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Construction of acylhydrazidate-extended metal–organic frameworks† Yan-Ning Wang,a,b Qing-Feng Yang,c Guang-Hua Li,a,b Ping Zhang,*a Jie-Hui Yu*a,b and Ji-Qing Xua,b Under hydrothermal conditions, the reactions of Ba2+/Zn2+, aromatic polycarboxylic acids and N2H4 with or without oxalic acid were carried out, affording four new acylhydrazidate-extended metal–organic frameworks (MOFs) [Ba(pmdh)] ( pmdh = pyromellitdihydrazidate) 1, [Ba(sdpth)(H2O)2]·0.5H2O (sdpth = 4,4’-sulfoyldiphthalhydrazidate) 2, [Ba2(cpth)2(H2O)2] (cpth = 4-carboxylphthalhydrazidate) 3 and [Zn2( pdh)2(ox)]·H2O (ox = oxalate, pdh = pyridine-2,3-dicarboxylhydrazidate) 4. The acylhydrazidate molecules pmdh, sdpth, cpth and pdh in compounds 1–4 derived from the hydrothermal in situ acylation of N2H4 with aromatic polycarboxylic acids. X-ray single-crystal diffraction analysis revealed that (i) in compound 1, the pmdh I molecules link the Ba2+ ions into a two-dimensional (2D) layer with a (4,4) topology, and then the pmdh II molecules extend these layers into a three-dimensional (3D) network; (ii) in compound 2, the sdpth molecules link the Ba2+ ions to form a one-dimensional (1D) square tube. Interestingly, the tubes are further linked into a 3D supramolecular network via the N–H⋯O interactions, creating synchronously big channels; (iii) in compound 3, the cpth I molecules link the Ba1 ions into a 3D network with a (10,3) topology. Ba2 and cpth II are distributed on the channels; (iv) in compound 4, Zn2+ and pdh aggregate to form two types of Zn4( pdh)4 clusters. The ox molecules act as the secondary
Received 15th March 2014, Accepted 9th May 2014
linkers, extending the Zn4( pdh)4 secondary building units (SBUs) into a 3D network with a 66 topology. The photoluminescence analysis indicates that compounds 3 and 4 emit green light with maxima at
DOI: 10.1039/c4dt00780h
495 nm for 3 (λex = 397 nm), and 522 nm for 4 (λex = 395 nm), respectively. At 77 K, the activated 2 and 4
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can adsorb N2 in amounts of 58.31 cm3 g−1 for 2 and 38.38 cm3 g−1 for 4, respectively.
Introduction Considerable effort has been taken in the design and synthesis of novel metal–organic frameworks (MOFs) due to their intriguing structures and topologies,1 and their potential applications in some fields such as adsorption, optics, magnetism and catalysis.2 Although so many MOFs have been obtained through self-assembly between metal ions and organic N/Odonor ligands, it is still a challenge to obtain a target MOF with a pre-designed structure and desirable property, because the self-assembly process is so complicated, and dominated by multiple factors including the geometric configuration of the metal ions, the nature of the organic ligands (size, shape, a College of Chemistry, Jilin University, Jiefang Road 2519, Changchun, 130023, P.R. China. E-mail: [email protected], [email protected] b State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Qianjin Street 2699, Changchun, 130023, P.R. China c Key Laboratory of Energy Resources and Chemical Engineering, Ningxia University, Yinchuan, Ningxia 750021, P.R. China † Electronic supplementary information (ESI) available. CCDC 991312–991314 for 1–3, and 976685 for 4. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt00780h
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flexibility/rigidity), and the detailed experimental conditions (solvent, pH, reaction temperature, ratio of metal-to-ligand). Over the last two decades, multidentate N-heterocyclic molecules and polycarboxylic acid molecules have been extensively employed in the construction of novel MOFs.3 Recently, as a new type of bridging ligand, the organic acylhydrazide molecules have attracted some attention.4 The diacylhydrazide molecules such as H2pmdh ( pmdh = pyromellitdihydrazidate) and H2sdpth (sdpth = 4,4′-sulfoyldiphthalhydrazidate) (see Scheme 1), having four acylamino groups, possess the potential to form extended networks with metal ions, which has been confirmed by the reported 3D compound [Co( pmdh)(H2O)2].5 Different from the diacylhydrazide molecules, the monoacylhydrazidate molecules such as H2pth ( pth = phthalhydrazidate) and Hpdh ( pdh = pyridine-2,3-dicarboxylhydrazidate) (see also Scheme 1) tend to form various discrete or chained oligomers with metal ions.6 But on the one hand, a simple modification to the monoacylhydrazide molecule is helpful to the further extension of these oligomers into a highD network. For example, the 3,4-pdh-extended Pb2+ compound [Pb(3,4-pdh)] (3,4-pdh = pyridine-3,4-dicarboxylhydrazidate) shows a 3D network structure. Here a meso-position C atom of
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Dalton Transactions
Scheme 1
Paper
Structures of acylhydrazidate molecules in 1–4.
the benzene ring is replaced by a N heteroatom.7 For another example, compound [Fe3(dcpth)2(H2O)4]·2H2O (dcpth = 4,5dicarboxylphthahydrazidate) also exhibits a 3D network structure. Here two carboxyl substituents on the 4- and 5-positions for H3dcpth play a key role in the formation of this extended network.8 On the other hand, the incorporation of another kind of organic bridging molecule can also extend these oligomers into a high-D network. As observed in the reported 3D compound [Zn2( pth)(atez)2] (atez = 5-aminotetrazolate), the atez molecules act as secondary linkers, extending the Zn2+pth chains into a 3D network.9 Note that all of the di(mono)acylhydrazide molecules mentioned above originated from the hydrothermal in situ acylation of N2H4 with aromatic polycarboxylic acids. Based on these considerations, two tetracarboxylic acid molecules ( pyromellitic acid, pma; 4,4′sulfonyldiphthalic dianhydride, sdphda) and one carboxylmodified phthalic acid (4-carboxylphthalic acid, cpha) were selected to serve as the acidic precursors. The reactions of Ba2+, aromatic polycarboxylic acids and N2H4 were carried out, affording three new acylhydrazidate-containing coordination polymers [Ba( pmdh)] 1, [Ba(sdpth)(H2O)2]·0.5H2O 2 and [Ba2(cpth)2(H2O)2] (cpth = 4-carboxylphthalhydrazidate) 3. Moreover, the incorporation of the H2(ox) (ox = oxalate) molecule into the reaction of Zn2+, pyridine-2,3-dicarboxylic acid ( pdca) and N2H4 was also performed, creating compound [Zn2( pdh)2(ox)]·H2O 4.
