Symmetrical non-chelating poly-N-heterocyclic carbenes

Article abstract of DOI:10.1039/c4ra12597e

A facile synthetic protocol, which utilized dehydrative condensation of phthalic anhydride and primary amines, for symmetrical non-chelating poly-N-heterocyclic carbene ligands was established. With the designed bi-functional building block, poly-NHC ligands having various denticities and geometries could be readily achieved by varying the structure of poly-amino compounds. Examination of catalytic activity of a series of poly-NHC-Ni complexes revealed the lack of electronic interference among NHC-metal units. This journal is

Full text of DOI:10.1039/c4ra12597e

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Symmetrical non-chelating poly-N-heterocyclic
carbenes†
Cite this: RSC Adv., 2014, 4, 62789
Ying-Hsuan Chen, Kai-En Peng, Gene-Hsiang Lee, Shie-Ming Peng
and Ching-Wen Chiu*
Received 17th October 2014
Accepted 7th November 2014
DOI: 10.1039/c4ra12597e
www.rsc.org/advances
A facile synthetic protocol, which utilized dehydrative condensation of method for highly symmetrical poly-benzimidazolium salts
phthalic anhydride and primary amines, for symmetrical non-chelating from assembly of simple molecular building parts. To show that
poly-N-heterocyclic carbene ligands was established. With the these poly-benzimidazolium salts can serve as precursors for
designed bi-functional building block, poly-NHC ligands having poly-NHCs, we have also prepared the corresponding poly-
various denticities and geometries could be readily achieved by nuclear metal complexes and investigated their application in
varying the structure of poly-amino compounds. Examination of hydrothiolation reaction.
catalytic activity of a series of poly-NHC–Ni complexes revealed the
In our previous study on the spiroborate-linked anionic bis-
40
lack of electronic interference among NHC–metal units.
NHC ligand (type D), we have shown that a linear twisted bis-
benzimidazolium salt could be readily prepared from reaction
of benzimidazolium derivatives and arylboronic acid. Unfortu-
nately, such assembly protocol was limited to the spiroborate
linkage that restricted the development of the catechol-
containing benzimidazolium molecule in poly-NHCs. To
access a large ligand library of highly symmetrical poly-NHC
ligands, we have designed a new bi-functional molecular
building block, which features an imidazole moiety on one side
of the benzene ring and a phthalic anhydride functionality on
the other side. This molecular LEGO brick can be assembled
with poly-amino compounds to yield the respective poly-
phthalimide derivatives, which are stable under normal pH
range or in the presence of bulky bases. The anticipated poly-
benzimidazolium salts can be obtained aer the second alkyl-
ation. With such method, the dimension, denticity, and
symmetry of the poly-benzimidazolium salts can be readily
controlled by the structure of poly-amino compounds. The
corresponding poly-nuclear metal complexes can then be
prepared aer metalation. Since all ylidene–metal bonds lie on
the symmetry axes of ligands, free rotation of chemical bonds
has no impact on the relative metal–metal distances and
orientations. Similar design of ligand has also been applied in
Being one of the most versatile ligands in stabilizing transition
metals and main group elements, N-heterocyclic carbene (NHC)
has shown tremendous impacts on both applied and funda-
1
–9
mental chemical researches. Nowadays, part of the interests
of NHC ligands has shied towards developments in chelating-
10–21
and bridging-type multi-topic NHC ligands.
One of the
recent advances of poly-NHCs is its application in constructing
the corresponding poly-nuclear metal complexes and organo-
22–28
metallic suparmolecules.
To extend the metal–carbene
chemistry to microporous organometallic polymers, non-
chelating rigid poly-NHCs are indispensable. As shown in the
works on porous NHC–metal polymers, the use of tris-NHCs
featuring symmetrical and rigid molecular backbones like
triptycene and triphenylene was imperative in realizing micro-
2
9,30
porous polymers.
However, syntheses of symmetrical tris-
31
32–34
35
NHCs such as type A and B
are not trivial (Scheme 1).
On the other hand, coupling of halobenzene and imidazoles has
proved to be useful in expanding the ligand library of poly-
NHCs (type C), which tend to form discrete molecular struc-
36–39
tures.
