Three-coordinate, cyclic bent allene iron complexes

Article abstract of DOI:10.1021/om400166t

Novel three-coordinate Fe complexes featuring 1,2-diphenyl-3,5-bis(2,6- dimethylphenoxy)pyrazolin-4-ylidine or "cyclic bent allene" (CBA) have been synthesized. Reaction with FeCl₂(PPh₃)₂ results in the loss of both phosph

Full text of DOI:10.1021/om400166t

Article
pubs.acs.org/Organometallics
Three-Coordinate, Cyclic Bent Allene Iron Complexes
Conor Pranckevicius and Douglas W. Stephan*
Department of Chemistry, University of Toronto, Toronto, Ontario, Canada
S
* Supporting Information
ABSTRACT: Novel three-coordinate Fe complexes featuring 1,2-diphenyl-3,5-bis(2,6-
dimethylphenoxy)pyrazolin-4-ylidine or “cyclic bent allene” (CBA) have been synthesized.
Reaction with FeCl2(PPh3)2 results in the loss of both phosphines to yield monomeric
Fe(CBA)Cl2. Treatment with 2 equiv of benzyl Grignard affords the three-coordinate alkylated
product Fe(CBA)(Bn)2. Exposure of the alkylated species to an atmosphere of CO results in
reductive elimination of 1,3-diphenylpropanone and formation of trigonal-bipyramidal Fe(CBA)-
(CO)4.
described as linked bis-carbenes ligating a carbon(0) atom.
Despite their unique properties, only a very small number of
reports21,26 describing their coordination chemistry have
appeared in the literature.
INTRODUCTION
■
N-heterocyclic carbenes (NHCs) have found widespread use as
ligands in a number of transition-metal-catalyzed trans-
formations.1−9 Their strong σ-donating ability can facilitate
oxidative addition, increase metal back-donation to π-acidic
ligands, and assist in ancillary ligand dissociation: important
steps in many catalytic cycles. Recently, interest has expanded
to include divalent carbon donors other than imidazolin-2-
ylidine. Metal-bound mesoionic carbenes based on imidazolin-
4-ylidine,10−12 triazole,13−15 and pyrazole,16−19 as well as cyclic
alkylamino carbenes.20 have been prepared and have been
shown to exhibit σ-donor and π-acceptor properties different
from those of NHCs, allowing refined electronic tuning of the
metal center.
In 2008, Bertrand and co-workers reported the isolation of
pyrazolin-4-ylidenes, termed “cyclic bent allenes” (CBAs).21,22
The very strong donating nature of this ligand was evidenced
by the average νCO value of Rh(CO)2Cl(CBA), which was
among the lowest reported. Additionally, computational and
experimental data have confirmed that CBAs are dibasic and are
capable of behaving as four-electron donors at the central
carbon.23−25 This ability stems from the introduction of
heteroatoms at the 3,5-positions on pyrazole; exocyclic
delocalization of the ring π electrons through these positions
disrupts aromaticity and localizes two lone pairs of electrons on
the central carbon (Scheme 1). CBAs can be equivalently
We have had a continuing interest in sterically demanding
ligands that provide strong donors but also afford steric
protection of an active site on a metal. One approach to such
systems has involved the use of hemilabile chelates which
stabilize reactive metal centers by the interaction of pendant
weakly bound donors.27 In our own work, we had previously
examined systems that incorporate pendant arene fragments
that shield the respective metal centers. In this regard we have
probed biphenyl-phosphinimide Ti complexes28 and Al
biphenylamide derivatives.29 In considering other ligand
systems with the potential for such shielding of a metal center,
our attention focused on CBAs, as the pendant aryloxide
substituents are positioned to provide the possibility of
bracketing the metal, thus affording an avenue to low-
coordinate species.