Experimental Materials and physical measurement All chemicals were reagent grade quality, obtained from commercial sources and used without further purification. Elemental analysis was performed on a Perkin-Elmer 2400LS II elemental analyzer. Infrared (IR) spectra were recorded on a Perkin Elmer Spectrum 1 spectrophotometer in the 4000–400 cm−1 region using a powdered sample on a KBr plate. Ultraviolet-visible (UV-vis) spectrum was obtained on a Rigaku-UV-3100 spectrophotometer. Powder X-ray diffraction (XRD) data were collected on a Rigaku/max-2550 diffractometer with Cu-Kα radiation (λ = 1.5418 Å). Thermogravimetric (TG) behavior was investigated on a Perkin-Elmer TGA-7 instrument with a heating rate of 10 °C min−1 in air. Fluorescence spectrum was obtained on a LS 55 florescence/phosphorescence spectrophotometer at room temperature. The measurements
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of N2/CO2 adsorption were carried out on asap-2010 and autosorb-IQ2 apparatus. Synthesis of the title compounds [Ba( pmdh)] 1. The yellow block crystals of 1 were obtained from a simple hydrothermal self-assembly of Ba(NO3)2 (78 mg, 0.3 mmol), pma (65 mg, 0.3 mmol) and N2H4·H2O (0.15 mL) in a 10 mL aqueous solution ( pH = 5 adjusted by H2(ox)) at 160 °C for 4 days. Yield: ca. 15% based on Ba(II). Anal. Calcd C10H4N4O4Ba 1: C 31.48, H 1.057, N 14.69. Found: C 30.59, H 1.051, N 14.57%. IR (cm−1): 1639 s, 1535 s, 1507 w, 1482 m, 1435 m, 1384 s, 1300 m, 1176 m, 1144 m, 816 m, 694 m, 650 s. [Ba(sdpth)(H2O)2]·0.5H2O 2. The yellow block crystals of 2 were obtained from a simple hydrothermal self-assembly of Ba(NO3)2 (131 mg, 0.5 mmol), sdphda (179 mg, 0.5 mmol) and N2H4·H2O (0.2 mL) in a 15 mL aqueous solution ( pH = 8 adjusted by N2H4) at 170 °C for 4 days. Yield: ca. 25% based on Ba(II). Anal. Calcd C16H13N4O8.5SBa 2: C 33.90, H 2.312, N 9.89. Found: C 34.19, H 2.305, N 9.81%. IR (cm−1): 1643 s, 1573 s, 1480 s, 1381 m, 1242 w, 1145 s, 1057 s, 884 w, 813 s, 716 w, 662 m, 559 m. [Ba2(cpth)2(H2O)2] 3. The yellow block crystals of 3 were obtained from a simple hydrothermal self-assembly of Ba(NO3)2 (131 mg, 0.5 mmol), cpha (96 mg, 0.5 mmol) and N2H4·H2O (0.2 mL) in a 15 mL aqueous solution ( pH = 7 adjusted by N2H4) at 170 °C for 4 days. Yield: ca. 20% based on Ba(II). Anal. Calcd C18H12N4O10Ba2 3: C 30.07, H 1.682, N 7.79. Found: C 29.74, H 1.691, N 7.78%. IR (cm−1): 1646 m, 1618 w, 1579 s, 1448 m, 1415 s, 1366 w, 1210 m, 1059 w, 820 m, 781 s, 710 s, 617 w, 524 m. [Zn2( pdh)2(ox)]·H2O 4. The yellow block crystals of 4 were obtained from a simple hydrothermal self-assembly of Zn(CH3COO)2·2H2O (110 mg, 0.5 mmol), pdca (83 mg, 0.5 mmol), N2H4·H2O (0.2 mL) and H2ox (63 mg, 0.5 mmol) in a 15 mL aqueous solution ( pH = 5 adjusted by H2(ox)) at 160 °C for 4 days. Yield: ca. 25% based on Zn(II). Anal. Calcd C16H10N6O9Zn2 4: C 34.25, H 1.797, N 14.98. Found: C 34.57, H 1.787, N 15.18%. IR (cm−1): 1659 s, 1567 s, 1524 m, 1477 m, 1262 w, 1161 w, 1119 s, 781 s, 713 w, 675 w, 535 s, 510 w, 495 s. X-ray crystallography The data were collected with Mo-Kα radiation (λ = 0.71073 Å) on a Rigaku R-AXIS RAPID IP diffractometer for 1, and on a Siemens SMART CCD diffractometer for 2–4. With the
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Table 1
Dalton Transactions Crystallographic data for 1–4
Formula M T (K) Crystal system Space group a (Å) b (Å) c (Å) β (°) V (Å3) Z Dc (g cm−3) μ (mm−1) Reflections collected Unique reflections Rint Gof R1, I > 2σ(I) wR2, all data
1
2
3
4
C10H4N4O4Ba 381.51 293(2) Monoclinic P21/c 8.2417(16) 10.946(2) 11.113(2) 93.86(3) 1000.3(3) 4 2.533 3.989 9586 2288 0.0172 1.119 0.0204 0.0602
C16H13N4O8.5SBa 566.69 293(2) Monoclinic C2/m 19.372(3) 23.436(3) 4.7214(5) 94.727(11) 2136.2(5) 4 1.762 2.009 6086 1963 0.0671 1.093 0.0600 0.2010
C18H12N4O10Ba2 719.00 293(2) Orthorhombic Pna21 6.9812(4) 14.8221(9) 19.0460(17)
C16H10N6O9Zn2 561.04 293(2) Tetragonal I41/a 17.6446(9) 17.6446(9) 22.742(3)
1970.8(2) 4 2.423 4.043 10 417 2649 0.0708 1.055 0.0307 0.0863
7080.4(10) 16 2.105 2.784 19 798 3130 0.0961 1.119 0.0418 0.1180
SHELXTL program, the structures of 1–3 were solved using direct methods, whereas the structure of 4 was solved using heavy atom methods.10 The non-hydrogen atoms were assigned anisotropic displacement parameters in the refinement. The H atoms on the N atoms in 1 were obtained from the difference Fourier map. The H atoms on the water molecules in 2–4 were not located. The other H atoms were treated using a riding model. Maybe training a larger-size single crystal can avoid the appearance of the B-level alerts in the cif-checking report of 3. The structures were then refined on F2 using SHELXL-97.10 CCDC numbers are 991312–991314 for 1–3, and 976685 for 4. The crystallographic data for 1–4 are summarized in Table 1.
Results and discussion Synthetic analysis All of the reactions were performed under hydrothermal conditions. The reactions of Ba(NO3)2, aromatic polycarboxylic acids ( pma, sdphda, cpha) and N2H4 afforded compounds 1–3, whereas the reaction of Zn(CH3COO)2, pdca, N2H4 and H2(ox) produced compound 4. Ox was introduced into the final framework of compound 4. The title compounds 1–4 are confirmed to be acylhydrazide-containing compounds, indicating that the hydrothermal in situ acylation of N2H4 with various aromatic polycarboxylic acids has occurred, yielding the acylhydrazide molecules. The acylhydrazide molecules do not exist in a diketo form in the complexes. Part of the acylamino groups will isomerize, so the keto-hydroxyl form is the stable existing form for the acylhydrazidate molecules (see Scheme S1†). The acylation of N2H4 with aromatic polycarboxylic acids can be carried out in a wide pH range (5–8), but the single-crystal growth for compounds 1–4 was strictly controlled by the pH level of the reactive system: pH = 5 for 1,
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pH = 8 for 2, pH = 7 for 3, and pH = 5 for 4. It is noteworthy that only at pH = 5, could compound 4 be obtained. At pH = 6–8, the product is the reported compound [Zn( pdh)2(H2O)2], where ox was not mixed into the Zn2+–pdh system.11 In the reactions, N2H4 is largely in excess in order to ensure thorough acylation of the carboxyls. The other metal ions of group IIA have also been used instead of Ba2+, but crystals suitable for X-ray single-crystal diffraction were not obtained. Structural description [Ba( pmdh)] 1. Compound 1 is a pmdh-extended 3D Ba2+ coordination polymer. It crystallizes in the space group P21/c, and the asymmetric unit is found to be composed of one Ba2+ ion (Ba1) and two types of pmdh molecules (pmdh I, pmdh II). Note that only a half of each pmdh molecule appears in the asymmetric unit of compound 1. As shown in Fig. 1a, Ba1 is in a 6-fold coordinated site, surrounded by three hydroxylimino O atoms (O1, O1a, O4b), two acylamino O atoms (O2c, O3) and one hydroxylimino N atom (N4d). The Ba1–Oacylamino bond length range of 2.6093(18)–2.811(2) Å is wider than that of Ba1–Ohydroxylimino (2.6596(17)–2.7165(18) Å). The Ba1–N4d distance is 2.859(2) Å. Two types of pmdh molecules are involved in the different μ6 coordination modes. For pmdh I, only the O atoms participate in the coordination to the Ba2+ ions. Each hydroxylimino O atom interacts with two Ba2+ ions, while each acylamino O atom coordinates to one Ba2+ ion. For pmdh II, four O atoms together with two hydroxylimino N atoms are involved in coordination to the Ba2+ ions. Each donor monodentately binds to one Ba2+ ion. The Ba2+ ion and two types of pmdh molecules aggregate together into a 3D network of compound 1. Fig. 1b is the projection plot of compound 1 in the (001) direction. The μ6-mode pmdh I molecules link the Ba2+ ions to form a 2D layer network (see inserted plot above). The pmdh I molecules with hydroxylimino O atoms as the donors first link the Ba2+ ions into a 1D chain, synchro-
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Fig. 1 Projection plots in the bc plane (inserted plot shows the coordination environment around Ba1) (a) and in the ab plane (inserted plots exhibit a layer structure (above) and interactions between layers (bottom)) (b) for 1 (a: −x + 1, −y, −z + 1; b: −x, y + 1/2, −z + 3/2; c: x, −y + 1/2, z − 1/2; d: x, −y − 1/2, z−1/2; e: x, −y + 1/2, z + 1/2; f: −x + 1, −y + 1, −z + 1; g: −x, −y, −z + 2; h: −x, y − 1/2, −z + 3/2).
nously producing the rhombic Ba2O2 rings. The Ba2O2 ring is planar, and the Ba⋯Ba contact is 4.055 Å. Then via the interactions between Ba2+ and acylamino O, the chains are linked together into this 2D layer. The topological method can be employed to better understand this layer structure. The Ba2O2 unit can be regarded as a 4-connected node, while each pmdh I molecule can also be viewed as a 4-connected node. So this layer can be described as a simple (4,4) net. The pmdh II molecules are distributed on the space between the layers (see inserted diagram below), extending the 2D layers into a 3D network of compound 1. Along the a-axial direction, two types of pmdh molecules array alternately in a parallel way, stabilizing the 3D network of compound 1. [Ba(sdpth)(H2O)2]·0.5H2O 2. Compound 2 is a sdpth-propagated 1D tubed coordination polymer. It crystallizes in the space group C2/m, and the asymmetric unit is found to be composed of a half of a Ba2+ ion (Ba1), a half of a sdpth molecule, one coordinated water molecule (Ow1) and a half of a lattice water molecule (Ow2). As shown in Fig. 2a, the hydroxy-
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limino O atoms of the sdpth molecules double bridge the Ba2+ ions into a 1D infinite single chain, which is based on the Ba2O2 units. The Ba2O2 unit shows a slight fold with a dihedral angle of 48.5°. Furthermore, two such single chains are linked together by the sdpth molecules to form a square tube with a size of ca. 8 × 8 Å2, running down the c-axial direction (see inserted plot). Along the same direction, the adjacent sdpth molecules form the π⋯π packing (contact: ca. 3.56 Å), stabilizing the tube structure. The sdpth molecule utilizes the hydroxylimino groups to chelate the Ba2+ ions. Two acylamino groups are non-coordinated. Interestingly, the non-coordinated acylamino group forms a dimer with the neighboring acylamino group via the N–H⋯O interactions (O2⋯N2e = 2.802(8) Å). Via this kind of hydrogen-bonded synthon, the 1D tubes are linked together into a 3D supramolecular network, creating synchronously a big rectangle channel with a size of ca. 16 × 8 Å2. In the tube, no water molecules were observed. The water molecules (Ow1, Ow2) actually occupy the space of the big channels. The Ba1–Ow1 distance of 2.938 Å suggests that
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Fig. 2
Dalton Transactions
1D tube structure (a) and 3D supramolecular network (b) for 2 (a: x, −y, z; e: −x + 1/2, −y + 1/2, −z).
