Highly modular access to functionalised metal-carbenes via post-modifications of a single bromoalkyl-substituted NHC-Pd(ii) complex

Article abstract of DOI:10.1039/c2cc33789d

The synthesis of a bromopropyl-substituted NHC-Pd(ii) complex, which can undergo exemplary and versatile 2nd and 3rd generation post-modifications easily affording 7 new functionalised NHC complexes, is demonstrated.

Full text of DOI:10.1039/c2cc33789d

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Highly modular access to functionalised
metal-carbenes via post-modifications of a single
bromoalkyl-substituted NHC–Pd(II) complex†‡
Cite this: Chem. Commun., 2013,
4
9, 4244
Received 26th May 2012,
Accepted 19th July 2012
Han Vinh Huynh* and Qiaoqiao Teng
DOI: 10.1039/c2cc33789d
www.rsc.org/chemcomm
The synthesis of a bromopropyl-substituted NHC–Pd(II) complex,
which can undergo exemplary and versatile 2nd and 3rd genera-
tion post-modifications easily affording 7 new functionalised NHC
complexes, is demonstrated.
1
–3
The rich chemistry of N-heterocyclic carbenes (NHCs)
does
4
not only stem from their strong s-donating ability, but also
from the ease of their modifications via the introduction of
various N-substituents. By doing so, the sterical demand and
the electronic properties of the resulting ligand can be tuned,
and perhaps more importantly, functional groups can be Scheme 1 Synthesis of ligand precursor A and complex precursors 1–4.
introduced. Generally, functionalised NHC complexes are
5
–9
obtained via metallation of prefunctionalised azolium salts.
In this approach, the functional group may interfere negatively Pd(OAc)
2
/NaBr gave rise to an ester-functionalised Pd–NHC
with the coordination process leading to the undesired formation dimer 1, which has been fully characterised by spectroscopic
of by-products and lower yields. Furthermore, versatility and and spectrometric methods including single crystal X-ray
modularity are limited, and only a small number of complexes diffraction (vide infra). Apparently, one equivalent of acetate
can be obtained from a single functionalised ligand precursor. deprotonated the benzimidazolium salt, while the 2nd equivalent
N
Although, the majority of transition metal carbenes are usually underwent a S 2 reaction with the bromopropyl arm forming the
stable, a highly versatile post-functionalisation strategy has not ester function. In order to prevent such interferences the milder
12,13
been reported thus far. As our contribution to this field we herein Ag-carbene transfer
report the synthesis of a bromopropyl-substituted NHC–Pd(II) salt A with Ag O and subsequent transfer to [PdBr
complex, which can undergo exemplary post-modifications easily the desired product 2 in 86% yield. The dissolution of complex 2
affording 7 new functionalised NHC complexes. in CH CN leads to the formation of the mononuclear acetonitrile
was investigated. Indeed, the reaction of
2
2
(CH CN) ] gave
3
2
3
The suitable ligand precursor A that contains a bromopropyl complex 3. Similarly dimeric 2 can undergo bridge-cleavage
N-substituent can be prepared by reacting benzylbenzimidazole reaction with pyridine affording the more stable pyridine adduct
1
0
with an excess of 1,3-dibromopropane (Scheme 1). It was 4 (Scheme 1), which was anticipated to be better suitable for post-
anticipated that the flexible bromoalkyl substituent could be modifications.
used to install a variety of functional groups via nucleophilic
substitution reactions (e.g. S 2) after NHC-complex formation. respective ester- and azido-functionalised NHC complexes 5
Initial attempts to synthesise a dimeric monocarbene–Pd(II) species and 6 in high yields without affecting the bromido ligands
3
The treatment of complex 4 with KOAc or NaN gave the
N
11
according to a previously published standard procedure using (Scheme 2). Notably, possible olefinic byproducts due to base-
catalyzed HBr elimination of the bromopropyl sidearm were
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
17543, Singapore. E-mail: chmhhv@nus.edu.sg; Fax: +65 6779 1691;
Tel: +65 6516 2670
This article is part of the ChemComm ‘Emerging Investigators 2013’ themed
issue.
not observed. Complex 5 can also be obtained by bridge-
cleavage reaction of dimer 1 with pyridine. Reaction with NaI
on the other hand is expected to affect both bromo-substituents
as well as the bromido ligands. In order to avoid halide
scrambling in this reaction, 10 equiv. of NaI was added to 4,
which cleanly afforded the Pd(II)–diiodido complex 7 bearing an
1
†
‡
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c2cc33789d
4
244 Chem. Commun., 2013, 49, 4244--4246
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a
Table 1 Comparison of a-CH
2
and Ccarbene NMR resonances
1
13
Complex
d ( H)
d (
C
carbene
)
Functional group
1
4.26
3.76
3.65
3.67
4.31
3.60
3.43
3.60
3.28
3.14
3.05
160.6
/
OAc
Br
Br
Br
OAc
b
d
2
3
c
161.8
164.6
164.3
164.4
163.3
163.0
163.9
162.9
174.8
4
5
6
7
8
9
N
I
3
N
3
SCN
SAc
1
1
0
1
À
S
a
d
b
c
Measured in CDCl
3
.
6 3
Measured in d -DMSO. Measured in CD CN.
The Ccarbene signal could not be resolved.
Scheme 2 Post-functionalisations of complexes 4 and 7.
iodo-functionalised NHC. The combination of a better iodo
leaving group at the sidearm with stronger coordinating iodido
ligands makes 7 an even better precursor for (here 2nd generation)
post-modifications. Thus, its S
NaN , KSCN and KSAc yielded three new derivatives bearing azido
8), thiocyanato (9) and thioester (10) functional groups. Notably,
complexes 9 and 10 are not obtainable via palladation of the
