ARTICLE

DOI: 10.1002/zaac.201000442

Syntheses, Crystal Structures, and Thermal Behaviors of Two New Metallo-Organically Templated Pentaborates

Yang Yang,[a] Yu Wang,[a] Jiang Zhu,[a] Rui-Bin Liu,[a] Jun Xu,[a] andChang-Gong Meng*[a]

Keywords: Cadmium borate; Solvothermal synthesis; Crystal structure; Hydrogen bonds; Thermal analysis

Abstract. Two new cadmium borates, [Cd(en)3][B5O6(OH)4]2·2H2O(en = ethylenediamine) (1) and [Cd(DETA)2][B5O6(OH)4]2 (DETA =diethylenetriamine) (2) were synthesized in a novel procedure undermild solvothermal conditions and characterized by single-crystal X-raydiffraction, IR spectroscopy, elemental analysis, and TG–DTA. Thecompound 1 crystallizes in monoclinic system, space group P21/c (No.14) with a = 8.526(2) Å, b = 23.127(6) Å, c = 15.438(4) Å, β =94.320(3) °, V = 3035.5(13) Å3, Z = 4. Compound 2 is triclinic, spacegroup P1̄ (No. 2), a = 8.632(5) Å, b = 9.418(6) Å, c = 27.856(18) Å,

IntroductionBorate materials have attracted great deal of attention in re-cent decades because of their rich structural chemistry[1–5] andpotential applications in mineralogy, UV laser, and nonlinearoptical materials.[1–3,6–12] The structural diversity of borates isa result of the flexibility of the boron atoms to adopt eithertrigonal or tetrahedral coordination. The two kinds of boronunits have the general propensity to polymerize by sharing cor-ners to form a wide range of polyanions including isolatedrings/cages, infinite chains, sheets, and frameworks,[13–17]

which can occlude interstitial cations and other guest species.It is a long-standing goal to synthesize specific crystal structureswith expected special performances. However, it is hardlyachieved by self-assembly processes of organic and inorganicmoieties under certain conditions.[18–20] Borate materials with var-ious alkali metals, alkaline earth metals, main group metals, rareearths, and transition metals have been widely explored.[14,16,21–24]

In contrast, less work has been carried out on metal-organic bo-rates. To date, only few metals coordinated by amines were suc-cessfully introduced into borate systems, such as[Cu(en)2][B7O13H3]n,[25] [Mn(C10H18N6)][B5O6(OH)4]2,[26]

[Ni(C4H10N2)(C2H8N2)2][B5O6(OH)4]2,[27] [Zn(DIEN)2]-[B5O6(OH)4]2, [B5O7(OH)3Zn(TREN)] [DIEN = diethylenetriam-

* Prof. Dr. C.-G. MengFax: +86-411-84708545E-Mail: cgmeng@dlut.edu.cn

[a] Department of ChemistryDalian University of TechnologyDalian 116024, P. R. ChinaSupporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/zaac.201000442 or from theauthor.

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α = 95.415(8) °, β = 91.891(7) °, γ = 93.563 (7) °, V = 2248(2) Å3,Z = 3. The anionic units of the both structures, [B5O6(OH)4]– are linkedby hydrogen bonds to form a three-dimensional framework with largechannels, in which the templating cadmium complex cations are lo-cated. The thermal decomposition performance of compound 1 re-quires three steps, whereas only two steps are needed for compound2, which all lead to amorphous phases. These processes are well ex-plained considering the structure and the change in the Cd2+ coordina-tion during heating.

ine and TREN = tris(2-aminoethyl)amine],[28] and [Co2(TETA)3]-[B5O6(OH)4]4 (TETA = triethylenetetramine).[29] Furthermore,these metal-organic borates were traditionally prepared undermild hydrothermal/solvothermal conditions with pyridine or pyri-dine-H2O mixtures as common solvents. With these methods, theobtained products were accidental, uncontrollable and often ac-companied by the formation of side products, because all of thereactants were added to the solvents simultaneously. For example,when [Ni(C2H8N2)3][B5O6(OH)4]2 was synthesized under hydro-thermal conditions, the by-product of the general formula[Ni(C2H8N2)3](NO3)2 was generated much easier. In the metal-organic borates, the metallo-organic complex ions are used astemplating agents and are located in the structural channels of theframeworks, so their shapes and sizes have important influenceon the size of the voids. On the above basis, we propound thatthe crystal structure and the shape and size of the channels canbe regulated by modifying the respective metallo-organic com-plex ions, to achieve the desired objectives and crystal structures.In this work, we successfully prepared two new cadmiumborates, [Cd(en)3][B5O6(OH)4]2·2H2O (1) and[Cd(DETA)2][B5O6(OH)4]2 (2), by a new synthetic procedure.The prepared metallo-organic complex ion and H3BO3 wereadded to 1-methyl-2-pyrrolidinone, and reacted under mildconditions. In this way, different templated compounds withthe same inorganic framework connectivity can be prepared byjust varying the organic molecules. Meanwhile, the two titlecompounds were generated just at 115 °C for 12 hours,whereas in the traditional hydrothermal/solvothermal condi-tions with pyridine or pyridine-H2O mixtures as solvents thesynthesis of borate materials required 180 °C for one week.

