Cyclometalated and alkoxyphenyl-substituted palladium imidazolin-2-ylidene complexes. synthetic, structural, and catalytic studies

Article abstract of DOI:10.1021/om700603d

Some new N,N′-bis(aryl)imidazolinium salts (3a-h), in which the N-aryl groups have one unsubstituted Ortho position and alkyl (Me, Pr ⁱ) or alkoxy (OMe, OPrⁱ) substitution at the other ortho and/or para positions, have been prepared. They were used for the synthesis of N-heterocyclic carbene complexes of palladium in which the N-heterocyclic carbene is attached to cyclometalated or electron-rich aromatic rings (4, 5, 11d,e). The reaction of Pd(tmed)(CH₃)₂ with the N-heterocyclic carbene generated from 3b and base gave the dimethyl complex 6, which quantitatively eliminated ethane to form the Pd(O) complex 7. The latter was converted to a new type of "pincer" complex (8) by facile cyclometalation of both aromatic groups of the N-heterocyclic carbene ligand. The activities of the new complexes 4 and 9d,e in the Heck reaction of aryl halides were compared. At higher temperatures the complexes 9d,e show low activity in the coupling of aryl chlorides.

Full text of DOI:10.1021/om700603d

Organometallics 2007, 26, 5627-5635
5627
Cyclometalated and Alkoxyphenyl-Substituted Palladium
Imidazolin-2-ylidene Complexes. Synthetic, Structural, and Catalytic
Studies
Neoklis Stylianides,† Andreas A. Danopoulos,*,† David Pugh,† Fred Hancock,‡ and
Antonio Zanotti-Gerosa‡
School of Chemistry, UniVersity of Southampton, Highfield, Southampton, U.K. SO17 1BJ, and
Johnson Matthey Catalysis and Chiral Technologies, 28 Cambridge Science Park, Milton Road,
Cambridge, U.K. CB4 0FP
ReceiVed June 19, 2007
Some new N,N′-bis(aryl)imidazolinium salts (3a-h), in which the N-aryl groups have one unsubstituted
ortho position and alkyl (Me, Pri) or alkoxy (OMe, OPri) substitution at the other ortho and/or para
positions, have been prepared. They were used for the synthesis of N-heterocyclic carbene complexes of
palladium in which the N-heterocyclic carbene is attached to cyclometalated or electron-rich aromatic
rings (4, 5, 11d,e). The reaction of Pd(tmed)(CH3)2 with the N-heterocyclic carbene generated from 3b
and base gave the dimethyl complex 6, which quantitatively eliminated ethane to form the Pd(0) complex
7. The latter was converted to a new type of “pincer” complex (8) by facile cyclometalation of both
aromatic groups of the N-heterocyclic carbene ligand. The activities of the new complexes 4 and 9d,e in
the Heck reaction of aryl halides were compared. At higher temperatures the complexes 9d,e show low
activity in the coupling of aryl chlorides.
aryl substituents.6 Furthermore, the aliphatic or aromatic sub-
stituents of NHCs coordinated to low-oxidation-state rhodium,
iridium, and ruthenium metal centers can sometimes undergo
metalation under thermal or photochemical conditions.7 The
majority of the coordination chemistry of the NHC ligands has
been based on “unsaturated” imidazol-2-ylidenes. This is due
to the convenient synthetic methods leading to their precursor
imidazolium salts and the easier handling of the free NHCs
(dimerization to Wanzlick alkenes is completely suppressed).
Recently, ligand design aspects of “saturated” NHCs have been
investigated,8,4c especially in relation to the development of
highly active and chiral Ru metathesis catalysts. These studies
included the variation of the nature and size of NHC substitution
and the attachment of other heteroatom donors to the NHC
functionality. In comparison to the unsaturated analogues, the
Introduction
The use of N-heterocyclic carbenes (NHCs) as ligands in
homogeneous catalysis is still an area of intensive research.1
The originally proposed electronic “analogy” between NHCs
and trialkylphosphines2 has been the guiding idea for the design
of new catalysts with enhanced activity and stability, by
replacing phosphine ligands with NHCs. However, the detailed
picture that is emerging from this work reveals many subtle
differences associated with the electronically similar ligand
types. For example, in late-transition-metal complexes with
organometallic coligands (σ-alkyls, alkylidenes, hydrides, etc.)
the coordinated NHC can participate in unusual rearrangement
reactions leading to imidazolium or alkylimidazolium salt
formation (formally reductive eliminations),3 conversion to
â-heteroatom alkyls by migration of anionic ligands to the
carbene carbon,4 cleavage of the heterocyclic ring,5 or C-N
bond cleavage between the heterocyclic ring and the alkyl or
(6) Burling, S.; Mahon, M. F.; Powell, R. E.; Whittlesey, M. K.; Williams,
J. M. J. J. Am. Chem. Soc. 2006, 128, 13702.
(7) (a) Hitchcock, P. B.; Lappert, M. F.; Pye, P. L.; Thomas, S. J. Chem.
Soc., Dalton Trans. 1979, 1929 (Ru, aryl C-H). (b) Huang, J.; Stevens, E.
D.; Nolan, S. P. Organometallics 2000, 19, 1194 (Rh, mesityl CH2-H).
(c) Trnka, T. M.; Morgan, J. P.; Sanford, M. S.; Wilhelm, T. E.; Scholl,
M.; Choi, T.-L.; Ding, S.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc.
2003, 125, 2546 (Ru mesityl CH2-H). (d) Abdur-Rashid, K.; Fedorkiw,
T.; Lough, A. J.; Morris, R. H. Organometallics 2004, 23, 86, (Ru mesityl
CH2-H, Ru But C-H). (e) Scott, N. M.; Dorta, R.; Stevens, E. D.; Correa,
A.; Cavallo, L.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 3516, (Rh, Ir,
But C-H). (f) Ampt, K. A. M.; Burling, S.; Donald, S. M. A.; Douglas, S.;
Duchet, S. B.; Macgregor, S. A.; Perutz, R. N.; Whittlesey, M. K. J. Am.
Chem. Soc. 2006, 128, 7452 (Ru mesityl CH2-H, photolytic). (g) Corbera´n,
R.; Sanau´, M.; Peris, E. J. Am. Chem. Soc. 2006, 128, 3974.
(8) (a) Blum, A. P.; Ritter, T.; Grubbs, R. H. Organometallics 2007, 26,
2122 (fluoroaryl-substituted imidazolin-2-ylidene Ru complexes). (b) Funk,
T. W.; Berlin, J. M.; Grubbs, R. H. J. Am. Chem. Soc. 2006, 128, 1840
(Ru, with chiral imidazolin-2-ylidenes). (c) Larsen, A. O.; Leu, W.;
Oberhuber, C. N.; Cambell, L. E.; Hoveyda, A. H. J. Am. Chem. Soc. 2004,
126, 11130. (d) Delaude, L.; Szypa, M.; Demonceau, A.; Noels, A. F. AdV.
Synth. Catal. 2002, 344, 749.
* To whom correspondence should be addressed. E-mail: ad1@soton.ac.uk.
† University of Southampton.
‡ Johnson Matthey Catalysis and Chiral Technologies.
(1) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (b)
Bourisou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV. 2000,
100, 39. (c) Nolan, S. P., Ed. N-Heterocyclic Carbenes in Synthesis; Wiley-
VCH: Weinheim, Germany, 2006. (d) Scott, N. M.; Nolan, S. P. Eur. J.
Inorg. Chem. 2005, 1815.
(2) Green, J. C.; Scurr, G.; Arnold, P. L.; Cloke, F. G. N. Chem. Commun.
1997, 1663.
(3) McGuiness, D. S.; Saendig, N.; Yates, B. F.; Cavell, K. J. J. Am.
Chem. Soc. 2001, 123, 4029.
(4) (a) Danopoulos, A. A.; Tsoureas, N.; Green, J. C.; Hursthouse, M.