In probing this hypothesis, we focused on Fe chemistry. In
general, Fe carbene complexes have drawn less attention than
their heavier congeners. Nonetheless, several reports have
described the isolation of Fe complexes featuring chelating
ligands incorporating NHCs30−41 as well as a handful of
tetracoordinate Fe species containing monodentate car-
benes.42−46 Combinations of sterically demanding NHCs and
anionic ligands have previously been exploited to access three-
coordinate trigonal-planar species, including Fe(NHC)X2 (X =
Bn, N(SiMe3)2, Mes, OSiPh3, NH(Dipp))45−48 and Fe(NHC)-
Cl(R) (R = Bn, N(SiMe3)2, CH2TMS).47 In this report, we
show that the steric demands of CBA ligands also provide for
Scheme 1. Resonance Forms of 3,5-Bis(aryloxy)pyrazolin-4-
ylidine
Received: February 28, 2013
Published: April 16, 2013
© 2013 American Chemical Society
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low-coordinate Fe complexes by describing the first examples of
Fe−CBA complexes, including the first example of a three-
coordinate Fe−dihalide species. In addition, subsequent
reactivity with the prototypical organometallic ligand CO is
probed.
RESULTS AND DISCUSSION
■
Employing a slightly modified synthetic procedure,21 1,2-
diphenoxy-3,5-bis(2,6-dimethylphenyl)pyrazolin-4-ylidine (1)
was isolated in high yield by deprotonation of the
corresponding pyrazolium tetrafluoroborate salt with K[N-
(SiMe3)2] at −47 °C as a pale yellow crystalline solid.
Subsequent reaction of FeCl2(PPh3)2 with a slight excess of 1
in toluene over the course of 2 h (Scheme 2) resulted in the
Scheme 2. Synthesis of 2−4
Figure 2. Molecular structure of 2: orange, Fe; green, Cl; red, O; blue,
N. Ellipsoids are shown at the 50% probability level. Hydrogen atoms
and cocrystallized toluene are omitted for clarity. Selected bond
lengths (Å) and angles (deg): Fe−C(1) = 2.065(4), Fe−Cl(1) =
2.214(1), Fe−Cl(2) = 2.244(1); C(1)−Fe−Cl(1) = 125.7(1), C(1)−
Fe−Cl(2) = 115.8(1), Cl(1)−Fe−Cl(2) = 118.28(5), C−O(1)−C =
115.0(3), C−O(2)−C = 116.3(3).
are canted toward the Fe center; however, the shortest Fe−
Cxylyl distance is over 3.0 Å in the molecular structure and the
xylyl groups remain equivalent in the 1H NMR upon cooling to
−80 °C. NMR inequivalence would be anticipated for Fe
bonded to either one or both xylyl rings. Complex 2 was also
characterized by Mossbauer spectroscopy. At 80 K in the zero
̈
field, a doublet was observed with a quadrupole splitting of 1.06
mm/s and an isomer shift of 0.65 mm/s (see the Supporting
Information for the spectrum), close to what has been
previously reported for three-coordinate Fe(II) where S =
2.50−52 To our knowledge, this is the first example of a
monomeric three-coordinate Fe−dihalide species, as related
bulky NHC−Fe complexes are chloride-bridged dimers.47
Treatment of a suspension of 2 at −47 °C with 2 equiv of
benzyl Grignard in diethyl ether resulted in an immediate color
change to red. Within minutes, highly air sensitive golden
brown crystals of 3 suitable for X-ray diffraction separated from
the solution and were isolated in 64% yield (Scheme 2). The
1H NMR displays additional paramagnetically shifted reso-
nances at +32.5, −36.8, and −68.4 ppm, which can be assigned
respectively to the meta, ortho, and para protons of the benzyl
moiety. The ortho signals are broadened, likely due to their
proximity to the metal center, and signals for the phenyl-
methylene protons are absent, typical for protons α to high-spin
three-coordinate Fe(II).53 The magnetic moment of 3 was
determined to be 5.3 μB, similar to that measured for 2. Single-
crystal X-ray diffraction confirmed the identity of the complex
as three-coordinate Fe(CBA)(Bn)2 (3) (Figure 3). The Fe−
C(1) distance is 2.1059(2) Å, and the Fe−C(32) and Fe−
C(39) distances are 2.082(2) and 2.092(2) Å, respectively. The
Fe−C(1) distance is contracted, and the Fe−benzyl distances
are slightly elongated in comparison to those in the three-
coordinate bis-benzyl NHC complex Fe(SIDipp)(Bn)2, per-
haps reflecting the greater donating power of the CBA.47 The
orientation of the pendant arene rings in both 2 and 3 suggests
electrostatic van der Waals interactions of the electron-rich
arene rings with the electrophilic Fe.
