An inorganic–organic hybrid material with a novel oxometallic framework: Hydrothermal synthesis and characterization of [Zn 3 O 3 (C 13 H 14 N 2 ) 3 ]V 6 O 15

]V 15 ( 1 ) with extended framework structure has been synthe- sized hydrothermally and characterized by vibrational spectroscopy, thermogravimetry and complete single crystal X-ray diffraction analysis. The compound has a complex three-dimensional covalent frame- work structure. It exhibits a fully oxidized novel oxometallic framework containing 10-membered {V 4 ZnO 5 } oxometalate rings and 4,4 0 -trimethylene dipyridine ligands (C 13 H 14 N 2 ) that connect pairs of crystallographically equivalent zinc atoms. The extended structure of 1 may also be viewed as containing a framework of corner-sharing {VO 4 } and {ZnO 2 N 2 } polyhedra together with 4,4’-trimethylene dipyridine ligands linking zinc centers. The hybrid material is thermally stable up to 323 (cid:2) C. It contains metal centers and coordination geometry that make it a potentially attractive model compound for investigating the structures of metallo-organic biomolecules by use of solid state NMR spectroscopic techniques. Crystal data for C 39 H 42 N O 18 V Zn : monoclinicic P 2 1 , a = 10.9894(9) Å, b = 18.1493 (15) Å, c = 13.0903 (11) Å, b = 109.8880(10) (cid:2)


a b s t r a c t
A new hybrid material [Zn 3 O 3 (C 13 H 14 N 2 ) 3 ]V 6 O 15 (1) with extended framework structure has been synthesized hydrothermally and characterized by vibrational spectroscopy, thermogravimetry and complete single crystal X-ray diffraction analysis. The compound has a complex three-dimensional covalent framework structure. It exhibits a fully oxidized novel oxometallic framework containing 10-membered {V 4 ZnO 5 } oxometalate rings and 4,4 0 -trimethylene dipyridine ligands (C 13 H 14 N 2 ) that connect pairs of crystallographically equivalent zinc atoms. The extended structure of 1 may also be viewed as containing a framework of corner-sharing {VO 4 } and {ZnO 2 N 2 } polyhedra together with 4,4'-trimethylene dipyridine ligands linking zinc centers. The hybrid material is thermally stable up to 323°C. It contains metal centers and coordination geometry that make it a potentially attractive model compound for investigating the structures of metallo-organic biomolecules by use of solid state NMR spectroscopic techniques. Crystal data for C 39 H 42 N 6 O 18 V 6  Design and synthesis of functional materials has attracted considerable attention in recent years [1]. This has led to a new class of inorganic-organic hybrid materials with rich topological diversity and potential applications in catalysis, gas storage, sorption and photochemistry [2][3][4][5]. Hybrid materials have shown excellent laser efficiencies and photostabilities [6], fast photochromic responses [7], as well as high and stable optical responses [8]. These materials can also be used as pH sensors [9] and many of them have shown attractive gas storage capacity [10][11][12][13][14].
We have been interested in the design and synthesis of inorganic-organic hybrid framework materials by combining metalorganic moieties and oxometalate motifs. We have successfully employed synthetic strategies using metal-organic complexes as linkers to cross-link vanadium oxide chains and layers to generate a series of open-framework hybrids and neutral networks. The metal-organic complex also provides the charge balance required to generate a neutral framework [15][16][17][18][19][20][21]. In this approach the organic components act as ligands to secondary transition metal centers giving rise to bimetallic, organically templated hybrid materials. Here we report a new hybrid material containing a novel oxometallic framework [Zn 3 O 3 (C 13 H 14 N 2 ) 3 ]V 6 O 15 (1) which was prepared hydrothermally and characterized by FT-IR, elemental analysis, manganometric titration, thermogravimetric analysis and complete single crystal X-ray diffraction analysis.
