Ionothermal synthesis of homochiral framework with acetate-pillared cobalt-camphorate architecture

Article abstract of DOI:10.1021/ic8007798

A new organically templated homochiral material (EMIm)[Co ₂-(D-cam)₂(ac)] (1; D-H₂cam = D-camphoric acid; ac = acetate; EMIm = 1-ethyl-3-methylimidazolium) has been ionothermally synthesized, and it features an unusual acetate-pillared cobalt-camphorate architecture encapsulating the cationic component of the ionic liquid.

Full text of DOI:10.1021/ic8007798

Inorg. Chem. 2008, 47, 5567-5569
Ionothermal Synthesis of Homochiral Framework with Acetate-Pillared
Cobalt-Camphorate Architecture
Shumei Chen, Jian Zhang, and Xianhui Bu*
Department of Chemistry and Biochemistry, California State UniVersity, Long Beach,
1
250 Bellflower BouleVard, Long Beach, California 90840
Received April 29, 2008
A new organically templated homochiral material (EMIm)[Co
2
-
dimethylamine resulting from the decomposition of DMF
serves as the cationic structure-directing agent to direct the
formation of the homochiral anionic framework. It is worth
noting that the anionic framework, while very common in
zeolites, phosphates, and many other oxides and chalco-
2 2
(D-cam) (ac)] (1; D-H cam ) D-camphoric acid; ac ) acetate; EMIm
)
1-ethyl-3-methylimidazolium) has been ionothermally synthe-
sized, and it features an unusual acetate-pillared cobalt-camphorate
architecture encapsulating the cationic component of the ionic
liquid.
5
,6
genides, only accounts for an unusually small fraction of
MOFs. Homochiral anionic MOFs are even rarer.
The synthesis of the homochiral MOFs with the anionic
framework, in a way similar to the synthesis of zeolites, could
bring new applications associated with the unique compo-
sitional and topological features of such materials. The use
of DMF for the synthesis of homochiral anionic frameworks
mentioned above does not represent a general synthetic
method. We are therefore in search of other methods that
are generally applicable to different compositional domains.
The ionothermal synthesis, which has undergone significant
Homochiral metal-organic frameworks (MOFs) have
attracted increasing attention because of their potential
applications in asymmetric catalysis and enantioselective
1–3
separation. The use of organic enantiopure chiral building
blocks for the construction of extended frameworks has been
shown to be a useful method for the synthesis of homochiral
1–4
materials. With few exceptions, known three-dimensional
3D) homochiral MOFs usually have neutral frameworks and
(
7–10
occasionally cationic frameworks charge-balanced with
development in the past several years,
represents a viable
2
simple inorganic anions. In our own work with chiral
option for our purpose. The ionic liquid can serve as both
the solvent and the template (sometimes also called a
camphoric acid, all resulting camphorate frameworks found
so far are neutral when the syntheses are performed using
7
b
structure-directing agent). Of particular interest is a recent
study by Morris et al. on the use of a chiral ionic liquid to
4
a–d
water or mixed water-alcohol as the solvent.
One rare
occurrence of the anionic homochiral framework is when a
mixture of dimethylformamide (DMF)-alcohol (ethanol or
butanol) was used as the solvent. In this case, the protonated
(5) (a) Attfield, M. P.; Al-Ebini, Y.; Pritchard, R. G.; Andrews, E. M.;
Charlesworth, R. J.; Hung, W.; Masheder, B. J.; Royal, D. S. Chem.
Mater. 2007, 19, 316. (b) Harvey, G.; Herve, A. C.; Hailes, H. C.;
Attfield, M. P. Chem. Mater. 2004, 16, 3756. (c) Harvey, G.; Teat,
S. J.; Tang, C. C.; Cranswick, L. M.; Attfield, M. P. Inorg. Chem.
2003, 42, 2428.
4e
*
To whom correspondence should be addressed. E-mail: xbu@csulb.edu.