1
formation of the white precipitate 2. Upon purification, the H
NMR spectrum of 2 displayed six paramagnetically shifted
resonances (Figure 1), integrating appropriately for the
1
Figure 1. H NMR spectra of 2: (s) residual CHDCl2; (x) solvent
impurities.
heterotopic protons present in the ligand. The magnetic
moment of 2 was determined to be 5.2 μB by the Evans
method,49 close to the spin-only value (4.9 μB), indicative of an
S = 2 spin state. A single-crystal X-ray diffraction study of 2
identified the complex as three-coordinate Fe(CBA)Cl2 (2)
(Figure 2). The ligands are disposed in a slightly distorted
trigonal planar geometry about the metal center, where the sum
of the angles about Fe is 359.9°. The Fe−C(1) bond length is
2.065(4) Å, shorter than distances typically reported for three-
coordinate Fe−NHC complexes.45,47 The pendant xylyl groups
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Figure 4. Molecular structure of 4. Ellipsoids are shown at the 50%
probability level. Hydrogen atoms and cocrystallized benzene are
omitted for clarity. Selected bond lengths (Å): Fe−C(5) =
2.0259(18); Fe−C(4) = 1.781(2); O(4)−C(4) = 1.151(3); C(5)−
C(6) = 1.402(2); C(5)−C(7) = 1.386(2); N(2)−C(6) = 1.360(2);
N(1)−C(7) = 1.383(2).
Figure 3. Molecular structure of 3. Ellipsoids are shown at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Fe−C(32) = 2.082(2), Fe−C(39)
= 2.092(2), Fe−C(1) = 2.105(2); C(32)−Fe−C(39) = 112.65(7),
C(32)−Fe−C(1) = 121.21(7), C(39)−Fe−C(1) = 125.52(7), C−
O(1)−C = 119.0(1), C−O(2)−C(18) = 119.5(1).
In summary, we have demonstrated that the strongly
donating CBA ligands which incorporate pendant aryloxide
substituents can shield the metal center, providing access to the
low-valent Fe complexes 1 and 2. The corresponding dibenzyl
species 3 reacts with CO, affording 4. This latter species
illustrates an alternate orientation of the aryloxide substituents
and an unusual localization of double bonds within the pyrazole
ring. We are continuing to explore ligands that display such
conformational flexibility and the potential utility of this feature
in catalytic applications.
Compound 3 reacts in toluene solution with 1 atm of CO,
resulting in the immediate reductive elimination of dibenzyl
ketone, as observed by GC-MS, and formation of diamagnetic
Fe(CBA)(CO)4 (4). This insertion−elimination mechanism is
not unprecedented,54,55 as Deng and co-workers recently
isolated (Mes)2CO from the reaction of Fe(Me2IPr)(Mes)2
with CO.46
The 13C NMR spectrum of 4 displays a single peak for the
carbonyl ligands at 219.6 ppm, implying a fast equilibrium
between axially and equatorially disposed CO ligands. IR bands
assignable to A1 CO stretches were observed at 2026 and 1950
cm−1, frequencies lower than those reported for Fe(IMe)(CO)4
of 2043 and 1962 cm−1.56 Interestingly, the degenerate E mode
CO stretch is split into a doublet with peaks at 1905 and 1893
cm−1, presumably due to a structural perturbation in the C3v
symmetry. This splitting has also been observed in related
Fe(0) NHC tetracarbonyl species.56
A crystallographic study of 4 confirmed the formulation and
illustrated that the aryloxide substituents are oriented away
from the Fe(CO)4 fragment (Figure 4). This orientation has
been observed previously by Bertrand et al. in sterically
congested Ru alkylidene complexes.25 Interestingly, one of the
backbone nitrogen atoms is nearly planar while the other is
significantly pyramidalized: ∑N(1) = 345.7°; ∑N(2) = 354.9°.
