Mixed carbene-isocyanide Pd(ii) complexes: Synthesis, structures and reactivity towards nucleophiles

Article abstract of DOI:10.1039/b816471a

Reaction of the bromo-bridged dimeric monocarbene complex [PdBr ₂(ⁱPr₂-bimy)]₂ (1) ( ⁱPr₂-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) with various isocyanides afforded a series of mixed carbene-isocyanide complexes [PdBr₂(ⁱPr₂-bimy)(CN-R)] (2a: R = Cy; 2b: R = ⁿBu; 2c: R = Xyl) as inseparable mixtures of trans- and cis-isomers. The reactivity of 2c towards nucleophiles was studied. A new mixed NHC-ADC Pd(ii) complex (4) was obtained in low yield when 2c was reacted with 2,6-dimethylaniline. Reaction of 2c with hydrazine yielded a hydrazine-bridged complex (6) via ligand substitution. In addition, salt metathesis of 2c with AgO₂C₂F₃ afforded a cis arranged complex [Pd(O₂CCF₃)₂(ⁱPr₂-bimy) (CN-Xyl)] (cis-7). Most complexes were fully characterized by multinuclei NMR spectroscopies, ESI or FAB mass spectrometry and X-ray diffraction analysis.

Full text of DOI:10.1039/b816471a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Mixed carbene–isocyanide Pd(II) complexes: synthesis, structures and
reactivity towards nucleophiles†
Yuan Han and Han Vinh Huynh*
Received 19th September 2008, Accepted 18th December 2008
First published as an Advance Article on the web 5th February 2009
DOI: 10.1039/b816471a
Reaction of the bromo-bridged dimeric monocarbene complex [PdBr2(iPr2-bimy)]2 (1) (iPr2-bimy =
1,3-diisopropylbenzimidazolin-2-ylidene) with various isocyanides afforded a series of mixed
n
carbene–isocyanide complexes [PdBr2(iPr2-bimy)(CN-R)] (2a: R = Cy; 2b: R = Bu; 2c: R = Xyl) as
inseparable mixtures of trans- and cis-isomers. The reactivity of 2c towards nucleophiles was studied. A
new mixed NHC–ADC Pd(II) complex (4) was obtained in low yield when 2c was reacted with
2,6-dimethylaniline. Reaction of 2c with hydrazine yielded a hydrazine-bridged complex (6) via ligand
substitution. In addition, salt metathesis of 2c with AgO2C2F3 afforded a cis arranged complex
[Pd(O2CCF3)2(iPr2-bimy)(CN-Xyl)] (cis-7). Most complexes were fully characterized by multinuclei
NMR spectroscopies, ESI or FAB mass spectrometry and X-ray diffraction analysis.
dazolin-2-ylidene) can be readily cleaved by various ligands to
give Pd(II) monocarbene species.7 We anticipated that the cleavage
Introduction
∫
Although isocyanides (C N–R) are versatile ligands in
of complex 1 with isocyanide ligands should afford the desired
mixed carbene–isocyanide complexes. Consequently, an initial
attempt was carried out by reacting complex 1 with 4 equiv. of
cyclohexyl isocyanide (CN–Cy) in CH2Cl2 at ambient temperature
under aerobic conditions. However, this reaction afforded a
complex product mixture from which the desired product cis-
[PdBr2(iPr2-bimy)(CN-Cy)] (cis-2a) and an unexpected dimeric
Pd(I) complex [PdBr(CN-Cy)2]2 (3) were crystallized out in very
low yields. A comparison of the 1H NMR spectrum of the
product mixture with that of the pure benzimidazolium salt
(iPr2-bimyH+Br-)7a revealed a substantial amount of the latter in
the product mixture. Apparently, the benzimidazolium salt was
formed due to decomposition of complex 1 or 2a and subsequent
hydrolysis of the carbene ligand. Although the mechanism for
the formation of the dimeric Pd(I) complex 3 is still unclear, it is
evident that hydrolysis of the carbene ligand and the presence
of excess cyclohexyl isocyanide contributed to its formation.8
Hence, dry solvents and stoichiometric amounts of isocyanides
were used in the optimized synthetic pathway shown in Scheme 1.
The mixed carbene–isocyanide complexes [PdBr2(iPr2-bimy)(CN-
organometallic chemistry, their role has always been overshad-
owed by the prominent CO ligand. Compared to the latter,
isocyanides exhibit stronger s-donor and weaker p-acceptor char-
acter, which can be tuned by the substituent R and the complex
fragment they are coordinated to. Thus the metal fragment
also has substantial influence on the reactivity and stability of
coordinated isocyanide ligands. In relation to our research on
N-heterocyclic carbenes (NHCs), we became interested in mixed
NHC–isocyanide complexes. In particular we wanted to study
the influence of a metal–NHC fragment on the character of
coordinated isocyanide ligands. Given the current popularity of
NHCs1 in organometallic chemistry it is surprising to note that
such complexes remain very rare.2 Complexes bearing both acyclic
diaminocarbene3,4 (ADC) and isocyanide ligands, on the other
hand, are relatively common due to the fact that they are usually
derived from 1,2-addition of protic nucleophiles to homoleptic
isocyanide complexes.5 This metal-template directed approach is
also successful for the preparation of NHC complexes and has
been reviewed.6 Herein, we report the preparation, properties
and reactivities of isocyanide Pd(II) complexes supported by a
benzimidazolin-2-ylidene ligand.
n
R)] (2a: R = Cy; 2b: R = Bu; 2c: R = Xyl) were obtained in
high yields by stirring 1 with two equivalents of the respective
isocyanide ligand in dry THF at ambient temperature under an
nitrogen atmosphere for 5 h.
Results and discussion
In all cases 2a–c, the product was obtained as an inseparable
mixture of trans- and cis-isomers as indicated by two sets of signals
Synthesis and characterization of mixed
benzimidazolin-2-ylidene–isocyanide complexes
1
in their H and 13C NMR spectra. The chemical shift for the
isopropyl C–H protons in these complexes resonate in the narrow
range of ~5.9–6.1 ppm and are shifted highfield by ~0.5 ppm
compared to that in the precursor complex 1. Interestingly, the
methyl groups of the isopropyl substituents in trans-isomers give
rise to one doublet, whereas those in cis-isomers give rise to
two doublets due to a more unsymmetrical surrounding and
allowing for a differentiation of these isomers. Correspondingly,
two resonances for these methyl groups are observed in 13C NMR
We recently reported that the dimeric monocarbene com-
plex [PdBr2(iPr2-bimy)]2 (1) (iPr2-bimy = 1,3-diisopropylbenzimi-
Department of Chemistry, 3 Science Drive 3, National University of
Singapore, Singapore 117543. E-mail: chmhhv@nus.edu.sg; Fax: +65
67791691; Tel: +65 65162670
† CCDC reference numbers 703027–703035. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b816471a
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a
b
˜
˜
Table 1 n(CN) and Dn(CN) for complexes 2a–c
2a
2b
2c
˜
n(CN)
2234
94
2229
79
2194
74
˜
Dn(CN)
a
.
b Measured as KBr pellet (cm-1).
