CH Bond Activation of Methane by a Transient η²‑Cyclopropene/Metallabicyclobutane Complex of Niobium

This study challenges the problem of the activation of a CH bond of methane by soluble transition metal complexes. High pressure solution NMR, isotopic labeling studies, and kinetic analyses of the degenerate exchange of methane in the methyl complex [Tp^Me2NbCH₃(<i>c</i>-C₃H₅)­(MeCCMe)] (<b>1</b>) are reported. Stoichiometric methane activation by the mesitylene complex [Tp^Me2Nb­(CH₂-3,5-C₆H₃Me₂)­(<i>c</i>-C₃H₅) (MeCCMe)] (<b>2</b>) giving <b>1</b> is also realized. Evidence is provided that these reactions proceed via an intramolecular abstraction of a β-H of the cyclopropyl group to form either methane or mesitylene from <b>1</b> or <b>2</b>, respectively, yielding the transient unsaturated η²-cyclopropene/metallabicyclobutane intermediate [Tp^Me2Nb­(η²-<i>c</i>-C₃H₄) (MeCCMe)] <b>A</b>. This is followed by its mechanistic reverse 1,3-CH bond addition of methane yielding the product

Highly Fluorinated Tris(indazolyl)borate Hydrocarbyl Complexes of Calcium and Magnesium: Synthesis and Structural Studies

Heteroleptic phenylacetylide complexes [{F₁₂-Tp^4Bo,3Ph}­Ae­(CCPh)]_<i>x</i> of calcium (Ae = Ca, <i>x</i> = 2; <b>2</b>) and magnesium (Ae = Mg, <i>x</i> = 1; <b>4</b>) containing the highly fluorinated 3-phenyl hydrotris­(indazolyl)­borate {F₁₂-Tp^4Bo,3Ph}⁻ ligand have been synthesized by acid–base reactions between the corresponding silylamido derivatives [{F₁₂-Tp^4Bo,3Ph}­Ae­{N­(SiRMe₂)₂}] (R = Me, Ae = Ca (<b>1</b>); R = H, Ae = Mg (<b>3</b>)) and PhCCH. Compounds <b>2</b> and <b>4</b> have been characterized by NMR spectroscopy and X-ray diffraction analysis. <b>2</b> crystallizes as a dinuclear complex, showing two nonsymmetrical “side-on” (π-type) interactions between the acetylide units and the Ca centers, whereas <b>4</b> crystallizes as a mononuclear complex, displaying a four-coordinate magnesium. The molecular structure of the complex [{F₁₂-Tp^4Bo,3Ph}­Mg­{N­(SiMe₂H)₂}] (<b>3</b>), obtained by the salt metathesis reaction between [Mg­{N­(SiMe₂H)₂}₂] and [Tl­{F₁₂-Tp^4Bo,3Ph}], is also reported. <b>3</b> is also four-coordinate and exhibits a Mg···β-Si–H agostic distortion. The synthesis and in situ characterization of the heteroleptic alkyl complex [{F₁₂-Tp^4Bo,3Ph}­Ca­{CH­(SiMe₃)₂}­(THF)] (<b>5</b>) is also reported, although attempts to isolate this compound failed due to its extreme sensitivity to temperature

CH Bond Activation of Unsaturated Hydrocarbons by a Niobium Methyl Cyclopropyl Precursor. Cyclopropyl Ring Opening and Alkyne Coupling Reaction

