NHC ligands with a secondary pyrimidyl donor for electron-rich palladium(0) complexes

Article abstract of DOI:10.1021/om100670u

Several (N-2-pyrimidyl-NHC)-palladium(0) complexes have been synthesized in very high yields and purities by transmetalation from the related silver(I) complexes with Pd⁰(tBuDAB)(ma). The coordination behavior of the heteroditopic ligands was analyzed, and the structure of their complexes was confirmed by NMR and X-ray diffraction studies. The complexes are active catalysts for the transfer hydrogenation of alkynes to Z-alkenes, with activity and selectivity depending on the pyrimidine substituents and the NHC nitrogen substituent. Selectivities toward the Z-alkene as high as 97% were observed.

Full text of DOI:10.1021/om100670u

Organometallics 2010, 29, 4555–4561 4555
DOI: 10.1021/om100670u
NHC Ligands with a Secondary Pyrimidyl Donor
for Electron-Rich Palladium(0) Complexes
Stefan Warsink,† I-Hsin Chang,‡ Jan J. Weigand,§ Peter Hauwert,† Jwu-Ting Chen,‡ and
Cornelis J. Elsevier*,†
†Molecular Inorganic Chemistry, van’t Hoff Institute for Molecular Sciences, University of Amsterdam,
Science Park 904, 1090 GD, Amsterdam, The Netherlands., ‡Department of Chemistry, National Taiwan
University, Taipei 106, Taiwan, Republic of China, and Institut fu€r Anorganische und Analytische Chemie,
§
€ €
€
Westfalische Universitat Munster, Germany
Received July 9, 2010
Several (N-2-pyrimidyl-NHC)-palladium(0) complexes have been synthesized in very high yields
and purities by transmetalation from the related silver(I) complexes with Pd0(tBuDAB)(ma). The
coordination behavior of the heteroditopic ligands was analyzed, and the structure of their complexes
was confirmed by NMR and X-ray diffraction studies. The complexes are active catalysts for the
transfer hydrogenation of alkynes to Z-alkenes, with activity and selectivity depending on the
pyrimidine substituents and the NHC nitrogen substituent. Selectivities toward the Z-alkene as high
as 97% were observed.
Introduction
the development and synthesis of heterobidentate NHC
ligands containing a secondary nitrogen donor.6-8 With
the combination of a strong donor and a more weakly
coordinating donor we intend to gain access to a class of
catalysts that benefits from the hemilabile behavior of one of
its ligands.9 Additionally, the basicity of the secondary
nitrogen donor is easily varied to obtain the desired func-
tionality. We endeavored the synthesis of electron-rich pal-
ladium complexes bearing an NHC ligand functionalized
with a pyrimidyl donor because of the ease with which they
are introduced and the fact that its basicity is lower than that
of the more often employed pyridyl moiety. These ligands
have been prepared before,10,11 but a thorough investigation
of the influence of the substituents on both the NHC and the
nitrogen donor has not been performed. The pyrimidyl
functionality in combination with the NHC scaffold gives
rise to a five-membered chelate ring when N,C-coordinated
to a metal. Because all nonmetal atoms in the ring are
sp2-hybridized, a considerable amount of strain is expected,
likely enhancing the hemilability of the system.
N-Heterocyclic carbenes (NHCs) have been used fre-
quently in organometallic chemistry and catalysis for almost
two decades,1,2 and the field shows no sign of saturation.3
The high stability these ligands impart on their transition
metal (TM) complexes and the catalytic activity of those
complexes justify the amount of attention to this class of
ligands. The strong donating character of NHCs combined
with their ability to act as acceptors enables them to form
complexes with a large number of TMs in various oxidation
states, as well as with many main group elements.4 The bulk
of the research performed with monodentate carbenes
focuses on catalysis, but also homo- and heteropolyden-
tate ligands containing NHCs are reported frequently.5
Currently, one of the focal points of our research concerns
*To whom correspondence should be addressed. E-mail: c.j.elsevier@
uva.nl.
€
(1) Herrmann, W. A.; Kocher, C. Angew. Chem., Int. Ed. Engl. 1997,
€
36, 2162–2187. Weskamp, T.; Bohm, V. P. W.; Herrmann, W. A. J.
Organomet. Chem. 2000, 600, 12–22. Bourissou, D.; Guerret, O.; Gabbai,
F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39–91. Herrmann, W. A. Angew.
Chem., Int. Ed. 2002, 41, 1290–1309. Crabtree, R. H. J. Organomet. Chem.
2005, 690, 5451–5457.
(8) Warsink, S.; Drost, R. M.; Lutz, M.; Spek, A. L.; Elsevier, C. J.
Organometallics 2010, 29, 3109–3116.
(9) Braunstein, P.; Naud, F. Angew. Chem., Int. Ed. 2001, 40, 680–
699.
(2) Arduengo, A. J., III; Harlow, R. H.; Kline, M. J. Am. Chem. Soc.
1991, 113, 361–363.
(10) Lee, K.-M.; Chen, J. C. C.; Lin, I. J. B. J. Organomet. Chem.
2001, 617-618, 364–375. Chen, D.; Banphavichit, V.; Reibenspies, J.;
Burgess, K. Organometallics 2007, 26, 855–859. Lee, K.-M.; Chen,
J. C. C.; Huang, C.-J.; Lin, I. J. B. CrystEngComm 2007, 9, 278–281. Ye,
J.; Chen, W.; Wang, D. Dalton Trans. 2008, 4015–4022. Chen, C.; Qiu, H.;
Chen, W.; Wang, D. J. Organomet. Chem. 2008, 693, 3273–3280.
Gnanamgari, D.; Sauer, E. L. O.; Schley, N. D.; Butler, C.; Incarvito,
C. D.; Crabtree, R. H. Organometallics 2009, 28, 321–325. Zhang, X.;
Liu, B.; Liu, A.; Xie, W.; Chen, W. Organometallics 2009, 28, 1336–1349.
Zhang, X.; Xia, Q.; Chen, W. Dalton Trans. 2009, 7045–7054. Xi, Z.; Liu,
B.; Lu, C.; Chen, W. Dalton Trans. 2009, 7008–7014. Lu, C.; Gu, S.; Chen,
W.; Qiu, H. Dalton Trans. 2010, 39, 4198–4204.
(3) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem.,
Int. Ed. 2007, 46, 2768–2813. Hahn, F. E.; Jahnke, M. C. Angew. Chem.,
Int. Ed. 2008, 47, 3122–3172.
(4) Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L.
Coord. Chem. Rev. 2009, 253, 687–703.
(5) Lee, H. M.; Lee, C.-C.; Cheng, P.-Y. Curr. Org. Chem. 2007, 11,
1491–1524. Normand, A. T.; Cavell, K. J. Eur. J. Inorg. Chem. 2008, 2008,
2781–2800. Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239.
