Highly stable naphthalene core based novel cleft-shaped strain molecule: influence of intermolecular H-bonding architectures

Abstract

The significance of intermolecular classical and non-classical H-bonding interactions in the stabilization of a naphthalene core based conformationally rigid cleft-shaped 1,5-dioxocin (BNAP) is presented here. The importance of H-bonding interactions to account for the unusual stability of a catalytically important novel molecule is reported for the first time. In addition to strong CH⋯π interactions, the formation of the unique intermolecular seven-membered H-bonded ring in the crystalline state through classical and non-classical H-bonding interaction was found to provide the unusual stability. This supramolecular structure was also found to impart stability in the presence of a strong acid as evident from the detailed UV-Visible spectroscopic studies. In addition to DFT calculations, the Hirshfeld surfaces, mapped with d_norm, and 2D fingerprint plots, support the existence of these classical and non-classical H-bonding and CH⋯π interactions. Furthermore, BNAP shows its remarkable catalytic activity for the Knoevenagel condensation reaction.

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Introduction

Dithiocin is a new and interesting class of conformationally rigid, cleft-shaped molecule. This class of molecule usually has two aromatic rings interlocked in an almost perpendicular fashion. Though interesting, these molecules are scantly reported in the literature primarily because of their inflexible synthetic protocol and instability.¹ Toste and co-workers were the first to report the synthetic protocol for Dithiocin derivatives.² Such well-defined cleft molecules have potential applications in molecular recognition, self-assembly and catalysis.³ This type of molecule is also subjected to clinical research as DNA probes⁴ due to their different modes of interaction with DNA and for their chiral properties.

Studies on the interactions in this class of cleft-shaped molecules have generated interest in recent times as evident from the detailed X-ray crystallographic studies on Trölger's base derivatives⁵ and complementary functionalized chiral carbocyclic cleft molecules.⁶ H-bonding (X–H⋯A) plays an important role in the chemistry⁷ and biology⁸ of various chemical and biological processes.^9,10 Exemplified in water networks¹¹ and peptide interactions,¹² classical H-bonding involves highly polar donors in the presence of strongly electronegative acceptors (X = A = N, O, F). Many detailed crystallographic analyses of both inorganic and organic systems have revealed close CH⋯O/N contacts, showing the stabilization afforded by “non-classical” H-bonds (NCHBs).¹³ NCHBs are often found in biological systems, an example being the thiamine–adenine base pair interaction in RNA.¹⁴ These NCHBs, while weaker than classical H-bonds, are still found to provide enough thermal stabilization to render complete control of the selectivity in chemical reactions.^15–17

In addition, CH⋯π interactions are another important non-covalent interaction present in biomolecules as well as materials.^18–20 This specific interaction also plays a major role in biomolecular recognition processes.^19–22 Quantitative knowledge of this interaction is very much important for understanding, as well as building the architecture of self-assembly in supra-molecules. These interactions can generally be described as the attractive molecular force that occurs between polarized CH fragments and aromatic rings. According to a gas phase theoretical study, a major dispersive component, together with a smaller, but still significant, electrostatic contribution (which increases with the polarization of the CH groups involved) determines the stability of the resulting complexes.²³ However, the structural determinants and driving forces behind CH⋯π bonds have proved more difficult to dissect and therefore have remained poorly understood. Recently, Das et al. has made a significant contribution to understanding the role of CH⋯π interactions in the formation of homo and hetero dimers in the gas phase.²⁴

In addition, the supramolecular structures of biomolecules as well as their important biological recognition processes are governed by a subtle balance among the various weak non-covalent interactions. These non-covalent interactions, which are also the backbones of the structures of the materials, generally ranging from conventional strong hydrogen bonding (OH⋯O and NH⋯O) to unconventional weak H-bonding (CH⋯O and CH⋯N). Undoubtedly, these intermolecular interactions, which govern the biological recognition processes, as well as the supramolecular structures in biology and materials, are well recognized by the scientific community for many decades.

In this study, the unusual stability of a conformationally rigid, cleft-shaped naphthalene moiety based stained molecule (BNAP) has been presented for the first time (Chart 1). The classical and non-classical H-bonding interactions along with CH⋯π interactions were found to hold the key for the stability of the compound.

Chart 1 Schematic of the chemical structure of BNAP.

