Inorganica Chimica Acta 404 (2013) 219–223

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Inorganica Chimica Acta

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Synthesis, characterization and fluorescence properties of two novelinorganic–organic hybrid gallium/indium borates

0020-1693/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.ica.2013.03.028

⇑ Corresponding author. Tel.: +86 29 81530805; fax: +86 29 81530727.E-mail address: liuzh@snnu.edu.cn (Z.-H. Liu).

Sa-Ying Li, Zhi-Hong Liu ⇑Key Laboratory for Macromolecular Science of Shaanxi Province, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, PR China

a r t i c l e i n f o

Article history:Received 24 August 2012Received in revised form 16 February 2013Accepted 17 March 2013Available online 26 March 2013

Keywords:Gallium/indium borateSynthesisIonothermalCrystal structureFluorescence properties

a b s t r a c t

Two new gallium/indium borates, Ga(en)2[B5O8(OH)2]�H2O (I) and In(en)2[B5O8(OH)2]�H2O (II), have beensynthesized under ionothermal conditions and characterized by elemental analysis, FT-IR, XRD and ther-mal analysis. Single X-ray diffraction studies reveal that compounds I and II are isostructural and isomor-phous. Both borates crystallize in the monoclinic system with space group P21/c. Their structures arecharacterized as an inorganic–organic hybrid 1D helical chain constructed by Ga/In–O–B motif. The 1Dhelical chains are held together through the O–H� � �O and N–H� � �O hydrogen bonds, forming a 3D supra-molecular structure. Furthermore, their fluorescence properties have also been studied.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Borate materials have attracted a great deal of attention in thepast decades owing to their structural chemistry and potentialapplications in mineralogy, luminescence and nonlinear opticalproperties [1]. From the structural point of view, boron atomscan coordinate with oxygen not only in triangular BO3(D) but alsoin tetrahedral BO4(T). These BO3 and BO4 groups may further linktogether via common oxygen atoms to form isolated rings or poly-merize into infinite chains, sheets and networks. So far, many bo-rate materials concerning alkali metal, alkaline earth metal, rareearth and transition metal have been synthesized under hydro-thermal or high temperature solid-state conditions. In contrast,very limited organic–inorganic hybrid borates have been obtained,such as zinc borates for [B5O7(OH)3Zn(TREN)] and [Zn(dab)0.5(-dab0)0.5(B4O6(OH)2)]�H2O [2].

Gallium/indium atoms also have flexible coordination geome-tries, as exemplified by tetrahedral GaO4, trigonal bipyramidalGaO5 and octahedral GaO6. It is expected that the combination ofthe B and Ga/In atoms in the same crystalline material may gener-ate a new class of materials with novel structures and useful prop-erties. Some crystalline materials with novel structures and usefulproperties in Ga–O–B system have been obtained, such as K2Ga2-

O(BO3)2[3], Sr3Ga3(BO3)4O(OH) [4], Ba4Ga2B8O18Cl2�NaCl [5], K2-

[Ga(B5O10)]�4H2O [6] and Rb2[Ga(B5O10)]�4H2O [7], which were

synthesized by high temperature solid state reaction and the mildsolvothermal methods.

During our investigation of such borate materials, we have syn-thesized two novel gallium/indium borates, Ga(en)2[B5O8(OH)2]-�H2O (I) and In(en)2[B5O8(OH)2]�H2O (II), under ionothermalconditions, which present the first inorganic–organic hybrid 13group borates. Herein, we describe their syntheses, crystal struc-tures and fluorescence properties.

2. Experimental

2.1. Synthesis and characterization

A mixture of Ga(NO3)3�xH2O/In2O3, NH4F, Mg(NO3)2�6H2O,H3BO3, C6H11N2Br (1-ethyl-3-methylimidazolium bromide) anden (ethylenediamine) in the molar ratio of 1:3:6:28:13:113 for Iand 1:2:3:12:6:50 for II was stirred for several hours at room tem-perature, and transferred into Teflon-lined autoclave and heated at180 �C for 7 days, and then cooled to room temperature naturally.Colorless single crystals were recovered by filtration, washed withdeionized water and ethanol, and dried in air at ambienttemperature.