In order to broaden the application of poly-NHCs in
material chemistry, we decided to develop a general synthetic
41
the synthesis of dianionic Janus-type bis(maloNHC).
The synthesis of the molecular LEGO brick is straightfor-
ward. Starting from the commercially available 5,6-dimethyl
benzimidazole, the N-ethyl-benzimidazole-5,6-dicarboxylic
anhydride (1) could be prepared in multi-gram scale in four
Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd.,
Taipei, 10617, Taiwan. E-mail: cwchiu@ntu.edu.tw; Fax: +886-2-2363-6359; Tel:
+
886-2-3366-8191
†
Electronic supplementary information (ESI) available: Experimental details for steps (Scheme 2). Synthesis of the corresponding benzimida-
all reported compounds and X-ray crystallographic data of [2a][I]
2
, [2a-(NiCpI)
2
], zolium salt was found to be difficult, plausibly due to the
and [5][I]. CCDC 1020160–1020162. For ESI and crystallographic data in CIF or
presence of electron withdrawing acid anhydride group that
other electronic format see DOI: 10.1039/c4ra12597e
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yield of [2b][I]
2
was considerably lower than its para-isomer.
Single crystal X-ray diffraction analysis of [2a][I]2 further
conrmed the identity of the compound. As shown in Fig. 1, the
two benzimidazolium units are essentially co-planar with
distance between the two C1 carbons of nearly 19 A˚ .
Synthesis of tris-benzimidazolium salt ([3][I] ) was achieved
3
with the same procedure by utilizing 1,3,5-tri-(4-aminophenyl)
benzene as the assembly platform. The symmetric structure of
1
[3][I]
3
was conrmed with H NMR spectrum, which contained
only one set of signals for the benzimidazolium unit. The tri-
cationic molecule was further characterized with HRTOF-MS
spectrometry, where three signals were detected within one
amu, as expected for molecules bearing three positive charges.
Similar reaction was carried out using tetra-(4-aminophenyl)
methane as starting material for the preparation of three-
dimensional tetra-dentate ligand, the tetra-benzimidazolium
Scheme 1
4 4
salt ([4][I] ). Once again, the existence of [4][I] and its
1
symmetric nature were conrmed with HRTOF-MS and H NMR
spectroscopy, respectively. In all cases, the denticity and
geometry of the resulting poly-benzimidazolium salts were
governed by the structure of poly-amino compounds. This result
reemphasizes that this assembly procedure is an extreme
versatile method for the synthesis of poly-benzimidazolium
salts.
2 3 4
With compound [2a][I] , [3][I] , and [4][I] in hand, we then
Scheme 2 Synthesis of 1.
began to examine whether these phthalimide-containing ben-
zimidazolium salts could function as precursors for poly-NHCs.
Reaction of these benzimidazolium salts with nickelocene
resulted in isolation of poly-nuclear nickel carbene complexes
in moderate to good yield. The reactions were carried out under
reduced the nucleophilicity of the nitrogen atom. Therefore, the
second alkylation was carried out aer dehydrative condensa-
tion of 1 and poly-amino compounds to afford the anticipated
poly-benzimidazolium salts.
nitrogen atmosphere in DMF and THF mixed solvent system at
ꢀ
1
00 C. The brick-red compound [2a-(NiCpI)
2
], [3-(NiCpI)
3
], and
As shown in Scheme 3, linear bis-benzimidazolium salt ([2a]
[
4-(NiCpI)
4
] were characterized using multi-nuclear NMR spec-
[I]2) was synthesized in good yield from condensation of 1 and
1
troscopies and mass spectrometry. Two common H NMR
para-phenylenediamine, followed by alkylation with ethyl
1
spectral features for compound [2a-(NiCpI) ], [3-(NiCpI) ], and
2
3
iodide. The H NMR spectroscopic characterization of [2a][I]
2
2
+
[4-(NiCpI) ] were observed. The rst one was the disappearance
4
revealed that [2a] could freely rotate in solution leading to
detection of only one set of signals. The same assembly proce-
dure could also be applied to ortho-phenylenediamine to afford
1
+
of the characteristic H resonance of the [N–CH–N] proton. The
1
overall up-eld shied H NMR spectrum also suggested the
formation of nickel-NHC complexes. The second spectral
feature was the simplicity of the signals, which was indicative of
the presence of symmetrical molecule in solution. The forma-
tion of the Ni-ylidene bond was further veried with HMBC
2
the ortho-bis-benzimidazolium salt ([2b][I] ), which has also
been characterized by NMR and high-resolution mass spec-
troscopies. With two benzimidazolium groups in proximity, the
spectroscopy. The ylidene centre of compound [2a-(NiCpI) ], [3-
2
(
NiCpI) ], and [4-(NiCpI) ] were detected at 191.7 ppm, 191.5
3 4
ppm, and 191.6 ppm, respectively. These values were essentially
Fig. 1 Molecular structure of [2a][I]
Syntheses of poly-benzimidazolium salts. Reaction 50% probability level. Hydrogen atoms, counter anions and solvent
conditions: (i) DMF–CH molecules were omitted for clarity.