(6) Warsink, S.; Hauwert, P.; Siegler, M. A.; Spek, A. L.; Elsevier,
C. J. Appl. Organomet. Chem. 2009, 23, 225–228.
(7) Warsink, S.; Boer, S. Y. d.; Jongens, L. M.; Fu, C.-F.; Liu, S.-T.;
Chen, J.-T.; Lutz, M.; Spek, A. L.; Elsevier, C. J. Dalton Trans. 2009,
7080–7086.
(11) Meyer, D.; Taige, M. A.; Zeller, A.; Hohlfeld, K.; Ahrens, S.;
Strassner, T. Organometallics 2009, 28, 2142–2149.
r
2010 American Chemical Society
Published on Web 10/01/2010
pubs.acs.org/Organometallics

4556 Organometallics, Vol. 29, No. 20, 2010
Scheme 1. Synthesis of N-Arylimidazoles
Warsink et al.
Scheme 2. Synthesis of Pyrimidyl-Imidazolium Salts
By combining the NHC and the pyrimidine in one ligand
with various substituents, we expect to gain insight into the
effects of the substituents on the coordination chemistry and,
ultimately, the performance of its zerovalent palladium
complexes in catalysis. We present here the synthesis and
structural characterization of the N-pyrimidyl-functiona-
lized imidazolium salts and the NHC complexes derived
thereof with silver(I) and palladium(0). The palladium-
(NHC) complexes are then applied to the transfer hydro-
genation of alkynes.
Scheme 3. Carbene Generation with Silver(I) Oxide
Results and Discussion
Ligand Synthesis. The N-arylimidazoles 1-4 were synthe-
sized according to a literature procedure in yields higher than
or equal to those reported (Scheme 1).12
Quaternization to the imidazolium salts 5-7 was per-
formed in two ways. In the case of mesityl (2,4,6-trimethyl-
phenyl, Mes) imidazole 1 and 2,6-diisopropylphenyl (DiPP)
imidazole 2, the pyrimidyl functionality was introduced by
heating the imidazole with 2-chloropyrimidine (PymCl, a) or
2-chloro-4,6-dimethylpyrimidine (PymMe2Cl, b) under sol-
vent-free conditions to obtain 5 and 6.11 For N-(2-pyrimi-
dyl)imidazole 3 and N-(2-(4,6-dimethyl)pyrimidyl)imidazole
4, quaternization with isopropyl iodide gave 7 in high yields
(Scheme 2).
complexes with a reaction time of 16 h. The pure products
were obtained after filtration and concentration of the reac-
tion mixture. Successful conversion is observed in the
1H NMR, where the disappearance of the signal of the acidic
imidazolium proton signifies carbene formation. However,
positive proof is seen only in 13C NMR and MS experiments.
For the complexes 8-10, the signal for the carbene carbon is
observed around 184 ppm in the 13C NMR, which is a
common value for Ag(NHC) complexes. In solution, dy-
namic exchange processes can take place; this is the reason
that instead of doublets due to 107,109Ag coupling, a singlet is
the only observed signal for the carbene carbon.15 In contrast,
this coupling is observed in more rigid polydentate systems.16 In
mass spectrometry experiments a biscarbene silver cation was
observed, which is an often seen architecture. However, the
results from MS experiments cannot be directly correlated with
a solution structure because of the dynamic nature of Ag(NHC)
silver complexes in the solution phase.
The progress of the reaction was monitored by 1H NMR,
where the imidazolium signal is observed between 10.6 and
10.4 ppm. Yields for the salts 5 and 6 are reasonably good
and comparable to those reported in the literature,11 whereas
those for 7 are excellent.
Silver(NHC) Complexes. To gain access to the NHCs,
carbene generation with basic silver(I) oxide was chosen.13
This reliable method works for most ligand precursors and
generally leads to high yields and stable products.6,7,14 With
the NHC pyrimidyl scaffold this is not different; high yields
are routinely obtained (Scheme 3). It has to be noted though
that the solubility of complexes 10 in dichloromethane is
lower than that of complexes 8 and 9, and a precipitate was
seen in the reaction mixture that disappeared upon dilution.
This observation might be best explained by the formation of
charged biscarbene silver complexes that are favored for
ligands with small substituents. With the larger aryl substit-
uents of 8 and 9, the monocarbene silver(I) complex is
favored. This neutral species would have a better solubility
in relatively apolar solvents.
Palladium(NHC) Complexes. The silver complexes were
then subjected to transmetalation with a zerovalent palla-
dium precursor17 to obtain the zerovalent NHC palladium
complexes 11-13.6 Yields were consistently high (91-99%),
as we have observed before, and excellent purities were
The reactions could be monitored visually by the uptake of
silver(I) oxide, but optimum results were obtained for all
ꢀ
(15) de Fremont, P.; Scott, N. M.; Stevens, E. D.; Ramnial, T.;
(12) Liu, J.; Chen, J.; Zhao, J.; Zhao, Y.; Li, L.; Zhang, H. Synthesis
2003, 2003, 2661–2666.
(13) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972–975.
Lightbody, O. C.; Macdonald, C. L. B.; Clyburne, J. A. C.; Abernethy,
C. D.; Nolan, S. P. Organometallics 2005, 24, 6301–6309.
(16) Hahn, F. E.; Radloff, C.; Pape, T.; Hepp, A. Chem.-Eur. J.
2008, 14, 10900–10904. Rit, A.; Pape, T.; Hahn, F. E. J. Am. Chem. Soc.
2010, 132, 4572–4573.
ꢀ
Hayes, J. M.; Viciano, M.; Peris, E.; Ujaque, G.; Lledos, A. Organometallics
2007, 26, 6170–6183.
(14) Garrison, J. C.; Youngs, W. J. Chem. Rev. 2005, 105, 3978–4008.
Lin, I. J. B.; Vasam, C. S. Coord. Chem. Rev. 2007, 251, 642–670.
(17) Cavell, K. J.; Stufkens, D. J.; Vrieze, K. Inorg. Chim. Acta 1980,
47, 67–76.

Article
Scheme 4. Transmetalation to Zerovalent Palladium
Organometallics, Vol. 29, No. 20, 2010 4557
obtained by precipitation of the products with pentane
(Scheme 4).
The signal for the carbene carbon in 11-13 exhibits a low-
field shift compared to that of the silver(I) complexes and is
observed between 188 and 194 ppm in the 13C NMR.
Because of the dissymmetry of the complexes, all hydrogens
and carbons give unique signals in the NMR. For the
methylpyrimidyl-substituted ligands 11b-13b, the signal
for the methyl ortho to the coordinating nitrogen shows a
downfield shift to around 2.8 ppm in the 1H NMR, as
opposed to the other methyl, which does not show a sig-
nificant difference in chemical shift from the corresponding
silver complex (2.6 ppm). In the nonsubstituted cases
11a-13a the difference in chemical shift for the hydrogens
neighboring the nitrogen is smaller, from around 8.8 ppm in
the silver complexes to 8.9 ppm in the palladium complexes.
