Construction of Two Inorganic-organic Hybrid Vanadogermanates Based on Di-Cd-Substituted Ge-V-O Cluster and Transition-metal Complex Bridges①

RU Jing-Jing GU Y-Nn MA Xing CHEN Jin-Zhong ZHENG Shou-Tin LI Xin-Xiong

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Construction of Two Inorganic-organic Hybrid Vanadogermanates Based on Di-Cd-Substituted Ge-V-O Cluster and Transition-metal Complex Bridges①

RU Jing-Jinga, bGU Ya-NanaMA XiangaCHEN Jian-ZhongaZHENG Shou-TianaLI Xin-Xionga②

a(350116)b(352100)

Two new inorganic-organic hybrid vanadogermanates H[Cd(en)(phen)(H2O)]-[Cd(en)(phen)]{[Cd(phen)]2[Ge8V12O41(OH)7]}·5H2O (1) and[Cd(dien)2][Cd(dien)]2{[Cd(phen)]2-Ge8V12O42(H2O)(OH)6}·6.5H2O (2)(en = ethylenediamine, dien = diethylenetriamine, phen = 1,10-phenanthroline) have been synthesized by hydrothermal method. Their structures were measured by single-crystal X-ray diffractions, thermogravimetric analysis, powder X-ray diffractions and infrared spectra. Structural analysis reveals that compound 1 is an infrequent dimeric structure based on di-Cd-substituted Ge-V-O cluster and transition-metal complexbridges, while compound 2 is an infinite 1-D chain constructed from di-Cd-substituted Ge-V-O clustersand dinuclear bridging complexes. Magnetic measurement indicated that both 1 and 2 exhibit antiferromagnetic behaviors.

vanadogermanates, hydrothermal synthesis, crystal structure, magnetic property;

1 INTRODUCTION

Nowadays, more and more researchers are interested in polyoxometalates (POMs) because of their superior properties in catalysis, optics, magne- tism, sorption, ion exchange and so on[1-4]. Com- pared with polyoxomolybdates and polyoxotungs- tates, polyoxovanadates (POVs) are less investigated relatively[5-8]. The most important characteristic in polyoxovanadate compounds is that there are various polyhedron construction units such as {VO4}, {VO5}, {VO6} and so on, which can form different vanadium-oxygen clusters through sharing vertexes, edges and planes. One of the most effective approaches for the development of POVs is the incorporation of heteroatoms into the clusters. Up to now, most studies focus on the incorporation of group 15 elements (As, Sb) into the well-known Keggin {V18O42} cluster anion, which are named as vanadoarsenates[9-15]and vanadoantimonates[16-21]. The {As2O5}/{SbIII2O5} dimers are formed by two vertex-sharing {AsO3}/{SbO3} trigonal pyramids, which substitute the VO5groups on the {V18O42} shell. Compared with vanadoarsenates and vana- doantimonates, the research of vanadosilica- tes(VSOs) and vanadogermanates(VGOs) is rela- tively scarce. The germanium-vanadium oxide clusters based on {V18O42} shell mainly include {Ge4V16O46} cluster[7, 22], {Ge6V15O48} cluster[23-24], {Ge8V14O50} cluster[7, 24]and {Ge8V12O48} cluster[7]. These clusters are mainly made up of {VO5} square pyramids and {Ge2O7} dumbbell dimers. The {Ge2O7} units are formed by two vertex-sharing {GeO4} tetrahedra, which are superior to the trigonal pyramid of {AsO3} and {SbO3}. The {AsO3} and {SbO3} trigonal pyramids usually tend to form isolated structures because of the weak coordination capability of As/SbIIIlone pairs[25-27].

