Synthesis and characterization of tungsten alkylidene and alkylidyne complexes supported by a new pyrrolide-centered trianionic ONO3- pincer-type ligand

Article abstract of DOI:10.1021/om4009422

Synthetic protocols for a pyrrolide-centered ONO^3- trianionic pincer-type ligand are presented. Treating (^tBuO)₃Wi - C^tBu with the proligand [pyr-ONO]H₃ (2) results in the formation of the trianionic pincer alkylidene complex [pyr-ONO]W=CH ^tBu(O^tBu) (3). Addition of a mild base to complex 3 provides the trianionic pincer alkylidyne complex {MePPh₃}{[pyr-ONO] Wi - C^tBu(O^tBu)} (4). All new compounds were characterized by NMR spectroscopy, combustion analysis, and, in the case of complex 4, single-crystal X-ray crystallography. DFT calculations performed on 4 provide insight into its electronic structure and indicate that the HOMO is ligand-based and localized on the pyrrolide π orbitals.

Full text of DOI:10.1021/om4009422

Communication
pubs.acs.org/Organometallics
Synthesis and Characterization of Tungsten Alkylidene and
Alkylidyne Complexes Supported by a New Pyrrolide-Centered
Trianionic ONO3− Pincer-Type Ligand
Matthew E. O’Reilly, Soufiane S. Nadif, Ion Ghiviriga, Khalil A. Abboud, and Adam S. Veige*
Department of Chemistry, Center for Catalysis, University of Florida, P.O. Box 117200, Gainesville, Florida 32611, United States
S
* Supporting Information
ABSTRACT: Synthetic protocols for a pyrrolide-centered
ONO3− trianionic pincer-type ligand are presented. Treating
(tBuO)3WCtBu with the proligand [pyr-ONO]H3 (2) results
in the formation of the trianionic pincer alkylidene complex
[pyr-ONO]WCHtBu(OtBu) (3). Addition of a mild base to
complex 3 provides the trianionic pincer alkylidyne complex
{MePPh3}{[pyr-ONO]WCtBu(OtBu)} (4). All new com-
pounds were characterized by NMR spectroscopy, combustion
analysis, and, in the case of complex 4, single-crystal X-ray
crystallography. DFT calculations performed on 4 provide insight into its electronic structure and indicate that the HOMO is
ligand-based and localized on the pyrrolide π orbitals.
rianionic pincer and pincer-type ligands are an emerging
ligand class for high-valent metal catalysis, which now
T
includes aerobic oxidation,1−3 nitrene and carbene transfer,4−6
ethylene and alkyne polymerization,7−9 and alkene isomer-
ization.10 One potential application for trianionic pincer and
pincer-type ligands is to improve tungsten- and molybdenum-
catalyzed alkyne cross metathesis.11−15 Theoretical calculations
by Jia and Lin16 show that a locked T-shaped geometry of the
ancillary ligands should mitigate the relative transition state
barrier by ∼24 kcal/mol. Trianionic pincer ligands, with their
rigid tridentate framework, lock the supporting anionic donors
in a T-shaped geometry, thereby minimizing the cycloaddition
energy barrier and providing an open coordination site for the
alkyne to access.17
Our previous attempts to realize an alkyne metathesis catalyst
supported by a [CF3-ONO] pincer-type ligand yielded unusually
stable tungstenacyclobutadiene complexes (Figure 1; B).18 Factors
contributing to their stability are poor steric pressure by the
pendant alkoxide arms toward the WC3 ring and the “inorganic
enamine”18,19 effect arising from an N atom lone pair in the
ligand backbone that aligns with the WC bond. The inorganic
enamine effect significantly destabilizes the ground state of the
W−alkylidyne (A), thus rendering the metallacyclobutadiene
intermediate (B) too thermodynamically stable to be com-
petent for retrocycloaddition. An ideal trianionic ONO3−
pincer-type ligand suitable for alkyne metathesis should enlarge
the steric groups on the pendant arms while effectively remov-
ing the influence of the N atom lone pair. A pyrrole-based tri-
anionic ONO3− pincer-type ligand offers the potential to
remove the nitrogen lone pair by sequestering it within π bonds
of the aromatic ring and to provide more electronically deficient
complexes. Additionally, ample evidence indicates pyrrolide-
ligated molybdenum and tungsten alkylidene complexes are
Figure 1. (A) Destabilized ONO3− pincer alkylidyne as a consequence
of inorganic enamine effect. (B) Stable metallacyclobutadiene
illustrating poor steric congestion between ligand pendant arms and
the metallacycle.
