Syntheses and crystal structures of two new sodium borates [Na₂(H₂O)₃][B₅O₈(OH)₂] and Na[enH₂][B₇O₁₀(OH)₄]

2.1 General

All reactants were used as purchased without further purification. IR spectra were measured on a Nicolet FT 1703X spectrophotometer in the 4000–400 cm^–1 region using KBr pellets. The thermogravimetric analysis (TGA) was performed using a TA-SDT Q600 thermal analyzer under N₂ atmosphere with a heating rate of 10 °C min^–1. The elemental analysis was carried out on an Elemental Vario EL III CHN elemental analyzer.

2.2 Evaporation synthesis of [Na₂(H₂O)₃][B₅O₈(OH)₂] (1)

Crystals of [Na₂(H₂O)₃][B₅O₈(OH)₂] (1) were grown from the aqueous solution of stoichiometric composition by evaporation at nearly 80 °C for 8 h. The raw materials Na₂B₄O₇·10H₂O (3.81 g, 10.0 mmol) and H₃BO₃ (3.09 g, 50.0 mmol) were added to 20 mL distilled water, and the resultant solution was stirred until it became clear. After 3 weeks, large colorless crystals, with smooth faces, were obtained (yield: 60 % based on Na⁺). – Analysis for B₁₀H₁₆O₂₆Na₂: calcd. H 2.75; found H 2.71. – IR (KBr disk, cm^–1): ν = 3600–3180 (br), 1750 (m), 1480 (s), 1400 (s), 1206 (s), 1184 (s), 950 (m), 752 (m), 684 (m), 510 (m).

2.3 Solvothermal synthesis of Na[enH₂][B₇O₁₀(OH)₄] (2)

A mixture of NaOH (0.040 g, 1.0 mmol), H₃BO₃ (0.618 g, 10.0 mmol), ethylenediamine (en) (0.20 mL, 3.0 mmol), pyridine (3.0 mL, 37.3 mmol), and H₂O (1.0 mL, 55.6 mmol) in a molar ratio of about 1:10:3:37.3:55.6 was sealed in a 23 mL Teflon-lined stainless steel reactor and kept at 200 °C for 7 days and then cooled to room temperature. Colorless block crystals of 2 were recovered by filtration, washed with distilled water, and dried in air (yield: 63 % based on Na⁺). – Analysis for C₂H₁₄N₂B₇O₁₄Na: calcd. C 6.18, H 3.63, N 7.21; found C 6.15, H 3.60, N 7.25. – IR (KBr disk, cm^–1): ν = 3330 (br), 3102 (w), 2640 (w), 2472 (w), 1740 (m), 1500 (s), 1386 (s), 1254 (m), 1206 (s), 1184 (s), 1000 (m), 904 (s), 800 (m), 706 (m), 472 (m).

2.4 Determination of crystal structures

Single crystals of 1 and 2 were carefully selected under an optical microscope and glued to thin glass fibers with epoxy resin. Intensity data were collected on a Bruker SMART APEX 2000 CCD diffractometer using graphite-monochromatized MoK_α radiation (λ = 0.71073 Å) at 293(2) K. The collected frames were processed with the software SAINT [18]. The data were corrected for absorption using the program Sadabs [19]. The structures were solved by Direct Methods and refined by full-matrix least squares on F² using the Shelxtl software package [20, 21]. All non-hydrogen atoms were refined anisotropically. The crystallographic data and experimental details for structural analyses of 1 and 2 are summarized in Table 1. Selected bond lengths and angles for 1 and 2 are listed in Tables 2 and 3, respectively.

CCDC 1030466 (1) and 1030467 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

3.1 Synthesis

It seems that the formation of [Na₂(H₂O)₃][B₅O₈(OH)₂] (1) was greatly influenced by the raw materials and the ratio of Na₂B₄O₇·10H₂O to H₃BO₃. Using NaBO₃·4H₂O or Na₂B₄O₇·10H₂O as only raw materials, the known compound [Na₂(H₂O)₈][B₄O₅(OH)₄] [22] was isolated in our hands. After optimizing the reaction conditions, we found that only when the ratio of Na₂B₄O₇·10H₂O to H₃BO₃ is 1:5, can compound 1 be isolated as the sole product in this reaction system.

In comparison with the preparation process of compound 1, Na[enH₂][B₇O₁₀(OH)₄] (2) was successfully obtained by solvothermal methods, the solvent being an essential factor for the formation of this compound. When using water as solvent, the previously reported compound Na₆[B₄O₅(OH)₄]₃·8H₂O [23, 24] was obtained. However, when ethylenediamine (en) acted as structure directing reagent, and pyridine/water were used as co-solvents, compound 2 could be successfully isolated.