B. Chem. Commun. 2003, 756. (b) Becker, E.; Stingl, V.; Dazinger, G.;
Puchberger, M.; Mereiter, K.; Kirchner, K. J. Am. Chem. Soc. 2006, 128,
6572. (c) Waltman, A. W.; Ritter, T.; Grubbs, R. H. Organometallics 2006,
25, 4238.
(5) Galan, B. R.; Gembicky, M.; Dominiak P. M.; Keister, J. B.; Diver,
S. T. J. Am. Chem. Soc. 2005, 127, 15702.
10.1021/om700603d CCC: $37.00 © 2007 American Chemical Society
Publication on Web 10/02/2007

5628 Organometallics, Vol. 26, No. 23, 2007
Scheme 1. Synthesis of Imidazolinium Salts Described in This Paper
Stylianides et al.
Scheme 2. Synthesis of Cyclometalated Palladium Acetates and Halides
saturated NHCs are stronger σ-donors, giving rise to reactive
low-oxidation-state metal centers.1b,9
In this paper we report (i) the synthesis of N,N′-bis(aryl)-
imidazolinium salts in which the aryl groups have one vacant
ortho position and alkyl (Me, Pri) or alkoxy (OMe, OPri)
substitution at the other ortho and/or para positions (see Scheme
1), (ii) the synthesis and full characterization of new Pd
complexes with “saturated” cyclometalated NHC ligands
(Schemes 2 and 3), and (iii) studies of the catalytic activity of
selected new complexes in the Heck reaction.
As a continuation of our work aiming at the development of
well-defined NHC complexes suitable for cross-coupling reac-
tions, we initiated a study of Pd complexes with “saturated”
NHCs substituted with electron-rich or cyclometalated aromatic
rings. Palladium complexes with cyclometalated phosphines and
phosphites have shown activity in Suzuki and C-heteroatom
bond formation reactions.10 Surprisingly, there has been a very
limited number of spectroscopically characterized cyclometa-
lated Pd(NHC) complexes reported by Hiraki et al.11
(10) (a) Elsevier, C. J.; Eberhard, M. R. In ComprehensiVe Organome-
tallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier:
Amsterdam, 2007; Vol. 8, p 280. (b) Bedford, R. B. Chem. Commun. 2003,
1787.
(11) (a) Hiraki, K.; Onishi, M.; Sugino, K. J. Organomet. Chem. 1979,
171, C50. (b) Hiraki, K.; Sugino, K. J. Organomet. Chem. 1980, 201, 469.
(9) Alder, R. W. In Carbene Chemistry: From Fleeting Intermediates
to Powerful Reagents; Bertrand, G., Ed.; Fontis Media/Marcel Dekker: New
York, 2002.

Palladium Imidazolin-2-yl Complexes
Organometallics, Vol. 26, No. 23, 2007 5629
Scheme 3. Synthesis and Reactivity of Cyclometalated
Palladium Alkyl Complexes
Figure 1. ORTEP representation of the structure of 4 showing
50% probability ellipsoids. H atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg) with estimated standard
deviations in parentheses: O1-Pd2 ) 2.090(3), O2-Pd1 ) 2.123-
(4), O3-Pd2 ) 2.127(4), O4-Pd1 ) 2.094(3), Pd1-Pd2 ) 2.8688-
(6), C5-Pd2 ) 1.946(5), C7-Pd2 ) 1.963(5), C22-Pd1 )
1.934(5), C33-Pd1 ) 1.972(5); C22-Pd1-C33 ) 79.5(2), C22-
Pd1-O4 ) 173.32(19), C33-Pd1-O4 ) 93.91(18), C22-Pd1-
O2 ) 98.78(19), C33-Pd1-O2 ) 178.29(18), O4-Pd1-O2 )
87.77(13), C22-Pd1-Pd2 ) 101.06(15), C33-Pd1-Pd2 ) 103.02-
(14), O4-Pd1-Pd2 ) 81.59(10), O2-Pd1-Pd2 ) 77.56(10), C5-
Pd2-C7 ) 79.6(2), C5-Pd2-O1 ) 173.48(19), C7-Pd2-O1 )
93.89(18), C5-Pd2-O3 ) 99.72(19), C7-Pd2-O3 ) 178.46(19),
O1-Pd2-O3 ) 86.72(13), C5-Pd2-Pd1 ) 95.26(14), C7-Pd2-
Pd1 ) 97.93(15), O1-Pd2-Pd1 ) 84.78(10), O3-Pd2-Pd1 )
80.72(10).
Results and Discussion
Proligand Synthesis. The imidazolinium salts 3c-h were
prepared by the cyclization of the corresponding bidentate
anilinium salts with triethyl orthoformate. The anilines 2a,b were
prepared by modification of literature methods and 1c-h by
reduction of the oxalamides with LiAlH4 or BH3.12 All new
salts were characterized by analytical and spectroscopic methods.
The isolated chlorides 3c-h were sparingly soluble in non-
aqueous polar solvents, exhibiting rather broad 1H NMR spectra.
However, characteristic peaks in the range δ 9.0-10.5 assign-
able to the C2 attached proton of the heterocyclic ring were
easily seen in all spectra. The identities of 3f,h′ were unequivo-
cally established crystallographically (see the Supporting In-
formation). All salts were hygroscopic solids that were dried
azeotropically and stored under nitrogen prior to deprotonation.
Attempts to isolate the free N-heterocyclic carbenes by the
reaction of the imidazolinium salts with various bases were not
successful; therefore, for the preparation of the metal complexes
described below the NHC was generated in situ in the presence
of a Pd precursor.
metrical data. The coordination environment of each Pd
comprises one CNHC, one Caryl, and two Pd-O(acetato) bonds.
The Pd‚‚‚Pd separation is ca. 2.87 Å. The Pd-CNHC (1.946(5)
and 1.972(5) Å) and Pd-Ctolyl (1.972(5) and 1.963(5) Å) bond
lengths fall in the short end of the range reported for similar
complexes. The Pd-O(acetato) bond lengths trans to the CNHC
and Ctolyl donor atoms are virtually equal, indicating similar trans
influences of the two organometallic ligands. Complex 4 is the
first structurally characterized palladium complex with cyclo-
metalated NHC ligands. Cyclometalated Pd-NHC complexes
have been previously obtained by the reaction in boiling
m-xylene of the electron-rich (Wanzlick) olefin 1,3-diphenyl-
2-imidazolidinylidene with Pd complexes bearing a donor-
stabilized σ-alkyl.11 Palladium complexes with bridging acetato
and phosphine or carbene coligands have been previously
prepared by the reaction of Pd(OAc)2 with phosphines or NHCs
in specific ratios; reversible cyclometalation of the phosphines
has been observed in [Pd(PPri3)2(OAc)]+.12
Reaction of Pd(OAc)2 with the NHC generated in situ from
the salt 3b and KOBut gave complex 5. It is not known whether
the reaction proceeds via the generation of the free imidazolin-
2-ylidene which was trapped by the Pd source or by elimination
of ButOH from the tert-butoxide adduct formed by the reaction
of the imidazolinium salt with the nucleophilic ButO-.7c,13 The
presence of two types of NHC ligands and the occurrence of
cyclometalation was evidenced from the two CNHC signals (δ
202.23 and 198.40) and one Caryl signal (δ 162), respectively,
in the 13C{1H} NMR spectrum of 5. The structure of 5 was
determined crystallographically, and a diagram of the molecule
is shown in Figure 2.
Synthesis of Palladium Complexes. (i) Alkylaryl-Substi-
tuted NHCs. The reaction of the NHC generated from 3a and
NaH with Pd(OAc)2 in toluene at 50 °C led to facile metalation
of the aromatic o-C-H bond of the o-tolyl substituent and
formation of the acetato-bridged dimer 4. The complex was
characterized by analytical and spectroscopic methods. The low
symmetry of the metal coordination sphere, which was evident
from the H and 13C{1H} NMR spectra and the appearance of
1
the CNHC-Pd (δ 198.25) and Ctolyl-Pd resonances (δ 136.12)
in the 13C{1H} NMR spectrum, supported the presence of one
metalated aromatic ring attached to the NHC heterocycle.