CN (v/v ¼ 1 : 1), reflux. (ii) EtI, DMF, reflux.
2
. Thermal ellipsoids were set at the
Scheme
3
3
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identical, suggesting the similarity in molecular electronics for
all poly-nuclear nickel complexes in the series. In addition, the
correlation between carbene carbon and Cp hydrogen in [3-
(
NiCpI) ] was identied in the HMBC spectrum, conrming the
3
2
formation of the C(NHC)–Ni bonds. The identity of [2a-(NiCpI) ]
was further corroborated with single crystal X-ray diffraction
˚
analysis. The observed Ni-ylidene bond distance of 1.881 A was
42,43
and the two nickel centres were separated by 22.7 A˚
typical,
with the two Ni–I bonds pointing at opposite direction (Fig. 2).
In addition to the poly-NHC nickel complexes, we have also
prepared the corresponding NHC–Rh(I) complexes. In all cases,
t
the poly-benzimidazolium salt was deprotonated with KO Bu in
THF/DMF, and was allowed to react with [Rh(COD)Cl] at
2
Fig. 3 Reaction time profile of the NHC–Ni catalysed hydrothiolation.
Reaction condition: 1-heptyne 1 eq., thiophenol 2.5 eq., Ni atom 3
ambient temperature. The formation of NHC–Rh(COD)I
complexes was conrmed with NMR and mass spectroscopies.
Once again, only one set of 1H resonance signals was observed
for all poly-NHC rhodium complexes. The 13C NMR signals of
the ylidene centre were detected at 204.2 ppm, 203.9 ppm, and
ꢀ
mol%, Et
3
N 6 mol%, CDCl
3
, 60 C.
this assembly approach, the size, denticity, geometry, and
dimension of symmetrical non-chelating poly-NHC ligands for
application in organometallic supramolecules can be readily
prepared from simple molecular building units. Extension of
this synthetic approach to amino acid containing NHC–metal
complexes and application of the reported poly-NHC in organ-
ometallic polymers are currently under investigation.
2
03.9 ppm for [2a-(Rh(COD)I) ], [3-(Rh(COD)I) ], and [4-
2 3
44–46
(Rh(COD)I) ], respectively.
Correlation between the ylidene
4
centre and hydrogen atoms of COD ligand was also recognized
in the HMBC spectrum of [2a-(Rh(COD)I) ], supporting the
2
formation of the anticipated complex.
To examine whether the assembly of multiple metal centres
affects the catalytic performance of each individual NHC-metal
unit, nickel catalysed hydrothiolation of alkyne was carried out Acknowledgements
47
using the aforementioned Ni–NHC complexes. In order to
evaluate the catalytic activity of Ni complex supported by the
phthalimide-containing NHC ligand, mononuclear nickel
complex, [5-(NiCpI)], was also prepared. As shown in Fig. 3, the
reaction time prole of the hydrothiolation showed insigni-
cant correlation of the product yield to the molecular backbone
of the poly-nuclear nickel complexes. In other word, the cata-
lytic activity of every [NiCpI] moiety in the poly-NHC nickel
complexes was independent of each other. This result suggests
that poly-NHC ligands reported in this work are great candi-
dates for construction of organometallic catalysts embedded
extended networks, in which the interference between catalytic
This work is supported by the Ministry of Science and Tech-
nology of Taiwan (NSC 101- 2113-M-002-013-MY2). C.-W. Chiu is
grateful to Kenda Foundation for the Golden-Jade Fellowship.
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