Reactivity differences of palladium(II) dimers bearing heterocyclic carbenes with two, one, or no α-nitrogen atoms toward isocyanides

Article abstract of DOI:10.1021/om101169x

Pd(II) dimers [PdI₂(rNHC)]₂ (1a,b) of pyrazole-based remote N-heterocyclic carbenes (rNHCs) have been synthesized through oxidative addition of 4-iodopyrazolium iodides (A, B) to [Pd₂(dba) ₃]. Reaction of 1a with aromatic (CN-Xyl) or aliphatic (CN-Cy) isocyanides led to the template-assisted formation of novel Pd(II) dimers 8 and 9 bearing betainic C-imino ligands via isocyanide insertion into Pd-C _rNHC bonds and subsequent dimerization. In contrast, both isocyanides reacted with the dimers [PdI₂(Me₂-indy)]₂ (2) and [PdI₂(Me₂-bimy)]₂ (3) bearing indazolin-3-ylidenes and benzimidazolin-2-ylidenes with formation of mononuclear mixed carbene/isocyanide complexes 4-7. Notably, only dimer 8 underwent further bridge cleavage with excess isocyanide, yielding the mixed C-imino/CN-Xyl complex 10, while dimer 9 remained intact. These results highlight the uniquely different reactivities of complexes with carbenes having no α-nitrogen versus those with one or two α-nitrogen atoms, as a result of their decreasing donor abilities.

Full text of DOI:10.1021/om101169x

1224 Organometallics 2011, 30, 1224–1230
DOI: 10.1021/om101169x
Reactivity Differences of Palladium(II) Dimers Bearing Heterocyclic
Carbenes with Two, One, or No r-Nitrogen Atoms toward Isocyanides
Yuan Han, Dan Yuan, Qiaoqiao Teng, and Han Vinh Huynh*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
Received December 11, 2010
Pd(II) dimers [PdI2(rNHC)]2 (1a,b) of pyrazole-based remote N-heterocyclic carbenes (rNHCs)
have been synthesized through oxidative addition of 4-iodopyrazolium iodides (A, B) to [Pd2(dba)3].
Reaction of 1a with aromatic (CN-Xyl) or aliphatic (CN-Cy) isocyanides led to the template-assisted
formation of novel Pd(II) dimers 8 and 9 bearing betainic C-imino ligands via isocyanide insertion
into Pd-CrNHC bonds and subsequent dimerization. In contrast, both isocyanides reacted with the
dimers [PdI2(Me2-indy)]2 (2) and [PdI2(Me2-bimy)]2 (3) bearing indazolin-3-ylidenes and benzimi-
dazolin-2-ylidenes with formation of mononuclear mixed carbene/isocyanide complexes 4-7.
Notably, only dimer 8 underwent further bridge cleavage with excess isocyanide, yielding the mixed
C-imino/CN-Xyl complex 10, while dimer 9 remained intact. These results highlight the uniquely
different reactivities of complexes with carbenes having no R-nitrogen versus those with one or two
R-nitrogen atoms, as a result of their decreasing donor abilities.
Introduction
that were obtained via oxidative addition of 4-iodopyrazo-
lium salts to zerovalent palladium precursors in the presence
of additional donors such as phosphine and pyridine.5 Due
to the fact that it is not possible to represent these pyrazolin-
4-ylidenes with any neutral carbene resonance structure, they
were classified as “abnormal” carbenes. However, it was
recently suggested to use the more appropriate term “me-
soionic” to describe such species.6
In our search for carbenes that are more donating than
classical NHCs, we have also communicated the versatile
coordination chemistry of indazolin-3-ylidenes.7 In contrast,
their carbon donor is adjacent to only one R-nitrogen atom,
making them ligands with reduced heteroatom stabiliza-
tion.8
N-heterocyclic carbenes (NHCs) have met spectacular
success in organometallic chemistry and transition-metal-
mediated catalysis, primarily due to their strong electron-
donating nature.1 The stability of such classical NHCs has
been primarily attributed to the push-pull electronic effect
exerted by two R-nitrogen atoms. The more recent search for
even stronger organometallic ligands has led to the discovery
of nonconventional carbenes, including one-N, six-mem-
bered remote NHCs (rNHCs) by Raubenheimer et al.2 and
“carbodicarbenes” (CDCs) by Bertrand et al.,3 in which the
carbon donors do not contain any R-heteroatoms. As our
contribution toward this development, we introduced two-
N, five-membered rNHCs derived from pyrazole (pyrazolin-
4-ylidenes) that are isomeric with classical imidazolin-2-
ylidenes.4 Likewise, these rNHC ligands contain carbon
donors without adjacent R-nitrogen atoms. Instead, the
electron-withdrawing heteroatoms are located two bonds
away from the carbenoid center, which makes them strongly
donating ligands. Since then, we have reported a series of
structurally diverse mono-, di-, and tetranuclear Pd(II)
complexes with mono- and bidentate bridging rNHC ligands
Mesoionic carbenes and carbenes with reduced heteroatom
stabilization are increasingly gaining attention, and there are
(5) (a) Han, Y.; Huynh, H. V.; Tan, G. K. Organometallics 2007, 26,
6581. (b) Han, Y.; Lee, L. J.; Huynh, H. V. Organometallics 2009, 28,
2778. (c) Han, Y.; Lee, L. J.; Huynh, H. V. Chem. ur. J. 2010, 16, 771. (c)
Han, Y.; Huynh, H. V. Dalton Trans. 2011, DOI: 10.1039/c0dt01037e.
(6) (a) Guisado-Barrios, G.; Bouffard, J.; Donnadieu, B.; Bertrand,
G. Angew. Chem., Int. Ed. 2010, 49, 4759. (b) Iglesias, M.; Albrecht, M.
Dalton Trans. 2010, 39, 5213.
(7) Jothibasu, R.; Huynh, H. V. Chem. Commun. 2010, 46, 2986.
(8) Schuster, O.; Yang, L.; Raubenheimer, H. G.; Albrecht, M.
Chem. Rev. 2009, 109, 3445.
*To whom correspondence should be addressed. E-mail: chmhhv@
nus.edu.sg.
(9) (a) Lebel, H.; Janes, M. K.; Charette, A. B.; Nolan, S. P. J. Am.
ꢀ
Chem. Soc. 2004, 126, 5046. (b) Prades, A.; Viciano, M.; Sanau, M.;
(1) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122.
(2) (a) Meyer, W. H.; Deetlefs, M.; Pohlmann, M.; Scholz, R.;
Esterhuysen, M. W.; Julius, G. R.; Raubenheimer, H. G. Dalton Trans.
2004, 413. (b) Schneider, S. K.; Roembke, P.; Julius, G. R.; Loschen, C.;
Raubenheimer, H. G.; Frenking, G.; Herrmann, W. A. Eur. J. Inorg.
