Planar tris-N-heterocyclic carbenes

Article abstract of DOI:10.1039/c3cc43278e

To further explore the coordination chemistry of poly-carbene complexes, two triphenylene-based planar tridentate N-heterocyclic carbenes and their corresponding tri-gold complexes were synthesized. Molecular structures of the tert-butyl substituted tritopic free carbene and the tri-nuclear gold complex were determined experimentally. A silver-dicarbene organometallic polymer was also prepared with the newly synthesized tris-NHC.

Full text of DOI:10.1039/c3cc43278e

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Planar tris-N-heterocyclic carbenes†
Yun-Ting Wang, Meng-Ting Chang, Gene-Hsiang Lee, Shie-Ming Peng and
Ching-Wen Chiu*
Cite this: Chem. Commun., 2013,
49, 7258
Received 3rd May 2013,
Accepted 17th June 2013
DOI: 10.1039/c3cc43278e
www.rsc.org/chemcomm
To further explore the coordination chemistry of poly-carbene been reported. Since intermolecular p–p interactions could be
complexes, two triphenylene-based planar tridentate N-heterocyclic extremely helpful in attaining long-range ordered structure, angular
carbenes and their corresponding tri-gold complexes were synthe- poly-NHC ligands in planar aromatic systems could have better
sized. Molecular structures of the tert-butyl substituted tritopic free chance in realizing crystalline coordination polymers. Therefore, we
carbene and the tri-nuclear gold complex were determined experi- decided to prepare rigid, planar tridentate NHC ligands and to
mentally. A silver-dicarbene organometallic polymer was also pre- examine their application in the syntheses of tri-nuclear gold
pared with the newly synthesized tris-NHC.
complexes and silver–NHC organometallic polymer. During the
reviewing process of this work, Peris’ group published the syn-
Since the isolation of Arduengo’s carbene1 in the early 90s, research thesis of compound 1a and its corresponding tri-gold (3a) and
on divalent carbon species stabilized within the nitrogen hetero- tri-palladium complexes. Catalytic activities of both tri-nuclear metal
cycles has shown tremendous advances in both organic and complexes were also evaluated.38
organometallic catalyses.2–8 Their superb sigma donating ability in
As shown in Scheme 1, hexa-amino triphenylenes obtained
conjunction with imposed steric protection of the bonded elements from Buchwald–Hartwig amination of hexabromo triphenylene
also led to the discovery of several otherwise unstable low-valent with tert-butylamine or mesitylamine were subject to formylative
main-group derivatives.9–11 Not only does the monodentate NHC cyclization with triethylorthoformate in the presence of HBF4 to
play an imperative role in recent chemical advances, molecules afford compounds [1a][BF4]3 and [1b][BF4]3 in good yield. The
featuring multiple NHC units have also captivated considerable characteristic 1H and 13C NMR resonances of the [N–CH–N]+
attention. While the chelating type poly-NHCs facilitate the units of [1]3+ are typical ([1a]3+: 9.17 ppm and 142.6 ppm; [1b]3+:
formation of highly reactive metal catalysts,12 the bridging type 10.39 ppm and 148.1 ppm).39 The symmetric nature of [1a]3+ and
1
poly-NHCs are utilized in various supramolecules that feature [1b]3+ was evident in the H NMR spectra of both compounds,
interesting molecular structures, ranging from one-dimensional
organic13–18 and organometallic polymers19–23 to molecular
squares,24–26 three-dimensional molecular cages27–29 and organo-
metallic polymers.30 These poly-NHCs containing supramolecules
are not only structurally fascinating but also function as active
catalysts in heterogeneous catalysis30–34 and semiconducting
materials.14,18,35,36
For potential application of poly-NHCs in porous coordination
polymers, rigid poly-NHC ligands having angular geometry should
be favourable. In fact, the annulated tritopic NHC ligand based
on triptycene has been prepared by Bielawski and co-workers.37
However, the corresponding organometallic polymers have not
Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd.,
Taipei, Taiwan 10617. E-mail: cwchiu@ntu.edu.tw; Fax: +886-2-2363-6359;
Tel: +886-2-3366-8191
† Electronic supplementary information (ESI) available: Experimental details of
all reported compounds. X-ray crystallographic data of 1b, 2a and 3a, 13C SSNMR
Scheme 1 Synthetic procedures for 1 and 2. Reaction conditions: (a) Pd(OAc)2,
IPrꢀHCl, NaOtBu, RNH2 (R: t-Bu (a); Mes (b)), toluene, 100 1C, 48 h; (b) HC(OEt)3,
of 2a-Ag. CCDC 936892–936894. For ESI and crystallographic data in CIF or other
HBF4ꢀOEt2, r.t. (1a: 18 h; 1b: 72 h); (c) KH, KOtBu, THF, 2 h.
electronic format see DOI: 10.1039/c3cc43278e
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in which only one set of proton resonances for the triphenylene to those observed in the mono-dentate ligand, suggesting that
backbone ([1a]3+: 9.19 ppm; [1b]3+: 8.81 ppm) and the azolium the annulation of three carbene moieties on triphenylene poses
units was observed ([1a]3+: 2.04 ppm; [1b]3+: 7.32, 2.45 ( para-Me), negligible electronic constraints on each individual NHC unit.
2.11 ppm (ortho-Me)). Single crystals of [1b][BF4]3 were obtained as
The lack of electronic constraints is also evident in the
