Synthesis of a zinc(II) imidazolium dicarboxylate ligand metal-organic framework (MOF): A potential precursor to MOF-tethered N-heterocyclic carbene compounds

Article abstract of DOI:10.1021/ic9021118

Two new isomeric dinitrile ligands containing imidazoliumsalt cores have been synthesized from cyanoanilines in good yield. These have been converted to the corresponding dicarboxylic acids using hydrobromic acid, or the dicarboxylic acids were synthesized directly from the analogous cyanoaniline starting material in a two-step, one-pot reaction. The crystal structures of three of the four compounds are reported. Two reaction pathways with metals are possible for the dicarboxylic acids, initially giving rise to metal carboxylate based metal-organic frameworks (MOFs) as described in this work or N-heterocyclic carbene (NHC) metal complexes en route to the synthesis of MOFs containing tethered NHC complexes. The reaction of 1, 3-bis(4-carboxyphenyl)imidazolium bromide with zinc nitrate in dimethylformamide (DMF) gave a one-dimensional (1D) MOF containing the intact imidazolium salt. The potentially porous structure, formed from close packing of the 1D necklace-type polymers, contains channels occupied by disordered DMF solvate molecules. A formate anion also bridges the zinc secondary building units in the 1D polymer, which has an undulating structure resulting from the U-shaped conformation of the dicarboxylate imidazolium ligand.

Full text of DOI:10.1021/ic9021118

1712 Inorg. Chem. 2010, 49, 1712–1719
DOI: 10.1021/ic9021118
Synthesis of a Zinc(II) Imidazolium Dicarboxylate Ligand Metal-Organic
Framework (MOF): a Potential Precursor to MOF-Tethered N-Heterocyclic Carbene
Compounds
Rachel S. Crees,† Marcus L. Cole,‡ Lyall R. Hanton,§ and Christopher J. Sumby*,†
† School of Chemistry & Physics, The University of Adelaide, Adelaide, South Australia 5005, Australia,
‡ School of Chemistry, University of New South Wales, Sydney, Australia, and § Department of Chemistry,
University of Otago, Dunedin, New Zealand
Received October 25, 2009
Two new isomeric dinitrile ligands containing imidazolium salt cores have been synthesized from cyanoanilines in good
yield. These have been converted to the corresponding dicarboxylic acids using hydrobromic acid, or the dicarboxylic
acids were synthesized directly from the analogous cyanoaniline starting material in a two-step, one-pot reaction. The
crystal structures of three of the four compounds are reported. Two reaction pathways with metals are possible for the
dicarboxylic acids, initially giving rise to metal carboxylate based metal-organic frameworks (MOFs) as described in
this work or N-heterocyclic carbene (NHC) metal complexes en route to the synthesis of MOFs containing tethered
NHC complexes. The reaction of 1,3-bis(4-carboxyphenyl)imidazolium bromide with zinc nitrate in dimethylformamide
(DMF) gave a one-dimensional (1D) MOF containing the intact imidazolium salt. The potentially porous structure,
formed from close packing of the 1D necklace-type polymers, contains channels occupied by disordered DMF solvate
molecules. A formate anion also bridges the zinc secondary building units in the 1D polymer, which has an undulating
structure resulting from the U-shaped conformation of the dicarboxylate imidazolium ligand.
Introduction
The latter two properties are intimately linked with the ability
to achieve permanent porosity in a material. In the field of
MOF chemistry, work over the last 10-15 years has been
focused on the synthesis and study of new MOFs (or coordina-
tion polymers). In this regard, new ligands, new ligand and
metal combinations, synthetic methods, and structures have
been a primary focus, with the ultimate goal of a more rational
design approach.1,2,14 However, in recent years, attention has
shifted to investigating and developing the aforementioned
properties of known and new MOFs.14 Recently, during the
course of this study, a supramolecular network of a related
imidazolium dicarboxylate was reported by Chun et al. with
copper(II) that contained a N-heterocyclic carbene (NHC)-
copper complex.16
The field of metal-organic framework (MOF) chemistry
has been an area of intense study over the past decade.1-10 The
interest in these solid-state materials stems from their appli-
cations11-15 in conductivity, luminescence, magnetism, non-
linear optics, chirality, catalysis, and physi- or chemisorption.
*To whom correspondence should be addressed. E-mail: christopher.
sumby@adelaide.edu.au. Phone: þ61 8 8303 7406. Fax: þ61 8 8303 4358.
(1) Ferey, G. Chem. Soc. Rev. 2008, 37, 191.
(2) Yaghi, O. M.; O0Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi,
M.; Kim, J. Nature 2003, 423, 705.
(3) Rosseinsky, M. J. Microporous Mesoporous Mater. 2004, 73, 15.
(4) Ferey, G.; Mellot-Draznicks, C.; Serre, C.; Millange, F. Acc. Chem.
Res. 2005, 38, 217.
The catalytic activity within a MOF has been investigated
for a number of years,10-13 where these materials are seen as
“designable” alternatives to zeolites. A MOF can confer the
desirable catalyst properties of size and shape selectivity and
ease of recycling. The catalytic activity of a MOF can be
derived in a number of ways. First, unsaturated structural
metal centers within the MOF can be the active catalysts,11,12
or the porous MOF itself can act as the catalyst (through the
(5) Maspoch, D.; Ruiz-Molina, D.; Veciana, J. J. Mater. Chem. 2004, 14,
2713.
