Introduction

Thiocyanuric acid (TCUA) is a tribasic, weak acid with acid first, second, and third dissociation constants of 1.99 × 10−6, 3.98 × 10−9, and 3.98 × 10−12, respectively. This polyprotic nature of TCUA permits different degree of deprotonation, and hence, various forms of thiolate and amide ligands for metals and TCUA salts. It is a planar, highly symmetric molecule (D 3h point group) and exists as two tautomers: the trithiol form (I) and the trithione form (II), as illustrated in Eq. (1) [7,8,9]:

(1)

Each tautomer has three hydrogen bond donor and three hydrogen acceptors at its nitrogen and sulfur atoms. This unique feature enables TCUA to construct self-assembled, supramolecular 1-D chain and 2-D porous structures based on hydrogen bonding [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]. The trithiol form (I) is predominant in metal complexes [8–11, 15], whereas the trithione form(II) has been observed to exist in the mono-lithiated hexamethylphosphoramide complex and in the ammonium salts and solvent adducts of TCUA [7, 8, 12,13,14, 16,17,18, 22]. For instance, the triethylammonium salt of TCUA has two polymorphs with 1-D chain structures based on N–H–S and N–H–N hydrogen bonds [16, 23]. On the other hand, the tripropylammonium salt of TCUA has a 2-D structure stabilized by N–H–S and N–H–N hydrogen bonds. The adduct between melamine and TCUA adopts 2-D porous sheets based on planar hexamers connected by N–H–S and N–H–N hydrogen bonds. The melamine/TCUA sheets are stacked to form a 3-D channeled, layered structure [14]. The adduct of 4,4′-bipyridine with TCUA is also constructed by 2-D porous sheets layered into 3-D channeled network, where N–H–S and N–H–N hydrogen bonds are responsible for the self-assembly of 4,4′-bipyridine and TCUA into 2-D sheets [12]. The acetone-solvate of TCUA, [(TCUA)2 (Me2CO)], is made of 2-D, pseudo-hexagonal planar net, generated by three pairs of N–H–S hydrogen bonds without involvement of acetone in hydrogen bonding. The acetone molecules are hydrogen bonded to hexagons by N–H–O bonds. The acetone molecules also act as pillars between the hexagon layers and result in 3-D channeled network, where the channel cavity are capped by acetone methyl groups [7, 12]. On the other hand, the methanol solvate of TCUA, [(TCUA)3 (MeOH)], is composed of 1-D, zigzag chains of TCUA via two pairs of N–H–S hydrogen bonds. The TCUA chains are crossed by C–H–S and S–S interactions and are, at the same time, separated by methanol molecules which are also hydrogen bonded with each other and hydrogen bonded to TCUA molecules via N–H–O bonds, generating a 3-D hydrogen-bonded ntework [7].

We report in this paper the crystal structure of N, N-dimethylformamide (DMF) solvate of TCUA, which was discovered accidently during our attempt to prepare covalent organic framework (COF) of TCUA through the oxidation of thiol functional groups to disulfide functional groups in DMF using a mild oxidant of sodium nitrate at room temperature.

Experimental

Materials

Thiocyanuricacid (TCUA, C3H3N3S3, > 98.0%, TCI), sodium nitrate (NaNO3, ReagentPlus, ≥ 99.0%, Sigma-Aldrich), N,N-dimethylformamide (DMF, HPLC, ≥ 99.9%, Sigma-Aldrich) were commercially available and were used without further purification. Ultrapure deionized water (18.2 MΩ cm) was obtained from a Milli-Q water purification system (Millipore, Billerica, MA, USA).

Synthesis procedure

The synthesis of DMF solvate of TCUA (RA5403) was carried out by combining a solution of TCUA (21.27 mg) in DMF (2.0 mL) with an aqueous solution of NaNO3 (2.0 mL, 1.0 M). The reaction solution was left at room temperature for 3 days, whereupon yellow crystals of RA5403 were obtained. The resulting crystals were filtered off and were washed three times with DMF/water (1:1 v/v). The needle-shaped crystals were obtained in 91% yield.

Analytical and physical characterizations

Proton (600 MHz) and carbon (150 MHz) nuclear magnetic resonance spectra were taken using JEOL ECA-600 Spectrometer. All chemical shifts were referenced to the residual solvent protons/carbons of the solvent indicated. 1H NMR (600 MHz, DMSO-6, δ) 7.94 (s, 1H), 2.88 (s, 3H), 2.72 (d, 3H). 13C NMR (150 MHz, DMSO-6, δ) 171.87, 162.32, 35.81, 30.79.