However, we do observe that the signals for the two protons are
not fully resolved on the NMR time scale. This is probably due
to dynamic processes at room temperature, involving exchange
of the two nitrogen donor atoms of the N-pyrimidyl-NHC
ligand. This hemilability is also expected to occur in the
methyl-substituted ligands, but the activation barrier is appar-
ently too high to let this process occur at room temperature. As
for the maleic anhydride ligand, we observe that in the less bulky
11a-13a the two signals coalesce to a broad singlet at room
Figure 1. Displacement ellipsoid plot (50% probability level) of
11a. H atoms are omitted for the sake of clarity. Selected bond
ꢀ
˚
lengths (A) andangles(deg):Pd1-C1 2.045(2), Pd1-C192.054(2),
Pd1-C18 2.110(2), Pd1-N3 2.197(2); C1-Pd1-N3 76.99(8),
C1-Pd1-C19 120.38(9), C1-Pd1-C18 160.16(9), C19-Pd1-
C18 39.7(1), C19-Pd1-N3 162.60(9), C18-Pd1-N3 122.74(9),
C5-N3-Pd1 131.1(2), C4-N3-Pd1 113.5(2), C20-C19-Pd1
108.4(2), N1-C1-Pd1 141.6(2), N2-C1-Pd1 115.0(2).
Scheme 5. Transfer Hydrogenation of Alkynes Catalyzed
by Pd(NHC)(ma) Complexes
1
temperature in the H NMR spectra, whereas for the more
bulky 11b-13b the signals are resolved into two doublets with a
coupling of 3.6 Hz. We attribute this to rotation of the maleic
anhydride facilitated by decoordination of the secondary donor.
For 11b and 12b the two alkene signals are very close together,
whereas for 13b they are over 0.5 ppm apart. The origin of this
difference remains obscure. In 13C NMR the same trends are
observed, albeit less pronounced.
X-ray Crystallography. To unequivocally establish the
structure of this type of complex, a crystal structure analysis
of complex 11a was performed. X-ray quality crystals were
obtained by vapor diffusion of diethyl ether into a dichloro-
methane solution of 11a (Figure 1).
˚
C19-Pd1 (2.054(3) A), reflect the difference in trans influ-
ence of the two donors in the bidentate NHC ligand. The
˚
alkene bond length C5-C19 (1.424(3) A) is elongated com-
˚
pared to the free ligand (1.332(3) A), indicating a lower bond
order caused by a significant d-π* back-bonding. The NHC
is a stronger donor than the pyrimidine moiety, and as a
consequence, the alkene is tilted away from the NHC ligand.
The bite angle (C1-Pd1-N3 76.97(8)°) of the NHC ligand is
similar to that observed in an analogous divalent palladium
complex.11 The distances between the atoms C1-Pd1 and
N1-Pd1 (2.045(2) and 2.197(2) A) are longer compared to
the divalent analogue, reflecting a higher electron density of
the zerovalent palladium.
The solid-state structure of 11a exhibits structural simila-
rities that are consistent with those of other NHC-Pd deri-
vatives featuring a maleic anhydride ligand.6 The bidentate
carbene ligand and the double bond of the maleic anhydride
define a plane where the anhydride moiety is coordinated
side-on, with an angle of 108.4° with respect to the coordina-
tion plane. The geometrical parameters of the maleic anhy-
dride molecule with the Pd center resemble features that lie
between a Pd metallacycle and a metal-olefin bond. Two
slightly different bond lengths between the maleic anhydride
˚
Catalytic Transfer Hydrogenation. As a probe for the
activity and selectivity of complexes 11-13, transfer hydro-
genation of 1-phenyl-1-propyne was performed (Scheme 5).
˚
ligand and the Pd center, C18-Pd1 (2.110(2) A) and

4558 Organometallics, Vol. 29, No. 20, 2010
Warsink et al.
Table 1. Catalytic Transfer Hydrogenation of 1-Phenyl-
1-propynea
evidenced by the broadening of the signals of the pyrimidyl
hydrogens. For the methyl-substituted pyrimidine donor,
this behavior was not seen at room temperature. Catalytic
transfer hydrogenation of 1-phenyl-1-propyne to (Z)-1-
phenyl-1-propene has been performed to gauge the influence
of the substituents on activity and selectivity. It appeared
that more bulk on either the secondary nitrogen donor or the
carbene nitrogen was beneficial for selectivity, but impeded
the conversion. The donor strength of the pyrimidyl func-
tionality appeared to be not sufficient to retain high selec-
tivity after full consumption of the starting material. The
results from the catalytic experiments show that the substrate
binds in the position occupied by the N-pyrimidyl donor,
which is in line with the hemilabile behavior observed in
NMR experiments. For the smaller N-isopropyl substituent,
deactivation took place. The large influence of the N-pyr-
imidyl donor on the catalytic results supports our interest in
the effect of different nitrogen donors in this type of complex
on reactivity and stability.
initial TOF
(mol sub/mol cat/h)
selectivity
Z/E/alkane (%)
entry
complex
1
2
3
4
5
6
11a
11b
12a
12b
13a
13b
43.9
29.8
24.0
14.4
35.1
18.7
90/6/4
95/3/2
97/2/1
97/2/1
85/3/12
90/3/7
a Reaction conditions: 1% Pd complex, 150 mM alkyne, 750 mM
triethylammonium formate, and 150 mM p-xylene (internal standard) in
15 mL of refluxing acetonitrile. Selectivity measured at full conversion,
or after 24 h (entries 5 and 6).
In previous research we have found that mono- and hetero-
bidentate (NHC)palladium(0) complexes are excellent catalysts
for the conversion of the alkyne to the Z-alkene.8,18,19 This
transformation shows a high chemo- and stereoselectivity that
is unique to this type of complexes. When a secondary amine
donor is tethered to the NHC, selectivities of more than 99%
toward (Z)-1-phenyl-1-propene were observed.6 The results of
our screening with differently substituted N-pyrimidyl-NHC
ligands are summarized in Table 1.
Experimental Section
General Procedures and Instrumentation. All reactions invol-
ving transition metal complexes were performed using standard
Schlenk techniques under an atmosphere of dry nitrogen.
Solvents were distilled using standard procedures.20 All chem-
icals were used as received, with the exception of palladium bis-
tert-butyldiazabutadiene maleic anhydride, which was synthe-
sized according to a literature procedure.17 NMR measurements
were performed on Varian Mercury 300 (1H: 300.13 MHz, 13C:
75.47 MHz), Bruker DRX 300 (1H: 300.11 MHz, 13C: 75.47
MHz), and Varian Inova 500 (1H: 499.86 MHz, 13C: 125.70
MHz) spectrometers. 13C NMR spectra were measured with 1H
decoupling. Positive chemical shifts (δ) are denoted for high-
frequency shifts relative to the external TMS reference. High-
resolution mass spectra were recorded on a JEOL JMS SX/
SX102A four-sector mass spectrometer; mass samples were
loaded in a matrix solution (3-nitrobenzyl alcohol) onto a
stainless steel probe and bombarded with xenon atoms with an
energy of 3 KeV. During the high-resolution FAB-MS measure-
ments a resolving power of 10 000 (10% valley definition) was used.