It is well known that the second transition- metal(TM) ions incorporated into POVs skeletons may produce attractive structures and some special properties. But, fewer investigations have been carried out on TM-substituted POVs. Up to now, there are only a few cases such as {[(en)2Cd2Ge8V12O40(OH)8(H2O)][Cd(en)2]2}·6H2O[23]reported in 2010, {(CdX)4Ge8VIV10O46(H2O)[VIII(H2O)2]4(GeO2)4}·8H2O (X = en, 1,2-dap)[28]reported by Yang’s group at 2014 and {[Cd(en)2][Cd(en)]2Ge8V12O40(OH)8}2{[Cd(en)(H2O)]2}·en·2H2O[29]made by Liu’s group at 2016. Recently, our group have reported two novel transition-metal substituted vanadogermanates[Cd(en)(H2O)2][Cd(en)2][Cd(en)]{[Cd(en)]2[Ge8V12O42.5(OH)5]}·2H2O and[Cd(en)3][Cd(en)]2{[Cd(en)]2[Ge8V12O42(OH)6]}·10H2O[30]. Inspired by this result, we decide to further explore this system.

In this work, we report the hydrothermal synthe- ses, crystal structures and magnetic properties of another two new transition-metal substituted vana- dogermanates H [Cd(en)(phen)(H2O)] [Cd(en)(phen)]-{[Cd(phen)]2[Ge8V12O41(OH)7]}·5H2O (1) and [Cd(dien)2][Cd(dien)]2{[Cd(phen)]2Ge8V12O42(H2O)(OH)6}·6.5H2O (2). 1 is an uncommon dimeric struc- ture based on di-Cd-substituted Ge-V-O cluster and transition-metal complexbridges. 2 exhibits an infinite 1-D chain structure built by di-Cd-subs- titu- ted Ge-V-O clustersand dinuclear bridging complexes.

2 EXPERIMENTAL

2. 1 Materials and measurements

All chemicals with analytical grade were pur- chased from commercial sources without any further purification. Elemental analyses of C, H and N were carried out with a Vario EL III elemental analyzer. Powder X-ray diffraction(PXRD) patterns of the samples were measured by a computer automated diffractometer (Bruker D8 Advance) equipped with Cu-radiation (= 1.54056 ?) at room tempera- ture with 2values from5 to 50° at a step of 0.02°.Fouriertransforminfrared(FT-IR) spectroscopy (4000～400 cm-1) were recorded on a Nicoletis 10 spectrometer with KBr pellets. Thermogravimetric (TG) analysis was examinedin the temperature of 20～1000°C at a heating rate of 10°C/min under ?owing N2atmosphere on a NETZSCH PC409 simultaneous thermal analyzer. The magnetic susceptibility data were measured with a Quantum Design MPMS SQUID VSM magnetometer in an external magnetic ?eld of 1 KOe at 2～300 K.

2. 2 Synthesis

During our exploration, we found that some factors including pH value, temperature and reaction time can affect the formation and crystal growth of the final products. Lots of parallel experiments prove that crystals of 1 and 2 were sensitive to the pH values of the reaction. The optimal pH values for crystal growth are 8.8～9.0 for 1 and 8.5～8.8 for 2, respectively. What’s more, the reaction temperature plays a vital role in the formation 1 and 2. When the reaction temperature was adjusted out of 160～180oC, 1 and 2 can not be obtained. Finally, the best reaction time should be in 3～5 days.

2. 2. 1 Synthesis of 1

A mixture of NH4VO3(0.0345 g, 0.29 mmol), GeO2(0.0384 g, 0.37 mmol), CdCl2·2.5H2O(0.0242 g, 0.11 mmol), phen(0.0306 g, 0.17 mmol), en(0.1 mL) and H2O(5mL) was stirred for 0.5h. Then it was transferred to a 25mL Teflon-lined stainless- steel autoclave and sealed. The mixture was heated at 170°C for 4 days and then cooled to room temperature. Brown block-shaped crystals were recovered by filtration, washed with distilled water, and dried at ambient temperature (Yield: 0.0711 g,46% on the base of GeO2). Elemental analysis (%) calcd. forH68C52N12O54V12Ge8Cd4(3366.97): C, 18.55; N, 4.99; H, 2.02. Found (%): C, 18.30; N, 5.01; H, 2.33. IR (KBr, cm-1): 3438(s), 3275(w), 3048(w), 1617(m), 1574(m), 1516(m), 1427(s), 1350(w), 1145(w), 1103(w), 984(s), 793(s), 729(m), 668(m), 542(m).

