African Journal of
Pure and Applied Chemistry

  • Abbreviation: Afr. J. Pure Appl. Chem.
  • Language: English
  • ISSN: 1996-0840
  • DOI: 10.5897/AJPAC
  • Start Year: 2007
  • Published Articles: 357

Full Length Research Paper

Preparation, structural and thermal studies of boroxine adducts having aryl boronic acids and pyrazoles

Hezil Hassan
  • Hezil Hassan
  • Department of Chemistry, Iran University of Science and Technology, Narmak, 16846-13114, Tehran, Iran.
  • Google Scholar

  •  Received: 02 April 2016
  •  Accepted: 27 May 2016
  •  Published: 31 August 2016


Four new boroxine adducts ((B3O3(Ph)3PzH) (1), (B3O3(Ph)3(PztBu,iPrH)2) (2), (B3O3(PhF2)3PzH). PzH (3) and (B3O3(PhF2)3(PztBu,iPrH)2) (4)) using phenylboronic acid, 3,5-difluorophenylboronic acid, 1H-pyrazole (PzH) and 3-tert-butyl-5-isopropyl pyrazole (PztBu,iPrH) were prepared and characterized by elemental analysis, IR, 1H-NMR and X-ray diffraction. The crystallographic study reveals that PzH and PztBu,iPrH are bonded to boroxine molecule through B-N dative bond. It also demonstrates the different type of hydrogen bond interactions between adjacent molecules. The thermal stability of these adducts was investigated by TGA.

Key words: Boroxine, crystal structures, hydrogen bonding, thermal study.


Because of importance in different synthetic reactions and significant applications in diverse areas, boronic acids  are  of  great  interest  (Phillips  and  James,  2004; Davis and James, 2005; James, 2005; Striegler, 2003; Elfeky et al., 2010; Wimmer et al., 2009; Li et al., 2008). In recent years, boronic acids  have  also  been  used  as exclusive building blocks in supramolecular chemistry (Fujita et al., 2008; Nishiyabu et al., 2011; Fossey and James, 2011). Boronic acids have –B(OH)2 group, and form the six-membered cyclic ring by simple dehydration of boronic acid. It is well established that boron in a cyclic ring (R3B3O3, R = alkyl or aryl group) acts as a lewis acid and has a tendency to accept the lone pair of electrons from the N-donor ligands (lewis base), and involved in the formation of the B-N dative bond in adducts (Icli et al., 2011; Höpfi, 1999; Sheepwash et al., 2011, 2013; Jorge et al., 2012). N-donor ligands easily form 1:1 adducts with arylboronic acids even under mild reaction conditions, and thermodynamically favored over 1:2 or 1:3 adducts due to relief in boroxine ring strain. Adducts performing 1:2 and 1:3 boroxine-N-donor ligands stoichiometry are very limited (Beckett et al., 1995; Höpfi, 1999; Kua and Iovine, 2005; Domingo et al., 2008; Saha et al., 2013; Jorge et al., 2016). In 1958, Synder et al. synthesized an adduct by using triphenylboroxine and pyridine by simple warming in anhydrous solvent (Snyder et al., 1958). In 2005, Cote et al. investigated highly stable and porous boronic acid derived covalent organic frameworks with large surface area (Cote et al., 2005). A large number of boroxine adducts with N–containing compounds have been studied due to wide commercial uses in various field like flame retardant materials, dopants, in Suzuki-Miyaura coupling reactions, non-linear optical materials, biosensors, covalent organic frameworks etc. (Bhat et al., 2011; Iovine et al., 2008; Morgan et al., 2000; Mehta and Fujinami, 1997; Yang et al., 2002; Miyaura and Suzuki, 1995; Cote et al., 2005; Türker et al., 2009), but the structural characterization and thermal study of boroxine adducts with pyrazoles are not reported till now. This paper presents the synthesis, structural, and thermal study of four new boroxine adducts. The main purpose is to see the effect of  substitution  in  phenyl  boronic  acids and pyrazoles ligands on the structure and crystal packing of these adducts.