Gas chromatography analyses were performed with a Carlo Erba
HRGC 8000 Top instrument using a DB-5 capillary column and
p-xylene as internal standard.
For the catalysts tested, introduction of methyl groups
onto the pyridimyl donor leads to lower reaction rates
(entries 1, 3, 5 compared to entries 2, 4, 6, respectively). At
the same time, the reaction selectivity increased when moving
from iPr (entries 5 and 6) to Mes (entries 1 and 2) and DiPP
(entries 3 and 4). The major drawback of this ligand type in
catalysis seems to be that overreduction and isomerization
after full conversion take place. In contrast, we did not
observe any overreduction or isomerization when a tertiary
amine donor was tethered to the NHC;6 selectivity and
activity were comparable to entries 2 and 3 for this type of
complex. The occurrence of side reactions in the system
described here implies that the affinity of the complex for the
Z-alkene product is high enough relative to the secondary
donor and the solvent, acetonitrile, for further reaction to take
place.18 As long as any alkyne is present, this coordinates
preferentially to palladium, precluding unwanted side reac-
tions. The donor strength of the pyrimidyl N atom is not large
enough to preclude this secondary reaction from occurring. For
the iPr-substituted ligands (entries 5 and 6) it has to be noted
that a deactivation takes place and complete conversion is not
reached after 24 h, whereas the aryl-substituted ligands show
full conversion within 12 hours.
General Synthesis of N-Arylimidazoles. 1-(2,6-Diisopropyl-
phenyl)imidazole 2 and 1-(2-(4,6-dimethylpyrimidyl))imidazole
4 were synthesized according to a literature procedure.12
1-(2,4,6-Methylphenyl)imidazole and 1-(2-pyrimidyl)imidazole
were synthesized according to a modified literature procedure.12
1-Mesityl-1H-imidazole 1. The product (27.3 g, 73%) was
obtained as a light beige crystalline solid after recrystallization
of the crude product from tetrahydrofuran. 1H NMR (CDCl3):
δ 7.53 (s, 1H, NCH), 7.28 (s, 1H, NCH), 6.96 (s, 2H, aryl-CH),
6.93 (s, 1H, NCHN), 2.36 (s, 3H, p-aryl-CH3), 2.01 (s, 6H,
o-aryl-CH3).
Conclusions
We have synthesized electron-rich zerovalent palladium
complexes containing one maleic anhydride ligand and a
heteroditopic NHC-pyrimidine ligand. These spatially very
open complexes were obtained in high yields and purities.
Complete characterization has been performed, and a repre-
sentative compound has also been characterized by X-ray
crystallography, showing the very planar nature of the ligand
and the coordination sphere. From the 1H NMR of the
palladium complexes it was concluded that the secondary
donor shows hemilabile behavior at room temperature,
1-(2-Pyrimidyl)imidazole 3.21 The product (1.34 g, 99%) was
obtained as a pale yellow solid from the reaction mixture with-
1
out the need for column chromatography. H NMR (CDCl3):
δ 8.69 (d, 3J(HH) = 4.8 Hz, 2H, m-pym-H), 8.62 (s, 1H, NCHN),
7.89 (d, 3J(HH) = 1.2 Hz, 1H, NCH), 7.20 (t, 3J(HH) = 4.8 Hz,
1H, p-pym-H), 7.17 (d, 3J(HH) = 1.2 Hz, 1H, NCH).
General Synthesis of (1-(2-Pyrimidyl)-3-aryl)imidazolium Salts.
A literature procedure11 was modified as follows: equimolar
(18) Hauwert, P.; Maestri, G.; Sprengers, J. W.; Catellani, M.;
Elsevier, C. J. Angew. Chem., Int. Ed. 2008, 47, 3223–3226.
(19) Sprengers, J. W.; Wassenaar, J.; Clement, J. D.; Cavell, K. J.;
Elsevier, C. J. Angew. Chem., Int. Ed. 2005, 44, 2026–2029.
(20) Perrin, D. D.; Armagero, L. F. Purification of Laboratory
Chemicals; Pergamon Press: Oxford, 1998.
(21) Huang, H.; Yan, X.; Zhu, W.; Liu, H.; Jiang, H.; Chen, K.
J. Comb. Chem. 2008, 10, 617–619.

Article
Organometallics, Vol. 29, No. 20, 2010 4559
amounts of N-aryl imidazole and the 2-chloropyrimidine were
heated overnight in a sealed vessel at 120 °C. The reaction mixture
was then allowed to cool to room temperature and washed with
diethyl ether until the supernatant was colorless. The product was
then dried in vacuo.
General Synthesis of (NHC)Silver(I) Complexes. A literature
procedure11 was modified as follows: imidazolium salt and
silver(I) oxide (0.55 equiv) were suspended in dichloromethane
to 0.1 M. After stirring overnight at room temperature, the
reaction mixture was filtered through Celite. The products were
obtained by concentrating the resulting solution in vacuo. Yields
are calculated on the basis of the imidazolium salt.
(1-(2-Pyrimidyl)-3-mesityl)imidazolium Chloride 5a.11. The pro-
duct (2.45 g, 67%) was obtained as a beige solid. 1H NMR (DMSO-
d6):δ10.45 (s, 1H, NCHN), 9.11 (d, 3J(HH) = 4.5 Hz, 2H, m-pym-
H), 8.81 (d, 3J(HH) = 1.8 Hz, 1H, CH), 8.16 (t, 3J(HH) = 1.8 Hz,
Bis[(1-(2-pyrimidyl)-3-mesityl)imidazol-2-ylidene]silver(I) Dichloro-
argentate(I) 8a.11 The product (0.12 g, 89%) was obtained as a pale
yellow solid. 1H NMR (CD2Cl2): δ 8.84 (d, 3J(HH) = 4.8 Hz, 2H,
m-pym-H), 8.35 (d, 3J(HH) = 2.1 Hz, 1H, CH), 7.46 (t, 3J(HH) =
3
1H, CH), 7.79 (t, J(HH) = 4.5 Hz, 1H, p-pym-H), 7.14 (s, 2H,
aryl-H), 2.32 (s, 3H, p-aryl-CH3), 2.08 (s, 6H, o-aryl-CH3).
(1-(2-(4,6-Dimethylpyrimidyl))-3-mesityl)imidazolium Chloride
5b. The product (0.30 g, 68%) was obtained as a purple solid. 1H
NMR (DMSO-d6): δ 10.41 (dd, 4J(HH) = 1.6 Hz, 4J(HH) =
1.6 Hz, 1H, NCHN), 8.70 (dd, 3J(HH) = 1.6 Hz, 4J(HH) = 1.6
Hz, 1H, CH), 8.15 (dd, 3J(HH) = 1.6 Hz, 4J(HH) = 1.6 Hz, 1H,
CH), 7.57 (s, 1H, pym-H), 7.16 (s, 2H, aryl-H), 2.58 (s, 6H, pym-
CH3), 2.34 (s, 3H, p-aryl-CH3), 2.10 (s, 6H, o-aryl-CH3). 13C NMR
(CDCl3): δ 170.8 (m-pym-C), 151.7 (i-pym-C), 141.8 (p-aryl-C),
135.2 (i-aryl-C), 134.3 (NCHN), 130.8 (o-aryl-C), 130.2 (m-aryl-C),
126.9 (CH), 121.8 (p-pym-CH), 120.9 (CH), 24.1 (pym-CH3), 21.4
(p-aryl-CH3), 18.0 (o-aryl-CH3). MS(FABþ, m/z): calcd for
C18H21N4 [M - Cl]þ 293.1766; found 293.1762.
