Spontaneous resolution to absolute chiral induction: Pseudo-kagomé type homochiral Zn(II)/Co(II) coordination polymers with achiral precursors

Article abstract of DOI:10.1021/ja4075369

It is observed that conglomerate crystallization of achiral precursors yielding racemate metal organic frameworks/coordination polymers (MOFs/CPs) can be driven to absolute homochiral crystallization of the desired enantiomorph by utilizing a suitable chiral induction agent. In a series of crystallization experiments isostructural Zn and Co homochiral CPs (1P, 1M and 2P, 2M) are prepared using the achiral precursors. In the presence of enantiopure camphoric acid, the crystallization process prefers absolute chiral induction over conglomerate formation which is established by single crystal X-ray diffraction and CD spectroscopy.

Full text of DOI:10.1021/ja4075369

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Communication
Spontaneous Resolution to Absolute Chiral Induction: Pseudo-Kagomé
Type Homochiral Zn(II)/Co(II) Coordination Polymers with Achiral Precursors
Kamal Kumar Bisht, and Eringathodi Suresh
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja4075369 • Publication Date (Web): 09 Oct 2013
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Spontaneous Resolution to Absolute Chiral Induction: Pseu-
do-Kagomé Type Homochiral Zn(II)/Co(II) Coordination Poly-
mers with Achiral Precursors
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†
,‡
,†,‡
Kamal Kumar Bisht and Eringathodi Suresh*
†
Analytical Department and Centralized Instrument Facility, Central Salt and Marine Chemicals Research Institute, Council
of Scientific and Industrial Research (CSIR–CSMCRI) G. B. Marg, Bhavnagar –364 002, Gujarat, India.
‡
Academy of Scientific and Innovative Research (AcSIR–CSMCRI) G. B. Marg, Bhavnagar –364 002, Gujarat, India.
Supporting Information
chiral induction agent to design catalytically active homochiral
ABSTRACT: It is observed that conglomerate crystallization of
achiral precursors yielding racemate MOFs/CPs can be driven to
absolute homochiral crystallization of desired enantiomorph by
7b
CPs from achiral precursors. Moreover, there are a couple of
examples in which an enantiopure solvent has smartly been emꢀ
8,9
ployed as chirality induction agent.
utilizing suitable chiral induction agent. In
a series of
crystallization experiments isostructural Zn and Co homochiral
CPs (1P, 1M and 2P, 2M) are prepared using the achiral
precursors. In presence of enantiopure camphoric acid,
crystallization process prefers absolute chiral induction over conꢀ
glomerate formation which is established by single crystal X–ray
diffraction and CD spectroscopy.
In a remarkable report, Bu et al, exploited the chirality induction
competence of camphoric acid to direct the absolute chirality of
intrinsically chiral MOF composed of achiral building blocks.
They have shown that the racemic form of camphoric acid inducꢀ
es the formation of conglomerate MOF; however, enantiopure
camphoric acid could induce absolute bulk chirality in the same
MOFs. Interestingly, a different achiral MOF was obtained in the
absence of chiral induction agents.
1
0
Chirality is an eminent feature of nature and plays an indispenꢀ
sable role in biological functions and material sciences. Designed
synthesis of homochiral metal organic frameworks (MOFs) or
coordination polymers (CPs) has been a frontier research field in
recent years due to their prime importance in the field of asymꢀ
Scheme 1. Cartoon depiction of the chiral induction (CI)
agent driven conglomeration and bulk asymmetric crystal-
lization processes (for example, reference 10) (a); sponta-
neous resolution driven conglomeration and the asymmet-
ric coercing of same process via CI agent leading to abso-
lute asymmetric crystallization (present study) (b).
metric catalysis, magnetism, non–linear optics and chiral recogniꢀ
1–4
tion.
The straight forward way to synthesize homochiral
CPs/MOFs is the use of enantiopure building blocks which transꢀ
late their inherent chirality to the resultant framework by “chiraliꢀ
ty conservation” process. This methodology suffers from the high
costs of enantiopure organic linkers. However, there are a few
excellent reports on naturally occurring chiral organic molecules
3
,4
as linker for the construction of homochiral MOFs. The prime
indirect ways to synthesize chiral MOFs by achiral building
1
blocks are ‘spontaneous resolution’ and ‘chirality induction’(CI).