All synthesis was performed in air, and solvents were used as received. Phenylboronic acid, 3,5-difluorophenylboronic acid, and 1H-pyrazole were purchased from Aldrich Chemical Co. 3-tert-butyl-5-isopropyl pyrazole was synthesized by previously reported method (Imai et al., 1998). Elemental analysis was carried out on PerkinElmer Elemental analyzer. IR and 1H-NMR spectra were recorded on Bruker ALPHA FT-IR and Bruker AM 400 MHz spectrometers, respectively. Thermal analysis was performed on PerkinElmer thermogravimetric analyzer.


Synthesis of adducts 1-4

Adducts 1-4, were synthesized according to scheme 1.




Synthesis of 1

A methanolic (10 ml) solution of phenylboronic acid (0.36 g, 3.00 mmol) and PzH (0.06 g, 1.00 mmol) was refluxed at 70°C for 4 h. Colorless crystals of 1 were obtained by the slow evaporation of solvent at room temperature in 0.24 g (62.5%) yield. Anal. Calcd for C21H19N2O3B3: C, 66.40; H, 5.04; N, 7.37. Found: C, 65.12; H, 4.99; N, 7.17. IR (KBr, cm-1): 3381, 3196, 3012, 2889, 2626, 2317, 2029, 1921, 1814, 1709, 1627, 1461, 1235, 969, 699, 527. 1H-NMR (400 MHz, CDCl3, ppm, 25oC): 6.21 (t, 1H, CH, Pz), 7.57 (d, 2H, CH, Pz), 12.61 (s, br, 1H, NH, Pz), 8.03 (dd, 6H, Ph), 7.35 (m, 9H, Ph).


Synthesis of 2

2 was obtained in 0.38 g (59%) yield by the same method as applied for 1 using PztBu,iPrH (0.17 g, 1.00 mmol). Anal. Calcd for C38H51N4O3B3: C, 70.84; H, 7.97; N, 8.69. Found: C, 70.19; H, 7.91; N, 8.49. IR (KBr, cm-1): 3418, 3143, 2956, 2865, 2247, 2139, 2069, 1971, 1829, 1786, 1609, 1573, 1437,  1296,  1049,  963,  827,  739, 645, 593, 497. 1H-NMR (400 MHz, CDCl3, ppm, 25oC): 6.51 (s, 2H, CH, Pz), 4.49 (m, 2H, CH, Pz), 1.39 (d, 12H, CH3, Pz), 1.54 (s, 18H, CH3, Pz), 12.64 (s, br, 2H, NH, Pz), 8.03 (dd, 6H, Ph), 7.37 (m, 9H, Ph).


Synthesis of 3

3 was prepared in 0.53 g (64%) yield by using the method as outlined for 1 using 3,5-difluorophenylboronic acid (0.48 g, 3.00 mmol) and PzH (0.06 g, 1.0 mmol). Anal. Calcd for C24H17N4O3F6B3: C, 51.86; H, 2.91; N, 10.08. Found: C, 50.91; H, 2.89; N, 9.97. IR (KBr, cm-1): 3467, 3139, 3089, 2971, 2849, 2597,  2257, 2091, 1969, 1781, 1624, 1579, 1446, 1223, 1321, 1199, 1163, 1077, 929, 829, 644, 547. 1H-NMR (400 MHz, CDCl3, ppm, 25oC): 6.27 (t, 2H, CH, Pz), 7.64 (d, 4H, CH, Pz), 12.67 (s, br, 2H, NH, Pz), 8.04 (dd, 6H, Ph), 7.40 (m, 3H, Ph).


