Silanetriols as Powerful Starting Materials for Selective Condensation to Bulky POSS Cages

Controlled condensation reactions of tertiary silanetriols CH₃(CH₂)_<i>n</i>(CH₃)₂CSi­(OH)₃ (<b>1b</b>–<b>f</b>; <i>n</i> = 1–5) in the presence of trifluoroacetic acid and the hydrolysis of CH₃(CH₂)₆(CH₃)₂CSiCl₃ (<b>3</b>) lead to the selective formation of the corresponding disiloxane tetrols [CH₃(CH₂)_<i>n</i>(CH₃)₂CSi­(OH)₂]₂O (<b>2b</b>–<b>g</b>; <i>n</i> = 1–6) in good yields. The TBAF-driven condensation reactions of the silanetriols CH₃(CH₂)_<i>n</i>(CH₃)₂CSi­(OH)₃ (<b>1a</b>–<b>c</b>; <i>n</i> = 0–2) as well as of the disiloxane-1,1,3,3-tetrol <b>2d</b> (<i>n</i> = 3) yield in the selective formation of the first T₈ cages bearing tertiary carbon substituents, CH₃(CH₂)_<i>n</i>(CH₃)₂C (<b>4a</b>–<b>d</b>; <i>n</i> = 0–3), which was not possible via the condensation of their alkoxysilane counterparts so far. The resulting compounds <b>2b</b>–<b>g</b> and <b>4a</b>–<b>d</b> have been characterized by multinuclear NMR, MS, and single-crystal X-ray diffraction

A Structural Comparison of Organoterminated Selenide, Diselenide, and Triselenide

<div><p>GRAPHICAL ABSTRACT</p><p></p></div

Oxorhenium(V) Complexes with Phenolate–Pyrazole Ligands for Olefin Epoxidation Using Hydrogen Peroxide

Oxorhenium­(V) complexes of the general formula [ReOCl₂(PPh₃)­(L)] (<b>2a</b>–<b>c</b>) and [ReOCl­(L)₂] (<b>3a</b>–<b>c</b>) with L being monoanionic, bidentate phenolate–pyrazole ligands <b>1a</b>–<b>c</b> that bear substituents with various electronic features on the phenol ring (<b>1a</b> Br, <b>1b</b> NO₂, <b>1c</b> OMe) were prepared. The compounds are stable toward moisture and air, allowing them to be handled in a normal lab atmosphere. All complexes were fully characterized by spectroscopic means and, in the case of <b>2b</b>, <b>2c</b>, <b>3b</b>, and <b>3c</b>, also by single-crystal X-ray diffraction analyses. Electrochemical investigations by cyclic voltammetry of complexes <b>3a</b>–<b>c</b> showed a shift to more positive potentials for the Re­(V)/Re­(VI) redox couple in the order of <b>3b</b> > <b>3a</b> > <b>3c</b> (R= NO₂ > Br > OMe), reflecting the higher electrophilic character of the Re atom caused by the ligands <b>1a</b>–<b>c</b>. Complexes <b>2a</b>–<b>c</b> and <b>3a</b>–<b>c</b> display excellent catalytic activity in the epoxidation of cyclooctene, where all six complexes give quantitative conversions to the epoxide within 3 h if <i>tert</i>-butylhydroperoxide (TBHP) is employed as oxidant. Moreover, they represent rare examples of oxorhenium­(V) catalysts capable of using the green oxidant hydrogen peroxide, leading to high yields up to 74%. Also, green solvents such as diethylcarbonate can be used successfully in epoxidation reactions, albeit resulting in lower yields (up to 30%)

Oxidorhenium(V) Complexes with Tetradentate Iminophenolate Ligands: Influence of Ligand Flexibility on the Coordination Motif and Oxygen-Atom-Transfer Activity

