Integrated molecular chirality, absolute helicity, and intrinsic chiral topology in three-dimensional open-framework materials

Full text of DOI:10.1021/ja8075692

Published on Web 12/02/2008
Integrated Molecular Chirality, Absolute Helicity, and Intrinsic Chiral Topology
in Three-Dimensional Open-Framework Materials
Jian Zhang, Shumei Chen, Areg Zingiryan, and Xianhui Bu*
Department of Chemistry and Biochemistry, California State UniVersity, Long Beach, 1250 Bellflower BouleVard,
Long Beach, California 90840
Received September 23, 2008; E-mail: xbu@csulb.edu
Homochirality and helicity are two essential features of life and
are integral to various biological functions. Biopolymers such as
proteins and DNA have acquired a definite helical sense (e.g., right-
handed R-helix) associated with the chirality of their molecular
building blocks (e.g., L-amino acids or D-sugars). Such biopolymers
exhibit dual homochiral features with both molecular chirality and
absolute helicity (Scheme 1).
Helicity is also common in the nonliving world, as evidenced
by the ubiquity of quartz. There is even speculation that biological
homochirality may be linked to the interactions between biomol-
ecules and resolved inorganic chiral solids such as quartz.1 Unlike
1-D polymeric DNA, intertwined helices in quartz are embedded
in a 3-D net that is intrinsically chiral.2 Also unlike biopolymers
such as DNA, quartz does not possess a chiral molecular building
block and is known to exist in both D- and L-forms. Similarly,
several metal-organic or chalcogenide materials are known to adopt
the quartz topology.3 Prior to this work, the bulk products of such
Figure 1. Absolute helicity control and coexistence of 3-D chiral quartz
net (c and d; here the balls represent In3+ centers and the rods represent
camphorate ligands) with enantiopure chiral building blocks (a and b) in
1D and 1L, respectively.
quartz-type materials tended to be racemic and the handedness of
the helix could not be controlled to be homohelical.
Materials with an intrinsically chiral topology2 such as quartz
and zeolite ꢀ have attracted interest for decades because of their
obvious potential in enantioselective applications. However, the
current inability to prepare zeolite ꢀ with reasonable enantiomeric
excess prevents its industrial application in chiral catalysis.4 There
is clearly an urgent need to prepare homochiral framework materials
with an intrinsically chiral topology.2
such study lead to functional framework materials for efficient chiral
recognition processes, but it also may help to provide fresh
opportunities for better understanding the origin of asymmetry in
the living system.
Here we describe unusual integrated homochirality features in
six new 3-D framework materials (1D, 1L, and 2-5) containing
enantiopure building blocks embedded in intrinsically chiral
topological nets, of which quartz net is the most well-known. Each
material has three homochiral features: 3-D intrinsically homochiral
net,2 homohelicity, and enantiopure molecular chirality (Scheme
1). It is shown here that the absolute helicity can be controlled by
the chirality of molecular building blocks.
Recent successes with metal-organic frameworks (MOFs) have
resulted in some very interesting chiral materials.5-8 With few
exceptions,9 bulk homochiral MOFs contain enantiopure molecular
building blocks (e.g., chiral carboxylates) with a framework
topology (e.g., diamond) that is not intrinsically chiral,6-8 a situation
opposite to that in quartz or zeolite ꢀ. Despite decades of research,
there are few examples of 3-D open-framework materials in which
an intrinsically chiral topology2 (e.g., quartz and zeolite ꢀ) coexists
with the chirality of enantiopure framework building blocks. It is
of great interest to explore 3-D framework materials that integrate
molecular chirality and intrinsic chiral topology; not only could
In 1D, 1L and 2, the enantiopure monomers coexist with the 3-D
intrinsically chiral topology of quartz (denoted qtz) net. For each
structure, the basic coordination chemistry of the In3+ site is four-
connected despite its eight-coordinate geometry, because each In3+
site is bonded to four carboxylate ligands and each carboxylate
group chelates to just one In3+ site (Figure 1a,b). The negative
Scheme 1. Unusual Integrated Homochirality Features
n-
framework [In(cam)2]n in each crystal (Figure 1c,d) is balanced
by guest organic ions (Table 1).
A striking feature in 1D and 1L is the helicity induction effect of
the enantiopure ligands, resulting in the formation of the homohe-
lical quartz topology. 1D and 1L are built from enantiopure D- and
L-camphoric acid, respectively. In both structures, the absolute
helicity is controlled by the handedness of the enantiopure ligand.
D-Camphorate leads to the right-handed (P-helix) quartz net in 1D
(denoted P-D form), whereas L-camphorate leads to the left-handed
(M-helix) quartz net in 1L (denoted M-L form). The chirality and
helicity of compound 1D are comparable to DNA, because both
adopt the P-D form.