Synthesis of 4

4 was synthesized in 0.45 g (59.8%) yield by the same method as described for 3 using PztBu,iPrH (0.17 g, 1.00 mmol). Anal. Calcd for C38H45N4O3F6B3: C, 60.84; H, 5.77; N, 7.46. Found: C, 60.13; H, 5.68; N, 7.07. IR (KBr, cm-1): 3418, 3129, 3018, 2917,  2755,  2601, 2239, 2069, 1911, 1837, 1755, 1629, 1457, 1213, 913, 729, 653, 547, 499. 1H-NMR (400 MHz, CDCl3, ppm, 25oC): 6.53 (s, 2H, CH, Pz), 4.51 (m, 2H, CH, Pz), 1.42 (d, 12H, CH3, Pz), 1.57 (s, 18H, CH3, Pz), 12.69 (s, br, 2H, NH, Pz), 8.07 (dd, 6H, Ph), 7.39 (m, 3H, Ph).


X-ray diffraction analysis

All crystals were obtained from the slow evaporation of methanolic solutions, and mounted on glass capillaries. All data were collected on a Bruker Kappa four circle-CCD diffractometer with graphite-monochromated MoKα radiation, operated at 50 kV and 40 mA at 25°C. Data were corrected for Lorentz and polarization effects (Sheldrick, 1996), and the SHELXTL program package was used for the structure solution and refinement (Sheldrick 1990, 2000). The hydrogen atoms were placed in geometrically calculated positions by using a riding model, and non-hydrogen atoms were refined anisotropically. Diamond and Mercury softwares were used for the formation of images and hydrogen bonding interactions (Brandenburg, 2000). 1, 3 and 4 are crystallized in triclinic system with P-1 space group, while 2 in monoclinic system with C2/c space group. The crystallographic data, hydrogen bond distances, selected bond lengths and angles are shown in Tables 1, 2, Appendix S1 and S2, respectively.




All adducts (B3O3(Ph)3PzH) (1), (B3O3(Ph)3(PztBu,iPrH)2) (2), (B3O3(PhF2)3PzH).PzH (3) and (B3O3(PhF2)3(PztBu,iPrH)2) (4) have been prepared by using phenylboronic acid, 3,5-difluorophenylboronic acid and corresponding pyrazoles (PzH/PztBu,iPrH) in methanol, and the different formulations were confirmed by elemental analysis, IR, NMR and crystallographic structure analysis. 2 and 4 are  rare  1:2  adducts  of  3,5-difluorotriphenylboroxine and PztBu,iPrH, whereas 1 and 3 are 1:1 adducts of triphenylboroxine and PzH but 3 is crystallized with free pyrazole as solvate.


Infrared and NMR spectroscopy

1-4 show strong bands in the region 1460-1250 cm-1, and at 1255 cm-1 due to B-O and B-N stretching bands, respectively (Smith and  Northrop,  2014).  NH  stretching  bands (3500-3400 cm-1) are shifted at 3100-3055 cm-1 due to formation of adjacent B-N dative bond. The IR spectra do not show any O-H stretching vibration in the region of 3300-3200 cm-1 that suggests the absence of O-H bands (Faniran and Shurvell, 1968). The formation of adducts have also been confirmed by the 1H-NMR spectra, showing the prominent downfield shift in each case with respect to free compounds as shown in Table S3.