there is a binding interaction between Ba1 and Ow1. Ba1 is in a 8-fold coordinated site. The Ba1–Ohydroxylimino distances of 2.740 Å and 2.977 Å are comparable with those observed in compound 1. The Ba1–N1 distance is 2.898 Å. The shortest Ba⋯Ba separation is 4.721 Å. [Ba2(cpth)2(H2O)2] 3. Compound 3 is a cpth-propagated 3D Ba2+ coordination polymer. It crystallizes in the space group Pna21, and the asymmetric unit is found to be composed of two types of Ba2+ ions (Ba1, Ba2), two types of cpth molecules (cpth I, cpth II) and two types of coordinated water molecules (Ow1, Ow2). As shown in Fig. 3a (inserted plots), Ba1 and Ba2 are both involved in a 9-fold coordinated site, but the detailed coordination environments are different. Ba1 is coordinated with two hydroxylimino O atoms (O1, O5), one acylamino O atom (O2c), four carboxyl O atoms (O7a, O3b, O4b, O8c), one hydroxylimino N atom (N2c) and one water molecule (Ow1), while Ba2 is surrounded by three acylamino O atoms (O2, O6d, O6e), two carboxyl atoms (O7, O8), one hydroxylimino N atom (N4d) and three water molecules (Ow2, Ow1c, Ow2f ). The Ba–O distances span a wide range from 2.665(6) Å to 3.076(7) Å. The Ba2–N4d distance of 2.987(8) Å is slightly
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longer than that of Ba1–N2c (2.888(7) Å). Cpth I and cpth II adopt different coordination modes (see Scheme 2). Cpth I adopts a μ4 coordination mode: the hydroxylimino group chelates one Ba2+ ion, the acylamino O atom binds to one Ba2+ ion, the carboxyl chelates one Ba2+ ion, and the hydroxylimino O atom interacts further with another Ba2+ ion. Different from cpth I, each carboxyl O atom for cpth II further binds to another Ba2+ ion, so cpth II adopts a μ6 coordination mode. Bridged by cpth, compound 3 exhibits a 3D network structure (also see Fig. 3a). In fact, Ba1 and cpth I aggregate to form a 3D network with a (10,3) topology, in which both Ba1 and cpth I are regarded as the 3-connected nodes (see Fig. 2b). Ba2 and cpth II occupy the space of the channel (inserted plot above). The inserted plot below shows the interactions of cpth II with Ba1 or Ba2. With O6, O7, O8 and N4 as the donors, cpth II interacts with Ba2, while with O5, O7 and O8 as the donors, cpth II interacts with Ba1. Note that the symmetry-related O6 atoms link Ba2 into a 1D endless zigzag chain, extending along the a-axial direction. Based on the topological viewpoint, compound 3 exhibits a 3D (44,62)(48,67)topology.
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Fig. 3 3D network of 3 (inset plot shows the coordination environments around Ba1 and Ba2) (a) and the 3D network constructed up from Ba1 and cpth I (inset plot shows the interactions of cpth II with Ba1 or Ba2) (b) (a: x + 1/2, −y − 1/2, z; b: −x + 3/2, y + 1/2, z + 1/2; c: x − 1/2, −y − 1/2, z; d: −x + 3/2, y − 1/2, z − 1/2; e: −x + 2, −y − 1, z − 1/2; f: x + 1/2, −y − 3/2, z; g: −x + 2, −y − 1, z + 1/2).
[Zn2( pdh)2(ox)]·H2O 4. Compound 4 is an ox-extended 3D Zn2+-pdh coordination polymers. It crystallizes in the space group I41/a, and the asymmetric unit is found to be composed of two types of Zn2+ ions (Zn1, Zn2), two types of pdh molecules ( pdh I, pdh II), one ox molecule and one lattice water molecule (Ow1). As shown in Fig. 4a, pdh I and pdh II adopt different doubly-bridged coordination modes. Both use the pyridyl N and hydroxylimino O atoms to chelate one Zn(II) center. The difference is that for pdh II, the hydroxylimino O atom further binds to the other Zn2+ ion, while for pdh I, the hydroxylimino N atom acts as the donor. For this reason, (i) the detailed coordination for Zn1 and Zn2 is different, even though Zn1 and Zn2 are both involved in a 5-fold coordinated site. Apart from coordination by two ox O atoms (O5 and O6 for Zn1, O7 and O8 for Zn2), one pyridyl N atom (N3 for Zn1, N6 for Zn2) and one hydroxylimino O atom (O1 for Zn1, O3 for Zn2), the 5-fold coordinated Zn1 is completed by one hydroxy-
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limino N atom (N1a), while the 5-fold coordinated Zn2 is completed by one hydroxylimino O atom (O3b); (ii) two types of Zn4( pdh)4 clusters are observed in compound 4. Zn1 and pdh I form one Zn4( pdh)4 cluster (labeled as Zn14( pdhI)4), while Zn2 and pdh II form the other Zn4( pdh)4 cluster (labeled as Zn24( pdhII)4). Four Zn2+ ions in each Zn4( pdh)4 cluster form a tetrahedron. Zn14 is a normal tetrahedron with a Zn⋯Zn range of 4.957–5.713 Å, while Zn24 is severely distorted (the Zn⋯Zn range: 3.642–4.990 Å). Ox adopts a common μ2 bridging-mode. Fig. 4b is the projection plot of compound 4 in the (001) direction. With ox as the linkers, each Zn14( pdhI)4 cluster is connected with four Zn24( pdhII)4 clusters. Similarly, each Zn24( pdhII)4 cluster is connected with four Zn14( pdhI)4 clusters. The green and yellow ox molecules extend the Zn4( pdh)4 into a 2D layer network (see Fig. 4c). Along the a-axial direction, through an alternate linkage of the pink and red ox molecules, the 2D layers are further propagated into a 3D network.
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Scheme 2
Dalton Transactions
Coordination modes of acylhydrazidate molecules in 1–4.
Based on the topological method, each Zn4( pdh)4 cluster can be regarded as a 4-connected node (tetrahedral configuration), while the ox molecule can be considered as the linker. Compound 4 exhibits a diamond-like 66-topology. The interpenetration phenomenon is found in the network structure of compound 4. Compound 4 displays a 2-fold interpenetrated net (see Fig. S1†). Due to the interpenetration, the pore turns out to be small. The lattice water molecules are distributed on the pores. As expected, (i) four acylamino groups for pmdh are all involved in coordination to the Ba2+ ions, so compound 1 shows a 3D network structure. The 3D network of compound 1 is different from that of the reported 3D compound [Co( pmdh)(H2O)2] due to the difference in the geometries of the metal ions. Also owing to this reason, the pmdh molecules adopt different coordination modes in both compounds;5 (ii) compound 3 also possesses a 3D network structure. Clearly, in the formation of this 3D network, the carboxyl on the benzene ring plays a crucial role. So far, the pth-coordinated Ba2+ compound has not been obtained. The metal ion also has an effect on the final structure architecture. For example, the cpthextended Ba2+ compound 3 exhibits a 3D network, while the reported cpth-propagated Pb2+ compound [Pb4(OH)2(cpth)3(H2O)3]·2H2O] shows a 2D layer network;12 (iii) in compound 4, the pdh molecules link the Zn2+ ions to form two types of Zn4( pdh)4 clusters, namely the secondary building units. The ox molecules act as the secondary linkers, extending the Zn4( pdh)4 tetranuclear clusters into a 3D network. This is different from the situation observed in the widely investigated metal–polycarboxylate system, in which the metal ions and the carboxyl groups form the various SBUs, and the polycarboxylate molecules further propagate the SBUs into various frameworks.3 Unexpectedly, compound 2 only possesses a 1D chain
11652 | Dalton Trans., 2014, 43, 11646–11657
structure. For sdpth, only two acylamino groups are involved in coordination to the Ba2+ ions. This may be related to the spacer of the sdpth molecule. The centric S atom adopts a tetrahedral configuration, so sdpth shows a V shape, which directly influences the resulting framework of compound 2. But interestingly, the sdpth molecules link the Ba2+ ions into a 1D tube structure, and no solvent molecules are found in the tube. More interestingly, the non-coordinated acylamino group forms a new synthon with the neighboring non-coordinated acylamino group via the N–H⋯O interactions, via which all of the tubes are linked together into a 3D supramolecular network. At the same time, the big channels are formed. The water molecules in the channels can be removed. Moreover, (i) the acylhydrazidate molecules in compounds 1–4 exist in the keto-hydroxyl form which is confirmed by the different C–O distances. X-ray single-crystal diffraction analysis revealed that one C–O distance (CvO) is obviously shorter than the other (C–O−) (see Table S1†), suggesting that one acylamino group has isomerized into the hydroxylimino group. With reaction to the metal ion, the hydroxyl deprotonates to balance the metal charge. So pmdh, sdpth and cpth in compounds 1–3 have a −2 charge, while pdh in compound 4 shows a −1 oxidation state. Characterization The TG behaviors of compounds 2–4 were investigated. Fig. 5 presents the temperature vs. weight-loss curves. Based on the TG curves, we can see that (i) all three compounds underwent two steps of weight loss. The initial minor weight loss (calcd: 8.0%, found: ca. 9.0% for 2; calcd: 5.0%, found: ca. 6.0% for 3; calcd: 3.2%, found: ca. 3.9% for 4) should be ascribed to the sublimation of the coordinated and/or lattice water molecules. The second step of weight loss corresponds to the decomposition of the organic molecules in the compounds; (ii) for
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Fig. 5
TG curves of 2–4.