3
4.8 Hz, 1H, p-pym-H), 7.15 (d, J(HH) = 2.1 Hz, 1H, CH), 7.07
(s, 2H, aryl-H), 2.39 (s, 3H, p-aryl-CH3), 2.07 (s, 6H, o-aryl-CH3).
Bis[(1-(2-(4,6-dimethylpyrimidyl))-3-mesityl)imidazol-2-ylidene]-
silver(I) Dichloroargentate(I) 8b. The product (0.12 g, 95%) was
obtained as an orange solid. 1H NMR (CD2Cl2): δ 8.29 (d,
3J(HH) = 1.5 Hz, 1H, CH), 7.07 (s, 1H, pym-H), 7.02 (d,
3J(HH)=1.5 Hz, 1H, CH), 6.95 (s, 2H, aryl-H), 2.59 (s, 6H, pym-
CH3), 2.33 (s, 3H, p-aryl-CH3), 2.01 (s, 6H, o-aryl-CH3). 13C NMR
(CD2Cl2): δ = 183.4 (NCN), 169.4 (m-pym-C), 154.7 (i-pym-C),
139.6 (p-aryl-C), 136.1 (i-aryl-C), 134.8 (o-aryl-C), 129.2 (m-aryl-
CH), 123.0 (CH), 119.6 (CH), 119.0 (p-pym-CH), 23.3 (pym-CH3),
21.0 (p-aryl-CH3), 17.6 (o-aryl-CH3). MS(FABþ, m/z): calcd for
C36H40N8Ag [M - AgCl2]þ 693.2423; found 693.2432.
(1-(2-Pyrimidyl)-3-(2,6-diisopropylphenyl))imidazolium Chloride
6a.11 The product (1.87 g, 62%) was obtained as a light brown
Bis[(1-(2-pyrimidyl)-3-(2,6-diisopropylphenyl))imidazol-2-ylidene]-
11
1
silver(I) Dichloroargentate(I) 9a. The product (0.17 g, 88%) was
solid. H NMR (DMSO-d6): δ 10.60 (s, 1H, NCHN), 9.11 (d,
obtained as a brown solid. 1H NMR (CD2Cl2): δ 8.84 (d, 3J(HH) =
4.8 Hz, 2H, m-pym-H), 8.33 (s, 1H, CH), 7.50-7.38 (m, 2H, p-aryl-
H, p-pym-H), 7.26 (d, 3J(HH) = 8.0 Hz, 2H, m-aryl-H), 7.10 (s, 1H,
3J(HH) = 4.5 Hz, 2H, m-pym-H), 8.82 (s, 1H, CH), 8.35 (s, 1H,
CH), 7.83 (t, 3J(HH) = 4.5 Hz, 1H, p-pym-H), 7.66 (t, 3J(HH) =
7.8 Hz, 1H, p-aryl-H), 7.49 (d, 3J(HH) = 7.8 Hz, 2H, m-aryl-H),
2.49 (m, 2H, iPr-CH), 1.17 (d, 3J(HH) = 6.9 Hz, 12H, iPr-CH3).
(1-(2-(4,6-Dimethylpyrimidyl))-3-(2,6-diisopropylphenyl))imidazo-
lium Chloride 6b. The product (0.26 g, 69%) was obtained as a dark
brown solid. 1H NMR (DMSO-d6):δ10.55 (s, 1H, NCHN), 8.73 (s,
1H, CH), 8.30 (s, 1H, CH), 7.64 (t, 3J(HH) = 7.6 Hz, 1H, p-aryl-H),
7.57 (s, 1H, pym-H), 7.46 (d, 3J(HH) = 7.6 Hz, 2H, m-aryl-H), 2.59
(s, 6H, pym-CH3), 2.44 (m, 2H, iPr-CH), 1.18 (d, 3J(HH) = 6.8 Hz,
iPr-CH3), 1.14 (d, 3J(HH) = 6.8 Hz, iPr-CH3). 13C NMR (CDCl3):
δ 171.0 (m-pym-C), 151.6 (i-pym-C), 145.3 (o-aryl-CH), 134.7
(i-aryl-C), 132.5 (NCHN), 130.3 (p-aryl-C), 127.9 (CH), 125.0
(m-aryl-CH), 122.1 (CH), 121.4 (p-pym-CH), 29.0 (iPr-CH), 24.6
(iPr-CH3), 24.5 (iPr-CH3), 24.1 (pym-CH3). MS(FABþ, m/z): calcd
for C21H27N4 [M - Cl]þ 335.2236; found 335.2231.
3
i
CH), 2.48 (septet, J(HH) = 6.8 Hz, 1H, Pr-CH), 2.38 (septet,
3J(HH) = 6.8 Hz, 1H, Pr-CH), 1.24 (d, J(HH) = 6.8 Hz, 6H,
i
3
iPr-CH3), 1.11 (d, 3J(HH)=6.8 Hz, 6H, iPr-CH3).
Bis[(1-(2-(4,6-dimethylpyrimidyl))-3-(2,6-diisopropylphenyl))-
imidazol-2-ylidene] Dichloroargentate(I) 9b. The product (0.24 g,
94%) was obtained as a brown solid. 1H NMR (CD2Cl2): δ 8.32
(s, 1H, CH), 7.45 (d, 3J(HH) = 5.7 Hz, 1H, p-aryl-H), 7.25 (t,
3J(HH) = 5.7 Hz, 2H, m-aryl-H), 7.08 (s, 1H, pym-H), 7.06 (s,
1H, CH), 2.63 (s, 6H, pym-CH3), 2.49 (septet, 3J(HH) = 5.1 Hz,
2H, iPr-CH), 1.23 (d, 3J(HH) = 5.1 Hz, 2H, iPr-CH3), 1.12 (d,
3J(HH) = 5.1 Hz, 2H, iPr-CH3). 13C NMR (CD2Cl2): δ 184.3
(NCN), 169.7 (m-pym-C), 155.1 (i-pym-C), 145.8 (o-aryl-CH),
135.8 (i-aryl-C), 130.6 (p-aryl-C), 124.8 (CH), 124.4 (m-aryl-
CH), 119.9 (CH), 119.0 (p-pym-CH), 28.5 (iPr-CH3), 24.4 (pym-
CH3), 24.1 (iPr-CH3), 23.8 (iPr-CH3). MS(FABþ, m/z): calcd
for C42H52N8Ag [M - AgCl2]þ 777.3362; found 777.3380.