Further confirmation of this was obtained by a single-crystal
X-ray diffraction study of 4. An ORTEP diagram of the
molecule is shown in Figure 1.
The molecule is dimeric, with two acetato ligands bridging
two distorted-square-planar Pd centers having very similar
(12) (a) Stephenson, T. A.; Wilkinson, G. J. Inorg. Nucl. Chem. 1967,
29, 2122. (b) Thirupathi, N.; Amoroso, D.; Bell, A.; Protasiewicz, J. D.
Organometallics 2005, 24, 4099. (c) Slootweg, J. C.; Chen, P. Organome-
tallics 2006, 25, 5863.
(13) Waltman, A. W.; Grubbs, R. H. Organometallics 2004, 23, 3105
and references cited therein.

5630 Organometallics, Vol. 26, No. 23, 2007
Stylianides et al.
Figure 3. ORTEP representation of the structure of 6 showing
50% probability ellipsoids. H atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg) with estimated standard
deviations in parentheses: Pd1-C1 ) 2.045(3), Pd1-C10 ) 2.088-
(3); C1′-Pd1-C1 ) 97.09(14), C1′-Pd1-C10 ) 88.79(10), C1-
Pd1-C10 ) 173.81(10), C10-Pd1-C10′ ) 85.42(15).
Figure 2. ORTEP representation of the structure of 5 showing
50% probability ellipsoids. H atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg) with estimated standard
deviations in parentheses: C10-Pd1 ) 2.029(8), C31-Pd1 ) 1.93-
(6), C39-Pd1 ) 2.024(8), Cl1-Pd1 ) 2.366(4); C31-Pd1-C39
) 83.1(16), C31-Pd1-C10 ) 168.7(16), C39-Pd1-C10 ) 91.9-
(3), C31-Pd1-Cl1 ) 96.0(16), C39-Pd1-Cl1 ) 179.1(2), C10-
Pd1-Cl1 ) 89.0(2).
Complex 5 is monomeric, with a square-planar Pd. The
coordination sphere includes one chloride and one cyclometa-
lated and one monodentate NHC ligand. The angle between the
N(1)-C(10)-N(2) plane of the monodentate NHC and the
coordinate plane is ca. 80°. The N(3)-C(31)-N(4) plane of
the cyclometalated NHC coincides with the coordination plane.
All Pd-C bond lengths to the NHC and aryl C fall in the
expected range.4a,14
The mechanism that could account for the different natures
of 4 and 5 is not obvious. The formation of an intermediate of
the type bis(carbene)PdX2, X ) acetate, halide (vide infra),
which could further rearrange to the cyclometalated product 5,
may be controlled kinetically when KOBut is used. In contrast,
use of NaH may result in the preferential formation of (carbene)-
PdX2, X ) acetate, which after cyclometalation could give 4
by dimerization. It is interesting to notice that attempts to
introduce the carbene 3a by the reaction of KOBut with the
imidazolinium salt in the presence of a Pd source gave
intractable mixtures. Further metalation of 5 by prolonged
heating or by carrying out the reaction under different stoichi-
ometries gave only reduced yields of 5.
Reaction of the NHC generated in situ from the salt 3b and
KOBut with Pd(tmed)(CH3)2 was critically dependent on the
reaction temperature. If the reaction mixture was kept below 0
°C, the isolated product was 6, in which the Pd is coordinated
by two monodentate NHC ligands and two methyls. This
complex was characterized spectroscopically, analytically, and
crystallographically. The 1H NMR data of 6 support a C2-
symmetric structure in solution (one singlet for Pd-CH3 and
two doublets for the diastereotopic CH(CH3)2 in addition to
signals due to the ligand backbone and aromatic protons). The
solution structure is maintained in the solid state, as is shown
in the ORTEP diagram of the molecule obtained from a single-
crystal X-ray diffraction study (Figure 3).
Figure 4. ORTEP representation of the structure of 7 showing
50% probability ellipsoids. H atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg) with estimated standard
deviations in parentheses: C1-Pd1 ) 2.022(6); C1-Pd1-C1′ )
180.000(1).
square-planar Pd. All Pd-C bond lengths are very similar, as
expected for the σ-donating NHCs and methyls, and are within
the range reported in the literature.14
Reaction of the NHC generated in situ from 3b and KOBut
with Pd(tmed)(CH3)2 at room temperature gave good yields of
the linear air-sensitive Pd(0) species 7. The structure of the
molecule is shown in Figure 4.
In the solid state 7 shows a C2h molecular symmetry and a
linear Pd(0) center. The N1-C1-N2 and N1′-C1′-N2′ planes
of the NHC heterocycles adopt an eclipsed conformation with
the bulky isopropyl substituents stacked on the same side of
the molecule. The Pd-C bond lengths are ca. 2.022 Å.
Complex 7 is the first structurally characterized Pd(0) NHC
complex with a saturated NHC backbone. Linear Pd(0) com-
plexes analogous to 7 with bulky imidazol-2-ylidene ligands
(unsaturated NHCs) have previously been reported.15 All adopt
a staggered conformation of the two NHC planes in order to
minimize steric interactions. The preferred eclipsed conforma-
tion of 7 in the solid state may be due to crystal-packing factors
and highlights the reduced steric congestion in this linear
electronically unsaturated molecule.
The N(1)-C(1)-N(2) plane of the NHC ligand forms a
dihedral angle of ca. 52° with the coordination plane of the
(14) Tulloch, A. A. D.; Winston, S.; Danopoulos, A. A.; Eastham, G.;
Hursthouse, M. B. Dalton Trans. 2003, 699.

Palladium Imidazolin-2-yl Complexes
Organometallics, Vol. 26, No. 23, 2007 5631
Scheme 4. Plausible Mechanism Leading to the Formation
of 8
(11)°) involving the tridentate species. Metalations leading to
bis-NHC “pincer” complexes have been reported for Rh and
Zr.16
Better insight into the mechanism of the formation of 6-8
was obtained by monitoring the thermolysis of 6 in d8-THF by
1H NMR spectroscopy. At room temperature 6 was cleanly
transformed to 7 and ethane. Thermolysis of 7 at 55-60 °C
gave the “pincer” complex 8. No other products (i.e., meth-
ylimidazolinium salts) were observed. However, conditions for
the clean monometalation of 7 have not been found.
It has been established on many occasions that a dominant
decomposition pathway of Pd(NHC) alkyls involves reductive
coupling of the alkyl and NHC ligands to form imidazolium
salts.3 This reaction has been studied by experimental and
computational methods for “unsaturated” NHCs.17 It is surpris-
ing that the reductive elimination of ethane responsible for the
conversion of 6 to 7 occurs in preference to imidazolinium salt
reductive elimination.
On the other hand, the preferred double metalation of aromatic
rings associated with the formation of 8 is also remarkable; 2,6-
diphenylpyridines have been bis-cyclometalated at the aromatic
rings by Au(III) following a transmetalation methodology.18 Bis-
cyclometalated phosphines and phosphites complexed to Pd are
not known, even though a limited number of examples with
iridium and platinum have been reported.19 Metalation of the
alkyl substituents of two different alkyl-NHC ligands coordi-
nated to the same Ir center has been reported by Nolan.19e
However, double metalation on the same NHC has not been
observed before. A plausible mechanism accounting for the
formation of products 6-8 may proceed via a sequence of Pd-
(0)-Pd(II)//Pd(II)-Pd(IV) or, more likely, by Pd(0)-Pd(II) and
σ-bond metathesis steps (see Scheme 4). The former sequence
has been postulated in the presence of co-oxidants.20
Figure 5. ORTEP representation of the structure of 8 showing
50% probability ellipsoids. H atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg) with estimated standard
deviations in parentheses: C1-Pd1 ) 1.901(3), C5-Pd1 ) 2.101-
(3), C14-Pd1 ) 2.092(3), C22-Pd1 ) 2.038(3); C1-Pd1-C22
) 176.06(11), C1-Pd1-C14 ) 77.59(11), C22-Pd1-C14 )
101.68(10), C1-Pd1-C5 ) 77.72(11), C22-Pd1-C5 ) 102.85-
(10), C14-Pd1-C5 ) 155.27(10).