˜
˜
˜
Dn(CN) = n(CN)complex - n(CN)free ligand
frequency with respect to that of the corresponding free iso-
cyanides, indicating successful coordination and a primarily
s-donating character of the isocyanide ligands. It may be worth
noting that, although the NMR data clearly shows the presence
∫
of trans-/cis-isomers in solution, only one C N vibration band
was observed for each pair of isomers both in CHCl3 solution and
∫
as KBr pellet. It was found that the C N–R stretching frequency
n
decreases in the order of Cy > Bu > Xyl (Table 1), which may
∫
be explained by a higher C N bond order due to the higher
+I effect of the cyclohexyl substituent compared to the other
two.10 The same trend was observed when comparing Dn(CN)
˜
(Table 1).
Scheme 1 Synthesis of mixed carbene–isocyanide complexes.
Single crystals of 2a–c suitable for X-ray diffraction analysis
were obtained by diffusion of pentane into a chloroform solu-
tion (2a), by evaporation of a dichloromethane–hexane solution
(2b) or a THF solution (2c). Their molecular structures are
depicted in Fig. 1–3 and crystallographic data are summarized in
Table 2. Interestingly, complexes cis-2a and trans-2a crystallized
co-instantaneously as needles and square blocks, respectively. On
the other hand, complex 2b crystallized as cis- while complex
2c crystallized as trans-isomers. The Pd–Ccarbene bond lengths
spectra of the cis-isomers. This observation is consistent with a
more sterically hindered rotation around the Pd–Ccarbene bond in
the cis-isomers compared to that in the trans-isomers. In addition,
the aromatic protons of the benzimidazolin-2-ylidene ligand also
give rise to two sets of signals in the 1H NMR spectra with
those for the trans-isomers being more upfield than those for
the cis-isomers. A comparison of their integrals revealed that the
trans form is generally the major product with trans : cis ratios
of ~3 : 1 (2a), ~3.5 : 1 (2b) and ~8 : 1 (2c). These ratios remained
˚
˚
˚
amounting to 1.979(4) A (cis-2a), 1.984(9) A (trans-2a), 1.978(3) A
1
˚
largely unchanged over a few days as monitored by H NMR
(cis-2b) and 1.992(2) A (trans-2c) are slightly longer than that
˚
spectroscopy in CDCl3. However, prolonging the reaction time
from 5 h to 5 d, ceteris paribus, afforded very different trans : cis
ratios of ~0.2 : 1 (2a), ~9.7 : 1 (2b) and ~4.9 : 1 (2c). Interestingly,
the supposedly most bulky and donating isocyanide ligand
(CN–Cy) leads preferably to the cis-isomer in CDCl3 solution,
whereas the least bulky one (CN–nBu) prefers to form the trans-
isomer. Surprisingly, the isomer ratio for complexes formed with
the least donating, but somewhat also bulky aromatic isocyanide
(CN–Xyl) is found in between those of the aliphatic ones,
indicating that this isomerization is not simply driven by the
stereoelectronic properties of the isocyanide ligands. Instead, the
fine interplay between different polarities of the geometric isomers
and those of the solvent used should influence their distribution
to a great extent.
As expected, 13C NMR spectroscopy also revealed two isomeric
carbene signals. Those in the trans-isomers resonate at ~168 ppm,
which are generally more downfield than those at ~164 ppm found
for the cis-isomers. The same trend when comparing trans and
cis-isomers has also been found in bis(carbene) complexes.9 The
signal for the carbon donor of the isocyanide ligand in complex
trans-2a appears at 129.7 ppm as a triplet due to the coupling
with the adjacent quadrupolar 14N atom and is shifted highfield
by 24.3 ppm compared to that in the free ligand. The respective
signals for complexes 2b and 2c, on the other hand, could not be
resolved under the given conditions.
found in the precursor complex 1 (1.947(3) A) suggesting a
less Lewis acidic metal center upon cleavage of the dimer by
coordination of an additional donor. As commonly observed,
the carbene ring planes of all complexes are oriented almost
perpendicularly to the PdC2Br2 coordination plane with dihedral
angles ranging from 85.17◦ to 89.46◦ to relieve steric congestion.
The Pd–CCN bonds in complex cis-2a and cis-2b are 1.930(4)
˚
and 1.939(3) A, respectively. These bond distances are notably
˚
shorter than those found in complex trans-2a [2.020(9) A] and
˚
trans-2c [2.000(2) A], in which the isocyanide ligand experiences
the strong trans influence of the NHC ligand. The CN bonds
in all isocyanide complexes 2a–c fall in the narrow range of
˚
1.130–1.145 A indicating triple-bond character and thus negligible
p-back donation from the metal. In agreement with this, all
∫
isocyanide ligands display a nearly-linear geometry with C N–C
angles ranging from 174.1(2)◦ to 179.4(11)◦.11
The molecular structure of dipalladium(I) complex 3 has also
been determined by X-ray diffraction study. As shown in Fig. 4,
it contains two directly bonded Pd atoms with a Pd–Pd bond
˚
of 2.5282(3) A, which is similar to reported values for related
complexes [PdX(CN-R)2]2 (X = Br, R = Xyl;12 X = Cl, R = Bu;13
t
X = I, R = Me14). The two coordination planes of each Pd(I)
center are almost perpendicular with a dihedral angle of 83.21◦.
˚
The four Pd–C bond distances amount to 1.957(3)~1.968(3) A.
In addition, the four C–N–C angles of the isocyanides fall in a
considerably wide range of 162.3(3)–177.8(3)◦ and deviate from
linearity to different extents.
∫
Complexes 2a–c all show strong C N IR absorption bands
in the range of 2234–2194 cm-1, which were shifted to higher
2202 | Dalton Trans., 2009, 2201–2209
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Table 2 Selected X-ray crystallographic data for compounds 2–7
cis-2a
trans-2a
cis-2b
trans-2c
3
Formula
Formula weight
Color, habit
Crystal size/mm
Temperature/K
Crystal system
Space group
C20H29Br2N3Pd
577.68
Colorless, needle
0.30 ¥ 0.26 ¥ 0.12
223(2)
Orthorhombic
P212121
0.031(7)
10.1903(7)
13.3919(9)
17.0572(11)
90
90
90
2327.8(3)
4
1.648
Mo-Ka
4.239
1.93–27.50
30682
5350 (Rint = 0.0455)
C20H29Br2N3Pd
577.68
Yellow, block
0.56 ¥ 0.40 ¥ 0.16
223(2)
Monoclinic
P21/c
—
15.2666(17)
10.6396(12)
14.0629(16)
90
96.277(3)
90
2270.5(4)
4
1.690
C18H27Br2N3Pd
551.65
Pale yellow, block
0.40 ¥ 0.30 ¥ 0.14
293(2)
Orthorhombic
P212121
0.041(5)
10.1986(5)
13.0958(6)
16.2190(7)
90
90
90
2166.19(17)
4
1.692
Mo-Ka
4.550
2.00–27.49
15395
4938 (Rint = 0.0270)
C22H27Br2N3Pd
599.69
Pale yellow, block
0.40 ¥ 0.30 ¥ 0.14
223(2)
C28H44Br2N4Pd2
809.29
Orange, block
0.40 ¥ 0.24 ¥ 0.14
223(2)
Monoclinic
Cc
0.010(4)
15.2383(6)
23.7826(9)
10.3642(4)
90
123.4320(10)
90
3134.6(2)
4
1.715
Triclinic
¯
P1
Flack parameter
—
˚
a/A
8.8532(4)
10.2693(4)
14.0845(6)
79.1370(10)
77.3210(10)
70.0450(10)
1165.24(9)
2
1.709
Mo-Ka
4.238
1.49–27.50
15312
5358 (Rint = 0.0198)
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
Dc/g cm-3
Radiation used
Mo-Ka
4.346
2.34–27.50
15491
5208 (Rint = 0.0750)
Mo-Ka
3.720
1.82–27.50
16865
7106 (Rint = 0.0198)
m/mm-1
q range/◦
Reflection collected
Independent
reflections
Max., min.