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17246 J. AM. CHEM. SOC. 2008, 130, 17246–17247
10.1021/ja8075692 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Crystal Data and Refinement Results
formulaa
space group
a (Å)
b (Å)
c (Å)
R(F)
Flack param
chiral netsb
helicityc
1D
1L
2
3
4
(TMA)[In(D-cam)2]·2H2O
(TMA)[In(L-cam)2]·2H2O
(choline)[In(D-cam)2]·2H2O
Co(D-Cam)1/2(bdc)1/2(tmdpy)
Ni(D-cam)(H2O)2
P3121
P3221
P3121
P3121
P212121
P43
12.7624(2)
12.9913(3)
12.8708(1)
10.4498(5)
7.2182(7)
7.943(1)
12.7624(2)
12.9913(3)
12.8708(1)
10.4498(5)
12.175(1)
7.943(1)
17.6291(5)
17.3911(1)
17.2433(4)
31.896(2)
14.182(2)
12.8125(2)
0.0495
0.0688
0.0513
0.0457
0.0454
0.0460
-0.01(7)
0.05(9)
-0.02(6)
0.01(4)
0.00(4)
0.10(6)
qtz
qtz
qtz
qzd
srs
P-helix
M-helix
P-helix
P-helix
M-helix
M-helix
5
Mg(L-ma)(H2O)2 ·(H2O)
srs
a
D-H2Cam ) D-camphoric acid; bdc ) 1,4-benzenedicarboxylate; tmdpy ) 4,4′-trimethylenedipyridine; L-H2ma ) L-malic acid; TMA ) (CH3)4N+;
choline ) [(CH3)3NCH2CH2OH]+. b For definitions of three-letter abbreviations, see Reticular Chemistry Structure Resource (http://rcsr.anu.edu.au/). c M
) minus (left-handed) helicity; P ) plus (right-handed) helicity.
centers, leading to the formation of the 3-D homochiral [Mg(L-
ma)(H2O)2]n framework with srs topology (Figure 2f; Supporting
Information, Figure S8). Along the c axis, there are 1-D channels
filled by the guest water molecules. Here, the L-ma ligands induces
absolute left-handed helicity in the srs net and controls the
homochirality of the srs net (-γ*).
In conclusion, we report here a series of 3-D open-framework
materials with unusual integration of molecular chirality, absolute
helicity, and 3-D intrinsic chiral net. The helicity of the intrinsically
chiral net can be dictated by enantiopure framework building units,
in a manner mimicking the chirality-helicity relationship in
biopolymers. Three-dimensional framework materials with inte-
Figure 2. Coexistence of the enantiopure building block (a) and chiral
grated homochiral features have the potential for enhanced chiral
qzd net (b) in 3, and of the enantiopure building block (c and e) and chiral
recognition and enantioselectivity. The exploration of such multi-
srs net (d and f) in 4 and 5.
chirality materials and chirality-helicity relationship could lead to
new insights into the generation and control of helicity and
homochirality in both solid-state materials and biopolymers.
Acknowledgment. We acknowledge support of this work by
NIH (2 S06 GM063119-05, X.B.).
In addition to the chiral quartz type, other intrinsically chiral
nets (quartz dual and srs) have also been made in this work by
using D-camphoric acid (D-cam) or L-malic acid (L-ma) (compounds
3-5). In 3, the enantiopure building blocks coexist with the 3-D
homochiral topology of the quartz dual (denoted qzd, maximum
symmetry P6222) net. The framework of 3 is built by cross-linking
dinuclear cobalt sites with two different carboxylate linkers: chiral
D-cam ligands and achiral 1,4-benzenedicarboxylate (BDC) ligands
(Supporting Information, Figure S3). Each carboxylate group of
the BDC ligand bridges two symmetry-related Co sites to form a
linear chain. Unlike the BDC ligand, in which each carboxylate
group bridges two Co sites, each carboxlyate group of the D-cam
ligand chelates to only one cobalt site (Figure 2a). In this mode,
the D-cam ligands link the dinuclear Co units into a right-handed
31 helix along the c axis (Supporting Information, Figure S4b). The
linear BDC-Co chains and helical D-cam-Co 31 chains share the
common dinuclear cobalt unit, resulting in the formation of a 3-D
framework (Figure 2b; Supporting Information, Figure S5) with
the dinuclear Co unit serving as the planar four-connected node.
In 4 and 5, the enantiopure ligands (D-cam in 4 and L-ma in 5)
coexist with the 3-D intrinsically chiral topology of the (10,3)-a (also
denoted srs, maximum symmetry I4132) net.10,11 In 4, the octahedral
Ni centers are doubly linked by one µ2-aqua ligand and one µ2-bridging
carboxylate group from the D-cam ligand into a 1-D chain (Supporting
Information, Figure S7a). Two remaining sites of each octahedral Ni
site are occupied by a terminal H2O ligand and one O atom from the
monodentate carboxylate group of a bridging D-cam ligand (Figure
2c). The above 1-D Ni-carboxylate chain is further linked to four
neighboring chains by the chiral five-rings of the D-cam ligands, to
generate a 3-D homochiral framework with srs topology (Figure 2d;
Supporting Information, Figure S7). In this srs net, all helices exhibit
the same left-handedness (denoted as -γ*).11
Supporting Information Available: Experimental details, additional
crystallographic figures, TGA diagram, magnetic properties of 3, and
CIF files. This material is available free of charge via the Internet at
http://pubs.acs.org.
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In 5, both metal center and L-ma ligand act as three-connected
nodes (Figure 2e), although the Mg2+ center is six-coordinate
through coordination to four oxygen atoms from three L-ma ligands
and two terminal aqua ligands. Each L-ma ligand links three Mg2+
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