Previous methods leading to linear Pd0(carbene)2 included
metal vapor synthesis,15a substitution of P(o-tol)3 from Pd(P(o-
15c,e
tol)3)2
by free NHC, and reaction of [Pd(allyl)Cl]2 with
sodium malonate in the presence of NHC.15b All require the
use of thermally stable NHC ligands, which is generally feasible
with imidazol-2-ylidenes and bulky imidazolin-2-ylidenes. The
synthesis of 7 constitutes a mild and clean method for the
reduction of Pd(II), compatible with sterically less hindered
saturated NHCs; however, its scope is still under investigation
and may be limited due to other competing reactions, as
exemplified below.
Reaction of the NHC generated in situ from 3b and KOBut
with Pd(tmed)(CH3)2 at 50 °C afforded good yields of the
complex 8 as yellow air-stable crystals. The identity of 8 was
established crystallographically. An ORTEP diagram of the
molecule is given in Figure 5.
Complex 8 is a square-planar Pd(II) complex in which
cyclometalation has taken place on the two aromatic substituents
of the same NHC ligand, leading to a unique, formally dianionic
tridentate “pincer”. The fourth coordination site of the Pd is
occupied by an ordinary monodentate NHC ligand. The triden-
tate “pincer” is almost planar, while the N3-C22-N4 plane
of the monodenate carbene forms an angle of ca. 81° with the
coordination plane. The Pd-CNHC bond length within the
“pincer” (1.901(3) Å) is shorter than that of the monodentate
NHC (2.038(3) Å). Furthermore, the N1-C1-N2 angle of the
tridentate NHC (113.1(2)°) is much larger than the correspond-
ing angle for the monodentate species (106.5(2)°), possibly due
to steric constraints. Distortions are also seen in the coordination
angles C1-Pd1-C5 (77.72(11)°) and C1-Pd1-C14 (77.59-
(ii) Alkoxyaryl-Substituted NHCs. Imidazolin-2-ylidene
complexes bearing alkoxy-substituted aromatic rings have only
(16) Rubio, R. J.; Andawan, G. T. S.; Bauer, E. B.; Hollis, T. K.; Cho,
J.; Tham, F. S.; Donnadieu, B. J. Organomet. Chem. 2005, 690, 5353.
(17) Graham, D. C.; Cavell, K. J.; Yates, B. F. Dalton Trans. 2006, 1768.
(18) Henderson, W. AdV. Organomet. Chem. 2006, 54, 207.
(19) (a) Dunina, V. V.; Gorunova, O. N. Russ. Chem. ReV. 2004, 73,
309. (b) Bedford, R. B.; Castillon, S.; Chaloner, P. A.; Claver, C.; Fernandez,
E.; Hitchcock, P. B. Ruiz, A. Organometallics 1996, 15, 3990. (c)
Mohammad, H. A. Y.; Grimm, J. C.; Eichele, K.; Mack, H.-G.; Speiser,
B.; Novak, F.; Quintanilla, M. G.; Kaska, W. C.; Mayer, H. A. Organo-
metallics 2002, 21, 5775. (d) Cheney, A. J.; McDonald, W. S.; O’Flynn,
K.; Shaw, B. L.; Turtle, B. L. J. Chem. Soc., Chem. Commun. 1973, 128.
(e) Scott, N. M.; Dorta, R.; Sevens, E. D.; Correa, A.; Cavallo, L.; Nolan,
S. P. J. Am. Chem. Soc. 2005, 127, 3516.
(15) (a) Arnold, P. L.; Cloke, F. G. N.; Geldbach, T.; Hitchcock, P. B.
Organometallics 1999, 18, 3228. (b) Caddick, S.; Cloke, F. G. N.;
Clentsmith, G. K. B.; Hitchcock, P. B.; McKerrecher, D.; Titcomb, L. R.;
Williams, M. R. W. J. Organomet. Chem. 2001, 617-618, 635. (c)
Gsto¨ttmayr, C. W. K.; Bo¨hm, V. P. W.; Herdtweck, E.; Grosche, M.;
Herrmann, W. A. Angew. Chem., Int, Ed. 2002, 41, 1363. (d) Konick, M.
M.; Guzei, I. A.; Stahl, S. S. J. Am. Chem. Soc. 2004, 126, 10212. (e)
Yamashita, M.; Goto, K.; Kawashima, T. J. Am. Chem. Soc. 2005, 127,
7294.
(20) Dick, A. R.; Hull, K. M.; Salford, M. S. J. Am. Chem. Soc. 2004,
126, 2300 and references cited therein.

5632 Organometallics, Vol. 26, No. 23, 2007
Stylianides et al.
Figure 6. ORTEP representation of the structure of 9d showing
50% probability ellipsoids. H atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg) with estimated standard
deviations in parentheses: Pd1-C18 ) 2.036(3), Pd1-C12 )
2.048(3), Pd1-C1 ) 2.053(3), Pd1-C34 ) 2.086(3); C18-Pd1-
C12 ) 178.53(10), C18-Pd1-C1 ) 98.64(10), C12-Pd1-C1 )
80.26(11) C18-Pd1-C34 ) 88.40(10), C12-Pd1-C34 ) 92.61-
(11), C1-Pd1-C34 ) 171.33(11).
Figure 7. ORTEP representation of the structure of 9e showing
50% probability ellipsoids. H atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg) with estimated standard
deviations in parentheses: Pd1-C21 ) 2.038(3), Pd1-C13 )
2.044(3), Pd1-C1 ) 2.054(4), Pd1-C20 ) 2.095(4); C21-Pd1-
C13 ) 176.17(14), C21-Pd1-C1 ) 103.88(14), C13-Pd1-C1
) 79.86(14), C21-Pd1-C20 ) 84.57(14), C13-Pd1-C20 )
91.80(14), C1-Pd1-C20 ) 169.63(14).
initial work by Herrmann and co-workers21 was focused on
reactive aryl iodides. More recent efforts have been extended
to the less reactive bromides and chlorides.22 There is no
generally applicable, high-TON and -TOF homogeneous catalyst
for the non- or deactivated aryl chlorides. The best phosphine-
containing system is based on PBut3.23 Furthermore, there is
still ongoing debate about the nature of the active species formed
from soluble Pd complexes in high-temperature Heck reactions
under catalytic conditions (nanoparticles, molecular underligated
species, or both).24
recently been reported in relation to the development of
ruthenium metathesis catalysts.8a The electron-releasing proper-
ties, the easily tunable bulk, and the possible hemilability of
the alkoxo substituents provide a wide scope of the alkoxo-
substituted NHCs ligands for catalyst tuning. Our initial results
in this area are reported here, focusing on the synthesis of
selected Pd complexes and their application as catalysts in the
Heck reactions. One feature that became apparent from our study
is the propensity of the ligands without ortho substituents on
the aromatic ring to undergo facile cyclometalation.
Reaction of Pd(tmed)(CH3)2 with the NHC generated in situ
from 3d,e and KOBut (vide supra) gave good yields of
complexes 9d,e, respectively. The 1H NMR spectra of the new
complexes suggest a low molecular symmetry and the presence
of one methyl group on Pd. In addition, signals associated with
two Pd-CNHC and one Pd-Caryl point to cyclometalation of
the NHC ligand. The structures of 9d,e were determined by
single-crystal X-ray diffraction. ORTEP diagrams of the com-
plexes are shown in Figures 6 and 7, respectively.