The synthesis of oxidorhenium­(V) complexes <b>1</b>–<b>3</b> coordinated by tetradentate iminophenolate ligands <b>H</b>_<b>2</b><b>L1</b>–<b>H</b>_<b>2</b><b>L3</b> bearing backbones of different rigidity (alkyl, cycloalkyl, and phenyl bridges) allows for the formation of distinct geometric isomers, including a symmetric <i>trans</i>-oxidochlorido coordination motif in complex <b>3</b>. The complex employing a cycloalkyl-bridged ligand (<b>2</b>) of intermediate rigidity exhibits an interesting solvent- and temperature-dependent equilibrium between a symmetric (trans) isomer and an asymmetric (cis) isomer in solution. The occurrence of a symmetric isomer for <b>2</b> and <b>3</b> is confirmed by single-crystal X-ray diffraction analysis. Chlorido abstraction from <b>2</b> with AgOTf yields the corresponding cationic complex <b>2a</b>, which does not exhibit an isomeric equilibrium in solution but adopts the isomeric form predominant for <b>2</b> in a given solvent. All complexes were, furthermore, employed in three benchmark oxygen-atom-transfer (OAT) reactions, namely, the reduction of perchlorate, the epoxidation of cyclooctene, and OAT from dimethyl sulfoxide (DMSO) to triphenylphosphane (PPh₃), to assess the influence of the isomeric structure on the reactivity in these reactions. In perchlorate reduction, a clear structural influence was observed, where the trans arrangement in <b>3</b> led to the complete absence of activity. In the epoxidation reaction, all complexes led to comparable epoxide yields, albeit higher catalytic activity but lower overall stability of the catalysts with a trans arrangement was observed. In OAT from DMSO to PPh₃, also a clear structural dependence was observed, where the trans complex <b>3</b> led to full phosphane conversion with an excess of oxidant, while the cis compound <b>1</b> was completely inactive

Oxidorhenium(V) Complexes with Tetradentate Iminophenolate Ligands: Influence of Ligand Flexibility on the Coordination Motif and Oxygen-Atom-Transfer Activity

The synthesis of oxidorhenium­(V) complexes <b>1</b>–<b>3</b> coordinated by tetradentate iminophenolate ligands <b>H</b>_<b>2</b><b>L1</b>–<b>H</b>_<b>2</b><b>L3</b> bearing backbones of different rigidity (alkyl, cycloalkyl, and phenyl bridges) allows for the formation of distinct geometric isomers, including a symmetric <i>trans</i>-oxidochlorido coordination motif in complex <b>3</b>. The complex employing a cycloalkyl-bridged ligand (<b>2</b>) of intermediate rigidity exhibits an interesting solvent- and temperature-dependent equilibrium between a symmetric (trans) isomer and an asymmetric (cis) isomer in solution. The occurrence of a symmetric isomer for <b>2</b> and <b>3</b> is confirmed by single-crystal X-ray diffraction analysis. Chlorido abstraction from <b>2</b> with AgOTf yields the corresponding cationic complex <b>2a</b>, which does not exhibit an isomeric equilibrium in solution but adopts the isomeric form predominant for <b>2</b> in a given solvent. All complexes were, furthermore, employed in three benchmark oxygen-atom-transfer (OAT) reactions, namely, the reduction of perchlorate, the epoxidation of cyclooctene, and OAT from dimethyl sulfoxide (DMSO) to triphenylphosphane (PPh₃), to assess the influence of the isomeric structure on the reactivity in these reactions. In perchlorate reduction, a clear structural influence was observed, where the trans arrangement in <b>3</b> led to the complete absence of activity. In the epoxidation reaction, all complexes led to comparable epoxide yields, albeit higher catalytic activity but lower overall stability of the catalysts with a trans arrangement was observed. In OAT from DMSO to PPh₃, also a clear structural dependence was observed, where the trans complex <b>3</b> led to full phosphane conversion with an excess of oxidant, while the cis compound <b>1</b> was completely inactive

Photoinduced Reactivity of the Soft Hydrotris(6-<i>tert</i>-butyl-3-thiopyridazinyl)borate Scorpionate Ligand in Sodium, Potassium, and Thallium Salts

