15 research outputs found
A New ONO<sup>3ā</sup> Trianionic Pincer-Type Ligand for Generating Highly Nucleophilic MetalāCarbon Multiple Bonds
Appending an amine to a Cī»C double bond drastically
increases
the nucleophilicity of the Ī²-carbon atom of the alkene to form
an enamine. In this report, we present the synthesis and characterization
of a novel CF<sub>3</sub>āONO<sup>3ā</sup> trianionic
pincer-type ligand, rationally designed to mimic enamines within a
metal coordination sphere. Presented is a synthetic strategy to create
enhanced nucleophilic tungstenāalkylidene and āalkylidyne
complexes. Specifically, we present the synthesis and characterization
of the new CF<sub>3</sub>āONO<sup>3ā</sup> trianionic
pincer tungstenāalkylidene [CF<sub>3</sub>āONO]ĀWī»CHĀ(Et)Ā(O<sup><i>t</i></sup>Bu) (<b>2</b>) and āalkylidyne
{MePPh<sub>3</sub>}Ā{[CF<sub>3</sub>āONO]ĀWī¼CĀ(Et)Ā(O<sup><i>t</i></sup>Bu)} (<b>3</b>) complexes. Characterization
involves a combination of multinuclear NMR spectroscopy, combustion
analysis, DFT computations, and single crystal X-ray analysis for
complexes <b>2</b> and <b>3</b>. Exhibiting unique nucleophilic
reactivity, <b>3</b> reacts with MeOTf to yield [CF<sub>3</sub>āONO]ĀWī»CĀ(Me)Ā(Et)Ā(O<sup><i>t</i></sup>Bu)
(<b>4</b>), but the bulkier Me<sub>3</sub>SiOTf silylates the <i>tert</i>-butoxide, which subsequently undergoes isobutylene
expulsion to form [CF<sub>3</sub>āONO]ĀWī»CHĀ(Et)Ā(OSiMe<sub>3</sub>) (<b>5</b>). A DFT calculation performed on a model
complex of <b>3</b>, namely, [CF<sub>3</sub>āONO]ĀWī¼CĀ(Et)Ā(O<sup><i>t</i></sup>Bu) (<b>3</b>ā²), reveals the
amide participates in an enamine-type bonding combination. For complex <b>2</b>, the Lewis acids MeOTf, Me<sub>3</sub>SiOTf, and BĀ(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> catalyze isobutylene expulsion
to yield the tungstenāoxo complex [CF<sub>3</sub>āONO]ĀWĀ(O)Ā(<sup><i>n</i></sup>Pr) (<b>6</b>)
Synthesis and Characterization of Group 4 Trianionic ONO<sup>3ā</sup> Pincer-Type Ligand Complexes and a Rare Case of Through-Space <sup>19</sup>Fā<sup>19</sup>F Coupling
This report describes the synthesis
and characterization of a new
series of group 4 complexes supported by a trianionic ONO<sup>3ā</sup> pincer-type ligand. Treating TiCl<sub>4</sub> with the proligand
[CF<sub>3</sub>āONO]ĀH<sub>3</sub> (<b>1</b>) and NEt<sub>3</sub> in benzene afforded {[CF<sub>3</sub>āONO]ĀTiCl<sub>3</sub>}Ā{HNEt<sub>3</sub>}<sub>2</sub> (<b>2</b>). By means
of a lithium transmetalation route, the neutral monochloride complex
[CF<sub>3</sub>āONO]ĀTiClĀ(THF) (<b>3</b>) was synthesized
in 91% yield. The analogous HfĀ(IV) derivative could not be obtained
using this method. Instead, transmetalation with thalliumĀ(I) resulted
in the formation of the seven-coordinate complex [CF<sub>3</sub>āONHO]ĀHfCl<sub>2</sub>(THF)<sub>2</sub> (<b>4-(THF)</b><sub><b>2</b></sub>), which was characterized by combustion analysis and X-ray
crystallography. Applying vacuum to <b>4-(THF)</b><sub><b>2</b></sub> liberated the THF ligands to provide the five-coordinate
THF-free complex [CF<sub>3</sub>āONHO]ĀHfCl<sub>2</sub> (<b>4</b>). Alkylation of complex <b>4</b> with alkyllithium
or Grignard reagents resulted in a mixture of unidentifiable products.