In view of the success of cyclometalated Pd phosphine and
phosphite complexes in various C-C coupling reactions,10 we
compared the activity of 4 and 9d,e in the Heck reaction of
aryl bromides and chlorides with n-butyl acrylate under different
conditions. Selected data are compiled in Table 1.
The data show that 4 and 9d,e are good catalysts for the
coupling of nonactivated and deactivated aryl bromides with
acrylates at low catalyst loading (1.5 mol %). With the activated
aryl chlorides the best conversions are observed at higher
temperatures (160 °C) and in Bu4NBr. Complexes 9d,e catalyze
the coupling of chlorobenzene in Bu4NBr, albeit in low yields.
In comparison to other NHC22a and phosphine23 catalysts, the
new complexes show inferior activity for the coupling of aryl
chlorides and operate under more forcing conditions.
Both complexes are square planar, the coordination sphere
comprising one singly metalated NHC, one monodentate NHC,
and one methyl group. The planes of the NHC and the
cyclometalated rings coincide with the coordination plane, while
the plane of the monodentate NHC forms an angle of ca. 67°
with the coordination plane. The Pd-CNHC and Pd-Caryl bond
lengths are virtually equal and are slightly shorter than the Pd-
Conclusions
Cmethyl bond length. There is no effect of the additional methoxy
group in 9e on the metrical data. In contrast to the case for 8,
complexes 9d,e do not undergo further metalation, even if they
are heated to 100 °C.
Palladium complexes with saturated NHCs bearing cyclom-
etalated aromatic substituents can be prepared under mild
conditions. The ease of cyclometalation was dependent on the
It is interesting to notice that in this reaction a Pd(0) product
analogous to 7 has not been observed. The reason for this may
be the increased electron-releasing nature of the alkoxy-
substituted aromatic rings, which is expected to destabilize lower
(i.e., Pd(0)) and stabilize higher oxidation states (i.e., Pd(II),
Pd(IV)). Therefore, the metalations leading to 9d,e and 8 are
probably different mechanistically.
(21) Herrmann, W. A.; Elison, M.; Fischer, J.; Ko¨cher, C.; Artus, G. J.
R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2371.
(22) (a) Selvakumar, K.; Zapf, A.; Beller, M. Org. Lett. 2002, 4, 3031.
(b) Fiddy, S. G.; Evans, J.; Neisius, T.; Newton, M. A.; Tsoureas, N.;
Tulloch, A. A. D.; Danopoulos, A. A. Chem. Eur. J. 2007, 13, 3652 and
references cited therein.
(23) Littke, A. F.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 6989.
(24) (a) Astruc, D. Inorg. Chem. 2007, 46, 1884 and references cited
therein. (b) de Vries, J. G. Dalton Trans. 2006, 421 and references cited
therein.
Catalytic Studies. The use of Pd-NHC complexes as
catalysts in the Heck reaction has been intensively studied. The

Palladium Imidazolin-2-yl Complexes
Organometallics, Vol. 26, No. 23, 2007 5633
Table 1. Comparable Activity Data in the Coupling of
Various Aryl Halides with n-Butyl Acrylate Catalyzed by the
Complexes Described in the Papera
chloride salts with triethyl orthoformate. Full experimental details
for each individual compound have been included as Supporting
Information. The imidazolinium salt 3h was prepared by cyclization
of N,N′-bis(2,4,6-trimethoxyphenyl)ethylenediamine (2h) with tri-
ethyl orthoformate in the presence of NH4BF4.
N,N′-Bis(aryl)oxalamides 1c-h. To a diethyl ether solution (200
mL) of substituted aniline (22.00 mmol) and Et3N (2.90 mL) was
added dropwise at 0 °C a solution of oxalyl chloride (0.90 mL,
10.00 mmol) in ether (50 mL). The mixture was then stirred at
room temperature overnight. The precipitate that formed was filtered
and triturated in methanol to remove triethylammonium chloride.
The solid was then filtered, washed with ether, and dried under
vacuum.
N,N′-Bis(aryl)ethylenebis(ammonium) Dichlorides 2c-h. These
compounds were prepared by the reduction of the oxalamides 1c-h
with LiAlH4. The oxalamide (8.10 mmol) was dissolved in THF,
and the solution was added dropwise to a precooled (0 °C) LiAlH4
(0.61 g, 16.00 mmol) suspension in THF (10 mL). The mixture
was refluxed overnight. After this mixture was cooled to 0 °C,
methanol was added dropwise until the effervescence ceased. The
mixture was further hydrolyzed with a solution of NaOH (2.0 g)
and MgSO4 (2.0 g) in water (10 mL). The resulting slurry was
filtered, and the solid was washed with THF. The filtrate was
evaporated, and the resulting oil was dissolved in methanol.
Concentrated HCl was then added, and the solvents were removed
to yield a solid which was collected, washed with acetone, and
dried under vacuum.
N,N′-Bis(2,4,6-trimethoxyphenyl)ethylenediamine (2h′). N,N′-
Bis(2,4,6-trimethoxyphenyl)oxalamide (3.40 g, 8.00 mmol) was
dissolved in THF, and to this was added a BH3·THF solution (10
mL). The mixture was refluxed overnight. Methanol was then added
dropwise, after cooling, until the bubbling ceased. The solvents
were removed, and methanol was added again and removed under
vacuum. This was repeated three times. The methanol-insoluble
solid was then filtered and dried under vacuum. Yield: 2.35 g (6.00
mmol), 75%. ES+ MS (m/z (%)): 391.9, 393.0, and 394.0 [M +
H]+. 1H NMR (300 MHz, d6-DMSO; δ): 9.30 (2H, s, -NH), 6.30
(4H, s, m-H aromatics), 6.10 (4H, s, CH2 ethylene), 3.90 (6H, s,
p-OCH3), 3.80 (12H, s, o-OCH3). 13C NMR data could not be
collected, as the compound was almost insoluble in all common
deuterated solvents.
1,3-Bis(aryl)imidazolinium Chlorides 3c-h. The salts 2c-h
(6.89 mmol) were suspended in triethyl orthoformate (5 mL) along
with formic acid (2 drops), and these suspensions were refluxed at
120 °C for 2 h. The mixtures were cooled to room temperature,
and ether was added. The precipitated products were filtered,
washed with ether, and dried azeotropically before storing in a
glovebox.
1,3-Bis(2,4,6-trimethoxyphenyl)imidazolinium Tetrafluorobo-
rate (3h′). N-N′-Bis(2,4,6-trimethoxyphenyl)ethylenediamine (2h′;
0.30 g, 0.74 mmol) and ammonium tetrafluoroborate (0.10 g, 0.90
mmol) were suspended in triethyl orthoformate (5 mL) and refluxed
overnight at 120 °C. The mixture was cooled to room temperature,
and the precipitated product was filtered, washed with ether, and
dried under vacuum. Yield: 0.10 g (0.20 mmol), 27%. ES+ MS
(m/z (%)): 403.2 and 404.2 [M]+. HRMS (ES+; m/z): calcd for
[C21H27N2O6]+, 403.1869; found, 403.1872. Anal. Calcd for C21H27-
BF4N2O6: C, 51.45; H, 5.55; N, 5.71. Found: C, 51.41; H, 5.57;
N, 5.70. 1H NMR (300 MHz, d6-DMSO; δ): 9.30 (1H, s,
imidazolinium H), 6.30 (4H, s, aromatic CH), 3.85 (4H, s, backbone
CH2), 3.80 (6H, s, p-OCH3), 3.35 (12H, s, o-OCH3). 13C NMR
data could not be collected, as the compound was almost insoluble
in all common deuterated solvents.