0.6302, 0.3628
0.5431, 0.1946
0.5683, 0.2633
0.5884, 0.2820
0.6240, 0.3177
transmission
Final R indices
[I > 2s(I)]
R1 = 0.0313,
wR2 = 0.0725
R1 = 0.0365,
wR2 = 0.0745
1.052
R1 = 0.0794,
wR2 = 0.1891
R1 = 0.1143,
wR2 = 0.2014
1.136
R1 = 0.0227,
wR2 = 0.0487
R1 = 0.0256,
wR2 = 0.0493
0.989
R1 = 0.0257,
wR2 = 0.0659
R1 = 0.0320,
wR2 = 0.0681
1.069
R1 = 0.0199,
wR2 = 0.0393
R1 = 0.0219,
wR2 = 0.0397
0.841
R indices (all data)
Goodness-of-fit on F2
-3
˚
Peak/hole/e A
0.708/ -0.512
1.723/ -1.330
0.542/ -0.383
0.901/ -0.434
0.286/ -0.477
4
5
6·2CHCl3
7·CH2Cl2
Formula
Formula weight
Color, habit
Crystal size/mm
Temperature/K
Crystal system
Space group
C30H38Br2N4Pd
720.86
Yellow, needle
0.30 ¥ 0.06 ¥ 0.04
243(2)
Orthorhombic
Pbcn
—
27.983(3)
16.8551(17)
13.0760(13)
90
90
90
6167.3(11)
8
1.553
Mo-Ka
3.218
1.41–27.50
41611
7085 (Rint = 0.0868)
C25H29N3
371.51
Yellow, block
0.26 ¥ 0.14 ¥ 0.10
243(2)
Monoclinic
C2/c
—
23.2291(15)
13.0532(9)
17.1124(12)
90
122.6700(10)
90
4367.8(5)
8
1.130
C28H42Br4Cl6N6Pd2
1207.82
Yellow, needle
0.46 ¥ 0.08 ¥ 0.06
223(2)
Orthorhombic
Pccn
—
18.1241(9)
26.0307(12)
9.1762(4)
90
90
90
4329.2(3)
4
1.853
Mo-Ka
4.920
1.56–25.00
23435
3814 (Rint = 0.0471)
C27H29Cl2F6N3O4Pd
750.83
Colorless, block
0.60 ¥ 0.40 ¥ 0.34
295(2)
Triclinic
¯
P1
Flack parameter
—
˚
a/A
8.5994(10)
10.3393(12)
19.243(2)
88.083(2)
85.830(2)
73.500(2)
1635.9(3)
2
1.524
Mo-Ka
0.800
2.05–27.50
21260
7488 (Rint = 0.0246)
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
Dc/g cm-3
Radiation used
Mo-Ka
0.067
1.88—25.00
12607
3844 (Rint = 0.0464)
m/mm-1
q range/◦
Reflection collected
Independent
reflections
Max., min.
0.8821, 0.4452
0.9934, 0.9829
0.7567, 0.2105
0.7727, 0.6454
transmission
Final R indices
[I > 2s(I)]
R1 = 0.0581,
wR2 = 0.1251
R1 = 0.1063,
wR2 = 0.1414
1.029
R1 = 0.0652,
wR2 = 0.1282
R1 = 0.1228,
wR2 = 0.1486
1.032
R1 = 0.0679,
wR2 = 0.1614
R1 = 0.0792,
wR2 = 0.1682
1.066
R1 = 0.0389,
wR2 = 0.1015
R1 = 0.0437,
wR2 = 0.1049
1.051
R indices (all data)
Goodness-of-fit on F2
-3
˚
Peak/hole/e A
0.890/ -0.542
0.127/ -0.095
3.755/ -1.328
0.431/ -0.467
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Fig. 2 Molecular structure of complex cis-2b showing 50% probability el-
˚
lipsoids; hydrogen atoms are omitted for clarity. Selected bond lengths (A)
and angles (◦): Pd1–C1 1.978(3), Pd1–C14 1.939(3), Pd1–Br1 2.4422(4),
Pd1–Br2 2.4745(3), N1–C1 1.336(3), N1–C2 1.399(3), N1–C8 1.481(3),
N2–C1 1.346(3), N2–C7 1.400(3), N2–C11 1.474(3), N3–C14 1.133(4),
N3–C15 1.444(4), C2–C7 1.383(4); C14–Pd1–C1 88.68(11), C14–Pd1–Br2
90.92(8), C1–Pd1–Br1 85.53(7), Br1–Pd1–Br2 94.874(13), C1–N1–C2
109.2(2), C1–N2–C7 109.1(2), C14–N3–C15 178.0(3).
Fig. 1 Molecular structures of complexes cis-2a (a) and trans-2a
(b) showing 50% probability ellipsoids; hydrogen atoms are omitted for
◦
˚
clarity. Selected bond lengths (A) and angles ( ): cis-2a: Pd1–C1 1.979(4),
Pd1–C14 1.930(4), Pd1–Br1 2.4347(5), Pd1–Br2 2.4556(5), N1–C1
1.348(5), N1–C2 1.398(5), N1–C8 1.490(5), N2–C1 1.324(5), N2–C7
1.402(5), N2–C11 1.486(5), N3–C14 1.134(5), N3–C15 1.459(5), C2–C7
1.395(6); C14–Pd1–C1 86.45(15), C14–Pd1–Br2 89.84(11), C1–Pd1–Br1
88.22(10), Br1–Pd1–Br2 95.49(2), C1–N1–C2 109.1(3), C1–N2–C7
109.9(3), C14–N3–C15 175.4(4). trans-2a: Pd1–C1 1.984(9), Pd1–C14
2.020(9), Pd1–Br1 2.4072(16), Pd1–Br2 2.4312(14), N1–C1 1.350(11),
N1–C2 1.398(11), N1–C8 1.482(11), N2–C1 1.346(11), N2–C7 1.393(11),
N2–C11 1.492(11), N3–C14 1.130(12), N3–C15 1.452(13), C2–C7
1.417(12); Br1–Pd1–C1 86.7(3), C14–Pd1–Br1 89.6(3), C1–Pd1–Br2
87.9(3), C14–Pd1–Br2 95.9(3), C1–N1–C2 109.5(7), C1–N2–C7 110.1(7),
C14–N3–C15 179.4(11).
Fig. 3 Molecular structure of complex trans-2c showing 50% proba-
bility ellipsoids; hydrog◦en atoms are omitted for clarity. Selected bond
˚
lengths (A) and angles ( ): Pd1–C1 1.992(2), Pd1–C14 2.000(2), Pd1–Br1
2.4263(3), Pd1–Br2 2.4278(3), N1–C1 1.347(3), N1–C2 1.398(3), N1–C8
1.477(2), N2–C1 1.349(3), N2–C7 1.398(3), N2–C11 1.479(3), N3–C14
1.145(3), N3–C15 1.405(3), C2–C7 1.389(3); C14–Pd1–Br1 91.71(7),
C14–Pd1–Br2 91.11(7), C1–Pd1–Br1 88.03(6), C1–Pd1–Br2 89.23(6),
C1–N1–C2 109.70(17), C1–N2–C7 109.77(17), C14–N3–C15 174.1(2).
