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

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.

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 (R 3 B 3 O 3 , 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., 2011Sheepwash et al., , 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.

MATERIALS AND METHODS
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 1 H-NMR spectra were recorded on Bruker ALPHA FT-IR and Bruker AM 400 MHz spectrometers, respectively.Thermal analysis was performed on PerkinElmer thermogravimetric analyzer.

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 graphitemonochromated 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(Sheldrick , 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.4) have been prepared by using phenylboronic acid, 3,5-difluorophenylboronic acid and corresponding pyrazoles (PzH/Pz tBu,iPr H) 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 Pz tBu,iPr H, 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 ) 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 1 H-NMR spectra, showing the prominent downfield shift in each case with respect to free compounds as shown in Table S3.

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.

Conclusions
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-tertbutyl-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.

APPENDIX Preparation, structural and thermal studies of boroxine adducts having aryl boronic acids and pyrazoles
Scheme 1.General method for the synthesis of 1-4.

Table S3 .
Comparison of 1 H NMR spectra.