Reactivity study of imino-N-heterocyclic carbene palladium(II) methyl complexes

Article abstract of DOI:10.1021/om400533d

Neutral and cationic palladium(II) complexes containing three different heteroditopic bis(aryl)-substituted imino-N-heterocyclic carbene ligands have been synthesized and structurally characterized. Modifications made to the substituent at the iminic carbon allow for independent tuning of the steric and electronic environment around the metal center. The nature of this substituent profoundly affects the thermal stability of the neutral palladium complexes. While the tert-butyl derivative decomposes at 60 °C over a period of 24 h to give the corresponding 1-mesityl-2-methyl-1H-imidazole and the corresponding imidoyl chloride as the major products, the phenyl and methyl analogues show no sign of decomposition. Likewise, the corresponding cationic complexes of all three carbene derivatives are stable under the same conditions, with no evidence of decomposition. While inactive for ethylene polymerization, these palladium methyl complexes react with CO and isocyanides to form a variety of products, including structurally characterized simple adducts and single and multiple insertion products.

Full text of DOI:10.1021/om400533d

Article
pubs.acs.org/Organometallics
Reactivity Study of Imino‑N‑Heterocyclic Carbene Palladium(II)
Methyl Complexes
Anna C. Badaj and Gino G. Lavoie*
Department of Chemistry, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada
S
* Supporting Information
ABSTRACT: Neutral and cationic palladium(II) complexes
containing three different heteroditopic bis(aryl)-substituted
imino-N-heterocyclic carbene ligands have been synthesized
and structurally characterized. Modifications made to the
substituent at the iminic carbon allow for independent tuning
of the steric and electronic environment around the metal
center. The nature of this substituent profoundly affects the
thermal stability of the neutral palladium complexes. While the
tert-butyl derivative decomposes at 60 °C over a period of 24 h
to give the corresponding 1-mesityl-2-methyl-1H-imidazole
and the corresponding imidoyl chloride as the major products, the phenyl and methyl analogues show no sign of decomposition.
Likewise, the corresponding cationic complexes of all three carbene derivatives are stable under the same conditions, with no evidence
of decomposition. While inactive for ethylene polymerization, these palladium methyl complexes react with CO and isocyanides to
form a variety of products, including structurally characterized simple adducts and single and multiple insertion products.
catalysts capable of copolymerizing ethylene and functionalized
olefins, such as acrylates.18,19 The C∧Imine ligands are
conceptually a hybrid of the α-diimine ligands in which one of
the two imine fragments has been replaced by a stronger σ-donor
and a poorer π-acceptor carbene. The NHC moiety was chosen
due to its demonstrated ability to impart thermal stability in
several complexes used in a variety of catalytic applications.20−22
Nickel complexes of these C∧Imine ligands were however found
to be inactive for ethylene polymerization.15
Considering the importance of palladium catalysts in cross-
coupling reactions20,21,23 and their ability to polymerize
functionalized olefins,18,19,24 we decided to extend our study to
diamagnetic palladium−methyl complexes to gain insight into
their thermal stability and into their insertion chemistry. Herein,
we present the synthesis, structural characterization, thermal
stability, and reactivity of both neutral and cationic Pd(II) methyl
complexes of C∧ImineR (where R = Me, Ph, or tBu) ligands
toward ethylene, isocyanides, and carbon monoxide.
INTRODUCTION
■
The reactivity of the metal−carbon bond plays a critical role in the
stoichiometric and catalytic synthesis of organic compounds.1
While carbon monoxide has historically played an important role as
substrate due to its role in hydroformylation,2 the related isocyanide
has received relatively far less attention.3 Isocyanides are of special
interest thanks to the addition of a biologically important nitrogen
atom and thanks to the presence of a new iminic reactive site for
subsequent reaction on the molecule. Palladium methyl complexes
of chelating nitrogen4−8 and phosphine9−11 ligands have proven
capable of inserting not only one but also multiple equivalents
of aryl and alkyl isocyanides.8 In many cases, the use of excess
isocyanide, however, leads to dissociation of the ancillary ligand
used, such as monodentate phosphine,11 and chelating P−P9 and
N−N5,9 ligands, with a decrease in control over the reactivity of the
transition metal complex.
Coordination of strong σ donors such as N-heterocyclic carbenes
(NHCs) may mitigate or eliminate this undesired dissociation of
the ancillary ligand. To our knowledge, only one NHC palladium
methyl complex has been shown to insert one equivalent of
isocyanide, even when used in excess.12 In the absence of an alkyl
migratory group, the isocyanide inserts into the palladium−carbene
bond in palladium iodide dimers coordinated with remote
N-heterocyclic carbenes.13 Interestingly, addition of 2,6-dimethy-
laniline to dibromo(NHC)(isocyanide)palladium gives a complex
that contains a new acyclic carbene, the result of a nucleophilic
attack of the aniline on the coordinated isocyanide.14
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Palladium(II)
Imino−NHC Complexes. Imidazolium salts, 1a and 1b, were
synthesized following literature procedures (Scheme 1).16 The
phenyl derivative C∧IminePh·HBF4 (1c) was prepared in 67%
and necessitated activation of the imidoyl chloride by reaction
with sodium tetrafluoroborate to form the corresponding
1
nitrilium salt in situ. The H NMR (CDCl3) resonance for the
central imidazolium proton of 1c is observed at δ 8.81, while
We have recently reported the synthesis and coordination of
aryl-substituted heteroditopic imino-N-heterocyclic carbenes
(C∧Imine).15−17 These are closely related to the α-diimine
ligands that have been installed on nickel and palladium to form
Received: June 10, 2013
© XXXX American Chemical Society
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Scheme 1. Synthesis of Imidazolium Salts (1a−1c) and of the Corresponding Free Carbenes (2b and 2c)
Scheme 2. Synthesis of Neutral (4a−4c) and Cationic (5a−5c) Palladium Complexes
Table 1. Selected Bond Distances (Å) and Angles (deg) for Complexes 4a−4c, 5b, and 12
4a
4b
4c
5b
12
bond lengths
Pd1−C1
Pd1−N3
Pd1−N4
Pd1−Cl1
1.969(4)
2.184(3)

1.939(5)
2.165(4)

1.969(3)
2.170(3)

1.949(3)
2.146(2)

1.977(4)
2.191(3)

2.3605(10)
2.033(4)

2.3612(12)
2.036(5)

2.3237(9)
2.040(4)


2.3268(12)

Pd1−CMe
2.039(3)

Pd1−C27
Pd1−C42
C4−N1
C4−N3
C27−O1
C32−C24
bond angles
C1−Pd1−N3
C1−Pd1−Cl1
C42−Pd1−Cl1
Pd1−N3−Cipso
1.972(5)





1.406(5)
1.273(5)

1.449(6)
1.295(6)

1.412(4)
1.295(4)

1.435(3)
1.278(3)

1.436(5)
1.282(5)
1.205(6)





77.93(14)
174.97(12)

77.22(17)
172.75(16)

78.54(13)
174.26(18)

77.47(10)

77.45(14)
172.69(12)