cata-
lyst
time
yield
(%)
aryl halide
C6H5Br
C6H5Br
C6H5Br
solvent temp (h)
base
1
2
3
4
5
6
7
8
9
4
9e
4
9e
4
9d
9e
4
dioxane 110
dioxane 110
18 NEt3
18 NEt3
90
99
96
99
90
99
99
50
70
38
84
79
0
NMP
NMP
NMP
NMP
NMP
DMF
140
140
140
140
140
140
18 NEt3
18 NEt3
18 NEt3
18 NEt3
18 NEt3
18 NEt3
18 NaOAc
18 NEt3
18 NaOAc
18 NaOAc
18 NEt3
18 NEt3
18 NBu3
18 NaOAc
18 NaOAc
C6H5Br
4-CH3C6H4Br
4-CH3C6H4Br
4-CH3C6H4Br
4-CH3COC6H4Cl
4-CH3COC6H4Cl
4
Bu4NBr 140
DMF 140
Bu4NBr 160
Bu4NBr 160
10 4-CH3COC6H4Cl 9d
11 4-CH3COC6H4Cl 9d
12 4-CH3COC6H4Cl 9e
13 C6H5Cl
14 C6H5Cl
15 C6H5Cl
16 C6H5Cl
17 C6H5Cl
4
NMP
DMF
DMF
140
140
160
9d
9d
9d
9e
15
25
35
25
Bu4NBr 160
Bu4NBr 160
a See the Experimental Section for details.
nature of the substitution of the aromatic ring. The ring
substitution and the coligands are important factors that
determine the nature of the products and the mechanism of the
cyclometalation. In fact, in one case cyclometalation was
preceded by the formation of an isolable Pd(NHC)2, suggesting
that the metalation step involved C-H oxidative addition. In
contrast, the isolation of cylometalated PdII(NHC)(CH3) com-
plexes when Pd(tmed)(CH3)2 was reacted with alkoxy-substi-
tuted NHCs suggests that the metalation in this case may involve
Pd(II)/Pd(IV) or σ-bond metathesis steps. Finally, the reaction
with Pd(OAc)2 may proceed via electrophilic attack or agostic
intermediates.25 The new cyclometalated complexes show good
activity in the Heck coupling of aryl bromides with acrylates
and low to moderate activity in the coupling of aryl chlorides
with acrylates.
Experimental Section
General Methods. Elemental analyses were carried out by the
London Metropolitan University microanalytical laboratory. All
manipulations were performed under nitrogen in a Braun glovebox
or using standard Schlenk techniques, unless stated otherwise.
Solvents were dried using standard methods and distilled under
nitrogen prior use. The light petroleum used throughout had a
boiling point of 40-60 °C.
The starting materials 2-methoxyaniline, 4-methoxyaniline, 2,4-
dimethoxyaniline, 2,4,6-trimethoxyaniline, 2-isopropoxyaniline, and
4-isopropoxyaniline were prepared by the reduction of the corre-
sponding nitro compounds by modification of literature procedures
(see the Supporting Information for full details).26-29 NMR data
were recorded on Bruker AV-300 and DPX-400 spectrometers,
operating at 300 and 400 MHz (1H), respectively. The spectra were
referenced internally using the signal from the residual protio solvent
(1H) or the signals of the solvent (13C).
General Method for the Synthesis of the Imidazolinium Salts
3c-h. The alkoxy-substituted imidazolinium salts 3c-g were
prepared by cyclization of the suitably substituted bis anilinium
(25) Davies, D. L.; Donald, S. M. A.; Macgregor, S. A. J. Am. Chem.
Soc. 2005, 127, 13754.
(26) Dubiel, S. V., Jr.; Zuffanti, S. J. Org. Chem. 1954, 19, 1359.
(27) Knolker, H. J.; Baumb, G.; Foitzika, H.; Goesmann, H.; Gonser,
P.; Jones, P. G.; Rottel, H. Eur. J. Inorg. Chem. 1998, 993.
(28) Kamer, P. C. J.; Nolte, R. J. M.; Drenth, W. J. Am. Chem. Soc.
1988, 110, 6818.
Bis(µ2-Acetato-κ2O,O′)bis{2-[3-(2-methylphenyl)imidazolin-
2-ylidene]-3-methylphenyl-κ2C,C′}dipalladium(II) (4). The imi-
dazolinium chloride 3a (0.25 g, 0.89 mmol), Pd(OAc)2 (0.20 g,
0.89 mmol), and NaH (0.24 g, 1.0 mmol) were placed in an ampule.
(29) Wolin, R. L.; Santillan, A., Jr.; Tang, L.; Huang, C.; Jiang, X.;
Lovenberg, T. W. Bioorg. Med. Chem. 2004, 12, 4511.

5634 Organometallics, Vol. 26, No. 23, 2007
Stylianides et al.
Toluene (15 mL) was added, the ampule was sealed under partial
vacuum, and the mixture was heated at 50 °C for 3 h. The resulting
suspension was filtered through Celite, and the filtrate was reduced
to ca. 5 mL and layered with petroleum ether to yield yellow
crystals. Yield: 0.28 g (0.68 mmol), 76%. Anal. Calcd for
C97H104N8O8Pd4: C, 60.19; H, 5.42; N, 5.79. Found: C, 60.33; H,
at -78 °C was added a solution of KOtBu (0.22 g, 1.99 mmol) in
THF (15 mL). The mixture was warmed to -10 °C and stirred for
2 h. It was then recooled to -78 °C and added to a cold (-78 °C)
solution of Pd(TMEDA)Me2 (0.20 g, 0.79 mmol) in THF (10 mL).
The reaction mixture was warmed to 25 °C and stirred at this
temperature overnight. The resulting deep red solution was
evaporated to dryness, the solid residue was extracted into petroleum
ether, and the extracts were filtered through Celite concentrated to
ca. 3 mL, and cooled to -35 °C to give dark red crystals. Yield:
0.42 g (0.58 mmol), 74%. Anal. Calcd for C42H52N4Pd: C, 69.93;
1
5.53; N, 5.87. H NMR (300 MHz, C6D5CD3; δ): 7.52 (2H, d, J
) 7.2 Hz, aromatic CH), 7.13 (2H, d, J ) 6.3 Hz, aromatic CH),
7.00-6.40 (10H, m, aromatic CH), 6.21 (2H, d, J ) 5.9 Hz,
aromatic CH), 3.40 (4H, t, J ) 10.2 Hz, 2 × backbone CH2), 2.68
(4H, t, J ) 10.2 Hz, 2 × backbone CH2), 2.31 (6H, s, CH3COO),
1.60 (6H, s, aromatic CH3), 1.20 (6H, s, aromatic CH3). 13C{1H}
NMR (75 MHz, C6D5CD3; δ): 198.25 (carbene C-Pd), 136.12
(aromatic C-Pd), 130.33 (aromatic C-N), 130.16 (aromatic C-N),
129.84, 129.06, 124.07, 123.75, 123.43, 120.67, 118.27, 115.82,
and 107.73 (aromatic carbons), 52.58 (acetate CCH3), 44.04
(backbone CH2), 41.72 (backbone CH2), 22.32 (acetate CH3), 16.28
(o-CH3), 15.94 (o-CH3).
1
H, 7.55; N, 7.77. Found: C, 69.86; H, 7.58; N, 7.79. H NMR
(300 MHz, C6D6; δ): 7.45 (2H, d, J ) 9.0 Hz, aromatic CH), 7.31
(4H, m, aromatic CH), 7.12 (2H, m, aromatic CH), 3.43 (2H, septet,
J ) 6.9 Hz, 2 × CH(CH3)2), 3.30 (4H, s, 2 × backbone CH2),
1.31 (12H, d, J ) 6.9 Hz, 2 × CH(CH3)2). 13C{1H} NMR (75 MHz,
C6D6; δ): 142.56 (aromatic ipso C-N), 131.29 (aromatic C-CH-
(CH3)2), 129.97, 128.29, 126.98, and 125.95 (4 × aromatic CH),
53.54 (backbone CH2), 28.29 (CH(CH3)2), 24.23 (CH(CH3)2).