Chem. 2005, 2973. (c) Schneider, S. K.; Julius, G. R.; Loschen, C.;
Raubenheimer, H. G.; Frenking, G.; Herrmann, W. A. Dalton Trans.
2006, 1226.
€
Peris, E. Organometallics 2008, 27, 4254. (c) Yang, L.; Kruger, A.; Neels,
A.; Albrecht, M. Organometallics 2008, 27, 3161. (d) Kong, Y.; Ren, H.;
Xu, S.; Song, H.; Liu, B.; Wang, B. Organometallics 2009, 28, 5934. (e)
Heckenroth, M.; Kluser, E.; Neels, A.; Albrecht, M. Angew. Chem., Int.
Ed. 2007, 46, 6293. (f) Bacciu, D.; Cavell, K. J.; Fallis, I. A.; Ooi, L.
Angew. Chem., Int. Ed. 2005, 44, 5282. (g) Heckenroth, M.; Neels, A.;
Garnier, M. G.; Aebi, P.; Ehlers, A. W.; Albrecht, M. Chem. Eur. J.
2009, 15, 9375. (h) Stander-Grobler, E.; Schuster, O.; Heydenrych, G.;
Cronje, S.; Tosh, E.; Albrecht, M.; Frenking, G.; Raubenheimer, H. G.
Organometallics 2010, 29, 5821. (i) Raubenheimer, H. G.; Cronje, S.
Dalton Trans. 2008, 1265.
(3) (a) Dyker, C. A.; Lavallo, V.; Donnadieu, B.; Bertrand, G. Angew.
Chem., Int. Ed. 2008, 47, 3206. (b) Kaufhold, O; Hahn, F. E. Angew.
Chem., Int. Ed. 2008, 47, 4057.
(4) Han, Y.; Huynh, H. V. Chem. Commun. 2007, 1089.
r
pubs.acs.org/Organometallics
Published on Web 02/10/2011
2011 American Chemical Society

Article
Scheme 1. Synthesis of Dimeric Pd(II) Pyrazolin-4-ylidene
Complexes 1a,b
Organometallics, Vol. 30, No. 5, 2011 1225
several studies, mainly on the catalytic activities, comparing
them to classical NHCs.9,6b Apart from catalysis, we are not
aware of any unified study on the different chemical reactiv-
ities of all three types of carbenes bearing two, one, or no R-
nitrogen atom. Herein, we report on the syntheses and
stabilities of phosphine- and pyridine-free pyrazolin-4-ylidene
Pd(II) dimers, which broadens the structural scope of these
unique ligands. Furthermore, we compare their reactivity
toward isocyanides with that of their analogues bearing either
NHCs with reduced heteroatom stabilization (indazolin-
3-ylidene) or classical NHCs (benzimdazolin-2-ylidene). The
different reactivities of Pd(II) pyrazolin-4-ylidene dimers
shown here highlight their nature as one of the strongest
neutral carbon donors known to date.
Figure 1. Molecular structure of 1a 4CH2Cl2 showing 50%
3
probability ellipsoids. Hydrogen atoms and solvent molecules
are omitted for clarity. Selected bond lengths (A) and angles
(deg): Pd1-C1 1.980(6), Pd1-I1 2.5719(15), Pd1-I2 2.5572(15),
Pd1-I1A 2.6774(17), C1-C2 1.354(9), C1-C3 1.374(9), C3-
N2 1.334(8), C2-N1 1.321(8), N1-N2 1.361(8); C1-Pd1-I1
89.25(19), C1-Pd1-I2 89.10(19), I1-Pd1-I1A 87.76(5),
I2-Pd1-I1A 93.90(5), C2-C1-C3 106.0(6).
The identity of 1a could be confirmed by an X-ray dif-
fraction analysis on single crystals obtained by slow evapora-
tion of a saturated CH2Cl2 solution. As depicted in Figure 1,
each of the two Pd(II) centers is coordinated by one rNHC
ligand and one terminal iodo and two bridging iodo ligands
in a square-planar fashion. As is commonly observed, the
rNHC ligands are oriented almost perpendicularly to the
coordination plane with a dihedral angle of 77.45°. The
Pd-Ccarbene bond length of 1.980(6) A is identical with those
found in the mononuclear phosphine complexes [PdI2-
(rNHC)(PPh3)] (1.996(7) A) and [PdI(rNHC)(PPh3)2]þ-
OTf- (2.004(9) A) with the same rNHC ligand within the
3σ limit.4,5a Furthermore, among the three different types of
Pd-I bonds, those trans to the rNHC ligand are significantly
longer than the other two types due to the strong trans
influence of the rNHC ligand.
Reactivity of Palladium(II) Dimers toward Isocyanides.
Monocarbene palladium(II) dimers can be easily cleaved
by various donors to give mixed carbene/coligand com-
plexes10 that can be employed as useful precatalysts for
certain organic transformations.11 A range of such com-
plexes bearing a constant carbene probe has also been
synthesized to determine the donor strength of the respective
coligand.12 Of particular interest is the bridge-cleavage reac-
tion with isocyanides, giving complexes with two different
types of palladium-carbon bonds.13 Alternatively, such
complexes have also been synthesized through template-
assisted reaction pathways.14 In certain cases, insertion
reactions of isocyanides into metal-carbon bonds have been
observed. Such insertions have been extensively studied
since the 1960s, owing to their fundamental importance in
Results and Discussion
Palladium(II) Pyrazolin-4-ylidene Dimers. We previously
reported the successful preparation of pyrazolin-4-ylidene
complexes by oxidative addition of 4-iodopyrazolium iodide
salts to [Pd2(dba)3] in the presence of PPh3 or pyridine as
supporting ligand.4,5 In the absence of any additional ligand,
the formation of iodo-bridged dimeric complexes of the type
[PdI2(rNHC)]2 is anticipated. Indeed, the reaction of 2 equiv
of 4-iodopyrazolium iodides (A, B) with 1 equiv of [Pd2-
(dba)3] in dry CH2Cl2 under reflux conditions proceeded
smoothly to yield the desired dimeric products [PdI2-
(rNHC)]2 (rNHC = 1-ethyl-2,3,5-trimethylpyrazolin-4-yli-
dene (1a), 2-ethyl-3,5-dimethyl-1-phenylpyrazolin-4-ylidene
(1b)) as red-brown solids in high yields of 75 and 86%,
respectively (Scheme 1).
