Nickel(II) and palladium(II) complexes with chelating N-heterocyclic carbene amidate ligands: Interplay between normal and abnormal coordination modes

Article abstract of DOI:10.1021/om4000133

A series of six Ni(II) and Pd(II) complexes of two bidentate and two tetradentate N-heterocyclic carbene (NHC)/amidate ligands have been prepared. The complexes are uncharged, with square-planar coordination geometries, and the ligands are bound via the NHC groups and the deprotonated amide nitrogen atoms. Pd(II) complexes were prepared for the bidentate ligands, and in each case, two chelating bidentate ligands were bound to the metal center, yielding cis/trans geometric isomeric forms. The Pd(II) complexes of the tetradentate ligands were obtained as a series of constitutional isomeric forms that were separable by fractional crystallization. The constitutional isomers differed in the coordination mode of the NHC groups, which were bound as either normal (nNHC) or abnormal (aNHC) carbenes. Density functional theory (DFT) studies show that the energies of the isomeric forms increase in the order nNHC/nNHC < nNHC/aNHC < aNHC/aNHC and suggest that the abnormal NHC coordination mode occurs in kinetic rather than thermodynamic reaction products. The Ni(II) complexes of the tetradentate ligand showed only normal NHC coordination, suggesting that the mechanism by which aNHC binding occurs is metal dependent. The Ni(II) and Pd(II) complexes with nNHC donors displayed distorted-square-planar coordination geometries and axial chirality.

Full text of DOI:10.1021/om4000133

Article
pubs.acs.org/Organometallics
Nickel(II) and Palladium(II) Complexes with Chelating N‑Heterocyclic
Carbene Amidate Ligands: Interplay between Normal and Abnormal
Coordination Modes
Kel Vin Tan,† Jason L. Dutton,† Brian W. Skelton,‡ David J. D. Wilson,† and Peter J. Barnard*,†
†Department of Chemistry, La Trobe Institute for Molecular Science, La Trobe University, Victoria 3086, Australia
‡Centre for Microscopy, Characterization and Analysis, The University of Western Australia, 35 Stirling Highway, Crawley, Western
Australia 6009, Australia
S
* Supporting Information
ABSTRACT: A series of six Ni(II) and Pd(II) complexes of two
bidentate and two tetradentate N-heterocyclic carbene (NHC)/
amidate ligands have been prepared. The complexes are uncharged,
with square-planar coordination geometries, and the ligands are
bound via the NHC groups and the deprotonated amide nitrogen
atoms. Pd(II) complexes were prepared for the bidentate ligands,
and in each case, two chelating bidentate ligands were bound to the
metal center, yielding cis/trans geometric isomeric forms. The
Pd(II) complexes of the tetradentate ligands were obtained as a
series of constitutional isomeric forms that were separable by
fractional crystallization. The constitutional isomers differed in the
coordination mode of the NHC groups, which were bound as either
“normal” (nNHC) or “abnormal” (aNHC) carbenes. Density
functional theory (DFT) studies show that the energies of the
isomeric forms increase in the order nNHC/nNHC < nNHC/aNHC < aNHC/aNHC and suggest that the “abnormal” NHC
coordination mode occurs in kinetic rather than thermodynamic reaction products. The Ni(II) complexes of the tetradentate
ligand showed only “normal” NHC coordination, suggesting that the mechanism by which aNHC binding occurs is metal
dependent. The Ni(II) and Pd(II) complexes with nNHC donors displayed distorted-square-planar coordination geometries and
axial chirality.
to the metal-bound carbon, as opposed to the normal mode,
where the heteroatoms are located in the α-positions.15 The
abnormal binding mode for NHCs was first described by
Crabtree and co-workers in 2001,21 and this initial report was
followed by a series of further studies on the nature of
aNHCs.22−24 It was demonstrated that aNHCs are stronger σ-
donors than their C2-bound counterparts and that metalation
at the C4 position may occur via C−H bond activation.23
Subsequently, both experimental16 and theoretical10,25,26
studies have shown that aNHCs are among the most basic of
all known neutral donors. Recently, a metal-free aNHC was
isolated and crystallographically characterized.19
Given the very strong σ-donating properties of aNHCs, there
is growing interest in the potential catalytic application of these
ligands. PdII−aNHC complexes have been evaluated in
Suzuki−Miyaura and Heck27,28 and Sonogashira29 coupling
reactions. IrIII−aNHC complexes have been shown to catalyze
the cross-coupling of alcohols and amines30 and the benzylation
of arenes.31 Albrecht and co-workers have demonstrated in a
INTRODUCTION
■
N-heterocyclic carbenes (NHCs) are among the most
important and widely studied ligand types in contemporary
organometallic chemistry.1 The main reason for this high level
of interest is the outstanding activity of NHC−metal complexes
in homogeneous catalysis.2−7 NHCs are particularly attractive,
as they provide excellent framework flexibility and, in
comparison to phosphines, NHC−metal complexes often
show enhanced stability to air, moisture, and heat.8 The high
stability and catalytic activity displayed by NHC−metal
complexes can be attributed to the robust metal−carbene
bond and the strong σ-donating properties of NHCs.7,9 New
applications of NHCs in organometallic chemistry are
constantly being developed.10−12
Despite the very large number of known NHC complexes,
the synthesis of NHCs with tunable electronic properties
remains a challenge.11,13 In the vast majority of reported cases,
the five-membered imidazole-based NHC coordinates to the
metal via the C2 carbon. However, there has been growing
interest in so-called “abnormal” NHCs (aNHCs), where
coordination is via the C4 carbon.14−20 The abnormal bonding
mode provides an α,β′ relationship for the heteroatoms relative
Received: January 13, 2013
Published: March 8, 2013
© 2013 American Chemical Society
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series of studies that for catalytic applications where an
electron-rich metal center is advantageous, aNHCs produce
more active systems than “normal” NHCs (nNHCs). For
example, the Pd(II) aNHC complex i was shown to be an
active catalyst in the hydrogenation of olefins, while the
“normal” NHC analogue ii was inactive.16,32
Scheme 1. Synthesis of Imidazolium Salts [I]·Cl, [II]·Cl,
a
[III]·Cl2, and [IV]·Cl2
a
Conditions: CH3CN, 100 °C, 16 h.
Chelating NHC-based ligands, functionalized with anionic
donors (e.g., amide, alkoxide, and phenoxide) are useful, as they
allow for the synthesis of NHC complexes of metals in high
oxidation states.33−35 Of particular interest in our research is
the combination of NHCs with deprotonated amide (amidate
or amido) donors. Ligands of this type and their metal
complexes have been the subject of a number of studies.36−42 In
particular, Ghosh and co-workers have investigated both the
potential catalytic and biomedical application of such ligands
with a variety of metals.43−47 These researchers have described
a series of dianionic amidate functionalized NHC ligands
designed to form highly stable Ni(II) complexes (e.g., iii) as
potential immunotolerance agents.45,47
the proligands the methylene group proton resonances
occurred as singlet signals within the range 5.27−5.57 ppm.
The square-planar Pd(II) complexes (1 and 2) of the
bidentate NHC ligands were prepared by heating the chosen
proligand ([I]Cl or [II]Cl) and K2PdCl4 in a 2/1 ratio with the
mild base K2CO3 in anhydrous DMF at 110 °C (Scheme 2).
Scheme 2. Synthesis of the cis and trans Stereoisomeric
a
forms of the Pd(II) Complexes 1 and 2
In this paper we report the synthesis and characterization of a
series of Ni(II) and Pd(II) complexes of bidentate and
tetradentate NHC/amidate ligands. For the Pd(II) complexes
of the tetradentate ligands, a remarkable interplay is seen in the
nature of the NHC coordination mode, where these groups are
bound to the metal as either “normal” or “abnormal” NHCs.