The homochiral crystallization of CPs/MOFs from achiral buildꢀ
ing blocks still remains a scantily explored area. Self assembly of
chiral MOFs consisting of achiral building blocks by means of
5
spontaneous resolution is an interesting phenomenon. However,
the handedness of individual nuclei during crystallization remains
uncontrollable in the case of spontaneous resolution and the prodꢀ
ucts tend to be conglomerate except in a few cases where a rare
Though there have been a few excellent reports demonstrating
the phenomenon of chiral induction to synthesize bulk homochiral
MOFs, the method remains elusive due to the unpredictability of
6
course of symmetry breaking operates. These unresolved conꢀ
glomerates remain of less practical use. The other methodology,
i.e., the use of chiral induction agents to achieve bulk homochiral
MOFs is a powerful strategy as it can accomplish absolute bulk
chirality in the system. Camphoric acid and cinchona alkaloids are
11
suitable chiral induction agent for a given set of precursors.
Aforesaid elegant reports indicate that the chirality induction
agents work specifically on individual systems and often remain
seminal for the construction of desired network. Evidently, there
is a realistic requirement to develop protocols to synthesize bulk
homochiral MOFs.
7
most celebrated chiral inducing agents. The use of cinchona alkaꢀ
loids as chiral induction agent was first demonstrated by Zhang, et
7a
al. for the construction of homochiral microporous MOFs. Reꢀ
cently Duan et.al, have further explored the cinchona alkaloids as
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In our pursuit of novel chiral CPs/MOFs comprising of achiral
cal coil in 1P and 1M respectively (Figures 1b, 2 and S3). Adjaꢀ
cent [Zn(H O) (ꢁ–SO )] zigzag chains are connected by the biꢀ
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building blocks, we accentuate on the cooperative operation of
spontaneous resolution and chiral induction. To be precise, we
have employed a chiral induction agent to maneuver the fate of
reaction from spontaneous resolution to the bulk homochiral crysꢀ
tallization of CPs as depicted in Scheme 1. As far as we know
there have been no reports on utilizing the enantiopure agents in
crystallization process to yield bulk absolute chirality in
CPs/MOFs from the achiral precursors which would otherwise
yield spontaneous resolution in the absence of those enantiopure
agents.
2
2
4
n
nodal ligands L resulting into [Zn(ꢁ–SO )(L)(H O) ] two dimenꢀ
4
2
2 n
sional network (Figure 1c) in which the same MꢂꢂꢂM cross–chain
distance (Zn1ꢂꢂꢂZn1 / Zn2ꢂꢂꢂZn2 = 15.59Å) is retained for both the
molecules present in 1P. Accordingly, the trans axial position
occupied by the terminal nitrogen atoms from two different L
+2
molecules completes the octahedral coordination around M in
which ZnꢂꢂꢂN distance range from 2.140(3) to 2.120(3) and
2.125(3) to 2.151(3) for Zn1 and Zn2 respectively. Average diheꢀ
dral angle between the two peripheral pyridine rings of the rigid
ligand L being 8.69° and 8.78°, indicating only marginal twist in
these ligands to make effective coordination with the metal in
both the zinc complexes.
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Scheme 2. Depiction of studied crystallization processes:
Contribution of enantiopure camphoric acid induces the
absolute asymmetric crystallization of 2D homochiral CPs
and absence of CI agent leads to conglomerate products.
Herein we report two enantiomeric pairs (1P, 1M and 2P, 2M)
of isostructural 2–dimensional metal organic frameworks
[
Zn(SO )(L)(H O) ] (1) and [Co(SO )(L)(H O) ] (2) (where L =
4 2 2 n 4 2 2 n
1
,4–bis(4–pyridyl)–2,3–diaza–1,3–butadiene) comprising of achiꢀ
ral building blocks by the spatial orientation of the ligand moiety
coordinated to the metal. Either of the 1P/1M and 2P/2M pairs of
chiral MOFs can be synthesized as a conglomerate by spontaneꢀ
ous resolution occurred upon diffusing the alcoholic solution of
ligand (L) into the aqueous metal sulphate solution. We further
show that the enantiopure camphoric acid (cam) can act as chiraliꢀ
ty inducing agent to achieve bulk absolute chiral CPs with desired
helicity (1P, 1M, 2P and 2M). In the chirality induction experiꢀ
ments enantiopure camphoric acid can coerce the enantio–
enrichment of bulk samples. Thus, MOFs 1P and 2P with absoꢀ
lute positive homohelicity have been obtained in the presence of
(—)–cam whereas (+)–cam directs the exclusive formation of
absolute negative homohelical CPs 1M and 2M (supporting inꢀ
formation S1).
Figure 1. (a) [Zn(H O) (ꢁ–SO )] chain running parallel to the b–
2
2
4
axis; (b) coupling of [Zn(H O) (ꢁ–SO )] chains by L at alternate
2
2
4
metal centres fabricate a right (P) handed helical coil in 1P; (c) P–
homohelices fuse together to generate 2D metal organic network;
(d) the underlying intrinsically chiral pseudo–Kagomé type topolꢀ
2
3
ogy is depicted with short vertex symbol (4 .6 .8).
