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Cite this: DOI: 10.1039/x0xx00000x
Pb3[C6(CH3)3(CO2)3H6]2[DMF]3:FirstlayeredPb‐ Kemp’striacidcomplex
Published on 15 July 2015. Downloaded by Yale University Library on 15/07/2015 15:35:45.
Saet Byeol Kim, Dong Woo Lee, Suk-Kyu Chang and Kang Min Ok* Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
An unprecedented layered Pb-Kemp’s triacid compound, Pb3[C6(CH3)3(CO2)3H6]2[DMF]3 has been synthesized. The material exhibits a selective CO2 adsorption, a reversible DMF coordination, and an intercalative hexanal addition reaction. A competing crystallization reaction under solvothermal condition using Pb2+ and Cd2+ with Kemp’s triacid reveals higher selectivity of Cd2+ over Pb2+.
intercalative coordination reactions for CAUMOF-17 are also presented, as is interesting heavy metal cation selectivity during the solvothermal synthesis.
CAUMOF-17 was synthesized solvothermally by combining Pb(NO3)2, cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic
acid
(Kemp’s
triacid), and HCON(CH3)2 in a vial with a screw cap at 80 C for 3
days.‡ Colorless hexagonal plate crystals of CAUMOF-17 have been
grown and the structure has been determined using single crystal X-ray The design and development of efficient molecular probes for the
diffraction.§ CAUMOF-17 is a novel layered material that is
recognition, binding, and removing of toxic metal ions remain an
crystallizing in a trigonal space group, R-3 (No. 148). As seen in Fig. 1,
ongoing challenge for synthetic chemists. Especially, the separation and
the central heavy metal cation, Pb2+ is connected by the Kemp’s triacid
isolation of heavy metal cations that are occurring from industrial waste
and DMF molecules through oxygen atoms. The unique Pb2+ cation is
waters are particularly important.1 Thus far, a number of selective metal
in a seven-coordinate environment with oxygen atoms with PbO bond
complexation agents have been suggested.2 Among many compounds,
lengths
synthetic molecular clefts containing Kemp’s triacid moieties have
environment of the Pb2+ cation is attributable to the stereoactive lone
demonstrated remarkable selectivity toward specific heavy metal
pair. The CC and CO bond distances in the Kemp’s triacid range
of
2.473(7)2.738(7)
Å.
The asymmetric
coordination
cations attributable to their peculiar structural characteristics.3 While
1.479(14)1.566(13) Å and 1.256(12)1.277(12) Å, respectively. In
the carboxylate groups pointing toward in one direction interact
addition, the CN and CO bond lengths in the coordinated DMF
strongly with metal ions, the methyl groups directing to opposite
molecule are 1.332(16)1.442(14) Å and 1.260(16) Å, respectively.
orientation present steric barriers. Meanwhile, other important materials
Bond valence sum calculations6 on CAUMOF-17 result in a value of
exhibiting versatile separation, sensing, and long term storage of metal
2.02 for the Pb2+ cation.
ions are porous coordination polymers (CPs) or metal-organic frameworks (MOFs).4 Pores with different framework environments in CPs can be tailored, once suitable metal cations and proper organic linkers are carefully designed. Interestingly, a large number of ligating functional groups found from the organic linkers of CPs are carboxylate groups. To our surprise, however, materials revealing extended structures with Kemp’s triacid linkers are rarely observed, although a few Uranyl complexes with nano-tubular, 1D, and 2D structures have been recently reported.5 Herein, we present the first Pb-Kemp’s triacid complex, Pb3[C6(CH3)3(CO2)3H6]2[DMF]3 (CAUMOF-17) with an unprecedented layered structure. We will describe detailed structural features for the coordination polymer containing the toxic heavy metal cation, Pb2+ and the molecular carrier, Kemp’s triacid. Thermal properties, selective gas adsorption, reversible solvent exchange, and
Thisjournalis©TheRoyalSocietyofChemistry2012
Fig. 1 ORTEP (50% probability ellipsoids) drawing for CAUMOF‐17. Hydrogen atomshavebeenomittedforclarity.