tures for the onset of the collapse of the host frameworks are higher (400 °C for 2, 480 °C for 3, 455 °C for 4); (iv) for compound 2, the water molecules in the channels can be easily removed. Once heated, the water molecules began to sublimate. In a narrow temperature range (30–140 °C), the water molecules were completely lost; (v) the TG curve suggests that there is one water molecule in a symmetric unit of 4, even though X-ray single-crystal diffraction analysis revealed that a half may be more reasonable. The appearance of the strong peaks at 1639 cm−1 for 1, 1643 cm−1 for 2, 1646 cm−1 for 3, and 1659 cm−1 for 4 in the IR spectra (see Fig. S2†) implies that the acylation of N2H4 with aromatic polycarboxylic acids has occurred, because the ν(COO) peaks are generally either larger than 1680 cm−1 or smaller than 1610 cm−1, whereas the ν(CONH) peaks appear in the range of 1620–1680 cm−1.13 The experimental powder XRD pattern for each compound is consistent with the simulated one generated on the basis of single crystal X-ray diffraction structural data, confirming that the as-synthesized product is a pure phase (see Fig. 6). Photoluminescence property
Fig. 4 Two types of Zn4( pdh)4 clusters (a), projection plots in (001) (b) and (100) directions (c) for 4.
acylhydrazidate-extended Ba2+ compounds 2 and 3, the final residues proved to be BaCO3, rather than BaO (calcd: 35.4%; found: ca. 34.7% for 2; calcd: 54.9%; found: ca. 54.1% for 3), whereas for acylhydrazidate-propagated Zn2+ compound 4, the residue was confirmed to be ZnO (Calcd: 28.4%, Found: ca. 29.5%); (iii) compounds 1–4 are stable, because the tempera-
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The solid-state photoluminescence property of 1–4 was investigated. Fig. 7 shows the corresponding emission and excitation spectra as well as the decay curves. Obviously, the solid-state compounds 3 and 4 possess photoluminescence properties. Upon excitation, both emit a similar green light with maxima at 495 nm (λex = 397 nm) and 522 nm (λex = 395 nm), for 3 and 4, respectively. So far, several pdh/derivative-coordinated transition-metal complexes have been obtained. The photoluminescence analysis indicates that almost all emit green light, and the peaks appear in the range of 500–530 nm.4i,6,11,14 Only a mononuclear Pb2+ compound [Pb(mpdh)2] (mpdh = 6-methylpyridine-2,3-dicarboxylhydrazidate) is an exception, emitting yellow light with a maximum at ca. 600 nm. The exception may be due to the special geometry for the Pb2+ center as well as the hydrogen-bonded interactions between the pdh molecules.15 The density functional theory
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Fig. 6 Experimental (red) and simulated (black) powder XRD patterns for 1–4.
(DFT) calculations suggest that the green-light emissions originate from the intra-acylhydrazidate transitions, corresponding to the charge transfer from the π* orbitals of the pyridine/ benzene ring moiety to the π orbitals of the acylhydrazidate ring moiety.4i,6a,11,14 Therefore, the green-light emission of 4 should have the same attribution. The green-light emission of 3 should also be assigned to this attribution, and only a slight blue shift by 27 nm is observed. Compounds 1 and 2 do not emit light, which is related to the close packing of the molecules. The close packing of the molecules tends to lead to luminescence quenching. Due to this reason, the majority of diacylhydrazide-coordinated compounds do not emit light.15 Only two reported complexes emit light, which should be related
Fig. 7
to their special space structures. For example, in compound [Cd(sdpth)( phen)(H2O)]·H2O ( phen = 1,10-phenanthrolion), no π⋯π interactions between the sdpth molecules are observed, so it emits light;16 in compound [Zn(N2H4)(dphkh)]·H2O (dphkh = 4,4′-diphthalhydrazidatoketone hydrazone), the incorporation of N2H4 molecule weakens the interactions between the dphkh molecules, so it also emits light.17 For compounds 3 and 4, the molecular stacking is closer yet. However, the presence of the carboxyl substituent and the N heteroatom on the benzene ring assists the electronic transitions from the acylhydrazidate ring moiety to the pyridine/ benzene ring moiety. On the other hand, the acidic protons in the acylhydrazidates (NH protons) may participate in the excited-state proton transfer (ESPT), which would also quench any emission process.18 So the luminescence quenching of compounds 1 and 2 may originate from two factors: the interligand charge transfer of stacked ligands and the ESPT. The decay curve for 3 fits into a double exponential function, and the lifetimes were calculated to be τ1 = 1.20 ns and τ2 = 2.54 ns, respectively. The luminescence lifetime for 4 was calculated to be τ1 = 1.50 ns. Fig. S3† provides the UV-vis spectra of the title compounds. Sorption study To assess the porosity property of the as-synthesized MOFs, the N2 adsorption isotherms of 2–4 at 77 K were measured. Fig. 8 illustrates the corresponding adsorption–desorption isotherms. The N2 uptakes of 2 and 4 follow a similar increasing trend. And the adsorption and desorption isotherm curves do not overlap with each other with small hysteresis. The BET surface areas are estimated to be 58.81 m2 g−1 for 2 and 39.04 m2 g−1 for 4, respectively. The measured pore volumes
Emission (a) and excitation spectra (b) as well as their decay curves for 3 and 4.
11654 | Dalton Trans., 2014, 43, 11646–11657
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hydrogen-bonded interaction between the acylamino groups, the acylhydrazidate-extended metal complex may self-assemble into a 3D porous supramolecular network. The TG analysis indicates that the host metal–acylhydrazidate frameworks are rather stable. Some metal–acylhydrazidate complexes possess photoluminescence properties. The emissions derive from the charge transfers associated with the acylhydrazidate molecules. The close packing of the molecules as well as the ESPT is prone to leading to luminescence quenching, but the carboxyl substituent and the N heteroatom on the benzene ring contribute to the electronic transitions. The N2 adsorption– desorption isotherms indicate that activated 2 and 4 could adsorb N2 at 77 K with an order of adsorption of 2 >4.
Fig. 8
N2 adsorption–desorption isotherms of 2–4 at 77 K.
Acknowledgements This research was supported by the National Natural Science Foundation of China (no. 21271083).
are ca. 0.123 cm3 g−1 for 2 and ca. 0.095 m2 g−1 for 4, respectively. The capacity of 2 to adsorb N2 at 77 K and 1 bar is 58.31 cm3 g−1, a value surpassing that of 4 (38.38 cm3 g−1). Compound 3 exhibits a nil adsorption to N2 at 77 K. The CO2 adsorption isotherm at 273 K for 2 was also studied. As shown in Fig. S4,† the adsorption amount is negligible (3.32 cm3 g−1), suggesting that compound 2 does not adsorb CO2 at 273 K. Based on the statement above, we know that (i) the activated 2 and 4 could adsorb N2 at 77 K; (ii) the adsorption amount of 2 is slightly larger than that of 4; (iii) the N2 adsorption isotherms at 77 K for 2 and 4 do not show the typical type-I, observed for most MOFs;2o,3e,h,g,i,s,19 (iv) the hysteresis loop is observed in the N2 adsorption–desorption isotherms for 2 and 4.