The air- and moisture-stable complexes 1a,b are generally
soluble in chlorinated and polar coordinating solvents such
as CH2Cl2, CH3CN, and DMSO but insoluble in the non-
polar solvents diethyl ether and toluene. Their identities were
supported by the positive ESI mass spectra, which show
isotopic envelopes centered at m/z 871 (1a) and 995 (1b),
respectively, corresponding to the [M - I]þ fragments. Their
1H NMR spectra show little changes compared to those of
their precursor salts (A, B). In the 13C NMR spectrum of
complex 1b, the carbenoid signal was found at 112.7 ppm,
which is more upfield than those of the complexes
[Pd(O2CCF3)2(rNHC)(PPh3)] (115.1 ppm) and [PdI(rNHC)-
(PPh3)2]þOTf- (128.2 ppm) with the same pyrazolin-4-yli-
dene ligand,4,5a in line with a more Lewis acidic metal center
due to the absence of additional phosphine donors. Never-
theless, this C4 signal in 1b is still significantly more down-
field by 40.1 ppm compared to the analogous resonance of
the precursor salt B, suggesting a successful coordination of
the pyrazolin-4-ylidene ligand. On the other hand, the 13C
carbene signal for complex 1a could not be resolved, despite
prolonged acquisition time, due to its poor solubility.
(10) Huynh, H. V.; Han, Y.; Ho, J. H. H.; Tan, G. K. Organometallics
2006, 25, 3267.
(11) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem.,
Int. Ed. 2007, 46, 2768.
(12) Huynh, H. V.; Han, Y.; Jothibasu, R.; Yang, J. A. Organome-
tallics 2009, 28, 5395.
(13) Han, Y.; Huynh, H. V. Dalton Trans. 2009, 2201.
(14) (a) Tamm, M.; Hahn, F. E. Coord. Chem. Rev. 1999, 182, 175. (b)
Tamm, M.; Hahn, F. E. J. Chem. Soc., Chem. Commun. 1995, 569. (c)
Flores-Figueroa, A.; Kaufhold, O.; Feldmann, K; Hahn, F. E. Dalton
Trans. 2009, 9334.

1226 Organometallics, Vol. 30, No. 5, 2011
Chart 1. Dimeric Monocarbene Complexes with Different Degrees of Heteroatom Stabilization
Han et al.
Scheme 2. Bridge-Cleavage Reactions of a Dimeric Pd(II) Benzimidazolin-2-ylidene Complex
metal-mediated polymerization reactions.15 Most studies in
this field involved insertions into metal-aryl or metal-alkyl
bonds, and only limited examples have been reported invol-
ving metal-carbene16 and metal-heteroaryl compounds.17
Therefore, we became interested in investigating how the
Pd-C bond of the pyrazolin-4-ylidene dimer 1a would interact
with aromatic and aliphatic isocyanides, in comparison with its
analogues 27 and 3 bearing indazole- and benzimidazole-
derived carbenes (Chart 1).
Palladium(II) Benzimidazolin-2-ylidene Dimer. The pre-
viously unknown dimer [PdI2(Me2-bimy)]2 (3; Me2-bimy =
1,3-dimethylbenzimidazolin-2-ylidene) was straightforwardly
synthesized by reacting 1 equiv of 1,3-dimethylbenzimidazo-
lium iodide with 1 equiv of Pd(OAc)2 with the addition of 4
equiv of NaI in DMSO at 90 °C, in analogy to the procedure for
a bromido derivative.10 The subsequent reaction of 3 with 2
equiv of 2,6-dimethylphenyl or cyclohexyl isocyanide (CN-Xyl
or CN-Cy) in CH2Cl2 at ambient temperature afforded the
mononuclear mixed carbene/isocyanide complexes trans-[PdI2-
(CN-Xyl)(Me2-bimy)] (4) and trans-[PdI2(CN-Cy)(Me2-bimy)]
(5) in high yields of 80 and 94%, respectively (Scheme 2). A
color change from red-brown to orange optically indicates the
successful bridge-cleavage reactions.
always afforded mixtures of cis and trans isomers despite
containing a more bulky NHC ligand.13 It appears that the
larger and more electron rich iodido ligands prefer a trans
arrangement. The successful cleavage reaction with trans selec-
tivity is supported by 1H and 13C NMR spectra of both com-
pounds, which show only one set of signals for the carbene and
isocyanide ligands, respectively. Upon coordination of the
latter, the carbene signal in both complexes shifted significantly
downfield to 171.3 ppm (Δδ=14.3 ppm), indicating a more
electron rich palladium center. In addition, strong absorption
bands observed at 2189 cm-1 (4) and 2216 cm-1 (5) by IR
spectroscopy (KBr pellet) indicate the presence of coordinated
isocyanides.
The identities of 4 and 5 as trans complexes were further
confirmed by X-ray diffraction analyses on single crystals
obtained by slow evaporation of saturated dichloromethane
solutions.18
Palladium(II) Indazolin-3-ylidene Dimer. Indazolin-3-yli-
denes are isomeric with benzimidazolin-2-ylidenes but pos-
sess stronger donor ability due to the presence of only one
electron-withdrawing R-nitrogen with respect to the car-
bene carbon.7,12 In order to evaluate if such changes would
induce any reactivity differences, the palladium(II) indazo-
lin-3-ylidene dimer [PdI2(Me2-indy)]2 (2; Me2-indy = 1,2-
dimethylindazolin-3-ylidene) was also treated with 2 equiv of
CN-Xyl and CN-Cy, respectively. In analogy to its isomer 3,
bridge-cleavage reactions of the brown complex 2 were
observed for both isocyanides, giving the two new mixed
indazolin-3-ylidene/isocyanide complexes trans-[PdI2(CN-
Xyl)(Me2-indy)] (6) and trans-[PdI2(CN-Cy)(Me2-indy)]
(7), which were isolated as yellow powders in yields of 83
and 65%, respectively (Scheme 3). In comparison to the
parent dimer 2, the solubilities of 6 and 7 in common organic
solvents, with the exception of nonpolar hexane and diethyl
ether, have also improved significantly.
These reactions turned out to be very selective, giving rise
to only trans isomers, which is in stark contrast to analogous
reactions of the bromido-bridged dimer [PdBr2(iPr2-bimy)]2
(iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene), which
(15) (a) Yamamoto, Y.; Yamazaki, H. Coord. Chem. Rev. 1972, 8,
225. (b) Yamamoto, Y.; Yamazaki, H. Inorg. Chem. 1974, 13, 439. (c)
Yamamoto, Y.; Tanase, T.; Yanai, T.; Asano, T.; Kobayashi, K. J.
Organomet. Chem. 1993, 456, 287. (d) Delis, J. G. P.; Aubel, P. G.;
Vrieze, K.; van Leeuwen, P. W. N. M.; Veldman, N.; Spek, A. L.; van
Neer, F. J. P. Organometallics 1997, 16, 2948. (e) Vicente, J.; Saura-
€
Llamas, I.; Grunwald, C.; Alcaraz, C.; Jones, P. G.; Bautista, D.
Organometallics 2002, 21, 3587.