Bis[(1-(2-pyrimidyl)-3-isopropyl)imidazol-2-ylidene]silver(I)
Diiodoargentate(I) 10a. The product (90%, 0.16 g) was obtained
as an ochre solid. 1H NMR (CD2Cl2): δ 8.77 (d, 3J(HH) = 4.8
Hz, 2H, m-pym-H), 8.16 (d, 3J(HH) = 1.8 Hz, 1H, CH), 7.41 (t,
3J(HH) = 4.8 Hz, 1H, p-pym-H), 7.23 (d, 3J(HH) = 1.8 Hz, 1H,
CH), 4.88 (septet, 3J(HH) = 6.9 Hz, 1H, iPr), 1.57 (d, 3J(HH) =
General Synthesis of 1-(2-Pyrimidyl)-3-isopropyl)imidazolium
Salts. The N-(2-pyrimidyl)imidazole was dissolved in 10 equiv of
2-iodopropane and heated to reflux for 16 h. After the reaction
mixture was allowed to cool, all volatiles were removed under reduced
pressure. The resulting solid was washed with diethyl ether until the
supernatant was colorless. The product was then dried in vacuo.
(1-(2-Pyrimidyl)-3-isopropyl)imidazolium Iodide 7a. The pro-
1
duct (0.43 g, 93%) was obtained as an ochre solid. H NMR
(CDCl3): δ 10.49 (dd, 4J(HH) = 1.8 Hz, 4J(HH) = 1.8 Hz, 1H,
3
i
6.9 Hz, 6H, Pr). 13C NMR (CD2Cl2): δ 183.8 (NCN), 159.1
NCHN), 8.88 (d, J(HH) = 4.8 Hz, 2H, m-pym-H), 8.26 (dd,
3J(HH)=1.6 Hz, 4J(HH)=1.6 Hz, 1H, CH), 7.85 (dd, 3J(HH) =
1.6 Hz, 4J(HH) = 1.6 Hz, 1H, CH), 7.55 (t, 3J(HH) = 4.8 Hz,
1H, p-pym-H), 5.51 (septet, 3J(HH) = 6.6 Hz, 1H, iPr), 1.72 (d,
3J(HH) = 6.6 Hz, 6H, iPr). 13C NMR (CDCl3): δ 160.0 (m-pym-
CH), 152.2 (i-pym-C), 134.0 (NCHN), 122.8 (CH), 122.5 (CH),
119.5 (p-pym-CH), 54.8 (iPr-CH), 23.2 (iPr-CH3). MS(FABþ,
m/z): calcd for C10H13N4 [M - I]þ 189.1140; found 189.1144.
(1-(2-(4,6-Dimethylpyrimidyl)-3-isopropyl)imidazolium Iodide
7b. The product (0.19 g, 87%) was obtained as a gray solid. 1H
NMR (CDCl3): δ 10.37 (dd, 4J(HH) = 1.8 Hz, 4J(HH) = 1.8
Hz, 1H, NCHN), 8.24 (dd, 3J(HH) = 1.6 Hz, 4J(HH) = 1.6 Hz,
1H, CH), 7.89 (dd, J(HH) = 1.6 Hz, J(HH) = 1.6 Hz, 1H,
CH), 7.17 (s, 1H, pym-H), 5.47 (septet, 3J(HH) = 6.6 Hz, 1H,
iPr), 2.60 (s, 6H, pym-CH3), 1.72 (d, 3J(HH) = 6.6 Hz, 6H, iPr).
13C NMR (CDCl3): δ 170.6 (m-pym-C), 151.7 (i-pym-C), 133.8
(NCHN), 122.3 (CH), 122.4 (CH), 119.4 (p-pym-C), 54.7 (iPr-CH),
24.2 (pym-CH3), 23.3 (iPr-CH3). MS(FABþ, m/z): calcd for
C12H17N4 [M - I]þ 217.1453; found 217.1456.
(m-pym-CH), 155.9 (i-pym-C), 121.0 (CH), 119.6 (CH), 118.9
(p-pym-CH), 55.7 (iPr-CH), 23.6 (iPr-CH3). MS(FABþ, m/z):
calcd for C20H24N8Ag [M - AgI2]þ 483.1175; found 483.1180.
Bis[(1-(2-(4,6-dimethylpyrimidyl))-3-isopropyl)imidazol-2-ylidene]-
silver(I) Diiodoargentate(I) 10b. The product (93%, 32 mg) was
obtained as a yellow solid. 1H NMR (CD2Cl2): δ 8.15 (d, 3J(HH) =
2.0 Hz, 1H, CH), 7.19 (d, 3J(HH) = 2.0 Hz, 1H, CH), 7.08 (s, 1H,
pym-H), 4.86 (septet, 3J(HH) = 6.6 Hz, 1H, iPr), 2.57 (s, 6H, pym-
3
i
CH3), 1.55 (d, J(HH) = 6.9 Hz, 6H, Pr). 13C NMR (CD2Cl2):
δ 183.9 (NCN), 169.5 (m-pym-C), 155.1 (i-pym-C), 119.5 (CH),
119.4 (CH), 118.3 (p-pym-CH), 55.5 (iPr-CH), 23.9 (pym-CH3),23.5
(iPr-CH3). MS(FABþ, m/z): calcd for C24H32N8Ag [M - AgI2]þ
539.1801; found 539.1809.
3
4
General Procedure for the Synthesis of [NHC-pyrimidyl]-
palladium(0) Maleic Anhydride Complexes. Palladium (bis-tert-
butyldiazabutadiene)(maleic anhydride) and bis[NHC-pyrimidyl]-
silver(I) complex (0.55 equiv) were dissolved in dichloromethane to
0.1 M. Immediate precipitation of silver(I) halide was observed.

4560 Organometallics, Vol. 29, No. 20, 2010
Warsink et al.
After 1 h, the reaction mixture was filtered through Celite and
concentrated to a small volume at reduced pressure. The solution
was then layered with pentane to precipitate the product. After
decanting the supernatant, the product was washed with a small
volume of pentane and dried in vacuo. Alternatively, the reaction
mixture can be fully concentrated and only then washed with
pentane. Thisprocedure ispreferable ifthe product islessstable in
solution. Yields are calculated on the basis of palladium.
[(1-(2-Pyrimidyl)-3-mesityl)imidazol-2-ylidene]palladium(0) Maleic
Anhydride 11a. The product (0.11 g, 91%) was obtained as a yellow
solid. 1HNMR(CD2Cl2):δ8.93 (bd, 3J(HH) = 5.1 Hz, 2H, m-pym-
H), 8.01 (d, 3J(HH) = 2.1 Hz, 1H, CH), 7.45 (t, 3J(HH) = 5.1 Hz,
1H, p-pym-H), 7.16-7.02 (2 br s, 2H, aryl-H), 7.07 (d, 3J(HH) =2.1
Hz, 2H, CH), 3.65-3.50 (2 br s, 2H, ma), 2.42 (s, 3H, p-aryl-CH3),
2.07 (br s, 6H, o-aryl-CH3). 13C NMR (CD2Cl2): δ 193.4 (NCN),
173.4, 172.4 (2 CO), 160.8, 160.7 (2 m-pym-C), 158.0 (i-pym-C), 139.6
(p-aryl-C), 135.8 (i-aryl-C), 134.5, 134.4 (2 o-aryl-C), 129.9, 129.1 (2
m-aryl-C), 124.8 (CH), 120.8 (CH), 117.2 (p-pym-C), 43.2, 40.7 (2
alkene-CH), 21.3 (p-aryl-CH3), 17.9 (br, o-aryl-CH3). MS(FABþ,
m/z): calcd for C20H19N4O3Pd [M þ H]þ 469.0492; found 469.0493.