Conclusion In summary, we report the synthesis, structural characterization, thermal behavior, and photoluminescence property of four acylhydrazidate-containing coordination polymers. They were obtained from the hydrothermal reactions of Ba2+/Zn2+, aromatic polycarboxylic acids, N2H4 with or without the ox molecule. The acylhydrazidate molecules were derived from the hydrothermal in situ acylation of N2H4 with aromatic polycarboxylic acids. X-ray single-crystal diffraction analysis revealed that compounds 1 and 3 are acylhydrazidate-extended 3D MOFs. For compound 4, although the monoacylhydrazidate molecules link the metal ions to form a discrete oligomer, the introduced ox molecules act as secondary linkers, propagating the oligomers into a 3D MOF. These results suggest that the strategies stated below are feasible for the construction of acylhydrazidate-extended MOFs: (i) using diacylhydrazide molecule to react with the metal ion; (ii) modifying the monoacylhydrazide molecule, especially its 4- and 5-positions; (iii) incorporating another kind of bridging-type organic molecule into the metal–monoacylhydrazide system. Moreover, via the
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Construction of acylhydrazidate-extended metal–organic frameworks† Yan-Ning Wang,a,b Qing-Feng Yang,c Guang-Hua Li,a,b Ping Zhang,*a Jie-Hui Yu*a,b and Ji-Qing Xua,b Under hydrothermal conditions, the reactions of Ba2+/Zn2+, aromatic polycarboxylic acids and N2H4 with or without oxalic acid were carried out, affording four new acylhydrazidate-extended metal–organic frameworks (MOFs) [Ba(pmdh)] ( pmdh = pyromellitdihydrazidate) 1, [Ba(sdpth)(H2O)2]·0.5H2O (sdpth = 4,4’-sulfoyldiphthalhydrazidate) 2, [Ba2(cpth)2(H2O)2] (cpth = 4-carboxylphthalhydrazidate) 3 and [Zn2( pdh)2(ox)]·H2O (ox = oxalate, pdh = pyridine-2,3-dicarboxylhydrazidate) 4. The acylhydrazidate molecules pmdh, sdpth, cpth and pdh in compounds 1–4 derived from the hydrothermal in situ acylation of N2H4 with aromatic polycarboxylic acids. X-ray single-crystal diffraction analysis revealed that (i) in compound 1, the pmdh I molecules link the Ba2+ ions into a two-dimensional (2D) layer with a (4,4) topology, and then the pmdh II molecules extend these layers into a three-dimensional (3D) network; (ii) in compound 2, the sdpth molecules link the Ba2+ ions to form a one-dimensional (1D) square tube. Interestingly, the tubes are further linked into a 3D supramolecular network via the N–H⋯O interactions, creating synchronously big channels; (iii) in compound 3, the cpth I molecules link the Ba1 ions into a 3D network with a (10,3) topology. Ba2 and cpth II are distributed on the channels; (iv) in compound 4, Zn2+ and pdh aggregate to form two types of Zn4( pdh)4 clusters. The ox molecules act as the secondary
Received 15th March 2014, Accepted 9th May 2014
linkers, extending the Zn4( pdh)4 secondary building units (SBUs) into a 3D network with a 66 topology. The photoluminescence analysis indicates that compounds 3 and 4 emit green light with maxima at
DOI: 10.1039/c4dt00780h
495 nm for 3 (λex = 397 nm), and 522 nm for 4 (λex = 395 nm), respectively. At 77 K, the activated 2 and 4
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can adsorb N2 in amounts of 58.31 cm3 g−1 for 2 and 38.38 cm3 g−1 for 4, respectively.
Introduction Considerable effort has been taken in the design and synthesis of novel metal–organic frameworks (MOFs) due to their intriguing structures and topologies,1 and their potential applications in some fields such as adsorption, optics, magnetism and catalysis.2 Although so many MOFs have been obtained through self-assembly between metal ions and organic N/Odonor ligands, it is still a challenge to obtain a target MOF with a pre-designed structure and desirable property, because the self-assembly process is so complicated, and dominated by multiple factors including the geometric configuration of the metal ions, the nature of the organic ligands (size, shape, a College of Chemistry, Jilin University, Jiefang Road 2519, Changchun, 130023, P.R. China. E-mail: [email protected], [email protected] b State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Qianjin Street 2699, Changchun, 130023, P.R. China c Key Laboratory of Energy Resources and Chemical Engineering, Ningxia University, Yinchuan, Ningxia 750021, P.R. China † Electronic supplementary information (ESI) available. CCDC 991312–991314 for 1–3, and 976685 for 4. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt00780h
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flexibility/rigidity), and the detailed experimental conditions (solvent, pH, reaction temperature, ratio of metal-to-ligand). Over the last two decades, multidentate N-heterocyclic molecules and polycarboxylic acid molecules have been extensively employed in the construction of novel MOFs.3 Recently, as a new type of bridging ligand, the organic acylhydrazide molecules have attracted some attention.4 The diacylhydrazide molecules such as H2pmdh ( pmdh = pyromellitdihydrazidate) and H2sdpth (sdpth = 4,4′-sulfoyldiphthalhydrazidate) (see Scheme 1), having four acylamino groups, possess the potential to form extended networks with metal ions, which has been confirmed by the reported 3D compound [Co( pmdh)(H2O)2].5 Different from the diacylhydrazide molecules, the monoacylhydrazidate molecules such as H2pth ( pth = phthalhydrazidate) and Hpdh ( pdh = pyridine-2,3-dicarboxylhydrazidate) (see also Scheme 1) tend to form various discrete or chained oligomers with metal ions.6 But on the one hand, a simple modification to the monoacylhydrazide molecule is helpful to the further extension of these oligomers into a highD network. For example, the 3,4-pdh-extended Pb2+ compound [Pb(3,4-pdh)] (3,4-pdh = pyridine-3,4-dicarboxylhydrazidate) shows a 3D network structure. Here a meso-position C atom of
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Dalton Transactions
Scheme 1
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Structures of acylhydrazidate molecules in 1–4.
the benzene ring is replaced by a N heteroatom.7 For another example, compound [Fe3(dcpth)2(H2O)4]·2H2O (dcpth = 4,5dicarboxylphthahydrazidate) also exhibits a 3D network structure. Here two carboxyl substituents on the 4- and 5-positions for H3dcpth play a key role in the formation of this extended network.8 On the other hand, the incorporation of another kind of organic bridging molecule can also extend these oligomers into a high-D network. As observed in the reported 3D compound [Zn2( pth)(atez)2] (atez = 5-aminotetrazolate), the atez molecules act as secondary linkers, extending the Zn2+pth chains into a 3D network.9 Note that all of the di(mono)acylhydrazide molecules mentioned above originated from the hydrothermal in situ acylation of N2H4 with aromatic polycarboxylic acids. Based on these considerations, two tetracarboxylic acid molecules ( pyromellitic acid, pma; 4,4′sulfonyldiphthalic dianhydride, sdphda) and one carboxylmodified phthalic acid (4-carboxylphthalic acid, cpha) were selected to serve as the acidic precursors. The reactions of Ba2+, aromatic polycarboxylic acids and N2H4 were carried out, affording three new acylhydrazidate-containing coordination polymers [Ba( pmdh)] 1, [Ba(sdpth)(H2O)2]·0.5H2O 2 and [Ba2(cpth)2(H2O)2] (cpth = 4-carboxylphthalhydrazidate) 3. Moreover, the incorporation of the H2(ox) (ox = oxalate) molecule into the reaction of Zn2+, pyridine-2,3-dicarboxylic acid ( pdca) and N2H4 was also performed, creating compound [Zn2( pdh)2(ox)]·H2O 4.