Experimental section

Chemicals

2-Hydroxynaphthaldehyde, DDQ (dichloro-dicyanoquinone) and β-naphthalene were purchased from Sigma-Aldrich and used as received. The solvents were used after standard purification.²⁵

Synthetic procedure

2-Hydroxynaphthaldehyde (0.50 g 2.89 mmol) dissolved in absolute ethanol (20 mL) was treated with ammonium acetate (0.25 g 3.24 mmol) and a catalytic amount of β-naphthol and DDQ (dichlorodicyanoquinone) providing a red colour solution. The solution was refluxed for 3.5 h and cooled to room temperature, resulting in the formation of a white precipitate. The precipitate was washed several times with ethanol (3 × 10 mL) and recrystallized from acetonitrile.

Characterization of BNAP. State: solid; colour: white; yield: 90%; melting point: 235.0 °C; CHN analysis: C, 81.10% (81.21%); H, 4.50% (4.65%); N, 4.20% (4.30%); FTIR (KBr, cm⁻¹): 3333.6, 2924.8, 2853.3, 1624.8, 1468.1, 1232.6; ¹H-NMR (300 MHz, CDCl₃): δ (ppm) 8.16 (d, J = 7.8 Hz, 2H), 7.72–7.65 (m, 6H), 7.35 (s, 3H), 7.02 (d, J = 8.4 Hz, 2H), 6.60 (s, 2H), 3.16 (s, 1H); ¹³C-NMR (75 MHz, CDCl₃): δ (ppm) 155.8; 132.4, 130.6, 129.9, 127.6, 124.9, 118.2, 114.0, 95.6.

Spectroscopic measurements

A UV-Visible spectrophotometer (CARY 100 BIO in the range of 200–800 nm) was used for electronic absorption spectroscopy measurements, which has photometric linearity till an absorbance of 3.5 and wavelength resolution of 0.2 nm. A Shimadzu FTIR-8900 spectrophotometer was used for obtaining the FTIR spectra in the 4000–400 cm⁻¹ region. The ¹H and ¹³C-NMR spectra were obtained on NMR (JEOL-300L) spectrometers.

TGA was performed using a Perkin-Elmer STA 6000 instrument under a nitrogen atmosphere at the heating rate of 10 °C min⁻¹. DSC was carried out under a nitrogen atmosphere using a Mettler STAR SW 10.00 instrument (10 °C min⁻¹).

The single crystal X-ray diffraction data were collected on an OXFORD X Caliber EoS diffractometer using graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å) at 298 K. The data were reduced using the CrysAlisPro software provided with the instrument and a multi-scan absorption correction were also performed using the same programme.²⁶ The structures were solved by direct methods and refined by full matrix least squares on F² using SHELX-2013. The drawings were made using ORTEP-III and Mercury.²⁷

Raman spectra were obtained with a home-made Raman microscopic setup using a CW 633 nm He–Ne laser as the excitation source. Details of the experimental setup are described elsewhere.²⁸ Raman spectra were obtained with a low laser power (∼1.2 mW) with 10 s exposure.

Quantum chemical calculations

DFT calculations with a hybrid functional B3LYP (Becke's three parameter hybrid functional using the LYP correlation functional) at the 6-311G (d, p) basis set were performed with the Gaussian 09W software package.²⁹ The electronic absorption spectra were obtained using time-dependent density functional theory (TD-DFT) method in the gas phase. Natural bond orbital (NBO) calculations and the molecular electrostatic potential (MEP) were also calculated to understand the electron density and strength of the bonding interactions present in the molecule.

Results and discussion

Initially, the thermal stability of the newly synthesized BNAP, which is structurally similar to dithiocin, has been investigated. A detailed analysis of the thermal properties helps us to understand the stability and basic structural information of the molecule and its interactions therein.

Thermal studies: TGA and DSC

The thermal stability of BNAP was investigated by performing thermal gravimetric analysis (TGA). No weight loss was observed up to ∼225 °C in the TGA. However, at higher temperature, three types of weight loss were observed as can be observed in Fig. 1a (DTA: the differential thermal analysis result is given in ESI-Fig. 1†). The first step of weight loss was ∼22% and occurred over the temperature range from 225 °C to 250 °C. The second step of weight loss was ∼32% and occurred from 251 °C to 377 °C. The final step of weight loss was ∼46% and started from 378 °C.

Fig. 1 (a) TGA plot and (b): DSC plot of BNAP.