This series of metal complexes is significant, as it provides a
potential avenue for a comparison of the influence that aNHC
versus nNHC binding has on structural and electronic
properties. Density functional theory (DFT) calculations
provide insight into the structure, bonding, and properties of
these aNHC and nNHC metal complexes.
a
Conditions: DMF, 110 °C, 16 h.
RESULTS AND DISCUSSION
■
Two classes of amide-functionalized imidazolium salts (NHC
ligand precursors) were prepared; the bidentate proligands
[I]Cl, [II]Cl and the tetradentate proligands [III]Cl2 and
[IV]Cl2. These salts were prepared in a similar manner from
the α-chloroamide starting materials N-(2-chloroacetyl)-
aniline37 and N,N′-bis(2-chloroacetyl)-1,2-phenylenediamine48
and the substituted imidazoles 1-methylimidazole and 1-
phenylimidazole (Scheme 1). The methyl-substituted imidazo-
lium salt [I]Cl and the Ni(II) complex of the NHC ligand
The complexes formed under these conditions were uncharged,
with two chelating ligands coordinated to the Pd(II) center via
the NHC group and the deprotonated amide nitrogen atom
(amidate), as has been found previously for similar systems.37
Analysis of the 1H NMR spectra indicated that two geometric
isomers were formed, with the cis/trans ratios being 0.66/0.34
and 0.63/0.37 for 1 and 2, respectively. The geometric isomers
were separated by stirring a suspension of the crude product in
methanol, with the trans isomers of 1 and 2 being less soluble
and readily isolated. For compound 1, removal of the solvent
from the filtrate and recrystallization of the residue from
methanol afforded pure cis-1. However, the cis isomer of 2
could not be purified in this way. The Pd(II) complexes of a
related series of bidentate ligands have been described
previously, and the ratio of cis to trans isomers was found to
be influenced by the polarity of the chosen reaction solvent.37
derived from it have been described previously.39 Similar H
1
NMR spectra were obtained for the imidazolium salts [I]·Cl,
[II]·Cl, [III]·Cl2, and [IV]·Cl2 in d6-DMSO. In each case
characteristic downfield resonances for the deshielded pro-
carbenic and amide N−H protons were observed, with
resonances occurring within the ranges of 9.19−10.18 and
10.75−11.15 ppm for these groups, respectively. For each of
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The numbers of signals observed in the 1H NMR spectra for
trans-1, cis-1, and trans-2 are consistent with the ligand
environments being magnetically equivalent. The equivalence
arises from an inversion center relationship for trans-1 and
trans-2 (point group Ci), with the inversion center lying on the
Pd(II) atom. In the case of cis-1 there is a 2-fold axis of rotation,
which interconverts the ligand environments (point group C2).
The signal for the pro-carbenic proton was absent from the 1H
NMR spectra, and a characteristic downfield chemical shift was
observed for the carbenic carbon atom, occurring at 171.4,
163.1, and 167.2 ppm for trans-1, cis-1, and trans-2, respectively.
Additionally, no amide N−H was observed in the spectra of the
complexes, which indicates that this group is deprotonated and
bound to the Pd(II) center. Upon coordination of the NHC
and amidate groups a six-membered chelate ring is formed, and
in all cases the methylene protons appear as sharp, well-
separated AX patterns, implying that the structures are rigid in
solution and the rate of interconversion of the endo and exo
proton environments is slow with respect to the NMR time
scale.
Scheme 4. Synthesis of the Geometric Isomeric forms of the
Pd(II) Complexes 5 and 6
a
a
n = normal NHC and a = abnormal NHC. Isolated yields are
reported. Conditions: DMF, 70 °C, 16 h.
The Ni(II) complexes (3 and 4) and the Pd(II) complexes
(5 and 6) formed from the tetradentate proligands [III]Cl2 and
[IV]Cl2 were prepared by heating a 1/1 mixture of the metal
source (anhydrous NiCl2 or K2PdCl4) and the chosen
proligand in DMF. Higher reaction temperatures (∼110 °C)
were required for formation of 3 and 4. In contrast, 70 °C was
sufficient for the formation of 5 and 6, with higher
temperatures leading to the formation of decomposition
products. After recrystallization from dichloromethane and
diethyl ether, the Ni(II) complexes were obtained as dark
brown powders in low to moderate yields (Scheme 3).
yield of each isomeric form (Scheme 4) shows the aNHC
coordination mode to be favored over the nNHC coordination
mode. The synthesis of complexes 5 and 6 at different
temperatures (55−85 °C) had little effect on the ratio of
isomers formed. However, higher temperatures (110−120 °C)
resulted in the formation of decomposition products, which
made metal complex isolation difficult. Previously, the synthesis
and separation of a similar constitutional isomeric mixture
(aNHC/nNHC and nNHC/nNHC) has been described for a
Rh(III) complex of a trimethylene-bridged bis-NHC ligand.49
Upon formation of the Ni(II) and Pd(II) complexes, signals
for the pro-carbenic proton or the amide N−H were no longer
present, indicating that these groups are deprotonated and
coordinated to the metal center. The methylene protons occur
as a well-separated AX pattern for 3 and an AB pattern for 4.
For both 3 and 4 the signals for each pair of equivalent Ar-H
protons on the phenylene diamine linker group are separated
by ∼2 ppm, with those closer to the carbonyl oxygen of the
amide group being more strongly deshielded, probably as a
result of an interaction with this group. The chemical shift
difference for the signals attributed to the imidazolium group
protons is small, indicating that these protons are in similar
magnetic environments.
a
Scheme 3. Synthesis of the Ni(II) Complexes 3 and 4
a
Conditions: DMF, 110 °C, 16 h.
Significant variation was seen in the 1H NMR spectra
1H NMR analysis of the crude products obtained in the
synthesis of 5 and 6 showed that in each case a mixture of
isomeric species was formed. With careful fractional crystal-
lization, complex 5 (methyl substituents) could be separated
into two structural (constitutional) isomers. These isomeric
forms differed in the coordination mode of the NHC group,
where either both of the NHC units are coordinated as
“abnormal” carbenes (5a,a) or as a combination of the “normal”
and “abnormal” coordination modes (5n,a). The ratio of
isomers (5n,a/5a,a) formed in the reaction was 0.39/0.61, as
attributable to the different isomeric forms of 5 and 6. The H
1
NMR spectra for the three isomers of 6 (R = Ph; 6n,n, 6n,a, and
6a,a) are shown in Figure 1. The spectrum for 6n,n is similar to
that seen for the analogous Ni(II) complex 4, indicating that
the NHC groups are bound in the normal manner for this
isomeric form. The number of signals observed for 6n,n is
consistent with an effective plane of symmetry passing through
the phenylenediamine linker and the metal center. The same
symmetry relationship is apparent for 5a,a and 6a,a, with both
NHC groups now bound as abnormal NHCs. In contrast to the
case for 6n,n, the methylene protons for 5a,a and 6a,a appear as a
singlet signal, rather than an AB/AX pattern, indicating a
fluxional molecule for which the endo and exo proton
environments are exchanging. Also of note is the signal for
the equivalent H1/H1′ protons, which appear as a highly
deshielded singlet at 9.54 ppm. The NMR data for the mixed
coordination mode complex 5n,a is consistent with a structure
lacking the symmetry relationships seen for the bis-normal and
1
estimated from the H NMR spectrum of the crude product
(before purification).
