INTRODUCTION

Schiff bases and related N-ligands attract considerable interest and play an important role in the development of the chemistry of chelate systems as potential ligands towards a large number of metal ions^1–8. Growing interest in the synthesis of such a type of compounds are due to their potential application as catalyst in many organic transformations^9–12 and diverse useful properties, such as solvatochromism¹³, molecular switching^14,15, crystal engineering¹⁶, etc. Noncovalent interactions (hydrogen, aerogen, halogen, chalcogen, pnictogen, tetrel and icosagen bonds, as well as cation-π, anion-π, lone pair-π, π-π stacking, agostic, pseudo-agostic, anagostic, dispersion-driven, lipophilic, etc.) concern weak forces of attraction formed between different molecules (intermolecular) or fragments of the same molecule (intramolecular). While these weak interactions were firstly taken into consideration by van der Waals in 1873 ¹⁷, the understanding of their crucial role in synthesis, catalysis, crystal engineering, pharmaceutical design, molecular biology, molecular recognition, materials, etc. has been increasingly explored in the last few decades.

Herein we found strong charge assisted hydrogen bond and halogen bonding in (E)-5-phenyl-3-((4-(trifluoromethyl)benzylidene)amino)thiazolidin-2-iminium bromide (scheme 1).

Scheme 1 

EXPERIMENTAL

NMR spectra were recorded at room temperature on a Bruker Avance II + 300 (UltraShield™ Magnet) spectrometer operating at 300.130 and 75.468 MHz for proton and carbon-13, respectively. All NMR spectra are reported in parts per million (ppm, d) relative to tetramethylsilane (Me₄Si) for ¹H and ¹³C NMR spectra, with the residual solvent proton and carbon resonances used as internal standards. Coupling constants (J) are reported in Hertz (Hz), and integrations are reported as number of protons.

The following abbreviations are used to describe peak patterns: s = singlet, d = doublet, t = triplet, m = multiplet, br = broad. ¹H and ¹³C NMR chemical shift assignments are supported by data obtained from ¹H-¹H COSY, ¹H-¹³C HMQC, and ¹H-¹³C. Electrospray mass spectra (ESI-MS) were run with an ion-trap instrument (Varian 500-MS LC Ion Trap Mass Spectrometer) equipped with an electrospray ion source. For electrospray ionization, the drying gas and flow rate were optimized according to the particular sample with 35 p.s.i. nebulizer pressure. Scanning was performed from m/z 0 to 1100 in methanol solution. The compounds were observed in the positive mode (capillary voltage = 80-105 V).

For the molecular structure of title compound, H atoms were located in the difference Fourier map, refined with fixed individual displacement parameters, using a riding model with C—H distances of 0.93 Å (for aromatic rings), 0.92 Å; 0.96 Å (for CH₃, CH₂), with U(H) values of 1.2U_eq(C, N) (for CH in aromatic rings and -NH₂+), and 1.5U_eq(C, O) (for CH₃ and -OH). Solvent molecules were restrained using Rigid body (RIGU) restrains (O1S, C1S). trifluoromethyl moiety (-CF₃) was restrained using SADI. All sigma for 1-2 distances of 0.004 Å and sigma for 1-3 distances of 0.004 Å. Finally the several disordered -CF₃, was treated with FVAR and SUMP.

X-ray diffraction patterns of title compound were collected using a Bruker SMART APEX-II CCD area detector equipped with graphite-monochromated Mo-Κα radiation (λ = 0.71073 Å) at room temperature. The diffraction frames were integrated using the APEX3 package¹⁸. The structure of were solved by intrinsic phasing¹⁹ using the OLEX 2 program²⁰.

The structure was then refined with full-matrix least-square methods based on F² (SHELXL-2014)¹⁹. For C₁₄H₈BrCl₂FN₂, non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were included in their calculated positions, assigned fixed isotropic thermal parameters and constrained to ride on their parent atoms. A summary of the details about crystal data, collection parameters and refinement are documented in Table 1, and additional crystallographic details are in the CIF files. ORTEP views were drawn using OLEX2 software²⁰. CCDC 1912059 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.as.uk/data_request/cif.