2. 2. 2 Synthesis of 2

A mixture of NH4VO3(0.0200 g, 0.17 mmol), GeO2(0.0380 g, 0.36 mmol), CdCl2·2.5H2O(0.0800 g, 0.35 mmol), phen(0.0300 g, 0.17 mmol), dien(0.2 mL) and H2O(5mL) was stirred for 0.5h. Then it was transferred to a 25mL Teflon-lined stainless- steel autoclave and sealed. The mixture was heated at 170 °C for 4 days and then cooled to room temperature. Brown block-shaped crystals were recovered by filtration, washed with distilled water, and dried at ambient temperature(Yield: 0.0448 g, 29% based on GeO2). Elemental analysis (%) calcd. for H89C40N16O55.5V12Ge8Cd5(3436.44): C, 13.98; N, 6.52; H, 2.59. Found (%): C, 14.01; N, 6.57; H, 2.98.IR (KBr, cm-1): 3422(w), 3352 (w), 3276(w), 2924(w), 2876(w), 1589(m), 1516(m), 1426(m), 1341(w), 1143(w), 1101(w), 984(s), 790(s), 728(w), 669(m), 552(m).

2. 3 Single-crystal X-ray crystallography

X-ray diffraction data for1 and 2 were collected on a Bruker APEX II CCD diffractometer at 296(2) K equipped with a fine focus, 2.0 kW sealed tube X-ray source (Moradiation,= 0.71073 ?) operating at 50 kV and 30 mA. The empirical absorption correction was based on equivalent reflections. The structures were solved by direct methods and refined on2by full-matrix least-squares methods using the SHELXS-97 and SHELXL-97 programs, respectively. All hydrogen atoms attached to carbon, nitrogen and oxygen atoms were geometrically placed. All non-H atoms were refined anisotropicallyexcept O7w, O8w and O9w in compound 1. The final formulas of 1 and 2 were determined by single-crystal X-ray diffraction together with elemental analysis and thermogra-vimetric analysis.Meanwhile, to balance the charge of 1, one proton should be added. The proton cannot be located and it was assumed to be delocalized on the overall structure, which is often observed in POM chemistry.All these crystaldata and structure refinement details are summarized in Table 1.

Table 1. Crystal Data and Structure Refinement for 1 and 2

= ∑||| – |||/∑||,= [∑(2–2)2/∑(2)2]1/2;= 1/[2(o2) + ()2+],

where= (2+ 22)/3.= 0.1852 and= 65.1392 for 1;=0.0942 and= 185.5212 for 2

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of 1

Table 2. Bond Valence Sum Calculation for 1

Fig. 1. a)-b) Ball and stick and polyhedral representations of [Cd(en)(phen)(H2O)]{[Cd(phen)]2[Ge8V12O41(OH)7]}3-cluster in 1; c) Structure of bridging complex[CdO2(en)(phen)]2-; d) View of the dimeric structure of 1.

All hydrogen atoms are omitted for clarity. Color codes: GeO4, purple; VO5, olive