127.8(2)
117.1(3)
126.0(2)
116.27(16)
117.0(2)
those for the inequivalent backbone protons of the heterocycle
appear at δ 8.09 and 7.52. The iminic and the central imidazolium
carbon nuclei resonate at δ 148.1 and 136.1, respectively. The
Fourier transform-infrared (FT-IR) stretching frequency for the
imine group of 1c was observed at 1675 cm−1, consistent with
values reported for 1a and 1b.16 Deprotonation of 1c with
NaHMDS cleanly gave the free carbene C∧IminePh (2c) in 72%
yield, with 1H and 13C NMR spectra consistent with the desired
product. The stretching frequency of the imine group has
decreased from that observed in 1c to 1652 cm−1.
(for 4a) or ligand displacement (for 4b and 4c) (Scheme 2). In
contrast to the corresponding (C∧ImineR)NiBr2 complexes,15
these complexes are diamagnetic, indicating a square planar
geometry. All three complexes are stable in solution on exposure
to air and moisture at room temperature. Their 1H NMR spectra
showed one set of resonances, consistent with the formation of
only one isomer, with the methyl group attached to the metal
center resonating at δ 0.23−0.42 in CDCl3. The conformation of
all complexes was determined by one-dimensional (1D)-nuclear
Overhauser effect spectroscopy (NOESY) nuclear magnetic
resonance (NMR) experiments. Selective excitation on the
methyl group bound to palladium resulted in a nuclear
The neutral palladium complexes were prepared in good
yields, 80%−98%, either from copper(I) transmetalation15,25
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Figure 1. ORTEP plot of the molecular structure of (C∧ImineMe)PdMeCl (4a) (50% probability level). Hydrogen atoms and a solvent molecule of
chloroform omitted for clarity.
Figure 2. ORTEP plot of the molecular structure of (C∧Iminet‑Bu)PdMeCl (4b) (50% probability level). Hydrogen atoms and a solvent molecule of
chloroform omitted for clarity.
Overhauser effect (NOE) for the mesityl ring directly attached to
the azole ring, consistent with the methyl group trans to the
imine nitrogen and the chloro ligand trans to the carbene.
The palladium methyl carbon and the central imidazolium
(NCN) carbon nuclei resonate at approximately δ 9 and 177,
respectively, for complexes 4a−4c. While the resonance of the
iminic carbon for both 4a and 4c appear at δ 156, that of the tert-
butyl derivative 4b is found downfield at δ 161.2. The decrease in
the FTIR νCN absorption for 4a−4c (1626−1652 cm−1) from
the corresponding imidazolium salt strongly suggests coordina-
tion of the ligand through both the carbenoid carbon and the
iminic nitrogen atoms. These values are also in agreement with
those observed in the related nickel complexes.8
The crystal structures for all three palladium complexes 4a−4c
were obtained, and the selected bond angles and bond lengths are
provided in Table 1. Complexes 4a (Figure 1) and 4c (Figure 3)
both crystallized in the space group P21/n, while complex 4b
crystallized in the space group P212121 (Figure 2). All three
complexes show a slightly distorted square planar structure
where the chloro ligand is trans to the carbene, in agreement with
the assignment based on solution NMR spectroscopy and
consistent with the trans influence. The palladium−methyl bond
lengths are observed in the range of 1.939(5)−1.969(4) Å,
similar to those reported for related complexes of NHC∧pyridine
bidentate ligands.26 The palladium−chloride bond lengths are in
the range of 2.3237(9)−2.3612(12) Å. In all three complexes
4a−4c, the C1−Pd1−N3 bite angles are comparable, with values
ranging from 77.22(17)° to 78.54(13)°. In contrast, the nature of
the substituent has a marked impact on the Pd1−N3−Cipso
angles, with the large tert-butyl group pushing the xylyl ring
toward the metal, resulting in the smallest angle of 117.1(3)°.
The smaller methyl group yields a larger angle of 127.8(2)°,
which is only slightly further reduced to 126.0(2)° upon
substituting the iminic carbon with a phenyl ring. These
differences further manifest themselves in the yaw distortion,27
with the smallest one observed in the tert-butyl derivative
(θ = 11.9°) and the largest ones in the methyl (θ = 13.5°) and phenyl
(θ = 13.6°) analogues. This is consistent with our previous report
on nickel analogues.15 The Pd1−N3 bond length is the smallest
for the tert-butyl [4b, 2.165(4) Å] and the phenyl [4c, 2.170(3)
Å] derivatives and the largest [2.184(3) Å] for the methyl
derivative (4a). Interestingly, the Pd−C1 bond length in 4a and
4c are also statistically equivalent and longer at 1.969 Å
compared to 4b [1.939(5) Å]. These metrics clearly impact
the coordination sphere around the metal, possibly resulting in
differences in reactivity toward various substrates. Finally, the
solid-state structures clearly show the axial positions of the
square planar structures well-protected by the ortho-methyl
substituents of both aryl rings, which are tilted by 85.8° and 74.0°
(4a), 87.2° and 84.6°(4b), and 79.0° and 69.8° (4c), with respect
to the mean plane formed by the metallacycle. Protection of the
axial positions is well-known to be essential for α-diimine
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Figure 3. ORTEP plot of the molecular structure of (C∧IminePh)PdMeCl (4c) (50% probability level). Hydrogen atoms omitted for clarity.
−
Figure 4. ORTEP diagram of [(C∧Iminet‑Bu)PdMe(MeCN)]PF6 (5b) (50% probability level). Counteranion (PF6 ) and hydrogen atoms omitted for
clarity.
palladium (and nickel) catalysts to achieve high molecular weight
polymer, effectively mitigating chain transfer from an associative
displacement of an alkene terminated polymer chain by an
incoming monomer.28
chemical shifts in chloroform-d ranging from δ −0.10 to 0.10.
The acetronitrile protons appears at approximately δ 1.6 for all
three cationic complexes, consistent with its coordination to the
metal center. The carbenoid carbon (NCN) nucleus of
compounds 5a−5c resonates at approximately δ 172 and upfield
shift from that of 4a−4c. Interestingly, no significant changes in
chemical shifts were observed for the palladium methyl carbon
nucleus. The lower electron density on the cationic metal center
of compounds 5a−5c led to a decrease in π-backdonation, as
evidenced by the FTIR νCN stretching frequencies (1635−
1660 cm−1) that are 5−9 cm−1 larger than those observed in
4a−4c. The values of these stretching frequencies are also
consistent with bidentate coordination of the ligand through the
carbenoid carbon and the imine nitrogen.
The related cationic methyl complexes were also prepared as
model compounds for the cationic hydrocarbyl propagating
species that are present in the catalytic cycle of olefin
polymerization. Addition of one equivalent of silver hexafluor-
ophosphate to compounds 4a−4c in acetonitrile resulted in the
formation of the cationic complexes 5a−5c in excellent yields
(Scheme 2). Similar to the neutral complexes, all cationic
complexes are stable in solution on exposure to air and moisture
at room temperature. Similarly, only one set of resonances is
observed in the proton NMR spectra, consistent with the
presence of only one isomer. 1D-NOESY NMR spectra are
consistent with the methyl group trans to the imine nitrogen,
with an acetonitrile molecule trans to the carbene.
X-ray quality crystals of 5b were successfully grown at room
temperature by slow vapor diffusion of diethyl ether into a
saturated dichloromethane solution. Compound 5b crystallized
in a monoclinic crystal system in the P21/n space group. The
complex adopts a distorted square planar geometry, with the
The palladium-bound methyl protons in 5a−5c resonate at
lower frequencies to those of the neutral analogues 4a−4c, with
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Scheme 3. Proposed Mechanisms for the Thermal Decomposition of Neutral 4b
acetonitrile bound trans to the carbene, as predicted from NMR
spectroscopy experiments (Figure 4). Abstraction of the chloro
ligand and its replacement with a molecule of acetonitrile caused
a decrease in both the Pd1−N3 and C4−N3 bond lengths from
2.165(4) Å in 4b to 2.146(2) Å in 5b and from 1.295(6) Å in 4b
to 1.278(3) Å in 5b, respectively, with very little change to the
Pd1−C1 bond length. This substitution also resulted in the
Pd1−N3−Cipso bond angle decreasing from 117.1(3) to
116.27(16)°, with little change to either the C1−Pd1−N3 bite
angle or the yaw distortion (θ12.2°in 5b compared to θ 11.9°in 4b).
Reactivity Studies of Palladium Complexes 4 and 5.
The activity of the compounds 4a−4c in the polymerization of
ethylene was tested at atmospheric pressure and room
temperature using 1000 equivalents methylaluminoxane
(MAO) as cocatalyst to generate in situ the putative catalytically
active cationic species. Palladium black readily formed and no
ethylene uptake was noted in any of the trials. While palladium
black was not observed in reactions with 5a−5c, these complexes
did not either produce polyethylene with and without MAO added.
Considering reports of reductive elimination in other NHC
palladium alkyl complexes,12,29,30 thermal studies of complexes 4
and 5 were undertaken to determine whether these complexes also
suffered from this undesired side-reaction, limiting their utility in
catalysis involving migratory insertion of an alkyl group.
All complexes were heated to 60 °C in CDCl3 and monitored
