Non-chelating polydentate N -heterocyclic carbenes through assembly approaches

Article abstract of DOI:10.1080/10426507.2015.1128918

Bridging-type polydentate N-heterocyclic carbene (NHC) ligands are valuable building blocks for organometallic supramolecules. In order to quickly expand the ligand library of poly-NHC, we have assembled several bi-functional carbene precursors with spiroborate, phthalimide and metal-carboxylate to yield poly-NHC ligands with various denticities and geometries. Condensation of 4,5-dihyxdroxy-N,N′-diaryl-benzimidazolium and arylboronic acids leads to a spiroborate-linked bis-benzimidazolium salt, which can be further deprotonated to give a linear anionic bis-NHC. The second approach utilizes the dehydrative condensation of phthalic anhydride functionalized benzimidazole with poly-anilines to afford a series of poly-benzimidazole compounds, which can be converted into the corresponding poly-nuclear metal-NHC complexes after alkylation and metalation. The introduction of a benzoate group to bi-functional carbene precursor facilitates the assembly of poly-benzimidazolium salts with metal salts leading to MCl₂ (M=Zn, Co)-linked C_2h symmetric bis-benzimidazoliums and a penta-Zn cluster-linked tetra-benzimidazolium.

Full text of DOI:10.1080/10426507.2015.1128918

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Non-chelating polydentate N-heterocyclic
carbenes through assembly approaches
Ya-Fan Lin, Chih-Hsun Liu, Chung-Hao Hu & Ching-Wen Chiu
To cite this article: Ya-Fan Lin, Chih-Hsun Liu, Chung-Hao Hu & Ching-Wen Chiu (2016)
Non-chelating polydentate N-heterocyclic carbenes through assembly approaches,
Phosphorus, Sulfur, and Silicon and the Related Elements, 191:4, 613-617, DOI:
10.1080/10426507.2015.1128918
To link to this article: http://dx.doi.org/10.1080/10426507.2015.1128918
Accepted author version posted online: 06
Feb 2016.
Submit your article to this journal
Article views: 9
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=gpss20
Download by: [Brunel University London]
Date: 20 April 2016, At: 03:48

PHOSPHORUS, SULFUR, AND SILICON
, VOL. , NO. , –
http://dx.doi.org/./..
Non-chelating polydentate N-heterocyclic carbenes through assembly approaches
Ya-Fan Lin, Chih-Hsun Liu, Chung-Hao Hu, and Ching-Wen Chiu
Department of Chemistry, National Taiwan University, Taipei, Taiwan
ABSTRACT
ARTICLE HISTORY
Received  December 
Accepted  December 
Bridging-type polydentate N-heterocyclic carbene (NHC) ligands are valuable building blocks for
organometallic supramolecules. In order to quickly expand the ligand library of poly-NHC, we have assem-
bled several bi-functional carbene precursors with spiroborate, phthalimide and metal-carboxylate to yield
poly-NHC ligands with various denticities and geometries. Condensation of 4,5-dihyxdroxy-N,Nꢀ-diaryl-
benzimidazolium and arylboronic acids leads to a spiroborate-linked bis-benzimidazolium salt, which can
be further deprotonated to give a linear anionic bis-NHC. The second approach utilizes the dehydrative
condensation of phthalic anhydride functionalized benzimidazole with poly-anilines to afford a series of
poly-benzimidazole compounds, which can be converted into the corresponding poly-nuclear metal-NHC
complexes after alkylation and metalation. The introduction of a benzoate group to bi-functional carbene
precursor facilitates the assembly of poly-benzimidazolium salts with metal salts leading to MCl2 (M=Zn,
Co)-linked C2h symmetric bis-benzimidazoliums and a penta-Zn cluster-linked tetra-benzimidazolium.
KEYWORDS
N-Heterocyclic carbine;
multi-dentate ligands;
non-chelating
GRAPHICAL ABSTRACT
Recently, we have tried to prepare symmetrical multi-dentate
NHC ligands through assembly methods. To this end, we
have designed three bi-functional benzimidazolium deriva-
tives that serve as carbene precursors and assembly linkages
simultaneously (Figure 1). Assembly of these bi-functional
building blocks with appropriate molecular platforms affords
multi-dentate carbene ligands with various denticities and
geometries. Besides, since the ylidene carbon in all cases
lie on the rotational axis of the building block, the twist-
ing angles between the benzimidazolium units and the cen-
tral molecular platform do not affect the relative position
and orientation of the metal–carbene bond. Such molecu-
lar design could potentially lead to crystalline organometallic
polymers.
Introduction
Being a strong σ-donating ligand to metals, N-heterocyclic car-
benes (NHCs) have been widely applied in transition metal
and main-group metal complexes.1–10 In addition to mono-
dentate NHC, poly-functional NHC ligands have also attracted
much attention for their ability to stabilize highly reactive low-
valent metal centers through chelation.11–22 Another impor-
tant advance of poly-NHC ligands is their application in the
construction of organometallic supramolecules, including lin-
ear polymers, planar molecular rectangles, three-dimensional
(3D) molecular cages, and porous organometallic polymers.23–31
However, the synthesis of poly-NHC ligands is not trivial, espe-
cially the one that maintains the relative ylidene position after
metalation.32–36
CONTACT Ching-Wen Chiu
cwchiu@ntu.edu.tw
No. , Sec. , Roosevelt Rd., Taipei , Taiwan.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gpss.
©  Taylor & Francis Group, LLC