(6) Fletcher, A. J.; Thomas, K. M.; Rosseinsky, M. J. J. Solid State Chem.
2005, 178, 2491.
(7) Kitagawa, S.; Uemura, K. Chem. Soc. Rev. 2005, 34, 109.
(8) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 1629.
(9) Zaworotko, M. J. Chem. Commun. 2001, 1.
(10) Bradshaw, D.; Claridge, J. B.; Cussen, E. J.; Prior, T. J.; Rosseinsky,
M. J. Acc. Chem. Res. 2005, 38, 273.
(11) Janiak, C. Dalton Trans. 2003, 2781.
(12) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43,
2334.
(13) Tanaka, D.; Kitagawa, S. Chem. Mater. 2008, 20, 922.
(14) Ferey, G. Dalton Trans. 2009, 4400.
(15) Farrusseng, D.; Aguado, S.; Pinel, C. Angew. Chem., Int. Ed. 2009,
48, 7502.
(16) Chun, J.; Jung, I. G.; Kim, H. J.; Park, M.; Lah, M. S.; Son, S. U.
Inorg. Chem. 2009, 48, 6353.
(17) Tanaka, K.; Oda, S.; Shiro, M. Chem. Commun. 2008, 820.
r
pubs.acs.org/IC
Published on Web 01/19/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 4, 2010 1713
Scheme 1. Synthetic Routes to Imidazolium Salt Derived MOFs (A)
for Anion or Neutral Small-Molecule Binding, Discrete NHC Com-
plexes (B), and, Ultimately, NHC Organometallic MOFs (C) with
Potential Applications as Catalytically Active Materials
binding anionic guests30,31 or enhancing physisorption of
small molecules.32 In the appropriate frameworks, these will
also be available for tethering catalytically active metal com-
plexes by unmasking the imidazolium moiety in a postsyn-
thetic modification.16
Recently, Chun et al.16 reported the synthesis of a closely
related imidazolium carboxylate ligand and the concomitant
formation of NHC-copper complexes in a MOF. We report
our recent work in this area, including the syntheses of four
new imidazolium-containing ligands, which, although they
do not contain the steric bulk to protect the NHC, are
potential precursors to MOF-tethered NHCs. Herein, we
demonstrate the differential reactivity of one of the carboxy-
late derivatives and the synthesis of a new one-dimensional
(1D) MOF material with zinc(II) salts.
Experimental Section
General Procedures. Melting points were recorded on an
Electrothermal melting point apparatus. IR spectra were col-
lected on a Perkin-Elmer BX FTIR spectrometer as KBr disks.
Electrospray (ES) mass spectra were recorded using a Finnigan
LCQ mass spectrometer. High-resolution ES mass spectrometry
was performed on a Micromass LCT-TOF mass spectrometer at
the University of Canterbury. Unless otherwise stated, NMR
spectra were recorded on a Varian Gemini 200 or 300 MHz or
Varian Unity 600 MHz spectrometer at 20 °C using a 5 mm
probe. 1H NMR spectra recorded in dimethyl sulfoxide
(DMSO)-d6, methanol-d4, and D2O were referenced to the
residual solvent peaks 2.50, 3.31, and 4.79 ppm. When required,
two-dimensional NOESY, TOCSY, ROESY, and COSY
experiments were performed using standard pulse sequences.
Unless otherwise stated, the values given for chemical shifts are
to the center of a multiplet and muliplicities are listed based on
three-bond coupling only. All coupling constants are given in
hertz. The Campbell microanalytical laboratory at the Univer-
sity of Otago performed elemental analyses. Unless otherwise
stated, reagents were obtained from commercial sources and
used as received.
incorporation of chiral ligands, for example).13,17 A third
alternative is to generate a MOF that can be used to tether a
catalytically active metal complex (in a nonstructural role).18
NHCs represent one of the most studied members of the
class of compounds referred to as nucleophilic carbenes.19
NHCs have been known for nearly 50 years,20 although
reports of the first stable NHCs came 30 years later.21 Since
this work was published, NHCs have been reported to be key
ligands in a variety of organometallic complexes capable of
catalyzing numerous reactions.22,23 A prominent example is
the development of NHC-ruthenium complexes capable of
olefin metathesis.24,25
As part of our broader research program studying ligands
containing electron-deficient [3]radialenes,26-28 or (thio)-
amides29 and (thio)ureas suitable for forming interactions
with anions, we have developed the synthesis of cationic imida-
zolium compounds bearing nitrile, pyridyl, and carboxylate
donor groups. When incorporated into MOFs (Scheme 1),
these organic groups may generate pockets that are suitable for
Ligand Synthesis. 1,3-Bis(4-cyanophenyl)imidazolium Chloride,
3(Cl). To a stirred solution of paraformaldehyde (0.98 g, 33 mmol)
in toluene (50 mL) was added 4-cyanoaniline (1; 7.72 g, 65.2 mmol)
and aqueous glyoxal [40% (v/v), 4.74 g, 32.6 mmol] followed by the
dropwise addition of HCl [37% (v/v), 3.72 mL, 32.6 mmol]. The
mixture was refluxed until ca. 6.5 mL of water was recovered via a
Dean-Stark apparatus. The solvent was removed in vacuo and the
resulting oil triturated with acetonitrile to yield a pale-brown
powder (7.50 g, 75%). Conversion to the yellow hexafluoro-
phosphate salt was achieved by stirring the chloride salt in water
with excess NH4PF6 and recovering the precipitate by filtration.