In the case of complex 6 (phenyl substituents), structural
isomeric forms representing all three possible combinations of
coordination modes were obtained (6n,n, 6n,a, and 6a,a), and
these were also separated by fractional crystallization (Scheme
4). The ratio of isomers (6n,n/6n,a/6a,a) formed in the reaction
was 0.25/0.35/0.40. For both complexes 5 and 6, the crude
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1
Figure 1. H NMR spectra for the three geometric (constitutional) isomeric forms of the Pd(II) complex 6 (6n,n, 6n,a, and 6a,a). The proton
numbering scheme is shown in the chemical structures (Im = imidazolyl, Ar = aryl).
Table 1. Selected Bond Distance (Å) and Bond Angle (deg) Data for Complexes cis-1, trans-1, trans-2, 3, 4, 5n,a, 5a,a, 6n,n, and 6a,a
metal−Ccarbene
metal−Namide
bond angle
complex
metal
M1−C1
M1−C2
M1−C1′
M1−C2′
M1−N3
M1−N3′
Ccarb−M− Ccarb
N3−M−N3′
cis-1
trans-1
trans-2
3
Pd(II)
Pd(II)
Pd(II)
Ni(II)
Ni(II)
Pd(II)
Pd(II)
Pd(II)
Pd(II)
1.970(2)
2.020(3)
2.024(3)
1.857(3)
1.860(2)
1.977(3)
1.962(2)
2.014(3)
2.024(3)
1.851(3)
1.865(2)
2.097(2)
2.064(3)
2.033(3)
1.917(2)
1.908(2)
2.052(2)
2.056(2)
2.031(2)
2.060(3)
2.094(2)
2.061(3)
2.033(3)
1.906(2)
1.911(2)
2.051(2)
2.044(2)
2.034(2)
2.046(3)
94.22(9)
178.8(1)
180.0
93.80(7)
178.9(1)
179.99(1)
86.01(9)
85.49(8)
81.90(9)
80.34(8)
83.23(8)
81.1(1)
93.9(1)
92.5(1)
95.6(1)
98.7(1)
95.4(1)
94.6(1)
4
5n,a
1.978(3)
1.990(2)
5a,a
1.993(2)
1.985(3)
6n,n
6a,a
1.958(2)
1.963(2)
1.976(3)
bis-abnormal coordination modes. The methylene groups are
nonequivalent and appear as two singlets, indicating conforma-
tional freedom similar to that seen for 5a,a and 6a,a. The
imidazolyl H3 proton for compound 6n,a is shielded and
resonates at 5.99 ppm (Figure 1). This upfield shift is likely to
be due to shielding from the phenyl R group of the normally
bound NHC unit, which may form a π−π stacking interaction
with the abnormally bound NHC unit (see Structural Studies).
Structural Studies. X-ray crystal structures were obtained
for the following complexes: trans-1, cis-1, trans-2, 3, 4, 5a,a, 5n,a,
6n,n, and 6a,a. Selected bond distances and crystallographic
refinement data for all of the structures solved are given in
Table 1 and Table S1 (Supporting Information), respectively.
The X-ray crystal structures of trans-1, cis-1, and trans-2 are
shown in Figure 2.
particularly important in catalytic applications.51 Amidate
donors are also known for their strong σ-donating properties
and their capacity to form stable metal complexes with metal
ions in high oxidation states.52−56 Analysis of the cis and trans
forms of 1 show little evidence of a marked NHC trans
influence. In the case of cis-1, the NHCs are trans to the
amidate groups and the average Pd−Namidate bond distance is
2.096(2) Å, while for trans-1, where the amidate donor groups
adopt a mutually trans disposition, the average Pd−Namidate
bond distance is 2.063(2) Å. Given the strong σ-donating
properties of the anionic amidate groups and the metallocyclic
nature of these complexes, the NHC trans influence is not
expected to be as pronounced as that seen in other systems.
Complex 3 crystallizes in the chiral space group P43212
(tetragonal), and two views of the molecular structure are
shown in Figure 3. The Ni(II) center adopts a distorted-square-
planar geometry (Figure 3a), with the distortion resulting from
steric interactions between the R groups (methyl) of the NHC
NHCs are well-known as very strong σ-donating ligands7,9
and for their marked trans influence (structural) and trans effect
(kinetic), with the trans effect exerted by NHCs being
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Scheme 5. Stereoisomeric Forms of the Complexes 3, 4, and
a
57
,
6n,n, Which Possess Axial Chirality
a
Stereodescriptors: M, left-handed helical twist; P, right-handed helical
twist.
1H NMR studies show that the methylene protons are
nonequivalent (AX pattern), indicating that the stereoisomeric
forms of these complexes do not interconvert.
Representative crystal structures were obtained for the three
possible constitutional isomeric forms of 5 and 6, these being
Figure 2. ORTEP50 representations of the X-ray crystal structures of
(a) trans-1, (b) cis-1, and (c) trans-2. Cocrystallized DMF (trans-1),
methanol (cis-1), and DCM and H2O (trans-2) are omitted for clarity.
Atomic displacement ellipsoids are shown at the 40% probability level.
5
n,a and 5a,a (5n,n was not formed) and 6n,n and 6a,a (Figure 4).
Compound 6n,n adopts a structure closely resembling that of 4
and also crystallizes in space group Pa3. The Pd−C and
̅
carbene
the Pd−Namidate bond distances (Table 1) are longer than those
of 4; however, they are the shortest among the Pd(II)
complexes prepared in this work. The distortion associated with
the R group (Ph) steric interactions is less pronounced than
that seen for 4, with the carbene carbon atoms C1 and C1′
being displaced by 0.340 and 0.349 Å from the plane defined by
N3, N3′, and Pd1. For compound 5n,a, in addition to the
amidate donors, the Pd center is bound to the “normal” C1
carbon of one NHC group and the “abnormal” C2′ carbon of
the other NHC. The square-planar geometry is distorted, with
C1 and C2′ being displaced by 0.324 and 0.385 Å above and
below the plane defined by N3, N3′, and Pd1, respectively.
Coordination of the abnormal carbene appears to relieve the
congestion associated with steric interactions between the
NHC methyl groups. This result is consistent with those of
several other studies, which have demonstrated that the
abnormal coordination mode often arises to lessen steric
congestion at the metal center.21,58−61 Decreased steric
Figure 3. ORTEP50 representations of the X-ray crystal structures of
(a) 3 (top view), (b) 3 (edge view) (cocrystallized methanol molecule
omitted for clarity), and (c) 4 (hydrogen atoms omitted for clarity;
one of the carbonyl oxygen atoms is disordered, and the site
occupancy factors are 0.62 (O1) and 0.38 (O2)). Atomic displacement
ellipsoids are shown at the 40% probability level.
1
congestion for 5n,a is also clearly evidenced by the H NMR
data. Singlets are observed for each pair of methylene protons
on carbons C4 and C4′, indicating that the NHC groups are
able to reorientate freely. As discussed in the Introduction,
there is evidence to suggest that aNHCs are among the most
basic of all known neutral donors. As such, a difference between
the Pd−Namidate bond distances trans to the abnormal carbene
in comparison to those for the normal carbene might be
expected. However, the bond distance data (Table 1) show that
the Pd−Namidate bond distances are identical.