614
Y.-F. LIN ET AL.
Figure . Design of building blocks.
[Rh(COD)Cl]2 afforded the corresponding [4-(BH3)2]− and
[4-(Rh(COD)Cl)2]− complexes, respectively. Unfortunately,
assembly of [1]+ with arylboronic acid could only lead to the
formation of a twisted linear ditopic NHC ligand [4]−.
Results and discussion
Spiro-borate linkage
Inspired by the work of boronate-based covalent organic frame-
work,37–40 we have designed a benzimidazolium salt ([1]+)
that contains two hydroxyl groups at 4,5-position for dehy-
drative condensation with arylboronic acid.30 The synthesis
of the catechol-benzimidazolium building unit is straightfor-
ward. After the amination of 4,5-dibromoveratrole, formyla-
tive cyclization of diamine compound afforded 4,5-dimethoxy-
N,Nꢀ-dimesitylbenzimidazolium, which can then be demethy-
lated with BBr3 to yield the anticipated building unit [1][Br]
(Scheme 1). Initial attempt to assemble building unit [1]+ with
benzenediboronic acid (BDBA) in 2:1 ratio in ethanol to give
the corresponding linear bis-benzimidazolium salt ([2]2+) was
unsuccessful due to the undesired cleavage of the aryl-boron
bonds of BDBA. Instead, the mono-cationic spiroborate-linked
bis-benzimidazolium salt ([3][Br]) was isolated exclusively. The
11B resonance of [3]+ detected at 15 ppm is consistent with the
formation of a [BO4]− unit. The structural connectivity of [3]+
is further verified by X-ray diffraction analysis on a single crystal
of [3][Br]. Two benzimidazolium units are brought together by a
[BO4]− linkage with a dihedral angle of nearly 90°, showing the
lack of electronic communication between two azolium units.
Therefore, [4]− could serve as a bridging ligand to two metal
centers with dis-connected electronic communication between
the two metal centers.
Phthalimide linkage
The second bi-functional building block that we designed is
a phthalic anhydride containing benzimidazole (5), which can
be assembled with polyamines and then converted into the
phthalimide-linked poly-benzimidazoliums.31 Since phthalim-
ide functionality is inert under normal pH and in the pres-
ence of bulky bases, metalation of these phthalimide-linked
poly-azolium salts can be carried out to give the correspond-
ing poly-nuclear NHC–metal complexes. Compound 5 can be
readily obtained in multi-gram scale in few steps. Dehydra-
tive condensation of 5 and aniline derivatives followed by sec-
ond alkylation with ethyl iodide afforded the anticipated poly-
benzimidazolium salts in good to excellent yield (Scheme 2).
Linear bis-benzimidaozlium ([6]2+), 2D tris-benzimidaozlium
([7]3+), and tetrahedral tetrakis-benzimidaozlium ([8]4+) were
prepared from the assembly of bi-functional building block
with di-amine, tri-amine, and tetra-amine, respectively. In other
words, the size, denticity, and geometry of poly-benzidazolium
salts could be easily controlled by the structure of the poly-
amino assembly platform.
To show that these phthalimide-linked poly-benzimi-
dazoliums could serve as poly-NHC precursors, the corre-
sponding poly-nuclear nickel ([6-(NiCpI)2], [7-(NiCpI)3],
and [8-(NiCpI)4]) and rhodium complexes ([6-(Rh(cod)I)2],
[7-(Rh(cod)I)3], and [8-(Rh(cod)I)4]) have also been prepared.
Interestingly, reaction kinetics of these poly-nickel carbene
Interestingly, the [BO4]− unit is robust toward KHMDS
and KOtBu, and the deprotonation of [3]+ in tetrahydrofu-
ran (THF) leads to the isolation of an anionic bis-NHC (K[4])
in excellent yield. The 13C NMR signal of the ylidene center
observed at 224.5 ppm confirms the formation of free carbene
in the solution.41 Further reaction of K[4] with BH3(SMe2) and
Scheme . Synthesis of bifunctional ligand [1][Br] and its reaction with BDBA to yield [3][Br].