Mp: 235-236 °C. 1H NMR (600 MHz, DMSO-d6): δ 8.13 (d, J=
8.4, 4H, H20/H60), 8.25 (d, J = 8.4, 4H, H30/H50), 8.67 (s, 2H,
H4/H5), 10.58 (s, 1H, H2). 13C NMR (600 MHz, DMSO-d6):
δ 113.2, 118.2, 122.4, 123.2, 135.0, 136.4, 138.2. IR (Nujol):
ν
max/cm-1 2230 (conjugated CN). ESI-MS: C17H11N4 [Mþ] 273
(18) Lassalle-Kaiser, B.; Guillot, R.; Aukauloo, A. Tetrahedron Lett.
2007, 48, 7004.
(19) Bourissou, D.; Guerret, O.; Gabbaie, F. P.; Bertrand, G. Chem. Rev.
2000, 100, 39.
(20) Wanzlick, H. W.; Schikora, E. Angew. Chem. 1960, 72, 494.
(21) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc.
1991, 114, 7034.
(22) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290.
(23) Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1776.
(24) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18.
(25) Colacino, E.; Martinez, J.; Lamaty, F. Coord. Chem. Rev. 2007, 251,
726.
(26) Hollis, C. A.; Hanton, L. R.; Morris, J. C.; Sumby, C. J. Cryst.
Growth Des. 2009, 9, 2911.
(27) Steel, P. J.; Sumby, C. J. Chem. Commun. 2002, 322.
(28) Steel, P. J.; Sumby, C. J. Inorg. Chem. Commun. 2002, 5, 323.
(29) Sumby, C. J.; Hanton, L. R. Tetrahedron 2009, 65, 4681.
(1%), 272 (16), 271 (83). Anal. Calcd for C17H11N4F6P: C, 49.05;
H, 2.67; N, 13.46. Found: C, 49.12; H, 2.70; N, 13.52. Crystals of
3(NO3) were obtained by the slow evaporation of a 1:1 acetone/
acetonitrile solution of 3(PF6) and silver(I) nitrate.
1,3-Bis(3-cyanophenyl)imidazolium Chloride, 4(Cl). Using the
method outlined for the synthesis of 3(Cl), imidazolium salt
4(Cl) was obtained as an off-white powder following trituration
(30) Custelcean, R.; Moyer, B. A. Eur. J. Inorg. Chem. 2007, 1321.
(31) Custelcean, R. Chem. Commun. 2008, 295.
(32) Higuchi, M.; Tanaka, D.; Horike, S.; Sakamoto, H.; Nakamura, K.;
Takashima, Y.; Hijikata, Y.; Yanai, N.; Kim, J.; Kato, K.; Kubota, Y.;
Takata, M.; Kitagawa, S. J. Am. Chem. Soc. 2009, 131, 10336.

1714 Inorganic Chemistry, Vol. 49, No. 4, 2010
Crees et al.
Table 1. Crystal Data and X-ray Experimental Data for {3(NO3)}2, 5(ClO4), 6(FeCl4), and 7
{3(NO3)}2
5(ClO4)
6(FeCl4)
7
empirical formula
fw
cryst syst
C34H22N10O6
666.62
triclinic
P1
C17H13ClN2O8
408.74
monoclinic
P21/c
C17H13Cl4FeN2O4
506.94
orthorhombic
Pnma
C19.75H16.75N3.25O7.25Zn
480.98
monoclinic
C2/m
space group
unit cell dimens
˚
a (A)
9.0944(9)
12.7988(13)
14.6289(14)
109.386(6)
100.961(6)
97.844(6)
1539.9(3)
2
1.438
0.103
688
0.42/0.34/0.12
27.74
33 987
9.5620(19)
10.638(2)
17.505(5)
14.693(3)
19.809(4)
7.3570(15)
20.796(4)
18.019(4)
10.943(2)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
111.00(3)
91.907(8)
3
˚
V (A )
Z
1662.3(7)
4
1.633
0.284
840
0.19/0.17/0.07
27.07
22 462
3575 [0.1020]
3377
2141.3(8)
4
1.573
1.228
1020
0.17/0.10/0.08
24.99
24 590
1857 [0.1085]
1792
4098.3(14)
8
1.559
1.249
1968
0.26/0.24/0.15
27.00
35 436
4612 [0.0538]
4150
density (calcd) (Mg/m3)
abs coeff (mm-1
F(000)
cryst size (mm)
)
θ
max of data collection (deg)
reflns collected
indep reflns [Rint
]
7013 [0.1084]
5178
7013/0/451
0.996
0.0472
obsd reflns [I > 2σ(I)]
data/restraints/param
GOF on F2
R1 [I > 2σ(I)]
wR2 (all data)
3575/0/253
1.039
0.0592
0.1593
1857/0/133
1.094
0.0964
0.2249
4612/2/300
1.078
0.0393
0.1172
0.1267
1
from acetonitrile (7.50 g, 75%). Mp: 220 °C (dec). H NMR
(600 MHz, CD3OD): δ 7.90 (t, J=7.8, 2H, H50), 8.04 (d, J=7.2,
2H, H40), 8.20 (d, J=7.8, 2H, H60), 8.34 (s, 2H, H20), 8.43 (s, 2H,
H4/H5), 10.51 (s, 1H, H2). 13C NMR (600 MHz, CD3OD): δ
116.2, 118.6, 124.4, 127.9, 128.7, 133.4, 135.8, 137.9, one signal
overlapped. IR (Nujol): νmax/cm-1 2234 (conjugated CN). ESI-
MS: C17H11N4 [Mþ] 273 (4%), 272 (18), 271 (78). Anal. Calcd
13C NMR [600 MHz, DMSO-d6, with K2CO3 added to dissolve
6(Cl)]: δ 122.1, 123.0, 126.4, 130.55, 130.6, 132.9, 135.0, 135.6,
166.1. IR (Nujol): νmax/cm-1 1693 (CdO). HRMS. Calcd for
C17H13N2O4 [Mþ]: 309.0875. Found: 309.0869. Anal. Calcd for
C17H13N2O4Cl: C, 59.22; H, 3.81; N, 8.13. Found: C, 60.70; H,
3.66; N, 8.10 (analysis for carbon consistently high). Crystals of
6(FeCl4) were obtained by the slow evaporation of a methanol
solution of 6(Cl) and ferric chloride.