Additionally, the N(2)−C(6) distance (1.360(2) Å) is
somewhat shorter than N(1)−C(7) (1.383(2) Å), and similarly
C(5)−C(7) (1.386(2) Å) is shorter than the C(5)−C(6) bond
length (1.402(2) Å) (Figure 4). These data imply a degree of
localization of double bonds in the pyrazole ring. The Fe−C(5)
distance is 2.026(2) Å, slightly longer than that reported for the
sterically unhindered Fe(IMe)(CO)4.57 However, the trans C−
O distance (1.151(3) Å) is also longer than that reported for
Fe(IMe)(CO)4, consistent with the more strongly donating
nature of the CBA ligand.
EXPERIMENTAL SECTION
■
General Remarks. All manipulations were carried out under an
atmosphere of dry, O2-free N2 employing a Vacuum Atmospheres
glovebox or a Schlenk vacuum line. Solvents were purified with a
Grubbs-type column system manufactured by Innovative Technology
and dispensed into thick-walled Straus flasks equipped with Teflon-
valve stopcocks. Deuterated dichloromethane was distilled under
reduced pressure from CaH2 and degassed by successive freeze−
pump−thaw cycles. Deuterated benzene was distilled from purple
sodium benzophenone ketyl. 1H and 13C NMR spectra were recorded
at 25 °C on Bruker 400 MHz spectrometers, unless otherwise noted.
Chemical shifts are reported in parts per million and are given relative
to SiMe4 and referenced to the residual solvent signal. All signals in
paramagnetic spectra are singlets unless otherwise stated. Combustion
analyses were performed in house, employing a Perkin-Elmer CHN
Analyzer. IR spectra were collected on a Perkin-Elmer Spectrum One
FT-IR instrument. Zero-field 57Fe Mossbauer spectra were recorded
̈
on a SEE Co. Mossbauer spectrometer (MS4) at 80 K in constant
̈
acceleration mode. 57Co/Rh was used as the radiation source.
WMOSS software was used for the quantitative evaluation of the S5
spectral parameters (least-squares fitting to Lorentzian peaks). The
minimum experimental line widths were 0.23 mm/s. The temperature
of the sample was controlled by a Janis Research Co. CCS-850 He/N2
cryostat within an accuracy of 0.3 K. Potassium hexamethyldisilazide,
benzylmagnesium chloride, and carbon monoxide were purchased
from Aldrich in the highest available grade and used without
subsequent purification. 1,2-Diphenyl-3,5-(2,6-dimethylphenoxy)-
pyrazolium tetrafluoroborate21 and FeCl2(PPh3)2 were prepared
according to literature procedures.
58
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Synthesis of 1,2-Diphenyl-3,5-bis(2,6-dimethylphenoxy)-
pyrazolin-4-ylidine (1). The cyclic bent allene 1 was synthesized
by a slightly modified literature procedure.21 Finely powdered 1,2-
diphenyl-3,5-(2,6-dimethylphenoxy)pyrazolium tetrafluoroborate (800
mg, 1.46 mmol) was combined with potassium hexamethyldisilazide
(292 mg, 1.46 mmol), and the mixture was cooled to −47 °C. Diethyl
ether (10 mL) was added at this temperature, and the stirred mixture
was warmed to room temperature over the course of 50 min. Diethyl
ether (15 mL) was then added, and the resultant suspension was
filtered through a fine-porosity glass frit. The solution was
concentrated under high vacuum until light yellow crystals just
began to form (approximately 5 mL), and pentane (10 mL) was
added. The solution was reconcentrated to 5 mL, resulting in the
precipitation of a large crop of light yellow crystals. This process was
repeated once more to effect complete precipitation of the product.
The remaining supernatant was decanted, and the light yellow
precipitate was washed with pentane (2 × 10 mL) and dried under
high vacuum to yield pure 2 (525 mg, 88%). Spectroscopic data are
identical with those previously reported.22
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for D.W.S.: dstephan@chem.utoronto.ca.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Brian Schaefer in Professor Paul Chirik’s laboratory
at Princeton University for the Mossbauer spectrum of 2.
D.W.S. is grateful for the financial support of the NSERC of
Canada and the award of a Canada Research Chair.
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NOTE ADDED AFTER ASAP PUBLICATION
■
In the version of this paper published on April 16, 2013, the
wrong version of part of the Supporting Information appeared
with the paper. The version of this information that appears as
of May 3, 2013, is correct.
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dx.doi.org/10.1021/om400166t | Organometallics 2013, 32, 2693−2697