over time for any sign of decomposition. The nature of the
substituent on the iminic carbon impacted the thermal stability of
the neutral complexes 4a−4c. In the case of the tert-butyl
derivative 4b, N-(2,6-dimethylphenyl)pivalimidoyl chloride and
1-mesityl-2-methyl-1H-imidazole31 were produced as the major
species after 30 min, with complete decomposition of 4b within
24 h. In contrast, both the methyl (4a) and phenyl (4c)
derivatives and the cationic complexes (5a−5c) showed no
evidence of decomposition when heated to 60 °C in CDCl3. This
is in stark contrast to other palladium alkyl systems for which the
cationic species undergo reduction elimination more readily than
their neutral analogues.29,30,32
Coordination of the imine to the cationic metal center enhances
the electrophilicity of the carbon atom, making it more
susceptible to nucleophilic attack of the chloride to generate
the observed imidoyl chloride and a reactive two-coordinate
(imidazolyl)(methyl)palladium complex C that undergoes
reductive elimination to give the substituted imidazole.
To get a better understanding of the actual decomposition
mechanism, the imidazolium tetrafluoroborate salt 1b and both
the neutral (4b) and cationic (5b) palladium methyl complexes
were treated with chloride ions. Addition of tetrabutylammo-
nium chloride to 1b gave no reaction, even at elevated
temperature, ruling out the formation of compound A, and
consequently of path a. Compound 1b, however, rapidly
decomposed to N-mesitylimidazole and to the imidoyl chloride
upon addition of HCl in diethyl ether at room temperature,
supposedly due to protonation of the imine nitrogen, effectively
activating the imine carbon for nucleophilic attack by the chloride.
While palladium complex 4b remains intact at room
temperature when exposed to Bu4NCl, formation of N-(2,6-
dimethylphenyl)pivalimidoyl chloride and 1-mesityl-2-methyl-
1H-imidazole was observed upon heating the reaction mixture to
60 °C. The lower decomposition rate compared to that observed
in the absence of additional chloride ions is consistent with the
cationic metal complex B being a key intermediate in the
decomposition of 4b. These results, coupled with the rapid
decomposition of the cationic complex 5b observed upon
addition of Bu4NCl, support the mechanism proposed in path b
(Scheme 3). While dissociation of the chloride in 4a and 4c may
also occur, the stronger imidazole−imine bond in these two
complexes, indicated by their short C4−N1 bond lengths
[1.406(5) and 1.412(4) Å, respectively, compared to 1.449(6)
in 4b], may prevent further decomposition to compound C,
resulting in the reversible coordination of the chloride anion to
regenerate the stable four-coordinate neutral palladium complex.
The stability of all three cationic complexes 5a−5c at 60 °C,
however, suggested that the inability of these complexes to
catalyze the polymerization of ethylene was not due to
decomposition of a cationic metal alkyl propagating species but
possibly arose from their inability to undergo migratory
insertion. To get further insight into the ability of the palladium
methyl group to insert into electrophiles, reaction of complexes 4
and 5 toward isocyanides was investigated. Interestingly, despite
using similar experimental conditions, reaction of neutral
complexes 4a−4c with tert-butyl isocyanide in toluene all led
to different reaction products, further highlighting the important
role of the substituent on the imine carbon (Scheme 4). Addition
Two mechanisms for the formation of N-(2,6-
dimethylphenyl)pivalimidoyl chloride and 1-mesityl-2-methyl-
1H-imidazole from 4b are proposed in Scheme 3. In path a,
complex 4b undergoes reductive elimination to generate Pd(0)
and the 2-methylimidazolium salt (A), which then reacts with the
chloride anion to produce the observed products. While chloride
is a very poor nucleophile, we have observed similar cleavage of
the imidazole−imine bond in other related systems.33 In path b,
the chloride first dissociates from 4b to form complex B.
E
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trans to each other. The palladium atom lies on an inversion
center, which results in two sets of crystallographically equivalent
ligand sets and chlorine atoms, with linear angles for C1−Pd−
C1a and Cl1−Pd1−Cl1a. Other angles around the metal range
from 89.7(2)° to 90.3(2)° and add up to 180(3)°. The Pd1−C1
bond length is of 2.027(4) Å, within the range reported for other
(NHC)2PdCl2 complexes.34 The N3−C4 bond length measures
1.252(5) Å, statistically equivalent to that observed in 1a.17
Under similar conditions, when tert-butyl isocyanide was
added to (C∧Iminet‑Bu)PdMeCl (4b) in toluene at −35 °C and
allowedtowarmuptoroomtemperature, agradualcolorchangefrom
tan to yellow was observed, with the final product identified as the
coordinated isocyanide adduct (C∧Iminet‑Bu)PdMe(t-BuNC)Cl (7)
Scheme 4. Reactivity of Neutral Complexes 4a−4c with tert-
Butyl Isocyanides
1
(Scheme 4). The H NMR spectrum (C6D6) of compound 7 is
consistent with coordination of the isocyanide with no insertion, as
evidenced by a new resonance at δ0.70 attributed to the protons of the
coordinated isocyanide and the palladium methyl protons remaining
upfield at δ−0.50. Migratory insertion would have resulted in the latter
resonating at a higher frequency (δ 1.6−2.3), characteristic of the
iminoacyl complex.9 In the 13C NMR spectrum, the isocyanide carbon
atom that is directly attached to the metal center is observed at δ119.6,
consistent with simple coordination.7,35 The IR spectrum shows a
strong band at 2189 cm−1, assigned to the coordinated isocyanide.
Heating the reaction mixture to 70 °C to enforce migratory insertion
of the methyl group into the coordinated isocyanide led to the
formation of palladium black and unidentified products presumably
through decomposition (vide supra).
of one equivalent of tert-butyl isocyanide to (C∧ImineMe)-
PdMeCl (4a) in toluene at −35 °C, with subsequent warming to
room temperature and standard workup, resulted a yellow solid.
1
The H NMR spectrum was surprisingly simple, with only
resonances assigned to the coordinated C∧ImineMe ligand
present, with no other proton coming from either the palladium
methyl group of the isocyanide. The FTIR νCN stretching
frequency at 1674 cm−1 strongly suggests coordination of the
C∧ImineMe ligand exclusively through the carbenoid carbon
atom. X-ray quality crystals of the isolated product were
successfully obtained by slow vapor diffusion of pentane into a
concentrated solution of dichloromethane.
In contrast, reaction of tert-butyl isocyanide with (C∧IminePh)-
PdMeCl (4c) at room temperature yielded the insertion product
1
8, as a light orange solid (Scheme 4). The H NMR spectrum
(C6D6) of 8 is consistent with the structure, as evidenced by a
new resonance at δ 2.49 for the iminoacyl methyl protons, and
the disappearance of the initial Pd−Me resonance at δ −0.50.
The imine bond stretching frequency at 1638 cm−1 furthermore
indicates that C∧IminePh remains coordinated in a bidentate
fashion to satisfy the preferred electronics and tetracoordinate
nature of palladium(II).
The compound crystallized in the centrosymmetric P1 space
̅
group, with the solid-state structure consistent with the
spectroscopic data, in which two C∧ImineMe ligands are bound
to the metal center through the carbene, with the iminic arm
remaining uncoordinated (Figure 5), similar to that observed in
(C∧ImineMe)2NiCl2.8 Compound 6 adopts a slightly distorted
square planar geometry, with both carbene ligands coordinated
Replacing the tert-butyl group of the isocyanide by an aryl ring
has a profound effect on the chemistry displayed by complexes
Figure 5. ORTEP plot of (C∧ImineMe)2PdCl2 (6) (50% probability level). Hydrogen atoms omitted for clarity. Selected bond distances (Å) and angles
(deg): Pd1−C1 2.027(4), Pd1−Cl1 2.413(10), C4−N1 1.445(5), C4−N3 1.254(5), and C1−Pd1−Cl1 89.7(2).
F
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Scheme 5. Reactivity of Complex 4a with 2,6-Dimethylphenyl Isocyanide
Figure 6. ORTEP plot of chloro(C∧ImineMe)(1,2-bis[(2,6-dimethylphenyl)imino]-3-[(2,6-dimethylphenyl)imino-κ-N]butyl-κ-C]palladium(II) (9)
(50% probability level). Left: Full structure. Right: N1/N2 substituents are omitted for clarity. Hydrogen atoms and one solvent molecule of chloroform
omitted in both views for clarity. Selected bond distances (Å) and angles (deg): Pd1−C1 1.996(3), Pd1−N4 2.103(3), Pd1−Cl1 2.400(19), Pd1−C42
1.971(3), C4−N1 1.446(4), C4−N3 1.265(4), C24−C32 1.290(4), C1−Pd1−N3 79.71(12), C1−Pd1−Cl1 94.7(6), C42−Pd1−Cl1 166.9(5), Pd1−
N4−C24 123.6(2).
4a−4c. Reaction with an excess of 2,6-dimethylphenyl isocyanide
gave spectroscopic evidence of several products of multiple
insertions (Scheme 5). X-ray quality crystals were successfully
isolated from the reaction mixture with 4a. Compound 9
selectively crystallized in a monoclinic crystal system in the P21/c
space group. The palladium center adopts a distorted square
planar geometry with three molecules of isocyanide inserted into
the Pd−Me bond, and forming a new five-membered metalla-
cycle, effectively breaking the chelate formed by C∧ImineMe in 4a
(Figure 6). This demonstrates the hemilability of the imine
fragment, easily dissociating from the metal center to