Y. Yang, Y. Wang, J. Zhu, R.-B. Liu, J. Xu, C.-G. MengARTICLE

Results and DiscussionCrystal Structure

The asymmetric unit of 1 is shown in Figure 1, which alsoshows the configuration of the polyborate anion [B5O6(OH)4]–

and the coordination environment of the templating cation[Cd(en)3]2+. The cadmium atom is octahedrally coordinated bysix nitrogen atoms of three ethylenediamine molecules to formthe cation [Cd(en)3]2+ with Cd–N bond lengths in the range2.336(3)–2.418(4) Å and N–Cd–N bond angles varying from73.45(12) to 168.46(12)°. The structure of the fundamentalbuilding block (FBB), [B5O6(OH)4]–, is composed of four BO3triangles and one BO4 tetrahedron that link each other. Thisisolated pentaborate anion is characterized as two rings [B3O3]connected by a shared BO4 tetrahedron. Each ring is formedby two BO3 triangles and a slightly distorted common BO4tetrahedron. The terminal oxygen atoms are protonated. TheB–O distances of the trigonal BO3 unit are in the range1.342(4) and 1.375(5) Å (av. = 1.360 Å), and the B–O distan-ces of the tetrahedral BO4 unit are in the range 1.453(5) to1.484(5) Å (av. = 1.471 Å). The O–B–O angles of the trigonalunit are in the range 123.5(4)–115.7(3)°, whereas the O–B–Oangles involving the tetrahedral unit are in the range 111.6(3)–108.0(3)°. Selected interatomic distances and angles of 1 arelisted in Table S1 (Supporting Information).

Figure 1. Thermal ellipsoid plot (30 % probability) and atomic label-ing scheme for an asymmetric unit of 1.

In compound 2, the asymmetric unit consists of the templat-ing ion [Cd(DETA)2]2+ and the polyanion [B5O6(OH)4]–, asshown in Figure 2. The structure of the FBB is the same asthe polyborate anion in compound 1. Selected interatomic dis-tances and angles of 2 are listed in Table S2. The B–O distan-ces of the trigonal boron atoms are in the range 1.323(12) and1.404(12) Å (av. = 1.366 Å), and the B–O distances of thetetrahedral boron atom are in the range 1.450(12) to1.501(11) Å (av. = 1.476 Å). The O–B–O angles involving thetrigonal unit are in the range 123.3(9)–114.8(8)°, whereas theO–B–O angles involving the tetrahedral unit are in the range112.30(7)–106.9(7)°. The cadmium atom is also octahedrallycoordinated, bonded to six nitrogen atoms from two DETAmolecules. The Cd–N bond lengths range from 2.360(14) to2.417(13) Å with a mean value of 2.386 Å, whereas the N–

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Cd–N bond angles vary from 72.0(8) to 179.9(5)°. The N(7)and N(9) atoms are located on special positions and disorderedover two positions with a site occupation factor (SOF) of 0.5,respectively.

Figure 2. Thermal ellipsoid plot (30 % probability) and atomic label-ing scheme for an asymmetric unit of 2.

It is well known that multiple hydrogen bonds interactionsplay an important role in the formation and stability of low-dimensional structures.[10,30] The details of hydrogen bondingin 1 and 2 are listed in Tables S3 and S4. In both compounds1 and 2 the [B5O6(OH)4]– ions are connected by hydrogen-bonding interactions of the hydroxyl groups into a three-di-mensional structure with 12-membered boron ring channelsalong the x axis, as shown in Figure 3, Figure 4a, and Fig-ure 5a. The difference between the two compounds is that thelong edge of the rectangle-like 12-membered boron rings in 1is parallel to the y axis, whereas in 2 the long edge of therectangle-like 12-membered boron rings stands in an angle of45° to the y axis, which can be attributed to the template-effectof different complex cations and different hydrogen-bondinginteractions between the complex cations and the framework.