The soft scorpionate ligand hydrotris­(6-<i>tert</i>-butyl-3-thiopyridazinyl)­borate (<b>Tn</b>) was found to exhibit pronounced photoreactivity. Full elucidation of this process revealed the formation of 6-<i>tert</i>-butylpyridazine-3-thione (<b>PnH</b>) and 4,5-dihydro-6-<i>tert</i>-butylpyridazine-3-thione (<b>H</b>_<b>2</b><b>PnH</b>). Under exclusion of light, no solvolytic reactions occur, allowing the development of high-yield preparation protocols for the sodium, potassium, and thallium salts and improving the yield for their derived copper boratrane complex. The photoreactivity is relevant for all future studies with electron-deficient scorpionate ligands

Activation of Molecular Oxygen by a Molybdenum(IV) Imido Compound

Activation of molecular dioxygen at a molybdenum­(IV) imido compound led to the isolation and full characterization of a remarkably stable transition-metal imidoperoxido complex

Templated C–C and C–N Bond Formation Facilitated by a Molybdenum(VI) Metal Center

Preparation of molybdenum dioxido complexes with novel iminophenolate ligands bearing pendant secondary amide functionalities led to unprecedented C–C and C–N coupling reactions of two α-iminoamides upon coordination. The diastereoselective cyclization to asymmetric imidazolidines occurs at the metal center in two consecutive steps via a monocoupled intermediate. A meaningful mechanism is proposed on the basis of full characterization of intermediate and final molybdenum-containing products by spectroscopic means and by single-crystal X-ray diffraction analyses. This process constitutes the first example of a diastereoselective self-cyclization of two α-iminoamides

Unusual C–N Coupling Reactivity of Thiopyridazines: Efficient Synthesis of Iron Diorganotrisulfide Complexes

The reaction of iron­(II) triflate with 6-<i>tert</i>-butyl-3-thiopyridazine (PnH) and 4-methyl-6-<i>tert</i>-butyl-3-thiopyridazine (^MePnH) respectively led to iron bis­(diorganotrisulfide) complexes [Fe­(^RPnS₃Pn^R)₂]­(OTf)₂ [R = H (<b>1a</b>) and Me (<b>2a</b>)]. The corresponding perchlorate complexes were prepared by using the iron­(II) chloride precursor and the subsequent addition of 2 equiv of NaClO₄, giving [Fe­(^RPnS₃Pn^R)₂]­(ClO₄)₂ [R = H (<b>1b</b>) and Me (<b>2b</b>)]. The compounds were fully characterized including single-crystal X-ray diffraction analysis. All four compounds exhibit nearly perfect octahedral geometries with an iron center coordinated by four nitrogen atoms from two ^RPnS₃Pn^R ligands and by two sulfur atoms of the central atom in the S₃ unit. The diamagnetic complexes exhibit unusually high redox potentials for the Fe^2+/3+ couple at <i>E</i>_1/2 = 1.15 V (for <b>1a</b> and <b>1b</b>) and 1.08 V (for <b>2a</b> and <b>2b</b>) versus Fc/Fc⁺, respectively, as determined by cyclic voltammetry. Furthermore, the source of the extra sulfur atom within the S₃ unit was elucidated by isolation of C–N-coupled pyridazinylthiopyridazine products

Templated C–C and C–N Bond Formation Facilitated by a Molybdenum(VI) Metal Center

Preparation of molybdenum dioxido complexes with novel iminophenolate ligands bearing pendant secondary amide functionalities led to unprecedented C–C and C–N coupling reactions of two α-iminoamides upon coordination. The diastereoselective cyclization to asymmetric imidazolidines occurs at the metal center in two consecutive steps via a monocoupled intermediate. A meaningful mechanism is proposed on the basis of full characterization of intermediate and final molybdenum-containing products by spectroscopic means and by single-crystal X-ray diffraction analyses. This process constitutes the first example of a diastereoselective self-cyclization of two α-iminoamides