However, access to the neutral complex <b>3</b> enabled the
subsequent preparation of organotitanium complexes [CF<sub>3</sub>āONO]ĀTiRĀ(THF) (<b>5-R</b>; R = Me, Bn, Mes). Single-crystal
X-ray analysis of <b>5-Me</b> indicated that the organotitanium
complexes are mononuclear. Single-crystal X-ray diffraction and NMR
studies in solution confirmed that complex <b>5-Mes</b> exhibits
rare through-space <sup>19</sup>Fā<sup>19</sup>F coupling (5
Hz)
Fast āWittig-Likeā Reactions As a Consequence of the Inorganic Enamine Effect
The
tungsten alkylidyne [CF<sub>3</sub>āONO]ĀWī¼CCĀ(CH<sub>3</sub>)<sub>3</sub>(THF)<sub>2</sub> (<b>3</b>) {where CF<sub>3</sub>āONO = (MeC<sub>6</sub>H<sub>3</sub>[CĀ(CF<sub>3</sub>)<sub>2</sub>O])<sub>2</sub>N<sup>3ā</sup>} supported by a
trianionic pincer-type ligand demonstrates enhanced nucleophilicity
in unusually fast āWittig-likeā reactions. Experiments
are designed to provide support for an inorganic enamine effect that
is the origin of the enhanced nucleophilicity. Treating complex <b>3</b> with various carbonyl-containing substrates provides tungsten-oxo-vinyl
complexes upon oxygen atom transfer. The rates of reactivity of <b>3</b> are compared with the known alkylidyne (DIPP)<sub>3</sub>Wī¼CCĀ(CH<sub>3</sub>)<sub>3</sub> (DIPP = 2,6-diisopropylphenoxide).
In all cases (except acetone), complex <b>3</b> exhibits significantly
faster overall rates than (DIPP)<sub>3</sub>Wī¼CCĀ(CH<sub>3</sub>)<sub>3</sub>. New oxo-vinyl complexes are characterized by NMR,
combustion analysis and single crystal X-ray diffraction. Treating <b>3</b> with acid chlorides provides the tungsten oxo chloride species
[CF<sub>3</sub>āONO]ĀWĀ(O)Cl (<b>4</b>) and disubstituted
alkynes. In the case of acetone the oxo-vinyl complex results in two
rotational isomers <b>10</b><sub><i><b>syn</b></i></sub> and <b>10</b><sub><i><b>anti</b></i>.</sub> The rate of isomerization was determined for the forward
and reverse directions and was complimented with DFT calculations
Synthesis and Characterization of Tungsten Alkylidene and Alkylidyne Complexes Supported by a New Pyrrolide-Centered Trianionic ONO<sup>3ā</sup> Pincer-Type Ligand
Synthetic protocols for a pyrrolide-centered
ONO<sup>3ā</sup> trianionic pincer-type ligand are presented.
Treating (<sup><i>t</i></sup>BuO)<sub>3</sub>Wī¼C<sup><i>t</i></sup>Bu with the proligand [pyr-ONO]ĀH<sub>3</sub> (<b>2</b>) results in the formation of the trianionic pincer
alkylidene complex [pyr-ONO]ĀWī»CH<sup><i>t</i></sup>BuĀ(O<sup><i>t</i></sup>Bu) (<b>3</b>). Addition of
a mild base to complex <b>3</b> provides the trianionic pincer
alkylidyne complex {MePPh<sub>3</sub>}Ā{[pyr-ONO]ĀWī¼C<sup><i>t</i></sup>BuĀ(O<sup><i>t</i></sup>Bu)} (<b>4</b>). All new compounds were characterized by NMR spectroscopy, combustion
analysis, and, in the case of complex <b>4</b>, single-crystal
X-ray crystallography. DFT calculations performed on <b>4</b> provide insight into its electronic structure and indicate that
the HOMO is ligand-based and localized on the pyrrolide Ļ orbitals
Solid State Collapse of a High-Spin Square-Planar Fe(II) Complex, Solution Phase Dynamics, and Electronic Structure Characterization of an Fe(II)<sub>2</sub> Dimer
Square-planar high-spin FeĀ(II) molecular
compounds are rare, and until recently, the only four examples of
non-macrocyclic or sterically driven molecular compounds of this kind
shared a common FeO<sub>4</sub> core. The trianionic pincer-type ligand
[CF<sub>3</sub>-ONO]ĀH<sub>3</sub> (<b>1</b>) supports the high-spin
square-planar FeĀ(II) complex {[CF<sub>3</sub>-ONO]ĀFeCl}Ā{LiĀ(Sv)<sub>2</sub>}<sub>2</sub> (<b>2</b>). In the solid state, <b>2</b> forms the dimer complex {[CF<sub>3</sub>-ONO]ĀFe}<sub>2</sub>Ā{(Ī¼-Cl)<sub>2</sub>Ā(Ī¼-LiTHF)<sub>4</sub>}
(<b>3</b>) in 96% yield by simply applying a vacuum or stirring
it with pentane for 2 h. A detailed high-frequency electron paramagnetic