The obtained samples were characterized by elemental analyses(determined on Vario EL III Elemental Analyzer), FT-IR spectros-copy (recorded over the 400–4000 cm�1 region on a Nicolet NEXUS670 spectrometer with KBr pellets at room temperature), X-raypowder diffraction (Rigaku D/MAX-IIIC X-ray diffractometer withCu target at 8�min�1) and thermogravimetric analysis (TGA)

Table 1Crystal data and structure refinement for I and II.

Compounds I II

Empirical formula C4H20B5GaN4O11 C4H20B5InN4O11

Formula weight 424.01 469.11T (K) 293(2) 293(2)Crystal system monoclinic monoclinic,Space group P21/c P21/ca (Å) 9.2866(3) 9.3939(2)b (Å) 12.3169(3) 12.4805(3)c (Å) 14.2910(4) 14.5117(3)b (�) 96.123(3) 95.088(2)V (Å3) 1625.31(8) 1694.66(6)Z 4 4Dcalc (g cm�3) 1.733 1.839Absorption coefficient (mm�1) 1.754 1.453F(000) 864 936Crystal size (mm) 0.19 � 0.17 � 0.16 0.18 � 0.16 � 0.15Theta range for data collection (�) 3.21–25.99 3.15–26.00Limiting indices �6 6 h 6 11, �15 6 k 6 14, �17 6 l 6 17 �11 6 h 6 11, �9 6 k 6 15, �17 6 l 6 17Reflections collected 6242/3179 6741/3324Completeness to theta = 25.99 99.7% 99.8%Data/restraints/parameters 3179/2/229 3324/0/242Goodness-of-fit on F2 1.017 1.019Final R indices [I > 2r(I)] R1 = 0.0360, wR2 = 0.0721 R1 = 0.0207, wR2 = 0.0527R indices (all data) R1 = 0.0591, wR2 = 0.0804 R1 = 0.0242, wR2 = 0.0545Largest differences on peak and hole (e Å�3) 0.555 and �0.431 0.451 and �0.430

220 S.-Y. Li, Z.-H. Liu / Inorganica Chimica Acta 404 (2013) 219–223

(performed on a SDT Q600 thermal analyzer under air atmospherewith a heating rate of 10 �C min�1).

Tra

nsm

ittan

ce/%

a

b

2.2. Determination of crystal structures

Crystals of I (dimensions 0.19 � 0.17 � 0.16 mm3) and II(dimensions 0.18 � 0.16 � 0.15 mm3) were carefully selected un-der an optical microscope, and data collection were performedon a CrysAlisPro, Oxford Diffraction Ltd., Version 1.171.34.36 CCDautomatic diffractometer with graphite-monochromatized Mo Karadiation (k = 0.71073 Å) by using the x-scan mode at room tem-perature. An empirical absorption correction was applied usingthe semi-empirical from equivalents program [8]. Their structureswere solved by direct methods using SHELXS-97 [9]. Crystallographicdata for I and II are presented in Table 1.

4000 3500 3000 2500 2000 1500 1000 500

Wavenumbers/cm-1

Fig. 1. FT-IR spectra of compounds I (a) and II (b).

3. Results and discussion

3.1. Elemental analyses

Anal. Calc. for I: C, 11.33; H, 4.75; N, 13.21. Found: C, 11.50; H,4.91; N, 13.24%. Anal. Calc. for II: C, 10.24; H, 4.30; N, 11.94. Found:C, 10.28; H, 4.43; N, 11.45%. All the experimental results are consis-tent with the calculated values based on the formulae given by sin-gle X-ray crystal diffraction.

3.2. FT-IR spectra

As shown in Fig. 1, the IR spectrum of I is very similar to that ofII, which indicates their similar structures. The FT-IR spectrum ofcompound II exhibits the following absorption bands, which areassigned referring to literatures [10,11]. The peaks at 3440, 3240and 3140 cm�1 are the stretching modes of the O–H, N–H, andC–H bands. The peak at 1640 cm�1 is the H–O–H bending. Theband at 1227 cm�1 is the in-plane bending of B–O–H. The bandsat 1359 cm�1 and 941 cm�1 are attributed to the asymmetric andsymmetric stretching of B(3)–O, respectively. The bands at1053 cm�1 and 791 cm�1 are the asymmetric and symmetric bend-ing of B(4)–O, respectively. The peak at 738 cm�1 is the

out-of-plane bending mode of B(3)–O. The band at 470 cm�1 isattributed to the stretching mode of In–O.