Experimental Materials and physical measurement All chemicals were reagent grade quality, obtained from commercial sources and used without further purification. Elemental analysis was performed on a Perkin-Elmer 2400LS II elemental analyzer. Infrared (IR) spectra were recorded on a Perkin Elmer Spectrum 1 spectrophotometer in the 4000–400 cm−1 region using a powdered sample on a KBr plate. Ultraviolet-visible (UV-vis) spectrum was obtained on a Rigaku-UV-3100 spectrophotometer. Powder X-ray diffraction (XRD) data were collected on a Rigaku/max-2550 diffractometer with Cu-Kα radiation (λ = 1.5418 Å). Thermogravimetric (TG) behavior was investigated on a Perkin-Elmer TGA-7 instrument with a heating rate of 10 °C min−1 in air. Fluorescence spectrum was obtained on a LS 55 florescence/phosphorescence spectrophotometer at room temperature. The measurements
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of N2/CO2 adsorption were carried out on asap-2010 and autosorb-IQ2 apparatus. Synthesis of the title compounds [Ba( pmdh)] 1. The yellow block crystals of 1 were obtained from a simple hydrothermal self-assembly of Ba(NO3)2 (78 mg, 0.3 mmol), pma (65 mg, 0.3 mmol) and N2H4·H2O (0.15 mL) in a 10 mL aqueous solution ( pH = 5 adjusted by H2(ox)) at 160 °C for 4 days. Yield: ca. 15% based on Ba(II). Anal. Calcd C10H4N4O4Ba 1: C 31.48, H 1.057, N 14.69. Found: C 30.59, H 1.051, N 14.57%. IR (cm−1): 1639 s, 1535 s, 1507 w, 1482 m, 1435 m, 1384 s, 1300 m, 1176 m, 1144 m, 816 m, 694 m, 650 s. [Ba(sdpth)(H2O)2]·0.5H2O 2. The yellow block crystals of 2 were obtained from a simple hydrothermal self-assembly of Ba(NO3)2 (131 mg, 0.5 mmol), sdphda (179 mg, 0.5 mmol) and N2H4·H2O (0.2 mL) in a 15 mL aqueous solution ( pH = 8 adjusted by N2H4) at 170 °C for 4 days. Yield: ca. 25% based on Ba(II). Anal. Calcd C16H13N4O8.5SBa 2: C 33.90, H 2.312, N 9.89. Found: C 34.19, H 2.305, N 9.81%. IR (cm−1): 1643 s, 1573 s, 1480 s, 1381 m, 1242 w, 1145 s, 1057 s, 884 w, 813 s, 716 w, 662 m, 559 m. [Ba2(cpth)2(H2O)2] 3. The yellow block crystals of 3 were obtained from a simple hydrothermal self-assembly of Ba(NO3)2 (131 mg, 0.5 mmol), cpha (96 mg, 0.5 mmol) and N2H4·H2O (0.2 mL) in a 15 mL aqueous solution ( pH = 7 adjusted by N2H4) at 170 °C for 4 days. Yield: ca. 20% based on Ba(II). Anal. Calcd C18H12N4O10Ba2 3: C 30.07, H 1.682, N 7.79. Found: C 29.74, H 1.691, N 7.78%. IR (cm−1): 1646 m, 1618 w, 1579 s, 1448 m, 1415 s, 1366 w, 1210 m, 1059 w, 820 m, 781 s, 710 s, 617 w, 524 m. [Zn2( pdh)2(ox)]·H2O 4. The yellow block crystals of 4 were obtained from a simple hydrothermal self-assembly of Zn(CH3COO)2·2H2O (110 mg, 0.5 mmol), pdca (83 mg, 0.5 mmol), N2H4·H2O (0.2 mL) and H2ox (63 mg, 0.5 mmol) in a 15 mL aqueous solution ( pH = 5 adjusted by H2(ox)) at 160 °C for 4 days. Yield: ca. 25% based on Zn(II). Anal. Calcd C16H10N6O9Zn2 4: C 34.25, H 1.797, N 14.98. Found: C 34.57, H 1.787, N 15.18%. IR (cm−1): 1659 s, 1567 s, 1524 m, 1477 m, 1262 w, 1161 w, 1119 s, 781 s, 713 w, 675 w, 535 s, 510 w, 495 s. X-ray crystallography The data were collected with Mo-Kα radiation (λ = 0.71073 Å) on a Rigaku R-AXIS RAPID IP diffractometer for 1, and on a Siemens SMART CCD diffractometer for 2–4. With the
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Table 1
Dalton Transactions Crystallographic data for 1–4
Formula M T (K) Crystal system Space group a (Å) b (Å) c (Å) β (°) V (Å3) Z Dc (g cm−3) μ (mm−1) Reflections collected Unique reflections Rint Gof R1, I > 2σ(I) wR2, all data
1
2
3
4
C10H4N4O4Ba 381.51 293(2) Monoclinic P21/c 8.2417(16) 10.946(2) 11.113(2) 93.86(3) 1000.3(3) 4 2.533 3.989 9586 2288 0.0172 1.119 0.0204 0.0602
C16H13N4O8.5SBa 566.69 293(2) Monoclinic C2/m 19.372(3) 23.436(3) 4.7214(5) 94.727(11) 2136.2(5) 4 1.762 2.009 6086 1963 0.0671 1.093 0.0600 0.2010
C18H12N4O10Ba2 719.00 293(2) Orthorhombic Pna21 6.9812(4) 14.8221(9) 19.0460(17)
C16H10N6O9Zn2 561.04 293(2) Tetragonal I41/a 17.6446(9) 17.6446(9) 22.742(3)
1970.8(2) 4 2.423 4.043 10 417 2649 0.0708 1.055 0.0307 0.0863
7080.4(10) 16 2.105 2.784 19 798 3130 0.0961 1.119 0.0418 0.1180
SHELXTL program, the structures of 1–3 were solved using direct methods, whereas the structure of 4 was solved using heavy atom methods.10 The non-hydrogen atoms were assigned anisotropic displacement parameters in the refinement. The H atoms on the N atoms in 1 were obtained from the difference Fourier map. The H atoms on the water molecules in 2–4 were not located. The other H atoms were treated using a riding model. Maybe training a larger-size single crystal can avoid the appearance of the B-level alerts in the cif-checking report of 3. The structures were then refined on F2 using SHELXL-97.10 CCDC numbers are 991312–991314 for 1–3, and 976685 for 4. The crystallographic data for 1–4 are summarized in Table 1.
Results and discussion Synthetic analysis All of the reactions were performed under hydrothermal conditions. The reactions of Ba(NO3)2, aromatic polycarboxylic acids ( pma, sdphda, cpha) and N2H4 afforded compounds 1–3, whereas the reaction of Zn(CH3COO)2, pdca, N2H4 and H2(ox) produced compound 4. Ox was introduced into the final framework of compound 4. The title compounds 1–4 are confirmed to be acylhydrazide-containing compounds, indicating that the hydrothermal in situ acylation of N2H4 with various aromatic polycarboxylic acids has occurred, yielding the acylhydrazide molecules. The acylhydrazide molecules do not exist in a diketo form in the complexes. Part of the acylamino groups will isomerize, so the keto-hydroxyl form is the stable existing form for the acylhydrazidate molecules (see Scheme S1†). The acylation of N2H4 with aromatic polycarboxylic acids can be carried out in a wide pH range (5–8), but the single-crystal growth for compounds 1–4 was strictly controlled by the pH level of the reactive system: pH = 5 for 1,
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pH = 8 for 2, pH = 7 for 3, and pH = 5 for 4. It is noteworthy that only at pH = 5, could compound 4 be obtained. At pH = 6–8, the product is the reported compound [Zn( pdh)2(H2O)2], where ox was not mixed into the Zn2+–pdh system.11 In the reactions, N2H4 is largely in excess in order to ensure thorough acylation of the carboxyls. The other metal ions of group IIA have also been used instead of Ba2+, but crystals suitable for X-ray single-crystal diffraction were not obtained. Structural description [Ba( pmdh)] 1. Compound 1 is a pmdh-extended 3D Ba2+ coordination polymer. It crystallizes in the space group P21/c, and the asymmetric unit is found to be composed of one Ba2+ ion (Ba1) and two types of pmdh molecules (pmdh I, pmdh II). Note that only a half of each pmdh molecule appears in the asymmetric unit of compound 1. As shown in Fig. 1a, Ba1 is in a 6-fold coordinated site, surrounded by three hydroxylimino O atoms (O1, O1a, O4b), two acylamino O atoms (O2c, O3) and one hydroxylimino N atom (N4d). The Ba1–Oacylamino bond length range of 2.6093(18)–2.811(2) Å is wider than that of Ba1–Ohydroxylimino (2.6596(17)–2.7165(18) Å). The Ba1–N4d distance is 2.859(2) Å. Two types of pmdh molecules are involved in the different μ6 coordination modes. For pmdh I, only the O atoms participate in the coordination to the Ba2+ ions. Each hydroxylimino O atom interacts with two Ba2+ ions, while each acylamino O atom coordinates to one Ba2+ ion. For pmdh II, four O atoms together with two hydroxylimino N atoms are involved in coordination to the Ba2+ ions. Each donor monodentately binds to one Ba2+ ion. The Ba2+ ion and two types of pmdh molecules aggregate together into a 3D network of compound 1. Fig. 1b is the projection plot of compound 1 in the (001) direction. The μ6-mode pmdh I molecules link the Ba2+ ions to form a 2D layer network (see inserted plot above). The pmdh I molecules with hydroxylimino O atoms as the donors first link the Ba2+ ions into a 1D chain, synchro-
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Fig. 1 Projection plots in the bc plane (inserted plot shows the coordination environment around Ba1) (a) and in the ab plane (inserted plots exhibit a layer structure (above) and interactions between layers (bottom)) (b) for 1 (a: −x + 1, −y, −z + 1; b: −x, y + 1/2, −z + 3/2; c: x, −y + 1/2, z − 1/2; d: x, −y − 1/2, z−1/2; e: x, −y + 1/2, z + 1/2; f: −x + 1, −y + 1, −z + 1; g: −x, −y, −z + 2; h: −x, y − 1/2, −z + 3/2).
nously producing the rhombic Ba2O2 rings. The Ba2O2 ring is planar, and the Ba⋯Ba contact is 4.055 Å. Then via the interactions between Ba2+ and acylamino O, the chains are linked together into this 2D layer. The topological method can be employed to better understand this layer structure. The Ba2O2 unit can be regarded as a 4-connected node, while each pmdh I molecule can also be viewed as a 4-connected node. So this layer can be described as a simple (4,4) net. The pmdh II molecules are distributed on the space between the layers (see inserted diagram below), extending the 2D layers into a 3D network of compound 1. Along the a-axial direction, two types of pmdh molecules array alternately in a parallel way, stabilizing the 3D network of compound 1. [Ba(sdpth)(H2O)2]·0.5H2O 2. Compound 2 is a sdpth-propagated 1D tubed coordination polymer. It crystallizes in the space group C2/m, and the asymmetric unit is found to be composed of a half of a Ba2+ ion (Ba1), a half of a sdpth molecule, one coordinated water molecule (Ow1) and a half of a lattice water molecule (Ow2). As shown in Fig. 2a, the hydroxy-
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limino O atoms of the sdpth molecules double bridge the Ba2+ ions into a 1D infinite single chain, which is based on the Ba2O2 units. The Ba2O2 unit shows a slight fold with a dihedral angle of 48.5°. Furthermore, two such single chains are linked together by the sdpth molecules to form a square tube with a size of ca. 8 × 8 Å2, running down the c-axial direction (see inserted plot). Along the same direction, the adjacent sdpth molecules form the π⋯π packing (contact: ca. 3.56 Å), stabilizing the tube structure. The sdpth molecule utilizes the hydroxylimino groups to chelate the Ba2+ ions. Two acylamino groups are non-coordinated. Interestingly, the non-coordinated acylamino group forms a dimer with the neighboring acylamino group via the N–H⋯O interactions (O2⋯N2e = 2.802(8) Å). Via this kind of hydrogen-bonded synthon, the 1D tubes are linked together into a 3D supramolecular network, creating synchronously a big rectangle channel with a size of ca. 16 × 8 Å2. In the tube, no water molecules were observed. The water molecules (Ow1, Ow2) actually occupy the space of the big channels. The Ba1–Ow1 distance of 2.938 Å suggests that
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Fig. 2
Dalton Transactions
1D tube structure (a) and 3D supramolecular network (b) for 2 (a: x, −y, z; e: −x + 1/2, −y + 1/2, −z).