for C17H11N4Cl 1/4H2O: C, 65.69; H, 3.73; N, 18.00. Found: C,
3
65.23; H, 3.74; N, 18.05.
{[Zn2(μ2-HCOO)(6)2](NO3) 1.5DMF}n, 7. Ligand 6(Cl)
3
1,3-Bis(4-carboxyphenyl)imidazolium Bromide, 5(Br). To a
solution of paraformaldehyde (0.98 g, 33 mmol) in toluene
(40 mL) was added 4-cyanoaniline (7.72 g, 65.2 mmol) and
aqueous glyoxal [40% (v/v), 4.74 g, 32.6 mmol] followed by the
dropwise addition of HCl [37% (v/v), 3.72 mL, 32.6 mmol]. The
mixture was refluxed until ca. 6.5 mL of water was recovered
using a Dean-Stark apparatus. The solvent was removed in
vacuo, HBr (90 mL) added, and the solution heated at reflux
overnight. The product was collected as a pale-brown powder by
filtration, washed with ethyl acetate, and dried (8.60 g, 69%).
Mp: >300 °C. 1H NMR (200 MHz, CD3OD): δ 7.88 (d, J=8.4,
4H, H30/H50), 8.03 (d, J = 8.4, 4H, H20/H60), 8.48 (s, 2H,
H4/H5), 10.38 (s, 1H, H2). 1H NMR (200 MHz, DMSO-d6): δ
8.09 (d, J=8.5, 4H, H30/H50), 8.24 (d, J=8.5, 4H, H20/H60), 8.71
(s, 2H, H4/H5), 10.60 (s, 1H, H2), 13.31 (s, b, COOH). 13C
NMR [600 MHz, D2O with K2CO3 added to dissolve 5(Br)]:
δ 121.0, 122.1, 130.4, 130.6, 135.7, 138.1, 173.3. IR (Nujol):
(50.0 mg, 0.15 mmol) and zinc nitrate (60 mg, 0.30 mmol) were
sealed in a 20 mL glass vial with a Teflon-lined screw cap along
with dimethylformamide (DMF; 2 mL). The mixture was
sonicated for 5 min and then heated at 120 °C for 5 days, cooled,
and allowed to stand at room temperature for 7 days. The
mother liquor was decanted and the solid washed with DMF
and then CHCl3 to yield 7 as pale-brown powder. Yield: 50.0 mg
(69%). IR (KBr disk): νmax/cm-1 1581 (s), 1381 (s). Anal. Calcd
for Zn2C40.5H38.5N6.5O14.5Cl3 (CHCl3 solvate): C, 44.81; H,
3.58; N, 8.39. Found: C, 44.96; H, 3.55; N, 8.40. Crystals suitable
for X-ray crystallography were obtained by performing the
same reaction with a 16-fold excess of zinc nitrate at 100 °C.
Single-Crystal X-ray Crystallography. Crystals were mounted
under oil on a plastic loop. X-ray diffraction data were collected with
˚
(i) Mo KR radiation (λ=0.710 73 A) at 90(2) K using a Bruker AXS
Single Crystal Diffraction System fitted with an Apex II CCD
detector ({3(NO3)}2 and 7) or (ii) synchrotron radiation (λ =
˚
0.7107 A) at 150(2) K using the Protein Microcrystal and Small
ν
max/cm-1 1715 (CdO). HRMS. Calcd for C17H13N2O4 [Mþ]:
309.0875. Found: 309.0862. Anal. Calcd for C17H13N2O4Br: C,
52.46; H, 3.37; N, 7.20. Found: C, 52.64; H, 3.54; N, 8.27
(analysis for nitrogen consistently high). Crystals of 5(ClO4)
were obtained by the slow evaporation of a methanol solution of
5(Br) and zinc(II) perchlorate.