The transient intermediate η²-cyclopropene/bicyclobutane niobium complex [Tp^Me2Nb­(η²-<i>c</i>-C₃H₄)­(MeCCMe)] <b>A</b>, generated by an intramolecular β-H abstraction of methane from the methyl cyclopropyl complex [Tp^Me2NbMe­(<i>c</i>-C₃H₅)­(MeCCMe)] (<b>1</b>), is able to cleave the CH bond of a variety of unsaturated hydrocarbons RH in a selective manner to give the corresponding hydrocarbyl complexes [Tp^Me2NbR­(<i>c</i>-C₃H₅)­(MeCCMe)] (R = 2-furyl, 2-thienyl, 1-alkynyl, 1-cyclopentenyl, 1-ferrocenyl (Fc), pentafluorophenyl). The activation of the C–H bond occurs stereospecifically via a 1,3-CH addition across the Nb­(η²-cyclopropene) bond of <b>A</b>. Full characterization of several of these complexes includes multinuclear NMR spectroscopy, X-ray diffraction, UV/vis spectroscopy, and electrochemical data. A charge transfer between the ferrocenyl moiety and the niobium center is responsible for the characteristic purple color of the bimetallic complex [Tp^Me2NbFc­(<i>c</i>-C₃H₅)­(MeCCMe)]. The reactivity of these complexes with benzene follows qualitatively the strength and the p<i>K</i>_a of the CH bond that is cleaved. The pentafluorophenyl complex [Tp^Me2Nb­(C₆F₅)­(<i>c</i>-C₃H₅)­(MeCCMe)] undergoes cyclopropyl ring opening and alkyne coupling to give two isomeric η⁴-butadienyl complexes, with [Tp^Me2Nb­(C₆F₅)­(η⁴-CMeCMeCHCHMe)] as the major isomer

Highly Fluorinated Aryl-Substituted Tris(indazolyl)borate Thallium Complexes: Diverse Regiochemistry at the B–N Bond

The synthesis and characterization (mainly by ¹⁹F NMR and X-ray diffraction) of highly fluorinated aryl-4,5,6,7-tetrafluoroindazoles and their corresponding thallium hydrotris­(indazolyl)­borate complexes are reported [aryl = phenyl, pentafluorophenyl, 3,5-dimethylphenyl, 3,5-bis­(trifluoromethyl)­phenyl]. Thanks to N–H···N hydrogen bonds, the indazoles crystallize as dimers that pack differently depending on the nature of the aryl group. The thallium hydrotris­(indazolyl)­borate complexes Tl­[Fn-Tp^4Bo,3aryl] resulting from the reaction of aryl-4,5,6,7-tetrafluoroindazoles [aryl = phenyl, 3,5-dimethylphenyl, 3,5-bis­(trifluoromethyl)­phenyl] with thallium borohydride adopt overall <i>C</i>_3<i>v</i> symmetry with the indazolyl groups bound to boron via their N-1 nitrogen in a conventional manner. When the perfluorinated pentaphenyl-4,5,6,7-tetrafluoroindazole is reacted with thallium borohydride, a single regioisomer of <i>C</i>_<i>s</i> symmetry having one indazolyl ring bound to boron via its N-2 nitrogen, TlHB­(3-pentafluorophenyl-4,5,6,7-tetrafluoroindazol-1-yl)₂(3-pentafluorophenyl-4,5,6,7-tetrafluoroindazol-2-yl) Tl­[F27-Tp^(4Bo,3C6F5)*], is obtained for the first time. Surprisingly, the perfluorinated dihydrobis­(indazolyl)­borate complex Tl­[F₁₈-Bp^3Bo,3C6F5], an intermediate on the way to the hydrotris­(indazolyl)­borate complex, has <i>C</i>_<i>s</i> symmetry with two indazolyl rings bound to boron via N-2. The distortion of the coordination sphere around Tl and the arrangement of the complexes in the crystal are discussed

Highly Fluorinated Aryl-Substituted Tris(indazolyl)borate Thallium Complexes: Diverse Regiochemistry at the B–N Bond