(
1) (a) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.; Kim,
K. Nature 2000, 404, 982. (b) Vaidhyanathan, R.; Bradshaw, D.;
Rebilly, J.-N.; Barrio, J. P.; Gould, J. A.; Berry, N. G.; Rosseinsky,
M. J. Angew. Chem., Int. Ed. 2006, 45, 6495. (c) Kepert, C. J.; Prior,
T. J.; Rosseinsky, M. J. J. Am. Chem. Soc. 2000, 122, 5158. (d)
Bradshaw, D.; Prior, T. J.; Cussen, E. J.; Claridge, J. B.; Rosseinsky,
M. J. J. Am. Chem. Soc. 2004, 126, 6106.
(6) (a) Forster, P. M.; Eckert, J.; Chang, J.-S.; Park, S.-E.; F e´ rey, G.;
Cheetham, A. K. J. Am. Chem. Soc. 2003, 125, 1309. (b) Guillou, N.;
Gao, Q.; Forster, P. M.; Chang, J.-S.; Nogu e` s, M.; Park, S.-E.; F e´ rey,
G.; Cheetham, A. K. Angew. Chem., Int. Ed. 2001, 40, 2831.
(7) (a) Cooper, E. R.; Andrews, C. D.; Wheatley, P. S.; Webb, P. B.;
Wormald, P.; Morris, R. E. Nature 2004, 430, 1012. (b) Parnham,
E. R.; Morris, R. E. Acc. Chem. Res. 2007, 40, 1005. (c) Parnham,
E. R.; Morris, R. E. J. Am. Chem. Soc. 2006, 128, 2204. (d) Lin, Z.;
Slawin, A. M. Z.; Morris, R. E. J. Am. Chem. Soc. 2007, 129, 4880.
(e) Lin, Z.; Wragg, D. S.; Warren, J. E.; Morris, R. E. J. Am. Chem.
Soc. 2007, 129, 10334.
(
(
2) (a) Kesanli, B.; Lin, W. Coord. Chem. ReV. 2003, 246, 305. (b) Wu,
C.-D.; Lin, W. Chem. Commun. 2006, 3673. (c) Wu, C. D.; Lin, W.
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Int. Ed. 2003, 42, 5314. (b) Dybtsev, D. N.; Yutkin, M. P.;
Peresypkina, E. V.; Virovets, A. V.; Serre, C.; F e´ rey, G.; Fedin, V. P.
Inorg. Chem. 2007, 46, 6843.
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4) (a) Zhang, J.; Bu, X. Angew. Chem., Int. Ed. 2007, 46, 6115. (b) Zhang,
J.; Chen, S.; Valle, H.; Wong, M.; Austria, C.; Cruz, M.; Bu, X. J. Am.
Chem. Soc. 2007, 129, 14168. (c) Zhang, J.; Yao, Y.-G.; Bu, X. Chem.
Mater. 2007, 19, 5083. (d) Zhang, J.; Bu, X. Chem. Commun. 2008,
(10) (a) Jin, K.; Huang, X.; Pang, L.; Li, J.; Appel, A.; Wherland, S. Chem.
Commun. 2002, 2872. (b) Dybtsev, D. N.; Chun, H.; Kim, K. Chem.
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4
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2
X. Inorg. Chem. 2008, 47, 3495.
1
0.1021/ic8007798 CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 13, 2008 5567
Published on Web 06/07/2008

COMMUNICATION
induce homochirality in a structure constructed of entirely
7
d
achiral building blocks. However, there have been few
examples of homochiral MOFs prepared in ionic liquids by
using enantiopure structural building blocks.