among the most highly active alkene metathesis catalysts and
provide remarkable selectivity for Z-isomer alkenes.20−25 Also,
rapid conversion between η1-N and η5 coordination modes
imparts stability to the catalysts.26−28 Herein, we introduce the
first pyrrolide-centered trianionic pincer-type ligand and its
corresponding tungsten alkylidene and alkylidyne complexes.
Scheme 1 depicts the synthetic steps to achieve the new ligand
precursor incorporating a pyrrole moiety within the central ring.
Suzuki coupling of (3-(tert-butyl)-2-methoxyphenyl)boronic acid29
(2.3 equiv) with 2,5-dibromo-1H-pyrrole30 (1 equiv) appends the
Received: September 20, 2013
Published: November 1, 2013
© 2013 American Chemical Society
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Organometallics
Communication
t
benzene solution of 3 removes the liberated BuOH, yielding a
purple powder. Cooling a concentrated pentane solution of 3 to
−35 °C precipitates an analytically pure sample as a micro-
Scheme 1. Synthesis of [pyr-ONO]Me2 (1) and
[pyr-ONO]H3 (2)
a
1
crystalline solid. H NMR spectroscopy (C6D6) provides good
evidence for the assignment of 3 as an alkylidene. The alkyli-
dene proton appears at 7.67 ppm, and the corresponding Cα
resonates at 268.6 ppm. The pyrrolide CH protons appear as a
singlet well downfield at 7.77 ppm. Using indirect detection,
the pyrrolide N atom resonates at 191.4 ppm, which is a shift of
38.1 ppm downfield from proligand 2, a clear indication of
coordination to tungsten. The 13C{1H} NMR resonances for
the pyrrolide C atoms appear at 112.9 and 137.6 ppm and are
similar to those for proligand 2. Observation of similar
pyrrolide 13C resonances between 3 and proligand 2 suggests
that η1-N coordination of the pyrrolide dominates in solution.
For instance, in a series of isoelectronic Ti4+ complexes, η5-
pyrrolide 13C resonances experience a ∼15 ppm downfield shift
relative to the free ligand value, while the 13C resonances are
minimally affected by η1-N coordination.31,32 The coordination
mode within 3 is cautiously assigned. In tungsten alkylidene
complexes prepared by Schrock and co-workers that feature
two pyrrolide ligands, the solid-state structure contained both
η1-N and η5-pyrrolide coordination modes. However, in
solution at ambient temperature the ligands rapidly inter-
convert to yield a single set of pyrrolide resonances that are
very close to the chemical shift of free pyrrole.26−28
a
Legend: (i) 10% Pd(PPh3)4, 8 Na2CO3, 3 KCl, toluene, EtOH, and
H2O, HCl/dioxane; (ii) KOtBu, [HSCH2CH2NHMe2]Cl, and DMF,
HCl.