Reactivity study of mixed carbene–isocyanide complexes
It has been well documented that coordinated isocyanides with
depicted in Scheme 2, the equimolar reaction of 2c with 2,6-
dimethylaniline in CH2Cl2 at ambient temperature overnight,
followed by removal of the solvent in vacuo and recrystallization
from hexane, yielded single crystals of the desired mixed NHC–
ADC complex [PdBr2(iPr-bimy)(ADC)] (4) and N,N¢,N¢¢-tris(2,6-
dimethylphenyl)-guanidine (5) in very low yields.17 The majority
of the starting material 2c remained unreacted as evidenced by the
1H NMR spectrum of the reaction mixture.
To the best of our knowledge, complex 4 represents so far
the first Pd(II) complex which combines a cyclic and an acyclic
carbene ligand. Fig. 5a shows the crystal structure of complex
4 in which the palladium center is surrounded by two bromo,
one benzimidazolin-2-ylidene and one acyclic diamino carbene
ligands in a trans fashion. The carbene ring plane is positioned
-1
˜
Dn(CN) values greater than 40 cm can undergo nucleophilic at-
tack by amines or other nucleophiles to give carbene complexes.6b,15
We wanted to investigate whether the mixed carbene–isocyanide
complexes 2a–c would undergo such reactions and hence can
serve as metal-templates for the synthesis of mixed NHC–ADC
complexes. Crociani and coworkers have reported that aromatic
isocyanides undergo nucleophilic attack much more easily than
aliphatic ones due to the higher electron susceptibility induced by
the electron-withdrawing aromatic groups.16 Therefore, complex
2c was chosen as the supposedly most reactive compound to
conduct this reactivity study. 2,6-dimethylaniline was selected as
the nucleophile in order to form a symmetrical acyclic carbene
ligand and thus to ease crystallization of the product. As
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Fig. 4 Molecular structure of complex 3 showing 50% probability
ellipsoids; hydrogen atoms are omitted for clarity. Selected bond lengths
◦
˚
(A) and angles ( ): Pd(1)–C(2) 1.957(3), Pd(1)–C(1) 1.968(3), Pd(1)–Pd(2)
2.5282(3), Pd(1)–Br(1) 2.5301(3), Pd(2)–C(4) 1.967(3), Pd(2)–C(3)
1.967(3), Pd(2)–Br(2) 2.5285(4), N(1)–C(1) 1.140(3), N(1)–C(5) 1.458(3),
N(2)–C(2) 1.140(3), N(2)–C(17) 1.458(3), N(3)–C(3) 1.144(3), N(3)–C(11)
1.461(3), N(4)–C(4) 1.140(3), N(4)–C(23) 1.453(3); C(2)–Pd(1)–Pd(2)
85.11(8), C(1)–Pd(1)–Pd(2) 89.47(8), C(2)–Pd(1)–Br(1) 91.66(8),
C(1)–Pd(1)–Br(1) 93.80(8), C(4)–Pd(2)–Pd(1) 78.50(8), C(3)–Pd(2)–Pd(1)
90.78(8), C(4)–Pd(2)–Br(2) 99.08(8), C(3)–Pd(2)–Br(2) 91.64(8),
C(1)–N(1)–C(5) 177.8(3), C(2)–N(2)–C(17) 170.4(3), C(3)–N(3)–C(11)
169.9(3), C(4)–N(4)–C(23) 162.3(3).
Fig. 5 (a) Molecular structure of complex 4 showing 50% probability el-
˚
lipsoids; hydrogen atoms are omitted for clarity. Selected bond lengths (A)
and angles (◦): Pd1–C1 2.024(5), Pd1–C14 2.047(5), Pd1–Br1 2.4369(7),
Pd1–Br2 2.4224(7), N1–C1 1.342(6), N1–C2 1.389(7), N1–C8 1.458(7),
N2–C1 1.348(7), N2–C7 1.393(7), N2–C11 1.488(6), N3–C14 1.350(6),
N4–C14 1.303(6), C2–C7 1.383(8); C14–Pd1–Br1 89.74(14), C14–Pd1–Br2
93.81(13), C1–Pd1–Br1 86.48(14), C1–Pd1–Br2 89.92(14), C1–N1–C2
110.1(4), C1–N2–C7 111.0(4), C14–N3–C15 125.2(4), C14–N4–C23
124.7(4), N3–C14–N4 115.5(5). (b) Packing diagram of complex 4 along
b axis.
˚
3.682 A, respectively, indicating p–p stacking interactions with
intercentroid distances between the stacked aryl rings of 4.000
˚
and 4.354 A.
The N,N¢,N¢¢-tris(2,6-dimethylphenyl)-guanidine (5) obtained
in this reaction may be formed through condensation of 2,6-
dimethylaniline and 1,3-bis(2,6-dimethylphenyl)-urea, which in
turn could have formed by air-oxidation of the acyclic carbene
ligand. The molecular structure of compound 5 was determined
by X-ray diffraction analysis and depicted in Fig. 6 along with
key bond lengths and angles. Interestingly, the central CN3 unit
is surrounded by three nearly-perpendicular 2,6-dimethylphenyl
groups to generate an almost three-fold symmetry arround the
central carbon atom. Such an arrangement is similar to that found
for N,N¢,N¢¢-tris(2,6-diisopropylphenyl)-guanidine.20
Scheme 2 Synthesis of the mixed NHC–ADC carbene complex (4).
almost perpendicular with respect to the PdBr2C2 coordination
plane in a torsion angle of 82.83◦. The Pd–CNHC bond amounts
˚
˚
to 2.024(5) A and is slightly longer than that of 2.017(2) A in the
homo-bis(carbene) complex trans-[PdBr2(iPr-bimy)2],18 which is
in accord with acyclic diamino carbenes being generally stronger
s-donors than benzimidazolin-2-ylidenes.4 Finally, the Pd–CADC
We tried to improve the yield of 4 by prolonging the re-
action time, increasing the reaction temperature, using excess
2,6-dimethylaniline or changing the solvent to higher boiling
toluene. Unfortunately, all these attempts led to a complicated re-
action mixture from which no complex 4 could be isolated. Instead,
the simple aniline-complex [PdBr2(iPr-bimy)(2,6-dimethylaniline)]
˚
bond length of 2.047(5) A is comparable with the reported value
for a trans-bis(ADC) Pd(II) complex.19 In the crystal packing,
the two aromatic substituents of the acyclic carbene ligand are
oriented such that each of them is aligned parallel with the
aromatic substituents of a neighbouring molecule, as depicted
in Fig. 5b. The interplanar distances amount to 3.582 and
1
(X) was found to be the major product after comparing the H
NMR spectrum of the product mixture with that of an authentic
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NMR spectrum appears at 159.9 ppm, which is comparable with
the reported values for 1,3-diisopropylbenzimidazolin-2-ylidene
Pd(II) complexes containing other N-donor bridges.7b
The identity of complex 6 was further confirmed by single crystal
X-ray diffraction. To the best of our knowledge, complex 6 is the
first hydrazine-bridged Pd(II) complex to be crystallographically
characterized. As shown in Fig. 7, each of the palladium centers
adopts a nearly perfect square planar geometry and they are
arranged trans to each other with respect to the N–N bond
˚
of the hydrazine bridge. The Pd–N bond lengths of 2.133(8) A
and are comparable to reported values for other Pd(II)–hydrazine
complexes.22 Finally, the two carbene ring planes are oriented
almost perpendicularly with a dihedral angle of 85.47◦.