The melting point and phase behaviour of BNAP was monitored in DSC (Fig. 1b). The endothermic phase transition at 152.5 °C was due to a solid–solid phase transition.³⁰ The exothermic sharp solid to liquid (representing melting point) phase transition was observed at 235.0 °C. These thermal plots show that BNAP was highly stable even at high temperatures.

Structural investigation of the crystalline state: a SCXRD study

Because a suitable single crystal of BNAP could be obtained from acetonitrile under slow evaporation conditions at 20 °C, detailed single crystal X-ray diffraction (SCXRD) studies have been performed to understand the molecular structure and interactions in the solid state. BNAP was found to be crystallized in the monoclinic system with P̄_121/c1 space group (Fig. 2).

Fig. 2 The ORTEP diagram of BNAP (CCDC no. 910216).

Interestingly, the classical and non-classical H-bonding interactions were observed in the crystallographic study. These two types of H-bonding interactions were found to be important to form a unique intermolecular seven-membered H-bonding system, as shown in Fig. 3. The distances of the classical NH⋯O and non-classical CH⋯O H-bonding interaction sites were 2.33 Å and 2.62 Å, respectively (Fig. 3). These classical (NH⋯O) and non-classical (CH⋯O) H-bonding interactions play a vital role in constructing the unique homo trimer molecular system, as shown in Fig. 4.

Fig. 3 Formation of the cyclic seven-membered ring through H-bonding interactions.

Fig. 4 Trimer formation via cyclic seven-membered H-bonding interactions. For clarity, part of the naphthalene ring and hydrogen atoms are omitted.

Homo trimer formation via classical (NH⋯O) and non-classical (CH⋯O) H-bonding interactions is rare. In particular, this seven-membered ring formation via H-bonding architecture resulting in a trimer is reported for the first time for this type of molecular system.

In addition, a strong aromatic CH⋯π (2.63 Å) interaction was also observed, as shown in Fig. 5. These three different interactions constrictively make the unique close packing in the crystalline state (Fig. 5). It appears that the aromatic CH⋯π interaction (2.63 Å) was responsible for constructing the 2D layered type supra-molecular architecture with the help of the seven membered H-bonding interaction system (classical NH⋯O and non-classical CH⋯O H-bonding interactions, Fig. 3) and is presented in Fig. 5.

Fig. 5 The crystal packing diagram of BNAP showing the constructive role of CH⋯π interactions in the formation of the 2D supramolecular structure in the crystalline state.

FTIR and Raman spectroscopy

Vibrational spectroscopic investigations on BNAP have been performed and the different modes were assigned from the FTIR and Raman spectra, as depicted in Fig. 6, in the region of interest (2700–3500 cm⁻¹). DFT calculated the Raman and FTIR spectra of BNAP, which match well with those found experimentally, as shown in ESI-Fig. 2 and 3.† This close matching helped us to assign the bands in the experimental spectra. N–H, aromatic C–H and aliphatic C–H vibrational modes were observed in the 2800 cm⁻¹ to 3400 cm⁻¹ region.³¹ The symmetric N–H vibration mode was found to appear at 3332 cm⁻¹ in FTIR and at 3328 cm⁻¹ in the Raman spectrum. The symmetric and asymmetric aromatic C–H vibrational modes for the naphthalene ring are very close to each other and appeared as broad bands at ∼3069 and ∼3063 cm⁻¹ in FTIR and the Raman spectrum, respectively. Two different symmetric aliphatic C–H vibration modes appear at 2924 and 2853 cm⁻¹ in FTIR and at 3009 and 2989 cm⁻¹ in the Raman spectrum.

Fig. 6 The experimental FTIR (top scale) and Raman (bottom scale) spectra of BNAP (2700–3500 cm⁻¹ region).

Electronic properties: UV-Vis absorption and fluorescence spectra

The ground state electronic and fluorescence properties of BNAP have been investigated using UV-Vis absorption and steady state fluorescence spectroscopy in acetonitrile (ACN). The UV-Vis and steady state fluorescence spectra are presented in Fig. 7.

Fig. 7 The UV-Vis and fluorescence spectra of BNAP in ACN.