(16) (a) Aumann, R.; Fischer, E. O. Chem. Ber. 1968, 101, 954. (b)
Cramer, R. E.; Panchanatheswaran, K.; Gilje, J. W. Angew. Chem., Int.
Ed. Engl. 1984, 23, 912. (c) Mitsudo, T.-A.; Watanabe, H.; Komiya, Y.;
Watanabe, Y.; Takaegami, Y.; Nakatsu, K.; Kinoshita, K.; Miyagawa,
Y. J. Organomet. Chem. 1980, 190, C39. (d) Fryzuk, M. D.; Duval, P. B.;
ESI mass spectrometry and 1H and 13C NMR spectrosco-
py corroborated the similar reactivities, including trans
selectivities, of the isomeric Pd(II) dimers 2 and 3. For
example, a pronounced downfield shift of the carbene reso-
nance from 148.8 ppm (2)7 to 164.9 (6) and 165.1 ppm (7),
respectively, was also observed upon formation of the mono-
nuclear complexes. The molecular structure of 6 (Figure 2),
Mao, S. S. S. H.; Rettig, S. J.; Zaworotko, M. J.; MacGillivray, L. R.
J. Am. Chem. Soc. 1999, 121, 1707. (e) Cadierno, V.; Garcı
´
ꢀ
a-Alvarez, J.;
Gimeno, J.; Rubio-Garcıa, J. J. Organomet. Chem. 2005, 690, 5856. (f)
´
Basuli, F.; Bailey, B. C.; Watson, L. A.; Tomaszewski, J.; Huffman,
J. C.; Mindiola, D. J. Organometallics 2005, 24, 1886. (g) Cadierno, V.;
ꢀ
a-Alvarez, J.; Gimeno, J. Organometallics 2008, 27, 1809.
Dı
´
ez, J.; Garcı
´
(17) (a) Bertani, R.; Berton, A.; Bianca, D. F.; Crociani, B. J.
Organomet. Chem. 1986, 303, 283. (b) Crociani, B.; Sala, M. Organo-
metallics 1986, 5, 1369.
(18) The molecular structures of 4 and 5 are discussed separately in
the Supporting Information.

Article
Organometallics, Vol. 30, No. 5, 2011 1227
Scheme 3. Bridge-Cleavage Reactions of a Dimeric Pd(II) Indazolin-3-ylidene Complex
Scheme 4. Insertion of Isocyanides into a Pd-rNHC Bond
Figure 2. Molecular structure of complex 6 showing 50% prob-
ability ellipsoids. Hydrogen atoms are omitted for clarity.
Selected bond lengths (A) and angles (deg): Pd1-C1 2.002(5),
Pd1-C10 1.993(5), Pd1-I1 2.5925(6), Pd1-I2 2.5991(6), N1-C1
1.333(6), N1-C8 1.462(6), N1-N2 1.377(5), N2-C9 1.455(6),
N2-C7 1.350(6), C7-C2 1.406(7), C10-N3 1.136(6), N3-C11
1.409(6); C1-Pd1-I1 89.01(14), C1-Pd1-I2 89.96(14), C10-
Pd1-I1 89.95(17), C10-Pd1-I2 91.03(17), N1-C1-C2 105.0(4),
C10-N3-C11 175.0(6).
determined by X-ray diffraction on single crystals obtained
from a saturated dichloromethane solution, confirms the
trans arrangement of the indazolin-3-ylidene and the iso-
cyanide ligand around a square-planar palladium center with
Pd-Ccarbene and Pd-CCN bonds of 2.002(5) and 1.993(5) A,
respectively. The carbene plane is oriented almost perpendi-
cularly to the PdC2I2 coordination plane with an angle of
83.42°. These and all other bond parameters are essentially in
the range of those observed for benzimidazolin-2-ylidene
analogues 4 and 5 and require no further comments.
From these results it can be concluded that the increased
donor ability of carbenes with reduced heteroatom stabiliza-
tion compared to their classical NHC counterparts is still
insufficient to induce different reactivities toward isocy-
anides.
Palladium(II) Pyrazolin-4-ylidene Dimer. Although pyra-
zolin-4-ylidenes and related ligands are among the strongest
neutral donors reported to date, their chemistry is still
underdeveloped. In this context, the question arises whether
the superior electron donation would translate into any
unique reactivity of their complexes. Thus, reactions of the
dimeric pyrazolin-4-ylidene complex 1a with 2 equiv of CN-
Xyl and CN-Cy were carried out under the same conditions
as described for dimers 2 and 3 (Scheme 4).
bands for isocyanide CtN triple bonds are absent. Further-
more, the values of these bands fall within the commonly
observed range (1550-1650 cm-1) for products resulting
from isocyanide insertion into Pd-C bonds.15c-e,17a,17b Ad-
ditional support for an isocyanide insertion was found in the
1H NMR spectra of complexes 8 and 9, which show down-
field-shifted resonances for the pyrazole moiety with respect
to those in the parent complex 1a. The 13C NMR signal for
the new CdN-Xyl function of complex 8 arises at 148.7
ppm, whereas that for CdN-Cy in 9 was not resolved.
Overall, these spectroscopic data in combination with the
complex properties suggest that 8 and 9 are dimeric species,
which have formed via isocyanide precoordination,15d iso-
cyanide insertion, and subsequent dimerization (Scheme 4).
A final confirmation for this proposal was obtained by
X-ray diffraction analysis on single crystals of the complex
9 4CH3CN, grown by slow evaporation of a CH3CN solu-
3
A distinct color change from red-brown 1a to orange was
observed for both isocyanides, indicating a successful reac-
tion. However, the orange products 8 and 9 exhibit much
poorer solubilities in comparison to the mixed carbene/
isocyanide complexes 4-6 and precipitate from dichloro-
methane. Nevertheless, they can be dissolved in polar co-
ordinating solvents, i.e. CH3CN, DMSO, and DMF, which
resembles properties of the dimeric Pd(II) complexes 1a, 2,
and 3.
tion, and its molecular structure is shown in Figure 3.