accommodate the steric and electronic properties of the other
ligands. This feature may prove invaluable in catalytic trans-
formations when reactive coordinatively- and electronically
unsaturated intermediates are produced. These intermediates
could effectively be stabilized through coordination of the imine
nitrogen atom before re-entering the catalytic cycle upon
associative displacement of the nitrogen donor by an incoming
substrate.
metallacycle through coordination of the isocyanide nitrogen
atom N4 is preferred over coordination of N3 from C∧ImineMe,
possibly due to the higher strain that would otherwise result
based on the yaw distortion for 4 and 5 (vide supra). The
formation of the observed metallacycle furthermore generates a
more favorable metal bite angle of 79.71(12)°, compared to the
more acute 77.93(14)° in 4a. This also leads to a Pd−N4 bond
length of 2.103(3) Å markedly shorter than Pd1−N3 [2.184(3)
Å] in complex 4a, with enhanced π-backdonation from the metal
to the CN π* orbital, resulting in a C32−N4 bond [1.290(4) Å]
slightly longer than C4−N3 in 4a [1.273(5) Å].
Considering the better thermal stability of the palladium
complexes 5 and the importance of cationic complexes in olefin
polymerization catalysis, the reactivity of these complexes in
isocyanide insertion was also explored. Addition of one
equivalent of either tert-butyl or 2,6-dimethylphenyl isocyanide
to the palladium complex produced the isocyanide adduct in
good yield (83−96%) (Scheme 6). The 1H NMR spectra of the
tert-butyl isocyanide reaction products (10) showed sharp
upfield resonances (δ 0.01 to −0.10) integrating to three
protons, characteristic of the palladium methyl group. The FTIR
νCN absorption of the coordinated isocyanide were observed as
a strong sharp band at 2208−2211 cm−1, approximately 74 cm−1
Complex 9 resulted from three consecutive insertions of 2,6-
dimethylphenyl isocyanide into the palladium−methyl bond, as
previously observed and structurally characterized by other
groups.8,9,11,36 Interestingly, formation of a five-membered
G
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Scheme 6. Reactivity of Cationic Complexes 5a−5c with Isocyanides
higher than that of free isocyanide.6 The 1H NMR spectra for 11
are also consistent with isocyanide coordination with no
evidence of migratory insertion of the methyl group, as indicated
by the corresponding FTIR νCN absorption at 2181−2184
cm−1. The CN stretching frequency for the C∧ImineR ligand in
10 and 11 ranged from 1626 to 1656 cm−1, indicating bidentate
coordination.
Addition of two to five equivalents of tert-butyl or 2,6-
dimethylphenyl isocyanide to 5a−5c resulted in multiple
products. While unable to isolate each individual component,
mass spectrometry on the reaction mixture shows signals
corresponding to the isocyanide adduct of the palladium methyl
complex 5 and of palladium N-(2,6-dimethylphenyl)iminoacyl
insertion products, which are a result of single and multiple insertion
of the methyl group into the isocyanide. Signals from the organic
isocyanide oligomers, generated by cleavage of the growing chain
from the metal center by protonolysis, were also observed.
Considering the ability of isocyanide to insert multiple times
into a metal−carbon bond made and the challenge in isolating
and characterizing the reaction mixture, we decided to explore
the reactivity of complexes 4 and 5 toward CO, which does not
undergo multiple insertions. The tert-butyl derivative 4b reacts
with CO within a few minutes to form the acyl complex 12 in
77% yield (Scheme 7). The low energy FTIR νCN stretching
chelate angle of 77.45(14)° remained practically unchanged from
that of the starting tert-butyl derivative complex 4b, the greater
bulk of the acyl group further distorts the Pd−NHC bond, as
suggested by a larger yaw distortion angle of 12.6°.
CONCLUSIONS
■
Several neutral and cationic palladium complexes with an aryl-
substituted acyclic imino-N-heterocyclic carbene were prepared,
isolated, and characterized to get insight into the inactivity of
related nickel complexes in ethylene polymerization.15 None of
the new palladium complexes gave polyethylene at 1 atm C2H4
and room temperature. Their thermal stability and reactivity
toward electrophiles were thus studied. All three cationic
palladium methyl complexes 5, model compounds for active
olefin polymerization catalysts, and both neutral complexes 4a
and 4c were found to be stable at elevated temperatures. In
contrast, the tert-butyl derivative 4b rapidly decomposed under
similar conditions, possibly through dissociation of the chloride
followed by a series of nucleophilic attack and reductive
elimination to yield palladium black, 1-mesityl-2-methyl-1H-
imidazole, and the corresponding imidoyl chloride. Addition of
MAO to 4 at room temperature also caused formation of
palladium black. We further demonstrated the significant role of
the substituent on the imine carbon on the reactivity of 4 and 5
with isocyanides and carbon monoxide. Compounds 4 and 5
reacted with these electrophiles to give a variety of products,
including structurally characterized simple adducts and single
and multiple insertion products. On the basis of these
observations, the inactivity of these palladium methyl complexes
in ethylene polymerization may thus stem from their inability to
insert ethylene, rather than from their intrinsic instability,
especially for the cationic species.
Scheme 7. Reactivity of Complexes 4b with CO
frequency of complex 12 (1631 cm−1) indicates chelation of the
iminic nitrogen atom. The νCO absorption at 1689 cm−1 is
consistent with a related NHC palladium acyl complex reported
by Jordan.12 In contrast, neutral complexes 4a and 4c, and all
cationic 5a−5c remained intact upon exposure to one
atmosphere of carbon monoxide, with no evidence of CO
coordination, insertion, or reductive elimination, as observed by
Elsevier in a related system.37
X-ray quality crystals of 12 were grown at room temperature
by slow vapor diffusion of pentane in a saturated dichloro-
methane solution. Compound 12 crystallized in a monoclinic
crystal system in the P21/n space group. The palladium center
adopts a distorted square planar geometry with one equivalent of
CO inserted into the Pd−Me bond (Figure 7). The acyl group
(C27−O1−C26) is approximately orthogonal (85.2°) to the
mean plane formed by C1, N3, Pd1, and C27, allowing for
interactions with the metal dπ orbitals. This competing
π-accepting capability of the acyl group may account for the
greater Pd1−N3 and Pd1−C1 bond lengths compared to those
in the palladium methyl precursor 4b (Table 1). While the
EXPERIMENTAL SECTION
■
All manipulations were performed under a dinitrogen atmosphere in a
glovebox or using standard Schlenk techniques. Solvents used in the
preparation of air and/or moisture sensitive compounds were dried
using an MBraun solvent purification system fitted with alumina
columns and stored over molecular sieves under dinitrogen. 1-(2,4,6-
Trimethylphenyl)imidazole,38 N-(2,6-dimethylphenyl)acetimidoyl
chloride, N-(2,6-dimethylphenyl)pivalimidoyl chloride, N-(2,4,6-
trimethylphenyl)benzimidoyl chloride,39 2b16 and 315 were prepared
according to published procedures. PdCl2 and Pd(COD)MeCl were
purchased from Strem and used without further purification. Silver
hexafluorophosphate was purchased from Alfa Aesar and used as
received. tert-Butyl isocyanide, 2,6-dimethylphenyl isocyanide, sodium
bis(trimethylsilyl)amide were purchased from Sigma-Alrich and used as
received. Methylaluminoxane was graciously donated by Albermarle
Corp. Deuterated NMR solvents were purchased from Cambridge
Isotope Laboratories and were degassed using three freeze−pump−
thaw cycles. C6D6 and CDCl3 were vacuum distilled from sodium and
CaH2, respectively, and stored under dinitrogen. NMR spectra were
recorded on a Bruker AV 400 (1H at 400 MHz and 13C at 100 MHz) or
Bruker AV 300 (1H at 300 MHz and 13C at 75.5 MHz) spectrometer and
are at room temperature, unless otherwise stated. The spectra were
H
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Figure 7. ORTEP plot of (C∧ImineMe)Pd(COMe)Cl (12) (50% probability level). Hydrogen atoms omitted for clarity.
referenced internally relative to the residual protio-solvent (1H), and
solvent (13C) resonances and chemical shifts were reported with respect
to δ = 0 for tetramethylsilane. FTIR spectra were recorded on a Thermo
Scientific Nicolet 6700 FT-IR spectrometer. Elemental composition was
determined by Guelph Chemical Laboratories Incorporated.
(100 MHz, C6D6): δ 158.2 (CN), 146.0 (Cipso(N‑mesityl)), 137.9
(p-C(azole‑mesityl)), 136.8 (Cipso(azole‑mesityl)), 135.3 (o-C(azole‑mesityl)), 131.8
(p-C(N‑mesityl)), 130.1 (NCCN(azole‑mesityl)), 129.7 (o-C(phenyl)), 129.3
(p-C(phenyl)), 129.1 (Cipso(phenyl)), 128.9 (NCCN(azole‑mesityl)), 128.8
(m-CH(azole‑mesityl)), 127.6 (m-C(phenyl)), 125.0 (o-C(N‑mesityl)), 123.7
(m-CH(N‑mesityl)), 21.0 (p-CH3(azole‑mesityl)), 20.7 (p-CH3(N‑mesityl)), 18.4
(o-CH3(N‑mesityl)), 18.0 (o-CH3(azole‑mesityl)); resonance for NCN was not
observed. FT-IR (cast-solvent) νCN 1652 cm−1. Anal. Calcd for
C28H29N3 (%): C, 82.52; H, 7.17; N, 10.31. Found (%): C, 82.26; H,
6.90; N, 10.58.
1-[1-(2,4,6-Trimethylphenylimino)benzyl]-3-(2,4,6-
trimethylphenyl)imidazolium tetrafluoroborate, C∧IminePh