Anal. Calcd for C20H18N4O3Pd C, 51.24; H, 3.87; N, 11.95. Found: C,
51.30; H, 4.01; N, 11.76.
173.3 (CO), 172.0 (CO), 170.6 (m-pym-C), 157.5 (i-pym-C), 145.8
(o-aryl-C), 145.0 (o-aryl-C), 135.5 (i-aryl-C), 130.1 (p-aryl-CH),
125.3 (CH), 124.5 (m-aryl-CH), 123.7 (m-aryl-CH), 119.1 (p-pym-
CH), 116.9 (CH), 42.2 (alkene), 40.2 (alkene), 29.3 (pym-CH3), 28.8
(iPr-CH), 28.6 (iPr-CH), 27.2 (pym-CH3), 24.5 (iPr-CH3), 24.1 (iPr-
CH3), 23.6 (iPr-CH3), 23.1 (iPr-CH3). MS(FABþ, m/z): calcd for
C21H26N4Pd [M - ma]þ 440.1192; found 440.1196. Anal. Calcd for
C25H28N4O3Pd C, 55.71; H, 5.24; N, 10.40. Found: C, 56.06; H,
4.85; N, 10.82.
[(1-(2-Pyrimidyl)-3-isopropyl)imidazol-2-ylidene]palladium(0)
Maleic Anhydride 13a. The product (40 mg, 93%) was obtained
1
3
as a yellow solid. H NMR (CD2Cl2): δ 8.89 (bd, J(HH) =
5.0 Hz, 2H, m-pym-H), 7.84 (d, 3J(HH) = 1.8 Hz, 1H, CH), 7.40
(t, 3J(HH) = 5.0 Hz, 1H, p-pym-H), 7.16 (d, 3J(HH) = 1.8 Hz,
1H, CH), 4.86 (septet, 3J(HH) = 7.0 Hz, 1H, iPr-H), 4.00 (br s,
2H, ma), 1.56 (d, 3J(HH) = 7.0 Hz, 6H, iPr-CH3), 1.43 (s, 6H,
pym-CH3). 13C NMR (CD2Cl2): δ 189.4 (NCN), 173.1 (CO),
160.6 (m-pym-CH), 157.7 (i-pym-C), 120.2 (CH), 119.1 (p-pym-
CH), 117.2 (CH), 54.8 (iPr-CH), 41.1 (alkene-CH), 23.2 (iPr-
CH3). MS(FABþ, m/z): calcd for C14H15N4O3Pd [M þ H]þ
393.0179; found 393.0186. Anal. Calcd for C14H14N4O3Pd C,
42.82; H, 3.59; N, 14.27. Found: C, 43.29; H, 4.01; N, 14.11.
[(1-(2-(4,6-Dimethylpyrimidyl))-3-isopropyl)imidazol-2-ylidene]-
palladium(0) Maleic Anhydride 13b. The product (30 mg, 91%) was
obtained as pale yellow crystals. 1H NMR (CD2Cl2): δ 7.82
[(1-(2-(4,6-Dimethylpyrimidyl))-3-mesityl)imidazol-2-ylidene]-
palladium(0) Maleic Anhydride 11b. The product (0.12 g, 99%)
1
was obtained as a yellow solid. H NMR (CD2Cl2): δ 7.95 (d,
3
(d, J(HH) = 2.1 Hz, 1H, CH), 7.18 (s, 1H, pym-H), 7.12 (d,
3J(HH) = 1.6 Hz, 1H, CH), 7.17 (s, 1H, pym-H), 7.12 (s, 1H,
3J(HH) = 2.1 Hz, 1H, CH), 4.91 (septet, 3J(HH) = 6.6 Hz, 1H,
iPr-H), 4.21 (d, 3J(HH) = 3.6 Hz, 1H, ma), 3.67 (d, 3J(HH) = 3.6
Hz, 1H, ma), 2.79 (s, 3H, pym-CH3), 2.56 (s, 3H, pym-CH3), 1.54 (d,
3J(HH) = 6.6 Hz, 3H, iPr-CH3), 1.52 (d, 3J(HH) = 6.6 Hz, 3H,
iPr-CH3).13CNMR(CD2Cl2):δ188.4 (NCN), 173.5 (br, CO), 170.4
(br, m-pym-C), 157.4 (i-pym-C), 119.0 (CH), 118.4 (p-pym-C), 117.2
(CH), 55.1 (iPr-CH), 42.1 (alkene), 39.7 (alkene), 27.4 (pym-CH3),
24.1 (pym-CH3), 23.2 (iPr-CH). MS(FABþ, m/z): calcd for
C16H19N4O3Pd [M þ H]þ 421.0492; found 421.0492. Anal. Calcd
for C16H18N4O3Pd C, 45.67; H, 4.31; N, 13.32. Found: C, 45.30; H,
4.21; N, 13.21.