To our surprise, the dimeric structure of 9 was not cleaved
by CH3CN upon standing in solution for a few days. This is
in contrast to our previously reported dimeric Pd(II) benzi-
midazolin-2-ylidene complex [PdBr2(iPr2-bimy)]2, which
could be easily cleaved by CH3CN to form [PdBr2(CH3-
CN)(iPr2-bimy)].10 Apparently, this Pd(II) dimer with iodido
bridges is more stable than bromido analogues, possibly due
to stronger soft-soft attractive interactions. In complex 9,
each palladium is surrounded by one newly formed C-imino
and one terminal iodido and two bridging iodido ligands in a
More strikingly, their IR spectra show strong bands at
1627 and 1617 cm-1 indicative for CdN double bonds, while

1228 Organometallics, Vol. 30, No. 5, 2011
Han et al.
afforded the dimeric complex 9 along with some unidentified
byproduct. This may be due to the stronger þI effect and
more pronounced steric bulk of the cyclohexyl substituent
compared to the 2,6-dimethylphenyl substituent. It is antici-
pated that the formation of 10 occurs through insertion of
the aromatic isocyanide, giving rise to dimeric complex 8,
1
which is cleaved in situ by a second isocyanide. In its H
NMR spectrum, all resonances arising from the pyrazole
moiety are shifted slightly downfield compared to those in
1a. The resonances for all methyl groups of the two xylyl
groups in complex 10 coincidentally overlapped, giving rise
to one singlet at 2.19 ppm that integrates to 12 protons. The
13C NMR signal for the C-imino donor was detected at 151.3
ppm. Furthermore, the IR spectrum of complex 10 shows
two strong absorption bands at 2170 and 1623 cm-1 char-
acteristic of the σ-coordinated isocyanide and the C-imino
ligand, respectively. The formation of 10 is also supported by
ESI mass spectrometry, which shows an isotopic pattern
centered at m/z 633 for the [M - I]þ complex fragment.
Due to the aforementioned reasons, in situ bridge cleavage
reactions of dimeric 9 did not take place, which again high-
lights that changes in ligand donor ability can lead to
significantly different reactivities of the resulting complexes.
Figure 3. Molecular structure of 9 4CH3CN showing 50%
3
probability ellipsoids. Hydrogen atoms and solvent molecules
are omitted for clarity. Selected bond lengths (A) and angles
(deg): Pd1-C1 1.980(4), Pd1-I1 2.6271(4), Pd1-I2 2.6290(4),
Pd1-I2A 2.7765(4), C1-C3 1.489(5), C1-N3 1.261(5), C10-
N3 1.466(5), N1-N2 1.361(4), N1-C4 1.350(4), N2-C2
1.346(4), C2-C3 1.407(5), C3-C4 1.401(5); C1-Pd1-I1
90.42(10), C1-Pd1-I2 86.91(10), I1-Pd1-I2A 95.230(14),
I2-Pd1-I2A 87.937(13), C3-C1-N3 118.3(3), C1-N3-C10
120.9(3), C2-C3-C4 106.2(3).
Conclusion
In an attempt to broaden the structural and chemical scope
of rNHC complexes, we have successfully prepared the
iodido-bridged Pd(II) dimers [PdI2(rNHC)]2 (1a,b) with
terminal monodentate pyrazolin-4-ylidene ligands via direct
oxidative addition of 4-iodopyrazolium salts to Pd(0) in the
absence of additional supporting ligands. Further treatment
of complex 1a with CN-Xyl and CN-Cy led to the formation
of Pd(II) dimers 8 and 9, bearing novel betainic C-imino
ligands through isocyanide insertions. In contrast, analo-
gous reactions of the Pd(II) dimers [PdI2(Me2-bimy)]2 (3)
and [PdI2(Me2-indy)]2 (2), containing classical NHCs
(benzimidazolin-2-ylidenes) and NHCs with reduced hetero-
atom stabilization (indazolin-3-ylidenes), respectively, afforded
mixed NHC/isocyanide Pd(II) complexes 4-7 via simple
bridge-cleavage reactions. The remarkably different reactivity
of 1a is attributed to the superior donor ability of pyrazolin-4-
ylidenes with no R-nitrogen atoms compared to that of inda-
zolin-3-ylidenes and benzimdazolin-2-ylidenes with one and
two electron-withdrawing R-nitrogen atoms, respectively.
Efforts to reveal further differences or similarities between
nonclassical carbenes and their well-known conventional
NHC counterparts are underway.
nearly perfect square-planar fashion. The C3-C1-N3 imi-
no plane forms a torsion angle of 75.20° relative to the
[PdCI3] coordination plane. The Pd-Cimino bond length of
1.980(4) A is essentially identical with the Pd-Ccarbene bond
(1.980(6) A) in 1a. The Pd-I bond trans to the C-imino
ligand, amounting to 2.7765(4) A, is significantly longer than
in the parent rNHC complex 1a (2.6774(17) A), indicating a
stronger trans influence of the C-imino ligand. All bond
parameters of the five-membered pyrazole moiety indicate
aromatic character and are essentially the same as observed
for pyrazolin-4-ylidenes. The C1-C3 bond that has newly
formed through isocyanide insertion into the rNHC ligand
amounts to 1.489(5) A, which is in the range of a typical
Csp2-Csp2 single bond. On the other hand, the C1-N3
distance of 1.261(5) A has substantial double-bond charac-
ter, which is also in line with the observed sp-to-sp2 rehy-
bridization of this CN fragment. Overall, the newly formed
C-imino ligand can be considered as a betaine, in which the
anionic C-donor atom is charge-separated from the cationic
pyrazolium moiety.
Experimental Section
As noted above, the iodido bridges of dimer 9 are very
strong and remain intact in the presence of weak donors such
as CH3CN. This low tendency to undergo bridge-cleavage
reactions may be explained by the increased electron density
around the palladium center as a result of coordination of the
very strongly donating C-imino ligand in 9. In comparison,
the C-imino ligand in 8, containing a less electron donating
xylyl group, should be a slightly weaker σ donor, which in
turn may lead to a different reactivity. To test this concept,
the bridge-cleavage reaction of 1a was repeated, except with
4 equiv of both isocyanides (Scheme 4).
General Considerations. Unless otherwise noted, all opera-
tions were performed without taking precautions to exclude air
or moisture, and all solvents and chemicals were used as received
without any further treatment. If required, CH2Cl2 was dried
over calcium hydride and distilled under nitrogen prior to use.
Tris(dibenzylideneacetone)dipalladium(0) was received from
Alfa Aesar. 1-Ethyl-4-iodo-2,3,5-trimethylpyrazolium iodide
(A),5a 2-ethyl-4-iodo-3,5-dimethyl-1-phenylpyrazolium iodide
(B),5a 1,3-dimethylbenzimidazolium iodide (C),19 and the di-
meric indazolin-3-ylidene complex 27 were prepared according
to reported procedures. 1H and 13C NMR spectra were recorded
Interestingly, these isocyanides indeed reacted differently.
In the reaction with 2,6-dimethylphenyl isocyanide, the
monomeric complex 10 was isolated in a high yield of
70%, while the reaction with cyclohexyl isocyanide primarily
(19) (a) Fischer, O. Chem. Ber. 1901, 34, 930. (b) Begtrup, M.; Larsen,
P. Acta Chem. Scand. 1990, 44, 1050.

Article
Organometallics, Vol. 30, No. 5, 2011 1229
on a Bruker ACF 300 or AMX 500 spectrometer, and the
chemical shifts (δ) were internally referenced to the residual
protio solvent signals relative to tetramethylsilane. Mass spectra
were measured using a Finnigan MAT LCQ (ESI) spectrometer.
