Carr–Purcell Pulsed Electron Double Resonance with Shaped Inversion Pulses

Pulsed electron paramagnetic resonance (EPR) spectroscopy allows the determination of distances, in the range of 1.5–8 nm, between two spin-labels attached to macromolecules containing protons. Unfortunately, for hydrophobic lipid-bound or detergent-solubilized membrane proteins, the maximum distance accessible is much lower, because of a strongly reduced coherence time of the electron spins. Here we introduce a pulse sequence, based on a Carr–Purcell decoupling scheme on the observer spin, where each π-pulse is accompanied by a shaped sech/tanh inversion pulse applied to the second spin, to overcome this limitation. This pump/probe excitation scheme efficiently recouples the dipolar interaction, allowing a substantially longer observation time window to be achieved. This increases the upper limit and accuracy of distances that can be determined in membrane protein complexes. We validated the method on a bis-nitroxide model compound and applied this technique to the trimeric betaine transporter <i>BetP</i>. Interprotomer distances as long as 6 nm could be reliably determined, which is impossible with the existing methods

A Preorganized Ditopic Borane as Highly Efficient One- or Two-Electron Trap

Reduction of the bis­(9-borafluorenyl)­methane <b>1</b> with excess lithium furnishes the red dianion salt Li₂[<b>1</b>]. The corresponding dark green monoanion radical Li­[<b>1</b>] is accessible through the comproportionation reaction between <b>1</b> and Li₂[<b>1</b>]. EPR spectroscopy on Li­[<b>1</b>] reveals hyperfine coupling of the unpaired electron to two magnetically equivalent boron nuclei (<i>a</i>(¹¹B) = 5.1 ± 0.1 G, <i>a</i>(¹⁰B) = 1.7 ± 0.2 G). Further coupling is observed to the unique B–C<i>H</i>–B bridgehead proton (<i>a</i>(¹H) = 7.2 ± 0.2 G) and to eight aromatic protons (<i>a</i>(¹H) = 1.4 ± 0.1 G). According to X-ray crystallography, the B···B distances continuously decrease along the sequence <b>1</b> → [<b>1</b>]^•– → [<b>1</b>]^2– with values of 2.534(2), 2.166(4), and 1.906(3) Å, respectively. Protonation of Li₂[<b>1</b>] leads to the cyclic borohydride species Li­[<b>1H</b>] featuring a B–H–B two-electron-three-center bond. This result strongly indicates a nucleophilic character of the boron atoms; the reaction can also be viewed as rare example of the protonation of an element–element σ bond. According to NMR spectroscopy, EPR spectroscopy, and quantum-chemical calculations, [<b>1</b>]^2– represents a closed-shell singlet without any spin contamination. Detailed wave function analyses of [<b>1</b>]^•– and [<b>1</b>]^2– reveal strongly localized interactions of the two boron p_<i>z</i>-type orbitals, with small delocalized contributions of the 9-borafluorenyl π systems. Overall, our results provide evidence for a direct B–B one-electron and two-electron bonding interaction in [<b>1</b>]^•– and [<b>1</b>]^2–, respectively

Exhaustively Trichlorosilylated C₁ and C₂ Building Blocks: Beyond the Müller–Rochow Direct Process