·
HBF4 (1c). N-(2,4,6-Dimethylphenyl)benzimidoyl chloride (3.63 g,
16.2 mmol) was dissolved in a minimal amount of acetonitrile and added
to a suspension of acetonitrile (70 mL) solution of NaBF4 (1.78 g, 16.2
mmol) at room temperature and stirred for 36 h. 2,4,6-Trimethylphe-
nylimidazole (3.03 g, 16.3 mmol) was subsequently added, and the
reaction mixture was stirred for an additional 12 h. Volatiles were then
removed under vacuum, and the resulting brown solid was dissolved in a
minimal amount of dichloromethane. The solution was filtered and
pentane was added to the filtrate to precipitate the product. The solid
was further washed with pentane and dried under vacuum, giving the
product as a light brown solid (5.38 g, 10.9 mmol, 67%). 1H NMR (400
MHz, CDCl3): δ 8.81 (s, 1H, NCHN(azole)), 8.09 (s, 1H,
NCHCN(azole‑mesityl)), 7.52 (s, 1H, NCCHN(azole‑mesityl)), 7.37 (m, 5H,
CH(phenyl)), 7.01 (s, 2H, m-CH(azole‑mesityl)), 6.74 (s, 2H, m-CH(N‑mesityl)),
2.33 (s, 3H, p-CH3(azole‑mesityl)), 2.20 (s, 3H, p-CH3(N‑mesityl)), 2.17 (s, 6H,
o-CH3(N‑mesityl)), 2.02 (s, 6H, o-CH3(azole‑mesityl)).13C{1H} NMR (100
MHz, CDCl3): δ 148.1 (CN), 141.8 (p-C(azole‑mesityl)), 140.7
(Cipso(N‑mesityl)), 134.5 (C(phenyl)), 134.4 (o-C(azole‑mesityl)), 132.6 (C(phenyl)),
130.7 (Cipso(azole‑mesityl)), 130.1 (m-CH(azole‑mesityl)), 130.0 (p-C(N‑mesityl)),
129.5 (C(phenyl)), 129.0 (m-CH(N‑mesityl)), 128.0 (C(phenyl)), 126.6
(o-C(N‑mesityl)), 124.9 (NCCN(azole‑mesityl)), 121.9 (NCCN(azole‑mesityl)),
21.2 (p-CH3(azole‑mesityl)), 20.8 (p-CH3(N‑mesityl)), 18.4 (o-CH3(N‑mesityl)),
17.6 (o-CH3(azole‑mesityl)). FT-IR (cast-CDCl3) νCN 1675 cm−1. Anal.
Calcd for C28H30N3BF4 (%): C, 67.89; H, 6.10; N, 8.48. Found (%): C,
68.13; H, 5.86; N, 8.33.
Chloromethyl{1-[1-(2,6-dimethylphenylimino)ethyl]-3-
(2,4,6-trimethylphenyl)imidazol-2-ylidene}palladium, Pd-
(C∧ImineMe)MeCl (4a). A solution of [Cu(C∧ImineMe)I]2 (3) (233
mg, 0.224 mmol) in THF (12 mL) was added to a solution of
chloromethyl(1,5-cyclooctadiene)palladium(II) (118 mg, 0.447 mmol)
in THF (4 mL). The resulting mixture was stirred at room temperature
for 16 h, and then the solution was filtered through Celite. The mixture
was concentrated down to 6 mL and pentane was added to precipitate
the product. A beige solid was recovered and further washed with
pentane (2 × 6 mL) and dried under reduced pressure (173 mg, 0.354
mmol, 80%). Crystals suitable for X-ray diffraction were grown at room
temperature by slow vapor diffusion of pentane into a saturated
chloroform solution. 1H NMR (400 MHz, CDCl3): δ 7.49 (d, 3J = 1.9
Hz, 1H, NCHCN(mesityl)), 7.07−7.06 (m, 3H, p-CH(2,6‑xylyl) + m-
3
CH(2,6‑xylyl)), 6.98 (s, 2H, m-CH(mesityl)), 6.83 (d, J = 1.9 Hz, 1H,
NCCHN(mesityl)), 2.35 (s, 3H, p-CH3(mesityl)), 2.22 (s, 9H, CH3(imine) + o-
CH3(2,6‑xylyl)), 2.13 (s, 6H, o-CH3(mesityl)), 0.30 (s, 3H, Pd−CH3).
13C{1H} NMR (100 MHz, CDCl3): δ 176.0 (NCN), 156.2(CN),
142.2 (Cipso(2,6‑xylyl), 140.1 (p-C(mesityl)), 134.8 (Cipso(mesityl)), 134.5
(o-C(mesityl)), 129.7 (o-C(2,6‑xylyl)), 129.4 (m-CH(mesityl)), 128.2
(m-CH(2,6‑xylyl)), 126.2 (p-CH(2,6‑xylyl)), 124.2 (NCCN(mesityl)), 116.9
(NCCN(mesityl)), 21.4 (p-CH3(mesityl)), 18.9 (o-CH3(2,6‑xylyl)), 18.0
(o-CH3(mesityl)), 14.8 (CH3(imine)), −8.9 (Pd−CH3). FT-IR (cast-
DCM): νCN 1652 cm−1. Anal. Calcd for C23H28N3ClPd (%): C,
56.57; H, 5.78; N, 8.60. Found (%): C, 56.42; H, 6.07; N, 8.42.
1-[1-(2,4,6-Trimethylphenylimino)benzyl]-3-(2,4,6-
trimethylphenyl)imidazol-2-ylidene, C∧IminePh (2c). A suspen-
sion of C∧IminePh·HBF4 1c (96.7 mg, 0.195 mmol) and KN[Si(CH3)3]2
(39.1 mg, 0.196 mmol) in toluene (15 mL) was cooled to −78 °C and
stirred at this temperature for 30 min. The flask was then removed from
the bath and stirred at room temperature for an additional 1.5 h.
Volatiles were removed under reduced pressure, and pentane (20 mL)
was added to extract the product. The pentane solution was filtered and
the filtrate was dried under reduced pressure to give a yellow solid (57.2
mg, 0.140 mmol, 72%). 1H NMR (400 MHz, C6D6): δ 7.51 (d, 3J = 7.5
Hz, 2H, p-CH(phenyl)), 7.32 (s, 1H, NCHCN(azole‑mesityl)), 6.94−6.89 (m,
3H, m-CH(phenyl) + o-CH(phenyl)), 6.65 (s, 2H, m-CH(azole‑mesityl)), 6.48 (s,
1H, NCCHN(azole‑mesityl)), 6.48 (s, 2H, m-CH(N‑mesityl)), 2.07 (s, 6H,
o-CH3(azole‑mesityl)), 1.97 (s, 3H, p-CH3(azole‑mesityl)), 1.94 (s, 3H,
p-CH3(azole‑mesityl)), 1.80 (s, 6H, o-CH3(N‑mesityl)). 13C{1H} NMR
Chloromethyl{1-[1-(2,6-dimethylphenylimino)-2,2-dimethyl-
propyl]-3-(2,4,6-trimethylphenyl)imidazol-2-ylidene}-
palladium, Pd(C∧Iminet‑Bu)MeCl (4b). A solution of C∧Iminet‑Bu (2b)
(323 mg, 0.864 mmol) in THF (8 mL) and was added to a solution of
chloromethyl(1,5-cyclooctadiene)palladium(II) (207 mg, 0.782 mmol)
in THF (6 mL). The reaction mixture was stirred for 5 h. Volatiles were
removed under reduced pressure, and a pentane wash (2 × 8 mL) was
performed to give a beige solid (415 mg, 0.766 mmol, 98%). Crystals
suitable for X-ray diffraction were grown at room temperature by slow
1
vapor diffusion of pentane into a saturated chloroform solution. H
NMR (400 MHz, CDCl3): δ 7.81 (d, 3J = 2.0 Hz, 1H, NCHCN(mesityl)),
6.70 (m, 3H, p-CH(2,6‑xylyl) + m-CH(2,6‑xylyl)), 6.97 (s, 2H, m-CH(mesityl)),
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6.81 (d, 3J = 2.2 Hz, 1H, NCCHN(mesityl)), 2.34 (s, 3H, p-CH3(mesityl)),
2.29 (s, 6H, o-CH3(2,6‑xylyl)), 2.15 (s, 6H, o-CH3(mesityl)), 1.39 (s, 9H,
C(CH3)3(imine)), 0.23 (s, 3H, Pd−CH3). 13C{1H} NMR (100 MHz,
CDCl3): δ 177.8 (NCN(mesityl)), 161.2 (CN), 143.7 (Cipso(2,6‑xylyl),
139.9 (p-C(mesityl)), 135.1 (Cipso(mesityl)), 134.5 (o-C(mesityl)), 129.4 (m-
CH(mesityl)), 128.8 (o-C(2,6‑xylyl)), 127.3 (m-CH(2,6‑xylyl)), 125.4 (p-
CH(2,6‑xylyl)), 122.7 (NCCN(mesityl)), 119.5 (NCCN(mesityl)), 39.9
(C(CH3)3(imine)), 30.0 (C(CH3)3(imine)), 21.3 (p-CH3(mesityl)), 19.4 (o-
CH3(2,6‑xylyl)), 18.02 (o-CH3(mesityl)), −8.6 (Pd−CH3). FT-IR (cast-
DCM): νCN 1626 cm−1. Anal. Calcd for C26H34N3ClPd (%): C, 58.87;
H, 6.46; N, 7.92. Found (%): C, 59.10; H, 6.32; N, 8.09.
dried under reduced pressure to give a champagne-colored solid
(140 mg, 0.206 mmol, 89%). 1H NMR (400 MHz, CDCl3): δ 8.19 (d, 3J =
2.1 Hz, 1H, NCHCN(mesityl)), 7.11 (d, 3J = 7.4 Hz, 2H, m-CH(2,6‑xylyl)),
7.06 (m, 1H, p-CH(2,6‑xylyl)), 7.03 (d, 3J = 2.1 Hz, 1H, NCCHN(mesityl)),
6.97 (s, 2H, m-CH(mesityl)), 2.34 (s, 3H, p-CH3(mesityl)), 2.30 (s, 6H,
o-CH3(2,6‑xylyl)), 2.10 (s, 6H, o-CH3(mesityl)), 1.71 (s, 3H, CH3CN), 1.43
(s, 9H, C(CH3)3(imine)), −0.08 (s, 3H, Pd−CH3). 13C{1H} NMR
(100 MHz, CDCl3): δ 172.1 (NCN(mesityl)), 164.5 (CN), 143.6
(Cipso(2,6‑xylyl), 140.4 (p-C(mesityl)), 134.5 (Cipso(mesityl)), 134.2 (o-C(mesityl)),
129.5 (m-CH(mesityl)), 128.3 (o-C(2,6‑xylyl)), 128.0 (m-CH(2,6‑xylyl)), 126.0
(p-CH(2,6‑xylyl)), 124.5 (NCCN(mesityl)), 122.2 (NCCN(mesityl)), 118.7
(CH3CN), 40.4 (C(CH3)3(imine)), 29.7 (C(CH3)3(imine)), 21.3
(p-CH3(mesityl)), 19.0 (o-CH3(2,6‑xylyl)), 17.9 (o-CH3(mesityl)), 1.6
(CH3CN), −8.2 (Pd−CH3). FT-IR (cast-DCM): νCN 1634 cm−1.
Anal. Calcd for C28H37N4F6PPd (%): C, 49.38; H, 5.48; N, 8.23. Found
(%): C, 49.09; H, 5.41; N, 8.05.
Chloromethyl{1-[1-(2,4,6-trimethylphenylimino)benzyl]-3-
(2,4,6-trimethylphenyl)imidazol-2-ylidene}palladium, Pd-
(C∧IminePh)MeCl (4c). C∧IminePh (2c) (259 mg, 0.635 mmol)
dissolved in THF (8 mL) and was added to a solution of
chloromethyl(1,5-cyclooctadiene)palladium(II) (159 mg, 0.601
mmol) in THF (8 mL). The reaction mixture was stirred for 4 h.
Volatiles were removed under reduced pressure, and a pentane wash was
performed to give a light cream solid (307 mg, 0.544 mmol, 91%).
Crystals suitable for X-ray diffraction were grown at room temperature
{1-[1-(2,4,6-Trimethylphenylimino)benzyl]-3-(2,4,6-
trimethylphenyl)imidazol-2-ylidene}palladiummethyl Acetoni-
trile Hexafluorophosphate, [Pd(C∧IminePh)(Me)(MeCN)]PF6 (5c).
A solution of (C∧IminePh)PdMeCl (4c) (125 mg, 0.221 mmol) in
MeCN (10 mL) was added to a suspension of silver hexafluorophos-
phate (57.1 mg, 0.226 mmol) in MeCN (2 mL) and stirred at room
temperature for 1 h in the absence of light. The solution was filtered
through a plug of Celite, and the volatiles were removed under reduced
pressure. The resulting brown residue was washed with pentane and
dried under reduced pressure to give a brown solid (148 mg, 0.207
mmol, 94%). 1H NMR (400 MHz, CDCl3): δ 7.56 (t, 3J = 7.4 Hz, 1H,
p-CH(phenyl)), 7.48 (t, 3J = 7.4 Hz, 2H, m-CH(phenyl)), 7.41 (d, 3J = 7.4 Hz,
2H, o-CH(phenyl)), 7.37 (d, 3J = 2.0 Hz, 1H, NCHCN(azole‑mesityl)), 7.00 (s,
1
by slow diffusion of pentane into a saturated chloroform solution. H
NMR (400 MHz, CDCl3): δ 7.50 (t, 3J = 7.5 Hz, 1H, p-CH(phenyl)), 7.42
(t, 3J = 7.5 Hz, 2H, m-CH(phenyl)), 7.35 (d, 3J = 7.5 Hz, 2H, o-CH(phenyl)),
7.08 (d, 3J = 2.0 Hz, 1H, NCHCN(azole‑mesityl)), 7.02 (s, 2H, m-
CH(azole‑mesityl)), 6.75 (s, 3J = 2.0 Hz, 1H, NCCHN(azole‑mesityl)), 6.73 (s,
2H, m-CH(N‑mesityl)), 2.38 (s, 3H, p-CH3(azole‑mesityl)), 2.25 (s, 6H, o-
CH3(N‑mesityl)), 2.21 (s, 6H, o-CH3(azole‑mesityl)), 2.19 (s, 3H, p-
CH3(N‑mesityl)), 0.42 (s, 3H, Pd−CH3).13C{1H} NMR (100 MHz,
C6D6): δ 176.5 (NCN(azole‑mesityl)), 156.5 (CN), 140.1 (p-
C(azole‑mesityl)), 139.6 (Cipso(N‑mesityl)), 135.1 (p-C(N‑mesityl)), 134.9
(Cipso(azole‑mesityl)), 134.5 (o-C(azole‑mesityl)), 131.8 (p-CH(phenyl)), 129.4