Molecule X-ray Diffraction beamline (PX2) at the Australian Syn-
chrotron (Table 1). Data sets were corrected for absorption using a
multiscan method, and structures were solved by direct methods
using SHELXS-9733 and refined by full-matrix least squares on F2
by SHELXL-97,34 interfaced through the program X-Seed.35 In
general, all non-hydrogen atoms were refined anisotropically, and
hydrogen atoms were included as invariants at geometrically esti-
mated positions, unless specified otherwise in additional details
below. Figures were produced using the program POV-Ray,36
1,3-Bis(3-carboxyphenyl)imidazolium Chloride, 6(Cl). Using
the method described for 5(Br), compound 6(Cl) was isolated
from the reaction as an off-white powder by filtration, washed
1
with ethyl acetate, and dried (8.28 g, 76%). Mp: >300 °C. H
NMR (600 MHz, CD3OD): δ 7.82 (t, J=7.8, 2H, H50), 8.10 (d,
J=7.8, 2H, H60), 8.27 (d, J=7.8, 2H, H40), 8.38 (s, 2H, H4/H5),
8.48 (s, 2H, H20), 10.26 (s, 1H, H2). 1H NMR [600 MHz,
DMSO-d6, with K2CO3 added to dissolve 6(Cl)]: 7.84 (t, J =
7.8, 2H, H50), 8.15 (m, 4H, H40, H60), 8.48 (s, 2H, H20), 8.66 (s,
2H, H4/H5), 10.54 (s, 1H, H2). 13C NMR (600 MHz, CD3OD):
δ 124.4, 125.3, 128.4, 132.4, 133.0, 135.1, 137.0, 137.0, 168.2.
(33) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(34) Sheldrick, G. M. SHELXL-97; University of Gottingen: Gottingen,
Germany, 1997.
€
€
(35) Barbour, L. J. J. Supramol. Chem. 2001, 1, 189.
(36) POV-Ray 3.6; Persistence of Vision Raytracer Pty Ltd.: Williamstown,
Australia, 2004.

Article
Inorganic Chemistry, Vol. 49, No. 4, 2010 1715
Scheme 2. Preparation of Imidazolium Ligands 3(Cl), 4(Cl), 5(Br), and 6(Cl)
interfaced through the program X-Seed. Publication materials were
prepared using the program enCIFer.37 Details of data collections
and structure refinements are given below. CCDC-752222-CCDC-
752225 contain the supplementary crystallographic data for this
structure (see the Supporting Information). These data can be
obtained free of charge from The Cambridge Crystallographic Data
Center via www.ccdc.cam.ac.uk/data_request/cif.
The structure of compound 7 possesses large voids containing
a considerable number of diffuse electron density peaks that
could not be adequately modeled as a solvent. The SQUEEZE
routine of PLATON38 was applied to the collected data, which
resulted in reductions in R1 and particularly wR2. R1, wR2, and
GOF before the SQUEEZE routine: 5.19%, 17.35%, and 1.076.
R1, wR2, and GOF after the SQUEEZE routine: 3.93%,
11.72%, and 1.078.
aniline and an R-dicarbonyl such as glyoxal, before cycliza-
tion of DAB with formaldehyde. In the synthesis of the
imidazolium compounds reported here, we opted for the
latter method, Scheme 2. Thus, the reaction of the isomeric
cyanoanilines (1 and 2) with glyoxal and paraformaldehyde
yielded 3 and 4 in good yield (75%) for both compounds.
These could be isolated as either their chloride (as synthe-
sized, 3, and 4) or hexafluorophosphate salts. Conversion to
the dicarboxylic acid derivatives 5 and 6 was achieved by
treatment of dinitriles 3 and 4 with hydrobromic acid. Other
acids such as sulfuric acid were ineffective in this conversion.
The synthesis of 5 and 6 could also be carried out in excellent
yield in a one-pot, two-stage reaction without isolating the
dinitrile compounds. A closely related imidazolium dicarbo-
xylate ligand was recently synthesized from 1,3-bis(4-iodo-
phenyl)imidazolium chloride by a palladium-catalyzed reac-
tion with carbon monoxide in methanol.16
Powder X-ray Diffraction (PXD) Analysis. PXD data were
collected using an image-plate Guinier camera set using Co KR
radiation at the South Australian Museum. Diffraction data
were collected in the range 2θ=5-100° with a step size of 2θ=
0.005° at 298 K. Theoretical powder patterns were calculated
with MERCURY39 using the single-crystal diffraction data
generated for 7 as a model.
Compounds 3-6 were characterized by a combination of
NMR and IR spectroscopy, ES mass spectrometry, elemen-
tal analysis, and, in the case of 3(NO3), 5(ClO4), and 6
(FeCl4), X-ray crystallography. Compounds 3-6 are reason-
ably soluble in polar solvents, methanol, DMSO, and DMF,
and can be made more soluble in chlorinated organic solvents
Results and Discussion
Two general routes are available for the synthesis of the
diarylimidazolium precursors used for the generation of
NHCs. The first and less widely utilized route involves
appending the appropriate heteroaryl substituents to either
imidazole or N-methylimidazole,40 while the second, more
common approach, requires formation of the central hetero-
cyclic imidazole ring during the synthesis of the imidazolium
compound.41,42 This second approach requires the synthesis
of a diazabutadiene (DAB) intermediate from 2 equiv of an
1
by conversion to their hexafluorophosphate salts. The H
NMR spectra of 3-6 all have diagnostic peaks correspond-
ing to the imidazolium H2 proton in the range 10.20-
10.60 ppm. IR spectra have characteristic peaks for the
CtN stretch at 2230 and 2234 cm-1 for 3 and 4, respectively.
After conversion to the dicarboxylic acid derivatives 5 and 6,
the CtN stretch disappears and a CdO stretch is observed
for both compounds. Elemental analysis for 3 and 4 was
readily fitted to the calculated values, but despite consistent
analyses, good fits between the calculated and experimental
values were not obtained for 5 and 6. The analyses obtained
are consistently high in nitrogen for 5 and high in carbon
for 6 presumably because of contamination with residual
cyanoaniline [5(Br)] starting material or carboxylate [6(Cl)]
coproduct.