Complexes 5a,a and 6a,a adopt similar “saddle”-shaped
structures, with both NHC groups coordinated as abnormal
carbenes (Figure 4b,d). The bis-abnormal coordination mode
results in a structure free from strain associated with steric
interactions between the NHC R groups, and relatively
undistorted square-planar geometries about the Pd(II) atoms
are obtained. Significant curvature of the ligand is observed for
these structures, with the methylene carbon atoms (C4 and
C4′) being displaced from the best plane defined by C2, C2′,
N3, and N3′ by 1.094 and 1.272 Å, respectively, for 5a,a and
0.930 and 1.168 Å, respectively, for 6a,a. Additionally, the NHC
units are twisted out of the metal coordination plane by 17 and
36° for 5a,a and 14 and 29° for 6a,a. Slightly shorter Pd−Ccarbene
units. The carbene carbon atoms (C1 and C1′) are displaced by
0.511 and 0.512 Å above and below the plane defined by N3,
N3′, and Ni1 (Figure 3b). This distortion of the square-planar
geometry results in axial chirality or helicity. Here a right- or
left-handed helical twist is induced by one NHC group being
displaced “up” and the other “down” relative to a chirality axis
passing through the Ni(II) center (Scheme 5).
Complex 4 crystallizes in the cubic space group Pa3, and the
̅
crystal is composed of a racemic mixture of each stereoisomeric
form of the complex. The distortion of the square-planar
geometry is less pronounced than that seen for 3, with the
carbene carbon atoms displaced 0.479 and 0.460 Å above and
below the plane defined by N3, N3′, and Ni1. The decreased
distortion for 4 may result from a π−π stacking interaction
between the phenyl ring from one arm of the tetradentate
ligand with the NHC unit of the other arm. The intramolecular
distances between these groups are relatively short: C1···C9′ is
3.127 Å, and C1′···C9 is 3.118 Å. For compounds 3 and 4, the
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Figure 4. ORTEP50 representations of the X-ray crystal structures (a) 5n,a, (b) 5a,a, (c) 6n,n, and (d) 6a,a. For 6n,n and 6a,a a cocrystallized molecule of
methanol is omitted for clarity in each case. Atomic displacement ellipsoids are shown at the 40% probability level.
bond distances are seen for compound 6a,a in comparison to
those for 5a,a (Table 1), with the average values being 1.981(6)
and 1.992(2) Å, respectively. In both cases the Pd−Ccarbene
bond distances in the “abnormal” form are slightly longer than
those for the “normal” form (6n,n), where the average Pd−
Ccarbene bond distance is 1.961(4) Å. When the bis-abnormal
complexes 5a,a and 6a,a are compared to the bis-normal complex
6n,n, there does appear to be a slightly stronger trans influence
exerted by the abnormal NHCs in comparison to the normal
NHCs. For compounds 5a,a and 6a,a the average Pd−Namidate
bond distances (trans to the aNHCs) are 2.050(9) and 2.05(1)
Å, respectively. For 6n,n the average Pd−Namidate bond distance
(trans to the normal NHCs) is shorter at 2.033(2) Å.
Previously, Pd(II) complexes of a chelating 2-picolyl-
functionalized aNHC ligand62 and a methylene-bridged bis-
aNHC ligand16 have been structurally characterized. For both
ligand types, a similar twisting of the aNHC units and
displacement of the methylene group away from the metal
coordination plane was observed. The PdII−aNHC bond length
for the structurally characterized complexes were within the
range 1.961(7)−2.007(4) Å, with variation observed primarily
resulting from the trans influence from the opposing ligand: i.e.,
Cl− versus I−. Consistent with the present study, little
difference was seen between the PdII−aNHC bond lengths
for the bis-aNHC−PdII complex and those of an analogous
complex with “normally” coordinated NHCs.
Table 2. Electrochemical Anodic Peak Potentials (mV) for
6n,n, 6n,a, and 6a,a
complex
Epa(1)
Epa(2)
6n,n
6n,a
6a,a
313
249
203
457
442
360
cyclic voltammograms for 6n,n, 6n,a, and 6a,a (100 mV/s, 1 mM
in anhydrous DMF with 0.1 M [Bu4N][PF6] supporting
electrolyte) are shown in Figure 5. Similar voltammograms
were obtained for each isomer, with two irreversible oxidative
processes (Epa(1) and Epa(2)) evident. Studies of the scan rate
dependence of these oxidations (Figure S1, Supporting
Information) show evidence for quasi-reversibility for Epa(1)
at higher scan rates (20 V/s). The Epa values (Table 2) show
that the highest oxidation potentials are associated with the
normal coordination mode of the carbene donors (6n,n), with
an intermediate value obtained for the mixed coordination
mode (6n,a) and the lowest value associated with both NHC
groups being coordinated in an abnormal manner.
DFT studies (see below) for 6n,n, 6n,a, and 6a,a yield orbital
energies of the highest occupied molecular orbital (HOMO)
that are consistent with the oxidation potentials, with 6n,n
predicted to have the lowest energy HOMO (−5.38 eV)
followed by 6n,a (−5.31 eV) and then 6a,a (−5.26 eV); from
B3LYP/def2-TZVPP//B3LYP/6-31G(d),LANL2DZ with
DMF IEF-PCM solvent.
Electrochemistry. The redox properties of the three
isomeric forms of 6 were studied using cyclic voltammetry,
with results summarized in Table 2. The anodic portions of the
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Optimized geometries were determined for all possible NHC
coordination modes (nNHC/nNHC, nNHC/aNHC, and
aNHC/aNHC) using the B3LYP hybrid exchange correlation
functional with the 6-31G(d) basis set on nonmetal atoms and
the LANL2DZ basis set and effective core potential on metal
atoms. In the cases where X-ray crystal structures were
available, the calculated geometries are in good agreement
with the solid-state structures. The Cartesian coordinates of all
optimized geometries are available as Supporting Information.
Although the aNHC coordination mode was not observed
experimentally for the Ni(II) complexes of the tetradentate
ligands (3 and 4), similar optimized geometries were calculated
for both the Ni(II) and Pd(II) (5 and 6) complexes with the
three possible NHC coordination modes. For the isomers with
nNHC/nNHC coordination, distorted four-coordinate square-
planar geometries were obtained, with the distortion a result of
steric interactions between the ligand R groups (Me and Ph).
In contrast, the nNHC/aNHC and aNHC/aNHC coordination
modes result in decreased steric congestion and lower levels of
distortion of the square-planar structures.
Figure 5. Anodic portion of the cyclic voltammogram for 6n,n, 6n,a, and
6a,a. Conditions: complex concentrations 1 mM, scan rate 100 mV s−1,
DMF, 0.1 M [Bu4N][PF6], glassy-carbon (3 mm diameter) working
electrode.
Analysis of the MOs suggests that in all cases the HOMO is
predominantly (>90%) ligand-based (Figure 6). For the
isomeric forms of 6 the character of the HOMO is similar,
being primarily π-orbitals associated with the amide units and
the phenyl linker group of the ligand and, to a lesser extent, the
metal center.
Given the congestion associated with the NHC R-groups for
the nNHC/nNHC isomers, complexes displaying aNHC
coordination might be predicted to be more stable on the
basis of steric grounds. This notion was not supported by the
DFT results. B3LYP/6-31G(d),LANL2DZ calculated relative
energies for the nNHC/nNHC, nNHC/aNHC, and aNHC/
aNHC isomeric forms of 3−6 are summarized in Table 3.
Table 3. B3LYP/6-31G(d),LANL2DZ Calculated Relative
Electronic and Gibbs Free Energies (kJ/mol) for the nNHC/
nNHC, nNHC/aNHC, and aNHC/aNHC Isomeric Forms
a
of Complexes 3−6
coordination mode
Figure 6. Molecular orbital isosurfaces of the highest occupied
molecular orbitals (HOMO) for the isomeric forms of 6: (a) 6n,n; (b)
6n,a; (c) 6a,a.
complex
metal
nNHC/nNHC
nNHC/aNHC
aNHC/aNHC
3
4
5
6
Ni(II)
Ni(II)
Pd(II)
Pd(II)
0.0
0.0
0.0
0.0
25.0 (20.7)
28.6 (25.6)
22.3 (22.9)
25.3 (22.3)
50.3 (47.9)
55.5 (53.1)
39.6 (38.6)
41.2 (36.9)
a
Single-point energies with DMF solvent at gas-phase optimized
It is possible that the strong trend observed in the values of
the oxidation potentials may result from the trans influence
exerted by the carbenes on the amide groups. Here the
abnormally coordinated NHC groups exert the strongest trans
influence, thereby decreasing the oxidation potential, while the
anodic shift seen for 6n,a and 6n,n results from the less
pronounced trans influence exerted by the normally coordi-
nated NHC groups.