Fig. 6 Molecular structure of compound 5 showing 50% proba-
bility ellipsoids; hydrogen atoms are omitted for clarity. Selected
bond lengths (A) and angles (◦): N(1)–C(1) 1.273(3), N(3)–C(1)
˚
1.365(3), N(2)–C(1) 1.367(3), N(1)–C(2) 1.422(3), N(3)–C(18) 1.433(3),
N(2)–C(10) 1.427(3); N(1)–C(1)–N(3) 125.6(2), N(3)–C(1)–N(2) 113.8(2),
N(1)–C(1)–N(2) 120.6(2), C(1)–N(3)–C(18) 126.5(2), C(1)–N(1)–C(2)
119.5(2), C(1)–N(2)–C(10) 124.1(2).
Fig. 7 Molecular structure of complex 6 showing 50% probability
ellipsoids; hydrogen atoms and solvent molecules are omitted for clarity.
◦
˚
Selected bond lengths (A) and angles ( ): Pd1–C1 1.958(8), Pd1–N3
2.133(8), Pd1–Br1 2.4187(12), Pd1–Br2 2.4324(12), N1–C1 1.340(10),
N1–C2 1.389(11), N1–C8 1.488(12), N2–C1 1.344(11), N2–C7 1.391(11),
N2–C11 1.465(12), N3–N3A 1.437(14), C2–C7 1.381(14); N3–Pd1–Br1
92.3(2), N3–Pd1–Br2 90.3(2), C1–Pd1–Br1 90.5(2), C1–Pd1–Br2 87.0(2),
Pd1–N3–N3A 118.2(8).
sample prepared by reacting complex 1 with 2 equiv. of 2,6-
dimethylaniline. Apparently, a Pd(II)-centered ligand substitution
is preferred over nucleophilic attack on the coordinated isocyanide.
A similar reactivity was also observed with hydrazine, which is a
widely-used bifunctional nucleophile to generate chelating ADC
ligands.19,21 As depicted in Scheme 3, the reactions of complex 2c
with 1 or 2 equiv. of hydrazine both yielded the hydrazine bridged
complex {[PdBr2(iPr2-bimy)]2(m-N2H4)} (6) as the only isolated
product.
The difficulty encountered in the preparation of a mixed
NHC–ADC complex is probably due to the insufficient electron
susceptibility of the coordinated isocyanide in 2c, which in turn is
caused by the strong s-donating and poor p-accepting nature of
the carbene ligand. The replacement of the bromo ligands in 2c
with less electron-donating ligands such as trifluoroacetate should
result in a more Lewis acidic metal center, which may increase
the susceptibility of the isocyanide ligand for nucleophilic attack.
Therefore, the trifluoroacetato complex (7) was prepared via salt-
metathesis (Scheme 4).18,23 It is worth mentioning that only cis-
7 was obtained in a quantitative yield, although a mixture of
trans- and cis-2c was reacted. We have reported a cis preference
upon halo–carboxylato exchange in a series of bis(carbene) Pd(II)
complexes.18,23 The isolation of pure cis-7 clearly demonstrates that
such a preference can be extended to mixed carbene–isocyanide
complexes as well. Complex cis-7 is soluble in most of polar
organic solvents such as THF, CH2Cl2, CH3CN but insoluble
in nonpolar hexane and toluene. Upon standing in solution,
complex cis-7 slowly decomposes to yield palladium black under
aerobic conditions. Again, this decreased stability upon halo–
trifluoroacetato exchange was also observed in some of Pd(II)–
carbene complexes.24
Scheme 3 Synthesis of complex 6.
The 1H NMR spetrum of cis-7 in CDCl3 shows two doublets of
equal intenstity at 1.82 and 1.75 ppm for the two inequivalent
Me-groups. Furthermore, the 13C NMR spectrum shows two
quartets at 162.4 and 161.6 ppm with 2J(C,F) = 37.6 and 38.5 Hz,
repectively, for the CO groups and another two quartets at 116.0
The formation of complex 6 was corroborated by its positive
mode ESI mass spectrum which shows a base peak centered
at m/z = 889 corresponding to the [M - Br]+ fragment. In
the 1H NMR spectrum, the hydrazine protons resonate as a
singlet at 4.72 ppm. Moreover, the carbene signal in the 13C
1
and 115.5 ppm with J(C,F) = 290.0 and 290.5 Hz, respectively,
2206 | Dalton Trans., 2009, 2201–2209
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Fig. 8 Molecular structure of complex cis-7 showing 50% probabil-
ity ellipsoids; hydrogen atoms and solvent molecules are omitted for
◦
˚
clarity. Selected bond lengths (A) and angles ( ): Pd1–C1 1.964(2),
Pd1–C14 1.911(3), Pd1–O1 2.0494(19), Pd1–O3 2.0419(18), N1–C1
1.341(3), N1–C2 1.395(3), N1–C8 1.487(3), N2–C1 1.349(3), N2–C7
1.384(3), N2–C11 1.485(3), N3–C14 1.144(3), N3–C15 1.401(3), C2–C7
1.391(3); C14–Pd1–O1 97.02(10), C14–Pd1–C1 86.94(10), C1–Pd1–O3
91.41(9), O1–Pd1–O3 84.41(8), C1–N1–C2 109.4(2), C1–N2–C7 109.8(2),
C14–N3–C15 179.0(3).
Scheme 4 Synthesis of complex 7.
for the CF3 groups. The observation of two sets of quartets is
consistent with a cis geometry. However, only one singlet was
observed at 2.0 ppm in the 19F NMR spectrum. The carbene
carbon in complex cis-7 resonates at 155.9 ppm, which is shifted
significantly highfield compared to that in the precursor complex
2c. Finally, two broad 13C NMR signals at 127. 9 and 125.1 ppm
are assigned to the two carbon atoms adjacent to the nitrogen
atom in the isocyanide ligand, respectively.
spectroscopic studies revealed all these complexes exist in solution
as a mixture of trans- and cis-isomers. The reactivity of 2c
bearing an aromatic isocyanide ligand towards nucleophiles was
studied in detail. Reaction of 2c with 2,6-dimethylaniline yielded
the first mixed NHC–ADC complex 4. On the other hand,
a hydrazine-bridged Pd(II) complex 6 was obtained via ligand
substitution when 2c was reacted with hydrazine. In addition,
a mixed NHC-isocyanide-carboxylato complex cis-7 was also
prepared by salt metathesis of 2c with AgO2CCF3 in attempt to
increase the susceptibility of the coordinated isocyanide towards
nucleophiles.
