Hydrothermal Synthesis, Structural Investigation and Characterization of an Organic-inorganic Hybrid, [Co(BIIM)3]2H[BW12O40]·7H2O①

WANG Zi-Ling ZHAI Qiu-Ge

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Hydrothermal Synthesis, Structural Investigation and Characterization of an Organic-inorganic Hybrid, [Co(BIIM)3]2H[BW12O40]·7H2O①

WANG Zi-Lianga②ZHAI Qiu-Geb

a(475004)b(464000)

Anorganic-inorganic hybrid based on-Keggin-typed [BW12O40]5-anionic cluster,[Co(BIIM)3]2H[BW12O40]·7H2O (BIIM = 2,2′-biimidazole), was hydrothermally synthesized and structurally characterized by thermogravimetric analysis, IR and UV spectra. The title compound crystallizes in monoclinic, space group21/with= 12.3490(10),= 33.270(3),= 18.9660(16)?,= 105.9950(10)°,= 7490.5(11) ?3, C36H50BCo2N24O47W12,M= 3905.80,D= 3.451g/cm3,(Mo) = 18.886 mm-1,(000) = 6972,= 4,= 1.004, the final= 0.0404 and= 0.0914 for 14683 observed reflections (> 2()). The compound consists of two isolated [Co(BIIM)3]2+cations and one [BW12O40]5-anion, in which each Co2+ion is coordinated by six N atoms from three BIIM ligands displaying a regular CoN6octahedron. These components are finally linked togetherN–H???O(polyoxometalate and water)hydrogen bonds into a two-dimensional framework.Full investigation of the intermolecular interactions by Hirshfeld surface and fingerprint plots clearly indicates that there are rare W–Oterminal???imidazoleringinteractions which cooperate with the classical N–H???O hydrogen bonds to stabilize the structural packing.

crystal structure, cobalt, Keggin, Hirshfeld surface, fingerprint plots

1 INTRODUCTION

Different polyoxometalates (POMs) due to their structural differences have different reaction activi- ties in coordinating to metal ions. Their successful introduction into hybrids as the inorganic block is an important breakthrough in preparing hybrid materials, which greatly gives impetus tothe rapid development of this field so that lots of related complexes have been reported in the past deca- des[1–5]. Up to now, design and synthesis of various organic-inorganic hybrids based on POM have been still attracting great interest because of their beautiful architectures and potential applications in chemistry, physics and material science,[6–10].

-Keggin-typed POMs, namely,12{[P12O40]3-(= Mo, W)[11–18]and [12O40]4-(= Si, Ge;= Mo, W)[19–24]},are widely selected for their higher negative charges, good symmetry and better stability in a broad pH range. However, com- pared with [P12O40]3-and [12O40]4-anions, hybrids based on [BW12O40]5-anions[25–28]have been less reported due to the minor radius of the heteroatom B. In reported literatures, [BW12O40]5-anion like other Keggin anions[6]mainly takes the following roles. Firstly, the [BW12O40]5-anion acts as the charge-compensating or space-filling cons- tituents[27]. Secondly, the [BW12O40]5-anion adopts as the inorganic building-block coordinated to a secondary transition metal site or coordination com- plex cations forming isolated, one- or multi-dimen- sional structures[28]. The actions of [BW12O40]5-ion are firstly affected by the geometry of the coor- dination cation, derived from the geometrical requi- rement of the ligand and the coordination pre- ferences of the metal ion. Additionally, the func- tions of [BW12O40]5-polyanion are affected by the reaction conditions, such as metal to ligand ratio, temperature, starting pH value,.

2,2′-biimidazole possesses two N-coordination sites and two -NH donors. Both N-coordination sites have strong coordination ability and flexible coordination modes according to the metal geometry or metal to ligand ratio, which can adopt not only as a bidentate ligand chelating a transition metal, but also as bis(mono-dentate) bridging to two transition metal ions. Moreover, two imidazole rings can rotate along the C–C bond so that 2,2′- biimidazole has more flexible coordination abilities. Additionally, two -NH donors can interact with other hydrogen bonding acceptorshydrogen bonds, which can further lead the structural unit to extend through hydrogen bonding interaction. However, POM-based compounds related to 2,2′- biimidazole have not been much observed[29–31]. Herein we present the hydrothermal synthesis, struc-tural investigation and characterization of an organic- inorganic hybrid, [Co(BIIM)3]2H[BW12O40]·7H2O.

2 EXPERIMENTAL

2. 1 Materials and measurements

All chemicals were of reagent grade as received from commercial sources and used without further purification. C, H and N elemental analyses were performed on a Perkin-Elmer 240C elemental analyzer.The infrared spectrum was recorded from KBr pellets on a Nicolet 170SXFT-IR spectrometer in the range of 400～4000cm-1.UV spectrum was obtained on a Shimzu UV-250 spectrometer in the range of 190～400 nm.TG-DTA measurement was performed on a Perkin-Elmer7 thermal analyzer in flowing nitrogen gas with a heating rate of 10 ℃/min.