3.2 Crystal and molecular structures

3.2.1 The structure of compound 1

The structure of [Na₂(H₂O)₃][B₅O₈(OH)₂] (1) is built up of Na–O polyhedra and the [B₅O₈(OH)₂]^2– polyborate anions. Its asymmetric unit consists of 18 non-hydrogen atoms, including 2 Na, 5 B, and 11 O (see Fig. 1). The [B₅O₈(OH)₂]^2– polyborate anion consists of two six-membered rings; each six-membered ring consists of one BO₃ triangle (∆), one BO₂(OH) triangle (∆), and one common BO₄(□) tetrahedron. According to the classification of polyborate anions proposed by Burns, Grice, and Hawthorne [15–17], the shorthand notation for the fundamental building blocks [B₅O₈(OH)₂]^2– in [Na₂(H₂O)₃][B₅O₈(OH)₂] is 4∆□:(2∆□)–(2∆□).

Fig. 1:

Part of the crystal and molecular structure of compound 1. Displacement ellipsoids are drawn at the 40 % probability level.

In 1 there are two kinds of coordinating Na⁺ ions (see Fig. 2a). Na(1) exhibits a six-fold coordination and coordinates to two oxygen atoms [O(6) and O(6A)] from B–O–B bridges and two oxygen atoms [O(5) and O(5A)] from hydroxyl groups and two oxygen atoms [O(1S) and O(1SA)] from two water molecules, in which the Na–O distances range from 2.3101(12) to 2.9918(9) Å with a mean value of 2.523(1) Å. Na(2) also exhibits a six-fold coordination, with two oxygen atoms [O(10) and O(10A)] from the hydroxyl groups and four oxygen atoms [O(2S), O(2SA), O(3S), and O(3SA)] from water molecules, in which the Na–O distances range from 2.3770(10) to 2.4211(10) Å with a mean value of 2.400(1) Å. The FBBs are connected by sharing Na⁺ ions forming infinite {Na₂B₅O₈(OH)₂·3H₂O}_n chains (see Fig. 2b). The adjacent chains are further connected with each other through strong O–H···O hydrogen bond interactions, thus leading to a 3D assembly (see Fig. 2c). The parameters of the hydrogen bonds in 1 are given in Table 4.

Fig. 2:

(a) The two kinds of coordinated Na⁺ ions in the crystal structure of compound 1; (b) polyhedral representation of infinite {Na₂B₅O₈(OH)₂·3H₂O}_n chains; (c) polyhedral view down the crystallographic a axis (··· = H bonds).

Table 4

Hydrogen bond parameters for [Na₂(H₂O)₃][B₅O₈(OH)₂] (1).^a

D–H···A	d(D–H) (Å)	d(H···A) (Å)	d(D···A) (Å)	∠(DHA) (deg)
O7–H7···O1S #1	0.82	1.92	2.730(2)	170.0
O8–H8···O3S #2	0.82	1.98	2.772(2)	161.1
O1S–H1S···O2 #3	0.92(3)	1.93(3)	2.849(2)	172(2)
O1S–H1S···O3 #3	0.92(3)	2.64(2)	3.232(2)	122.8(19)
O1S–H2S···O2S #4	0.88(2)	1.86(2)	2.718(2)	165.3(19)
O2S–H3S···O4 #5	0.85(2)	1.94(2)	2.772(2)	164.4(19)
O2S–H4S···O8 #6	0.87(3)	1.88(3)	2.747(2)	178(2)
O3S–H5S···O7 #7	0.81(2)	2.04(2)	2.855(2)	177(2)
O3S–H6S···O3 #8	0.88(2)	1.90(2)	2.776(2)	176.8(19)

^aSymmetry transformations used to generate equivalent atoms: #1 x − ½, –y+1/2, z − ½; #2 x+½, y+½, z; #3 –x+³/₂, –y+½, –z+1; #4 –x+1, –y, –z+1; #5 –x+1, y, –z+½; #6 –x+1, –y, –z; #7 x, –y, z–½; #8 –x+³/₂, y − ½, –z+½.