there is a binding interaction between Ba1 and Ow1. Ba1 is in a 8-fold coordinated site. The Ba1–Ohydroxylimino distances of 2.740 Å and 2.977 Å are comparable with those observed in compound 1. The Ba1–N1 distance is 2.898 Å. The shortest Ba⋯Ba separation is 4.721 Å. [Ba2(cpth)2(H2O)2] 3. Compound 3 is a cpth-propagated 3D Ba2+ coordination polymer. It crystallizes in the space group Pna21, and the asymmetric unit is found to be composed of two types of Ba2+ ions (Ba1, Ba2), two types of cpth molecules (cpth I, cpth II) and two types of coordinated water molecules (Ow1, Ow2). As shown in Fig. 3a (inserted plots), Ba1 and Ba2 are both involved in a 9-fold coordinated site, but the detailed coordination environments are different. Ba1 is coordinated with two hydroxylimino O atoms (O1, O5), one acylamino O atom (O2c), four carboxyl O atoms (O7a, O3b, O4b, O8c), one hydroxylimino N atom (N2c) and one water molecule (Ow1), while Ba2 is surrounded by three acylamino O atoms (O2, O6d, O6e), two carboxyl atoms (O7, O8), one hydroxylimino N atom (N4d) and three water molecules (Ow2, Ow1c, Ow2f ). The Ba–O distances span a wide range from 2.665(6) Å to 3.076(7) Å. The Ba2–N4d distance of 2.987(8) Å is slightly
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longer than that of Ba1–N2c (2.888(7) Å). Cpth I and cpth II adopt different coordination modes (see Scheme 2). Cpth I adopts a μ4 coordination mode: the hydroxylimino group chelates one Ba2+ ion, the acylamino O atom binds to one Ba2+ ion, the carboxyl chelates one Ba2+ ion, and the hydroxylimino O atom interacts further with another Ba2+ ion. Different from cpth I, each carboxyl O atom for cpth II further binds to another Ba2+ ion, so cpth II adopts a μ6 coordination mode. Bridged by cpth, compound 3 exhibits a 3D network structure (also see Fig. 3a). In fact, Ba1 and cpth I aggregate to form a 3D network with a (10,3) topology, in which both Ba1 and cpth I are regarded as the 3-connected nodes (see Fig. 2b). Ba2 and cpth II occupy the space of the channel (inserted plot above). The inserted plot below shows the interactions of cpth II with Ba1 or Ba2. With O6, O7, O8 and N4 as the donors, cpth II interacts with Ba2, while with O5, O7 and O8 as the donors, cpth II interacts with Ba1. Note that the symmetry-related O6 atoms link Ba2 into a 1D endless zigzag chain, extending along the a-axial direction. Based on the topological viewpoint, compound 3 exhibits a 3D (44,62)(48,67)topology.
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Fig. 3 3D network of 3 (inset plot shows the coordination environments around Ba1 and Ba2) (a) and the 3D network constructed up from Ba1 and cpth I (inset plot shows the interactions of cpth II with Ba1 or Ba2) (b) (a: x + 1/2, −y − 1/2, z; b: −x + 3/2, y + 1/2, z + 1/2; c: x − 1/2, −y − 1/2, z; d: −x + 3/2, y − 1/2, z − 1/2; e: −x + 2, −y − 1, z − 1/2; f: x + 1/2, −y − 3/2, z; g: −x + 2, −y − 1, z + 1/2).
[Zn2( pdh)2(ox)]·H2O 4. Compound 4 is an ox-extended 3D Zn2+-pdh coordination polymers. It crystallizes in the space group I41/a, and the asymmetric unit is found to be composed of two types of Zn2+ ions (Zn1, Zn2), two types of pdh molecules ( pdh I, pdh II), one ox molecule and one lattice water molecule (Ow1). As shown in Fig. 4a, pdh I and pdh II adopt different doubly-bridged coordination modes. Both use the pyridyl N and hydroxylimino O atoms to chelate one Zn(II) center. The difference is that for pdh II, the hydroxylimino O atom further binds to the other Zn2+ ion, while for pdh I, the hydroxylimino N atom acts as the donor. For this reason, (i) the detailed coordination for Zn1 and Zn2 is different, even though Zn1 and Zn2 are both involved in a 5-fold coordinated site. Apart from coordination by two ox O atoms (O5 and O6 for Zn1, O7 and O8 for Zn2), one pyridyl N atom (N3 for Zn1, N6 for Zn2) and one hydroxylimino O atom (O1 for Zn1, O3 for Zn2), the 5-fold coordinated Zn1 is completed by one hydroxy-
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limino N atom (N1a), while the 5-fold coordinated Zn2 is completed by one hydroxylimino O atom (O3b); (ii) two types of Zn4( pdh)4 clusters are observed in compound 4. Zn1 and pdh I form one Zn4( pdh)4 cluster (labeled as Zn14( pdhI)4), while Zn2 and pdh II form the other Zn4( pdh)4 cluster (labeled as Zn24( pdhII)4). Four Zn2+ ions in each Zn4( pdh)4 cluster form a tetrahedron. Zn14 is a normal tetrahedron with a Zn⋯Zn range of 4.957–5.713 Å, while Zn24 is severely distorted (the Zn⋯Zn range: 3.642–4.990 Å). Ox adopts a common μ2 bridging-mode. Fig. 4b is the projection plot of compound 4 in the (001) direction. With ox as the linkers, each Zn14( pdhI)4 cluster is connected with four Zn24( pdhII)4 clusters. Similarly, each Zn24( pdhII)4 cluster is connected with four Zn14( pdhI)4 clusters. The green and yellow ox molecules extend the Zn4( pdh)4 into a 2D layer network (see Fig. 4c). Along the a-axial direction, through an alternate linkage of the pink and red ox molecules, the 2D layers are further propagated into a 3D network.
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Scheme 2
Dalton Transactions
Coordination modes of acylhydrazidate molecules in 1–4.
Based on the topological method, each Zn4( pdh)4 cluster can be regarded as a 4-connected node (tetrahedral configuration), while the ox molecule can be considered as the linker. Compound 4 exhibits a diamond-like 66-topology. The interpenetration phenomenon is found in the network structure of compound 4. Compound 4 displays a 2-fold interpenetrated net (see Fig. S1†). Due to the interpenetration, the pore turns out to be small. The lattice water molecules are distributed on the pores. As expected, (i) four acylamino groups for pmdh are all involved in coordination to the Ba2+ ions, so compound 1 shows a 3D network structure. The 3D network of compound 1 is different from that of the reported 3D compound [Co( pmdh)(H2O)2] due to the difference in the geometries of the metal ions. Also owing to this reason, the pmdh molecules adopt different coordination modes in both compounds;5 (ii) compound 3 also possesses a 3D network structure. Clearly, in the formation of this 3D network, the carboxyl on the benzene ring plays a crucial role. So far, the pth-coordinated Ba2+ compound has not been obtained. The metal ion also has an effect on the final structure architecture. For example, the cpthextended Ba2+ compound 3 exhibits a 3D network, while the reported cpth-propagated Pb2+ compound [Pb4(OH)2(cpth)3(H2O)3]·2H2O] shows a 2D layer network;12 (iii) in compound 4, the pdh molecules link the Zn2+ ions to form two types of Zn4( pdh)4 clusters, namely the secondary building units. The ox molecules act as the secondary linkers, extending the Zn4( pdh)4 tetranuclear clusters into a 3D network. This is different from the situation observed in the widely investigated metal–polycarboxylate system, in which the metal ions and the carboxyl groups form the various SBUs, and the polycarboxylate molecules further propagate the SBUs into various frameworks.3 Unexpectedly, compound 2 only possesses a 1D chain
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structure. For sdpth, only two acylamino groups are involved in coordination to the Ba2+ ions. This may be related to the spacer of the sdpth molecule. The centric S atom adopts a tetrahedral configuration, so sdpth shows a V shape, which directly influences the resulting framework of compound 2. But interestingly, the sdpth molecules link the Ba2+ ions into a 1D tube structure, and no solvent molecules are found in the tube. More interestingly, the non-coordinated acylamino group forms a new synthon with the neighboring non-coordinated acylamino group via the N–H⋯O interactions, via which all of the tubes are linked together into a 3D supramolecular network. At the same time, the big channels are formed. The water molecules in the channels can be removed. Moreover, (i) the acylhydrazidate molecules in compounds 1–4 exist in the keto-hydroxyl form which is confirmed by the different C–O distances. X-ray single-crystal diffraction analysis revealed that one C–O distance (CvO) is obviously shorter than the other (C–O−) (see Table S1†), suggesting that one acylamino group has isomerized into the hydroxylimino group. With reaction to the metal ion, the hydroxyl deprotonates to balance the metal charge. So pmdh, sdpth and cpth in compounds 1–3 have a −2 charge, while pdh in compound 4 shows a −1 oxidation state. Characterization The TG behaviors of compounds 2–4 were investigated. Fig. 5 presents the temperature vs. weight-loss curves. Based on the TG curves, we can see that (i) all three compounds underwent two steps of weight loss. The initial minor weight loss (calcd: 8.0%, found: ca. 9.0% for 2; calcd: 5.0%, found: ca. 6.0% for 3; calcd: 3.2%, found: ca. 3.9% for 4) should be ascribed to the sublimation of the coordinated and/or lattice water molecules. The second step of weight loss corresponds to the decomposition of the organic molecules in the compounds; (ii) for
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Fig. 5
TG curves of 2–4.