The UV-Vis spectra were measured and compared with 2-hydroxynaphthaldehyde in order to assign the peaks (ESI-Fig. 4†). It was found that two district low molar extinction coefficient absorption bands having vibrational features appeared at 277 nm and 335 nm. The electronic transition at 335 nm was due to the n → π* transition with a lower molar extinction coefficient (ε_335nm = ∼7.5 × 10³ dm³ mol⁻¹ cm⁻¹). The π → π* transition was observed at 277 nm with a vibrational feature (ε_277nm = 2.1 × 10⁴ dm³ mol⁻¹ cm⁻¹). Due to the presence of oxygen atoms, the electronic energy levels were shifted towards higher energy levels (shorter wavelength) when compared with the reactant (2-hydroxynaphthaldehyde).

Further electronic properties in the excited state of BNAP were investigated using steady state fluorescence spectroscopy. The fluorescence spectra were obtained with excitation at 300 nm in ACN and are presented in Fig. 7. The sharp emission band was observed with λ_emission at 356 nm.

The stability of BNAP in acid and base was also tested using UV-Vis absorption spectroscopy in ACN. In the presence of a strong acid (∼10⁻³ M HCl), no change was observed in the UV-Vis absorption spectrum (vide ESI Fig. 5†), indicating that this secondary amine (BNAP) was quite stable. This might be due to the unique intermolecular H-bonding network as already discussed (vide Structural investigation of the crystalline state: a SCXRD study). However, in the presence of a strong base (10⁻⁴ M KOH), a new species was found to form with a charge transfer band at 614 nm. Interestingly, this charge transfer band was found to survive for less than 25 min (as shown in ESI Fig. 6†).

Quantum chemical calculations: analysis of the electronic transition and charge density

Quantum chemical calculations have been performed to gain a better understanding of the electronic properties of BNAP. The optimized structure of BNAP and electronic transition level diagram as obtained by TD-DFT and FMO calculations are shown in Fig. 8a and b.

Fig. 8 (a) The optimized structure of BNAP. (b) The electronic transition level diagram of BANAP.

Experimentally the UV-Vis absorption band observed at 332 nm (ca. 317 nm, f = 0.08) corresponded to the HOMO to LUMO electronic transition. The shorter wavelength absorption band at 289 nm (ca. 283 nm, f = 0.03) corresponds to the HOMO (−1) to LUMO (+1) electronic transition.

Furthermore, NBO calculations, which help us to understand the delocalization of electron density from the occupied Lewis-type (donor) NBOs to unoccupied non-Lewis type (acceptor) NBOs, were also performed to understand the bonding character in BNAP. The stabilization of the orbital interactions is proportional to the energy difference between the interacting orbitals. Therefore, the interaction having the strongest stabilization occurs between the effective donors and effective acceptors. This bonding–antibonding interaction can be quantitatively described in terms of the NBO approach that is expressed by means of second order perturbation interaction energy E⁽²⁾. This energy represents an estimate of the off-diagonal NBO Fock matrix element. The stabilization energy E⁽²⁾ associated with i_donor − j_acceptor delocalization is estimated from the second order perturbation approach as given below:

where q_i is the donor orbital occupancy, e_i and e_j are the diagonal elements (orbital energies) and F(i, j) is the off-diagonal Fock matrix element. The Lewis and non-Lewis orbitals obtained by the valence hybrids calculated using NBO calculations and are reported in Table 1. From the NBO calculations, it was observed that the N–H σ bond has purely sp³ character having an electron density of 1.97 and charge distribution of 31.03% over the H atom and 68.97% over the N atom where the N–H σ* bond also has sp³ character having an electron density of only 0.01 and charge distribution of 68.97% over the H atom and 31.03% over the N atom.

Table 1 The Lewis and non-Lewis orbitals obtained by the valence hybrids from NBO calculations. The figure containing atomic number in molecule is presented in ESI-Fig. 7

Bond (A–B)	ED (e)	EDA (%)	EDB (%)	NBO	s (%)	p (%)
σ (H40–N6)	1.97	31.03	68.97	0.5570(s)H^+	99.94	0.06
				0.8305(sp^3.35)N	22.96	76.96
σ (C8–N6)	1.98	39.94	60.06	0.6320(sp^3.08)C^+	24.47	75.40
				0.7750(sp^2.37)N	29.62	70.31
σ (C23–O1)	1.98	30.93	69.07	0.5561(sp^4.17)C^+	19.30	80.42
				0.8311(sp^2.67)O	27.22	72.72
σ* (H40–N6)	0.01	68.97	31.03	−0.8305(s)H^+	99.94	0.06
				0.5570(sp^3.35)N	22.96	76.96
σ* (C8–N6)	0.03	60.06	39.94	−0.7750(sp^3.08)C^+	24.47	75.40
				0.6320(sp^2.37)N	29.62	70.31
σ* (C23–O1)	0.09	69.07	30.93	−0.8311(sp^4.17)C^+	19.30	80.42
				0.5561(sp^2.67)O	27.22	72.72