3. 2 Crystal structure of 2

The molecular structure of 2 contains one di-Cd- substituted Ge-V-O cluster {[Cd(phen)]2[Ge8V12O41(OH)6]}6-(2a,Fig. 2a-2b), one [Cd(dien)2]2+group, two [Cd(dien)]2+units and 6.5 lattice water molecules. The structure and configuration of 2a are similar to those of 1aexcept the number of hydroxyl groups on the cluster. The number of hydroxyl groups in 2a is six, while that of 1a is seven. The Ge–O/V–O bond distances for the terminal O atoms fall in the ranges of 1.706(1)~1.743(2)/1.593(8)~1.616(5) ?, and those for the bridging O atoms are in the ranges of 1.713(3)~1.799(1)/1.923(2)~2.009(4) ?. On the basis of BVS calculations (Table 3), the oxidation states of all V are +4 (4.03~4.22), and those of all Ge atoms are +4 (4.02~4.09) in 2. Three unique Cd2+ions (Cd(1), Cd(2), Cd(3)) within 2 exhibit three different coordination geometries: distorted trigonal bipyramid CdO2N3for Cd(1), trigonal prisms CdO4N2for Cd(2) and pentagonalbipyramid CdON6for Cd(3). The Cd(1) ion is composed of three N atoms of one dien ligand (Cd–N: 2.269(8)~2.416(8) ?) and two μ3-O atoms of GeO4tetrahedron from two neighboring 2a clusters. Interestingly, two symmetry-related Cd(1) ions are interconnected through sharing two μ3-O atoms, leading to the formation of a dinuclear complex [Cd2O2(en)2(phen)2] with centrosymmetry (Fig. 2c). The Cd(2) ion is defined by two N atoms of one phen ligand (Cd–N 2.339(7)~2.410(8) ?) and four μ3-O of 2acluster (Cd–O: 2.264(5)~2.331(6) ?). Cd(3) is surrounded by six N atoms of two dien ligands (Cd–N: 2.380(1)~2.47(2) ?) and one O atom of a VO5tetragonal pyramid of 2a (Cd–O: 2.618(9) ?), forming a decorative [CdO(dien)2] unit mounted on the exterior of 2a cluster. Worth a mention is the further assembly of 2aclusters. As depicted in Fig. 2c, each 2a cluster linked two neighboring ones by two dinuclear complexes [Cd2O2(dien)2] through sharing Ge–O–Cd bonds, leading to the formation of an infinite 1-D chain (Fig. 2d). The successful construction of such 1-D chain further proved that tetragonal {GeO4} groups are superior to the trigonal pyramid of {AsO3} and {SbO3} in building extended polyoxovanadate structures.

Fig.2. a) and b) Ball and stick/polyhedral representations of {[Cd(phen)]2[Ge8V12O42(OH)6]}6–cluster in 2; c) View of the dinuclear bridging complex [Cd2(dien)2O2]; d) View of the 1-D chain in 2. All hydrogen atoms are omitted for clarity. Symmetric codes: a: 1–,, 0.5–; b: 1.5–, –0.5–, 1–

Table 3. Bond Valence Sum Calculations for2

3. 3 PXRD analysis

In the PXRD patterns of 1 and 2 (Fig.3), the good accordance between the experimental patterns and the simulated ones indicates good phase purities.

Fig. 3. Experimental and simulated PXRD patterns of 1 and 2

3. 4 FT-IR analysis

IR spectra of 1and2 are shown in Fig.4. The NH2and CH2stretching bands are observed at 3383～3312 and2975～2890 cm–1, and the absorp- tion peaks at 1609～1570 cm-1can be attributed to the NH2bending bands. These signals confirm the presence of amino groups in 1 and2. The broad band at 3462～3391 cm-1can result from the OH stretching.The peaks lying in 1536～1401 and 738～705 cm-1can be assigned to the phen ligand. The strong peaks situated in 1016～896 cm-1belong to the stretching vibrations of V=O bonds, whereas those at 879～743 cm-1may be due to the Ge–O stretching vibration of the GeO4tetrahedra[23]. The peaks located at 2386～2289 cm-1are attributed to the asymmetric stretching vibration of CO2.