Infrared spectra were recorded with a Varian 3100 FT-IR
spectrometer. Elemental analyses were performed on a Perkin-
Elmer PE 2400 elemental analyzer at the Department of Chem-
istry, National University of Singapore.
to 4 from 3 (101 mg, 0.10 mmol) and cyclohexyl isocyanide (25
μL, 0.20 mmol), yielding the product as an orange solid (116 mg,
0.19 mmol, 94%). 1H NMR (300 MHz, CDCl3): δ 7.35 (dd, 2 H,
Ar H), 7.27 (dd, 2 H, Ar H), 4.01 (s, 6 H, NCH3), 3.97 (m, 1 H,
CtNCH), 1.95-1.74 (br m, 6 H, CH2), 1.52-1.36 (br m, 4 H,
CH2). 13C{1H} NMR (75.47 MHz, CDCl3): 171.3 (Ccarbene),
135.6 (Ar C), 128.0 (br t, CtN), 123.6, 110.6 (Ar C), 55.0
(CtNC), 36.0 (NCH3), 32.3, 25.4, 22.9 (CH2). IR (KBr pellet):
2216 cm-1 (s, CtN). Anal. Calcd for C16H21I2N3Pd: C, 31.22;
H, 3.44; N, 6.83. Found: C, 31.29; H, 3.38; N, 6.77. MS (ESI):
m/z 597 [M - I þ C7H11N]þ.
Bis(μ-iodo)bis(1-ethyl-2,3,5-trimethylpyrazolin-4-ylidene)diio-
dodipalladium(II) (1a). Tris(dibenzylideneacetone)dipalladium-
(0) (92 mg, 0.1 mmol) and A (78 mg, 0.2 mmol) were dissolved in
dry CH2Cl2 (30 mL) and heated under reflux for 4 h under an
inert nitrogen atmosphere. After it was cooled to ambient
temperature, the resulting mixture was filtered through Celite,
and the filtrate was extracted with H2O (4 ꢀ 30 mL). The CH2Cl2
layer was dried over MgSO4, and the solvent was removed under
reduced pressure. The residue was washed with pentane (3 ꢀ
30 mL) and dried in vacuo to give the crude product as a red-
brown powder. Diffusion of pentane into a concentrated CHCl3
solution afforded the pure product as a red-brown solid. Yield:
75 mg, 0.075 mmol, 75%. 1H NMR (500 MHz, CD2Cl2): δ 4.11
(m, 3J(H,H) = 7.2 Hz, 4 H, CH2), 3.70 (s, 6 H, NCH3), 2.59 (s,
6 H, CH3), 2.58 (s, 6 H, CH3), 1.30 (t, 3J(H,H) = 7.2 Hz, 6 H,
CH2CH3). 13C{1H} NMR (125.76 MHz, CD2Cl2): 147.7, 146.9
(CCH3), 42.2 (CH2), 33.6 (NCH3), 16.3, 16.0, 14.7 (CH3),
carbene signal not detected. Anal. Calcd for C16H28I4N4Pd2:
C, 19.28; H, 2.83; N, 5.62. Found: C, 19.31; H, 2.85; N, 5.53. MS
(ESI): m/z 871 [M - I]þ.
Diiodo(1,2-dimethylindazolin-3-ylidene)(2,6-dimethylphenyl
isocyanide)palladium(II) (6). 2,6-Dimethylphenyl isocyanide (14 mg,
0.108 mmol) was added to a suspension of indazolin-3-ylidene Pd(II)
dimer 2 (55 mg, 0.054 mmol) in dichloromethane (5 mL). The
resulting yellow solution was stirred at ambient temperature over-
night. All the volatiles were removed in vacuo, and the resulting solid
was washed with diethyl ether (3 ꢀ 10 mL) to give the product as a
yellow powder (57 mg, 0.089 mmol, 83%) upon drying. 1H NMR
(500 MHz, CDCl3): δ 8.22 (d, 1 H, Ar H), 7.59 (t, 1 H, Ar H),
7.29-7.19(m,3H,ArH),7.11(ps.d,2H,ArH),4.35(s,3H,NMe),
3.94 (s, 3 H, NMe), 2.57 (s, 6 H, Me). 13C{1H} NMR (125.76 MHz,
CDCl3): 164.9 (Ccarbene), 141.0, 137.1, 132.5, 131.0, 130.4, 130.0,
128.7, 126.7, 123.0, 109.3 (Ar C), 41.4, 34.1 (NCH3), 19.8 (Me),
CtN signal could not be detected. IR (KBr pellet): 2185 (s, CtN)
cm-1. Anal. Calcd for C18H19I2N3Pd: C, 33.91; H, 3.00; N, 6.59.
Found: C, 33.85; H, 2.71; N, 6.52. MS (ESI): m/z 642 [M - I þ
C9H9N]þ, 1017 [2 M - I - C9H9N]þ, 1148 [2 M - I]þ.
Diiodo(cyclohexyl isocyanide)(1,2-dimethylindazolin-3-ylidene)-
palladium(II) (7). Complex 7 was synthesized analogously to 6 by
reacting cyclohexyl isocyanide (7.5 μL, 0.060 mmol) with indazo-
lin-3-ylidene Pd(II) dimer 2 (30 mg, 0.030 mmol) in CH2Cl2 (5 mL)
to afford a yellow solid (24 mg, 0.039 mmol, 65%). 1H NMR (500
MHz, CDCl3): δ 8.18 (d, 1 H, Ar H), 7.57 (t, 1 H, Ar H), 7.26 (t,
1 H, Ar H), 7.18 (d, 1 H, Ar H), 4.32 (s, 3 H, NMe), 3.98 (br m, 1 H,
NCH), 3.94 (s, 3 H, NMe), 1.89 (br s, 6 H, CH2), 1.47 (br s, 4 H,
CH2). 13C{1H} NMR (125.76 MHz, CDCl3): 165.1 (Ccarbene),
141.0, 132.5, 131.1, 130.1, 123.0, 109.2 (Ar C), 55.0 (CH), 41.3,
34.0 (NCH3), 32.6, 25.6, 23.1 (CH2), CtN signal could not be
detected. IR (KBr pellet): 2209 (s, CtN) cm-1. Anal. Calcd for
C16H21I2N3Pd: C, 31.22; H, 3.44; N, 6.83. Found: C, 31.65; H, 3.40;
N, 6.78. MS (ESI): m/z 598 [M - I þ C7H11N]þ.