[1,3-Bis(2-isopropylphenyl)imidazolin-2-ylidene]chloro{2-[3-
(2-isopropylphenyl)imidazolin-2-ylidene]-3-isopropylphenyl-
κ2C,C′}palladium(II) (5). The salt 3b (0.62 g, 1.80 mmol),
Pd(OAc)2 (0.20 g, 0.89 mmol), and KOtBu (0.22 g, 2.0 mmol) were
placed in an ampule. Toluene (15 mL) was added, and the ampule
was sealed under partial vacuum and heated at 50 °C for 3 h. The
resulting suspension was filtered through Celite, and the filtrate
was reduced to ca. 3 mL and layered with petroleum ether to yield
colorless crystals. Yield: 0.49 g (0.65 mmol), 73%. Anal. Calcd
for C42H51ClN4Pd: C, 66.92; H, 6.82; N, 7.43. Found: C, 66.90;
H, 6.81; N, 7.42. 1H NMR (300 MHz, C6D6; δ): 7.41 (3H, d, J )
8.7 Hz, aromatic CH), 6.49 (3H, dd, J ) 8.7, 2.7 Hz, aromatic
CH), 6.42 (3H, d, J ) 2.7 Hz, aromatic CH), 6.31 (6H, m, aromatic
CH), 3.63 (4H, s, 2 × backbone CH2), 3.41 (4H, s, 2 × backbone
CH2), 3.37 and 3.26 (2 × 2H, septet, J ) 6.9 Hz, 4 × CH(CH3)2),
1.59, 1.56, 1.48, and 1.42 (4 × 6H, d, J ) 6.9 Hz, 4 × CH(CH3)2).
13C{1H} NMR (75 MHz, C6D6): δ, 202.23 (carbene C-Pd), 198.40
(carbene C-Pd), 161.99 (aromatic C-Pd), 155.32 (aromatic
quaternary C-N), 149.25 (aromatic quaternary C-N), 142.61
(aromatic quaternary C-CH(CH3)2), 129.69, 139.86, 137.45,
132.44, 129.56, 128.27, 126.92, 126.54, 126.02, 123.50, and 121.37
(aromatic), 56.26 and 54.01 (backbone CH2), 52.89 and 49.16
(backbone CH2), 29.01 and 27.99 (CH(CH3)2), 27.31 and 26.40
(CH(CH3)2), 25.89 and 24.91 (CH(CH3)2), 24.51 and 23.85 (CH-
(CH3)2).
[1,3-Bis(2-isopropylphenyl)imidazolin-2-ylidene]{1,3-bis(6-
isopropylphenyl-2-yl)imidazolin-2-ylidene-κ3C,C′,C′′}palladium-
(II) (8). To a solution of 3b (0.57 g, 1.66 mmol) in THF (20 mL)
at -78 °C was added a solution of KOtBu (0.22 g, 1.99 mmol) in
THF (15 mL). The mixture was warmed to -10 °C and stirred for
2 h. It was then recooled to -78 °C and added to a cold (-78 °C)
solution of Pd(TMEDA)Me2 (0.20 g, 0.79 mmol) in THF (10 mL).
The reaction mixture was warmed to 50 °C and stirred overnight
at this temperature. The solvent was removed from the resulting
yellow solution, the solid residue was extracted with benzene, and
the extracts were filtered through Celite, concentrated, and layered
with petroleum ether to give yellow crystals. Yield: 0.39 g (0.54
mmol), 68%. Anal. Calcd for C42H50N4Pd: C, 70.13; H, 7.29; N,
7.79. Found: C, 70.35; H, 7.38; N, 7.91. 1H NMR (300 MHz, CD2-
Cl2; δ): 8.09 (2H, m, aromatic CH), 7.61 (6H, m, aromatic CH),
7.32 (2H, m, aromatic CH), 7.10 (2H, m, aromatic CH), 6.95 (2H,
m, aromatic CH), 4.50 (4H, s, 2 × backbone CH2), 4.36 (4H, s, 2
× backbone CH2), 3.82 (2H, septet, J ) 6.9 Hz, 2 × CH(CH3)2),
3.43 (2H, septet, J ) 6.9 Hz, 2 × CH(CH3)2), 1.62 (12H, d, J )
6.9 Hz, 2 × CH(CH3)2), 1.53 (12H, d, J ) 6.9 Hz, 2 × CH(CH3)2).
13C{1H} NMR (75 MHz, CD2Cl2; δ): 214.21 (carbene C-Pd),
157.91 (aromatic C-Pd), 152.27 (aromatic C-N), 145.72 (aromatic
C-N), 140.60 (aromatic C-CH(CH3)2), 130.29 (aromatic C-CH-
(CH3)2), 138.38, 129.56, 128.27, 126.92, 126.54, 126.02, 123.50,
and 121.37 (7 × aromatic CH), 55.25 (backbone CH2), 51.99
(backbone CH2), 28.17 (CH(CH3)2), 27.23 (CH(CH3)2), 24.79 (CH-
(CH3)2), 24.50 (CH(CH3)2).
Bis[1,3-bis(2-isopropylphenyl)imidazolin-2-ylidene]dimeth-
ylpalladium(II) (6). To a solution of the salt 3b (0.57 g, 1.66 mmol)
dissolved in THF (20 mL) at -78 °C was added a solution of KOt-
Bu (0.22 g, 1.99 mmol) in THF (15 mL). The mixture was warmed
to -10 °C and stirred for 2 h. It was then recooled to -78 °C and
added to a cold (-78 °C) solution of Pd(TMEDA)Me2 (0.20 g,
0.79 mmol) in THF (10 mL). The reaction mixture was warmed to
0 °C and stirred overnight at this temperature. The volatiles were
removed under reduced pressure, and the solid residue was extracted
in toluene. The extracts were filtered through Celite, concentrated
to ca. 5 mL, and layered with petroleum ether to give white crystals.
Yield: 0.49 g (0.65 mmol), 83%. Anal. Calcd for C44H58N4Pd: C,
[1,3-Bis(4-methoxyphenyl)imidazolin-2-ylidene]{2-[3-(4-meth-
oxyphenyl)imidazolin-2-ylidene]-5-methoxyphenyl-κ2C,C′}-
methylpalladium(II) (9d). To a solution of 3d (0.53 g, 1.66 mmol)
in THF (20 mL) at -78 °C was added a solution of KOtBu (0.22
g, 1.99 mmol) in THF (15 mL). The mixture was warmed to -10
°C and stirred for 2 h. It was then recooled to -78 °C and added
to a cold (-78 °C) solution of Pd(TMEDA)Me2 (0.20 g, 0.79 mmol)
in THF (10 mL). The reaction mixture was warmed to room
temperature and stirred overnight. Evaporation of the volatiles under
reduced pressure followed by extraction of the residue in benzene
filtration through Celite, concentration, and layering with petroleum
ether gave brown crystals. Yield: 0.35 g (0.51 mmol), 64%. Anal.
Calcd for C35H38N4O4Pd: C, 61.36; H, 5.59; N, 8.18. Found: C,
61.29; H, 5.65; N, 8.08. 1H NMR (300 MHz, C6D6; δ): 7.52 (4H,
d, J ) 8.7 Hz, aromatic CH), 6.83 (8H, m, aromatic CH), 6.51
(3H, m, aromatic CH), 4.10 (4H, s, 2 × backbone CH2), 3.97 (4H,
s, 2 × backbone CH2), 2.34 (6H, s, O-CH3), 2.01 (3H, s, O-CH3),
1.85 (3H, s, O-CH3), 0.41 (3H, s, Pd-CH3). 13C{1H} NMR (75
MHz, C6D6; δ): 154.32 (aromatic C-N), 153.02 (aromatic C-N),
132.31 (aromatic C-OCH3), 131.53 (aromatic C-OCH3), 130.08,
1
70.33; H, 8.05; N, 7.46. Found: C, 70.51; H, 8.12; N, 7.52. H
NMR (300 MHz, C6D6; δ): 7.53 (2H, d, J ) 9.0 Hz, aromatic
CH), 7.40 (4H, m, aromatic CH), 7.18 (2H, m, aromatic CH), 4.45
(4H, s, 2 × backbone CH2), 3.40 (2H, septet, J ) 6.9 Hz, 2 ×
CH(CH3)2), 1.58 (12H, d, J ) 6.9 Hz, 2 × CH(CH3)2), 0.42 (3H,
s, Pd-CH3). 13C{1H} NMR (75 MHz, C6D6; δ): 211.33 (carbene
C-Pd), 143.58 (aromatic quaternary C-N), 132.38 (aromatic
quaternary C-CH(CH3)2), 128.56, 128.01, 127.51, and 126.90 (4
× aromatic CH), 52.98 (backbone CH2), 27.35 (CH(CH3)2), 23.28
(CH(CH3)2), 18.55 (Pd-CH3).
