Hemilabile behavior of a thioether-functionalized N-heterocyclic carbene ligand

Article abstract of DOI:10.1039/b608325k

The truely hemilabile nature of a novel thioether-functionalized N-heterocyclic carbene ligand is demonstrated in a range of Pd(ii) complexes. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b608325k

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Hemilabile behavior of a thioether-functionalized N-heterocyclic
carbene ligand
Han Vinh Huynh,* Chun Hui Yeo and Geok Kheng Tan
Received (in Cambridge, UK) 12th June 2006, Accepted 19th July 2006
First published as an Advance Article on the web 3rd August 2006
DOI: 10.1039/b608325k
m/z = 219 for the [HL]+ fragment. In an attempt to prepare
The truely hemilabile nature of a novel thioether-functionalized
palladium(II) complexes, Pd(OAc)2 was reacted with 2 equivalents
of [HL]Br in CH3CN or DMSO at 90 uC. However, these
reactions gave rise to complex mixtures, from which only the
bis(carbene) complex cis-1 could be isolated in a low yield of 38%.
Apparently, a deprotonation of the benzylic protons of [HL]Br
under the relatively harsh conditions led to the formation of
complex mixtures.
N-heterocyclic carbene ligand is demonstrated in a range of
Pd(II) complexes.
In recent years, N-heterocyclic carbenes (NHCs) have been the
focus of intense research in organometallic chemistry and catalysis
due to their unique properties.1 Of particular interest is a donor-
functionalization of NHC ligands at their nitrogen atoms, since it
offers both the possibility for hemilabile coordination as well as the
opportunity to immobilize2 the resulting catalysts on polymer
resins. To this point, several complexes with N-, O- and P-donor
functionalized NHCs have been investigated.3 Examples of NHCs
functionalized with a softer sulfur atom are surprisingly rare, and
to the best of our knowledge, limited to two thiolate-NHCs4 and
only one thioether-NHC.5 Although many of the these examples
are potentially hemilabile ligands, no such behavior has been
reported to date.
Consequently, the milder Ag–carbene transfer method was
explored.6 The reaction of Ag2O with 2 equivalents of [HL]Br in
CH2Cl2 afforded Ag–carbene species, which were directly filtered
into a solution of [PdBr2(CH3CN)2]. The carbene transfer onto
palladium readily occurs at ambient temperature and is accom-
panied by precipitation of AgBr, which can be easily removed. We
found that stoichiometry control is crucial for this reaction. Thus a
L : Pd ratio of 2 : 1 affords a mixture of the isomeric bis(carbene)
complexes cis-1, trans-anti-1 and trans-syn-1 (Scheme 2). Due to
different solubilities, cis and trans isomers can be separated by
washing with acetone. The remaining insoluble yellow powder
(53%) contains trans-anti-1 and trans-syn-1, which cannot be
further separated. 1H and 13C{1H} NMR spectra in CDCl3 of the
latters show two sets of signals with Ccarbene resonances at 169.9
and 169.8 ppm corresponding to the syn and anti isomers,
respectively. Removal of the solvent from the acetone solution and
subsequent recrystallization from CHCl3 yielded an off-white
With this contribution, we present a versatile synthetic route to
the thioether-functionalized imidazolium salt [HL]Br and a study
on the hemilabile coordination behavior of its corresponding
NHC. Ligand precursor [HL]Br was prepared by N-alkylation of
N-methylimidazole with 2-methylmercaptobenzyl bromide, which
in turn was obtained in a four-step procedure from commercially
available thiosalicyclic acid (Scheme 1). The straightforward
synthesis affords [HL]Br in a very good overall yield of 64%
based on thiosalicyclic acid.
1
powder of cis-1 (y45%). Its H NMR spectrum in CD3CN is
The formation of [HL]Br was corroborated by a characteristic
downfield signal in the 1H NMR spectrum at 10.37 ppm for
the NCHN proton and a base peak in the ESI mass spectrum at
dominated by broad signals indicating a restricted rotation of the
Pd–Ccarbene bonds in the cis isomer. 13C{1H} NMR data, on the
other hand, could not be obtained due to insufficient solubility.