Density Functional Theory Studies. To gain further
insights into the relative stabilities of complexes displaying
either “normal” or “abnormal” NHC coordination modes, DFT
was used to calculate the optimized geometries for 3−6.
For the complexes of the methyl-substituted ligands (3 and
5) both gas-phase and solvated (IEF-PCM SCRF solvation
model with DMF solvent) optimized geometries were
calculated to investigate the effect of solvent on geometries.
For complex 5, the relative single-point energies (inclusive of
solvent) at the gas phase and solvated optimized geometries
agreed to within 0.1 kJ/mol, while for the Ni complex 3 the
relative energies agreed to within 0.5 kJ/mol. Such variation is
insignificant in comparison to the relative isomeric energies
(25−50 kJ/mol). Subsequently, only gas-phase-optimized
geometries were calculated for the larger complexes of the
phenyl-substituted ligands (4 and 6).
geometries. Gibbs free energies are given in parentheses.
Solvent effects were found to be significant for relative energies,
for which only energies inclusive of solvent effects are reported
(gas-phase energies are available as Supporting Information,
Table S2). The aNHC/aNHC isomer is calculated to be the
least stable, with nNHC/nNHC being the most stable. It is
notable that the relative electronic and Gibbs free energies are
very similar. Basis set effects were considered with B3LYP/
def2-TZVP calculations, with relative energies for 3 and 5 (R =
Me) deviating by less than 1 kJ/mol while for 4 and 6 (R = Ph)
the difference is typically 3−5 kJ/mol. Test MP2/6-31G-
(d),LANL2DZ calculations for 5 give relative electronic
energies within 2.5 kJ/mol of the B3LYP/6-31G(d),LANL2DZ
results. In comparison to the relative isomer energies, the
variation with method and basis set is sufficiently small to give
confidence that the calculated trends are reliable. In all cases
complexes displaying aNHC coordination are predicted to be
less stable than their nNHC counterparts.
The association of “abnormal” coordination with higher
calculated energies is consistent with previous studies and is
likely to result from the different arrangements of the
heteroatoms in the imidazolylidene ring.15,63 A detailed analysis
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of the bonding and electronic structure is the subject of further
investigation.
NHC R groups and that these compounds are fluxional on the
NMR time scale. Abnormally coordinated NHCs are thought
to be among the most basic of all known neutral donors. The
structural studies conducted here show little evidence for a
difference in the trans influence exerted by the normally or
abnormally coordinated NHC groups. Cyclic voltammetric
measurements showed that “abnormal” NHC coordination
reduces the oxidation potential relative to the “normal” NHC
binding mode for constitutional isomeric forms of 6. DFT
studies indicate that aNHC coordination results in higher
energy structures relative to the nNHC counterparts and, taken
together with the experimental results, suggest that the
“abnormal” NHC binding occurs in kinetic rather than
thermodynamic reaction products.
Despite the DFT prediction of lower stability associated with
1
aNHC coordination, H NMR analysis of the crude products
obtained for both 5 and 6 showed that higher yields were
obtained for isomers displaying aNHC in comparison with
those displaying nNHC coordination. These findings are
consistent with the result of a previous study, which suggest
that the isomers displaying aNHC coordination are kinetic
rather than thermodynamic reaction products.21 In an effort to
experimentally define which coordination mode (nNHC/
nNHC, nNHC/aNHC, or aNHC/aNHC) represents the
actual thermodynamic product, solutions of 6n,n, 6n,a, and 6a,a
were prepared in d6-DMSO and these samples were incubated
at 80 °C for 5 days. It was anticipated that rearrangement of the
ligand binding mode might occur, yielding the thermodynami-
EXPERIMENTAL SECTION
■
1
cally more stable isomer. The samples were monitored by H
General Procedures. All reagents were purchased from Sigma-
Aldrich, were of analytical grade or higher, and were used without
further purification unless otherwise stated. Anhydrous DMF was
prepared by standing the analytical grade solvent over molecular sieves
(3 Å, activated at 300 °C) for 2 days, followed by vacuum distillation.
NMR spectra were recorded using a Bruker ARX-300 spectrometer
(300.14 MHz for 1H, 75.48 MHz for 13C) and were internally
referenced to solvent resonances. All reactions were performed under
nitrogen unless otherwise stated. 1-Phenylimidazole,65 N-(2-
chloroacetyl)aniline,37 and N,N′-bis(2-chloroacetyl)-1,2-phenylenedi-
amine48 were prepared according to the literature procedures.
Electrochemical experiments were performed using an AUTOLAB
type II electrochemical station potentiostat (MEP Instruments, North
Ryde, NSW, Australia) with General Purpose Electrochemical Systems
(GPES) software (version 4.9). A conventional three-electrode
configuration was used, consisting of a glassy-carbon 3 mm diameter
working electrode shrouded in Teflon (CH Instruments, Austin, TX,
USA), a 1 cm2 platinum-gauze auxiliary electrode, and a silver-wire
quasi reference electrode. Potentials were referenced to the ferrocene/
ferrocenium couple measured in situ (1 mM) in each case. Complexes
4, 6n,n, 6n,a, and 6a,a were prepared at a concentration of 1 mM in
distilled dimethylformamide, with 0.1 M [Bu4N][PF6] as the
supporting electrolyte.
NMR spectroscopy, and no changes in the spectra for any of
the isomeric forms were observed, showing that ligand
rearrangement does not occur under these conditions. An
identical NMR experiment was conducted with the addition of
1
the weak base Cs2CO3, and again no changes in the H NMR
spectra were observed. The lack of ligand rearrangement is
consistent with previous studies21,64 and is suggestive of
metalation at the aNHC position taking place via a process
that does not involve deprotonation and formation of the “free”
aNHC followed by coordination to the metal. Crabtree and co-
workers have postulated that aNHC binding occurs via a
mechanism involving C−H bond activation.23 It is possible that
differences between the Ni(II) and Pd(II) observed in this
work, i.e. Pd(II) induces aNHCs binding while Ni(II) does not,
result from Pd(II) being more active in C−H bond activation
processes than Ni(II). The preferential formation of the aNHC
isomers seems likely to result from steric congestion between
the NHC R groups during complex formation.60 Transition
state structures can be envisioned where steric congestion
between the NHC R groups favors the formation of
constitutional isomers displaying aNHC coordination (kinetic
products), while steric crowding in the transition state disfavors
nNHC coordination (thermodynamic product). Additionally,
complexes 1 and 2 of the much less sterically demanding
bidentate ligands show no evidence of aNHC binding.
Microanalyses were performed by the Microanalytical Laboratory at
the ANU Research School of Chemistry, Canberra, Australia.