PHOSPHORUS, SULFUR, AND SILICON
615
Scheme . Assembly of poly-benzimidazolium salts from building block 5 and poly-amino molecular platforms.
[9]+. However, heating ZnCl2 and [9a][I] in acetonitrile at 80 °C
for two days resulted in a colorless di-zinc cluster (10). A related
di-cobalt cluster (11) was also obtained in a similar manner
(Scheme 4). Single crystal X-ray diffraction studies revealed that
both 10 and 11 adopt a dimeric structure comprising two metal
ions and two zwitterionic ligands. As shown in Figure 2, the two
benzimidazolium moieties are anti-parallel to each other with an
inversion center located at the centroid of the eight-membered
ring, leading to a C2h symmetry of bis-benzimidazolium com-
plex. Two metal centers (Zn or Co) and two carboxylate groups
are linked in a syn-anti-fashion to form an eight-membered
ring in chair form conformation. Complex 10 features a Zn(II)–
complexes in hydrothiolation of terminal alkyne showed negli-
gible correlation with ligand denticity and complex geometry.
This observation implies that every [NHC-NiCpI] moiety of
poly-nuclear nickel complexes is independent of each other.
Metal-carboxylate linkage
In an attempt to attain ligand topologies that are inaccessi-
ble with organic molecular platform, we then explored the
potential to utilize the versatile coordination modes of car-
boxylate and transition metals to assemble poly-azolium units.
To bring a carboxylic acid group to the C2 axis of a benzim-
idazolium molecule, benzimidazole-5,6-dicarboxylic acid and
4-amino-benzoic acid were allowed to react in dimethylfor-
mamide (DMF) to yield the corresponding benzimidazole-
carboxylic acid, which can then be converted into the antic-
ipated benzimidazolium-carboxylic acid bi-functional ligand
([9a]+ and [9b]+) through stepwise deprotonation and alkyla-
tion with methyl iodide or benzyl bromide (Scheme 3).
To facilitate metal–carboxylate coordination, bases were
added to deprotonate carboxylic acid to give the corresponding
benzimidazolium-carboxylate zwitterion. Unfortunately, only
one type of ligand topology was achieved through the metal–
carboxylate coordination. In the presence of pyridine, neither
Zn(NO3)2 nor Zn(OAc)2 formed coordination compounds with
˚
Zn(II) distance of 3.870 A, while the two Co(II) are separated by
˚
3.725 A in 11. The biggest structural difference between 10 and
11 is the torsion angles formed by phenyl and benzimidazolium
plane (29.46° for 10 and 42.21° for 11). Thus, an intermolecu-
lar π−π stacking of phthalimide groups is found in the crystal
structure of 10 because of a better coplanarity in 10.
When [9b][Br] was allowed to react with two equivalents of
Zn(ClO4)2 in the presence of 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU) in THF/NMP (v/v = 8:1) at 140 °C under solvother-
mal condition, a tetrakis-benzimidazolium linked by a penta-
zinc cluster ([12][ClO4]2) was isolated. The aminobenzoic acid
group of [9b]+ was removed under the harsh reaction con-
dition to give the corresponding benzimidazolium-phthalate
Scheme . Synthesis of benzimidazolium-carboxylic acid ligands ([9][X]).