2. 2 Synthesis of [Co(BIIM)3]2H[BW12O40]·7H2O

A mixture of Na2WO4·2H2O (0.50 g, 1.5 mmol), Na2B4O7·4H2O (0.05 g, 0.13 mmol), CoCl2·6H2O (0.10 g, 0.42 mmol), Co(CH3COO)2·4H2O (0.12 g, 0.48 mmol), BIIM (0.03 g, 0.13 mmol) and H2O (15 mL) was sealed in a Teflon-lined stainless-steel reactor and heated at 160 ℃ for 4 d with the starting pH = 4 adjusted with hydrochloric acid (6 mol/L). After cooling slowly to room temperature for about 24 h, deep red crystals were obtained, filtered, washed several times with distilled water, and dried in air. Elemental analysis (%) calcd.: C, 11.07; H, 1.29; N, 8.61. Found (%): C, 10.89; H, 1.34; N, 8.47.

2. 3 X-ray crystallography

A red crystal with dimensions of 0.32mm × 0.25mm × 0.18mm was used for X-ray data collec- tion performed on a Bruker SMART APEX CCD area-detector diffractometer equipped with a gra-phitemonochromated Moradiation (= 0.71073 ?) at 293 (2) K. A total of 41228 reflections were collected in the range of 2.45≤≤28.24°, of which 14683 independent reflections were collected (int=0.0528). The intensities were corrected for Lorentz and polarization effects and for empirical absorption. The structure was solved by direct methods and refined by full-matrix least-squares techniques based on2using SHELXS-97 and SHELXL-97[32]programs, respectively. All non-hydrogen atoms were refined anisotropically. For the polyoxo- metalate system, H atoms bound to water are very difficult to locate in difference Fourier maps so that they are omitted from the final model. All remai- ning H atoms were positioned geometrically. The final= 0.0404,0.0914 (= 1/[2(F2) + (0.0433)2+ 13.3102], where= (F2+ 2F2)/3),= 1.004 and (Δ/)max= 0.001. Selected bond lengths and bond angles are given in Table 1.

Table 1. Selected Bond Distances (?) and Bong Angles (°)

3 RESULTS AND DISCUSSION

3. 1 Crystal structure description

As shown in Fig. 1, the complex consists of one [BW12O40]5-anion, two discrete [Co(BIIM)3]2+cations and seven crystallization water molecules. Like other-Keggin anions[6], the [BW12O40]5-anion has the following preferences: W–Ot1.699(6)～1.737(6), W–Ob,c1.854(6)～1.968(5) and W–Oa2.317(5)～2.409(5) ?. For BO4tetrahedron, the B–O distances are in the range of 1.521(10)～1.549(10) ? with mean length 1.531 ?, while O–B–O angles vary from 108.4(6) to 111.3(7)°, showing that the central BO4tetrahedron and all WO6octahedra were more regular. For two discrete [Co(BIIM)3]2+cations, each Co2+defined by six N atoms from three BIIM ligands forms a CoN6octahedron with relative Co(1)–N lengths falling in the 1.904(7)~1.936(7) ? range and the Co(2)–N lengths changing from 1.895(7) to 1.938(7) ? combining their related bond angles, which shows each Co2+ion also displays a regular octahe- dron. The results further indicate that weaker inter- molecular-interactions exist among these com- ponents. It is well known that the Co2+-based cation usually adopts octahedral coordination. Therefore, herein the Co2+ion defined by six N atoms has no extra site coordinating to the [BW12O40]5-anion. Compared with the compound [Cu(BIIM)2]- {[Cu(BIIM)2(H2O)][Cu(BIIM)2BW12O40]·3H2O}2[31]reported by us, two compounds under similar synthetic conditions have different structures. In[Cu(BIIM)2]{[Cu(BIIM)2(H2O)][Cu(BIIM)2BW12O40]·3H2O}2[31], one Cu2+site is in square planar and other two Cu2+sites are in the square pyramid in which one [Cu(BIIM)2]2+unit is supported by [BW12O40]5-polyanion through the terminal oxygen. The results indicate that the actions of [BW12O40]5-anion are obviously limited by the geometries of the coordination cations, derived from the geometrical requirements of the ligand and the coordination preferences of the metal ions. Generally, the Co2+- based cation adopts octahedral coordination, while a Cu2+-based cation fragment can exhibit octahedral, square pyramidal and square planar geometries.