However, an X-ray diffraction analysis on single crystals of the
solvate cis-1?2DMF,{ obtained by vapor diffusion of diethyl ether
into a concentrated DMF solution, confirms the cis arrangement
of the NHC ligands in the essentially square planar complex with
the bulky methylmercaptobenzyl groups anti to each other (Fig. 1).
In order to evaluate whether the thioether function can
coordinate to the Pd(II) center, the L : Pd ratio was changed to
1
1 : 1. The H NMR spectrum of the reaction product in CDCl3
shows very broad signals at ambient temperature. Upon cooling to
230 uC, some signals sharpen, and two doublets at 5.00 and
5.85 ppm are observed for the non-equivalent benzylic protons,
which indirectly indicates a coordination of the sulfur function and
thus the formation of the targeted complex 2. In addition, two sets
of signals are observed for the N–CH3 and the S–CH3 groups
pointing to a rather complex mixture (vide infra), although
elemental analysis results are found to be close to the expected
values. An X-ray diffraction study on single crystals obtained from
vapor diffusion of diethyl ether into a DMF solution finally
Scheme 1 Synthetic pathway for the preparation of [HL]Br.
Department of Chemistry, National University of Singapore, 3 Science
Drive 3, 117543, Singapore, Singapore. E-mail: chmhhv@nus.edu.sg;
Fax: +65 67791691; Tel: +65 65152670
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Fig. 2 Molecular structure of 2 showing 50% probability ellipsoids.
˚
Selected bond lengths [A] and angles [u]: Pd1–C1 1.971(3), Pd1–S1
2.3079(3), Pd1–Br1 2.4508(4), Pd1–Br2 2.4792(4), C1–N1 1.343(4), C1–N2
1.335(4); C1–Pd1–S1 93.65(9), C1–Pd1–Br1 86.53(8), S1–Pd1–Br2
86.94(2), Br1–Pd1–Br2 93.070(13), N1–C1–N2 105.8(3).
of 65.155(77)u between the carbene ring plane and the PdCSBr2
coordination plane. The longer Pd–Br2 bond [2.4792(4) vs.
˚
2.4508(4) A for Pd–Br1] trans to the carbene reflects its strong
trans influence.
We anticipated that complex 2 undergoes two dynamic
processes in solution. The first process involves flipping of the
seven-membered ring, which slows down upon cooling, giving rise
to the inequivalent benzylic protons observed at low temperature.
The second process involves reversible de- and recoordination of
the sulfur donor in a hemilabile fashion. Decoordination results in
a formally unsaturated metal center, which stabilizes itself upon
dimerization. The two dynamic processes are in equilibrium with
Scheme 2 Synthetic pathway for the preparation of palladium(II) NHC
complexes.
1
at least 3 species (Scheme 2), which may explain the complex H
NMR spectrum.
To confirm the feasibilty of this proposal and thus the truely
hemilabile behavior of the thioether, we added one equivalent
(based on Pd) of PPh3 to the mixture in CH3CN. The stronger
phosphine ligand is expected to cleave both the dimeric complex as
well as the weaker Pd–S(Ar)R bond in 2. Indeed, the addition of
PPh3 leads to a clean and well resolved 1H NMR spectrum
corroborating the formation of the mixed NHC–phosphine
complex 3 as the sole product. Two doublets are observed at
5.76 and 4.71 ppm, respectively, for the two inequivalent benzylic
protons with a geminal coupling constant of 2J(HH) = 14.3 Hz. This
observation is in line with a hindered rotation of the Pd–Ccarbene
bond mainly due to the steric bulk of the phosphine ligand and
indirectly confirms the cis arrangement of the NHC and PPh3
ligands. The 13C{1H} NMR signal for the carbenoic carbon
appears at 162.7 ppm and the phosphine donor resonances at
27.13 ppm in the 31P NMR spectrum.
Fig. 1 Molecular structure of cis-1 showing 50% probability ellipsoids.