Bis(μ-iodo)bis(2-ethyl-3,5-dimethyl-1-phenylpyrazolin-4-ylidene)-
diiododipalladium(II) (1b). 1b was prepared analogously to 1a from
tris(dibenzylideneacetone)dipalladium(0) (92 mg, 0.1 mmol) and B
(91 mg, 0.2 mmol). Yield: 97 mg, 0.086 mmol, 86%. 1H NMR (500
MHz, CDCl3): δ 7.64-6.62 (m, 6 H, Ar H), 7.31-7.30 (d, 4 H, Ar
H), 3.91 (m, 3J(H,H) = 7.2 Hz, 4 H, CH2), 2.71 (s, 6 H, CH3), 2.37
(s, 6 H, CH3), 1.11 (t, 3J(H,H) = 7.2 Hz, 6 H, CH2CH3). 13C{1H}
NMR (125.76 MHz, CDCl3): 150.1, 149.6 (CCH3), 133.7, 132.4,
131.2, 129.1 (Ar-C), 112.7 (Ccarbene), 43.1 (CH2), 17.2, 17.0, 15.5
(CH3). Anal. Calcd for C26H32I4N4Pd2: C, 27.86; H, 2.88; N, 5.00.
Found: C, 27.32; H, 2.80; N, 4.54%. MS (ESI): m/z 995 [M - I]þ.
Bis(μ-iodo)bis(1,3-dimethylbenzimidazolin-2-ylidene)diiododi-
palladium(II) (3). A mixture of 1,3-dimethylbenzimidazolium
iodide (C; 165 mg, 0.5 mmol), Pd(OAc)2 (113 mg, 0.5 mmol),
and NaI (300 mg, 2 mmol) in DMSO (10 mL) was stirred at
90 °C for 24 h. The reaction mixture was filtered over Celite, and
the solvent of the filtrate was removed by vacuum distillation.
The resulting residue was washed with H2O (4 ꢀ 20 mL) and
dried in vacuo to afford the product as a red-brown solid (177
mg, 0.18 mmol, 70%). 1H NMR (300 MHz, CDCl3): δ 7.67 (dd,
4 H, Ar H), 7.35 (dd, 4 H, Ar H), 4.02 (s, 12 H, NCH3). 13C{1H}
NMR (75.47 MHz, CDCl3): 157.0 (Ccarbene), 134.3, 123.1, 110.5
(Ar C), 35.4 (NCH3). Anal. Calcd for C18H20I4N4Pd2: C, 21.35;
H, 1.99; N, 5.53. Found: C, 21.45; H, 2.04; N, 5.16. MS (ESI):
m/z 1035 [M þ Na]þ.
Bis(μ-iodo)bis(2,6-dimethylphenyl-C-imino)diiododipalladium(II)
(8). 2,6-Dimethylphenyl isocyanide (20 mg, 0.15 mmol) was added
to a suspension of 1a (75 mg, 0.075 mmol) in dichloromethane (8 mL).
The reaction mixture was stirred at ambient temperature for 24 h.
The orange precipitate was collected by filtration, washed with
small portions of dichloromethane, and then dried in vacuo to give
the product as an orange powder. The dichloromethane filtrate was
concentrated to 2 mL, which upon stirring at ambient temperature
afforded a second crop of the product. Overall yield: 78 mg, 0.062
mmol, 83%. 1HNMR(500 MHz, d6-DMSO): δ6.92-6.83 (m, 3 H,
Ar H), 4.50 (m, 3J(H,H) = 7.0 Hz, 2 H, CH2), 4.00 (s, 3 H, CH3),
2.94 (s, 3 H, NCCH3), 2.92 (s, 3 H, NCCH3), 2.35 (s, 6 H, CH3),
1.33 (t, 3J(H,H) = 7.0 Hz, 3 H, CH2CH3). 13C{1H} NMR (125.76
MHz, d6-DMSO): 148.7 (CdN-Xy), 143.0, 142.0 (NCCH3),
128.1, 127.5, 126.5, 122.1 (Ar C), 41.4 (CH2), 33.4 (NCH3), 21.8,
18.0, 13.8, 13.5, 13.1 (CH3), C4-pyr not detected. IR (KBr pellet):
1627 (s, CdN-Xy) cm-1. Anal. Calcd for C34H46I4N6Pd2: C,
32.43; H, 3.68; N, 6.67. Found: C, 32.22; H, 3.77; N, 6.61. MS (ESI):
m/z 1261 [M þ H]þ, 1133 [M - I]þ, 580 [1/2 M - I þ DMSO]þ.
Bis(μ-iodo)bis(cyclohexyl-C-imino)diiododipalladium(II) (9).
Cyclohexyl isocyanide (19 μL, 0.15 mmol) was added to a
suspension of 1a (75 mg, 0.075 mmol) in dichloromethane
(8 mL). The reaction mixture was stirred at ambient temperature
for 24 h. The orange precipitate was collected by filtration,
washed with small portions of dichloromethane, and then dried
in vacuo to give the product as an orange powder (66 mg, 0.054
Diiodo(1,3-dimethylbenzimidazolin-2-ylidene)(2,6-dimethyl-
phenyl isocyanide)palladium(II) (4). 2,6-Dimethylphenyl isocyanide
(26 mg, 0.20 mmol) was added to a suspension of 3 (101 mg, 0.10
mmol) in dichloromethane (12 mL). The reaction mixture was
stirred at ambient temperature for 24 h. The volatiles were removed
under reduced pressure. The residue was washed with hexane (3 ꢀ
15 mL) and dried in vacuo to afford the product as an orange solid
(102 mg, 0.16 mmol, 80%). 1H NMR (300 MHz, CDCl3): δ 7.38
(dd, 2 H, Ar H), 7.31 (dd, 2 H, Ar H), 7.22 (ps. dd, 1 H, Ar H), 7.12
(ps. d, 2 H, Ar H), 4.07 (s, 6 H, NCH3), 2.56 (s, 6 H, Ar CH3).
13C{1H} NMR (75.47 MHz, CDCl3): 171.3 (Ccarbene), 137.1, 135.8,
130.6, 128.7, 126.4, 123.8, 110.7 (Ar C), 36.1 (NCH3), 19.7 (Ar CH3).
The CtN signal could not be detected. IR (KBr pellet): 2189 cm-1
(s, CtN). Anal. Calcd for C18H19I2N3Pd: C, 33.91; H, 3.00; N, 6.59.
Found: C, 33.88; H, 2.85; N, 6.58. MS (ESI): m/z 656 [M þ NH4]þ.
Diiodo(cyclohexyl isocyanide)(1,3-dimethylbenzimidazolin-2-
ylidene)palladium(II) (5). Complex 5 was prepared analogously
1
mmol, 72%). H NMR (300 MHz, CD3CN): δ 4.37 (m, 1 H,
CH), 4.29 (m, 3J(H,H) = 7.2 Hz, 2 H, CH2CH3), 3.80 (s, 3 H,

1230 Organometallics, Vol. 30, No. 5, 2011
Table 1. Selected X-ray Crystallographic Data for Complexes 1a 4CH2Cl2, 6, and 9 4CH3CN
Han et al.
