2N^CH2O]­H₃ (<b>2</b>) with (^<i>t</i>BuO)₃WCR (where R = Et, ^<i>t</i>Bu) results in the formation of the dianionic pincer complexes [O^CH2NH^CH2O]­WCR­(O^<i>t</i>Bu) (where R = Et (<b>3-Et</b>), ^<i>t</i>Bu (<b>3-</b>^{<i><b>t</b></i>}<b>Bu</b>_{<i><b>anti</b></i>})). Deprotonation of <b>3-</b>^{<i><b>t</b></i>}<b>Bu</b>_{<i><b>anti</b></i>} by treatment with Ph₃PCH₂ forms the anionic alkylidyne complex {[O^CH2N^CH2O]­WC^<i>t</i>Bu­(O^<i>t</i>Bu)}­{CH₃PPh₃} (<b>4-</b>^{<i><b>t</b></i>}<b>Bu</b>). DFT calculations modeling <b>4-</b>^{<i><b>t</b></i>}<b>Bu</b> reveal overlap of the N atom lone pair with a WC π bond. However, <b>4-</b>^{<i><b>t</b></i>}<b>Bu</b> reacts with electrophiles preferentially at the pincer N atom as opposed to the WC_α group. Multinuclear NMR spectroscopy, combustion analysis, and single-crystal X-ray crystallography are employed to characterize complexes <b>3-Et</b>, <b>3-</b>^{<i><b>t</b></i>}<b>Bu</b>_{<i><b>anti</b></i>}, and <b>4-</b>^{<i><b>t</b></i>}<b>Bu</b>

Highly Tactic Cyclic Polynorbornene: Stereoselective Ring Expansion Metathesis Polymerization of Norbornene Catalyzed by a New Tethered Tungsten-Alkylidene Catalyst

The tungsten alkylidyne [^<i>t</i>BuOCO]­WC­(^<i>t</i>Bu) (THF)₂ (<b>1</b>) reacts with CO₂, leading to complete cleavage of one CO bond, followed by migratory insertion to generate the tungsten-oxo alkylidene <b>2</b>. Complex <b>2</b> is the first catalyst to polymerize norbornene via ring expansion metathesis polymerization to yield highly <i>cis</i>-syndiotactic cyclic polynorbornene

Introducing “Ynene” Metathesis: Ring-Expansion Metathesis Polymerization Leads to Highly Cis and Syndiotactic Cyclic Polymers of Norbornene

Tungsten alkylidynes [CF₃–ONO]­WCC­(CH₃)₃(THF)₂ (<b>1</b>) and [^<i>t</i>BuOCO]­WCC­(CH₃)₃(THF)₂ (<b>3</b>) react with ethylene. Complex <b>1</b> reacts reversibly with ethylene to give the metallacyclobutene (<b>2</b>). Complex <b>3</b> reacts with ethylene to form the tethered alkylidene (<b>4</b>) featuring a tetraanionic pincer ligand. Complexes <b>1</b> and <b>3</b> initiate the polymerization of norbornene at room temperature. The polymerization of norbornene by <b>1</b> is not stereoselective, whereas <b>3</b> generates a highly cis and syndiotactic cyclic polynorbornene. Comparison of the intrinsic viscosity, radius of gyration, and elution time of the synthesized cyclic polynorbornene with those of linear analogues provides conclusive evidence for a cyclic topology