The Cl^–-induced heterolysis of the Si–Si bond in Si₂Cl₆ generates an [SiCl₃]⁻ ion as reactive intermediate. When carried out in the presence of CCl₄ or Cl₂CCCl₂ (CH₂Cl₂ solutions, room temperature or below), the reaction furnishes the monocarbanion [C­(SiCl₃)₃]⁻ ([<b>A</b>]⁻; 92%) or the vicinal dianion [(Cl₃Si)₂C–C­(SiCl₃)₂]^2– ([<b>B</b>]^2–; 85%) in excellent yields. Starting from [<b>B</b>]^2–, the tetrasilylethane (Cl₃Si)₂(H)­C–C­(H)­(SiCl₃)₂ (H₂<b>B</b>) and the tetrasilylethene (Cl₃Si)₂CC­(SiCl₃)₂ (<b>B</b>; 96%) are readily available through protonation (CF₃SO₃H) or oxidation (CuCl₂), respectively. Equimolar mixtures of H₂<b>B</b>/[<b>B</b>]^2– or <b>B</b>/[<b>B</b>]^2– quantitatively produce 2 equiv of the monoanion [H<b>B</b>]⁻ or the blue radical anion [<b>B</b>^<b>•</b>]⁻, respectively. Treatment of <b>B</b> with Cl^– ions in the presence of CuCl₂ furnishes the disilylethyne Cl₃SiCCSiCl₃ (<b>C</b>; 80%); in the presence of [HMe₃N]­Cl, the trisilylethene (Cl₃Si)₂CC­(H)­SiCl₃ (<b>D</b>; 72%) is obtained. Alkyne <b>C</b> undergoes a [4+2]-cycloaddition reaction with 2,3-dimethyl-1,3-butadiene (CH₂Cl₂, 50 °C, 3d) and thus provides access to 1,2-bis­(trichlorosilyl)-4,5-dimethylbenzene (<b>E1</b>; 80%) after oxidation with DDQ. The corresponding 1,2-bis­(trichlorosilyl)-3,4,5,6-tetraphenylbenzene (<b>E2</b>; 83%) was prepared from <b>C</b> and 2,3,4,5-tetraphenyl-2,4-cyclopentadien-1-one under CO extrusion at elevated temperatures (CH₂Cl₂, 180 °C, 4 d). All closed-shell products were characterized by ¹H, ¹³C­{¹H}, and ²⁹Si NMR spectroscopy; an EPR spectrum of [<i>n</i>Bu₄N]­[<b>B</b>^<b>•</b>] was recorded. The molecular structures of [<i>n</i>Bu₄N]­[<b>A</b>], [<i>n</i>Bu₄N]₂[<b>B</b>], <b>B</b>, <b>E1</b>, and <b>E2</b> were further confirmed by single-crystal X-ray diffraction. On the basis of detailed experimental investigations, augmented by quantum-chemical calculations, plausible reaction mechanisms for the formation of [<b>A</b>]⁻, [<b>B</b>]^2–, <b>C</b>, and <b>D</b> are postulated

Exhaustively Trichlorosilylated C₁ and C₂ Building Blocks: Beyond the Müller–Rochow Direct Process

The Cl^–-induced heterolysis of the Si–Si bond in Si₂Cl₆ generates an [SiCl₃]⁻ ion as reactive intermediate. When carried out in the presence of CCl₄ or Cl₂CCCl₂ (CH₂Cl₂ solutions, room temperature or below), the reaction furnishes the monocarbanion [C­(SiCl₃)₃]⁻ ([<b>A</b>]⁻; 92%) or the vicinal dianion [(Cl₃Si)₂C–C­(SiCl₃)₂]^2– ([<b>B</b>]^2–; 85%) in excellent yields. Starting from [<b>B</b>]^2–, the tetrasilylethane (Cl₃Si)₂(H)­C–C­(H)­(SiCl₃)₂ (H₂<b>B</b>) and the tetrasilylethene (Cl₃Si)₂CC­(SiCl₃)₂ (<b>B</b>; 96%) are readily available through protonation (CF₃SO₃H) or oxidation (CuCl₂), respectively. Equimolar mixtures of H₂<b>B</b>/[<b>B</b>]^2– or <b>B</b>/[<b>B</b>]^2– quantitatively produce 2 equiv of the monoanion [H<b>B</b>]⁻ or the blue radical anion [<b>B</b>^<b>•</b>]⁻, respectively. Treatment of <b>B</b> with Cl^– ions in the presence of CuCl₂ furnishes the disilylethyne Cl₃SiCCSiCl₃ (<b>C</b>; 80%); in the presence of [HMe₃N]­Cl, the trisilylethene (Cl₃Si)₂CC­(H)­SiCl₃ (<b>D</b>; 72%) is obtained. Alkyne <b>C</b> undergoes a [4+2]-cycloaddition reaction with 2,3-dimethyl-1,3-butadiene (CH₂Cl₂, 50 °C, 3d) and thus provides access to 1,2-bis­(trichlorosilyl)-4,5-dimethylbenzene (<b>E1</b>; 80%) after oxidation with DDQ. The corresponding 1,2-bis­(trichlorosilyl)-3,4,5,6-tetraphenylbenzene (<b>E2</b>; 83%) was prepared from <b>C</b> and 2,3,4,5-tetraphenyl-2,4-cyclopentadien-1-one under CO extrusion at elevated temperatures (CH₂Cl₂, 180 °C, 4 d). All closed-shell products were characterized by ¹H, ¹³C­{¹H}, and ²⁹Si NMR spectroscopy; an EPR spectrum of [<i>n</i>Bu₄N]­[<b>B</b>^<b>•</b>] was recorded. The molecular structures of [<i>n</i>Bu₄N]­[<b>A</b>], [<i>n</i>Bu₄N]₂[<b>B</b>], <b>B</b>, <b>E1</b>, and <b>E2</b> were further confirmed by single-crystal X-ray diffraction. On the basis of detailed experimental investigations, augmented by quantum-chemical calculations, plausible reaction mechanisms for the formation of [<b>A</b>]⁻, [<b>B</b>]^2–, <b>C</b>, and <b>D</b> are postulated