anisole fragments to the central pyrrole ring. The reaction
proceeds at 95 °C for 18 h. After several purification steps,
treating the product with hydrochloric acid in dioxane at 50 °C
removes the BOC protecting group, and recrystallization from
1
cold isopropyl alcohol provides [pyr-ONO]Me2 (1). H and
13C{1H} NMR spectroscopy confirm the identity of 1. The
pyrrole NH proton appears as a broad singlet at 9.90 ppm, and
the pyrrole aromatic protons resonate as a doublet (4J =
2.68 Hz) at 6.55 ppm.
Refluxing a DMF solution of 1 with KOtBu and 2-(dimethyl-
amino)ethanethiol hydrochloride removes the methyl groups to
provide [pyr-ONO]H3 (2) in modest yield (36%) after recry-
stallization from pentane. The 1H NMR spectrum of 2 exhibits
a resonance attributable to the −OH protons at 6.12 ppm,
the pyrrole NH resonance appears at 8.96 ppm, and indirect
15N detection indicates that the N atom resonates at 153.5 ppm
(see the Supporting Information). The 13C{1H} NMR spectrum
of 2 exhibits resonances for the pyrrole C atoms at 108.6 and
126.0 ppm.
Treating complex 3 with the mild base H2CPPh3 in pentane
rapidly deprotonates the alkylidene to afford the anionic
alkylidyne {MePPh3}{[pyr-ONO]WCtBu(OtBu)} (4),
which precipitates as a yellow pastelike solid. Filtration of the
solid and removal of all volatiles eventually afford a fine
powder. Complex 4 is sparingly soluble in benzene and diethyl
1
ether but readily dissolves in chloroform. The initial H NMR
spectrum of complex 4 in CDCl3 exhibits broad resonances, but
after 0.5 h in solution, the spectrum resolves into sharp
resonances. Supporting the assignment of a WCα bond, the
13C{1H} NMR spectrum of 4 in CDCl3 reveals a resonance at
301.1 ppm. Interestingly, the aromatic signals also shift
significantly upfield from those in 3; the pyrrole CH’s resonate
at 6.56 ppm, a total shift of 1.21 ppm. Moving in the opposite
direction, the 15N resonance of complex 4 appears downfield
from that of 3 at 212.2 ppm. Again, the 13C resonances for the
pyrrole ring carbons appear at 106.5 and 137.6 ppm; similar to
the resonances for both 2 and 3.
Combining benzene solutions of proligand 2 and (tBuO)3W
CtBu results in a color change to deep violet over 0.5 h. The
color change results from expulsion of 2 equiv of tert-butyl
alcohol and coordination of the trianionic pincer ligand
(Scheme 2). During metalation, a proton transfer to the
Scheme 2. Synthesis of [pyr-ONO]WCHtBu(OtBu) (3)
and {MePPh3}{[pyr-ONO]WCtBu(OtBu)} (4)
Single crystals deposit via slow evaporation from a
concentrated diethyl ether solution of 4. Figure 2 depicts the
Figure 2. Molecular structure of 4 with hydrogen atoms, diethyl ether
lattice molecule, and methyltriphenylphosphonium group removed for
clarity.
alkylidyne bond gives the trianionic pincer tungsten alkylidene
molecular structure of complex 4 obtained from a single-crystal
X-ray diffraction experiment. This is an important structure,
[pyr-ONO]WCHtBu(OtBu) (3). Applying vacuum to the
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1
since previous attempts to grow single crystals of another anionic
alkylidyne, {MePPh3}{[CF3−ONO]WCtBu(OtBu)} (5),
were unsuccessful.18 The anion of complex 4 is pseudo-Cs sym-
metric, and the tungsten ion adopts a distorted (τ = 0.09)33
square-pyramidal geometry, with the WC bond occupying
the axial position. The [pyr-ONO]3− trianionic pincer and
−OtBu ligands coordinate in the basal plane. The pyrrolide
binds through the σ-N coordination bond, supporting the 13C
NMR assignment. The W1−C29 bond length is 1.741(3) Å,
which is shorter by 0.013(3) Å than the corresponding bond
found in the only other ONO3− tungsten alkylidyne ([CF3-
ONO]WCtBu(OEt2)) to be crystallographically character-
ized.18 The W1−N1 bond length of 2.161(3) Å is consistent
with other complexes featuring an η1-pyrrolide coordinated to a
W(VI) ion.20−28 The W1−N1 bond length is longer, as expected,
than the Namido−W bond of 2.008(2) Å found in [CF3-
ONO]WCtBu(OEt2).
and the initial broadness in the H spectrum of 4 in CDCl3.