616
Y.-F. LIN ET AL.
Scheme . Synthesis of metal–carboxylate-linked bis- and tetrakis-benzimidazoliums.
fragment (13), which then coordinated with Zn metal cen- of [12][ClO4]2 in organic solvents prohibited further deproto-
ters (Scheme 4). Structural analysis of [12][ClO4]2 shows that nation or metalation of the molecule.
[12]2+ possesses five coplanar zinc ions, two terminal bro-
In conclusion, three different linkage groups were designed
mides, two capping NMP molecules, two μ -hydroxo ligands, to connect two or more benzimidazolium units with organic
3
and four benzimidazolium-phthalate units. The hydroxo group or inorganic assembly platforms. Although the condensation of
binds to three neighboring zinc ions with Zn-O distances var- [1]+ with arylboronic acid could only give the linearly opposed
˚
ied from 1.948(8) to 2.111(8) A. Interestingly, the two carboxy- bis-NHC, the orthogonal arrangement of two NHC moieties in
late groups of benzimidazolium-phthalate moiety have different [4]− may find interesting application in ferromagnetically cou-
linkage types. One bridges two zinc ions in a syn–syn orienta- pled bi-metallic complexes. Meanwhile, the reaction of 5 with
tion, and the other provides a single binding site to the metal. poly-amino compounds successfully afforded the anticipated
Each benzimidazolium-phthalate ligand is capable of binding poly-benzimidazolium salts that could be metalated to give the
to three zinc ions with ortho-dicarboxylate functionalities. As a corresponding poly-NHC metal complexes. The assembly con-
result, four benzimidazolium units are brought together through cept can be further extended to inorganic metal–carboxylate
a complicated coordination to five Zn(II) ions. The selected linkage. However, all attempts toward deprotonation or direct
structural parameters of [12]2+ are listed in Figure 3. The four metalation of the metal-carboxylate-linked bis- and tetrakis-
C2 carbons of benzimidazolium moieties are essentially copla- benzimidazolium were unsuccessful due to either the poor solu-
nar in a rectangular arrangement with the C2 …C2 distances of bility of these complexes or the undesired dissociation of metal–
˚
˚
11.3 A and 13.8 A. Unfortunately, the extremely poor solubility carboxylate linkages.
˚
Figure . ORTEP drawing of (a) 10, and (b) 11 with % probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond distances (A) and angles (deg) for
10: Zn–O .(), Zn–O .(), Zn–Cl .(), Zn–Cl .(), C–O–Zn .(), C–O–Zn .(), O–Zn–O .(), Cl–Zn–Cl .(), O–Zn–Cl
.(), O–Zn–Cl .(), O–Zn–Cl .(), O–Zn–Cl .(). For 11: Co–O .(), Co–O .(), Co–Cl .(), Co–Cl .(), C–O–Co .(),
C–O–Co .(), O–Co–O .(), Cl–Co–Cl .(), O–Co–Cl .(), O–Co–Cl .(), O–Co–Cl .(), O–Co–Cl .().