Figure 3. View of the 12-membered boron rings constructed by hydro-gen bonding.

In 1, the templating [Cd(en)3]2+ cation and two crystallizedwater molecules reside in the rectangle-like 12-membered bo-

Metallo-Organically Templated Pentaborates

Figure 4. (a) View of the hydrogen-bonded borate anion host latticewith 12-membered boron rings in 1. (b) View of the packing structureof 1 along the x axis. Hydrogen atoms of the cadmium complex cationsare omitted for clarity.

ron rings (Figure 4b), and both crystallized water moleculesand all nitrogen atoms of the ammonium groups interact withthe inorganic framework through extensive hydrogen bondswith N···O distances in the range 3.019(5)–3.303(4) Å (Fig-ure 6a). Figure S1 (Supporting Information) shows the packingstyle of the metal complex [Cd(en)3]2+ ion along the [100] di-rection. The enantiomers of the metal complex cation with Δand Λ configuration are alternatingly arranged down to thecrystallographic z axis and are similar to the chiral [Zn(en)3]2+ion in the Zn(en)3B5O7(OH)3.[31]

In 2, the [Cd(DETA)2]2+ guest templates are also located inthe rectangle-like 12-membered boron rings (Figure 5b), how-ever, unlike the nitrogen atoms of the ammonium groups in 1,only the two terminal nitrogen atoms of each ammonium groupinteract with the inorganic framework through hydrogen bondswith N···O distances in the range 3.017(16)–3.489(18) Å (Fig-ure 6b). The structure of 2 is similar to the previously reportedcobalt borate [Co(DIEN)2][B5O6(OH)4]2,[29] constructed fromthe same FBB. However, a distinction can be found in thearrangement of templating complex cations with hydrogen-bonded host lattices, which can be attributed to the differentspace groups they crystallize in.

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Figure 5. (a) View of the hydrogen-bonded borate anion host latticewith 12-membered boron rings in 2. (b) View of the packing structureof 2 along the x axis. Hydrogen atoms of the cadmium complex cationsare omitted for clarity.

Infrared (IR) Spectra

The IR spectra of 1 and 2 are similar and are shown in Fig-ure 7. The bands at 1406 and 1312 cm–1 might be assigned tothe asymmetric and symmetric stretching modes of B–O inBO3.[32–34] The band at 707 cm–1 is assigned to the out-of-plane bending mode of B–O in BO3. The band at 1162 cm–1

corresponds to the in-plane bending mode of B–O–H. Thebands at 1053, 1023, 920, and 773 cm–1 are characteristic ofthe asymmetric and the symmetric stretching modes of B–O inBO4.[32–34] The band at 482 cm–1 is assigned to the bendingmode of B–O in BO4. A signal at 1616 cm–1 corresponds tothe bending of NH2.[27] The bands at 2957 and 2890 cm–1 aredue to the stretching vibration of CH2 group. The broad bandsat 3520–3080 cm–1 correspond to the stretching vibrations ofthe O–H and N–H bands.

Thermal Properties

The thermal behaviors of 1 and 2 are shown in Figure 8. TheTG curve of 1 shows that the compound is stable up to about80 °C. On further heating, a three-step weight loss was ob-served. The initial weight loss between 80 and 170 °C corre-sponds to the removal of two water molecules (found 4.34 %;

Y. Yang, Y. Wang, J. Zhu, R.-B. Liu, J. Xu, C.-G. MengARTICLE

Figure 6. (a) Side view of the hydrogen-bond interactions between the[Cd(en)3]2+ complex and adjacent [B5O6(OH)4]– clusters in 1. (b) Sideview of the hydrogen bond interactions between [Cd(DETA)2]2+ cati-ons and adjacent [B5O6(OH)4]– clusters in 2. The hydrogen atoms oncarbon atoms are omitted for clarity.

Figure 7. The FT-IR spectra of 1 and 2.

calcd. 4.71 %). The second step, occurring between 170 and600 °C, is ascribed to the loss of three ethylenediamine mole-cules (found 22.86 %; calc. 23.57 %). The third weight lossbetween 600 and 1000 °C corresponds to the removal of fourwater molecules from the dehydration of hydroxyls (found11.48 %; calcd. 9.42 %). The residue after the calcinations for1 is amorphous and its phase is unidentified. Due to the lastloss of weight in the thermal degradation of 1 at relatively hightemperature, the difference between the found and calculatedmay be caused by a possible sublimation occurred for someanhydrous borates or for boric oxide.