All the four compounds crystallize in chiral space group C2 and
possess a two dimensional network with intrinsic chirality. The
Zn and Co CPs are isotypic; hence the structural description is
restricted to the Zn CPs 1P and 1M. 1P and 1M contain two crysꢀ
tallographically independent [Zn(SO )(L)(H O) ] entities with
4
2
2 n
identical coordination environment (Figure S2). Coordination
+2
around the Zn possesses a distorted octahedral N2O4 geometry.
The basal plane of each metal center is provided by two cis coorꢀ
dinated water molecules and oxygen atoms from two different
sulfato groups oriented in cis disposition linking the adjacent
2
screw related Zn(II) ions via ꢁ –κ binding mode to form a neuꢀ
2
tral zigzag [Zn(H O) (ꢁ–SO )] chain motif running along the b–
2
2
4
n
axis (Figure 1a). The adjoining MꢂꢂꢂM distance within zigzag
chain for both the molecules present in the asymmetric unit is
6
.96 Å (Zn1ꢂꢂꢂZn1) and 6.03Å (Zn2ꢂꢂꢂZn2) respectively. ZnꢂꢂꢂO
distance involving the water ranges from 2.087(2) to 2.170(2)Å,
.109(2) to 2.123(2)Å and the sulfato bridged oxygens is 2.071(2)
2
Figure 2. Mirror image relation of right and left handed helical
motifs composed of achiral building blocks in enantiomorphs 1P
and 1M.
to 2.098(2) Å, 2.150(2) Å, 2.165(2) Å apropos Zn1 and Zn2 reꢀ
spectively. The [Zn(H O) (ꢁ–SO )] chains running parallel to the
2
2
4
b–axis are further coupled by L with alternate metal centre in
cross chain mode fabricating a right (P) and left (M) handed heliꢀ
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Concomitant effect of ꢁ –κ bridging mode of the screw related
absorption spectra in the region ca 295 nm (Figure S5). Hence,
CD spectra for six random crystals from the both Zn and Co CPs
crystallized in the presence of enantiopure camphoric acids and
ten crystals from the each batch crystallized in the absence of
camphoric acid were recorded using Nujol mull (Figure S6).
Single crystals of both CPs grown in the presence of (—)–cam
and (+)–cam showed a positive and negative dichroic signal at ca
295 nm respectively. Representative CD patterns for 1P and 1M
possess virtually mirror symmetric relation as depicted in Figure
3. Hence, the absolute chirality of the CPs prepared in the
presence of enantiopure camphoric acid established by single
crystal X–ray studies are also complemeted by the CD data.
Randomly picked individual crystals from the batch crystallized
in the absence of enantiopure camphoric acid showed
positive/negative dichroic signals varying from crystal to crystal,
indicating the formation of conglomerate (Figure S6). As anticiꢀ
pated, mixture of a few crystals from the batch crystallized in the
absence of CI agent (cam) did not show any dichroic signal
(Figure S6). Thus,CD spectral data also supports the
conglomerate and predominent bulk homochiral crystallization of
CPs in the absence or presence of enantiopure camphoric acid
respectively. However, it is a challenging task to prove the absoꢀ
lute homochirality of the bulk sample because determination of
absolute configuration in each and every crystal by single crystal
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metal centers by the sulfate ligands and the cross chain bridging
of the zigzag chains by L with alternate metal ions, reinforce the
construction of intrinsically chiral pseudo–Kagomé type two diꢀ
2
3
mensional enantiomorphic networks (4 .6 .8) in 1P and 1M (Figꢀ
ure 1d). Packing diagram of 1P revealed that both 2D nets present
in the asymmetric unit (involving Zn1 and Zn2) are arranged in
ABAB pattern. Independent 2D nets are associated by O–HꢂꢂꢂO
hydrogen bonding interactions between the coordinated water
molecules and sulphate oxygen between the A and B sheets. In
addition to this O–HꢂꢂꢂO interaction, the lattice water molecules
are also involved in O–HꢂꢂꢂO interactions between the coordinated
water molecules and sulfate oxygens in bridging the 2D sheets
with different metal centers and stabilizing the molecule in the
crystal lattice. Albeit similar structural features including the heliꢀ
cal nature and H–bonding interactions are observed in Co(II)
complexes 2P and 2M also (Figure S4). Selected bond length and
bond angles; details of hydrogen bonding interactions for all the
four complexes with hydrogen bonding parameters are given in
Tables S2 and S3.