The synthesis and characterization (mainly by ¹⁹F NMR and X-ray diffraction) of highly fluorinated aryl-4,5,6,7-tetrafluoroindazoles and their corresponding thallium hydrotris­(indazolyl)­borate complexes are reported [aryl = phenyl, pentafluorophenyl, 3,5-dimethylphenyl, 3,5-bis­(trifluoromethyl)­phenyl]. Thanks to N–H···N hydrogen bonds, the indazoles crystallize as dimers that pack differently depending on the nature of the aryl group. The thallium hydrotris­(indazolyl)­borate complexes Tl­[Fn-Tp^4Bo,3aryl] resulting from the reaction of aryl-4,5,6,7-tetrafluoroindazoles [aryl = phenyl, 3,5-dimethylphenyl, 3,5-bis­(trifluoromethyl)­phenyl] with thallium borohydride adopt overall <i>C</i>_3<i>v</i> symmetry with the indazolyl groups bound to boron via their N-1 nitrogen in a conventional manner. When the perfluorinated pentaphenyl-4,5,6,7-tetrafluoroindazole is reacted with thallium borohydride, a single regioisomer of <i>C</i>_<i>s</i> symmetry having one indazolyl ring bound to boron via its N-2 nitrogen, TlHB­(3-pentafluorophenyl-4,5,6,7-tetrafluoroindazol-1-yl)₂(3-pentafluorophenyl-4,5,6,7-tetrafluoroindazol-2-yl) Tl­[F27-Tp^(4Bo,3C6F5)*], is obtained for the first time. Surprisingly, the perfluorinated dihydrobis­(indazolyl)­borate complex Tl­[F₁₈-Bp^3Bo,3C6F5], an intermediate on the way to the hydrotris­(indazolyl)­borate complex, has <i>C</i>_<i>s</i> symmetry with two indazolyl rings bound to boron via N-2. The distortion of the coordination sphere around Tl and the arrangement of the complexes in the crystal are discussed

β‑H Abstraction/1,3‑CH Bond Addition as a Mechanism for the Activation of CH Bonds at Early Transition Metal Centers

This article describes the generalization of an overlooked mechanism for CH bond activation at early transition metal centers, namely 1,3‑CH bond addition at an η²-alkene intermediate. The X-ray-characterized [Cp₂Zr­(<i>c</i>-C₃H₅)₂] eliminates cyclo­propane by a β‑H abstraction reaction to generate the transient η²-cyclo­propene [Cp₂Zr­(η²-<i>c</i>-C₃H₄)] intermediate <b>A</b>. <b>A</b> rapidly cleaves the CH bond of furan and thiophene to give the furyl and thienyl complexes [Cp₂Zr­(<i>c</i>-C₃H₅)­(2-C₄H₃X)] (X = O, S), respectively. Benzene is less cleanly activated. Mechanistic investigations including kinetic studies, isotope labeling, and DFT computation of the reaction profile all confirm that rapid stereo­specific 1,3‑CH bond addition across the Zr­(η²-alkene) bond of <b>A</b> follows the rate-determining β‑H abstraction reaction. DFT computations also suggest that an α‑CC agostic rotamer of [Cp₂Zr­(<i>c</i>-C₃H₅)₂] assists the β‑H abstraction of cyclo­propane. The nature of the α‑CC agostic interaction is discussed in the light of an NBO analysis

β‑H Abstraction/1,3‑CH Bond Addition as a Mechanism for the Activation of CH Bonds at Early Transition Metal Centers

This article describes the generalization of an overlooked mechanism for CH bond activation at early transition metal centers, namely 1,3‑CH bond addition at an η²-alkene intermediate. The X-ray-characterized [Cp₂Zr­(<i>c</i>-C₃H₅)₂] eliminates cyclo­propane by a β‑H abstraction reaction to generate the transient η²-cyclo­propene [Cp₂Zr­(η²-<i>c</i>-C₃H₄)] intermediate <b>A</b>. <b>A</b> rapidly cleaves the CH bond of furan and thiophene to give the furyl and thienyl complexes [Cp₂Zr­(<i>c</i>-C₃H₅)­(2-C₄H₃X)] (X = O, S), respectively. Benzene is less cleanly activated. Mechanistic investigations including kinetic studies, isotope labeling, and DFT computation of the reaction profile all confirm that rapid stereo­specific 1,3‑CH bond addition across the Zr­(η²-alkene) bond of <b>A</b> follows the rate-determining β‑H abstraction reaction. DFT computations also suggest that an α‑CC agostic rotamer of [Cp₂Zr­(<i>c</i>-C₃H₅)₂] assists the β‑H abstraction of cyclo­propane. The nature of the α‑CC agostic interaction is discussed in the light of an NBO analysis