Hydrogen-bonded assemblies of two organically templated borates: syntheses and crystal structures of [(1,10-phen)(H₃BO₃)₂] and [2-EtpyH][(B₅O₆(OH)₄]

2.1 General

B(OH)3, 1,10-phenanthroline monohydrate (1,10-phen·H2O), pyridine (py), and 2-ethyl-pyridine (2-Etpy) were purchased from Sinopharm Chemical Reagent Co., Ltd, Shanghai, China and used without further purification. FT-IR spectra were measured on a Nicolet FT 1703X spectrophotometer in the 4000–400 cm–1 region using KBr pellets (USA). The thermogravimetric analysis (TGA) was performed with a TA-SDT Q600 thermal analyzer under N2 atmosphere with a heating rate of 10 oC min–1 (USA). Elemental analyses were carried out using a Perkin-Elmer 2400 CHN analyzer (USA). The single-crystal X-ray diffraction was performed on SMART APEX II (Germany). The collected frames were processed with the software saint, Area Detector Control and Integration Software, Bruker Analytical X-ray Instruments Inc., Madison, Wisconsin (USA). An empirical absorption correction was applied using thesadabs program, Program for Empirical Absorption Correction of Area Detector Data, University of Gottingen, Gottingen, Germany. The structures were all solved by Direct Methods using the shelxs-97 program package. The structures were refined on F2 by full-matrix least-squares methods using the program shelxl-97, Software Reference Manual (version 5.1), Bruker Analytical X-ray Instruments Inc., Madison, Wisconsin (USA), also see G. M. Sheldrick, Acta Crystallogr. 2008, A64, 112.

2.2 Synthesis of [(1,10-phen)(H₃BO₃)₂] (1)

A mixture of B(OH)₃ (247.3 mg, 4.0 mmol), 1,10-phen anthroline monohydrate (198.2 mg, 1.0 mmol), pyridine (3955.2 mg, 50 mmol), and distilled water (900 mg, 50 mmol) was sealed in a 23 mL Teflon-lined stainless steel autoclave, heated to 160 °C under autogenous pressure for 3 days, and then cooled to room temperature at a rate of 5 °C/h. The large colorless prism-shaped crystals of 1 were filtered, washed with distilled water, and dried at ambient temperature in air (0.287 g, yield 79 % based on boron). – Anal. for C₁₂H₁₄B₂N₂O₆: Calcd. C 47.43, H 4.64, N 7.12; found: C 47.40, H 4.63, N 7.14.

2.3 Synthesis of [2-EtpyH][(B₅O₆(OH)₄](2)

A mixture of B(OH)₃ (494.4 mg, 8.0 mmol), 2-ethylpyridine (2-Etpy) (932.3 mg, 8.7 mmol), and N,N′-dimethylformamide (DMF) (3655.2 mg, 50 mmol) in a 23 mL Teflon-lined stainless steel autoclave was heated to 180 °C under autogenous pressure for 3 days and then cooled to room temperature at a rate of 5 °C h^–1. The colorless block-shaped crystals of 2 were filtered, washed with distilled water, and dried at ambient temperature (0.324 g, yield 62 % based on boron). – Anal. for C₇H₁₄B₅NO₁₀: Calcd. C 25.77, H 4.33, N 4.29; found: C 25.79, H 4.35, N, 4.26 %.

2.4 Determination of crystal structures

Single crystals of 1 (prism, dimensions 0.27 × 0.14 × 0.12 mm³) and 2 (block, dimensions 0.36 × 0.31 × 0.27 mm³) were carefully selected under an optical microscope and glued to thin glass fibers with epoxy resin. Single-crystal X-ray diffraction data were collected on a Siemens SMART CCD 2K diffractometer with graphite-monochromatized MoK_α radiation (λ = 0.71073 Å) in an ω-scanning mode at 293 K. The collected frames were processed with the software saint. An empirical absorption correction was applied using the sadabs program. The structures were all solved by Direct Methods using the shelxs-97 program package. The structures were refined on F² by full-matrix least-squares methods using the program shelxl-97. All the non-hydrogen atoms were refined anisotropically. The crystallographic data and experimental details are summarized in Table 1, while selected bond lengths and angles are given in the supplementary materials.

CCDC 1029042 (1) and 1029043 (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 and growth of crystals

Complex 1 was synthesized under mild conditions from the reaction of B(OH)₃ and 1,10-phen monohydrate in the presence of pyridine and distilled water. Unlike most organic templates, 1,10-phen does not take up protons from water molecules because its capacity of accepting protons is lower than that of pyridine. B(OH)₃ does not form polyborate anions through condensation (dehydration) reactions. Complex 2 was obtained from the similar reaction of B(OH)₃ and 2-Etpy in the presence of DMF solvent. Obviously, 2-Etpy is protonated to form [2-EtpyH]⁺ cation as an organic template for the pentaborate [B₅O₆(OH)₄]^– anion.