3.3. X-ray powder diffraction patterns

Figs. 2 and 3 show the powder XRD patterns of as-synthesizedcompounds and the simulated patterns on the basis of single-crys-tal structures of I and II, respectively. The diffraction peaks on pat-terns correspond well in position, indicating the phase purity of theas-synthesized samples.

3.4. Crystal structures

Single X-ray diffraction studies reveal that compounds I and IIare isostructural and isomorphous, and here compound II as exam-ple will be described in detail. In II, its asymmetric unit consists ofone crystallographically distinct indium atom, one [B5O8(OH)2]3�

cluster, two en molecules and a water molecule as shown in

Fig. 2. XRD pattern of synthetic sample I.

Fig. 3. XRD pattern of synthetic sample II.

Fig. 4. The asymmetric unit of compound II, drawn at 50% probability level.Relevant bond distances (Å) and bond angles (�): In(1)–O(1) 2.0953(15), In(1)–O(10) 2.1101(15), In(1)–N(2) 2.2545(19), In(1)–N(3) 2.259(2), In(1)–N(4)2.2825(19), In(1)–N(1) 2.314(2); O(1)–In(1)–N(3) 95.61(7), O(10)–In(1)–N(3)99.53(7), O(1)–In(1)–N(4) 171.45(7), O(10)–In(1)–N(4) 88.25(7), O(1)–In(1)–N(1)93.14(6), O(10)–In(1)–N(1) 172.06(6), O(1)–In(1)–O(10) 88.00(6), O(1)–In(1)–N(2)97.09(7), O(10)–In(1)–N(2) 94.78(7), N(2)–In(1)–N(1) 77.28(7), N(3)–In(1)–N(1)88.19(7), N(4)–In(1)–N(1) 91.64(7).

Fig. 5. Polyhedral view along the c axis of compound II.

S.-Y. Li, Z.-H. Liu / Inorganica Chimica Acta 404 (2013) 219–223 221

Fig. 4, which also shows the coordination environments of the Inatom and B atoms.

The [B5O8(OH)2]3� unit is formed through an in situ polymeriza-tion reaction of boric acid material under synthesized conditions.This pentaborate polyanion consists of two [B3O3] rings linked bya common [BO4] tetrahedron. Each [B3O3] ring is formed by twoBO3 triangles (D) [B(1) and B(2), B(4) and B(5)] and a slightly dis-torted common BO4 tetrahedron (T) (B(3)). Two of the terminaloxygen atoms (O4 and O8) are protonated. The B–O bond lengthsare in the region of 1.332(3)–1.410(3) Å for the BO3 triangles and1.465(3)–1.481(3) Å for the BO4 tetrahedra, respectively. The

B–O–B bond angles are distributed in the range of 114.6(2)–124.3(2)� for the BO3 triangles and 108.28(18)–110.97(19)� forthe BO4 tetrahedra, respectively. According to the classification ofpolyborate anions proposed by Heller [12] and Christ and Clark[13], the shorthand notation for [B5O8(OH)2]3� in this compoundis 5:/1[5:4D + T]. It need to point out that the present [B5O8(-OH)2]3� anion is different from those appeared in the known Li3B5-

O8(OH)2 and Na3B5O8(OH)2�H2O. The shorthand notation for theknown [B5O8(OH)2]3� is 5:/2[5:2D + 3T] [14].

The indium atom is six-coordinated with four nitrogen atomsfrom two en molecules as the chelate forms and two oxygen atoms(O1 and O10) from two individual [B5O8(OH)2]3� clusters in themonodentate fashion, forming a distorted octahedral environment(InO2N4). The In–O and In–N bond lengths are in the ranges of2.0953(15)–2.1101(15) Å and 2.2545(19)–2.3142(2) Å, respec-tively. The O–In–O bond angle is 88.00(6)�. The N–In–N bond an-gles change from 77.28(7)� to 161.16(7)�. The O–In–N bondangles change from 88.25(7)� to 172.06(6)�.

In the crystal, each asymmetric unit connects each other by In–O–B motif to the form an inorganic–organic hybrid 1D helix chainstructure along the c axis (Fig. 5), where the oxygen atoms comefrom the unprotonated terminal O1 and O10 in two individual [B5-

O8(OH)2]3� clusters, and boron atoms are the B1 and B5 in thesame [B5O8(OH)2]3� cluster.