(37) enCIFer; Cambridge Structural Database Centre: Cambridge, U.K.,
2005.
(38) Spek, A. L. Acta Crystallogr. 1990, A46, C34.
(39) Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.; Edgington, P. R.;
McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; Streek, J. v. d.;
Wood, P. A. J. Appl. Chem. 2008, 41, 466.
(40) Catalano, V. J.; Malwitz, M. A.; Etogo, A. O. Inorg. Chem. 2004, 43,
5714.
(41) Jung, I. G.; Seo, J.; Chung, Y. K.; Shin, D. M.; Chun, S.-H.; Son,
S. U. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 3042.
(42) Alexander, S. G.; Cole, M. L.; Morris, J. C. New J. Chem. 2009, 33,
720.
As part of an investigation of the coordination chemistry
of 3, crystals of the compound as its nitrate salt, {3(NO3)}2,
were obtained by the reaction of 3(Cl) with silver(I) nitrate.
The asymmetric unit contains two molecules of the cation 3

1716 Inorganic Chemistry, Vol. 49, No. 4, 2010
Crees et al.
Figure 2. Perspective view of 5(ClO4), confirming the structure of the
dicarboxylic acid and its rigid angular structure.
Figure 1. Perspective view of one of the molecules of 3 from the crystal
structure {3(NO3)}2, confirming the structure of the compound and its
rigid angular structure.
and two nitrate counterions. Aside from subtle differences in
hydrogen-bonding interactions, both molecules of 3 in the
asymmetric unit have near-identical bond lengths and angles.
Consequently, the remainder of the discussion focuses on
only one of the molecules in the asymmetric unit. The crystal
structure confirms the structure of the dinitrile compound 3
(Figure 1) and demonstrates the suitability of these types of
compounds as rigid components for MOF synthesis. Ligands
constructed on the imidazolium scaffold with donor groups
in the 4 position of the phenyl ring attached to the central
imidazole (i.e., 3 and 5) are angular components with
approximately 140° between the two donors. In the case of
compound 3, the angle between the two nitrile donors, which
Figure 3. Perspective view of 6(FeCl4), confirming the structure of the
dicarboxylic acid 6 and its U-shaped conformation.
˚
˚
DC16-O33=3.26 A and dH26-O34=2.62, DC26-O34=3.51 A.
The carboxylic acid moieties are also hydrogen-bonded, with
˚
much shorter hydrogen-bonding distances (1.87 and 1.94 A),
˚
are separated by approximately 14.99 A, is 143.1°.
to additional perchlorate molecules within the crystal lattice.
This prevents the formation of hydrogen bonds between
molecules of 5 in the solid state.
The salt 6(FeCl4) crystallized in the orthorhombic space
group Pmna with half a molecule of 5 and half a tetrachloro-
ferrate anion in the asymmetric unit. A perspective view of 6
(FeCl4) is shown in Figure 3, with only the shortest
Compound 3 is not planar in the solid state with torsion
angles C16-C11-N1-C2 and C2-N3-C21-C26 of 39.0
and 47.2°, respectively. In the solid state, 3 and the nitrate
anion form an intricate hydrogen-bonded network. Figure 1
illustrates C-H O hydrogen bonds involving only the
3 3 3
nitrate ion that sits in the cleft formed by the imidazolium
ring of 3. The C-H O hydrogen-bond distances are
C-H O hydrogen bonds involving the anion that interacts
3 3 3
3 3 3
˚
dH16A-O37=2.47, DC16-O37=3.22 A and dH2A-O38=2.28,
with the imidazolium pocket shown. As expected, the struc-
tureis once again similarto compounds3 and 5 but with quite
a different orientation of the two carboxylic acid groups; in
the solid-state structure of 6(FeCl4), compound 6 adopts a
U-shaped arrangement of the donors. Compound 6 is non-
planar in the solid state with a torsion angle for the biaryl
bonds between the central five-membered imidazolium ring
and the peripheral phenyl rings of 45.7° (C16-C11-
N1-C2). The tetrachloroferrate anion, which is formed in
the crystallization solution, lies in a central pocket formed by
the imidazolium salt and is weakly hydrogen-bonded to H16,
˚
DC2-O38=3.17 A.
Crystals of the two carboxylic acid derivatives 5 and 6 were
obtained from attempted reactions of these compounds with
transition-metal salts under ambient conditions in methanol.
Compound 5 crystallized as a perchlorate salt, 5(ClO4),
following reaction with zinc perchlorate, while 6(FeCl4)
crystals were obtained by the cocrystallization of compound
6 and ferric chloride. These structures support spectroscopic
data obtained for 5(Br) and 6(Cl) and confirm the expected
structures for these compounds.