N
2 reactions with the nucleophiles
3
(
1
4
respective pre-functionalised azolium salts. In this respect, we
have previously reported reactions of thioester-functionalised
imidazolium and benzimidazolium salts with Pd(OAc) that
2
directly give rise to dimeric thiolato–NHC complexes via in situ
hydrolysis of the thioester groups. This fact highlights the
importance of the current post-modification protocol, which
1
5
16
Fig. 1 Molecular structures of complexes 1, 3, 7, 9 and 11 showing 50%
probability ellipsoids; hydrogen atoms are omitted for clarity. ‘‘A’’ letter in the
atom labels for complex 1 indicates atoms at equivalent positions (Àx, 1Ày, Àz).
not only complements the traditional methodology, but also their inductive effects. The same trend can be observed for their
provides functionalised NHC compounds that are otherwise not respective mononuclear pyridine adducts 4 and 5. The reduced
accessible.
ÀI effect of the azido group in 6 and 8, on the other hand, leads
It is conceivable that the functional groups in complexes 8–10 to an upfield shift. The identical a-CH
2
chemical shift for
could be subjected to a 3rd generation post-functionalisation. To complexes 6 and 8 also reveals that substitution of bromido
demonstrate such feasibility, complex 10 was chosen for a base- versus iodido ligands has no significant effect on the post-
assisted hydrolysis reaction, which led to the formation of the modifications. Overall, the negative inductive effects of the
anticipated thiolato-donor functionalised NHC complex-dimer functional groups investigated here can be ranked in the order
1
1 in a good yield.
SAc o SCN o I o N o Br o OAc based on the comparison of
3
1
Further explorations along this line could include copper- the a-CH H NMR signals of their respective complexes.
2
1
3
catalyzed azide–alkyne cycloaddition (Click) reactions of the
The C NMR signals for the carbene atom of the pyridine
azido–NHC complex 8 to form triazole NHC-complexes, or a complexes 4–10 are found in the narrow range of 162.9–164.6
Staudinger reduction of 8 to form a NHC complex bearing an ppm. Those of the complexes 1 and 3 are shifted upfield
amino group. Complex 9 could be hydrolyzed to a thiocarbamate indicating more Lewis acidic metal centres, while that of the
in the Riemschneider thiocarbamate synthesis.
dimer 11 is found downfield at 174.8 ppm due to the electron-
1
6
Apart from the spectroscopic signatures of the functional rich thiolato donors.
group itself, such post-modifications can be best traced by
monitoring the H NMR signal of the a-methylene group with and 11 have also been determined by single crystal X-ray
respect to the reactive site. Other resonances of the ligands diffraction and are depicted in Fig. 1. As expected, complex 1
The solid state molecular structures of complexes 1, 3, 7, 9
1
remain largely unaffected by the post-modifications.
is a dimeric monocarbene–Pd(II) complex with two bridging
H NMR bromido ligands. The molecule lies at an inversion centre.
1
Table 1 summarizes and compares the a-CH
2
resonances for complexes 1, 2 and 4–10. Comparing those of The coordination sphere at each square planar Pd centre is
the dimers 1 and 2 reveals that the ester group in 1 is more completed by an ester–NHC and a terminal bromido ligand.
deshielding than the bromo substituent in 2, which reflects The two alkylester N-substituents point in different directions
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resulting in an anti-arrangement with respect to the coordination spacers as well as to other metal centres, which is currently
plane. The carbene plane deviates from the coordination plane ongoing in our laboratories, will result in a large library of new
with a dihedral angle of B831.
functionalised NHC complexes with various potential applications.
We thank the National University of Singapore and the
Apart from their different functional groups, the mono-
nuclear complexes 3, 7 and 9 have a very similar structure. All Singapore Ministry of Education for financial support (WBS
contain a square planar Pd(II) centre that is coordinated by one R-143-000-407-112). Technical support from staff at the
functionalised benzimidazolin-2-ylidene, two trans-halido and an CMMAC of our department is appreciated. In particular, we
acetonitrile (3) or a pyridine (7 and 9) ligand. As expected the thank Ms Geok Kheng Tan, Ms Yimian Hong and Prof. Lip Lin
carbene planes are oriented almost perpendicularly to the coordina- Koh for determining the X-ray molecular structures.
tion planes by angles of B751 (3) and B821 (7 and 9), respectively.
Notably, there is no correlation between the C–X bond length (X =
Notes and references
functional group) and the ÀI effects of the functional groups.
1
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3
2
3
4
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2
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5
6
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2
2
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2
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substituted NHC complex that provides easy and efficient access to
a range of 2nd and 3rd generation functionalised NHC complexes
via a versatile post-modification strategy. It is feasible that such a
strategy will be useful in the anchoring of NHC based complexes
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1
1
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246 Chem. Commun., 2013, 49, 4244--4246
This journal is c The Royal Society of Chemistry 2013