X-ray Crystallography. Single crystals suitable for X-ray
crystallography were grown as follows: trans-1, slow evaporation of a
DMF solution containing a mixture of cis- and trans-1; cis-1, 5aa, 6n,n
,
and 6a,a, slow evaporation of methanol solutions of the title
compounds; trans-2 and 5n,a, slow evaporation of dichloromethane
solutions of the title compounds; 3, diffusion of vapors between
hexane and a solution of 3 in methanol; 4, diffusion of vapors between
ether and a solution of 4 in dichloromethane. Refinement data are
given in Table S1 (Supporting Information). Diffraction data were
measured using an Oxford Gemini diffractometer mounted with Mo
Kα radiation (λ = 0.71073 Å). Following multiscan absorption
corrections and solution by direct methods or Patterson methods, the
structures were refined using full-matrix least-squares refinement
techniques on F2 using SHELXL97.66 The non-hydrogen atoms were
refined with anisotropic displacement parameters, with hydrogen
atoms placed geometrically and refined using a riding model. All
calculations were carried out using the program Olex2.67 Images were
generated by using ORTEP-3.50 CCDC 889432−889440 contain
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
CONCLUSION
■
A family of Ni(II) and Pd(II) complexes of bidentate and
tetradentate NHC/amidate ligands have been prepared. Two
geometric isomeric forms (cis and trans) of the Pd(II)
complexes of the bidenate ligands (1 and 2) were obtained.
The Pd(II) complexes of the tetradentate ligands (5 and 6)
showed a remarkable interplay in the nature of the NHC
coordination mode, where these groups were bound to the
metal as either “normal” or “abnormal” NHCs. In contrast, only
the “normal” NHC coordination mode was obtained for the
Ni(II) tetradentate complexes (3 and 4). The constitutional
isomeric forms of 5 and 6 provide an interesting series that
allow the influence that “abnormal” versus “normal” NHCs
have on structural and electronic properties of the resultant
isomers to be examined. The square-planar coordination
geometries of the Ni(II) and Pd(II) complexes with normally
coordinated NHCs were strongly distorted as a result of steric
congestion between the NHC R groups. 1H NMR and
structural studies show that the “abnormal” coordination
mode results in decreased steric interactions between the
Density Functional Theory. Density functional theory (DFT)
calculations were carried out within the Gaussian 09 suite of
programs.68 Ground state geometries were optimized in the absence
of solvent with the B3LYP69−71 hybrid exchange-correlation functional
in conjunction with the 6-31G(d) basis set72−74 for non-metal atoms
and the LANL2DZ basis set and core potential for nickel and
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Organometallics
Article
palladium.75,76 The def2-TZVP basis set and effective core potential77
was utilized in single-point energy calculations. The polarizable
continuum model (PCM)78 self-consistent reaction field (SCRF)
was used to model solvent effects, with a solvent of DMF to be
consistent with the experimental system. Select species were optimized
inclusive of solvent effects. Solvent effects are included in single-point
energies at the gas-phase optimized geometries. An SCF convergence
criteria of 10−8 au was employed throughout.
7.5 Hz, Haryl), 6.73 (d, 2H, 3JHH = 1.5 Hz, Himi), 6.94 (dd, 4H, 3JHH
=
7.5 Hz, 3JHH = 8.1 Hz, Haryl), 7.07 (d, 2H, 3JHH = 1.5 Hz, Himi), 7.62 (d,
3
4H, JHH = 8.1 Hz, Haryl) ppm. 13C NMR (d6-DMSO): δ 36.3 (CH3),
58.5 (CH2), 122.0 (Cimi), 122.3 (Cimi), 123.6 (Caryl), 126.4 (Caryl),
127.7 (Caryl), 151.0 (Cq), 171.1 (Pd-C), 171.4 (CO) ppm. Anal.
Found: C, 53.69; H, 4.73; N, 15.55. Calcd for C24H24N6O2Pd: C,
53.89; H, 4.52; N, 15.71.
Pd(II) Complex cis-1. The methanol solution (from the trituration
step in the synthesis of trans-1 above) was evaporated to dryness, and
the residue was further recrystallized from methanol to give cis-1 as a
white powder. Yield: 0.018 g (12%). 1H NMR (d6-DMSO): δ 3.31 (s,
6H, CH3), 4.31 (d, 2H, 2JHH = 14.4 Hz, CH2), 5.26 (d, 2H, 2JHH = 14.4
Hz, CH2), 6.69 (t, 2H, 3JHH = 7.2 Hz, Haryl), 6.82 (dd, 4H, 3JHH = 7.2
Synthesis. Imidazolium Salt [I]Cl.39 A solution of 2-chloro-N-
phenylacetamide (0.843 g, 4.97 mmol) and 1-methylimidazole (0.40
mL, 4.97 mmol) in acetonitrile (70 mL) was heated at reflux (100 °C)
for 16 h. A white precipitate formed, which was collected and washed
with diethyl ether (3 × 5 mL) to afford a white powder. Yield: 1.06 g
(85%). 1H NMR (d6-DMSO): δ 3.91 (s, 3H, CH3), 5.27 (s, 2H, CH2),
7.07 (t, 1H, 3JHH = 6.9 Hz, Haryl), 7.32 (dd, 2H, 3JHH = 6.9 Hz, J = 8.4
Hz, Haryl), 7.63 (d, 2H, 3JHH = 8.4 Hz, Haryl), 7.73 (s, 1H, Himi), 7.77 (s,
1H, Himi), 9.19 (s, 1H, NCHN), 11.04 (s, 1H, NH). 13C NMR (d6-
DMSO): δ 35.9 (CH3), 51.2 (CH2), 119.2 (Caryl), 123.0 (Cimi), 123.7
(Caryl), 123.9 (Cimi), 128.9 (Caryl), 137.8 (NCN), 138.5 (Cq), 163.7
(CO). Anal. Found: C, 57.41; H, 5.55; N, 16.78. Calcd for
C12H14ClN3O: C, 57.26; H, 5.61; N, 16.69.
3
3
Hz, JHH = 7.8 Hz, Haryl), 6.98 (d, 4H, JHH = 7.8 Hz, Haryl), 7.32 (d,
3
3
2H, JHH = 1.8 Hz, Himi), 7.61 (d, 2H, JHH = 1.8 Hz, Himi) ppm. 13C
NMR (d6-DMSO): δ 36.1 (CH3), 57.8 (CH2), 120.6 (Caryl), 122.0
(Cimi), 122.2 (Cimi), 125.7 (2 × Caryl), 147.8 (Cq), 163.1 (Pd-C), 166.5
(CO) ppm. Anal. Found: C, 51.55; H, 5.82; N, 13.10. Calcd for
C24H24N6O2Pd·3CH3OH: C, 51.39; H, 5.75; N, 13.32.
Pd(II) Complex trans-2. This compound was prepared from [II]·Cl
(0.11 g, 0.36 mmol), K2CO3 (0.25 g, 1.78 mmol), and K2PdCl4 (0.058
g, 0.18 mmol) in anhydrous dimethylformamide (25 mL) using the
method described for 1 to give trans-2 as a cream-colored powder.
