Molybdenum(VI) Dioxo and Oxo-Imido Complexes of Fluorinated β-Ketiminato Ligands and Their Use in OAT Reactions

Substitution of a methyl by a trifluoromethyl moiety in well-known β-ketimines afforded the ligands (Ar)­NC­(Me)­CH₂CO­(<i>CF</i>_<i>3</i>) (HL^H, Ar = C₆H₅; HL^Me, A r= 2,6-Me₂C₆H₃; HL^iPr, Ar = 2,6-ⁱPr₂C₆H₃). Subsequent complexation to the [MoO₂]²⁺ core leads to the formation of novel complexes of general formula [MoO₂(L^R)₂] (R = H, <b>1</b>; R = Me, <b>2</b>; R = iPr, <b>3</b>). For reasons of comparison the oxo-imido complex [MoO­(N^tBu)­(L^Me)₂] (<b>4</b>) has also been synthesized. Complexes <b>1</b>–<b>4</b> were investigated in oxygen atom transfer (OAT) reactions using the substrate trimethylphosphine. The respective products after OAT, the reduced Mo^IV complexes [MoO­(PMe₃)­(L^R)₂] (R = H, <b>5</b>; R = Me, <b>6</b>; R = iPr, <b>7</b>) and [Mo­(N^tBu)­(PMe₃)­(L^Me)₂] (<b>8</b>), were isolated. All complexes have been characterized by NMR spectroscopy, and <b>1</b>–<b>4</b> also by cyclic voltammetry. A positive shift of the Mo^VI–Mo^V reduction wave upon fluorination was observed. Furthermore, molecular structures of complexes <b>2</b>, <b>4</b>, <b>5</b>, and <b>8</b> have been determined via single crystal X-ray diffraction analysis. Complex <b>8</b> represents a rare example of a Mo^IV phosphino-imido complex. Kinetic measurements by UV–vis spectroscopy of the OAT reactions from complexes <b>1</b>–<b>4</b> to PMe₃ showed them to be more efficient than previously reported nonfluorinated ones, with ligand L′ = (Ar)­NC­(Me)­CH₂CO­(<i>CH</i>_<i>3</i>) [MoO₂(L′)₂] (<b>9</b>) and [MoO­(N^tBu)­(L′)₂] (<b>10</b>), respectively. Thermodynamic activation parameters Δ<i>H</i>^‡ and Δ<i>S</i>^‡ of the OAT reactions for complexes <b>2</b> and <b>4</b> have been determined. The activation enthalpy for the reaction employing <b>2</b> is significantly smaller (12.3 kJ/mol) compared to the reaction with the nonfluorinated complex <b>9</b> (60.8 kJ/mol). The change of the entropic term Δ<i>S</i>^‡ is small. The reaction of the oxo-imido complex <b>4</b> to <b>8</b> revealed a significant electron-donating contribution of the imido substituent

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

Molybdenum(VI) Dioxo Complexes Employing Schiff Base Ligands with an Intramolecular Donor for Highly Selective Olefin Epoxidation

Reaction of [MoO₂(η²-<i>t</i>Bu₂pz)₂] with Schiff base ligands HL^X (X = 1–5) gave molybdenum­(VI) dioxo complexes of the type <i>cis</i>-[MoO₂(L^X)₂] as yellow to light brown solids in moderate to good yields. All ligands coordinate via its phenolic O atom and the imine N atom in a bidentate manner to the metal center. The third donor atom (R₂ = OMe or NMe₂) in the side chain in complexes <b>1</b>–<b>4</b> is not involved in coordination and remains pendant. This was confirmed by X-ray diffraction analyses of complexes <b>1</b> and <b>3</b>. Complexes <b>1</b>, <b>3</b>, and <b>5</b> exist as a mixture of two isomers in solution, whereas complexes <b>2</b> and <b>4</b> with sterically less demanding substituents on the aromatics only show one isomer in solution. All complexes are active catalysts in the epoxidation of various internal and terminal alkenes, and epoxides in moderate to good yields with high selectivities are obtained. In the challenging epoxidation of styrene, complexes <b>1</b> and <b>2</b> prove to be very active and selective. The selectivity seems to be influenced by the pendant donor arm, as complex <b>5</b> without additional donor in the side chain is less selective. Experiments prove that the addition of <i>n</i>-butyl methyl ether as intermolecular donor per se has no influence on the selectivity. The basic conditions induced by the NMe₂ groups in complexes <b>3</b> and <b>4</b> lead to lower activity

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