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Figure 8. TG–DTA curves of 1(a) and 2(b).

The TG curve of 2 shows a continuous weight loss between220 and 590 °C, which is attributed to the removal of the twoDETA molecules (found 23.07 %; calcd. 27.33 %). The sec-ond weight loss between 590 and 1000 °C amounts 21.14 %,which corresponds to the loss of four water molecules due tothe condensation of hydroxyl groups (calcd. 9.54 %), and thesublimation of anhydrous borates or boric oxide due to therelatively high temperature. The residue after the calcinationsfor 2 is also amorphous and its phase is unidentified. The de-composing processes of the two compounds can be expressedas follows:

These explanations are confirmed by the DTA curves. In Fig-ure 8a, the endothermic peak appearing at 236 °C is related tothe loss of three ethylenediamine molecules. In Figure 8b, thetwo endothermic peaks appearing at 304 and 402 °C are attrib-

Metallo-Organically Templated Pentaborates

uted to the removal of the two DETA molecules, whereas thethird endothermic peak at 658 °C is related to the removal offour water molecules from the dehydration of hydroxyl groups.The weak endothermic peaks at 881 °C in Figure 8a and at865 °C in Figure 8b are related to the sublimation of anhy-drous borates or boric oxide.

ConclusionsThe two new cadmium borates, [Cd(en)3][B5O6(OH)4]2·2H2O(1) and [Cd(DETA)2][B5O6(OH)4]2 (2), were solvothermallysynthesized by a new synthetic procedure. Although both com-pounds 1 and 2 consist of the same polyborate anions[B5O6(OH)4]–, which are linked by hydrogen bonds to formthree-dimensional frameworks with large channels, in which thetemplating cadmium complex cations are located, the host latti-ces of the hydrogen-bonded borate anions are not the same be-cause of the template-effect of different complex cations anddifferent hydrogen-bonding interactions between the complexcations and the frameworks. Meanwhile, we found that in thenew synthetic procedure different templated compounds withthe same inorganic framework connectivity could be preparedjust by varying the organic molecules. The thermal behaviors ofthe two compounds were studied and the thermal decompositionperformances are well explained considering the structure andthe change in the Cd2+ coordination during the heating. Giventhe various transition-metal elements and other organic aminemolecules that can be introduced into the borate architecturewith different FBBs, it can be expected that many other novelstructural complex borate materials will be realized, and furtherwork on this subject is in progress.

Experimental SectionSynthesis of 1 and 2: All reagents in the synthesis were of analyticgrade and were used as received. Cd(NO3)2·2.5H2O (3.09 g, 10 mmol)was dissolved in deionized water (40 mL) whilst stirring. Afterwards,organic amine [2 mL, ethylene diamine (en) for 1, or diethylenetriam-ine (DETA) for 2] was dropped slowly into the reaction mixture. Uponcompletion of addition, the mixture was evaporated and condensed toabout 10 mL in a water bath, and a yellowish thick liquid was ob-tained. H3BO3 (0.185 g, 3 mmol) was added to 1-methyl-2-pyrrolidi-none (3 mL) in a 20 mL scintillation vial. After H3BO3 had completelydissolved, an aliquot of the above-synthesized liquid (0.5 mL) wasadded. The mixture was stirred to be homogeneous, sealed, and heatedat 115 °C for 12 h. After completion of the reaction, the mixture wascooled to room temperature (73.2 % yield for 1 and 64.7 % for 2 basedon boron). The as-synthesized colorless block crystals are insoluble inwater and common organic solvents.

Elemental analyses (wt %) for 1: found C 9.50; H 4.61; N 11.1 %;calcd. C 9.42; H 4.74; N 10.99 %. For 2: found C 12.65; H 4.71; N11.28 %; calcd. C 12.72; H 4.54; N 11.13 %. All experimental resultsare consistent with the calculated values based on the formula givenby X-ray single crystal diffraction (Figure S2, Supporting Informa-tion). The X-ray powder diffraction pattern for the bulk product is ingood agreement with the pattern based on single-crystal X-ray solutionin position, which indicates the phase purity of the as-synthesized sam-ples of the title compound. The agreement between the simulated and

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experimental diffraction patterns could prove that the reaction systemis able to avoid the generation of by-products, which are formed veryeasy in hydrothermal systems.