resonance and field-dependent <sup>57</sup>Fe MoĢssbauer investigation
of <b>3</b> revealed a weak antiferromagnetic exchange interaction
between the local iron spins which exhibit a zero-field splitting
tensor characterized by negative <i>D</i> parameter. In
solution, <b>2</b> is in equilibrium with the solvento complex
{[CF<sub>3</sub>-ONO]ĀFeClĀ(THF)}Ā{Li<sub>2</sub>(Sv)<sub>4</sub>} (<b>2Ā·Sv</b>) and the dimer <b>3</b>. A
combination of frozen solution <sup>57</sup>Fe MoĢssbauer spectroscopy
and single crystal X-ray crystallography helped elucidate the solvent
dependent equilibrium between these three species. The oxidation chemistry
of <b>2Ā·Sv</b> was investigated. Complex <b>2</b> reacts
readily with the one-electron oxidizing agent CuCl<sub>2</sub> to
give the FeĀ(III) complex {[CF<sub>3</sub>-ONO]ĀFeCl<sub>2</sub>}Ā{LiĀ(THF)<sub>2</sub>}<sub>2</sub> (<b>4</b>). Also, <b>2Ā·Sv</b> reacts with 2 equiv of TlPF<sub>6</sub> to form the FeĀ(III)
complex [CF<sub>3</sub>-ONO]ĀFeĀ(THF)<sub>3</sub> (<b>5</b>)
Solid State Collapse of a High-Spin Square-Planar Fe(II) Complex, Solution Phase Dynamics, and Electronic Structure Characterization of an Fe(II)<sub>2</sub> Dimer
Square-planar high-spin FeĀ(II) molecular
compounds are rare, and until recently, the only four examples of
non-macrocyclic or sterically driven molecular compounds of this kind
shared a common FeO<sub>4</sub> core. The trianionic pincer-type ligand
[CF<sub>3</sub>-ONO]ĀH<sub>3</sub> (<b>1</b>) supports the high-spin
square-planar FeĀ(II) complex {[CF<sub>3</sub>-ONO]ĀFeCl}Ā{LiĀ(Sv)<sub>2</sub>}<sub>2</sub> (<b>2</b>). In the solid state, <b>2</b> forms the dimer complex {[CF<sub>3</sub>-ONO]ĀFe}<sub>2</sub>Ā{(Ī¼-Cl)<sub>2</sub>Ā(Ī¼-LiTHF)<sub>4</sub>}
(<b>3</b>) in 96% yield by simply applying a vacuum or stirring
it with pentane for 2 h. A detailed high-frequency electron paramagnetic
resonance and field-dependent <sup>57</sup>Fe MoĢssbauer investigation
of <b>3</b> revealed a weak antiferromagnetic exchange interaction
between the local iron spins which exhibit a zero-field splitting
tensor characterized by negative <i>D</i> parameter. In
solution, <b>2</b> is in equilibrium with the solvento complex
{[CF<sub>3</sub>-ONO]ĀFeClĀ(THF)}Ā{Li<sub>2</sub>(Sv)<sub>4</sub>} (<b>2Ā·Sv</b>) and the dimer <b>3</b>. A
combination of frozen solution <sup>57</sup>Fe MoĢssbauer spectroscopy
and single crystal X-ray crystallography helped elucidate the solvent
dependent equilibrium between these three species. The oxidation chemistry
of <b>2Ā·Sv</b> was investigated. Complex <b>2</b> reacts
readily with the one-electron oxidizing agent CuCl<sub>2</sub> to
give the FeĀ(III) complex {[CF<sub>3</sub>-ONO]ĀFeCl<sub>2</sub>}Ā{LiĀ(THF)<sub>2</sub>}<sub>2</sub> (<b>4</b>). Also, <b>2Ā·Sv</b> reacts with 2 equiv of TlPF<sub>6</sub> to form the FeĀ(III)
complex [CF<sub>3</sub>-ONO]ĀFeĀ(THF)<sub>3</sub> (<b>5</b>)
An OCO<sup>3ā</sup> Trianionic Pincer Tungsten(VI) Alkylidyne: Rational Design of a Highly Active Alkyne Polymerization Catalyst
Synthesis, characterization, and catalytic alkyne polymerization
results for the first trianionic pincer alkylidyne complex, [<sup><i>t</i></sup>BuOCO]ĀWī¼CCĀ(CH<sub>3</sub>)<sub>3</sub>(THF)<sub>2</sub> (<b>6</b>), are described. Complex <b>6</b> is a highly active catalyst for the polymerization of acetylenes
and exhibits a high turnover number (4371), activity (1.05 Ć
10<sup>6</sup> g<sub>PPA</sub> mol<sub>cat</sub><sup>ā1</sup> h<sup>ā1</sup>), and yield (87%) for the polymerization of
1-ethynyl-4-fluorobenzene
New Alkylidyne Complexes Featuring a Flexible Trianionic ONO<sup>3ā</sup> Pincer-Type Ligand: Inorganic Enamine Effect versus Sterics in Electrophilic Additions
Metalācarbon
multiple bonds exhibit enhanced nucleophilicity
at the Ī± carbon within ONO trianionic pincer alkylidyne complexes.