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relative positions of adjacent layers stimulated by CO2 (see Fig. 4b).8 In
O(3), and O(4), and form a face-shared hexagonal ring (see Fig. 2a).
other words, more effective interactions of CO2 with the open-
The hexagonal ring composed of the face-shared PbO7 polyhedra is
coordination sites may induce the selectiveDOI: structural flexibility of 10.1039/C5CC05360A
then capped by Kemp’s triacid from the above and below along the
CAUMOF-17. Another possible cause for the observed differential gas
[001] direction (see Fig. 2b). Each hexagonal ring is further linked by
adsorption might be attributed to the dissimilar thermal energies
the carboxylate groups in Kemp’s triacid, which results in a novel
measured at different temperatures.4h-k
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layered structure in the ab-plane (see Fig. 2c). Thus, the Kemp’s triacid serves as both intra- and inter-subunit linkers. DMF molecules are also alternately coordinated to Pb2+ cations in between the linkers and completing the layered structure of CAUMOF-17 (see Fig. 2c). It should be noted that CAUMOF-17 is the first example revealing a layered structure containing both Pb2+ and Kemp’s triacid.
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ChemComm Accepted Manuscript
The PbO7 polyhedra share their faces through the oxygen atoms, O(1),
JournalName
Fig. 3 Ball‐and‐stick and polyhedral representation revealing the layered structureofCAUMOF‐17intheac‐plane.TheKemp’striacidsserveasbothintra‐ and inter‐subunit linkers. DMF molecules are alternately coordinated to Pb2+ cationsinbetweenthelinkers(green,Pb;gray,C;cyan,N;red,O).
Fig. 2 (a) Ball‐and‐stick model representing a face‐shared hexagonal ring composedofPbO7polyhedrainCAUMOF‐17.(b)Thehexagonalringiscappedby Kemp’s triacid from the above and below along the [001] direction. (c) Each hexagonal ring is further connected by Kemp’s triacids and DMF molecules, whichcompletesalayeredstructureintheab‐plane(green,Pb;gray,C;cyan,N; red,O).
Interestingly, the methyl groups in the Kemp’s triacid linkers and DMF are pointing to each other, upwards and downwards (see Fig. 3). The observed distance between two methyl groups of Kemp’s triacid linkers in different layers [C(4) and C(7)] is about 4.2 Å. Thus, van der Waals interactions are expected in between the layers of CAUMOF-17. The CALC SOLV command in PLATON7 crystallographic program suggests that CAUMOF-17 possesses about 39% of void space when the coordinated DMF molecules are excluded. However, if the DMF molecules are included, the space decreases to 6%. The observed void space in CAUMOF-17 and a number of reported two-dimensional frameworks exhibiting significant gas adsorption phenomena led us to monitor gas adsorption into the layers.8 Although the N2 and H2 diffusions into layers are not significant at 77 K, to our surprise, the adsorption of CO2 at 195 K is much greater for the heated sample of CAUMOF-17 at 150 C under vacuum (see Fig. 4a). The observed selective CO2 adsorption may be attributable to the change of
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Fig.4(a)AdsorptionisothermsofN2(▲(blue),77K),H2(■,77K),andCO2(● (red), 195 K) on CAUMOF‐17. (b) Ball‐and‐stick model representing how CO2 stimulate the layers by interacting with open‐coordination sites and adsorption occurs.
The infrared spectrum of CAUMOF-17 shows the CH stretching vibrations for the Kemp’s triacid at ca. 29302960 cm-1. The
carboxylate stretching vibrations are observed at around 13101650
cm-1. The peaks occurring at about 509565 cm-1 are attributable to the PbO bonds. The IR assignments are consistent with those previously
reported values.9 The IR spectrum for CAUMOF-17 can be found in the ESI.
The thermal property of CAUMOF-17 was investigated using thermogravimetric analysis. CAUMOF-17 exhibits an initial weight
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ChemComm Accepted Manuscript
loss of about 12% up to 240 C attributed to the removal of the coordinated DMF molecules. The framework of the material seems to
CAUMOF-17 crystals were immersed in ca. 3 mL of hexanal at 80 C
be thermally stable up to 300 C on the basis of the TGA diagram and
for two days. Interestingly, the interlayer distance of the resulting DOI: 10.1039/C5CC05360A
PXRD patterns measured at different temperatures. Above 300 C, the
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material increased to 4.18 Å attributed to the complete coordination of
framework of CAUMOF-17 starts thermally decomposing. PXRD
hexanal molecules to the available coordination sites of Pb2+ (see Figs.
pattern for the thermally decomposed product after heating to 800 C is
5a and 5b).
identified as PbO. The TGA diagram and PXRD data at different
In order to investigate the selectivity of Kemp’s triacid for the heavy
temperatures have been deposited in the ESI.
