24 research outputs found
Molybdenum(VI) Dioxo and Oxo-Imido Complexes of Fluorinated Ī²-Ketiminato Ligands and Their Use in OAT Reactions
Substitution of a methyl by a trifluoromethyl moiety
in well-known
Ī²-ketimines afforded the ligands (Ar)ĀNCĀ(Me)ĀCH<sub>2</sub>COĀ(<i>CF</i><sub><i>3</i></sub>) (HL<sup>H</sup>, Ar = C<sub>6</sub>H<sub>5</sub>; HL<sup>Me</sup>, A r= 2,6-Me<sub>2</sub>C<sub>6</sub>H<sub>3</sub>; HL<sup>iPr</sup>, Ar = 2,6-<sup>i</sup>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>). Subsequent complexation to the
[MoO<sub>2</sub>]<sup>2+</sup> core leads to the formation of novel
complexes of general formula [MoO<sub>2</sub>(L<sup>R</sup>)<sub>2</sub>] (R = H, <b>1</b>; R = Me, <b>2</b>; R = iPr, <b>3</b>). For reasons of comparison the oxo-imido complex [MoOĀ(N<sup>t</sup>Bu)Ā(L<sup>Me</sup>)<sub>2</sub>] (<b>4</b>) has also
been synthesized. Complexes <b>1</b>ā<b>4</b> were
investigated in oxygen atom transfer (OAT) reactions using the substrate
trimethylphosphine. The respective products after OAT, the reduced
Mo<sup>IV</sup> complexes [MoOĀ(PMe<sub>3</sub>)Ā(L<sup>R</sup>)<sub>2</sub>] (R = H, <b>5</b>; R = Me, <b>6</b>; R = iPr, <b>7</b>) and [MoĀ(N<sup>t</sup>Bu)Ā(PMe<sub>3</sub>)Ā(L<sup>Me</sup>)<sub>2</sub>] (<b>8</b>), were isolated. All complexes have
been characterized by NMR spectroscopy, and <b>1</b>ā<b>4</b> also by cyclic voltammetry. A positive shift of the Mo<sup>VI</sup>āMo<sup>V</sup> 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<sup>IV</sup> phosphino-imido complex.
Kinetic measurements by UVāvis spectroscopy of the OAT reactions
from complexes <b>1</b>ā<b>4</b> to PMe<sub>3</sub> showed them to be more efficient than previously reported nonfluorinated
ones, with ligand Lā² = (Ar)ĀNCĀ(Me)ĀCH<sub>2</sub>COĀ(<i>CH</i><sub><i>3</i></sub>) [MoO<sub>2</sub>(Lā²)<sub>2</sub>] (<b>9</b>) and [MoOĀ(N<sup>t</sup>Bu)Ā(Lā²)<sub>2</sub>] (<b>10</b>), respectively. Thermodynamic activation parameters
Ī<i>H</i><sup>ā”</sup> and Ī<i>S</i><sup>ā”</sup> 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 Ī<i>S</i><sup>ā”</sup> 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 PhenolateāPyrazole Ligands for Olefin Epoxidation Using Hydrogen Peroxide
OxorheniumĀ(V) complexes of the general
formula [ReOCl<sub>2</sub>(PPh<sub>3</sub>)Ā(L)] (<b>2a</b>ā<b>c</b>) and [ReOClĀ(L)<sub>2</sub>] (<b>3a</b>ā<b>c</b>) with L being monoanionic, bidentate phenolateāpyrazole
ligands <b>1a</b>ā<b>c</b> that bear substituents
with various electronic features on the phenol ring (<b>1a</b> Br, <b>1b</b> NO<sub>2</sub>, <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>ā<b>c</b> showed
a shift to more positive potentials for the ReĀ(V)/ReĀ(VI) redox couple
in the order of <b>3b</b> > <b>3a</b> > <b>3c</b> (R= NO<sub>2</sub> > Br > OMe), reflecting the higher electrophilic
character of the Re atom caused by the ligands <b>1a</b>ā<b>c</b>. Complexes <b>2a</b>ā<b>c</b> and <b>3a</b>ā<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 oxorheniumĀ(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
oxidorheniumĀ(V) complexes <b>1</b>ā<b>3</b> coordinated
by tetradentate iminophenolate ligands <b>H</b><sub><b>2</b></sub><b>L1</b>ā<b>H</b><sub><b>2</b></sub><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
(PPh<sub>3</sub>), 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 PPh<sub>3</sub>, 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 [MoO<sub>2</sub>(Ī·<sup>2</sup>-<i>t</i>Bu<sub>2</sub>pz)<sub>2</sub>] with Schiff base ligands
HL<sup>X</sup> (X = 1ā5) gave molybdenumĀ(VI) dioxo complexes
of the type <i>cis</i>-[MoO<sub>2</sub>(L<sup>X</sup>)<sub>2</sub>] 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 (R<sub>2</sub> = OMe or NMe<sub>2</sub>) in the side chain in complexes <b>1</b>ā<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 NMe<sub>2</sub> 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
oxidorheniumĀ(V) complexes <b>1</b>ā<b>3</b> coordinated
by tetradentate iminophenolate ligands <b>H</b><sub><b>2</b></sub><b>L1</b>ā<b>H</b><sub><b>2</b></sub><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
(PPh<sub>3</sub>), 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 PPh<sub>3</sub>, 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
Activation of Molecular Oxygen by a Molybdenum(IV) Imido Compound
Activation of molecular
dioxygen at a molybdenumĀ(IV) imido compound led to the isolation and
full characterization of a remarkably stable transition-metal imidoperoxido
complex
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 hydrotrisĀ(6-<i>tert</i>-butyl-3-thiopyridazinyl)Āborate (<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><sub><b>2</b></sub><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
Templated CāC and CāN 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
CāC and CāN coupling reactions of two Ī±-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 Ī±-iminoamides