Imidazolium Salt [II]Cl. This compound was prepared from 1-
phenylimidazole (0.50 g, 3.47 mmol) and 2-chloro-N-phenylacetamide
(0.59 g, 3.47 mmol) using the method described for [I]Cl. The
1
2
Yield: 0.026 g (23%). H NMR (d6-DMSO): δ 4.36 (d, 2H, JHH
=
=
product was obtained as a white powder. Yield: 0.82 g (75%). H
14.1 Hz, CH2), 5.41 (d, 2H, 2JHH = 14.1 Hz, CH2), 6.65 (t, 2H, 3JHH
1
3
NMR (d6-DMSO): δ 5.37 (s, 2H, CH2), 7.08 (t, 1H, JHH = 7.5 Hz,
7.1 Hz, Haryl), 6.82 (dd, 4H, 3JHH = 7.1 Hz, 3JHH = 7.5 Hz, Haryl), 6.95
(d, 4H, 3JHH = 7.5 Hz, Haryl), 7.40 (s, 2H, Himi), 7.51 (t, 4H, 3JHH = 7.2
Hz, Haryl and Himi), 7.63 (dd, 4H, 3JHH = 7.2 Hz, 3JHH = 7.8 Hz, Haryl),
7.82 (d, 4H, 3JHH = 7.8 Hz, Haryl) ppm. 13C NMR (d6-DMSO): δ 58.3
(CH2), 120.2 (Cimi), 121.3 (Caryl), 122.4 (Cimi), 123.4 (Caryl), 125.6
(Caryl), 126.0 (Caryl), 127.9 (Caryl), 128.6 (Caryl), 137.7 (Cq), 149.8 (Cq),
167.2 (Pd-C), 169.1 (CO) ppm. Anal. Found: C, 60.11; H, 4.33; N,
12.39. Calcd for C34H28N6O2Pd·H2O: C, 60.31; H, 4.47; N, 12.41.
Ni(II) Complex 3. To a solution of [III]Cl2 (0.13 g, 0.301 mmol) in
anhydrous dimethylformamide (15 mL) were added K2CO3 (0.21 g,
1.50 mmol) and a solution of NiCl2 (0.039 g, 0.301 mmol) in
anhydrous dimethylformamide (10 mL). The resultant mixture was
heated at 110 °C for 16 h, and after it was cooled to room
temperature, the mixture was filtered and the solvent was removed
under reduced pressure. The residue was recrystallized from a mixture
of dichloromethane and diethyl ether to give 3 as a light brown
3
3
Haryl), 7.33 (dd, 2H, JHH = 7.5 Hz, JHH = 8.1 Hz, Haryl), 7.60−7.65
3
(m, 5H, Haryl), 7.81 (d, 2H, JHH = 8.1 Hz, Haryl), 8.07 (s, 1H, Himi),
8.38 (s, 1H, Himi), 10.01 (s, 1H, NCHN), 11.15 (s, 1H, NH). 13C
NMR (d6-DMSO): δ 51.6 (CH2), 119.1 (Caryl), 120.7 (Cimi), 122.0
(Caryl), 123.7 (Caryl), 125.0 (Cimi), 128.8 (Caryl), 129.9 (Caryl), 130.5
(Caryl), 134.6 (Cq), 136.6 (NCN), 138.6 (Cq), 163.5 (CO). Anal.
Found: C, 65.00; H, 5.21; N, 13.45. Calcd for C17H16ClN3O: C, 65.07;
H, 5.14; N, 13.39.
Imidazolium Salt [III]Cl2. This compound was prepared from N,N′-
bis(chloroacetyl)1,2-phenylenediamine (0.6 g, 2.3 mmol) and 1-
methylimidazole (0.366 mL, 4.6 mmol) using the method described
for [I]Cl. The crude product was recrystallized from boiling ethanol to
1
give [III]Cl2 as a pale purple powder. Yield: 0.639 g (65%). H NMR
(d6-DMSO): δ 3.91 (s, 6H, CH3), 5.49 (s, 4H, CH2), 7.15−7.18 (m,
2H, Haryl), 7.58−7.62 (m, 2H, Haryl), 7.73 (s, 2H, Himi), 7.90 (s, 2H,
Himi), 9.35 (s, 2H, NCHN), 10.75 (s, 2H, NH). 13C NMR (d6-
DMSO): δ 35.9 (CH3), 51.5 (CH2), 123.0 (Cimi), 123.8 (Cimi), 124.7
(Caryl), 125.2 (Caryl), 129.8 (Cq), 137.8 (NCN), 164.3 (CO). Anal.
Found: C, 49.09; H, 5.19; N, 19.00. Calcd for C18H22Cl2N6O2·H2O:
C, 48.77; H, 5.46; N, 18.96.
1
powder. Yield: 0.027 g (21%). H NMR (d6-DMSO): δ 3.18 (s, 6H,
CH3), 4.66 (d, 2H, 2JHH = 17.1 Hz, CH2), 5.13 (d, 2H, 2JHH = 17.1 Hz,
CH2), 6.63−6.66 (m, 2H, Haryl), 7.32 (d, 2H, 3JHH = 1.8 Hz, Himi), 7.42
(d, 2H, 3JHH = 1.8 Hz, Himi), 8.30−8.33 (m, 2H, Haryl) ppm. 13C NMR
(d6-DMSO): δ 36.1 (CH3), 54.5 (CH2), 119.9 (Caryl), 121.3 (Cimi),
121.6 (Caryl), 123.7 (Cimi), 143.4 (Cq), 162.7 (Ni-C), 166.4 (CO)
ppm. Anal. Found: C, 49.57; H, 4.32; N, 18.75. Calcd for
C18H18N6O2Ni·0.5CH2Cl2: C, 49.21; H, 4.24; N, 18.61. The reported
elemental analysis results are provided to illustrate the best values
obtained to date. Copies of the NMR spectra (1H and 13C) are
provided in the Supporting Information (Figure S2) as additional
evidence of purity.
Imidazolium Salt [IV]Cl2. This compound was prepared from 1-
phenylimidazole (0.55 g, 3.81 mmol) and N,N′-bis(chloroacetyl)1,2-
phenylenediamine (0.50 g, 1.91 mmol) using the method described for
[III]Cl2 to give [IV]Cl2 as a pale pink powder. Yield: 0.735 g (70%).
1H NMR (d6-DMSO): δ 5.57 (s, 4H, CH2), 7.18−7.21 (m, 2H, Haryl),
7.57−7.69 (m, 8H, Haryl), 7.81−7.83 (m, 4H, Haryl), 8.18 (s, 2H, Himi),
8.37 (s, 2H, Himi), 10.18 (s, 2H, NCHN), 10.81 (s, 2H, NH). 13C
NMR (d6-DMSO): δ 52.0 (CH2), 120.6 (Cimi), 122.0 (Caryl), 124.8
(Cimi, Cq), 125.5 (Caryl), 129.9 (2 × Caryl), 130.5 (Caryl), 134.7 (Cq),
136.8 (NCN), 164.0 (CO). Anal. Found: C, 57.24; H, 5.43; N,
14.04. Calcd for C28H26Cl2N6O2·2H2O: C, 57.44; H, 5.16; N, 14.35.
Pd(II) Complex trans-1. To a solution of [I]Cl (0.15 g, 0.59 mmol)