It is possible to construct 3D homochiral frameworks by
4
e
cross-linking metal centers with only enantiopure ligands,
however, the number of 3D frameworks that can be produced
through this approach has been limited. It can be advanta-
geous to apply a dual-ligand synthetic strategy because the
use of two functionally different complementary ligands
provides an additional level of control in the framework
structure and charge density distribution. One successful
dual-ligand synthetic strategy in crystal engineering is to
combine a bi- or polydentate carboxylate ligand with a
Figure 1. Crystal structure of 1 showing the atom-labeling scheme with
3
0% probability. H atoms were omitted for clarity. [Symmetry code: (a)
1
1
dipyridine ligand. In our earlier work involving chiral
camphorates, a common structural mode resulting from the
use of this dual-ligand method is that divalent metal cations
1
3
1
1
1
-
/2 + x, /2 - y, -1 - z; (b) - /2 + x, /2 - y, -1 - z; (c) - /2 - x, 1
1
- y, - /2 + z.]
Four carboxylate groups from four D-cam ligands bridge the
dinuclear Co centers into a paddle-wheel unit with a
metal-metal separation of 2.803 Å. The apical site of each
Co center is occupied by one O atom of the pillaring acetate
ligand. The Co · · · O distances range from 2.006(6) to
(
or clusters) form neutral chains or sheets with deprotonated
chiral comphorates, which are then linked or pillared by
auxiliary neutral bipyridine ligands into 3D homochiral
frameworks.
4
d,f
In this work, we show that homochiral metal-camphorate
2
.059(7) Å. The cross-linking of adjacent dimers by D-cam
layers can also be pillared by the anionic carboxylate ligand.
ligands leads to a homochiral grid-like (4,4) layered sub-
structure parallel to the ab plane (Figure 2a). The auxiliary
acetate ligands with the anti-anti conformation serve as
We report here a new homochiral material, (EMIm)[Co
2
(D-
cam) (ac)] (1; D-H cam ) D-camphoric acid; ac ) acetate;
2
2
EMIm ) 1-ethyl-3-methylimidazolium), which was iono-
thermally synthesized and features an organically templated
n
pillars to link the [Co(D-cam)] layer into an anionic six-
connected 3D network (Figure 2b). The Co · · · Co distance
separated by the acetate ligand is 5.991 Å. This noninter-
penetrating framework can be simplified into the primitive
3
D framework with a pillared architecture. While it is
common for a mononegative acetate ligand to bridge two
metal sites, it is quite rare for it to function as pillars between
homochiral layers.
1
4
cubic lattice (denoted: pcu net ).
The pillared 3D architecture contains 2D channels parallel
to the ab plane. The channel along the a axis has the
dimension of 5.991 × 6.720 Å. The solvent-accessible
The ionothermal reaction of Co(ac)
in 1-ethyl-3-methylimidazolium bromide (EMIm-Br) at 120
C for 7 days generates purple crystals of 1. The phase
2
·6H
2 2
O and D-H cam
1
2
°
3
volume is approximately 1032.1 Å per unit cell volume,
purity is supported by powder X-ray diffraction (Figure S1
in the Supporting Information).
and the pore volume ratio is calculated to be 32.8% using
1
5
1
3
the PLATON program. The EMIm cations serve as extra-
framework guest molecules to fill the void space and charge-
balance the anionic framework. It is interesting to note that
there are two types of channels along the b axis. One is
occupied by the EMIm cations, and another is obstructed
by the protruding methyl groups of the organic ligands
Single-crystal X-ray diffraction shows that compound
crystallized in chiral space group P2 . The asymmetric
1
1 1 1
2 2
II
unit of 1 contains two Co ions, two deprotonated D-cam
ligands, one deprotonated acetate ligand, and one EMIm ion
Figure 1). The coexistence of the enantiopure ligand D-cam
and the achiral ligand acetate in the structure represents an
unusual structural mode in homochiral open frameworks.
Both Co1 and Co2 sites adopt square-pyramidal geometry.
(
(Figure 3b). The location of the EMIm cation in the channel
is also of interest. As shown in Figure 3a, each EMIm cation
is located at the center of a four-membered ring formed via
four paddle-wheel units, two D-cam ligands, and two ac
ligands. The R-methyl groups of two D-cam ligands and two
ac ligands all point away from the center of the four-
membered ring, which leaves enough space for the EMIm
cation.