3
aryl-H), 6.98 (s, 1H, aryl-H), 6.96 (d, J(HH) = 1.6 Hz, 1H,
CH), 3.53 (d, 3J(HH) = 3.6 Hz, 1H, ma), 3.49 (d, 3J(HH) = 3.6
Hz, 1H, ma), 2.80 (s, 3H, pym-CH3), 2.59 (s, 3H, pym-CH3), 2.40
(s, 3H, p-aryl-CH3), 2.06 (s, 3H, o-aryl-CH3), 2.04 (s, 3H, o-aryl-
CH3). 13C NMR (CD2Cl2): δ 191.8 (NCN), 173.1 (CO), 171.9
(CO), 170.2 (m-pym-C), 170.0 (m-pym-C), 157.8 (i-pym-C),
139.0 (p-aryl-C), 135.3 (i-aryl-C), 133.9 (o-aryl-C), 129.5 (m-aryl-
C), 128.5 (m-aryl-C), 123.8 (CH), 118.8 (CH), 117.0 (p-pym-C), 41.4
(alkene-CH), 39.6 (alkene-CH), 27.0 (pym-CH3), 23.9 (pym-CH3),
20.9 (p-aryl-CH3), 17.6 (o-aryl-CH3), 17.3 (o-aryl-CH3). MS-
(FABþ, m/z): calcd for C18H20N4Pd [M - ma]þ 398.0723; found
398.0730. Anal. Calcd for C22H22N4O3Pd: C, 53.18; H, 4.46; N,
11.28. Found: C, 53.30; H, 4.61; N, 11.06.
X-ray Data Collection, Reduction, and Refinement. A suitable
single crystal of 11a was coated with Paratone-N oil, mounted
using a glass fiber pin, and frozen in the cold nitrogen stream of
the goniometer. X-ray diffraction data was collected on a Bruker
AXS APEX CCD diffractometer equipped with a rotation anode
at 153(2) K using graphite-monochromated Mo KR radiation
[(1-(2-Pyrimidyl)-3-(2,6-diisopropylphenyl))imidazol-2-ylidene]-
palladium(0) Maleic Anhydride 12a. The product (80 mg, 93%)
was obtained as an ochre solid. 1H NMR (CD2Cl2): δ 8.95 (bd,
3J(HH) = 8.0 Hz, 2H, m-pym-H), 8.04 (d, 3J(HH) = 2.0 Hz, 1H,
CH), 7.58 (t, 3J(HH) = 8.0 Hz, 1H, p-aryl-H), 7.48 (t, 3J(HH) =
5.0 Hz, 1H, p-pym-H), 7.38 (d, 3J(HH) = 8.0 Hz, 1H, m-aryl-H),
7.16 (d, 3J(HH) = 2.0 Hz, 1H, CH), 3.58 (br s, 2H, ma), 2.68 (br s,
1H, iPr-CH), 2.57 (br s, 1H, iPr-CH), 1.29 (br s, 3H, iPr-CH3), 1.20
˚
(λ =0.71073A) witha scanwidthof 0.3° and exposure time of 1 s.
The generator setting was 50 kV and 180 mA. Diffraction data
were collected over the full sphere, and the frames were integrated
using the Bruker SMART22 software package using the narrow
frame algorithm. Data were correctedfor absorptioneffects using
the SADABS routine (empirical multiscan method). Atomic
scattering factors for non-hydrogen elements were taken from
the literature tabulations.23 The structure solution was found
by using direct methods as implemented in the SHELXS-97
package and was refined with SHELXL-9724 against F2 using
first isotropic and later anisotropic thermal parameters for all
non-hydrogen atoms. C-H atom positions were calculated and
allowed to ride on the carbon atom to which they are bonded,
(br s, 3H, iPr-CH3), 1.16 (d, 3J(HH) = 7.0 Hz, 6H, iPr-CH3). 13
C
NMR (CD2Cl2): δ 193.9 (NCN), 173.2 (CO), 172.0 (CO), 161.1
(m-pym-CH), 160.0 (m-pym-CH), 157.8 (i-pym-C), 145.8 (o-aryl-C),
145.1 (o-aryl-C), 135.4 (i-aryl-C), 130.3 (p-aryl-CH), 125.7 (CH),
124.4 (m-aryl-CH), 123.9 (m-aryl-CH), 120.7 (p-pym-CH), 116.7
(CH), 43.6 (alkene), 40.8 (alkene), 28.8 (br, iPr-CH), 24.3 (iPr-CH3),
23.9 (iPr-CH3), 23.3 (iPr-CH3). MS(FABþ, m/z): calcd for
C19H22N4Pd [M - ma]þ 412.0879; found 412.0889. Anal. Calcd
for C23H24N4O3Pd C, 54.07; H, 4.74; N, 10.97. Found: C, 53.81; H,
4.62; N, 11.13.
˚
assuming C-H bond length of 0.95 A for aromatic protons and
0.98 A for methyl groups. H atom temperature factors were fixed
˚
[(1-(2-(4,6-Dimethylpyrimidyl))-3-(2,6-diisopropylphenyl))-
imidazol-2-ylidene]palladium(0) Maleic Anhydride 12b. The product
(0.19 g, 87%) was obtained as an ochre solid. 1H NMR (CD2Cl2):
δ 7.97 (d, 3J(HH) = 2.0 Hz, 1H, CH), 7.54 (dd, 3J(HH) = 7.6 Hz,
1H, p-aryl-H), 7.36 (dd, 3J(HH) = 7.6 Hz, 1H, m-aryl-H), 7.31 (dd,
3J(HH) = 7.6 Hz, 1H, m-aryl-H), 7.18 (s, 1H, pym-H), 7.04 (d,
3J(HH) = 2.0 Hz, 1H, CH), 3.59 (d, 3J(HH) = 3.6 Hz, 1H, ma),
3.50 (d, 3J(HH) = 3.6 Hz, 1H, ma), 2.80 (s, 3H, pym-CH3), 2.70
(dq, 3J(HH) = 6.8 Hz, 3J(HH) = 6.8 Hz, 1H, iPr-CH), 2.59 (s, 3H,
at 1.20 (arom-H) or 1.50 (CH3) times the isotropic temperature
factor of the C atom to which they are bonded. The H atom
contributions were calculated but not refined. In the case of the
protons of the maleic anhydride moiety the protons (H18, H19)
wererefined isotropically. The locationsofthe largestpeaks inthe
final difference Fourier map calculated as well as the magnitude
of the residual electron densities in each case were of no chemical
3
3
(22) SMART; Bruker AXS: Madison, WI, 2000.
(23) Cromer, D. T.; Waber, J. T. International Tables for X-Ray
Crystallography; Kynoch Press: Birmingham, 1974; Vol. 4.
(24) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
2008, 64, 112–122.
pym-CH3), 2.47 (dq, J(HH) = 6.8 Hz, J(HH) = 6.8 Hz, 1H,
iPr-CH), 1.27 (d, 3J(HH) = 6.8 Hz, iPr-CH3), 1.20 (d, 3J(HH) =
i
3
i
6.8 Hz, Pr-CH3), 1.14 (d, J(HH) = 6.8 Hz, Pr-CH3), 1.10 (d,
3J(HH) = 6.8 Hz, iPr-CH3). 13C NMR (CD2Cl2): δ 192.7 (NCN),

Article
Organometallics, Vol. 29, No. 20, 2010 4561
significance. 11a (C20H18N4O3Pd): fw = 468.78, monoclinic, space
acetonitrile was heated to reflux. A solution of 1 mol % of
catalyst (Table 1) in 2 mL of acetonitrile was added to the
reaction mixture. Periodically, samples were taken over a period
of 24 h. Aliquots from the reaction mixtures were quenched by
dilution with EtOH and were analyzed by GC to determine
conversions and composition.
˚
group P21/n, Z = 4, a = 13.271(1) A, b = 10.2858(8) A, c =
˚
3
˚
˚
14.300(1) A, β = 105.392(1)°, V = 1882.1(3) A , F000 = 944, T =
153(2) °C, μ = 1.015, 19 366 reflections collected, 4687 reflections
unique (Rint = 0.0403), 3871 reflections observed (F>2σ(F)). The
final was R1 = 0.0287 and wR2 = 0.0694 (all data).
General Experimental Procedure for Catalytic Transfer Semi-
hydrogenation of Phenyl Propyne Using Formic Acid As Hydro-
gen Donor. A solution of 150 mM 1-phenyl-1-propyne, 750 mM
triethylammonium formiate, and 150 mM p-xylene in 12 mL of
Supporting Information Available: This material is available
free of charge via the Internet at http://pubs.acs.org.