3.2.2 The structure of compound 2

The crystal structure of Na[enH₂][B₇O₁₀(OH)₄] (2) consists of an isolated [B₇O₁₀(OH)₄]^3– polyborate anion, a sodium (Na⁺) ion, and a [NH₃CH₂CH₂NH₃]²⁺ ion (see Fig. 3). The [B₇O₁₀(OH)₄]^3– heptaborate group consists of three six-membered rings in which four trigonal BO₂(OH) units, one trigonal BO₃ unit, and two tetrahedral BO₄ units are linked by vertical oxygen atoms. In other words, BO₂(OH) triangles, the BO₃ triangle, and the BO₄ tetrahedra are linked by oxygen atoms, and the three six-membered rings are linked through their vertices [B(1) and B(5)] to form the heptaborate anion [B₇O₁₀(OH)₄]^3–. In the BO₂(OH) triangles, the B–O bond lengths lie from 1.342(2) to 1.390(2) Å, with an average one being 1.367(2) Å; in the BO₃ triangle, the B–O bond distances lie from 1.350(2) to 1.392(2) Å, with an average one being 1.369(2) Å; while in those tetrahedral boron atoms, the B–O bond lengths range from 1.418(2) to 1.506(2) Å, and the average bond length is 1.474(2) Å. These compare very well with the corresponding mean bond lengths observed in many hydrated or anhydrous borate compounds [8, 10, 11]. The FBBs’ shorthand notation for [B₇O₁₀(OH)₄]^3– is 7:5∆+2□. The polyborate anion with seven boron atoms is also found in [Cu(en)₂][B₇O₁₃H₃] [25], Cs₂B₇O₉(OH)₅ [26], and Rb₂B₇O₉(OH)₅ [27].

Fig. 3:

Part of the crystal and molecular structure of compound 2. Displacement ellipsoids are drawn at the 30 % probability level.

In 2, Na⁺ and [enH₂]²⁺ cations compensate the negative charge of the heptaborate. All sodium ions are octahedrally coordinated, Na exhibits a six-fold coordination and coordinates to three oxygen atoms from B–O–B bridges and three oxygen atoms from hydroxyl, in which the Na–O distances range from 2.379(2) to 2.827(2) Å with a mean value of 2.538(2) Å. It is known that extensive hydrogen bond interactions play an important role in the formation and stability of low-dimensional structures. As is shown in Fig. 4a, The FBBs are connected by sharing Na⁺ ions forming an infinite {Na[B₇O₁₀(OH)₄ ]}_n sheet-like structure. The [enH₂]²⁺ cations lie in the vacant space of the infinite {Na[B₇O₁₀(OH)₄]}_n sheets and form N–H···O hydrogen bonds with the [B₇O₁₀(OH)₄]^3– groups, thus leading again to a 3D assembly (see Fig. 4b). The parameters of the hydrogen bonds in 2 are presented in Table 5.

Fig. 4:

(a) Polyhedral view along the crystallographic c axis; (b) polyhedral view along the a axis (··· = H bonds).

Table 5

Hydrogen bond parameters for Na[enH₂][B₇O₁₀(OH)₄] (2).^a

D–H···A	d(D–H) (Å)	d(H···A) (Å)	d(D···A) (Å)	∠(DHA) (deg)
N1–H1A···O6 #1	0.89	2.37	3.247(3)	167.9
N1–H1B···O10	0.89	2.58	3.416(2)	156.2
N1–H1C···O14 #2	0.89	1.87	2.739(2)	165.4
N2–H2A···O13 #3	0.89	1.99	2.873(2)	171.9
N2–H2B···O6 #4	0.89	2.05	2.865(2)	151.7
N2–H2C···O14 #5	0.89	1.90	2.778(2)	170.5
N2–H2C···O12 #5	0.89	2.52	3.105(2)	123.8

^aSymmetry transformations used to generate equivalent atoms: #1 x – 1, y, z; #2 x, y+1, z; #3 –x, –y, –z+1; #4 –x+1, –y+1, –z+1; #5 –x+1, –y, –z+1.

3.3 FT-IR spectroscopy

The FT-IR spectra of compounds 1 and 2 exhibit the following absorption bands. The band at 3600–3200 cm⁻¹ is due to the stretching modes of O–H. The strong bands at around 1500–1200 cm⁻¹ might be the asymmetric and symmetric stretching modes of B–O in BO₃. The bands at 850–700 cm⁻¹ are assigned as the asymmetric and symmetric stretching of B–O in BO₄. The band at 700–400 cm⁻¹ is a combination of the out-of-plane bending mode of B–O in BO₃ and the bending mode of B–O in BO₄ [28]. In addition, for compound 2 the stretching vibrations of the N–H and C–H bands are observed at 3102 and 2640 cm⁻¹, respectively.

3.4 TGA analysis

The thermal behaviors of 1 and 2 are displayed in Figs. 5 and 6, respectively. TGA shows that 1 has a one-step weight loss at about 140–470 °C, corresponding to the removal of water (found: 23.55 %; calcd.: 24.57 %). For 2, there is a one-step weight loss of 31.93 % between 100 and 550 °C, which corresponds to the loss of water due to the condensation of hydroxyl groups and [enH₂]²⁺ and can be compared with a calculated value of 25.24 %.

Fig. 5:

TG curve of compound 1.

Fig. 6:

TG curve of compound 2.