As indicated by 1H and 13C NMR spectroscopy in solution, a cis
configuration was found by X-ray diffraction study on single crys-
˚
tals of cis-7 (Fig. 8) as well. Both the Pd–Ccarbene (1.964(2) A) and
˚
the Pd–CCN (1.911(3) A) bonds are shorter than the corresponding
ones in the precursor complex 2c, confirming a more Lewis
acidic metal center. The Pd1–O1 bond (2.0494(19) A) trans to
the carbene ligand is longer than the Pd1–O3 bond (2.0419(18) A)
trans to the isocyanide ligand, indicating again a stronger trans
influence of the former. Upon bromo-trifluoroacetato metathesis,
the coordinated carbon atom in the isocyanide ligand indeed
became more electrophilic, which is also evidenced by the IR
˚
˚
Experimental section
∫
spectrum of cis-7 where a strong C N vibration band is observed
General considerations
at a higher wave number of 2214 cm-1 (cf. 2194 cm-1 in complex
2c). Having confirmed the increased electron susceptibility of
the coordinated isocyanide in cis-7, we attempted to generate a
mixed NHC–ADC complex by reacting cis-7 with one equiv. of
2,6-dimethylaniline in CH2Cl2 at ambient temperature. However,
Unless otherwise noted all operations were performed without
taking precautions to exclude air and moisture. All solvents and
chemicals were used as received without any further treatment
if not noted otherwise. THF was dried with Na/benzophenone
and handled using standard Schlenk techniques. Complex 1 was
prepared as previously reported.7a 1H, 13C, and 19F NMR spectra
were recorded on a Bruker ACF 300 and AMX 500 spectrometer
and the chemical shifts (d) were internally referenced to the
residual solvent signals relative to tetramethylsilane (1H, 13C) or
externally to CF3CO2H (19F). Mass spectra were measured using
a Finnigan MAT LCQ (ESI) and Finnigan/MAT 95XL-T (FAB)
spectrometer. Infrared spectra were recorded with a Varian 3100
FT-IR spectrometer using KBr pellet. Elemental analyses were
performed on a Perkin-Elmer PE 2400 elemental analyzer at the
Department of Chemistry, National University of Singapore.
1
the H NMR spectrum of the reaction mixture again revealed a
rather incomplete and complicated reaction. Attempts to separate
unreacted complex cis-7 with the other products were hampered
by decomposition of the reaction mixture to palladium black upon
workup.
Conclusions
A series of mixed carbene–isocyanide complexes (2a–c) were
successfully prepared by reacting the bromo-bridged monocar-
bene complex (1) with various isocyanides. 1H and 13C NMR
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1
Syntheses
93%. trans-2c: H NMR (300 MHz, CDCl3): d 7.63–7.58 (dd,
2 H, Ar–H), 7.26–7.21 (m, 3 H, Ar–H), 7.12–7.10 (d, 2 H, Ar–H),
6.05 (m, 3J(H,H) = 7.1 Hz, 2 H, NCH(CH3)2), 2.52 (s, 6 H,
Dibromo-(cyclohexylisocyanide)-(1,3-diisopropylbenzimidazo-
lin-2-ylidene)palladium(II) (2a). Complex 1 (94 mg, 0.10 mmol)
and cyclohexyl isocyanide (25 ml, 0.2 mmol) were dissolved in
dry THF (10 ml) and stirred at ambient temperature for 5
h under nitrogen. The volatiles were removed under reduced
pressure. The residue was washed with hexane (15 ml ¥ 3) and
dried in vacuo. Crystallization by diffusion of pentane into a
concentrated CHCl3 solution afforded the product as yellow
crystals. Total yield: 98 mg, 0.17 mmol, 85%. trans-2a: 1H NMR
(300 MHz, CDCl3): d 7.57–7.54 (dd, 2 H, Ar–H), 7.22–7.19 (dd,
3
1
CH3), 1.77 (d, J(H,H) = 7.1 Hz, 12 H, NCH(CH3)2). 13C{ H}
NMR (75.47 MHz, CDCl3): 168.3 (s, Ccarbene), 136.2, 133.3, 130.0,
128.0, 122.6, 112.9 (s, Ar–C), 54.4 (s, NCH(CH3)2), 20.8 (s,
∫
NCH(CH3)2), 18.7 (s, CH3). The C N and Ar-CN signals could
not be detected. cis-2c: 1H NMR (300 MHz, CDCl3): d 7.68–7.65
(dd, 2 H, Ar–H), 7.34–7.31 (dd, 2 H, Ar–H), 7.25–7.21 (t, 1 H,
3
Ar–H), 7.08–7.05 (d, 2 H, Ar–H), 6.05 (m, J(H,H) = 7.1 Hz,
2 H, NCH(CH3)2), 2.28 (s, 6 H, CH3), 1.79 (d, 3J(H,H) = 7.1 Hz,
6 H, NCH(CH3)2), 1.72 (d, 3J(H,H) = 7.1 Hz, 6 H, NCH(CH3)2).
2 H, Ar–H), 5.98 (m, 3J(H,H) = 7.1 Hz, 2 H, NCH(CH3)2), 3.88
13C{ H} NMR (75.47 MHz, CDCl3): 163.9 (s, Ccarbene), 135.7,
1
3
∫
(m, 1 H, C NCH), 1.89–1.77 (m, 6 H, CH2), 1.72 (d, J(H,H) =
133.3, 130.5, 128.3, 123.4, 113.1 (s, Ar–C), 54.4 (s, NCH(CH3)2),
7.1 Hz, 12 H, NCH(CH3)2), 1.43–1.32 (m, 4 H, CH2). 13C{ H}
1
∫
21.1, 20.4 (s, NCH(CH3)2), 18.6 (s, CH3). The C N and Ar-CN
signals could not be detected. trans-2c and cis-2c: IR (KBr pellet)
n = 2194 (s, C N) cm . Anal. Calc. for C22H27N3Br2Pd: C,
44.06; H, 4.54; N, 7.01. Found: C, 44.03; H, 4.40; N, 6.86%. MS
(FAB): m/z = 651 [M - Br + C9H9N]+.
Tetrabromo-bis(1,3-diisopropylbenzimidazolin-2-ylidene)-l-(hy-
drazine)dipalladium(II) (6). Complex 2c (90 mg, 0.15 mmol) and
hydrazine monohydrate (3.6 ml, 0.075 mmol) were dissolved in
CH2Cl2 (5 ml) and stirred at ambient temperature overnight. The
resulting mixture was filtered over Celite and the solvent was
removed under reduced pressure. Upon adding CHCl3 (2 ml) to the
residue, yellow precipitates were formed and collected by filtration.