in anhydrous dimethylformamide (15 mL) were added K2CO3 (0.41 g,
2.96 mmol) and a solution of K2PdCl4 (0.097 g, 0.30 mmol) in
anhydrous dimethylformamide (10 mL). The resultant mixture was
heated at 110 °C for 16 h, and after it was cooled to room
temperature, the mixture was filtered and the solvent removed under
reduced pressure. 1H NMR analysis of the crude product showed it to
be a mixture of cis and trans isomers of the title compound. The trans
isomer was isolated by adding methanol (10 mL), and after stirring for
15 min the insoluble solid was collected and washed with diethyl ether
Ni(II) Complex 4. This compound was prepared from a solution of
[IV]Cl2 (0.12 g, 0.21 mmol), K2CO3 (0.15 g, 1.06 mmol), and NiCl2
(0.028 g, 0.21 mmol) using the method described for 3, except the
reaction was conducted at 125 °C for 16 h. The crude product was
recrystallized from a mixture of dichloromethane and diethyl ether to
give 4 as a brown powder. Yield: 0.026 g (23%). 1H NMR (d6-
DMSO): δ 4.55 (dd, 2H, 2JHH = 17.1 Hz, CH2), 4.65 (dd, 2H, 2JHH
=
17.1 Hz, CH2), 6.69−6.73 (m, 2H, Haryl), 7.07 (d, 2H, 3JHH = 1.8 Hz,
3
3
Himi), 7.25 (d, 2H, JHH = 1.8 Hz, Himi), 7.34 (t, 2H, JHH = 7.5 Hz,
Haryl), 7.56 (dd, 4H, 3JHH = 7.5 Hz, 3JHH = 7.8 Hz, Haryl), 7.81 (d, 4H,
3JHH = 7.8 Hz, Haryl), 8.40−8.43 (m, 2H, Haryl) ppm. 13C NMR (d6-
DMSO): δ 54.2 (CH2), 120.1 (Caryl), 121.5 (Cimi), 121.6 (Caryl), 122.4
(Cimi), 124.3 (Caryl), 127.9 (Caryl), 128.8 (Caryl), 138.8 (Cq), 143.6 (Cq),
164.1 (Ni-C), 166.0 (CO) ppm. Anal. Found: C, 62.03; H, 4.28; N,
15.40. Calcd for C28H22N6O2Ni·0.5H2O: C, 62.02; H, 4.28; N, 15.50.
The reported elemental analysis results are provided to illustrate the
best values obtained to date. Copies of the NMR spectra (1H and 13C)
1
to give trans-1 as a cream-colored powder. Yield: 0.032 g (20%). H
NMR (MeOD + d6-DMSO): δ 3.63 (s, 6H, CH3), 4.23 (d, 2H, 2JHH
14.1 Hz, CH2), 5.42 (d, 2H, 2JHH = 14.1 Hz, CH2), 6.70 (t, 2H, 3JHH
=
=
1921
dx.doi.org/10.1021/om4000133 | Organometallics 2013, 32, 1913−1923

Organometallics
Article
are provided in the Supporting Information (Figure S2) as additional
evidence of purity.
6
n,n. The acetonitrile filtrate (from the recrystallization of 6a,a
above) was evaporated to dryness, and the resultant residue was
recrystallized from methanol to afford 6n,n as a dark brown powder.
Yield: 0.015 g (4%)/ 1H NMR (d6-DMSO): δ 4.72 (dd, 2H, 2JHH = 9.9
Pd(II) Complex 5. 5a,a. To a solution of [III]Cl2 (0.16 g, 0.37
mmol) in anhydrous dimethylformamide (20 mL) were added K2CO3
(0.26 g, 1.9 mmol) and a solution of K2PdCl4 (0.12 g, 0.37 mmol) in
anhydrous dimethylformamide (20 mL). The resulting mixture was
heated at 70 °C for 16 h, and after it was cooled to room temperature,
the mixture was filtered and the solvent was removed under reduced
2
Hz, CH2), 4.97 (dd, 2H, JHH = 9.9 Hz, CH2), 6.76−6.78 (m, 2H,
Haryl), 7.25−7.28 (m, 2H, Haryl), 7.29 (d, 2H, 3JHH = 1.2 Hz, Himi), 7.36
(d, 2H, 3JHH = 1.2 Hz, Himi), 7.42−7.48 (m, 8H, Haryl), 8.40−8.42 (m,
2H, Haryl) ppm. 13C NMR (d6-DMSO): δ 56.4 (CH2), 120.5 (Caryl),
121.2 (Cimi), 121.8 (Cimi), 123.0 (Caryl), 125.0 (Caryl), 128.0 (Caryl),
128.1 (Caryl), 137.9 (Cq), 143.8 (Cq), 163.5 (CO), 165.1 (Pd−C)
ppm. Anal. Found: C, 52.88; H, 4.59; N, 13.29. Calcd for
C28H22N6O2Pd·3H2O: C, 52.96; H, 4.44; N, 13.24.
1
pressure. H NMR analysis of the crude product showed that two
isomers were formed (5n,a/5a,a) in the ratio 0.39/0.61. The crude
product was triturated with dichloromethane (30 mL), and a brown
solid that did not dissolve was collected and recrystallized from
methanol to give 5a,a as a pale brown powder. Yield: 0.046 g (27%). 1H
NMR (d6-DMSO): δ 3.76 (s, 6H, CH3), 4.63 (s, 4H, CH2), 6.58−6.60
(m, 2H, Haryl), 7.10 (s, 2H, Himi), 8.13−8.15 (m, 2H, Haryl), 8.84 (s,
2H, NCHN) ppm. 13C NMR (d6-DMSO): δ 34.4 (CH3), 56.7 (CH2),
119.8 (Caryl), 122.2 (Caryl), 124.5 (Cimi), 134.3 (NCN), 143.7 (Cq),
144.9 (Pd-C), 164.6 (CO) ppm. Anal. Found: C, 40.88; H, 4.68; N,
15.58. Calcd for C18H18N6O2Pd·4H2O: C, 40.88; H, 4.96; N, 15.89.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, tables, figures, and CIF files giving X-ray crystallographic
data for trans-1, cis-1, trans-2, 3, 4, 5a,a, 5n,a, 6n,n and 6a,a, scan
rate dependence of cyclic voltammograms for 6n,n, 6n,a, and 6a,a,
relative energies for calculated geometries, and the Cartesian
coordinates of optimized Ni(II) and Pd(II) complexes. This
material is available free of charge via the Internet at http://
pubs.acs.org.
5
n,a. The dichloromethane solution (from the trituration step in the
synthesis of 5a,a, above) was evaporated to dryness, and the residue
was further recrystallized from dichloromethane to give 5n,a as a cream-
colored powder. Yield: 0.007 g (4%). 1H NMR (d6-DMSO): δ 3.70 (s,
6H, CH3), 4.67 (s, 2H, CH2), 4.81 (s, 2H, CH2), 6.63 (s, 1H, Himi),
3
6.68−6.70 (m, 2H, Haryl), 7.28 (d, 1H, JHH= 0.9 Hz, Himi), 7.41 (d,
AUTHOR INFORMATION
1H, 3JHH= 0.9 Hz, Himi), 7.93−7.95 (m, 1H, Haryl), 8.54−8.56 (m, 1H,
Haryl), 8.69 (s, 1H, NCHN) ppm. 13C NMR (d6-DMSO + MeOD): δ
33.9 (2 × CH3), 120.2 (Caryl), 120.6 (Caryl), 121.5 (Cimi), 122.2 (Cimi),
122.5 (Caryl), 123.3 (Caryl), 126.0 (Cimi), 133.8 (NCN), 140.4 (Cq),
143.3 (Cq), 144.5 (Pd-C), 165.3 (Pd-C), 165.6 (CO), 165.8 (C
O) ppm. Anal. Found: C, 47.04; H, 3.81; N, 18.22. Calcd for
C18H18N6O2Pd: C, 47.33; H, 3.97; N, 18.40.
■
Corresponding Author
*E-mail for P.J.B.: p.barnard@latrobe.edu.au.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Pd(II) Complex 6. 6a,a. This compound was prepared from a
solution of [IV]Cl2 (0.39 g, 0.071 mmol), K2CO3 (0.49 g, 3.54 mmol),
and K2PdCl4 (0.23 g, 0.071 mmol) using the method described for 5.
1H NMR analysis of the crude product showed that three isomers were
formed (6n,n/6n,a/6a,a) in the ratio 0.25/0.35/0.40. 6a,a was obtained as
■
We thank Dr. Gregory Barbante and Dr. Conor Hogan for help
with the electrochemical measurements. We thank the
Australian Research Council for LIEF funding for the X-ray
diffractometer used in this work.
1
a dark brown powder. Yield: 0.085 g (21%). H NMR (d6-DMSO): δ
3
4.77 (s, 4H, CH2), 6.62−6.64 (m, 2H, Haryl), 7.52 (t, 2H, JHH= 4.5
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■
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