PHOSPHORUS, SULFUR, AND SILICON
617
Figure . ORTEP drawing of [12][ClO4]2 with % probability ellipsoids: (a) side view, and (b) top view. Hydrogen atoms, counter anions, and solvent molecules are
˚
omitted for clarity. Selected bond distances (A) and angles (deg): Zn–O .(), Zn–O .(), Zn–O .(), Zn–O .(), Zn–O .(), Zn–O .(),
Zn–Br .(), Zn–O .(), Zn–O .(), Zn–O .(), Zn–O .(), Zn–O–Zn .(), Zn–O–Zn .(), Zn–O–Zn .().
18. Blom, B.; Tan, G.; Enthaler, S.; Inoue, S.; Epping, J. D.; Driess, M. J. Am.
Chem. Soc. 2013, 135, 18108-18120.
Acknowledgment
We would like to thank Dr. G.-H. Lee, Y.-H. Liu, and Prof. S.-M. Peng for
structural determinations.
19. Darmawan, N.; Yang, C.-H.; Mauro, M.; Raynal, M.; Heun, S.; Pan, J.;
Buchholz, H.; Braunstein, P.; De Cola, L. Inorg. Chem. 2013, 52, 10756-
10765.
20. Howell, T. O.; Huckaba, A. J.; Hollis, T. K. Org. Lett. 2014, 16, 2570-
2572.
Funding
21. Schick, S.; Pape, T.; Hahn, F. E. Organometallics 2014, 33, 4035-4041.
22. Li, Y.; Tang, J.; Gu, J.; Wang, Q.; Sun, P.; Zhang, D. Organometallics
2014, 33, 876-884.
We would like to thank the Ministry of Science and Technology of Tai-
wan for funding (MOST 103-2113-M-002-005 and MOST 104-2811-M-
002-130).
23. Boydston, A. J.; Williams, K. A.; Bielawski, C. W. J. Am. Chem. Soc.
2005, 127, 12496-12497.
24. Boydston, A. J.; Bielawski, C. W. Dalton Trans. 2006, 4073-4077.
25. Boydston, A. J.; Rice, J. D.; Sanderson, M. D.; Dykhno, O. L.; Bielawski,
C. W. Organometallics 2006, 25, 6087-6098.
References
1. Díez-González, S. N-Heterocyclic Carbenes: From Laboratory Curiosi-
ties to Efficient Synthetic Tools; RSC Publishing, Cambridge, 2011.
2. Herrmann, W. A.; Köcher, C. Angew. Chem. Int. Ed. 1997, 36, 2162-
2187.
3. Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. Chem. Rev. 2000,
100, 39-91.
4. Enders, D.; Niemeier, O.; Henseler, A. Chem. Rev. 2007, 107, 5606-
5655.
26. Williams, K. A.; Boydston, A. J.; Bielawski, C. W. Chem. Soc. Rev. 2007,
36, 729-744.
27. Choi, J.; Yang, H. Y.; Kim, H. J.; Son, S. U. Angew. Chem. Int. Ed. 2010,
49, 7718-7722.
28. Schmidtendorf, M.; Pape, T.; Hahn, F. E. Angew. Chem. Int. Ed. 2012,
51, 2195-2198.
29. Mejuto, C.; Guisado-Barrios, G.; Peris, E. Organometallics 2014, 33,
3205-3211.
5. Hahn, F. E.; Jahnke, M. C. Angew. Chem. Int. Ed. 2008, 47, 3122-3172.
6. Alcaide, B.; Almendros, P.; Luna, A. Chem. Rev. 2009, 109, 3817-3858.
7. Díez-González, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612-
3676.
30. Su, J.-H.; Lee, G.-H.; Peng, S.-M.; Chiu, C.-W. Dalton Trans. 2014, 43,
3059-3062.
31. Chen, Y.-H.; Peng, K.-E.; Lee, G.-H.; Peng, S.-M.; Chiu, C.-W. RSC Adv.
2014, 4, 62789-62792.
8. Samojilowicz, C.; Bieniek, M.; Grela, K. Chem. Rev. 2009, 109, 3708-
3742.
32. Williams, K. A.; Bielawski, C. W. Chem. Commun. 2010, 5166-5168.
33. Gonell, S.; Poyatos, M.; Peris, E. Angew. Chem. Int. Ed. 2013, 52, 7009-
7013.
9. Fevre, M.; Pinaud, J.; Gnanou, Y.; Vignolle, J.; Taton, D. Chem. Soc. Rev.
2013, 42, 2142-2172.
34. Gonell, S.; Alabau, R. G.; Poyatos, M.; Peris, E. Chem. Commun. 2013,
49, 7126-7128.
10. Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. Nature 2014,
510, 485-496.
35. Wang, Y.-T.; Chang, M.-T.; Lee, G.-H.; Peng S.-M.; Chiu, C.-W. Chem.
Commun. 2013, 49, 7258-7260.
11. Poyatos, M.; Mata, J. A.; Peris, E. Chem. Rev. 2009, 109, 3677-3707.
12. Kernbach, U.; Ramm, M.; Luger, P.; Fehlhammer, W. P. Angew. Chem.
Int. Ed. 1996, 35, 310-312.
36. Segarra, C.; Linke, J.; Mas-Marza, E.; Kuck D.; Peris, E. Chem. Com-
mun. 2013, 49, 10572-10574.
13. Douthwaite, R. E.; Houghton, J.; Kariuki, B. M. Chem. Commun. 2004,
698-699.
37. Niu, W.; O’Sullivan, C.; Rambo, B. M.; Smith, M. D.; Lavigne, J. J. Chem.
Commun. 2005, 4342-4344.
14. Nieto, I.; Cervantes-Lee, F.; Smith, J. M. Chem. Commun. 2005, 3811-
3813.
38. Niu, W.; Smith, M. D.; Lavigne, J. J. J. Am. Chem. Soc. 2006, 128, 16466-
16467.
15. Muñoz, S. B.; Foster, W. K.; Lin, H.-J.; Margarit, C. G.; Dickie, D. A.;
Smith, J. M. Inorg. Chem. 2012, 51, 12660-12668.
16. Gonell, S.; Poyatos, M.; Mata, J. A.; Peris, E. Organometallics 2012, 31,
5606-5614.
39. Côté, A. P.; Benin, A. I.; Ockwig, N. W.; O’Keeffe, M.; Matzger, A. J.;
Yaghi, O. M. Science 2005, 310, 1166-1170.
40. El-Kaderi, H. M.; Hunt, J. R.; Mendoza-Cortés, J. L.; Côté, A. P.; Taylor,
R. E.; O’Keeffe, M.; Yaghi, O. M. Science 2007, 316, 268-272.
41. Tapu, D.; Dixon, D. A.; Roe, C. Chem. Rev. 2009, 109, 3385-3407.
17. Seitz, S. C.; Rominger, F.; Straub, B. F. Organometallics 2013, 32, 2427-
2434.