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The different types of donor–acceptor interactions and their stabilization energies were determined by second order perturbation analysis, as presented in Table 2. The hyperconjugative interactions were formed by the orbital overlap between the bonding orbital and anti-bonding orbital, which results in charge transfer that stabilizes the system. These interactions can be identified by finding the increase in electron density in the anti-bonding orbital. A very strong interaction has been observed between the lone pair LP N6 and σ* O₁–C₂₃ with an energy of 14.5 kcal mol⁻¹. The high value of energy for the interaction between LP N₆ and σ* O₁–C₂₃ indicates that the delocalization was more probable in the amide group supporting the SCXRD data.

Table 2 The second-order perturbation energies E^(2)a (donor → acceptor)

Donor NBO electron	Acceptor NBO (j) electron	E^(2) (kcal mol^−1)	E(i) − E(j) (a.u.)	F(i, j) (a.u.)
a E^(2) is the energy of the hyperconjugative interactions in kcal mol^−1.
LP N6	σ* O1–C23	14.50	0.56	0.08
LP N6	σ* O3–C8	5.32	0.58	0.05
LP O1	σ* N6–C23	2.06	0.93	0.03
LP O3	σ* N6–O8	0.84	0.91	0.02

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The molecular electrostatic potential V(r) was created in the space around a molecule by its nuclei and electrons and is well established as a guide to the molecular reactive behaviour. It is defined by:

where Z_A is the charge of the nucleus A, located at R_A, ρ(r′) is the electronic density function of the molecule and r′ is the dummy integration variable.³² The molecular electrostatic potential (MEP) map is related to the electron density and is a very useful descriptor in determining the sites for electrophilic and nucleophilic reactions, as well as hydrogen bonding interactions. Being a real physical property, V(r) can be determined experimentally using diffraction or computational methods.³³

The delocalization of electrons in the molecule can be established by MEP calculations and can also verify the experimental results obtained for the molecules. MEP simply measures how attractive (blue) or repulsive (red) any region of the molecule is to a proton placed at any point surrounding the molecule, as shown in Fig. 9.

Fig. 9 The 3D plots of the MEP map for BNAP. The yellow region indicates that the region is electron rich and the other region, which is blue indicates that the region is electron deficient.

It is observed in Fig. 9 that the colour of the oxygen atom part containing the naphthalene moiety is deeply yellow and the N–H region is deep blue in colour.

Hirshfeld surface analysis

The molecular Hirshfeld surfaces in the crystal structure are constructed on the basis of the electron distribution calculated as the sum of spherical atom electron densities. For a given crystal structure and set of spherical atomic electron densities, the Hirshfeld surface is unique.³⁴ The normalized contact distance (d_norm) based on both d_e and d_i, and the vdW radii of the atom, given by eqn (1) enables identification of the regions of particular importance to the intermolecular interactions.³⁵ The value of d_norm is negative or positive when the intermolecular contacts are shorter or longer than the vdW separations, respectively. The combination of d_e and d_i in the form of a 2D fingerprint plot³⁶ provides summary of the intermolecular contacts in the crystal.³⁵ The Hirshfeld surfaces are mapped with d_norm and the 2D fingerprint plots presented in this paper were generated using CrystalExplorer 2.1.