˚
The salt 5(ClO4) crystallized in the monoclinic space group
P21/c with one complete molecule of 5 and a perchlorate
anion in the asymmetric unit. A perspective view of 5(ClO4) is
where dH16-Cl22=2.75, DC16-Cl22=3.68 A. Once again the
carboxylic acid moieties are also hydrogen-bonded, this time
with additional molecules of 6, which results in an undulating
1D hydrogen-bonded tape that extends along the b axis of the
unit cell.
shown in Figure 2, with the shortest C-H O hydrogen
3 3 3
bonds shown. As expected, the structure is very similar to the
dinitrile ligand 3, with an almost identical angle of 143°
between the two carboxylic acid groups. Like the dinitrile 3,
compound 5 is not planar in the solid state, with torsion
angles for the biaryl bonds between the central five-membered
imidazolium ring and the peripheral phenyl rings of 32.1°
(C16-C11-N1-C2) and 38.0° (C2-N3-C21-C26). Once
again the anion lies in a central pocket formed by the
imidazolium salt, although it is not hydrogen-bonded to
Using solvothermal methods, compounds 5 and 6 have
-
been reacted with a number of different zinc salts (e.g., NO3
and CH3COO-), indifferent stoichiometries, using a range of
solvents (e.g., methanol, DMF, and diethylformamide) and
with a diverse range of coligands. These reactions invariably
led to amorphous or microcrystalline powders. When 6(Cl)
was reacted with zinc nitrate in DMF, a reaction product was
formed that is composed of colorless blocklike crystals and a
pale-brown-yellow powder. The crystals could be separa-
ted manually and were found to be suitable for single-crystal
H2. The shortest C-H O hydrogen bonds within this
3 3 3
pocket involve H16 and H26, where dH16-O33 = 2.60,

Article
Inorganic Chemistry, Vol. 49, No. 4, 2010 1717
Figure 4. Perspective view of the undulating structure of the 1D necklace-type coordination polymer in the crystal structure of 7.
X-ray crystallography. The structure of 7 was shown to have
the formula {[Zn2(μ2-HCOO)(6)2](NO3) 1.5DMF}n. The
3
phase purity of the bulk samples was confirmed by powder
diffraction and IR spectroscopy.
Unfortunately, the structure of 7 was not shown to be the
targeted type of three-dimensional MOF but a lower dimen-
sional 1D polymer (Figure 4). Nonetheless, the structure is
the first zinc-containing MOF structure incorporating an
imidazolium precursor and possesses a number of very
interesting features. In particular, it is interesting to note that
the imidazolium ring is unaffected by the harsh reaction
conditions used during the synthesis of 7. The 1D polymeric
structure is of the necklace type with two molecules of 6 and a
formate anion bridging between each dizinc carboxylate
secondary building unit (SBU). The cleft formed by the two
molecules of 6 houses a nitrate anion and a DMF solvate
molecule.
Figure 5. Perspective view of the Zn2 SBUs in 7. The μ2-bridging
formate anion, a hydrogen-bonded nitrate anion, and a DMF solvate
molecule are shown in the pocket formed by two molecules of 6, which
Compound 7 crystallizes in the monoclinic space group
C2/c with a zinc cation, two half-ligand components (both on
mirror planes), half a formate anion, half a nitrate anion, and
two disordered and partially occupied DMF molecules in the
asymmetric unit. The residual electron density was found to
be located in channels in the structure but was unable to be
accurately modeled. The SQUEEZE routine of PLATON38
was applied to the data.
˚
connect the Zn2 SBUs in the structure. Selected bond lengths (A) and
angles (deg): Zn1-O41 1.963(2), Zn1-O18 2.022(2), Zn1-O39 2.039(2),
Zn1-O19 2.060(2), Zn1-O38 2.062(2); O41-Zn1-O18 109.34(7),
O41-Zn1-O39 101.48(7), O18-Zn1-O39 88.38(7), O41-Zn1-O19
92.55(7), O18-Zn1-O19 158.10(8), O39-Zn1-O19 86.64(7),
O41-Zn1-O38 100.00(7), O18-Zn1-O38 90.17(7), O39-Zn1-O38
157.69(7), O19-Zn1-O38 86.45(7).
and DMF solvate (O50) are dH21-O67 = 2.10, DC21-O67
3.11 A and dH1-O50=2.31, DC1-O50=3.19 A.
=
The dizinc carboxylate SBUs are paddlewheel clusters43
that are commonly observed with carboxylate ligands
˚
˚
Formate has been previously used for the formation of
MOFs.45 The inclusion of a formate ion in the structure of7 is
serendipitous, although we have also tried introducing other
carboxylate ligands into the reaction mixture. It has two
probable origins, either from decomposition of the DMF
solvent46 or from decarboxylation of the ligand. With the
isomeric carboxylic acid ligand 5, we have also observed the
formation of MOFs containing only formate as a ligand
under quite similar conditions. No evidence for the decarbo-
xylation of either 5 or 6 has been observed, and presumably
the formate observed in 7 arose from decomposition of the
solvent during the reaction.46
The 1D necklace-type coordination polymers extend along
the b axis of the unit cell. The arrangement of the ligands
around the polymer leads to a cross-shaped structure (when
viewed along the polymer chain) that interdigitates to com-
plete the crystal packing (Figure 6a). The crystal structure of
˚
(Figure 5). The zinc atoms are separated by 3.01 A with
˚
typical Zn-O bonds of between 2.022(2) and 2.062(2) A. The
dizinc SBUs are capped by oxygen atoms from a bridging
˚
formate anion with a Zn-O bond length of 1.963(2) A. Thus,
the zinc atoms are five-coordinate with a τ5 value of 0,44
which indicates a square-pyramidal geometry. Within the 1D
polymer, each SBU is connected to an adjacent SBU by two
molecules of 6 and a μ2-bridging formate anion. Each dizinc
˚
SBU is separated from an adjacent SBU by 9.01 A.