(m-CH(azole‑mesityl)), 129.0 (m-CH(phenyl)), 128.6 (m-CH(N‑mesityl)), 128.3
(o-CH(phenyl)), 128.1 (Cipso(phenyl)), 123.5 (NCCN(azole‑mesityl)), 118.7
(NCCN(azole‑mesityl)), 21.4 (p-CH3(azole‑mesityl)), 21.1 (p-CH3(N‑mesityl)),
19.2 (o-CH3(N‑mesityl)), 18.0 (o-CH3(azole‑mesityl)), −8.8 (Pd−CH3),
resonance for (o-C(N‑mesityl)) was not observed. FT-IR (cast-DCM):
νCN 1635 cm−1. Anal. Calcd for C29H32N3ClPd (%): C, 61.71; H, 5.71;
N, 7.44. Found (%): C, 61.49; H, 5.45; N, 7.18.
3
2H, m-CH(azole‑mesityl)), 6.98 (s, J = 2.0 Hz, 1H, NCCHN(azole‑mesityl)),
6.83 (s, 2H, m-CH(N‑mesityl)), 2.36 (s, 3H, p-CH3(azole‑mesityl)), 2.22 (s, 3H,
p-CH3(N‑mesityl)), 2.21 (s, 6H, o-CH3(N‑mesityl)), 2.01 (s, 6H,
o-CH3(azole‑mesityl)), 1.75 (s, 3H, CH3CN), −0.10 (s, 3H, Pd−CH3).
13C{1H} NMR (100 MHz, CDCl3): δ 171.3 (NCN(mesityl)), 158.8 (C
N), 140.5 (p-C(azole‑mesityl)), 138.9 (Cipso(N‑mesityl)), 136.4 (p-C(N‑mesityl)),
134.4 (o-C(azole‑mesityl)), 133.0 (p-CH(phenyl)), 129.8 (o-C(N‑mesityl)), 129.6
(m-CH(azole‑mesityl)), 129.5 (m-CH(phenyl) + Cipso(azole‑mesityl)), 129.1 (m-
C(N‑mesityl)), 128.5 (o-CH(phenyl)), 126.2 (Cipso(phenyl)), 125.0
(NCCN(azole‑mesityl)), 121.1 (NCCN(azole‑mesityl)), 119.5 (CH3CN), 21.3
(p-CH3(azole‑mesityl)), 20.8 (p-CH3(N‑mesityl)), 18.7 (o-CH3(N‑mesityl)), 17.9
(o-CH3(azole‑mesityl)), 1.6 (CH3CN), −8.9 (Pd−CH3). FT-IR (cast-
DCM): νCN 1640 cm−1. Anal. Calcd for C31H35N4F6PPd (%): C,
52.07; H, 4.93; N, 7.84. Found (%): C, 51.86; H, 4.70; N, 8.01.
{1-[1-(2,6-Dimethylphenylimino)ethyl]-3-(2,4,6-
trimethylphenyl)imidazol-2-ylidene}palladium Methyl Aceto-
nitrile Hexafluorophosphate, Pd(C∧ImineMe)(Me)(MeCN)]PF6
(5a). A solution of (C∧ImineMe)PdMeCl (4a) (202 mg, 0.414 mmol)
in MeCN (6 mL) was added to a suspension of silver hexafluor-
ophosphate (105 mg, 0.415 mmol) in MeCN (4 mL) and stirred at
room temperature for 1 h in the absence of light. The solution was
filtered through a plug of Celite, and the volatiles were removed under
reduced pressure. The resulting tan residue was washed with pentane
(2 × 4 mL) and dried under reduced pressure to give a light beige solid
Dichloro(bis{1-[1-(2,6-dimethylphenylimino)ethyl]-3-(2,4,6-
trimethylphenyl)imidazol-2-ylidene})palladium, Pd-
(C∧ImineMe)2Cl2 (6). tert-Butyl isocyanide (6.0 μL, 0.053 mmol) was
syringed into a cooled (−35 °C) solution of 4a (26.0 mg, 0.053 mmol)
in toluene (4 mL) and stirred at room temperature for 16 h. Volatiles
were removed under reduced pressure, and a pentane wash was
performed to give a yellow-cream-colored solid. Sample was further
purified by slow vapor diffusion of pentane onto a saturated
dichloromethane solution over a period of several days to provide
1
(236 mg, 0.369 mmol, 89%). H NMR (400 MHz, CDCl3): δ 7.93
3
(d, J = 2.0 Hz, 1H, NCHCN(mesityl)), 7.13 (m, 3H, p-CH(2,6‑xylyl)
+
3
m-CH(2,6‑xylyl)), 6.98 (s, 2H, m-CH(mesityl)), 6.93 (d, J = 2.0 Hz, 1H,
NCCHN(mesityl)), 2.43 (s, 3H, CH3(imine)), 2.35 (s, 3H, p-CH3(mesityl)),
2.23 (s, 6H, o-CH3(2,6‑xylyl)), 2.09 (s, 6H, o-CH3(mesityl)), 1.72 (s, 3H,
CH3CN), 0.00 (s, 3H, Pd−CH3). 13C{1H} NMR (100 MHz, CDCl3): δ
170.3 (NCN(mesityl)), 160.1 (CN), 141.1 (C(2,6‑xylyl), 140.4
(p-C(mesityl)), 134.4 (o-C(mesityl)), 134.3 (C(mesityl)), 129.8 (o-C(2,6‑xylyl)),
129.5 (m-CH(mesityl)), 128.6 (m-CH(2,6‑xylyl)), 126.9 (p-CH(2,6‑xylyl)), 124.9
(NCCN(mesityl)), 120.1 (NCCN(mesityl)), 117.2 (CH3CN), 21.3
(p-CH3(mesityl)), 18.3 (o-CH3(2,6‑xylyl)), 17.8 (o-CH3(mesityl)), 14.7
(CH3(imine)), 1.6 (CH3CN), −9.1 (Pd−CH3). FT-IR (cast-DCM):
νCN 1662 cm−1. Anal. Calcd for C25H31N4F6PPd (%): C, 47.00; H,
4.89; N, 8.77. Found (%): C, 46.90; H, 5.15; N, 8.52.
1
small yellow crystals (14.2 mg, 0.0169 mmol, 64%). H NMR (400
MHz, CDCl3): δ 7.97 (s, 1H, NCHCN(mesityl)), 7.16 (d, 3J = 7.4 Hz, 2H,
m-CH(2,6‑xylyl)), 7.07 (m, 1H, p-CH(2,6‑xyly)), 6.97 (s, 2H, m-CH(mesityl)),
6.67 (s, 1H, NCCHN(mesityl)), 3.61 (s, 3H, CH3(imine)), 2.19 (s, 6H,
o-CH3(mesityl)), 2.10 (s, 3H, p-CH3(mesityl)), 2.07 (s, 6H, o-CH3(2,6‑xylyl)).
13C{1H} NMR (100 MHz, CDCl3): δ 168.2 (NCN), 155.1 (CN),
145.2 (Cipso(2,6‑xylyl), 139.3 (p-C(mesityl)), 137.0 (o-C(mesityl)), 135.7
(Cipso(mesityl)), 128.9 (m-CH(mesityl)), 127.6 (m-CH(2,6‑xylyl)), 126.5
(o-C(2,6‑xylyl)), 123.2 (p-CH(2,6‑xylyl)), 122.6 (NCCN(mesityl)), 119.5
(NCCN(mesityl)), 20.4 (p-CH3(mesityl)), 20.1 (CH3(imine)), 18.6
(o-CH3(mesityl)), 18.9 (o-CH3(2,6‑xylyl)). FT-IR (cast-DCM): νCN 1700
cm−1. Anal. Calcd for C44H50N6Cl2Pd (%): C, 62.90; H, 6.00; N, 10.00.
Found (%): C, 62.67; H, 5.74; N, 10.17.
{1-[1-(2,6-Dimethylphenylimino)-2,2-dimethylpropyl]-3-
(2,4,6-trimethylphenyl)imidazol-2-ylidene}palladium Methyl
Acetonitrile Hexafluorophosphate, [Pd(C∧Iminet‑Bu)(Me)(MeCN)]PF6
(5b). A solution of (C∧Iminet‑Bu)PdMeCl (4b) (122 mg, 0.230 mmol) in
MeCN (8 mL) was added to a suspension of silver hexafluorophosphate
(60.0 mg, 0.237 mmol) in MeCN (2 mL) and stirred at room
temperature for 1 h in the absence of light. The solution was filtered
through a plug of Celite, and the volatiles were removed under reduced
pressure. The resulting brown residue was washed with pentane and
Chloromethyl{1-[1-(2,6-Dimethylphenylimino)-2,2-dime-
thylpropyl]-3-(2,4,6-trimethylphenyl)imidazol-2-ylidene}palla-
dium tert-Butyl Isocyanide Adduct, Pd(C∧Iminet‑Bu)Me(tert-
BuNC)Cl (7). tert-Butyl isocyanide (30.0 μL, 0.265 mmol) was syringed
into a cooled (−35 °C) solution of 4b (67.5 mg, 0.127 mmol) in toluene
J
dx.doi.org/10.1021/om400533d | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(6 mL) and stirred at room temperature for 16 h. Volatiles were
removed under reduced pressure and a pentane wash was performed to
(s, 3H), 2.33 (s, 3H), 2.25 (s, 3H), 2.10 (s, 3H), 2.08 (s, 3H), 1.97 (s,
6H), 1.91 (s, 3H), 1.77 (s, 3H), 1.58 (s, 3H), 1.41 (s, 3H), 1.15 (s, 3H);
minor isomer (20%) δ 8.40 (s, 1H, NCHCHN(mesityl)), 7.04 (s + br, 3H;
accidental overlap with major isomer), 6.98 (m; accidental overlap with
major isomer), 6.88 (m; accidental overlap with major isomer), 6.78 (m;
1
give a cream-colored solid (60.0 mg, 0.0978 mmol, 77%). H NMR
(400 MHz, C6D6): δ 7.09 (s + br, 1H, p-CH(2,6‑xylyl)), 6.91 (d, 3J = 7.2 Hz,
2H, m-CH(2,6‑xylyl)), 6.62 (s, 1H, m-CH(mesityl)), 6.61 (d, 3J = 1.6 Hz, 1H,
NCHCN(mesityl)), 6.58 (s, 1H, m-CH(mesityl)), 5.98 (s, 1H,
NCCHN(mesityl)), 3.19 (s, 3H, o-CH3(2,6‑xylyl)), 2.60 (s, 3H, p-CH3(mesityl)),
1.96 (s, 3H, o-CH3(mesityl)), 1.80 (s, 3H, o-CH3(mesityl)), 1.79 (s, 9H,
∧
accidental overlap with major isomer), 3.32 (s, 3H, CH3(C Imine)), 2.37 (s,
3H; accidental overlap with major isomer), 2.35 (s, 3H), 2.29 (s, 3H),
2.06 (s, 3H), 2.01 (s, 3H), 1.91 (s, 3H; accidental overlap with major
isomer), 1.77 (s, 3H; accidental overlap with major isomer), 1.58 (s, 3H;
accidental overlap with major isomer), 1.41 (s, 6H; accidental overlap
with major isomer), 1.15 (s, 6H; accidental overlap with major isomer).
13C{1H} NMR (100 MHz, CDCl3; only resonances for the major isomer
are reported) δ 180.5, 170.9, 164.6, 154.3, 150.3, 146.8, 145.3, 143.0,
138.9, 137.7, 136.5, 135.2, 129.9, 129.5, 128.7, 128.5, 128.4, 128.1, 128.0,
127.9, 127.8, 127.7, 126.9, 124.0, 123.5, 122.3, 122.1, 120.4 (NCCN),
C(CH3)3(imine) + 3H, o-CH3(2,6‑xylyl)), 0.70 (s, 9H, CNC(CH3)3(isocyanide)
,
−0.50 (s, 3H, Pd−CH3). 13C{1H} NMR (100 MHz, C6D6): δ 157.6
(CN), 144.7 (Cipso(2,6‑xylyl), 138.6 (o-C(mesityl)), 137.5 (p-C(mesityl)),
136.6 (o-C(mesityl)), 134.3 (Cipso(mesityl)), 130.1 (m-CH(mesityl)), 129.0
(p-CH(2,6‑xylyl)), 128.5 (m-CH(mesityl)), 128.0 (m-CH(2,6‑xylyl)), 124.2
(m-CH(mesityl)), 123.2 (o-C(2,6‑xylyl)), 122.8 (o-C(2,6‑xylyl)), 120.5
(NCCN(mesityl)), 120.2 (NCCN(mesityl)), 119.6 (CNC(CH3)3(isocyanide)
,
∧
59.7 {Pd−[C(NXyl)]3CH3}, 21.2 (CH3(C Imine)), 19.7, 19.5, 19.0,
55.8 (CNC(CH3)3(isocyanide), 41.4 (C(CH3)3(imine)), 30.3
(C(CH3)3(imine)), 29.5 (CNC(CH3)3(isocyanide), 21.9(o-CH3(2,6‑xylyl)),
21.0 (o-CH3(mesityl)), 20.0 (p-CH3(mesityl)), 19.0 (o-CH3(2,6‑xylyl)), 18.1
(o-CH3(mesityl)), −14.9 (Pd-CH3); resonance for NCN(mesityl) was not
18.6, 18.4, 17.5, 16.8. Anal. Calcd for C50H55N6ClPd (%): C, 68.10; H,
6.29; N, 9.53. Found (%): C, 67.89; H, 6.24; N, 9.35.
{1-[1-(2,6-Dimethylphenylimino)ethyl]-3-(2,4,6-
trimethylphenyl)imidazol-2-ylidene}palladium Methyl tert-
Butyl Isocyanide Hexafluorophosphate, [Pd(C∧ImineMe)(Me)(t-
BuNC)]PF6 (10a). tert-Butyl isocyanide (8.0 μL, 0.071 mmol) was
syringed into a solution of 5a (45.5 mg, 0.0712 mmol) in THF (6 mL)
and stirred at room temperature for 1 h. Volatiles were removed under
reduced pressure, and a pentane wash was performed to give a brown
observed. FT-IR (cast-DCM): νCN‑ligand 1696 and 1684 cm−1
,
νCN‑isocyanide 2189 cm−1. Anal. Calcd for C31H43N4ClPd (%): C,
60.68; H, 7.06; N, 9.13. Found (%): C, 60.42; H, 7.22; N, 8.85.
Chloromethyl{1-[1-(2,4,6-trimethylphenylimino)benzyl]-3-
(2,4,6-trimethylphenyl)imidazol-2-ylidene}[1-(tert-butylimino)-
ethyl]palladium, Pd(C∧IminePh)Me(tBuNC)Cl (8). tert-Butyl iso-
cyanide (22 μL, 0.20 mmol) was syringed into a cooled (−35 °C)
solution of 4c (54.2 mg, 0.0960 mmol) in toluene (8 mL) and stirred at
room temperature for 24 h. Volatiles were removed under reduced
pressure to give a light orange solid. A minimal amount of DCM was
added to dissolve the crude material that was then filtered through a plug
of Celite. Pentane was added to precipitate the product as an orange
solid, which was further washed with pentane. The solid was dried under