˚
Selected bond lengths [A] and angles [u]: Pd1–C1 1.982(3), Pd1–C13
1.992(3), Pd1–Br1 2.4856(4), Pd1–Br2 2.4785(3), C1–N1 1.350(4), C1–N2
1.348(4), C13–N3 1.347(4), C13–N4 1.357(4); C1–Pd1–C13 92.84(13),
C1–Pd1–Br2 86.57(9), C13–Pd1–Br1 87.06(9), Br1–Pd1–Br2 93.539(16),
N1–C1–N2 104.6(3), N3–C13–N4 104.8(3).
confirmed the identity of complex 2,{ and its solid state molecular
structure is depicted in Fig. 2. The complex adopts a distorted
square planar geometry with the carbene and sulfur donors in cis
position resulting in a seven-membered chelate ring, which is in a
boat-like conformation. It is worth mentioning, that the
coordination of the relatively weakly donating thioether does not
require abstraction of a bromo ligand (e.g. with Ag salts) in order
to generate a more Lewis-acidic palladium center. The coordina-
tion of the sulfur atom leads to a significantly small dihedral angle
Single crystals obtained from a saturated CD3CN solution were
subjected to an X-ray diffraction analysis, and the molecular
structure of 3 is shown in Fig. 3.{ The metal center is coordinated
by one NHC, one phosphine and two bromo ligands in a distorted
square planar fashion. The former two are found in a cis
arrangement, which is thermodynamically favored.7 Furthermore,
it can be clearly seen that the steric bulk of the phosphine ligand
prevents a rotation of the Pd–Ccarbene bond.
3834 | Chem. Commun., 2006, 3833–3835
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Dc = 2.223 g cm21, m (Mo-Ka) = 6.928 mm21, 9558 reflections measured,
3324 unique (Rint = 0.0232) which were used in all calculations. The final
wR2 was 0.0708 (all data).
3: C30H29Br2N2PPdS, M = 746.80, monoclinic, a = 9.1662(7), b =
3
˚
˚
15.3599(12), c = 20.4797(15) A, b = 91.949(2)u, U = 2881.7(4) A ,
T = 223(2) K, space group P21/c (no. 14), Z = 4, Dc = 1.721 g cm21
,
=
m (Mo-Ka) = 3.568 mm21, 20290 reflections measured, 6595 unique (Rint
0.0385) which were used in all calculations. The final wR2 was 0.0929 (all
data).CCDC 610655–610657. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b608325k
{ Spectroscopic data: [HL]Br: 1H NMR (300 MHz, CDCl3): d 10.37 (s, 1 H,
NCHN), 7.65 (dd, 3J(H,H) = 7.5 Hz, d, 4J(H,H) = 1.2 Hz, 1 H, Ar–H), 7.43–
7.22 (m, 5 H, Ar–H), 5.64 (s, 2 H, CH2), 4.10 (s, 3 H, NCH3), 2.50 (s, 3 H,
SCH3). 13C{1H} NMR (75.47 MHz, CDCl3): d 138.4 (s, NCN), 137.4,
131.2, 130.7, 130.5, 127.0, 126.2 (s, Ar–C), 123.4, 121.8 (s, NCH), 51.1 (s,
CH2), 36.8 (s, NCH3), 16.1 (s, SCH3). cis-1: 1H NMR (300 MHz, CD3CN):
d 7.36–6.82 (m, br, 12 H, Ar–H), 5.45 (s, br, 4 H, CH2), 3.87 (s, br, 6 H,
NCH3), 2.55 (s, br, 6 H, SCH3). 2: 1H NMR (300 MHz, CDCl3): d 7.50–
6.54 (m, br, 6 H, Ar–H), 5.93 (s, br, 2 H, CH2), 4.04 (s, br, 3 H, NCH3),
2.83 (s, br, 3 H, SCH3). 3: 1H NMR (300 MHz, CDCl3): d 7.66–7.05 (m,
19 H, Ar–H), 6.56 (d, 3J(H,H) = 1.9 Hz, 1 H, CH), 6.40 (d, 3J(H,H) = 1.9 Hz,
1 H, CH), 5.76 (d, 2J(H,H) = 14.3 Hz, 1 H, CH2), 4.71 (d, 2J(H,H) = 14.3 Hz,
1 H, CH2), 3.61 (s, 3 H, NCH3), 2.42 (s, 3 H, SCH3). 31P NMR (121 MHz,
CDCl3): d 27.1 (s, 1 P, PPh3). 13C{1H} NMR (75.47 MHz, CDCl3): d 162.7
(s, NCN), 137.5 (s, Ar–C), 134.3 (d, 2/3J(P,C) = 11.0 Hz, Ar–C), 132.3 (s, Ar–
C), 131.2 (d, 1J(P,C) = 35.7, Ar–C), 131.0 (d, 4J(P,C) = 2.7 Hz, Ar–C), 130.2,
Fig. 3 Molecular structure of 3 showing 50% probability ellipsoids.