Crystal Structure Determination: A suitable single crystal with di-mensions of 0.30 × 0.20 × 0.20 mm for 1 and 0.25 × 0.20 × 0.15 mmfor 2 was carefully selected under an optical microscope and glued tothin glass fiber with epoxy resin. The intensities of the crystals datawere collected with a Bruker SMART APEX CCD diffractometer withgraphite-monochromated Mo-Kα (λ = 0.71073 Ǻ) using the SMARTand the SAINT programs.[35] All structure solutions were performedwith direct methods using SHELXS-97[36] and the structure refinementwas done against F2 using SHELXL-97.[37] All non-hydrogen atomswere found in the final difference Fourier map and refined with aniso-tropic thermal displacement coefficients. Hydrogen atoms were fixedgeometrically at calculated distances and allowed to ride on the paren-tal non-hydrogen atoms. Crystallographic data are summarized in Ta-ble 1.

Crystallographic data (excluding structure factors) for the structuresreported in this paper have been deposited with the Cambridge Crystal-lographic Data Centre as supplementary publication no. CCDC-779816and CCDC-781658 for 1 and 2, respectively. Copies of the data canbe obtained free of charge on application to CCDC, 12 Union Road,Cambridge CB2 1EZ, UK (Fax: +44-1223-336-033; E-Mail:deposit@ccdc.cam.ac.uk)

Materials and Physical Measurements: Elemental analysis was car-ried out with an Elemental Vario EL III microanalyzer. Powder X-raydiffraction (XRD) data were recorded with a Shimazu X-ray diffrac-tometer using Cu-Kα (λ = 1.5406 Å) radiation (40 kV, 30 mA) withscanning rate of 0.06°·s–1, over the 2θ range of 3–50°. IR spectra ofthe samples were recorded with a Nicolet Avatar 360 FT-IR spectrome-ter in the spectral region from 400 to 4000 cm–1 with a resolution of2 cm–1 using the KBr technique. The thermal analysis was performedwith a Mettler-Toledo SDTA 851 analyzer from ambient temperatureto 800 °C in nitrogen atmosphere with a heating rate of 10 °C·min–1.

Supporting Information (see footnote on the first page of this article):XRD patterns, selected bond lengths and angles for compounds 1 and2.

AcknowledgementWe are grateful to Dr. Cheng He and Dr. Yuan Lin for assistance withthe single-crystal X-ray diffraction and thermal behavior studies, re-spectively.

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Y. Yang, Y. Wang, J. Zhu, R.-B. Liu, J. Xu, C.-G. MengARTICLE

Table 1. Crystallographic data of 1 and 2.

Compounds 1 2

Formula C6H36B10CdN6O22 C8H34B10CdN6O20Formula weight 764.91 754.91Temperature /K 293 (2) 293 (2)Wavelength /Å 0.71073 0.71073Crystal system Monoclinic TriclinicSpace group P21/c P1̄a /Å 8.526(2) 8.632 (5)b /Å 23.127(6) 9.418 (6)c /Å 15.438(4) 27.856 (18)α /° 90 95.415 (8)β /° 94.320(3) 91.891 (7)γ /° 90 93.563 (7)Volume /Å3 3035.5(13) 2248 (2)Z 4 3ρcalcd. /g·cm–3 1.674 1.673Crystal color, habit colorless, block colorless, blockF(000) 1552 1146Θ range for date collection /° 2.25–24.99 2.25–24.99Limiting indices –7 ≤ h ≤ 10 –10 ≤ h ≤ 9

–27 ≤ k ≤ 19 –10 ≤ k ≤ 6–18 ≤ l ≤ 16 –32 ≤ l ≤ 32

Reflections collected 10459 8163Independent reflections 5235 7727Independent reflections with I > 2σ(I) 3664 5249Rint 0.025 0.031Completeness to Θ = 24.99 98.1 % 97.5 %Refinement method full-matrix least-squares on F2 full-matrix least-squares on F2Goodness-of-fit on F2 1.02 1.00Final R indices[I > 2σ(I)] R1 = 0.0368, wR2 = 0.0847 R1 = 0.0953, wR2 = 0.2622R indices (all data) R1 = 0.0541, wR2 = 0.0912 R1 = 0.1076, wR2 = 0.2739

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Received: December 21, 2010Published Online: March 4, 2011