reduced pressure to give the desired product and was a light orange solid
1
solid (46.1 mg, 0.0677 mmol, 96%). H NMR (400 MHz, CDCl3): δ
7.95 (d, 3J = 1.8 Hz, 1H, NCHCN(mesityl)), 7.12 (m, 3H, p-CH(2,6‑xylyl)
+
3
m-CH(2,6‑xylyl)), 6.99 (s, 2H, m-CH(mesityl)), 6.97 (d, J = 1.8 Hz, 1H,
NCCHN(mesityl)), 2.41 (s, 3H, CH3(imine)), 2.35 (s, 3H, p-CH3(mesityl)),
2.24 (s, 6H, o-CH3(2,6‑xylyl)), 2.10 (s, 6H, o-CH3(mesityl)), 1.11 (CNC-
(CH3)3(isocyanide), 0.01 (s, 3H, Pd−CH3).13C{1H} NMR (100 MHz,
CDCl3): δ 177.2 (NCN(mesityl)), 162.0 (CN), 143.9 (Cipso(2,6‑xylyl),
140.4 (p-C(mesityl)), 135.0 (CNC(CH3)3(isocyanide), 134.3 (o-C(mesityl)),
134.0 (Cipso(mesityl)), 129.5 (m-CH(mesityl)), 129.4 (o-C(2,6‑xylyl)), 128.7 (m-
CH(2,6‑xylyl)), 126.9 (p-CH(2,6‑xylyl)), 124.9 (NCCN(mesityl)), 120.4
(NCCN(mesityl)), 57.95 (CNC(CH3)3(isocyanide), 29.7 (CNC-
(CH3)3(isocyanide), 21.3 (p-CH3(mesityl)), 18.4 (o-CH3(2,6‑xylyl)), 17.8 (o-
CH3(mesityl)), 14.5 CH3(imine)), −12.6 (Pd−CH3). FT-IR (cast-DCM):
νCN(ligand) 1656 cm−1, νCN(isocyanide) 2208 cm−1. Anal. Calcd for
C28H37F6N4PPd (%): C, 49.38; H, 5.48; N, 8.23. Found (%): C, 49.14;
H, 5.52; N, 7.95.
1
(42.9 mg, 0.0555 mmol, 69%). H NMR (400 MHz, C6D6): δ 6.84
(m, 1H, p-CH(phenyl)), 6.80 (m, 4H, m-CH(phenyl) + o-CH(phenyl)), 6.72
(s, 1H, m-CH(azole‑mesityl)), 6.71 (s, 1H, m-CH(azole‑mesityl)), 6.62 (s, 1H,
m-CH(N‑mesityl)), 6.54 (s, 1H, m-CH(N‑mesityl)), 6.31 (s, 1H,
NCHCN(azole‑mesityl)), 5.80 (s, 1H, NCCHN(azole‑mesityl)), 2.48 (s, 3H,
p-CH3(azole‑mesityl)), 2.19 (s, 3H, o-CH3(azole‑mesityl)), 2.18 (s, 3H, Pd−C
N(CCH3)3CH3), 2.16 (s, 3H, o-CH3(N‑mesityl)), 2.07 (s, 3H,
o-CH3(azole‑mesityl)), 2.03 (s, 3H, p-CH3(N‑mesityl)), 2.01 (s, 3H,
o-CH3(N‑mesityl)), 1.89 (s, 9H, Pd−CN(CH3)3CH3). 13C{1H} NMR
(100 MHz, C6D6): δ 156.1 (CN), 140.2 (p-C(N‑mesityl)), 140.1
(p-C(azole‑mesityl)), 135.4 (o-C(azole‑mesityl)), 135.3 (Cipso(azole‑mesityl)), 134.8
(o-C(N‑mesityl)), 134.2 (o-C(N‑mesityl)), 131.4 (CH(phenyl)), 131.3
(CH(phenyl)), 129.9 (m-CH(N‑mesityl)), 129.6 (m-CH(azole‑mesityl)), 129.1
(m-CH(azole‑mesityl)), 129.0 (m-CH(N‑mesityl)), 128.8 (CH(phenyl)), 128.5
(CH(phenyl)), 123.0 (NCCN(azole‑mesityl)), 118.8 (NCCN(azole‑mesityl)), 55.5
(Pd−CN(CCH3)3CH3), 32.7 (Pd−CN(CCH3)3CH3), 32.5 (Pd−
CN(CCH3)3CH3), 21.0 (p-CH3(N‑mesityl)+ 19.2 o-CH3(N‑mesityl)), 19.8
(p-CH3(azole‑mesityl)), 18.9 (o-CH3 (azole‑mesityl)+ o-CH3(N‑mesityl)), 18.3
(o-CH3(azole‑mesityl)); resonances for NCN(azole‑mesityl), ipso-C(phenyl), and
Pd−CN(CCH3)3CH3 were not observed. FTIR (cast-DCM): νCN
1638 cm−1. Anal. Calcd for C34H41N4ClPd (%): C, 63.06; H, 6.38; N,
8.65. Found (%): C, 62.86; H, 6.32; N, 8.49.
{1-[1-(2,6-Dimethylphenylimino)-2,2-dimethylpropyl]-3-
(2,4,6-trimethylphenyl)imidazol-2-ylidene}palladium Methyl
tert-Butyl Isocyanide Hexafluorophosphate, [Pd-
(C∧Iminet‑Bu)(Me)(t-BuNC)]PF6 (10b). tert-Butyl isocyanide (5.3 μL,
0.047 mmol) was syringed into a solution of 5b (31.9 mg, 0.0468 mmol)
in THF (4 mL) and stirred at room temperature for 1 h. Volatiles were
removed under reduced pressure, and a pentane wash was performed to
give a brown solid (31.2 mg, 0.0431 mmol, 92%). 1H NMR (400 MHz,
CDCl3): δ 8.25 (d, 3J = 1.4 Hz, 1H, NCHCN(mesityl)), 7.08 (m, 3H, p-
CH(2,6‑xylyl) + m-CH(2,6‑xylyl)), 7.07 (s, 1H, NCCHN(mesityl)), 6.97 (s, 2H,
m-CH(mesityl)), 2.34 (s, 9H, p-CH3(mesityl) + o-CH3(2,6‑xylyl)), 2.10 (s, 6H,
o-CH3(mesityl)), 1.41 (s, 9H, C(CH3)3(imine)), 1.13 (CNC(CH3)3(isocyanide)
,
−0.07 (s, 3H, Pd−CH3). 13C{1H} NMR (100 MHz, CDCl3): δ 178.5
(NCN(mesityl)), 166.1 (CN), 147.0 (Cipso(2,6‑xylyl), 140.3 (p-C(mesityl)),
134.7 (CNC(CH3)3(isocyanide), 134.3 (o-C(mesityl)), 134.2 (Cipso(mesityl)),
129.5 (m-CH(mesityl)), 128.2 (m-CH(2,6‑xylyl)), 127.8 (o-C(2,6‑xylyl)), 126.1
(p-CH(2,6‑xylyl)), 124.6 (NCCN(mesityl)), 122.7 (NCCN(mesityl)), 57.8
CNC(CH3)3(isocyanide), 40.6 (C(CH3)3(imine)), 29.9 (C(CH3)3(imine)),
29.7 (CNC(CH3)3(isocyanide), 21.3 (p-CH3(mesityl)), 19.1 (o-CH3(2,6‑xylyl)),
17.8 (o-CH3(mesityl)), −11.4 (Pd-CH3). FT-IR (cast-DCM): νCN(ligand)
1626 cm−1, νCN(isocyanide) 2211 cm−1. Anal. Calcd for C31H43N4F6PPd
(%): C, 51.49; H, 5.99; N, 7.75. Found (%): C, 51.58; H, 6.12; N, 8.02.
{1-[1-(2,4,6-Trimethylphenylimino)benzyl]-3-(2,4,6-trime-
thylphenyl)-imidazol-2-ylidene}palladium Methyl tert-Butyl
Isocyanide Hexafluorophosphate, [Pd(C∧IminePh)(Me)(tBuNC)]-
PF6 (10c). tert-Butyl isocyanide (7.0 μL, 0.062 mmol) was syringed into
a solution of 5c (43.9 mg, 0.0614 mmol) in THF (6 mL) and stirred at
room temperature for 1 h. Volatiles were removed under reduced
pressure, and a pentane wash was performed to give a light brown solid
Chloro{1-[1-(2,6-dimethylphenylimino)ethyl]-3-(2,4,6-
trimethylphenyl)imidazol-2-ylidene}{1,2-bis[(2,6-
dimethylphenyl)imino]-3-[(2,6-dimethylphenyl)imino-κ-N]-
butyl-κ-C}palladium, (9). A cooled solution (−35 °C) solution of 2,6-
dimethylphenyl isocyanide (13.3 mg, 0.101 mmol) in THF (4 mL) was
added to a cooled (−35 °C) partially soluble solution of 4a (24.5 mg,
0.0502 mmol) in THF (2 mL). The mixture was dried in vacuo and
washed with pentane to give the product as an orange-pink solid (mass
recovered 32.4 mg). Crystals suitable for X-ray diffraction were grown at
room temperature by slow vapor diffusion of pentane into a saturated
chloroform solution. 1H NMR (400 MHz, CDCl3): major isomer (80%)
δ 8.35 (d, 3J = 1.3 Hz, 1H, NCHCHN(mesityl)), 7.04 (s + br; accidental
overlap with minor isomer), 6.98 (m; accidental overlap with minor
isomer), 6.88 (m; accidental overlap with minor isomer), 6.78 (m;
1
∧
accidental overlap with minor isomer), 3.35 (s, 3H, CH3(C Imine)), 2.37
(38.8 mg, 0.0512 mmol, 83%). H NMR (400 MHz, CDCl3): δ 7.54
K
dx.doi.org/10.1021/om400533d | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(t, 3J = 7.5 Hz, 1H, p-CH(phenyl)), 7.46 (t, 3J = 7.5 Hz, 2H, m-CH(phenyl)),
(o-CH3(mesityl)), −10.8 (Pd−CH3), (CN-(2,6-dimethylphenyl) carbon
missing. FT-IR (cast-DCM): νCN(ligand) 1628 cm−1, νCN(isocyanide)
2184 cm−1. Anal. Calcd for C35H43F6N4PPd (%): C, 54.51; H, 5.62;
N, 7.27. Found (%): C, 54.26; H, 5.45; N, 7.00.
3
3
7.40 (d, J = 7.5 Hz, 2H, o-CH(phenyl)), 7.36 (d, J = 1.8 Hz, 1H,
NCHCN(azole‑mesityl)), 7.03 (s, 3J = 1.8 Hz, 1H, NCCHN(azole‑mesityl)), 7.00
(s, 2H, m-CH(azole‑mesityl)), 6.80 (s, 2H, m-CH(N‑mesityl)), 2.36 (s, 3H,
p-CH3(azole‑mesityl)), 2.23 (s, 6H, o-CH3(N‑mesityl)), 2.20 (s, 3H,
p-CH3(N‑mesityl)), 2.15 (s, 6H, o-CH3(azole‑mesityl)), 1.13 (CNC-
(CH3)3(isocyanide), −0.10 (s, 3H, Pd−CH3).13C{1H} NMR (100 MHz,
CDCl3): δ 178.2 (NCN(mesityl)), 160.5 (CN), 141.7 (Cipso(N‑mesityl)),
140.4 (p-C(azole‑mesityl)), 136.3 (p-C(N‑mesityl)), 134.90 (CNC-
(CH3)3(isocyanide), 134.4 (o-C(azole‑mesityl)), 134.1 (Cipso(azole‑mesityl)), 133.0
(p-CH(phenyl)), 129.5 (m-CH(azole‑mesityl)), 129.5 (m-CH(phenyl)), 129.1
(m-C(N‑mesityl)), 129.1 (o-C(N‑mesityl)), 128.5 (o-CH(phenyl)), 126.0
(Cipso(phenyl)), 125.2 (NCCN(azole‑mesityl)), 121.5 (NCCN(azole‑mesityl)),
58.0 (CNC(CH3)3(isocyanide), 29.6 (CNC(CH3)3(isocyanide), 21.3
(p-CH3(azole‑mesityl)), 20.8 (p-CH3(N‑mesityl)), 18.8 (o-CH3(N‑mesityl)), 17.9
(o-CH3(azole‑mesityl)), −12.0 (Pd−CH3). FT-IR (cast-DCM): νCN(ligand)
1636 cm−1, νCN(isocyanide) 2208 cm−1. Anal. Calcd for C34H41F6N4PPd
(%): C, 53.94; H, 5.46; N, 7.40. Found (%): C, 54.02; H, 5.71; N, 7.28.
{1-[1-(2,6-Dimethylphenylimino)ethyl]-3-(2,4,6-
trimethylphenyl)imidazol-2-ylidene}palladium Methyl 2,6-Di-
methylphenyl Isocyanide Hexafluorophosphate, [Pd-
(C∧ImineMe)(Me)(ArNC)]PF6 (11a). A solution of 2,6-dimethylphenyl
isocyanide (10.7 mg, 0.0816 mmol) in THF (6 mL) was added to a
solution of 5a (52.0 mg, 0.0814 mmol) in THF (4 mL) and stirred at
room temperature for 1 h. The solution was filtered through a plug of
Celite, and the volatiles were removed under reduced pressure. The
resulting brown solid was further washed with pentane and dried under
reduced pressure to give the desired product (52.7 mg, 0.888 mmol,
{1-[1-(2,4,6-Trimethylphenylimino)benzyl]-3-(2,4,6-
trimethylphenyl)imidazol-2-ylidene}palladium Methyl 2,6-Di-
methylphenyl Isocyanide Hexafluorophosphate, [Pd-
(C∧IminePh)(Me)(ArNC)]PF6 (11c). A solution of 2,6-dimethylphenyl
isocyanide (9.20 mg, 0.070 mmol) in THF (4 mL) was added to a
solution of 5c (50.0 mg, 0.070 mmol) in THF (6 mL) and stirred at
room temperature for 1 h. The solution was filtered through a plug of
Celite, and the volatiles were removed under reduced pressure. The
resulting brown solid was further washed with pentane and dried under
reduced pressure to the desired product (48.6 mg, 0.0604 mmol, 86%).
1H NMR (400 MHz, CDCl3): δ 7.53 (t, 3J = 7.4 Hz, 1H, p-CH(phenyl)),
3
3
7.46 (t, J = 7.4 Hz, 2H, m-CH(phenyl)), 7.41 (d, J = 7.4 Hz, 2H, o-
CH(phenyl)), 7.39 (d, 3J = 1.8 Hz, 1H, NCHCN(azole‑mesityl)), 7.20 (t, 3J =
7.7 Hz, 1H, p-CH(2,6‑xylyl)‑isocyanide), 7.07 (s, 3J = 1.8 Hz, 1H,
NCCHN(azole‑mesityl)), 7.03 (s, 2H, m-CH(azole‑mesityl)), 7.01 (m, 2H,
m-CH(2,6‑xylyl)‑isocyanide), 6.72 (s, 2H, m-CH(N‑mesityl)), 2.37 (s, 3H,
p-CH3(azole‑mesityl)), 2.27 (s, 6H, o-CH3(N‑mesityl)), 2.20 (s, 6H,
o-CH3(azole‑mesityl)), 2.11 (s, 3H, p-CH3(N‑mesityl)), 1.97 (s, 6H,
o-CH3(2,6‑xylyl)‑isocyanide), 0.31 (s, 3H, Pd−CH3).13C{1H} NMR (100
MHz, CDCl3): δ 177.3 (NCN(mesityl)), 161.0 (CN), 141.9
(Cipso(N‑mesityl)), 140.5 (p-C(azole‑mesityl)), 136.4 (p-C(N‑mesityl)), 135.7
