imidazole - CIQ - UAEM

Journal of Molecular Structure 1099 (2015) 126e134

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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc

Novel synthesis, structural analysis, photophysical properties and theoretical study of 2,4,5-tris(2-pyridyl)imidazole ez-Castro a, Jesús Baldenebro-Lo pez a, Daniel Glossman-Mitnik b, Alberto Ba € pfl c, Adriana Cruz-Enríquez a, Valentín Miranda-Soto d, Miguel Parra-Hake d, Herbert Ho J. Campos-Gaxiola a, * Jose noma de Sinaloa, Fuente de Poseido n y Prol. A. Flores S/N C.U. Los Mochis, Sinaloa, 81223, Mexico Facultad de Ingeniería Mochis, Universidad Auto n en Materiales Avanzados, S.C., Miguel de Cervantes 120, Complejo Industrial Chihuahua, Chihuahua, 31136, Mexico Centro de Investigacio sicas e Ingeniería, Universidad Auto noma de Morelos, Av. Universidad 1001, Cuernavaca, Centro de Investigaciones Químicas, Instituto de Ciencias Ba 62209, Mexico d n, Instituto Tecnolo gico de Tijuana, Apartado Postal 1166, Tijuana, B.C., 22000, Mexico Centro de Graduados e Investigacio a

b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 December 2014 Received in revised form 22 May 2015 Accepted 22 May 2015 Available online 17 June 2015

2,4,5-Tris(2-pyridyl)imidazole has been successfully synthetized by a novel synthetic route and fully characterized by FT-IR,UVeVis and fluorescence spectroscopy, one- and two-dimensional NMR spectroscopy (1H, 13C{1H} ATP, 1He1H COSY, NOESY 1He13C HSQC and HMBC) high-resolution, mass spectrometry (HR-FABþ), and single-crystal X-ray diffraction analysis. Additionally, the molecular geometry, vibrational frequencies and infrared intensities were calculated by density functional theory using the M06/6-31G(d) level of theory, showing good agreement with the experimental results. The title compound showed interesting photophysical properties, which were studied experimentally in solution and in the solid state by UVeVis and fluorescence spectroscopy, and theoretically using TD-DFT calculations. Natural and Mulliken atomic charges and the molecular electrostatic potential (MEP) have been mapped. © 2015 Elsevier B.V. All rights reserved.

Keywords: Imidazole derivative Polypyridyl compound Non-covalent interactions X-ray structure Photophysical properties Theoretical calculations

1. Introduction In the field of heterocyclic chemistry, imidazole comprises a five-membered N-containing aromatic ring structure with a variety of interesting properties. Imidazole-related drugs have excellent therapeutic properties, which has encouraged medicinal chemists to synthesize a large number of novel active pharmaceutical ingredients (APIs) [1]. In this context addition and substitution reactions of imidazole and its derivatives continue receiving considerable attention not only for the preparation of biologically active compounds [2], but also for the design of new materials [3]. Imidazoles are also useful ligands in coordination chemistry and constitute still today an important area of scientific investigation [4,5]. Pyridine-substituted imidazole-based ligands are of special interest for research in the field of crystal engineering, particularly in form of polypyridine-type tectons (building blocks), which can

* Corresponding author. E-mail address: [email protected] (J.J. Campos-Gaxiola). http://dx.doi.org/10.1016/j.molstruc.2015.05.055 0022-2860/© 2015 Elsevier B.V. All rights reserved.

be easily protonated in an acidic medium to produce, frequently in combination with metal complexes, supramolecular networks through non-covalent interactions [6]. Existing methodologies for the synthesis of imidazoles are limited in terms of the starting materials, conversion and product selectivity [7e10]. There are some reports on synthetic procedures based on aromatic nitriles as starting materials. Among them, Vijendra and co-workers reported the synthesis of 2,4,5-tris(2pyridyl)imidazole, which was prepared from a 1:2 mixture of 2picolyamine and 2-cyanopyridine [11]. To overcome low conversions employing traditional synthetic methods and to minimize the amount of reaction byproducts and undesired impurities, the use of catalysts is a potentially useful strategy, which even promotes the formation of more complex imidazole derivatives by incorporation of additional functionalities [12]. Siamaki and Arndsten have described a palladium-catalyzed (5% mol) process for the synthesis of tetra-substituted imidazoles, involving two imines and an acid chloride under CO atmosphere. However, the corresponding imidazoles were obtained only in low to moderate yields [13]. Other