metal cations during the crystallization of the layered material, same amounts of Pb2+ and Cd2+ have been added to the solution of Kemp’s triacid and heated in a sealed vial under the identical solvothermal
synthetic condition for CAUMOF-17. The competing crystallization reaction reveals a higher selectivity of Cd2+ over Pb2+: the selectivity toward Cd2+ over Pb2+ has been found to be about 6, which was
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confirmed by EDX analysis for the isolated colorless microcrystals
from the reaction. The colorless microcrystals reveal a highly
crystalline phase with the similar unit cell parameters (a = b 14.445 Å, c 30.045 Å) to those of CAUMOF-17 on the basis of X-ray
diffraction. A full structural analysis is in progress to determine the crystal structure of the phase. EDX data and PXRD pattern for the obtained product are found in the ESI.
In summary, we have synthesized and fully characterized the first
example of layered Pb-Kemp’s triacid compound. The material reveals a selective CO2 adsorption, a reversible solvent exchange, and an
intercalative coordination phenomenon. An interesting heavy metal
cation selectivity of Kemp’s triacid has been successfully demonstrated under solvothermal reaction condition for the layered material.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No.
2013R1A2A2A01007170). S.B.K. thanks CAU for the Chung-Ang University Freshmen Academic Record Excellent Scholarship in 2014.
Notesandreferences
Fig. 5 (a) Powder X‐ray diffraction patterns and (b) a schematic representation revealing a reversible coordination of DMF molecules and intercalative coordinationofhexanalinCAUMOF‐17.Notethechangesofinterlayerdistances depending on the coordinated molecules on Pb2+ cations in the layers of CAUMOF‐17.
The loss of coordinated solvent, DMF from CAUMOF-17 turns out to be fully reversible. The as-synthesized CAUMOF-17 was evacuated at 150 C in order to remove the coordinated DMF molecules. The powder X-ray diffraction pattern for the activated sample reveals that the (003) peak of CAUMOF-17 is significantly shifted to the right-hand side; however, the crystallinity of the de-coordinated material is similar to that of the as-synthesized compound (see Fig. 5a). As seen in Fig. 5b, the interlayer distance decreases from 3.86 Å to 3.52 Å when the coordinated DMF molecules are removed from the framework of CAUMOF-17. And then the activated compound has been allowed to stand in DMF solvent at 80 C for two days. A complete recoordination of DMF to Pb
2+
cations in the layers occurred, which was
confirmed by PXRD (see Figs. 5a and 5b). Because CAUMOF-17 exhibited robust and reversible solvent exchange reactions, we thought that a longer linear molecule might be intercalatively added to the
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Department of Chemistry, Chung-Ang University, Seoul 156-756, Republic of Korea. Fax: 82 2 825 4736; Tel: 82 2 820 5197; E-mail: [emailprotected] † Electronic Supplementary Information (ESI) available: X-ray crystallographic file in CIF format, experimental and calculated powder XRD patterns, infrared spectrum, thermogravimetric analysis diagram,
and energy dispersive analysis by X-ray (EDX) data for the product of
competing crystallization reaction. CCDC 1409438. For ESI and crystallographic
data
DOI: 10.1039/c000000x/
in
CIF
or
other
electronic
format
see
‡ Experimental Section. CAUMOF-17 was synthesized by combining Pb(NO3)2 (0.099 g, 3.00 10-4 mol), Kemp’s triacid (cis,cis-1,3,5trimethylcyclohexane-1,3,5-tricarboxylic acid, 0.025 g, 1.00 10-4 mol),
and 1.0 mL of N,N-dimethylformamide (DMF). The reaction mixture was
transferred to a vial with a screw cap and was subsequently sealed tightly. The vial is initially heated to 50 °C for 12 h and then the reaction
temperature increased to 80 °C. Colorless hexagonal plate crystals were
grown in three days. After cooling, the hexagonal crystals were filtered and washed thoroughly with acetone. The isolated yield for CAUMOF-17 is about 33% on the basis of Pb(NO3)2. IR data (cm-1): 2964, 2933, 1535, 1467, 1388, 1359, 1336, 1311, 1257, 1209, 1097, 1058, 1035, 977, 894, 867, 833, 783, 669, 632, 565, 509, 459, 416. Elemental microanalysis for
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DOI: 10.1039/C5CC05360A
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