It is well known that multipoint hydrogen bond interactionsplay an important role in the formation and stability of low-dimen-sional structures. In this compound, the 1D helical chains are heldtogether through the O–H� � �O and N–H� � �O hydrogen bonds, form-ing a three-dimensional supramolecular structure (Fig. 6). More-over, the lattice water molecules are located among the chains,and interact with the chains through N–H� � �O(11) and O(11)–H� � �O hydrogen bonds. In addition, there also exist N–H� � �O hydro-gen bonds in the inorganic–organic hybrid 1D helix chain.

Fig. 6. Packing view of compound II along the a axis. Relevant bond distances (Å):N(1)–H(1C)� � �O(2) 2.878(3), N(4)–H(4D)� � �O(4) (�x + 1, y � 1/2, �z + 1/2) 2.876(3),N(2)–H(2C)� � �O(5)(x � 1, y, z) 2.925(3), N(2)–H(2D)� � �O(7)(�x + 1, �y + 1, �z)2.976(2), O(8)–H(2)� � �O(1)(�x + 1, �y + 1, �z) 2.669(2), O(11)–H(3)� � �O(8)(�x + 1,�y + 1, �z) 2.749(4), O(4)–H(1)� � �O(10)(x + 1, y, z) 2.606(2).

Temperature/°C

0 100 200 300 400 500 600 700 80065

70

75

80

85

90

95

100

Wei

ght/%

a

b

Fig. 7. TG curves of compounds I (a) and II (b).

300 400 500 600 700

0

200

400

600

800

1000

PL in

tens

ity(a

.u.)

Wavelength/nm

b

a

Fig. 8. Solid-state emission spectra of the compounds I (a) and II (b).

222 S.-Y. Li, Z.-H. Liu / Inorganica Chimica Acta 404 (2013) 219–223

3.5. Thermal properties

As shown in Fig. 7, the TG curves show that compounds I and IIhave the total weight losses of 32.16% and 30.43% between 30 and800 �C, respectively, which correspond to the removal of one lat-tice water molecule and two en molecules and can be comparedwith calculated values of 32.69% for I and 29.89% for II. The firstweight losses are 4.80% occurring from 30 to 200 �C for I and3.92% occurring from 30 to 230 �C for II, which correspond to theloss of 1 water molecule and can be compared with calculated val-ues of 4.25% for I and 3.84% for II.

3.6. Fluorescence properties

Some studies have shown that indium complex displays photo-luminescence properties [15]. The fluorescence spectra of com-pounds I and II were measured with a PE LS55 fluorescencespectrophotometer at room temperature using powder crystalsamples. As shown in Fig. 8, upon excitation of the solid samplesat 228 nm, respectively, both compounds exhibit a strong fluores-cent emission band at 399 nm. Meanwhile, the emission band wasobserved at 376 nm for the free en ligand with excitation at266 nm. It is obvious that the emission peaks of compounds Iand II are red-shift compared to the free en ligand. Therefore, these

emission bands of compounds I and II could be assigned to theemission of ligand-to-metal charge transfer [16]. It can be seen thatthe PL intensity of compound I is almost two times that of com-pound II, which might be resulted from the different ionic radiusof Ga3+ and In3+.

4. Conclusions

We have successfully synthesized two new gallium/indium bo-rates, Ga(en)2[B5O8(OH)2]�H2O(I) and In(en)2[B5O8(OH)2]�H2O (II),under ionothermal conditions, which present the first inorganic–organic hybrid 13 group borates. Both compounds are isostructur-al, which are characterized as a 1D helical chain structure con-structed by Ga/In–O–B motif. The 1D helical chains are heldtogether through the O–H� � �O and N–H� � �O hydrogen bonds, form-ing a 3D supramolecular structure. Moreover, they exhibit strongfluorescence properties at room temperature. It is anticipated thatmore 13 group borates with interesting structures as well as phys-ical properties will be synthesized under ionothermal conditions.

Acknowledgment

Project supported by National Natural Science Foundation ofChina (Nos. 20871078 and 21173143).

Appendix A. Supplementary material

CCDC 882603 and 882604 contain the supplementary crystallo-graphic data for this paper. These data can be obtained free ofcharge from The Cambridge Crystallographic Data Centre viahttp://www.ccdc.cam.ac.uk/data_request/cif.

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