Presumably time is required for the solvent to break apart the
lattice interactions. More importantly, the close C−H···π con-
tacts suggest significant electron density exists on the pyrrolide
portion of the [pyr-ONO]3− ligand. This physical observation
is consistent with both computational and experimental findings
(vide infra).
DFT geometry optimization of the anionic tungsten alkylidyne
matches very well the crystallographic structure of 4 (see the
Supporting Information). Figure 4 depicts selected molecular
Examining the close contact distances surrounding the
anionic fragment of complex 4 reveals some interesting features
within the crystal lattice. The [pyr-ONO]3− ligand is encap-
sulated by three nearby Ph3PCH3+ cations (Figure 3). Notably,
Figure 4. Single-point calculation of the [pyr-ONO]WCtBu-
(OtBu)− fragment 4′ (isovalue 0.051687).
orbitals obtained from single-point calculations. An important
observation is that an N atom lone pair is not within the HOMO
and results in two energetically similar tungsten alkylidyne π
bonds (HOMO-1 and HOMO-2). These results are in contrast
with the electronic structure of the alkylidyne anion {MePPh3}-
{[CF3-ONO]WCtBu(OtBu)} (5), featuring an ONO ligand
with a prominent N atom lone pair that aligns with the M−C
multiple bond to produce an inorganic enamine.18,19 Interestingly,
the HOMO-1 contains a slightly antibonding interaction to
the π combination within the pyrrolide ring, though it is a
significantly weaker interaction in comparison to that observed
within 5.18,19
To access a neutral species, the tert-butoxide ligand must be
eliminated. An established protocol9,18 is to add methyl triflate
to produce methyl tert-butyl ether (MeOtBu) and the salt
[MePPh3][OTf]. Adding methyl triflate to 4 in Et2O, with the
intention of removing the −OtBu ligand, produces copious
amounts of [MePPh3][OTf] precipitate, but a 1H NMR
spectrum of the supernatant reveals the presence of multiple
unidentifiable products. Within the complicated NMR mixture,
resonances attributable to pyrrole-Me protons were observed,
which provides some evidence that alkylation occurs at the
[pyr-ONO]3− ligand. In addition, alkylation of the pyrrolide is
consistent with the single-point calculations, indicating the π
orbitals of the central pyrrolide are the HOMO and are
relatively unhindered toward electrophilic attack. These results
are not surprising, considering literature precedent for electro-
philic aromatic substitution of pyrroles.35,36 A comparative
example is the protonation of a pyrrolide-ligated tungsten
alkylidene to afford pyrrolenine (eq 1).27
+
Figure 3. Close C−H···π contact between MePPh3 and [pyr-
ONO]WCtBu(OtBu)− in the solid-state structure of complex 4.
the distances between the calculated hydrogen atom positions
of the Ph3PCH3 are too close to the anion to be regular van
+
der Waals contacts (2.94 Å), suggesting an interaction between the
phosphonium protons and the π bonds of the [pyr-ONO]3−
+
ligand. The closest contacts to each Ph3PCH3 are depicted in
Figure 3, and their distances are 2.612(3), 2.658(3), and
+
2.716(2) Å. These distances are well within reported Ph3PCH3
C−H···π contact distances with a related furan.34 This inter-
action also explains the poor solubility of complex 4 in C6D6
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pyrrolide-centered trianionic pincer-type ligand. Considering
that the first step of the ligand synthesis employs a boronic acid
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Text, figures, tables, and a CIF file giving NMR spectra, X-ray
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AUTHOR INFORMATION
Corresponding Author
*E-mail for A.S.V.: veige@chem.ufl.edu.
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ACKNOWLEDGMENTS
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A.S.V. acknowledges the UF and the NSF (CHE-1265993), for
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NSF (CHE-0821346) for the purchase of X-ray equipment.
Computational resources were provided by the University of
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