X-ray crystallography

Data were collected on a Bruker APEX-II D8 Venture area diffractometer, equipped with graphite monochromatic Mo K α radiation, λ = 0.71073 Å at 100 (2) K. Cell refinement and data reduction were carried out by Bruker SAINT. SHELXT [13, 14] was used to solve structure. The final refinement was carried out by full-matrix least-squares techniques with anisotropic thermal data for nonhydrogen atoms on F. CCDC 1455209 contains the supplementary crystallographic data for this compound, which can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Discussion

Thiocyanuric acid (TCUA) is very well known to form polymers through extensive hydrogen bonds [7,8,9,10,11,12,13,14,15,16,17,18,19,20,, 8, 16, 18, 24]. Here, we were able to prepare a layered, 3D-structure of TCUA with DMF, where the TCUA molecule N–H bonds act as H-bond donors and TCUA molecule C–S bonds and DMF molecule C=O bonds serve as H-bond acceptors. The crystal structure of DMF-solvate of TCUA, reported herein, revealed that TCUA adopts the trithion form (II, Eq. 1). The asymmetric unit of the DMF solvate of TCUA is based on a hydrogen bond (N–H–O) bridge connecting a TCUA molecule and a DFM molecule solvent, as shown below in Fig. 1. The crystallographic data and refinement information are summarized in Table 1. The selected bond lengths and bond angles are listed in Table 2. The asymmetric unit is containing of one independent molecule as shown in Fig. 1. All the bond lengths and angles are in normal ranges [25]. In the crystal packing, Figs. 2 and 3, molecules are linked via intermolecular hydrogen bonds (Table 3).

Fig. 1
figure 1

Asymmetric unit of DMF solvate of TCUA (viewed along a-axis); yellow: sulphur, red: oxygen; sky blue: nitrogen, grey: carbon, white: hydrogen, dashed line: H-bond

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Table 1 Experimental details
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Table 2 Selected geometric parameters (Å, °)
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Fig. 2
figure 2

The crystal structure packing of hydrogen-bonded TCUA ribbons which are hydrogen bonded to DMF molecules forming a 2-D sheets, where these sheets are turned form a layered, 3-D structure (viewed along c-axis); yellow: sulphur, red: oxygen; sky blue: nitrogen, grey: carbon, white: hydrogen, dashed lines: H-bonds

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Fig. 3
figure 3

The parallel TCUA ribbons in the different sheets held by π stacking (viewed along a-axis); yellow: sulphur, sky blue: nitrogen, grey: carbon, white: hydrogen, dashed lines: H-bonds

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Table 3 Hydrogen-bond geometry (Å, °)
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The TCUA molecules form a one-dimensional zigzag ribbons, where the TCUA molecules within each ribbon are bridged by N–H–S hydrogen bonds. The TCUA ribbons are separated by DMF molecules, which are in turn hydrogen bonded to TCUA molecules through N–H–O bridges in an alternative fashion between two TCUA ribbons, as shown in Fig. 2.

The TCUA ribbons along with their H-bonded DMF molecules form a two-dimensional sheet, as shown in Fig. 2. These sheets are in turn paralleled to other sheets through Van der Waals forces (π stacking), converting the 2-D sheets into a layered, three-dimensional structure. Figure 3 shows the TCUA ribbons which are paralleled in the different sheets.

The crystal structure pattern of our DMF solvate of TCUA is similar to those observed previously for triethylammonium thiocyanurate [9], methanol solvate of TCUA [7], tripropylammonium thiocyanurate, and the second polymorph of triethylammonium thiocyanurate [18].

The formation of the DMF solvate of TCUA was also confirmed by both liquid-state proton and carbon-13 NMR investigations. The proton NMR spectrum showed only DMF peaks at (DMSO-6, δ) 7.94, 2.88, and 2.72 which could be due to the hydrogen bond, N–H–O, formation between DMF and TCUA, reflecting the partial preservation of the DMF solvate of TCUA in the liquid state. On the other hand, carbon-13 NMR spectrum showed both TCUA and DMF carbons at (DMSO-6, δ) 171.87, 162.32, 35.81, 30.79.

Conclusion

We prepared readily a layered, 3-D DMF solvate of TCUA at room temperature in the presence of a mild oxidant of NaNO3. The unsuccessful attempt for synthesizing a COF via oxidizing the TCUA thiol functional groups into disulphide functional groups could be attributed to the thermodynamic preference of TCUA to adopt the trithione form; and the probability need for harsher conditions of temperature, pressure, and oxidation. The structure of DMF solvate of TCUA was studied by SXRD. The NMR studies confirmed the formation of this solvate (see the supporting info).