Besides chelating to Co2+, BIIM ligand interacts with [BW12O40]5-and water moleculescomplex hydrogen bonds (Table 2). For [Co(1)(BIIM)3]2+ion (,,), N(7)–H and N(8)–H as hydrogen-bonding donors bond to water O(5W) (–1,,) and O(6W) (,,), N(11)–H and N(12)–H interact with O(3) and O(20) atoms (,,) and N(3)–H hydrogen bonds to O(13) (+1/2, –+1/2,–1/2) from the [BW12O40]5-ion, respectively. For the other [Co(2)(BIIM)3]2+ion (,,), N(16)–H interacts with water O(7W) (,,), while N(19)–H, N(20)–H and N(23)–H contact with water oxygen O(4W) (+1,,), O(3W) (+1/2, –+1/2,+1/2) and O(4W) (+1/2, –+1/2,–1/2), respectively. These components are finally linked together into a two-dimensional framework. It is noteworthy that PLATON[33]analysis shows that W–O???interac- tions appear between the terminal oxygen from [BW12O40]5-and the imidazole ring, like O(1) and N(18)-imidazole ring at (–1,,) with the distance 3.296(9)? from O(1) to the ring centroid, O(2) and N(6)-imidazole ring at (+1/2, –+1/2,+1/2) as well as O(5) and N(13)-imidazole ring at (–1/2, –+1/2,-1/2) with their related distances of 3.734(10) and 3.809(9)?, respectively. According to our knowledge, this kind of interaction has been less seen in those reported POM-based hybrids[31]. Cooperative N–H???O hydrogen bonds and W–O???interactions stabilize the structure packing.

Fig. 1. Crystal structure of the compound showing atom-numbering scheme adopted. Displacement ellipsoids are drawn at the 30% probability level. The hydrogen atoms attached to carbon and nitrogen atoms as well as water molecules are omitted for clarity

3. 2 Hirshfeld surface analysis

Hirshfeld surfaces combing fingerprint plots generated by the Crystalexplorer2.1 software[34]can identify the types and regions of intermolecular interactions and the proportion of this interaction to the total Hirshfeld surfaces area.Molecular Hirsh- feld surfaces in crystal structure are constructed from the electron distribution. The normalized contact distances (norm) based on bothd,dand theradii of the atom are listed in the following equation. The 2D fingerprint plot is the combination ofdandd[35, 36].

norm= (iivdw)/ivdw+ (eevdw)/evdw

Hirshfeld surfaces for the compound have been mapped overnormand shape index (Fig. 2), respec- tively. The interactions between polyoxometalate oxygen and the H atoms bound to N are shown as deep red areas in the Hirshfeld surfaces. Pale red areas on the surface correspond to weaker and longer contacts than hydrogen-bonding interactions. Visible spots are attributed to the H···H contacts. Obviously, a Hirshfeld surface can fully reflect the hydrogen-bonding information listed in Table 2. The intermolecular NH···O(polyoxometalate and water)interactions with 56.9% contribution to the total Hirshfeld surface appear as two distinct spikes in the 2D fingerprint plots (Fig. 3a, b). The lower spike attributes to O atoms interacting with H atoms bound to the nitrogen atoms, the upper spike being NH atoms interacting with O atoms. O···C/C···O contacts constitute 9.6% (Fig. 3c) of the total Hirshfeld surface attributing to the WO···C interactions, in which WO(terminal oxygen )···(imidazole ring)occurs in the fingerprint plots as a typical style. The scattered points spread up toi=e= 1.2 ? are corresponding to H···H interactions in the finger- print plots (Fig. 3d). Obviously, there are no CH···interactions because no pairs of typical ‘wings’ appear at the top left and bottom right of the two-dimensional fingerprint plot. Moreover,stacking interactions are not observed because the adjacent red and blue triangles are not present in the shape index surface.

Fig.2. Hirshfeld surface: (a)normand (b) shape index for the compound

Fig. 3. Fingerprint plots: (a) full and involving (b) H···O/O···H, (c) O···C/C···O, and (d) H···H contacts showingthe proportion of contacts contributing to the total Hirshfeld surface area of the compound

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°)

Symmetry codes: (a)+1/2, –+1/2,–1/2; (b)1,,; (c)1,,; (d)+1/2, –+1/2,+1/2

3. 3 IR and UV spectra

The IR spectrum exhibits absorption peaks at 3140 cm-1attributed to the characteristic of water molecules. Strong absorption peaks at 1425～1619cm-1are associated with the BIIM ligands. The band at 1001 cm-1is characteristic of B–O stretching vibrations. Peak at 957 cm-1corresponds to(W–Ot), and those at 908, 820 and 749 cm-1are attributed to(W–O–W). Compared with the IR spectrum of the parent-Keggin H5[BW12O40][28], the characteristic vibration frequency for the compound has slight shifts, affected mainly by the surrounding metal-coordination cations. The UV spectra exhibit one intense absorption peak at 266 nm, which corresponds to the Ob,c→ W charge transition. The Ot–W charge-transfer absorption band disappears due to the coordination mentioned above. This is characteristic of Keggin-type poly- anion, similar to that in literature[31].

3. 4 Thermal property

As shown in Fig. 4,the thermal stability was examined by TG and DTA techniques in the tem- perature range of 24～950 ℃. The thermal decom- position process displays one stage, which indicates the interactions are weaker among the metal- coordination cation and polyanion. Unfortu- nately, the rapid weight loss starts from room temperature attributed to the humidification of the sample. Additionally, a larger endothermic peak at 752 ℃ appears in the DTA curvedue to the skeleton decomposition of [BW12O40]5-anion.

Fig. 4. TG-DTA curve of the compound

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18 December 2013;

10 July 2014 (CCDC 922710)

the Natural Science Foundation of Henan Province (No. 102300410021)

. E-mail: zlwang@henu.edu.cn