ez-Castro et al. / Journal of Molecular Structure 1099 (2015) 126e134 A. Ba

procedures for imidazole synthesis start from imidazolines, which are prepared starting from diaminoethane and nitriles or methyl salicylate [14e16]. Herein, we report on a novel and efficient catalytic one-pot preparation of 2,4,5-tris(2-pyridyl) imidazole using cis-(±)-2,4,5tris(pyridin-2-yl)imidazoline as starting material (yield 64%), which has been prepared according to the procedure described by Anastassiadou et al. [16]. A literature search revealed that neither two-dimensional (2D) NMR nor UVeVis and fluorescence spectroscopic data of 2,4,5-tris(2-pyridyl) imidazole have been reported so far. Additionally, TD-DFT calculations have been performed to study the molecular structure characteristics and to predict and examine with more detail some experimental spectroscopic properties (IR and UVeVis spectra) of the title compound. The molecular structure and supramolecular arrangement of the title compound have been examined by means of low-temperature singe-crystal Xray diffraction analysis (T ¼ 100 K). We have become interested in the coordination chemistry of polypyridine ligands, since they have shown fluorescent properties [17], and might be employed as sensors for transition metal ions. Moreover, they are interesting building blocks for the construction of coordination polymers. Some related compounds that have been used for this purpose are: cis-(±)-2,4,5-tris(pyridin-2-yl)imidazoline [18,19] and 2,4,5-tris(4-pyridyl)imidazole [20]. As part of our ongoing research on the coordination chemistry of polypyridine ligands and fluorescent properties, we are interested in the use of the title compound 2,4,5-tris(2-pyridyl)imidazole, as ligand for the synthesis of transition metal complexes.

2. Experimental 2.1. Materials and methods All chemicals were purchased from Aldrich and used as received without further purification. The precursor cis-(±)-2,4,5tris(pyridin-2-yl)imidazoline was synthesized as reported elsewhere [21]. Infrared spectra were recorded on a Bruker Alpha Tensor 27 spectrophotometer with the spectral resolution of 2.0 cm1 using KBr pellets in the 4000e500 cm1 region. 1D (1H, 13 1 C{ H} ATP), and 2D NMR (1He1H COSY, NOESY, 1He13C HSQC and HMBC) spectra were recorded with a Bruker Ascend 400 NMR instrument. Chemical shifts are reported in ppm and were referenced to residual solvent resonances. High-resolution mass-spectrometric (HR-FABþ) studies were carried out on the JMS-700 MSTATION from JEOL. UVeVis absorption spectra were recorded on a Shimadzu UV-1800 UV spectrophotometer. Emission spectra in solution and in the solid state were obtained on a Perkin Elmer LS55 fluorescence spectrophotometer. The thermogravimetric analysis was performed under nitrogen (50 mL min1) in the temperature range of 25e600 C (10 C min1) using a TA SDT Q600 apparatus. The single-crystal X-ray structure was determined on a Bruker-APEX diffractometer equipped with a CCD area detector (lMo-Ka ¼ 0.71073 Å, monochromator: graphite). Frames were collected via u/f-rotation at 10 s per frame (SMART) [22]. The measured intensities were reduced to F2 and corrected for absorption with SADABS (SAINT-NT) [23]. Corrections were made for Lorentz and polarization effects. Structure solution, refinement and data output were carried out with the SHELXTL-NT program package [24]. CeH hydrogens were refined using a riding model, but the NH hydrogen atom has been localized by a difference Fourier map and refined with a distance restraint of 0.840(1) Å. Figures were created with ORTEP [25] and MERCURY [26].