Reducing the solvent of the filtrate to 1 ml followed by prolonged
stirring (3 h) afforded a second crop of a yellow precipitate. The
yellow precipitate was combined and dried in vacuo to give the
product (33 mg, 0.034 mmol, 45%). 1H NMR (500 MHz, CD2Cl2):
d 7.61–7.59 (m, 4 H, Ar–H), 7.25–7.23 (m, 4 H, Ar–H), 6.21
NMR (75.47 MHz, CDCl3): 168.5 (s, Ccarbene), 133.4 (s, Ar–C),
∫
∫
129.7 (br, C N), 122.5, 112.9 (s, Ar–C), 55.5 (s, C NC), 54.4 (s,
NCH(CH3)2), 31.9, 24.9, 22.5 (s, CH2), 20.8 (s, NCH(CH3)2). cis-
-1
∫
˜
1
2a: H NMR (300 MHz, CDCl3): d 7.64–7.61 (dd, 2 H, Ar–H),
3
7.31–7.28 (dd, 2 H, Ar–H), 5.92 (m, J(H,H) = 7.1 Hz, 2 H,
∫
NCH(CH3)2), 3.80 (m, 1 H, C NCH), 1.89–1.77 (m, 6 H, CH2),
1.73 (d, 3J(H,H) = 7.1 Hz, 6 H, NCH(CH3)2), 1.67 (d, 3J(H,H) =
7.1 Hz, 6 H, NCH(CH3)2), 1.43–1.32 (m, 4 H, CH2). 13C{ H}
1
NMR (75.47 MHz, CDCl3): 164.6 (s, Ccarbene), 133.5 (s, Ar–C),
∫
123.3, 113.1 (s, Ar–C), 54.5 (s, NCH(CH3)2), 52.4 (s, C NC), 32.0,
24.5, 22.6 (s, CH2), 21.2, 20.5 (s, NCH(CH3)2). The C N signal
∫
˜
could not be detected. trans-2a and cis-2a: IR (KBr pellet) n =
-1
∫
2234 (s, C N) cm . Anal. Calc. for C20H29N3Br2Pd: C, 41.58; H,
5.06; N, 7.27. Found: C, 41.94; H,5.07; N, 7.23%. MS (FAB):
m/z = 498 [M - Br]+.
Dibromo-(n-butylisocyanide)-(1,3-diisopropylbenzimidazolin-2-
ylidene)palladium(II) (2b). 2b was prepared in analogy to 2a from
complex 1 (94 mg, 0.10 mmol) and n-butyl isocyanide (21 ml,
3
(m, J(H,H) = 6.9 Hz, 4 H, NCH(CH3)2), 4.72 (s, 4 H, NH2),
1.73 (d, J(H,H) = 6.9 Hz, 24 H, NCH(CH3)2). 13C{ H} NMR
(125.76 MHz, CD2Cl2): 159.9 (s, Ccarbene), 133.7, 122.8, 113.0 (s,
Ar–C), 54.9 (s, NCH(CH3)2), 20.7 (s, NCH(CH3)2). IR (KBr
˜
3
1
1
0.2 mmol). Total yield: 105 mg, 0.19 mmol, 95%. trans-2b: H
NMR (300 MHz, CDCl3): d 7.57–7.54 (dd, 2 H, Ar–H), 7.22–7.19
(dd, 2 H, Ar–H), 5.97 (m, 3J(H,H) = 7.1 Hz, 2 H, NCH(CH3)2),
pellet)
n
=
3233, 3143 (m, NH) cm-1. Anal. Calc. for
∫
3.64 (t, 2 H, C NCH2), 1.74–1.65 (m, 2 H, CH2), 1.72 (d,
C26H40N6Br4Pd2: C, 32.22; H, 4.16; N, 8.67. Found: C, 31.46; H,
4.03; N, 8.24% (better results could not be obtained although the
1H and 13C NMR spectra do not show any impurities). MS (ESI):
m/z = 889 [M - Br]+.
cis-(1,3-Diisopropylbenzimidazolin-2-ylidene)-(2,6-dimethyl-phe-
nylisocyanide)-bis(trifluoroacetato)palladium(II) (7). A mixture
of complex 2c (120 mg, 0.20 mmol) and AgO2CCF3 (93 mg,
0.42 mmol) was suspended in acetonitrile (10 ml) and stirred
at ambient temperature overnight shielded from light. The
reaction mixture was filtered over Celite and the solvent of the
filtrate was removed under reduced pressure. To the residue was
added CHCl3 (15 ml) and the resulting mixture was filtered over
Celite. Removing the solvent of the filtrate in vacuo afforded the
3J(H,H) = 7.1 Hz, 12 H, NCH(CH3)2), 1.55–1.45 (m, 2 H, CH2),
0.95 (t, 3 H, CH3). 13C{ H} NMR (75.47 MHz, CDCl3): 168.3 (s,
1
Ccarbene), 133.4, 122.5, 113.0 (s, Ar–C), 54.4 (s, NCH(CH3)2), 43.9
∫
(br, C NC), 30.5 (s, CH2), 20.8 (s, NCH(CH3)2), 19.5 (s, CH2), 13.2
1
∫
(s, CH3). The C N signal could not be detected. cis-2b: H NMR
(300 MHz, CDCl3): d 7.64–7.61 (dd, 2 H, Ar–H), 7.31–7.28 (dd,
2 H, Ar–H), 5.91 (m, 3J(H,H) = 7.1 Hz, 2 H, NCH(CH3)2), 3.59
3
∫
(t, 2 H, C NCH2), 1.74–1.65 (m, 2 H, CH2), 1.73 (d, J(H,H) =
7.1 Hz, 6 H, NCH(CH3)2), 1.67 (d, 3J(H,H) = 7.1 Hz, 6 H,
NCH(CH3)2), 1.32–1.23 (m, 2 H, CH2), 0.81 (t, 3 H, CH3). 13C{ H}
1
NMR (75.47 MHz, CDCl3): 164.4 (s, Ccarbene), 133.5, 123.3, 113.1 (s,
∫
Ar–C), 54.6 (s, NCH(CH3)2), 44.7 (br, C NC), 30.4 (s, CH2), 21.2,
20.5 (s, NCH(CH3)2), 19.4 (s, CH2), 13.0 (s, CH3). The C N signal
∫
1
product as white powder (126 mg, 0.19 mmol, 95%). H NMR
˜
could not be detected. trans-2b and cis-2b: IR (KBr pellet) n=
(500 MHz, CDCl3): d 7.69–7.67 (m, 2 H, Ar–H), 7.37–7.35 (m,
2 H, Ar–H), 7.23 (t, 1 H, Ar–H), 7.06 (d, 2 H, Ar–H), 6.14 (m,
3J(H,H) = 6.9 Hz, 2 H, NCH(CH3)2), 2.17 (s, 6 H, CH3), 1.82
-1
∫
2229 (s, C N) cm . Anal. Calc. for C18H27N3Br2Pd: C, 39.19; H,
4.93; N, 7.62. Found: C, 38.98; H, 4.75; N, 7.50%. MS (FAB):
m/z = 472 [M-Br]+, 555 [M - Br + C5NH9]+.
3
3
(d, J(H,H) = 6.9 Hz, 6 H, NCH(CH3)2), 1.75 (d, J(H,H) =
1
Dibromo-(1,3-diisopropylbenzimidazolin-2-ylidene)-(2,6-dimethyl-
phenylisocyanide)palladium(II) (2c). 2c was prepared in analogy
to 2a from complex 1 (281 mg, 0.30 mmol) and 2,6-dimethylphenyl
isocyanide (79 mg, 0.60 mmol). Total yield: 335 mg, 0.56 mmol,
6.9 Hz, 6 H, NCH(CH3)2). 13C{ H} NMR (125.76 MHz, CDCl3):
162.4 (q, 2J(C,F) = 37.6 Hz, O2CCF3), 161.6 (q, 2J(C,F) =
38.5 Hz, O2CCF3), 155.9 (s, Ccarbene), 136.0, 133.2, 131.1, 128.6 (s,
∫
Ar–C), 127.9, 125.1 (br, Ar-CN and C N), 124.0 (s, Ar–C), 116.0
2208 | Dalton Trans., 2009, 2201–2209
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(q, 1J(C,F) = 290.0 Hz, CF3), 115.5 (q, 1J(C,F) = 290.5 Hz, CF3),
113.4 (s, Ar–C), 55.2 (s, NCH(CH3)2), 21.5, 21.2 (s, NCH(CH3)2),
2 (a) Y. Ito, T. Hirao and T. Saegusa, J. Organomet. Chem., 1974, 82,
C47; (b) Y. Ito, T. Hirao, K. Tsubata and T. Saegusa, Tetrahedron Lett.,
1978, 18, 1535.