Fig. 4. IR spectra of 1 and 2

3. 5 TG analysis

The TGA curves of 1 and 2 are shown in Fig. 5. For 1, a weight loss of 5.92% in the range of 20～300oC observed corresponds to the removal of five H2O molecules and two en ligands (calcd: 6.24%). The second weight loss about 4.40% from 300 to 500oC is assigned to the removal of seven hydroxyl groups and one coordinated water ligand (calcd: 4.06%). Then the weight loss about 21.27% from 500 to 813oC is caused by the decomposition of four phen molecules (calcd.: 21.38%). For 2, the weight loss of 7.87% from 20 to 125oC is due to the departure of 7.5 water molecules and removal of six hydroxyl groups (calcd: 6.90%). The weight loss of 11.23% from 125 to 415oC results from the loss of four dien molecules (calcd: 12.01%). After that, a sequence of weight loss about 10.49% from 415 to 585oC can be attributed to the decomposition of two phen ligands (calcd: 10.50%).

Fig. 5. TG curves of 1 and 2

3. 6 Magnetic properties

The variable-temperature magnetic susceptibilities of1and2 have been measured in the temperature range of 2～300 K with an external field of 1 KOe (Figs. 6 and 7). At room temperature, the experi- mentalmT values of 1and 2 are 1.36 and 1.36cm3·K·mol–1per formula unit respectively,much lower than the theoretical value of 4.5 cm3·K·mol–1for twelve uncoupled V4+ions considering g = 2.00. Upon cooling, themT values of 1 and 2decrease slowly to 1.08 cm3·K·mol–1at 48 K and 1.25 cm3·K·mol–1at 35 K. On cooling, the χmT values of 1 and 2 decrease abruptly, reaching the minima of 0.07 and 0.09 cm3·K·mol–1at 2 K. The above behaviors suggest the presence of antiferromagnetic coupling with the cluster in 1 and 2[33]. From the structural stand point of view, the bond length between the V···V pairs with double O bridges fall in the ranges of 2.852~3.025 ? for 1 and 2.847~3.027 ? for 2,which are expected for antiferromagnetic coupling[23, 34]. Additionally, the temperature dependence of the reciprocal susceptibility (1/m) obeys the Curie-Weiss law in the temperature range of 220 to 9 K and 245 to 5 K for 1 and 2, respectively. The Weiss and Curie constants are –8.61 K and 1.26 cm3·K·mol–1for 1 and –1.07 K and 1.27 cm3·K·mol–1for 2, which further support the presence of antiferromagnetic interactions between the V4+ions within the clusters. Therefore, antiferromagnetic properties are observed in the two compounds that are not unexpected.

Fig.6. Temperature dependence of χmT and 1/χmfor 1 between 2 and 300 K

Fig.7. Temperature dependence of χmT and 1/χmfor 2 between 2 and 300 K

4 CONCLUSION

We have successfully constructed two new inorga- nic-organic hybrid vanadogermanates under hydro- thermal conditions. Structural analysis reveals that compound 1 is an infrequent dimeric structure based on di-Cd-substituted Ge–V-O cluster [Cd(en)- (phen)(H2O)]{[Cd(phen)]2[Ge8V12O41(OH)7]}3–and transition-metal complex [CdO2(en)(phen)]2–bridges, while compound 2 is an infinite 1-D chain constructed from di-Cd-substituted Ge–V-O cluster {[Cd(phen)]2[Ge8V12O42(OH)6]}6–and dinuclear complex [Cd2(dien)2O2] bridges. Magnetic studies indicate that 1 and 2 have antiferromagnetic coupling between the metal ions within the clusters. This work not only enriches the structural diversity of inorganic-organic hybrid vanadogermanates, but also confirms the great potential for making novel functional materials by linking discrete tran- sition-metal substituted Ge–V–O clusters with various transition-metal complex bridges.

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2 June 2017;

23 August 2017 (CCDC 1549211, 1547131)

10.14102/j.cnki.0254-5861.2011-1741

①This work was supported by the National Natural Science Foundation of China (No. 21401195, 21671040), the Natural Science Foundation for Young Scholars of Fujian Province (2015J05041) and the Open Foundation of State Key Laboratory of Structural Chemistry (20160020)

②Li Xin-Xiong, Tel: 0086-0591-22863595, E-mail: lxx@fzu.edu.cn