˚
2/3
Selected bond lengths [A] and angles [u]: Pd1–C1 1.986(3), Pd1–P1
129.5 (s, Ar–C), 128.5 (d,
121.3 (s, Ar–C), 51.4 (s, CH2), 37.9 (s, NCH3), 16.4 (s, SCH3).
J
= 11.0 Hz, Ar–C), 126.7, 126.3, 122.8,
(P,C)
2.2642(8), Pd1–Br1 2.4775(4), Pd1–Br2 2.4785(3), C1–N1 1.349(4), C1–N2
1.350(4); C1–Pd1–P1 92.18(9), C1–Pd1–Br1 86.41(9), P1–Pd1–Br2
91.03(2), Br1–Pd1–Br2 90.763(15), N1–C1–N2 105.1(3).
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In conclusion, we have presented a straightforward synthesis of
the thioether-functionalized imidazolium salt [HL]Br. This mod-
ular synthetic route allows the introduction of a wide range of
alkyl substituents at the sulfur atom, which offers the possibility to
fine-tune both electronic and steric factors of the sulfur donor. We
have shown that palladium(II) complexes derived from [HL]Br are
easily accessible using the Ag–carbene transfer method. In our
case, the L : Pd ratio determines either the formation of dicarbene
(1) or monocarbene (2) complexes. Treatment of the latter with a
stronger phosphine ligand cleaves the Pd–S(Ar)R bond forming
complex 3, which demonstrates a truly hemilabile behavior of the
NHC ligand L. Studies on the catalytic activities of the complexes
and extension of the methodology to other donor-functionalized
NHCs are under way.{
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Notes and references
{ Crystal data: cis-1?2DMF: C30H42Br2N6O2PdS2, M = 849.04, triclinic,
˚
a = 10.0059(6), b = 11.4835(7), c = 16.1385(10) A, a = 76.031(1), b =
3
˚
78.306(2), c = 84.751(1)u, U = 1760.40(19) A , T = 223(2) K, space group
P1 (no. 2), Z = 2, Dc = 1.602 g cm21, m (Mo-Ka) = 2.951 mm21, 12426
¯
5 H. Seo, H.-J. Park, B. Y. Kim, J. H. Lee, S. U. Son and Y. K. Chung,
Organometallics, 2003, 22, 618.
6 H. M. J. Wang and I. J. B. Lin, Organometallics, 1998, 17, 972.
7 H. V. Huynh, Y. Han, J. H. H. Ho and G. K. Tan, Organometallics,
2006, 25, 3267.
reflections measured, 8012 unique (Rint = 0.0269) which were used in all
calculations. The final wR2 was 0.1006 (all data).
2: C12H14Br2N2PdS, M = 484.53, triclinic, a = 8.6338(5), b = 9.6410(6),
˚
c = 9.7985(10) A, a = 107.453(1), b = 103.622(1), c = 101.368(1)u,
3
U = 723.96(8) A , T = 223(2) K, space group P1 (no. 2), Z = 2,
˚
¯
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 3833–3835 | 3835