(
11) (a) Chen, B.; Yang, Y.; Zapata, F.; Lin, G.; Qian, G.; Lobkovsky,
E. B. AdV. Mater. 2007, 19, 1693. (b) Chen, B.; Ma, S.; Zapata, F.;
Fronczek, F. R.; Lobkovsky, E. B.; Zhou, H.-C. Inorg. Chem. 2007,
4
6, 1233. (c) Chen, B.; Ma, S.; Hurtado, E. J.; Lobkovsky, E. B.;
Liang, C.; Zhu, H.; Dai, S. Inorg. Chem. 2007, 46, 8705.
(
12) Synthesis of 1: Co(ac)2 ·6H2O (0.3822 g) and D-H2cam (0.2009 g) in
1
-ethyl-3-methylimidazolium bromide (EMIm-Br; 1.848012. g) were
mixed in a 23 mL Teflon cup. The vessel was then sealed and heated
at 120 °C for 7 days. The autoclave was subsequently allowed to cool
to room temperature. The purple crystals of 1 were obtained (85%
yield based on H2cam).
The thermal behavior of 1 was analyzed on a crystalline
-
1
sample from 30 to 1000 °C at a heating rate of 10 °C min ,
under a nitrogen atmosphere with a flowing rate of 100 mL
(
13) Crystal data for 1: C28H24Co2N2O10, Mr ) 684.50, orthorhombic, space
-
1
group P212121, a ) 13.3346(10) Å, b ) 13.4613(9) Å, c ) 17.5115(10)
min (Figure S2 in the Supporting Information). The
3
-3
Å, V ) 3143.3(4) Å , Z ) 4, Dc ) 1.446 g cm , Flack parameter )
thermogravimetric curve of 1 is largely unchanged up to 275
0
.05(4), R1 (wR2) ) 0.0628 (0.1655), and S ) 1.076 for 4042
reflections with I > 2σ(I). The structure was solved by direct methods
followed by successive difference Fourier methods. All non-H atoms
were refined anisotropically. Computations were performed using
(14) For definitions of the three-letter abbreviations, see Reticular Chemistry
Structure Resource (http://rcsr.anu.edu.au/).
2
SHELXTL, and final full-matrix refinements were against F .
(15) Spek, A. L. Acta Crystallogr., Sect. A 1990, 46, C34.
5568 Inorganic Chemistry, Vol. 47, No. 13, 2008

COMMUNICATION
Figure 3. (a) View of the EMIm cation in the four-membered ring (the
outside methyl groups are highlighted by green arrows). (b) View of the
3
D pillared framework of 1 along the b axis, showing two types of channels.
Figure 2. (a) Six-connected paddle-wheel unit in 1. (b) View of the pillared
D framework of 1 along the a axis, showing channels occupied by the
EMIm cations.
3
proach. It combines two of the most rapidly evolving research
areas (ionic liquid based materials synthesis and extended
homochiral materials) and presents a useful approach that
can be further expanded to create a variety of functional
materials.
°
C and then has two steps of weight loss. The first weight
loss of 25.2% from 275 to 375 °C corresponds to the loss of
the EMIm cations and the ac ligands (calcd 24.9%). The
second step from 375 to 515 °C is attributed to the further
decomposition of the D-cam ligands. The remaining weight
of 22.5% is likely that of CoO (calcd 21.9%).
Acknowledgment. We are thankful for support of this
work by the NIH (to X.B.; Grant 2 S06 GM063119-05).
In conclusion, a new anionic homochiral acetate-pillared
cobalt-camphorate architecture was successfully prepared
under ionothermal conditions through the unique pillaring
role of the acetate ligand. The work represents one of few
examples of the ionothermal synthesis of homochiral materi-
als using the enantiopure ligand-based building block ap-
Supporting Information Available: Powder X-ray diffraction
pattern, thermogravimetric analysis diagram, and X-ray crystal-
lographic files in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC8007798
Inorganic Chemistry, Vol. 47, No. 13, 2008 5569