To the solution of 1 mmol of 3-amino-5-phenylthiazolidin-2-iminium bromide in 20 mL ethanol was added 1 mmol of 4-(trifluoromethyl)benzaldehyde and refluxed for 2 hours. Then the reaction mixture was cooled down. Reaction products was precipitated from reaction mixture as a colorless single crystals, collected by filtration and washed with cold acetone.Yield 73%. Mp 231°C. Anal. Calcd. for C₁₇H₁₅BrF₃N₃S (Mr =430.29): C, 47.45; H, 3.51; N, 9.77. Found: C, 47.40; H, 3.48; N, 9.71 %. ¹H NMR (300MHz, DMSO-d₆): 4.58 (k, 1H, CH₂, ³J_H-H=6.6); 4,89 (t, 1H, CH₂, ³J_H-H=8.4); 5.60 (t, 1H, CH-Ar, ³J_H-H=7.5); 7.39-8.29 (m, 9H, 9Ar-H); 8.51 (s, 1H, CH=); 10.51 (s, 1H, NH=). ¹³C NMR(75MHz, DMSO-d₆): 45.45, 56.03, 125.74, 125.80, 127.86, 128.95, 129.15, 129.22, 130.72, 131.14, 136.85, 137.50, 149.54, 168.62. MS (ESI), m/z: 350.38 [C₁₇H₁₅F₃N₃S]⁺ and 79.88 Br⁻.

RESULTS AND DISCUSSION

The cation structure corresponds to the E isomer in the solid state. All the distances and angles are normal^21,22. The bond lengths range between measured C-C and C=N values for single and double bonds in C4-C5, C5-N1 and N1-N2 are slightly larger than the reported for similar organic compounds^23–25, depicted a potential electronic disconnection between the rings in the cation of the title compound.

Figure 1 ORTEP plot of the title compound. Thermal ellipsoid was drawn with 30% of probability. Some hydrogen atoms were omitted for sake. 

In the cation of the title salt, the central thiazolidine ring (S1/N2/C1-C3) adopts an envelope conformation with puckering parameters 0.300(6)Å, and φ(2)= 325,1(12)°. The dihedral angle between the mean plane of the central thiazolidine ring and the (4-trifluoromethyl)benzylidene ring (C5-C10) is 0.4(3) while this plane make a angle 10(2)° with the phenyl ring. The N2-N1-C4-C5 bridge that links the thiazolidine and (4-trifluoromethyl)benzylidene ring is 0.40 (3)° (see tables 3 and 4).

In the crystal N-H⋯Br hydrogen bonds link the components into a bi-dimensional network with the cations and anions stacked parallel to plane 101. The weak interactions are mainly constituted by H⋯F, Η⋯π and Η⋯Br. Moreover, In crystal structure it is found strong charge assisted hydrogen-halogen bonding and, intermolecular hydrogen-π bonding interaction along to [010] direction with 2.40 Å (Figure 2 and table 5)

Figure 2 Non-covalent interactions in the crystal structure of the title compound. 

A Hirshfeld surface analysis was conducted to verify the contributions of the different intermolecular interactions. This analysis was used to investigate the presence of hydrogen bonds and intermolecular interactions in the crystal structure. The Hirshfeld surface analysis ²⁶ was generated by CrystalExplorer 17.5 ²⁷ and comprised d_norm surface plots and 2D (two-dimensional) fingerprint plots ²⁸. The plots of the Hirshfeld surface confirms the presence of the non-covalent interaction described below (Figure 3.), taking account the several positional disorder in the molecular structure

Figure 3 The two-dimensional fingerprint plots of the title compound [d_e and d_i represent the distances from a point on the Hirshfeld surface to the nearest atoms outside (external) and inside (internal) the surface, respectively]. 

The weak intermolecular interactions are mainly constituted by H⋯F, Η⋯π and H⋯Br, with a contribution of 24.5, 19.3 and 6.8%, respectively. Where the reciprocal contacts appear as a wide stain for H⋯F, with d_e + d_i≃ 2.8 Å, for H⋯Br as a sharp spike with d_e + d_i≃ 2.3 Å and, π ⋯H as asymmetrical wings with d_e + d_i≃ 3.0. Å, according to the interaction depicted in figure 2. This type of weak interactions are also observed in isostructural compounds recently reported^23–25.

CONCLUSIONS.

In this study we offer the report of synthesis, characterization and structural studies of the title compounds, showing the E isomer in the solid state. The weaks intermolecular interactions, were successfully verified with Hirshfeld surface analyses, being mainly constituted by H⋯F, Η⋯π and H⋯Br, with a contribution of 24.5, 19.3 and 6.8%, respectively. The chemistry of this compound allow postulate as a good candidate for several applications such as potential biological, pharmacological and analytical applications, moreover heterocyclic amines are also widely used in the synthesis of Schiff bases, which provide different kinds of noncovalent interactions.