Defined as the <i>inorganic enamine effect</i>, this phenomenon
is a result of the overlap of the N atom lone pair within the ONO<sup>3ā</sup> ligand and a Ļ bond from the metalācarbon
multiple bond. Treating the proligand [O<sup>CH2</sup>N<sup>CH2</sup>O]ĀH<sub>3</sub> (<b>2</b>) with (<sup><i>t</i></sup>BuO)<sub>3</sub>Wī¼CR (where R = Et, <sup><i>t</i></sup>Bu) results in the formation of the dianionic pincer complexes
[O<sup>CH2</sup>NH<sup>CH2</sup>O]ĀWī¼CRĀ(O<sup><i>t</i></sup>Bu) (where R = Et (<b>3-Et</b>), <sup><i>t</i></sup>Bu (<b>3-</b><sup><i><b>t</b></i></sup><b>Bu</b><sub><i><b>anti</b></i></sub>)). Deprotonation
of <b>3-</b><sup><i><b>t</b></i></sup><b>Bu</b><sub><i><b>anti</b></i></sub> by treatment
with Ph<sub>3</sub>Pī»CH<sub>2</sub> forms the anionic alkylidyne
complex {[O<sup>CH2</sup>N<sup>CH2</sup>O]ĀWī¼C<sup><i>t</i></sup>BuĀ(O<sup><i>t</i></sup>Bu)}Ā{CH<sub>3</sub>PPh<sub>3</sub>} (<b>4-</b><sup><i><b>t</b></i></sup><b>Bu</b>). DFT calculations modeling <b>4-</b><sup><i><b>t</b></i></sup><b>Bu</b> reveal overlap of the
N atom lone pair with a Wī¼C Ļ bond. However, <b>4-</b><sup><i><b>t</b></i></sup><b>Bu</b> reacts
with electrophiles preferentially at the pincer N atom as opposed
to the Wī¼C<sub>Ī±</sub> group. Multinuclear NMR spectroscopy,
combustion analysis, and single-crystal X-ray crystallography are
employed to characterize complexes <b>3-Et</b>, <b>3-</b><sup><i><b>t</b></i></sup><b>Bu</b><sub><i><b>anti</b></i></sub>, and <b>4-</b><sup><i><b>t</b></i></sup><b>Bu</b>
Highly Tactic Cyclic Polynorbornene: Stereoselective Ring Expansion Metathesis Polymerization of Norbornene Catalyzed by a New Tethered Tungsten-Alkylidene Catalyst
The tungsten alkylidyne
[<sup><i>t</i></sup>BuOCO]ĀWī¼CĀ(<sup><i>t</i></sup>Bu) (THF)<sub>2</sub> (<b>1</b>) reacts
with CO<sub>2</sub>, leading to complete cleavage of one Cī»O
bond, followed by migratory insertion to generate the tungsten-oxo
alkylidene <b>2</b>. Complex <b>2</b> is the first catalyst
to polymerize norbornene via ring expansion metathesis polymerization
to yield highly <i>cis</i>-syndiotactic cyclic polynorbornene
Introducing āYneneā Metathesis: Ring-Expansion Metathesis Polymerization Leads to Highly Cis and Syndiotactic Cyclic Polymers of Norbornene
Tungsten
alkylidynes [CF<sub>3</sub>āONO]ĀWī¼CCĀ(CH<sub>3</sub>)<sub>3</sub>(THF)<sub>2</sub> (<b>1</b>) and [<sup><i>t</i></sup>BuOCO]ĀWī¼CCĀ(CH<sub>3</sub>)<sub>3</sub>(THF)<sub>2</sub> (<b>3</b>) react with ethylene. Complex <b>1</b> reacts
reversibly with ethylene to give the metallacyclobutene
(<b>2</b>). Complex <b>3</b> reacts with ethylene to form
the tethered alkylidene (<b>4</b>) featuring a tetraanionic
pincer ligand. Complexes <b>1</b> and <b>3</b> initiate
the polymerization of norbornene at room temperature. The polymerization
of norbornene by <b>1</b> is not stereoselective, whereas <b>3</b> generates a highly cis and syndiotactic cyclic polynorbornene.
Comparison of the intrinsic viscosity, radius of gyration, and elution
time of the synthesized cyclic polynorbornene with those of linear
analogues provides conclusive evidence for a cyclic topology