dark thin needles by slow diffusion of ether into a dichloromethane reaction of 2 and AuCl(SMe2). 3.5 equivalents of gold complex were
solution of [1b][BF4]3 at room temperature. Although the structural introduced to the tritopic free carbene (2) at room temperature
connectivity of [1b]3+ could be established using single crystal X-ray (Scheme 2). The resulting mixture was stirred overnight to afford the
diffraction analysis, the disordered inclusion solvent molecules in anticipated tri-gold complexes (3) in good yield (74% for 3a and 68%
the crystal lattice preclude the precise determination of numerical for 3b). The formation of the carbon–gold bond is verified using 13
C
parameters of [1b]3+ (Fig. S1, ESI†).‡ Given the fact that NMR spectroscopy, where the C2 carbons of 3a and 3b were
N,N0-dimesityl-ortho-phenylenediamine is notoriously reluctant respectively detected at 182.3 ppm and 185.2 ppm. The observed
to undergo formylative cyclization reaction, the relatively 13C resonance values are comparable to those reported for other
facile synthesis of [1b][BF4]3 at ambient temperature was very NHC–AuCl complexes.39,44 Examination of the crude reaction
surprising.40,41 An attempt at shortening the reaction time by mixture confirmed the absence of mono- and di-gold complexes,
raising the reaction temperature resulted in a complicated suggesting that the coordination of the first ylidene centre does not
reaction mixture, from which no [1b][BF4]3 was isolated.
With compound [1]3+ in hand, we then decided to isolate the
impact the reactivity of the rest of the carbene units.
Single crystals of 3a suitable for X-ray diffraction analysis
corresponding tridentate free carbenes. Generation of tritopic were obtained from diffusion of ether into a CH3CN–CH2Cl2
free NHCs (2a and 2b) was accomplished via deprotonation of solution of 3a at room temperature. The molecular structure of 3a
the corresponding azolium salts with potassium hydride in the shown in Fig. 2 confirms the formation of the desired tri-nuclear
presence of a catalytic amount of potassium tert-butoxide in gold carbene complex. Interestingly, the central triphenylene back-
THF. Upon deprotonation, the characteristic 1H resonance bone is no longer planar. The ruffled structure of the tri-gold
signal of the [N–CH–N]+ unit disappeared ([1a]3+: 9.17 ppm; complex could be the result of crystal packing and very weak
[1b]3+: 10.39 ppm), and the whole spectrum shifted up-field. aurophilic interactions (Fig. S2, ESI†). In each tri-gold complex,
The formation of the divalent carbon centers was corroborated two of the gold atoms deviate from the plane of the triphenylene
by the detection of the distinctive 13C resonance of 2a and 2b at ring and form weak interactions with adjacent tri-gold complexes
230.4 ppm and 231.4 ppm, respectively.39 The bulky N-substituents with Au–Au distances of 3.631 Å and 3.730 Å, which are considerably
(tert-butyl and mesityl) stabilize the active ylidene centres kinetically, longer than the typical aurophilic interaction distance of 2.5–3.5 Å.45
and prevent the newly prepared tritopic NHCs from polymerizing
through the formation of dibenzotetraazafulvalene.42,43
Encouraged by the isolation of 3 we then decided to investigate
whether ligand 2 is capable of forming three-dimensional organo-
The molecular structure of 2a was further confirmed by metallic polymers for potential application in heterogeneous
single crystal X-ray diffraction analysis. Single crystals of 2a catalysis.30–33 Since the silver–NHC complex is a common carbene
were obtained via slow evaporation of a benzene solution of 2a transfer reagent, the silver–NHC coordination polymer could
at room temperature in a dry-box. The 2a benzene solvate potentially be the starting material for various metal–carbene
crystallized as colourless blocks in the monoclinic P21/m space polymers.15 Upon mixing the THF solution of 2a and AgOTf,
group. As shown in Fig. 1, compound 2a adopts the C2v symmetry the light brown coordination polymer (2a-Ag) formed
with essentially planar central fused arenes. The distances from and precipitated out of the solution immediately. Formation
+
each ylidene carbon to the centroid of triphenylene are identical of the [C(NHC)–Ag–C(NHC)
]
linkage was corroborated by the
(5.861 Å). The averaged C–N distance of 1.373 Å and the N–C–N detection of a characteristic 13C resonance signal around
angle of 104.41 of the benzimidazol-2-ylidene unit are comparable
Fig. 1 Molecular structure of 2a. Thermal ellipsoids are drawn at 50% probability.
Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond
lengths (Å) and angles (deg): N(1)–C(1) 1.370(2), N(2)–C(1) 1.374(2), N(3)–C(2)
1.374(2), N(1)–C(1)–N(2) 104.33(14), N(3)–C(2)–N(3A) 104.6(2).
Scheme 2 Syntheses of tri-gold NHC complexes (3) and the silver-dicarbene
coordination polymer (2a-Ag).
c
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network by incorporation of more labile metal-ligand interactions.
This work was supported by the National Science Council in
Taiwan (NSC 99-2113-M-002-011-MY2 and NSC 101-2113-M-
002-013-MY2). Prof. Ching-Wen Chiu is grateful to the Kenda
Foundation for the Golden-Jade Fellowship. We also thank
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