Structure description of 1-4

According to Figure 1a, the crystal structure of 1 shows two molecular units. In each unit one boron atom (B(1)) has tetrahedral geometry, while other two boron atoms (B(2) and B(3)) shows trigonal planar geometry. B(1) has Sp3 hybridization due to the additional B-N dative bond. In one molecular unit, the B(2)-O(1), B(2)-O(3), B(3)-O(2) and B(3)-O(3) bond distances are in the range of 1.347(22) to 1.399(21) Å, and these are much smaller than that of B(1)-O(1) (1.468(20) Å) and B(1)-O(2) (1.456(23) Å) in B3O3 ring. In the same manner, B(2)-C(7) (1.542(21) Å) and B(3)-C(13) (1.548(24) Å) bond lengths are also shorter than the B(1)-C(1) (1.595(25) Å). The B(1)-N(1) bond length is (1.617(23) Å) which is nearly matched with the reported literature (Wu et al., 1999). Another unit also follows the same pattern as previous one. The crystal packing shows that the one unit is non-covalently hydrogen bonded to neighboring unit through the various weak C-H···B (C8-H8···B5, 3.153 (58) Å; C16-H16···B5, 3.058 (46) Å; C35-H35···B3, 3.154 (58) Å), N-HA···π (N2-H2A···π, 3.021 (46) Å; N4-H4A···π, 2.646 (36) Å) and  C-H···π  (C19-H19···π,  2.572  (35)  Å; C20-H20···π, 3.502 (54) Å; C21-H21···π, 2.897 (43) Å; C40-H40···π, 2.973 (45) Å; C41-H41···π, 2.889 (44) Å; C42-H42···π, 3.510 (37) Å) intermolecular interactions (Sarma and Baruah, 2009) (Appendix Figure S1). The angles between weak C-H···B bonds are 150.0 and 173.5o. All these interactions help to create the two dimensional sheet like framework (Figure 1(b)).



The asymmetric unit of 2 shows that two boron atoms B(1) and B(1i) have tetrahedral geometry (Sp3), while other boron atom B(2) has a trigonal planar geometry (Sp2). In this molecular structure, B1i, O2i, C1i, N1i lie on inversion centers with symmetry code (i = 2-x, y, 0.5-z) (Figure 2(a)). From the X-ray structure, it is clear that both PztBu,iPrH ligands in boroxine adduct are anti to each other which is more stable than syn configuration (Iovine et al., 2008). The B(1)-O(1) (1.437(13) Å) and B(1)-O(2) (1.446(4) Å) bond distances are greater than the B(2)-O(2) (1.362(10) Å). Similarly, the B(1)-C(1) (1.621(8) Å) bond length is slightly higher than that of B(1)-C(7) (1.579(6)Å). The B(1)-N(1) bond length is (1.655(11) Å), which is greater than the B-N bond of 1. The crystal structure analysis describes that one molecular unit is hydrogen bonded with other units through C17-H17B···B2, 3.141(15) Å; C17-H17C···B2, 3.358 (8) Å and C18-H18B···π, 3.005 (14) Å non covalent interactions (Melikova et al., 2002) (Appendix Figure S2), and the C-H···B bond angles in 2 are 119.1 and 103.7°C which are lesser than the C-H···B bond angles of 1 (Table 2). Three dimensional zig-zag layered network is obtained by all C-H···B and C-H···π non covalent interactions (Figure 2b).



3 contains an adduct having 3,5-difluorophenylboronic acid and PzH with one free PzH in lattice (Figure 3a). In this structure, B(1) has tetrahedral geometry (Sp3), while B(2)   and   B(3)   both  atoms   present   trigonal    planar geometry (Sp2). The B(1)-O(1) (1.447(5) Å) and B(1)-O(2) (1.447(4) Å) bond distances are greater than the B(2)-O(2) (1.346(6) Å), B(2)-O(3) (1.378(6) Å), B(3)-O(1) (1.349(5) Å), B(3)-O(3) (1.381(4) Å). B(1)-C(1) bond length is 1.608(6) Å, which is slightly higher than the B(2)-C(7) (1.566(5) Å) and B(3)-C(13) (1.549(5) Å). B(1)-N(1) bond distance is 1.632(5) Å, which is higher than 1 but less than 2 (Appendix Table S1). The packing of crystal shows that the molecular units are interconnected to each other via various intermolecular hydrogen bond interactions, that is, C20-H20···B1, 3.403(7) Å;  C4-H4···F3, 2.850(1) Å; C6-H6···F1, 2.685(6) Å; C16-H16···F2, 2.470(6) Å; C19-H19···F6, 2.728(4) Å; C22-H22···F3, 2.558(7) Å (Appendix Figures S3 and S4(a)). On the other side, the free PzH in lattice also shows the N4-H4A···F5, 2.337(7) Å and C22-H22···F2, 2.722(9) Å noncovalent interactions with adducts (Appendix Figure S4(b)). The C-H···B bond angle in 3 is 124.0° which are lesser than the C-H···B bond angles of 1 and greater than 2. All these interactions design a three dimensional ladder like framework (Figure 3b).