tures for the onset of the collapse of the host frameworks are higher (400 °C for 2, 480 °C for 3, 455 °C for 4); (iv) for compound 2, the water molecules in the channels can be easily removed. Once heated, the water molecules began to sublimate. In a narrow temperature range (30–140 °C), the water molecules were completely lost; (v) the TG curve suggests that there is one water molecule in a symmetric unit of 4, even though X-ray single-crystal diffraction analysis revealed that a half may be more reasonable. The appearance of the strong peaks at 1639 cm−1 for 1, 1643 cm−1 for 2, 1646 cm−1 for 3, and 1659 cm−1 for 4 in the IR spectra (see Fig. S2†) implies that the acylation of N2H4 with aromatic polycarboxylic acids has occurred, because the ν(COO) peaks are generally either larger than 1680 cm−1 or smaller than 1610 cm−1, whereas the ν(CONH) peaks appear in the range of 1620–1680 cm−1.13 The experimental powder XRD pattern for each compound is consistent with the simulated one generated on the basis of single crystal X-ray diffraction structural data, confirming that the as-synthesized product is a pure phase (see Fig. 6). Photoluminescence property
Fig. 4 Two types of Zn4( pdh)4 clusters (a), projection plots in (001) (b) and (100) directions (c) for 4.
acylhydrazidate-extended Ba2+ compounds 2 and 3, the final residues proved to be BaCO3, rather than BaO (calcd: 35.4%; found: ca. 34.7% for 2; calcd: 54.9%; found: ca. 54.1% for 3), whereas for acylhydrazidate-propagated Zn2+ compound 4, the residue was confirmed to be ZnO (Calcd: 28.4%, Found: ca. 29.5%); (iii) compounds 1–4 are stable, because the tempera-
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The solid-state photoluminescence property of 1–4 was investigated. Fig. 7 shows the corresponding emission and excitation spectra as well as the decay curves. Obviously, the solid-state compounds 3 and 4 possess photoluminescence properties. Upon excitation, both emit a similar green light with maxima at 495 nm (λex = 397 nm) and 522 nm (λex = 395 nm), for 3 and 4, respectively. So far, several pdh/derivative-coordinated transition-metal complexes have been obtained. The photoluminescence analysis indicates that almost all emit green light, and the peaks appear in the range of 500–530 nm.4i,6,11,14 Only a mononuclear Pb2+ compound [Pb(mpdh)2] (mpdh = 6-methylpyridine-2,3-dicarboxylhydrazidate) is an exception, emitting yellow light with a maximum at ca. 600 nm. The exception may be due to the special geometry for the Pb2+ center as well as the hydrogen-bonded interactions between the pdh molecules.15 The density functional theory
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Fig. 6 Experimental (red) and simulated (black) powder XRD patterns for 1–4.
(DFT) calculations suggest that the green-light emissions originate from the intra-acylhydrazidate transitions, corresponding to the charge transfer from the π* orbitals of the pyridine/ benzene ring moiety to the π orbitals of the acylhydrazidate ring moiety.4i,6a,11,14 Therefore, the green-light emission of 4 should have the same attribution. The green-light emission of 3 should also be assigned to this attribution, and only a slight blue shift by 27 nm is observed. Compounds 1 and 2 do not emit light, which is related to the close packing of the molecules. The close packing of the molecules tends to lead to luminescence quenching. Due to this reason, the majority of diacylhydrazide-coordinated compounds do not emit light.15 Only two reported complexes emit light, which should be related
Fig. 7
to their special space structures. For example, in compound [Cd(sdpth)( phen)(H2O)]·H2O ( phen = 1,10-phenanthrolion), no π⋯π interactions between the sdpth molecules are observed, so it emits light;16 in compound [Zn(N2H4)(dphkh)]·H2O (dphkh = 4,4′-diphthalhydrazidatoketone hydrazone), the incorporation of N2H4 molecule weakens the interactions between the dphkh molecules, so it also emits light.17 For compounds 3 and 4, the molecular stacking is closer yet. However, the presence of the carboxyl substituent and the N heteroatom on the benzene ring assists the electronic transitions from the acylhydrazidate ring moiety to the pyridine/ benzene ring moiety. On the other hand, the acidic protons in the acylhydrazidates (NH protons) may participate in the excited-state proton transfer (ESPT), which would also quench any emission process.18 So the luminescence quenching of compounds 1 and 2 may originate from two factors: the interligand charge transfer of stacked ligands and the ESPT. The decay curve for 3 fits into a double exponential function, and the lifetimes were calculated to be τ1 = 1.20 ns and τ2 = 2.54 ns, respectively. The luminescence lifetime for 4 was calculated to be τ1 = 1.50 ns. Fig. S3† provides the UV-vis spectra of the title compounds. Sorption study To assess the porosity property of the as-synthesized MOFs, the N2 adsorption isotherms of 2–4 at 77 K were measured. Fig. 8 illustrates the corresponding adsorption–desorption isotherms. The N2 uptakes of 2 and 4 follow a similar increasing trend. And the adsorption and desorption isotherm curves do not overlap with each other with small hysteresis. The BET surface areas are estimated to be 58.81 m2 g−1 for 2 and 39.04 m2 g−1 for 4, respectively. The measured pore volumes
Emission (a) and excitation spectra (b) as well as their decay curves for 3 and 4.
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hydrogen-bonded interaction between the acylamino groups, the acylhydrazidate-extended metal complex may self-assemble into a 3D porous supramolecular network. The TG analysis indicates that the host metal–acylhydrazidate frameworks are rather stable. Some metal–acylhydrazidate complexes possess photoluminescence properties. The emissions derive from the charge transfers associated with the acylhydrazidate molecules. The close packing of the molecules as well as the ESPT is prone to leading to luminescence quenching, but the carboxyl substituent and the N heteroatom on the benzene ring contribute to the electronic transitions. The N2 adsorption– desorption isotherms indicate that activated 2 and 4 could adsorb N2 at 77 K with an order of adsorption of 2 >4.
Fig. 8
N2 adsorption–desorption isotherms of 2–4 at 77 K.
Acknowledgements This research was supported by the National Natural Science Foundation of China (no. 21271083).
are ca. 0.123 cm3 g−1 for 2 and ca. 0.095 m2 g−1 for 4, respectively. The capacity of 2 to adsorb N2 at 77 K and 1 bar is 58.31 cm3 g−1, a value surpassing that of 4 (38.38 cm3 g−1). Compound 3 exhibits a nil adsorption to N2 at 77 K. The CO2 adsorption isotherm at 273 K for 2 was also studied. As shown in Fig. S4,† the adsorption amount is negligible (3.32 cm3 g−1), suggesting that compound 2 does not adsorb CO2 at 273 K. Based on the statement above, we know that (i) the activated 2 and 4 could adsorb N2 at 77 K; (ii) the adsorption amount of 2 is slightly larger than that of 4; (iii) the N2 adsorption isotherms at 77 K for 2 and 4 do not show the typical type-I, observed for most MOFs;2o,3e,h,g,i,s,19 (iv) the hysteresis loop is observed in the N2 adsorption–desorption isotherms for 2 and 4.
Conclusion In summary, we report the synthesis, structural characterization, thermal behavior, and photoluminescence property of four acylhydrazidate-containing coordination polymers. They were obtained from the hydrothermal reactions of Ba2+/Zn2+, aromatic polycarboxylic acids, N2H4 with or without the ox molecule. The acylhydrazidate molecules were derived from the hydrothermal in situ acylation of N2H4 with aromatic polycarboxylic acids. X-ray single-crystal diffraction analysis revealed that compounds 1 and 3 are acylhydrazidate-extended 3D MOFs. For compound 4, although the monoacylhydrazidate molecules link the metal ions to form a discrete oligomer, the introduced ox molecules act as secondary linkers, propagating the oligomers into a 3D MOF. These results suggest that the strategies stated below are feasible for the construction of acylhydrazidate-extended MOFs: (i) using diacylhydrazide molecule to react with the metal ion; (ii) modifying the monoacylhydrazide molecule, especially its 4- and 5-positions; (iii) incorporating another kind of bridging-type organic molecule into the metal–monoacylhydrazide system. Moreover, via the
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