(C(2,6‑xylyl)‑isocyanide), 134.5 (o-C(azole‑mesityl)), 134.1 (Cipso(azole‑mesityl)),
133.0 (p-CH(phenyl)), 130.4 (p-CH(2,6‑xylyl)‑isocyanide), 129.6
(m-CH(azole‑mesityl)), 129.4 (m-CH(phenyl)), 129.3 (m-C(N‑mesityl)), 128.9
(o-C(N‑mesityl)), 128.6 (o-CH(phenyl)), 128.2 (m-CH(2,6‑xylyl)‑isocyanide), 126.0
(C(phenyl)), 125.4 (NCCN(azole‑mesityl)), 121.7 (NCCN(azole‑mesityl)), 21.3
(p-CH3(azole‑mesityl)), 20.7 (p-CH3(N‑mesityl)), 18.9 (o-CH3(N‑mesityl)), 18.2
(o-CH3(2,6‑xylyl)‑isocyanide), 17.9 (o-CH3(azole‑mesityl)), −11.3 (Pd−CH3). FT-
IR (cast-DCM): νCN(ligand) 1635 cm−1, νCN(isocyanide) 2181 cm−1. Anal.
Calcd for C38H41N4F6PPd (%): C, 56.69; H, 5.13; N, 6.96. Found (%):
C, 56.91; H, 4.97; N, 7.10.
1
3
89%). H NMR (400 MHz, CDCl3): δ 7.99 (d, J = 1.6 Hz, 1H,
NCHCN(mesityl)), 7.19 (t, 3J = 7.7 Hz, 1H, p-CH(2,6‑xylyl)), 7.09 (m, 3H,
p-CH(2,6‑xylyl)‑isocyanide + m-CH(2,6‑xylyl)‑isocyanide), 7.02 (s, 2H, m-CH(mesityl)),
7.00 (s + br, 3H, NCCHN(mesityl) + m-CH(2,6‑xylyl)), 2.42 (s, 3H, CH3(imine)),
2.37 (s, 3H, p-CH3(mesityl)), 2.29 (s, 6H, o-CH3(2,6‑xylyl)), 2.14 (s, 6H,
o-CH3(mesityl)), 1.93 (s, 6H, o-CH3(2,6‑xylyl)‑isocyanide), 0.23 (s, 3H, Pd−CH3).
13C{1H} NMR (100 MHz, CDCl3): δ 176.7 (NCN(mesityl)), 162.5 (CN),
{Acetylchloro[1-(2,6-dimethylphenylimino)-2,2-dimethyl-
propyl]-3-(2,4,6-trimethylphenyl)imidazol-2-ylidene}-
palladium, Pd(C∧Iminet‑Bu)(COMe)Cl (12). A Schlenk tube
containing a solution of 4b (32.5 mg, 0.0613 mmol) in 6 mL of THF
was backfilled with CO and stirred for 15 min at atmospheric pressure
and room temperature. The reaction was dried under vacuum and
brought inside the glovebox. The solid residue was dissolved in a
minimal amount of THF, filtered through a plug of Celite, and the
yellow solution was dried under vacuum. The solid was washed with
pentane and dried under vacuum to give a yellow-green solid (26.5 mg,
0.0474 mmol, 77%). 1H NMR (400 MHz, CDCl3): δ 7.82 (d, 3J = 1.3
Hz, 1H, NCHCN(mesityl)), 6.97 (m, 3H, p-CH(2,6‑xylyl) + m-CH(2,6‑xylyl)),
6.94 (m, 2H, m-CH(mesityl)), 6.87 (s + br, 1H, NCCHN(mesityl)), 2.31
(s, 3H, p-CH3(mesityl)), 2.29 (s, 6H, o-CH3(2,6‑xylyl)), 2.14 (s, 6H,
o-CH3(mesityl)), 1.71 (s, 3H, Pd−CO−CH3), 1.36 (s, 9H, C(CH3)3(imine)).
13C{1H} NMR (100 MHz, CDCl3): δ 228.5 (Pd−CO−CH3), 179.4
(NCN(mesityl)), 161.7 (CN), 143.1 (C(2,6‑xylyl), 140.3 (p-C(mesityl)),
135.5 (o-C(mesityl)), 134.4 (C(mesityl)), 129.4 (m-CH(mesityl)), 127.9 (o-
C(2,6‑xylyl)), 127.4 (m-C(2,6‑xylyl)), 125.2 (p-CH(2,6‑xylyl)), 122.2
(NCCN(mesityl)), 120.0 (NCCN(mesityl)), 40.2 (C(CH3)3(imine)), 38.7
(Pd−CO−CH3), 29.9 (C(CH3)3(imine)), 21.3 (p-CH3(mesityl)), 19.6
(o-CH3(2,6‑xylyl)), 18.1 (o-CH3(mesityl)). FT-IR (cast-DCM): νCN 1632
cm−1, νCO 1690 cm−1. Anal. Calcd for C27H34N3OClPd (%): C, 58.07;
H, 6.14; N, 7.52. Found (%): C, 58.19; H, 6.28; N, 7.24.
General Procedure for Ethylene Polymerization. Ethylene
polymerization was performed at atmospheric pressure and room
temperature in a 500 mL Schlenk flask containing a magnetic stir bar.
The flask was conditioned in an oven at 160 °C for at least 12 h prior to
use. The hot flask was brought to room temperature under dynamic
vacuum and backfilled with ethylene. This cycle was repeated a total of
three times. Under an atmosphere of ethylene, the flask was charged
with 25 mL of dry toluene and 1000 equivalents of methylaluminoxane
(2 M in toluene). The solution was stirred for 15 min before the catalyst
(10.3 μmol) in 1 mL of toluene was introduced into the flask via syringe.
The reaction mixture was vigorously stirred for 10 min after the addition
144.2 (Cipso(2,6‑xylyl), 140.4 (p-C(mesityl)), 135.7 (o-C(2,6‑xylyl)‑isocyanide), 134.5
(o-C(mesityl)), 134.0 (Cipso(mesityl)), 130.4 (p-CH(2,6‑xylyl)), 129.5 (m-CH(mesityl)),
129.2 (o-C(2,6‑xylyl)), 129.0 (m-CH(2,6‑xylyl)‑isocyanide), 128.2 (m-CH(2,6‑xylyl)),
127.0 (p-CH(2,6‑xylyl)‑isocyanide), 124.1 (NCCN(mesityl) + C(2,6‑xylyl)‑isocyanide), 120.6
(NCCN(mesityl)), 21.3 (p-CH3(mesityl)), 18.4 (o-CH3(2,6‑xylyl)), 18.3
(o-CH3(2,6‑xylyl)‑isocyanide), 17.8 (o-CH3(mesityl)), 14.5 CH3(imine)), −11.8
(Pd-CH3); CN-(2,6-dimethylphenyl) carbon resonances not observed.
FTIR (cast-DCM): νCN(ligand) 1653 cm−1, νCN(isocyanide) 2181 cm−1.
Anal. Calcd for C32H37N4F6PPd (%): C, 52.72; H, 5.12; N, 7.68. Found (%):
C, 52.49; H, 5.03; N, 7.41.
{1-[1-(2,6-Dimethylphenylimino)-2,2-dimethylpropyl]-3-
(2,4,6-trimethylphenyl)imidazol-2-ylidene}palladium Methyl
2,6-Dimethylphenyl Isocyanide Hexafluorophosphate, [Pd-
(C∧Iminet‑Bu)(Me)(ArNC)]PF6 (11b). A solution of 2,6-dimethylphenyl
isocyanide (22.3 mg, 0.170 mmol) in THF (8 mL) was added to a
solution of 5b (112 mg, 0.165 mmol) in THF (6 mL) and stirred at
room temperature for 1 h. The solution was filtered through a plug of
Celite, and the volatiles were removed under reduced pressure. The
resulting brown solid was further washed with pentane and dried under
reduced pressure to the desired product (114 mg, 0.148 mmol, 89%). 1H
NMR (400 MHz, CDCl3): δ 8.31 (d, 3J = 1.8 Hz, 1H, NCHCN(mesityl)),
7.19 (t, 3J = 7.8 Hz, 1H, p-CH(2,6‑xylyl)‑isocyanide), 7.13 (d, 3J = 1.8 Hz, 1H,
NCCHN(mesityl)), 7.19 (d, 3J = 7.8 Hz, 2H, m-CH(2,6‑xylyl)‑isocyanide), 7.00
3
(s, 2H, m-CH(mesityl)), 6.95 (d, J = 7.6 Hz, 2H, m-CH(2,6‑xylyl)), 6.80
3
(t, J = 7.6 Hz, 3H, p-CH(2,6‑xylyl)), 2.38 (s, 6H, o-CH3(2,6‑xylyl)), 2.35
(s, 3H, p-CH3(mesityl)), 2.14 (s, 6H, o-CH3(mesityl)), 2.04 (s, 6H,
o-CH3(2,6‑xylyl)‑isocyanide), 1.43 (s, 9H, C(CH3)3(imine)), 0.11 (s, 3H, Pd−
CH3). 13C{1H} NMR (100 MHz, CDCl3): δ 177.9 (NCN(mesityl)), 166.5
(CN), 146.8 (Cipso(2,6‑xylyl), 140.4 (p-C(mesityl)), 135.2
(C(2,6‑xylyl)‑isocyanide), 134.3 (o-C(mesityl)), 134.2 (Cipso(mesityl)), 130.3
(p-CH(2,6‑xylyl)‑isocyanide), 129.6 (m-CH(mesityl)), 128.2 (m-CH(2,6‑xylyl)
+
m-CH(2,6‑xylyl)‑isocyanide), 127.5 (o-C(2,6‑xylyl)), 126.1 (p-CH(2,6‑xylyl)), 125.1
(o-C(2,6‑xylyl)‑isocyanide), 124.9 (NCCN(mesityl) + o-C(2,6‑xylyl)‑isocyanide), 123.0
(NCCN(mesityl)), 40.7 (C(CH3)3(imine)), 29.9 (C(CH3)3(imine)), 21.3
(p-CH3(mesityl)), 19.2 (o-CH3(2,6‑xylyl)), 18.6 (o-CH3(2,6‑xylyl)‑isocyanide), 17.9
L
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Organometallics
Article
of the catalyst and subsequently quenched with a 50:50 mixture of
concentrated hydrochloric acid and methanol. The resulting mixture
was filtered, and any solid collected was washed with distilled water.
Solids collected, if any, were dried under vacuum at approximately 60 °C
for several hours.
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X-ray Crystallography. Detailed crystallographic data for 4a−4c, 6,
and 9 (tables of atomic coordinates with isotropic and anisotropic
displacement parameters, bond lengths, and angles) are provided as
Supporting Information. Crystallographic data for 4a−4c, 6, and 9 were
collected at the University of Toronto on a Bruker−Nonius Kappa-
CCD diffractometer using monochromated Mo Kα radiation (λ =
0.71073 Å) at 150 K and were measured using a combination of ϕ scans
and ω scans with κ offsets to fill the Ewald sphere. Intensity data were
processed using Denzo−SMN.40 Absorption corrections were carried
out using SORTAV.41 Crystallographic data for 5b and 12 were
collected at the University of Toronto on a Bruker Kappa APEX-DUO
diffractometer using monochromated Mo Kα radiation (Bruker
Triumph) and were measured using a combination of ϕ scans and ω
scans. The data were processed using APEX2 and SAINT.42 Absorption
corrections were carried out using SADABS.42 All structures were solved
and refined using Superflip43 and refined with SHELXS-9744 for full-
matrix least-squares refinement that was based on F2. All H atoms were
included in calculated positions and allowed to refine in riding-motion
approximation with Uiso tied to the carrier atom.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data for 4a, 4b, 4c, 5b, 6, 9, and 12 (CCDC
reference numbers 940711−940717). Crystallographic data in
CIF and other electronic format, including tables of crystal data
and structure refinement, bond lengths, angles, atomic
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AUTHOR INFORMATION
■
Corresponding Author
(19) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996,
118, 267−268.
*E-mail: glavoie@yorku.ca. Tel: +1 416 736 2100, ext. 77728.
Fax: +1 416 736 5936.
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(21) Wurtz, S.; Glorius, F. Acc. Chem. Res. 2008, 41, 1523−1533.
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Notes
(22) (a) Khramov, D. M.; Lynch, V. M.; Bielawski, C. W.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
G.G.L. gratefully acknowledges financial support from the
Ontario Ministry of Research and Innovation for an Early
Researcher Award, the Natural Sciences and Engineering
Research Council for a Discovery Grant and a Research and
Tools Instrumentation Grant, the Canadian Foundation for
Innovation and the Ontario Research Fund for infrastructure
funding, and York University for start-up funds. A.C.B. wishes to
thank York University for a Provost Dissertation Scholarship.
The authors wish to thank Dr. Alan J. Lough of the University of
Toronto (Toronto, Ontario) and Dr. James F. Britten of
McMaster University (Hamilton, Ontario) for X-ray data
acquisition and/or for assistance in solving and refining the
solid-state structures.
(23) Díez-Gonzal
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