The  molecular  structure  of   4   has   same   structural dimension and geometry as 2, presented in Figure 4a. The B(1)-O(2) (1.438(18) Å), B(1)-O(3) (1.456(17) Å), B(2)-O(1) (1.461(18) Å) and B(2)-(O(3) (1.407(19) Å) bond distances are much greater than B(3)-O(1) (1.380(17) Å) and B(3)-O(2) (1.363(19) Å). Similarly, the B(1)-C(1) (1.607(20) Å) and B(2)- C(7) (1.619(17) Å) bond lengths are higher than that of B(3)-C(13) (1.569(20) Å). The B(1)-N(1) and B(2)-N(2) bond lengths are 1.644(25) and 1.658(28) Å, respectively, which are greater than that of 2. The X-ray crystal structural analysis shows that the molecule is intermolecular hydrogen bonded to adjacent molecules through C26-H26C···B3, 3.469 (60) Å; C34-H34B···B3, 3.213 (40) Å; C34-H34C···B3, 3.327(50) Å; C8-H8···F6, 2.731 (26) Å; C10-H10···F1, 2.818 (18) Å; C12-H12···F2, 2.565 (26) Å; C27-H27C···F4, 2.804 (25) Å; C28-H28B···F6, 2.832 (32) Å; C33-H33B···F5, 2.661 (36) Å and C37-H37A···F3, 2.662 (33) Å interactions (Appendix Figures S5 and S6). C-H···B bond angles in 4 are 125.3, 113.6 and 105.6° which are nearly matched with 2 and 3, but lesser than the C-H···B bond angles of 1. Three dimensional perspective view is created by involving all type of intermolecular interactions (Figure 4b).




Thermal study

All adducts 1-4 are stable at room temperature and their TGA plots are given in Figure 5. 1 and 2 show the one step decomposition. Adduct 1 decomposes completely in the temperature range of 169-236°C (~ 92.7% mass loss), while 2 shows the 91.2% mass loss in the temperature range of 221-377°C. 3 decompose in two steps. In the first step free PzH releases between 103-123°C temperature range with 11.4% mass loss, while the second step corresponds to the removal of adduct in the 127-223°C temperature range with 76.7% mass loss. 4 follows the same pattern as 2 with 93.4% weight loss in the temperature range of 305-389°C.



This study has synthesized and structurally characterized four new boroxine adducts with different stoichiometric compositions (1:1, and 1:2) having phenyl boronic acid, 3,5-difluorophenylboronic acid, 1H-pyrazole and 3-tert-butyl-5-isopropyl pyrazole. The X-ray crystal structure studies conclude that on increasing the substituents of phenyl boronic acids and pyrazoles, the stoichiometry, number of non-covalent interactions varies from 1 to 4, and the C-H···B bond angles of 2-4 are lesser than 1. The molecules are intermolecularly hydrogen bonded to each other through various noncovalent interactions, and gives two/three dimensional frameworks. From this it is clear that the packing changes with the substitution in aryl boronic acids, pyrazoles, and diversity in adduct stoichiometry. Thermal study shows that all adducts are stable at room temperature and decompose at high temperature.


The authors have not declared any conflict of interests.


Author gratefully acknowledges Iran University of Science and Technology for financial assistance and instrumental facilities.


The crystallographic data are available free of charge at [email protected] or Bond lengths and bond angles tables (Table S1, S2 and S3), hydrogen bond interaction Figures (Figure S1-S6) are also available.


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Preparation, structural and thermal studies of boroxine adducts having aryl boronic acids and pyrazoles