3.2 Infrared spectra

The infrared (IR) spectra of 1 and 2 exhibit the typical bands for the components (Fig. 1). In the IR spectrum of 1, the strong bands at 1427 and 1272 cm^–1 are consistent with the existence of trigonally coordinated boron. The stretching vibration of the O–H bond is observed at 3329 cm^–1 as a broad band. In the IR spectrum of 2, the strong bands at 1336 and 1125 cm^–1 are characteristic of trigonally coordinated boron, while the bands at 1032, 929, and 770 cm^–1 are characteristic of tetrahedrally coordinated boron [29]. The stretching vibration of O–H is observed at 3330 cm^–1. The broad band at 3200–2713 cm^–1 is attributed to the stretching vibrations of the O–H bonds and the combination of CH₂, CH₃, and NH groups. The IR spectra of 1 and 2 show bands in two ranges of 1559–1574 cm^–1 as the strong peaks and 1165–1200 cm^–1as the medium peaks, which can be obviously assigned to the vC–N) of the pyridine ligands.

Fig. 1:

IR spectra of complexes 1 (a) and 2 (b).

3.3 Crystal structures

Complex 1 crystallizes in the triclinic space group P1̅ with Z = 2. The molecular structure is illustrated in Fig. 2. Complex 1 is a co-crystal that consists of two B(OH)₃ molecules and one 1,10-phen molecule per formula unit. The B(OH)₃ and 1,10-phen molecules interact with each other, forming O–H···N hydrogen bonds with H···N distances of 1.93–1.96 Å, as shown in Fig. 3a. Further, as observed for B(OH)₃ in 1, the hydroxyl groups are involved in formation of the O–H···^.O hydrogen bonds with the H···O distances of 1.90–2.0 Å. As shown in Fig. 3b, the hydrogen bonds involved in the aggregation of the ensembles leads to crinkled tapes, which, in turn, are packed into a two-dimensional hydrogen-bonding framework. 1,10-Phen molecules are sandwiched between the [B(OH)₃]_n chains (see Fig. 3c). The details of hydrogen bonds of 1 are given in Table 2.

Fig. 2:

ortep plot of the principal structure components of complex 1.

Fig. 3:

(a) Recognition pattern between the co-crystal formers boronic acid and 1,10-phenanthroline; (b) O–H···O hydrogen bonds formed by B(OH)₃ molecules; (c) packing of molecules in the two-dimensional arrangement in the crystal structure of 1.

Complex 2 crystallizes in the monoclinic space group P2₁/c with Z = 4. Figure 4 displays an ortep plot of 2. Selected bond lengths and angles are given in Table 3. The formula unit of 2 consists of one protonated [2-EtpyH]⁺ cation and one pentaborate [B₅O₆(OH)₄]^– anion. As found in several known borates, such as hydrated A[B₅O₆(OH)₄]·xH₂O (A = Li⁺, Na⁺, K⁺, Cs⁺, [NH₄]⁺; x = 2, 3) [30], [Zn(dien)₂][B₅O₆(OH)₄]₂ (dien = diethylenetriamine) [31], [Me₃NCH₂CH₂OH][B₅O₆(OH)₄] [19], and [(4-MepyH)·(4-Mepy)][B₅O₆(OH)₄] [19], the polyanion [B₅O₆(OH)₄]^– is composed of one BO₄ tetrahedron and four BO₂(OH) triangles, which form two B₃O₃ rings linked by the common BO₄ tetrahedron. The trigonal boron centers have B–O distances in the range of 1.346(2)–1.383(2) Å (average of 1.363(2) Å), and the tetrahedral boron center has relatively longer B–O distances in the range of 1.462(2)–1.473(2) Å (average of 1.468(2) Å). These values are in good agreement with those of other borate compounds reported previously [19–21]. The O–B–O angles in the BO₄ tetrahedron lie in the range of 108.52(13)–110.91(12)°, and those in the BO₃ triangles span the range from 116.78(15)° to 122.74(15)°; the averages for the corresponding angles are very close to 109.47° and 120°, respectively.

Fig. 4:

ortep plot of the components of complex 2.

The pentaborate anions [B₅O₆(OH)₄]^– are linked together by O–H···O hydrogen bonds to form a three-dimensional supramolecular framework, as shown in Figs. 5a and 5b [32–34]. The templating [2-EtpyH]⁺ cations are situated in the cavities arising from the supramolecular pentaborate framework and interact with the inorganic framework through the N–H···O hydrogen bonds (N1···O6 = 2.725(2) Å), as shown in Fig. 5c. Hydrogen bond data for complex 2 are summarized in Table 4.

Fig. 5:

(a) The pentaborate anions [B₅O₆(OH)₄]^– linked by O–H···O hydrogen bonds; (b) a view along the crystallographic a axis showing a unidirectional rectangle-like borate anion host lattice with 12-membered boron rings; (c) packing of molecules in the three-dimensional arrangement in 2.

3.4 Thermal properties

TGA of complex 2 was carried out in nitrogen atmosphere from 40 to 800 °C with a heating rate of 10 °C min^–1. As shown in Fig. 6, the TGA curve of complex 2 shows a continuous weight loss between 270 and 597 °C, which was attributed to the removal of the 2-Etpy (C₇H₉N) and the dehydration of the boric acids (found: 47.93 %, calcd.: 44.20 %).

Fig. 6:

TGA curve of complex 2.