Bis[1,3-bis(2-isopropylphenyl)imidazolin-2-ylidene]palladium-
(0) (7). To a solution of 3b (0.57 g, 1.66 mmol) in THF (20 mL)

Palladium Imidazolin-2-yl Complexes
Organometallics, Vol. 26, No. 23, 2007 5635
Table 2. Crystallographic Data for the Complexes Described in the Paper
4
5
6
7
8
9d
9e
chem formula
formula wt
cryst syst
space group
a/Å
C97H104N8O8Pd4
1935.48
monoclinic
P2/c
16.7336(18)
11.2402(12)
22.3834(16)
90
100.888(7)
90
4134.3(7)
4
C42H51ClN4Pd
753.72
triclinic
P1h
12.309(2)
12.969(2)
13.570(2)
115.906(4)
97.093(4)
95.803(5)
1904.3(5)
2
C44H58N4Pd
749.34
monoclinic
P2/c
22.4014(7)
8.0114(3)
24.4471(8)
90
114.925(2)
90
3978.8(2)
4
C42H52N4Pd
718.88
monoclinic
C2/c
23.344(2)
8.0765(4)
19.6151(18)
90
100.456(3)
90
3636.8(5)
4
C42H50N4Pd
717.26
triclinic
P1h
10.063(2)
12.528(2)
14.840(3)
71.63(3)
86.25(3)
89.04(3)
1771.7(6)
2
C35H38N4O4Pd
685.09
monoclinic
P21/c
10.4722(5)
16.0850(9)
18.4923(9)
90
92.636(2)
90
3111.6(3)
4
C39H46N4O8Pd
805.24
triclinic
P1h
11.5140(2)
17.8268(4)
18.5840(5)
87.6150(10)
86.8100(10)
85.3690(10)
3793.52(15)
4
b/Å
c/Å
R/deg
â/deg
γ/deg
V/Å3
Z
T/K
120(2)
0.921
65 825
9583
0.1604
0.0617
0.1224
120(2)
0.591
15 723
5080
0.0524
0.0777
0.2129
120(2)
0.282
39 959
9055
0.0608
0.0421
0.0927
120(2)
0.545
9163
120(2)
0.559
120(2)
0.642
120(2)
0.546
80 693
17 373
0.0722
0.0492
0.1046
µ/mm-1
no. of data
no. of unique data
Rint
52 811
8200
0.0755
0.0392
0.0858
17 972
7094
0.0410
0.0393
0.0780
2219
0.0718
0.0560
0.1212
final R(|F|) for
final R(F2)
129.56, 128.13, and 109.10 (aromatic CH), 112.35 (aromatic
C-Pd), 58.18 and 56.32 (backbone CH2), 43.76 (O-CH3), 18.38
(O-CH3).
X-ray Crystallography. A summary of the crystal data and data
collection and refinement parameters for compounds 4-8 and 9d,e
are given in Table 2.
[1,3-Bis(2,4-dimethoxyphenyl)imidazolin-2-ylidene]]{2-[3-
(2,4-dimethoxyphenyl)imidazolin-2-ylidene]-3,5-dimethoxyphe-
nyl-κ2C,C′}methylpalladium(II) (9e). This was prepared in a way
analogous to that for complex 9e using 3e (0.63 g, 1.66 mmol),
KOtBu (0.22 g, 1.99 mmol), and Pd(TMEDA)Me2 (0.20 g, 0.79
mmol) to yield colorless crystals. Yield: 0.45 g (0.56 mmol), 71%.
Anal. Calcd for C39H46N4O8Pd: C, 58.17; H, 5.76; N, 6.96.
Found: C, 58.10; H, 5.95; N, 6.98. 1H NMR (300 MHz, C6D6; δ):
7.38 (4H, d, J ) 8.7 Hz, aromatic CH), 6.47 (4H, dd, J ) 8.7, 2.7
Hz, aromatic CH), 6.39 (3H, d, J ) 2.7 Hz, aromatic CH), 3.64
(6H, s, 2 × O-CH3), 3.58 (10H, s, 2 × backbone CH2 and 2 ×
O-CH3), 3.41 (10H, s, 2 × backbone CH2 and 2 × O-CH3), 2.10
(3H, s, O-CH3), 1.76 (3H, s, O-CH3), 0.60 (3H, s, Pd-CH3).
13C{1H} NMR (75 MHz, C6D6; δ): 155.89 (aromatic C-N), 153.68
(aromatic C-N), 129.93 (aromatic C-OCH3), 129.28, 124.16,
103.38, and 99.54 (aromatic CH), 117.59 (aromatic quaternary
C-Pd), 59.12 and 52.51 (backbone CH2), 55.06 (O-CH3), 47.76
(O-CH3).
Catalytic Studies. The aryl halide (1.0 mmol), butyl acrylate
(25.6 mg, 2.0 mmol), base (1.2 mmol), diglyme (internal standard
1.0 µmol), and the catalyst (15.0 µmol) were placed in a glass
ampule equipped with a Youngs tap. Solvent (2.0 mL) was added,
and the ampule was sealed under partial vacuum. It was then placed
in a preheated silicon oil bath and heated for 18 h. After the reaction
mixture was cooled, it was passed through a short pad of silica
and subjected to gas chromatography. The identification was carried
out by GC/MS and comparison of the retention data with those for
authentic compounds.
All data sets were collected on a Enraf-Nonius Kappa CCD area
detector diffractometer with an FR591 rotating anode (Mo KR
radiation) and an Oxford Cryosystems low-temperature device
operating in ω scanning mode with ψ and ω scans to fill the Ewald
sphere. The programs used for control and integration were Collect,
Scalepack, and Denzo.30 The crystals were mounted on a glass fiber
with silicon grease, from Fomblin vacuum oil. All solutions and
refinements were performed using the WinGX package31 and all
software packages within. All non-hydrogen atoms were refined
using anisotropic thermal parameters, and hydrogens were added
using a riding model.
Acknowledgment. We thank the Department of Trade and
Industry (DTI) for the support of this work under the Manu-
facturing Molecules Initiative (MMI) project and the partners
associated with the MMI project (University of Liverpool,
University College, London, Johnson-Matthey Catalysts, As-
traZeneca, GlaxoSmithKline, and Pfizer). We also thank Dr.
Joseph A. Wright for assistance in crystallography.
Supporting Information Available: Text, tables, and figures
giving full details of ligand syntheses and of the X-ray crystal
structures, including complete tables of crystal data, atomic
coordinates, bond lengths and angles, and positional and anisotropic
thermal parameters; CIF files also give the X-ray structural data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
When Bu4NBr was used instead of solvent, it was added as a
solid to the reaction mixture (2.0 g) After the reaction was stopped,
the mixture was cooled, internal standard was added, and this
mixture was diluted with water and extracted with ether (3 × 5
mL). Analysis was carried out as above.
OM700603D
(30) Hooft, R. COLLECT; Nonius BV, 1997-2000. Otwinoski, Z.;
Minor, W. SCALEPACK, DENZO. Methods Enzymol. 1997, 276, 307.
(31) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 83.