3
3
1a 4CH2Cl2
3
6
9 4CH3CN
3
formula
fw
C16H28I4N4Pd2 C4H8Cl8
3
1336.53
C18H19I2N3Pd
637.56
C30H50I4N6Pd2 C8H12N4
3
1379.38
color, habit
cryst size (mm)
temp (K)
cryst syst
space group
a (A)
orange, block
0.70 ꢀ 0.14 ꢀ 0.10
223(2)
triclinic
P1
9.306(8)
10.916(8)
10.961(9)
yellow, block
0.38 ꢀ 0.18 ꢀ 0.10
orange, block
0.54 ꢀ 0.16 ꢀ 0.08
100(2)
triclinic
P1
8.7000(11)
10.0247(13)
15.825(2)
223(2)
monoclinic
P21/n
17.9130(12)
7.7294(5)
15.4981(10)
b (A)
c (A)
R (deg)
β (deg)
γ (deg)
V (A3)
66.564(17)
75.026(16)
80.789(16)
984.9(13)
90
107.899(2)
90
2042.0(2)
81.020(3)
85.403(3)
66.200(20)
1247.1(3)
Z
1
2.253
Mo KR
4.609
4
2.074
Mo KR
3.937
1
1.837
Mo KR
3.232
Dc (g cm-3
radiation used
)
μ (mm-1
)
θ range (deg)
2.04-27.50
4446
0.6557, 0.1407
R1 = 0.0467, wR2 = 0.1265
R1 = 0.0610, wR2 = 0.1362
1.038
2.39-27.50
4676
0.6942, 0.3162
R1 = 0.0437, wR2 = 0.0986
R1 = 0.0560, wR2 = 0.1037
1.058
2.24-27.49
5698
0.7820, 0.2742
R1 = 0.0320, wR2 = 0.0805
R1 = 0.0368, wR2 = 0.0829
1.015
no. of unique data
max, min transmn
final R indices (I > 2σ(I))
R indices (all data)
goodness of fit on F2
peak/hole (e A-3
)
1.335/-0.951
2.113/-1.051
1.704/-0.568
NCH3), 2.94 (s, 3 H, NCCH3), 2.93 (s, 3 H, NCCH3), 2.05-2.02
(m, 2 H, CH2), 1.75-1.64 (m, 3 H, CH2), 1.45-1.39 (m, 4 H,
CH2), 1.34-1.29 (m, 4 H, CH2 and CH2CH3). 13C{1H} NMR
(125.76 MHz, CD3CN): 145.0, 143.9 (NCCH3), 125.5
(CCdN-Cy), 69.8 (CH), 42.7 (CH2CH3), 34.2 (NCH3), 33.2,
27.0, 25.3 (CH2), 14.7, 14.4, 14.3 (CH3), CdN-Cy not detected.
IR (KBr pellet): 1617 (s, CdN-Cy) cm-1. Anal. Calcd for
C30H50I4N6Pd2: C, 29.65; H, 4.15; N, 6.92. Found: C, 29.07; H,
4.21; N, 6.65. MS (ESI): m/z 649 [1/2 M þ CH3CN þ H]þ.
Diiodo(2,6-dimethylphenyl-C-imino)(2,6-dimethylphenyl isocya-
nide)palladium(II) (10). 2,6-Dimethylphenyl isocyanide (21 mg,
0.16 mmol) was added to a suspension of 1a (40 mg, 0.04 mmol)
in dichloromethane (5 mL). The reaction mixture was stirred at
ambient temperature for 24 h. The orange precipitate was
filtered off, and the solvent of the filtrate was removed under
reduced pressure to afford a yellow solid. Slow evaporation of a
concentrated dichloromethane/toluene solution afforded the
analytically pure product as yellow crystals (43 mg, 0.056 mmol,
70%). 1H NMR (500 MHz, CD2Cl2): δ 7.21 (t, 3J(H,H) = 7.6
Hz, 1 H, Ar H), 7.07 (d, 3J(H,H) = 7.6 Hz, 2 H, Ar H), 6.86-
6.82 (m, 3 H, Ar H), 4.38 (m, 3J(H,H) = 7.6 Hz, 2 H, CH2), 3.96
(s, 3 H, NCH3), 2.92 (s, 3 H, NCCH3), 2.91 (s, 3 H, NCCH3),
2.19 (s, 12 H, CH3), 1.49 (t, 3J(H,H) = 7.6 Hz, 3 H, CH2CH3).
13C{1H} NMR (125.76 MHz, CD2Cl2): 151.3 (CdN-Xy),
143.8, 143.0 (NCCH3), 135.0, 129.1, 128.3, 127.87, 127.83,
126.7, 123.3 (Ar C), 42.3 (CH2), 33.7 (NCH3), 19.8, 18.7,
14.4, 12.6, 12.4 (CH3), CtN-Xy and C4-pyr not detected. IR
(KBr pellet): 2170 (s, CtN-Xy) cm-1, 1623 (s, CdN-Xy)
cm-1. Anal. Calcd for C26H32I2N4Pd: C, 41.05; H, 4.24; N,
7.36. Found: C, 41.23; H, 4.22; N, 7.21. MS (ESI): m/z 633
[M - I]þ.
X-ray Diffraction Studies. X-ray data were collected with a
Bruker AXS SMART APEX diffractometer, using Mo KR
radiation at 100(2) K (4 and 9 4CH3CN) or 223(2) K
(1a 4CH2Cl2, 5, and 6), with the SMART suite of programs.20
3
3
Data were processed and corrected for Lorentz and polarization
effects with SAINT21 and for absorption effect with SADABS.22
Structure solution and refinement were carried out with the
SHELXTL suite of programs.23 The structure was solved by
direct methods to locate the heavy atoms, followed by difference
maps for the light, non-hydrogen atoms. All hydrogen atoms
were placed at calculated positions. All non-hydrogen atoms
were generally given anisotropic displacement parameters in the
final model. A summary of the most important crystallographic
data is given in Table 1.
Acknowledgment. We thank the National University
of Singapore for financial support (Grant No. R 143-000-
410-112) and the CMMAC staff of our department for
technical assistance.
Supporting Information Available: Text, figures, and a table
giving a detailed discussion, selected crystallographic data, and
molecular structures of 4 and 5 and CIF files giving crystal-
lographic data for 1a 4CH2Cl2, 4-6, and 9 4CH3CN. This
3
3
material is available free of charge via the Internet at http://
pubs.acs.org.
(21) SAINTþ version 6.22a; Bruker AXS Inc., Madison, WI, 2001.
(22) Sheldrick, G. W. SADABS version 2.10; University of
(20) SMART version 5.628; Bruker AXS Inc., Madison, WI, 2001.
(23) SHELXTL version 6.14; Bruker AXS Inc., Madison, WI, 2000.
€
€
Gottingen, Gottingen, Germany, 2001.