A mixture of NH 4 VO 3 (1 mmol), 4,4 0 -tmdp (1 mmol), ZnSO 4 Á 7H 2 O (0.5 mmol), C 2 H 5 OH (85.76 mmol, 5 ml), H 2 O (555.5 mmol, 10 ml) was placed in a 23 ml teflon-lined Parr reaction vessel and stirred continuously for 10 min. The reaction mixture (pH = 5.1) was then treated hydrothermally at 125°C for 5 days. The reaction was then cooled to room temperature for 24 h and colorless crystals of 1, which were covered with a black shiny impurity were isolated from the yellow mother liquor.
Use of the adipic acid (C 6 H 10 O 4 ), which is not incorporated in the final product, in the reaction mixture increased the purity of the compound (1) considerably (by decreasing the black impurity). It is possible that adipic acid helps to maintain suitable pH for the formation of the product in better yield and purity. Addition of ethanol to the reaction mixture (in addition to adipic acid) gave a similar result. Our study shows that under hydrothermal condition pH and temperature influence vanadate speciation. We have already reported the synthesis of [{Co 2 (4,4 0 -tmdp) 4  Compound 1 crystallizes in the monoclinic space group P2 1 , with two formula units of stoichiometry C 39 H 42 N 6 O 18 V 6 Zn 3 per unit cell. A summary of the crystal data and details of the intensity data collection and structure refinement are given in a footnote [22][23][24][25][26]. Full details have been deposited [27]. The compound has a novel structure composed of inorganic and organic motifs. The structure may be viewed as containing a framework of corner-sharing VO 4 and ZnO 2 N 2 polyhedra together with 4,4 0 -trimethylene dipyridine ligands linking pairs of Zn 2+ ions. Views of the asymmetric unit and of the oxometallate framework of 1 are shown in Figs. 1 and 2, respectively. A view of the unit cell contents of 1 is shown in Fig. 3. All bond angles and bond distances are within their expected ranges.
The compound has a complex three-dimensional structure that contains a 10-membered V 4 ZnO 5 ring. The ring atoms V2 and V3 are linked to the two non-ring zinc atoms by V2-O8-Zn2 and V3-O9-Zn3 bridges. Two 4,4'-trimethylene dipyridine molecules connect pairs of crystallographically equivalent zinc atoms. For Zn2 and Zn3, the linked Zn atoms are separated by pure unit cell translations parallel to the c-axis, while for Zn1 the relationship combines a twofold screw operation about b with a unit cell translation along c. The two non-ring vanadium atoms, V5 and V6, are part of a V 2 O 7 grouping that forms V5-O7-Zn2 and V6-O10-Zn3 bridges to the two non-ring zinc atoms. Finally, the ring vanadium atoms V1 and V4 form bridges through O6 and O11, respectively, to V 2 O 7 groupings in other asymmetric units.
Thermogravimetric analysis of compound 1 showed that it is stable up to 323°C. The compound loses weight in two consecutive steps. The first weight loss of 28.43% (calc. 28.63%) is observed at 323°C. This corresponds to the loss of two ligand molecules. The second weight loss of 30.93% (calc. 30.48%), which corresponds  to the loss of 14 oxygen atoms and one ligand, starts at $473°C and continues until 952°C. The IR spectrum of the blackish residue left after TGA study showed a broad band at 1638 cm À1 . This clearly indicates a complete decomposition of the framework structure (after heating up to 1000°C) and formation of a mixed metal oxide -{Zn/V/O} -phase which has not been further characterized.
Recent years have seen increased interest in the use of NMR spectroscopy to study the structure and coordination chemistry of metal ions in biological systems to understand the role of metal ions in chemical and biological processes in living organisms [28][29][30]. The hybrid material described here contains fully oxidized metal centers in suitable coordination geometry that make it a potentially attractive model compound for investigating the structures of metallo-biomolecules by use of solid state NMR techniques.
In conclusion, a new inorganic-organic hybrid material containing a novel framework has been synthesized hydrothermally and characterized. The combination of hydrothermal method with metal-organic complexes and oxometallic motifs provides a powerful tool for design and development of functional hybrid materials. The structures of such materials can 'in principle' be modified/tailored by changing the organic and/or oxometallic components along with variations in pH and temperature of the reactions. However, precise design and synthesis of materials with predictable structures is still in its infancy.