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Three different crystallization experiments particularly, in the
presence of (—)–cam; in presence of (+)–cam and in the absence
of camphoric acid were performed to distinguish and comprehend
the spontaneous resolution and cooperative chirality induction
phenomenon operative in the current system (Scheme 2). To
establish the chiral crystallization trend operating in various
experiments, six crystals were randomly picked from each
crystallization batch and analyzed by single crystal X–ray
diffraction techniques. Flack parameter value of the collected
crystal data sets revealed that Zn CP crystallized in the presence
of (—)–cam exclusively possess right handed helicity, i.e., all six
crystals were found to be 1P. Similarly, six crystals selected from
the batch crystallization carried out in the presence of (+)–cam
were identical to 1M , with left handed helicity. However, in the
absence of (+)– or (—)–cam equal distribution of 1P and 1M was
observed as three crystals were 1P and remaining were 1M.
Similar batch experiments carried out for Co CPs also showed
identical end results. Crystallographic details, Flack parameter
values and observed helicity for Zn and Co CPs obtained from
different batch crystallization experiments are presented in the
Tables S4–S9.
9
X–ray diffraction is not viable. The results presented here i.e., 36
(6×6) good quality single crystal data with refined Flack parameꢀ
ter values indicate the crystallization experiments carried out in
the presence of enantiopure camphoric acid results in the predomꢀ
inant formation of desired enantiomorph.
Both Zn and Co CPs showed thermal stability upto ca 280 ⁰C
after liberating the lattice guests in the temperature range 60–150
and 50–160 ⁰C respectively (Figure S7). Luminescent properties
of both CPs were also explored and observed that upon excitation
at 295 nm; both CPs emit at 365 nm which could be assigned to
the intra–ligand energy transfer. The overall quantum yields
(Φoverall) for Zn and Co CPs were 4.4 and 5.7 respectively which
indicate the excellent fluorescent ability of these materials (Figure
S8).
In summary, we demonstrated that the phenomenon of chirality
induction can be applied to the CP/MOF systems comprising of
achiral building blocks which otherwise display spontaneous resoꢀ
lution. Absolute chiral crystallization for the isostructural Zn and
Co coordination polymeric enantiomorphs [Zn(SO )(L) (H O) ]
2 n
4
2
and [Co(SO )(L)(H O) ] has been achieved in the presence of
4
2
2 n
chirality induction agent (enantiopure camphoric acids). In the
absence of camphoric acid conglomerate formation was observed.
The bulk homochiral crystallization and conglomerate formation
studies for the system are unambiguously established by crystalꢀ
lography and CD spectroscopy. Chirality in 1P /1M and 2P /2M
is originated by the self assembly of homoꢀhelical motifs in the
intrinsically chiral pseudo–Kagomé type 2D network. Synthesized
Zn and Co CPs show strong photoluminescence originating from
the ligand to ligand energy transfer. Present study highlights the
feasibility of chirality induction process on the systems exhibiting
spontaneous resolution and hence opens up new avenues for the
selective crystallization of desired enantiomorphs in MOFs/CPs
comprising of achiral ligands. Acquiring insight from the present
results, we are currently exploring the feasibility of chiral inducꢀ
tion techniques in ternary MOF/CP conglomerates involving achiꢀ
ral dicarboxylate and flexible N–donor ligands.
Figure 3. Representative CD spectra of enantiomorphs 1P and
1M depicting the mirror image relation for the dichroic signal at
295 nm.
In order to confirm the absolute configurations, individual
crystals of 1P, 1M; and 2P, 2M used for crystallographic studies
were further characterized by solid–state circular dichroism (CD)
spectroscopy, which furnished complementary results to the
crystallographic findings. The Zn and Co CPs show electronic
ASSOCIATED CONTENT
Supporting Information
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Experimental Details, Characterization Data, CIF files, Supportꢀ
ing Figures. This material is available free of charge via the Interꢀ
net at http://pubs.acs.org.
(9) Lin, Z.; Slawin, A. M. Z.; Morris, R. E. J. Am. Chem. Soc. 2007,
29, 4880–4881.
10) Zhang, J.; Chen, S.; Nieto, R. A.; Wu, T.; Feng, P.; Bu, X. Angew.
Chem. Int. Ed. 2010, 49, 1267–1270.
11) Kepert, C. J.; Prior, T. J.; Rosseinsky, M. J. J. Am. Chem. Soc.
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(
(
AUTHOR INFORMATION
2
Corresponding Author
esuresh@csmcri.org; sureshe123@rediffmail.com
Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENT
Authors acknowledge the CSIR, India (Grant No. NWP–0010) for
financial support, Mr. Batuk Bheel for NMR data, Mr. Satyaveer
Gothwal for TGA data, Mr. V. K. Agrawal for IR data, Mr. Viral
Vakani for CHN analysis and Dr. P. Paul for all round analytical
support. KKB acknowledges CSIR (India) for SRF.
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