3 R. W. Alder, P. R. Allen, M. Murray and A. G. Orpen, Angew. Chem.,
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4 K. Denk, P. Sirsch and W. A. Herrmann, J. Organomet. Chem., 2002,
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5 For a recent review, see:L. M. Slaughter, Comments Inorg. Chem., 2008,
29, 46.
6 (a) M. Tamm and F. E. Hahn, Coord. Chem. Rev., 1999, 182, 175 and
references therein; (b) R. A. Michelin, A. J. L. Pombeiro and M. F.
C. Guedes da Silva, Coord. Chem. Rev., 2001, 218, 75 and references
therein.
7 (a) H. V. Huynh, Y. Han, J. H. H. Ho and G. K. Tan, Organometallics,
2006, 25, 3267; (b) Y. Han, H. V. Huynh and G. K. Tan, Organometallics,
2007, 26, 6447.
8 This type of dimeric Pd(I) complexes are normally synthesized by
redox reations between Pd(RNC)2 and PdX2(RNC)2 (R= alkyl or aryl;
X = Cl, Br, I), see: S. Otsuka, Y. Tatsuno and K. Ataka, J. Am.
Chem. Soc., 1971, 6705.
9 H. V. Huynh, J. H. H. Ho, T. C. Neo and L. L. Koh, J. Organomet.
Chem., 2005, 690, 3854.
1
18.3 (s, CH3). 19F{ H} NMR (282.4 MHz, CDCl3): 2.0 (s, 6 F,
-1
∫
˜
CF3). IR (KBr pellet) n = 2214 (s, C N) cm . Anal. Calc. for
C26H27F6N3O4Pd·CH2Cl2: C, 43.19; H, 3.89; N, 5.60. Found: C,
43.70; H, 3.68; N, 5.68%. MS (ESI): m/z = 552 [M - O2CCF3]+.
Dibromo-(1,3-diisopropylbenzimidazolin-2-ylidene)-(2,6-dimethyl-
aniline)palladium(II) (X). Complex 1 (94 mg, 0.10 mmol) and
2,6-dimethylaniline (25 ml, 0.2 mmol) were dissolved in CHCl3
(10 ml) and stirred at ambient temperature for 5 h. The volatiles
were removed under reduced pressure. The residue was washed
with hexane (15 ml ¥ 3) and dried in vacuo to afford the product
as a yellow powder (112 mg, 0.19 mmol, 95%). 1H NMR
(300 MHz, CDCl3): d 7.57–7.54 (dd, 2 H, Ar–H), 7.21–7.18 (dd,
2 H, Ar–H), 7.07–6.95 (m, 3 H, Ar–H), 6.20 (m, 3J(H,H) =
6.9 Hz, 2 H, NCH(CH3)2), 4.12 (s, 2 H, NH2), 2.55 (s, 6 H,
3
1
CH3), 1.75 (d, J(H,H) = 6.9 Hz, 12 H, NCH(CH3)2). 13C{ H}
NMR (75.47 MHz, CDCl3): 160.1 (s, Ccarbene), 138.7, 134.1, 129.2,
128.7, 125.2, 123.0, 113.3 (s, Ar–C), 55.2 (s, NCH(CH3)2), 21.3 (s,
NCH(CH3)2), 19.8 (s, CH3).
10 B. Crociani, T. Boschi and U. Belluco, Inorg. Chem., 1970, 9, 2021.
11 It must be noted that linearity of coordinated isocyanide ligands alone
does not necessarily rule out p-back donation from the metal center
to the isocyanide ligands. See: C. Elschenbroich, in Organometallics,
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 3rd edn., 2006,
pp. 382–383.
X-Ray diffraction studies
12 J. Vicente, J. -A. Abad and E. Mart´ınez-Viviente, Organometallics, 2002,
21, 4454.
Diffraction data for compounds 2a–c and 3–7 were collected with
a Bruker AXS APEX CCD diffractometer at 223(2) (cis-2a, trans-
2a, trans-2c, 3), 243(2) (4, 5), 293(2) (cis-2b) or 295(2)K (7) using
13 (a) R. E. Marsh, Acta, Crystallogr., Sect. B, 1997, 53, 317; (b) Y.
Yamamoto and H. Yamazaki, Bull. Chem. Soc. Jpn., 1985, 58, 1843.
14 N. M. Rutherford, M. M. Olmstead and A. L. Balch, Inorg. Chem.,
1984, 23, 2833.
˚
graphite monochromated Mo-Ka radiation (l = 0.71073 A).
15 (a) R. J. Angelici, J. Organomet. Chem., 2008, 693, 847; (b) J. Chatt, R.
L. Richards and G. H. D. Royston, J. Chem. Soc., Dalton Trans., 1973,
1433.
16 (a) B. Crociani, T. Boschi, M. Nicolini and U. Belluco, Inorg. Chem.,
1972, 11, 1292; (b) T. Boschi, B. Crociani, M. Nicolini and U. Belluco,
Inorg. Chim. Acta, 1975, 12, 39.
Data were collected over the full sphere and were corrected
for absorption. Structure solutions were found by the Patterson
method. Structure refinement was carried out by full-matrix least
squares on F2 using SHELXL-9725 with first isotropic and later
anisotropic displacement parameters for all non-hydrogen atoms.
All hydrogen atoms were put at calculated positions. A summary
of the most important crystallographic data is given in Table 2.
17 Adequate characterizations on compounds 4 and 5 could not be done
due to their very low yields.
18 Y. Han, H. V. Huynh and L. L. Koh, J. Organomet. Chem., 2007, 692,
3606.
19 A. I. Moncada, J. M. Tanski and L. M. Slaughter, J. Organomet. Chem.,
2005, 690, 6247.
20 R. E. Boere´, J. Masuda and G. Wolmersha¨user, Can. J. Chem., 2000,
78, 1613.
Acknowledgements
We thank the National University of Singapore for financial
support (Grant No. R 143-000-327-133) and the CMMAC staff
of our department for technical assistance.
21 (a) A. I. Moncada, M. A. Khan and L. M. Slaughter, Tetrahedron Lett.,
2005, 46, 1399; (b) A. I. Moncada, S. Manne, J. M. Tanski and L. M.
Slaughter, Organometallics, 2006, 25, 491.
22 J. E. Kickham and S. J. Loeb, Inorg. Chem., 1995, 34, 5656.
23 H. V. Huynh, T. C. Neo and G. K. Tan, Organometallics, 2006, 25,
1298.
24 (a) H. V. Huynh, D. LeVan, F. E. Hahn and T. S. A. Hor, J. Organomet.
Chem., 2004, 689, 1766; (b) Y. Han and H. V. Huynh, Chem. Commun.,
2007, 1089.
Notes and references
1 For a recent review, see: F. E. Hahn and M. C. Jahnke, Angew. Chem.,
Int. Ed., 2008, 47, 3122.
25 G. M. Sheldrick, SHELXL-97; Universita¨t Go¨ttingen: Germany, 1997.
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