The U-shaped conformation of ligand 6, observed in the
structure of 6(FeCl4), is maintained in the MOF 7. This
results in an undulating or zigzag structure to the 1D
polymer. The nitrate anion and a DMF solvate molecule
are hydrogen-bonded within the U-shaped clefts of the two
molecules of 6 that connect two zinc SBUs. The C-H
O
3 3 3
hydrogen-bond distances involving the imidazolium H2
protons and the oxygen atoms of the nitrate anion (O67)
(45) Wang, Z.; Zhang, Y.; Kurmoo, M.; Liu, T.; Vilminot, S.; Zhao, B.;
Gao, S. Aust. J. Chem. 2006, 59, 617.
(46) Burrows, A. D.; Cassar, K.; Friend, R. M. W.; Mahon, M. F.; Rigby,
S. P.; Warren, J. E. CrystEngComm 2005, 7, 548.
(43) Vagin, S. I.; Ott, A. K.; Rieger, B. Chem.-Ing.-Tech. 2007, 79, 767.
(44) Addison, A. W.; Rao, T. N.; Reedijk, J.; Rijn, J. v.; Verschoor, G. C.
J. Chem. Soc., Dalton Trans. 1984, 1349.

1718 Inorganic Chemistry, Vol. 49, No. 4, 2010
Crees et al.
Figure 6. Perspective views of a space-filling representation of 7 viewed down (a) the b and (b) c axes, respectively. Disordered solvent molecules are not
shown.
Figure 7. Comparison of the experimental PXD data for a sample of as-synthesized 7 and the calculated pattern from single-crystal X-ray data for 7.
7 also reveals that the 1D polymers pack in the crystal with
weak π-stacking interactions between the chains. The struc-
ture has disordered DMF solvate molecules in channels that
extend along the b and c axes (Figure 6b). The channels that
are directed along the c axis are much larger, and we are in the
process of investigating the porosity and sorption characteri-
stics of 7.
for solvated samples from bulk preparations of 7. Com-
parison of the patterns for bulk samples of 7 [prepared
from 6(Cl) at 120 or 130 °C with 2 or 4 equiv of zinc nitrate]
and the calculated pattern for 7 (from the single-crystal
structure) showed that, with the exception of a few in-
tensity differences, the patterns were in good agreement
with each other (Figure 7).
The synthesis of compound 7 has been studied under a
number of different reaction conditions. The best yields of
7 as a pure phase were obtained by heating the reaction
mixture consisting of 6(Cl) and zinc nitrate (1:2-1:4 L:M
ratios) at 120-130 °C for 4-5 days. In this manner,
crystalline powders in up to 69% yield were obtained. At
higher temperatures and using higher ratios of zinc nitrate,
the isolated material contained very little 7 (although at
120-130 °C crystals suitable for X-ray crystallography
were obtained despite a vast excess of zinc nitrate). Reac-
tions undertaken at lower temperatures gave an amor-
phous material in low yield. PXD studies were undertaken
to investigate further the characteristics of 7 and, in
particular, to compare the single-crystal structure and
the bulk material of 7. The PXD patterns were recorded
Conclusion
In this paper, we have described the high-yielding synthesis
of four new imidazolium salt ligands 3-6 bearing additional
donor groups suitable for the formation of MOFs. These
ligands were prepared in one or two simple steps from
commercially available anilines. While these ligands do not
have the steric bulk around the imidazolium moiety com-
monly found in such compounds, they are useful and
effective structural analogues for more bulky and compli-
cated imidazolium ligands. The crystal structures of com-
pounds 3 and 5 reveal that these compounds are angular
components with approximately 143° between the donor
groups, while 6 has a U-shaped conformation in the solid
state.

Article
When 6(Cl) was reacted with zinc nitrate, using solvother-
Inorganic Chemistry, Vol. 49, No. 4, 2010 1719
Fellowship and a Future Fellowship (Grant FT099-
1910) and for support of this research through a Dis-
covery Project (DP0773011). The University of Otago is
acknowledged for providing access to facilities for X-ray
crystallography. The Australian Synchrotron is thanked
for funding travel and access to the PX1 and PX2
beamlines (AS091 and AS092). The views expressed
herein are those of the authors and are not necessarily
those of the owner or operator of the Australian Syn-
chrotron. The authors thank Prof. Allan Pring and
Dr. Jiafang Bei from the South Australian Museum
for recording the PXD data. Dr. Jonathan Morris is
acknowledged for helpful discussions.
mal conditions, an unusual dizinc paddlewheel-based MOF
was formed. In addition to having an undulating 1D necklace-
type structure that incorporates an anionic formate bridging
ligand, the 1D polymeric material is packed in the crystals with
a potentially porous structure. The imidazolium core is un-
affected by the harsh conditions employed in this reaction and
is available for subsequent postsynthetic modifications that
are a current focus for our research. This result, along with the
structures recently reported by Chun et al.,16 suggests that the
future for MOF materials formed from such imidazolium
ligands is bright. Our future investigations will focus on the
synthesis of further MOF structures derived from imidazo-
lium-containing ligands and, in particular, the study of post-
synthetic modifications or the physisorption of gases within
MOF 7 and other similar structures.
Supporting Information Available:
NMR spectra for
5(Br) and 6(Cl) and crystallographic information files in CIF
format for {3(NO3)}2, 5(ClO4), 6(FeCl4), and 7. This material
is available free of charge via the Internet at http://pubs.
acs.org.
Acknowledgment. C.J.S. thanks the Australian
Research Council for an Australian Post-Doctoral