The intermolecular interactions of the title compound were quantified using Hirshfeld surface analysis. This approach is a graphical tool used for the visualization and understanding of intermolecular interactions.³⁷ Herein, we estimate the intermolecular contacts, which are shown in Fig. 10a and b. The contribution of inter-contacts to the Hirshfeld surfaces are H⋯H (44.5%), N⋯H (2.4%), C⋯H (39.3%), O⋯H (9.1%), and others (C⋯C, N⋯O, C⋯O; 4.7%). These inter-contacts are highlighted by conventional mapping of d_norm on the molecular Hirshfeld surfaces are shown in Fig. 10a and b. The red spots over the surface indicate the inter-contacts involved in the hydrogen bonds. Furthermore, inter-contacts were plotted with fingerprint plots (Fig. 11a–d). H⋯H inter-contacts (Fig. 11a) shows large surfaces, whereas the O⋯H plot (Fig. 11d) shows the presence of O⋯H contact with the two characteristic wings. The N⋯H contact plot shows two narrow pointed wings that provide evidence for the N–H⋯O classical hydrogen bonds and the N⋯H plot reveals the intermolecular hydrogen bonding interactions shown in Fig. 11c. The inter-contacts O⋯H showing two narrow pointed wings provide evidence for CH⋯O non-classical hydrogen bonds and the C⋯H plot reveals the information of intermolecular H-bonds.

Fig. 10 (a) The electrostatic potential mapped on the Hirshfeld surface (different orientation) with ±0.15 au. (b) d_norm mapped on the Hirshfeld surface used to visualize the intercontacts.

Fig. 11 Fingerprint plots: full and resolved into the different intermolecular interactions showing the percentage of contact contribution to the total Hirshfeld surface area.

Application as a catalyst in the Knoevenagel condensation reaction

As it is a strained secondary amine, it is expected that the basicity of BNAP will be different from normal secondary amines such as piperidine. For the first time this unique strained molecule (BNAP) was used as a catalyst for the Knoevenagel condensation reaction between substituted benzaldehydes and active methylene derivatives in ethanol at room temperature for a stipulated time (Scheme 1 and Table 3).

Scheme 1 Synthesis of various derivatives following Knoevenagel condensation method using BNAO as catalyst.

Table 3 The derivatives synthesized using the Knoevenagel condensation reaction. The reaction time and yields are also mentioned (yield in % and time in minutes)

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This eco-friendly synthetic protocol for the Knoevenagel condensation was used to synthesize a series of important cyano group containing synthetic precursors, as shown in Table 3 that are used for the synthesis of biologically active molecules at room temperature using a minimum amount of BNAP as the catalyst (∼10 mol%) without the need for a chromatographic separation technique.

Conclusions

In conclusion, the synthesis and structural studies of a new conformationally rigid, cleft-shaped 1,5-dioxocin were presented herein. The unusual stability of this strained molecule was analysed based on the existence of classical and non-classical H-bonding interactions as well as CH⋯π interactions. A stable intermolecular seven-membered H-bonding ring was found to form via classical NH⋯O (2.33 Å) and strong non-classical CH⋯O (2.62 Å) H-bonding interactions. In addition, formation of the trimer occurred via classical NH⋯O and strong non-classical CH⋯O hydrogen bonding interactions, while the CH⋯π (2.63 Å) interactions helped it form a 2D supra-molecular structure in its crystal packing. We concluded that these interactions are responsible for the high thermal stability. The Hirshfeld surfaces provide support for the existence of classical and non-classical H-bonding interactions and formation of the trimer. The UV-Vis absorption spectrum and its detailed theoretical analysis were also presented. The remarkable catalytic activity of BNAP was observed in the Knoevenagel condensation reaction that was used to synthesize important cyano group containing synthetic precursor, which can be used for the synthesis of biologically active molecules at room temperature using a minimum amount of catalyst without the need for a chromatographic separation technique.

General procedure for the Knoevenagel reaction

The substituted aromatic aldehyde (1, 1 mmol), active methylene compound (2, 1.01 mmol) and catalyst (BNAP, 10 mol%) were taken in a round-bottom flask and stirred at room temperature for a stipulated period of time till the completion of the reaction (as monitored by TLC; the time and yield are mentioned in Table 3). After completion of the reaction, the resulting precipitate was filtered and washed with water (3 × 10 mL) followed by ethanol (3 × 10 mL) and dried under high vacuum to afford the NMR pure product. All the compounds are characterized by ¹H and ¹³C NMR spectroscopy.

Acknowledgements

We acknowledge the CSIR New Delhi (Project No. 01(2792)/14) for research grants and Dr N. V. Hemanth and Prof. Shinsuke Shigeto of NCTU, Taiwan for recording the Raman spectrum. Department of Chemistry, BHU is gratefully acknowledged for providing instrument and infrastructure facilities.

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedure, separation techniques, electronic spectra and NMR spectra of all compounds. CCDC 910216. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra06855c

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