127

2.2. Synthesis of the title compound A solution of cis-(±)-2,4,5-tris(pyridin-2-yl)imidazoline (1.00 g, 3.32 mmol) in toluene (50 mL) was treated with palladium on carbon (Pd/C 5%) (1.00 g) and refluxed for 72 h with constant stirring (Scheme 1). The resultant black suspension was filtered to remove the catalyst and the solvent was removed completely under reduced pressure to obtain a yellow solid. The solid residue was recrystallized from a solvent mixture of CH2Cl2/C5H12 to give pale yellow crystals. Yield: 0.64 g (64%). FT-IR (KBr): 3435, 3051, 3011, 1588, 1476, 1446, 1426, 1285, 1250, 1209, 1150, 988, 970, 738, 697 and 659 cm1. 1H NMR (400 MHz, CD2Cl2): d 11.48 (slbs, 1H, NeH), 8.85 (d, 3J ¼ 8.4 Hz, 1H, d00 ), 8.67 (bd, 3J ¼ 4.4 Hz, 1H, d), 8.63 (dd,4J ¼ 2.0 and 1.6 Hz, 1H, g0 ), 8.62 (dd,4J ¼ 2.8 and 0.8 Hz, 1H, g00 ), 8.25 (dt or ddd, 3J ¼ 8.0 and 4J ¼ 0.8e1.2 Hz, 1H, g), 8.17 (d,3J ¼ 8.0 Hz, 1H, d0 ), 7.83 (ddd or td, 3J ¼ 8.0 z 3J ¼ 7.6 and 4 J ¼ 1.6 Hz, 2H, f, e0 ), 7.71 (ddd or td,3J ¼ 8.0 Hz and 4J ¼ 2.0 Hz, 1H, e00 ), 7.31 (ddd,3J ¼ 7.6, 3J ¼ 4.8 and 4J ¼ 1.2 Hz, 1H, e), 7.27 (dd, two 3 J ¼ 6.8 Hz, 1H, f0 ), 7.21 (dd, 3J ¼ 6.8 and 3J ¼ 6.4 Hz, 1H, f00 ). 13C {1H} NMR (100 MHz,CD2Cl2): d 155.1 (a), 149.8 (g0 ), 149.5 (g00 ), 149.3 (h”), 148.9 (d), 148.7 (b), 145.9 (h, 140.6 (h0 ), 137.5 (e0 ), 137.0 (f), 136.9 (e00 ), 130.8 (c), 124.0 (e), 123.6 (d0 ), 123.5 (d00 ), 122.8 (f00 ), 122.7 (f0 ), 120.5 (g). High-resolution MS (HR-FABþ): m/z calcd. for C18H14N5 [MþH]þ, 300.1249; found: 300.1256. 3. Computational details All calculations were performed with the Gaussian09 package [27]. Minimum energy structures were calculated using density functional theory (DFT) [28,29] with the M06/6-31G(d) level of theory, and confirmed through a frequency calculation (no imaginary frequencies). The absorption spectrum and the transitions between the different orbitals were evaluated by time-dependent (TD) DFT calculations [30,31], using the M06 hybrid-meta-GGA functional [32] combined with the 6-31G(d) basis set [33]. The effects of a solvated environment were evaluated with the integral equation formalism for the polarizable continuum model (IEFPCM) and the implementation of the non-equilibrium solvation model [34]. The solvent considered for this analysis was ethanol. 4. Results and discussion Dehydrogenation of cis-(±)-2,4,5-tris(pyridin-2-yl)imidazoline with Pd/C 5%, provided the title compound 2,4,5-tris(2-pyridyl) imidazole in good yield (64%), which corresponds to 25% overall yield starting from 2-pyridinecarboxaldehyde and ammonium hydroxide. The product was then fully characterized by IR, NMR (1D and 2D) and UVeVis spectroscopy, high-resolution mass spectrometry (HR-FABþ) and single-crystal X-ray diffraction analysis. 4.1. NMR and IR spectroscopic analysis Room temperature

1

H NMR spectroscopic analysis of 2,4,5-

N

N HN N

Pd/C 5%

N

Toluene, reflux

HN

N N

N N Scheme 1. Preparation of the title compound.

128


tris(2-pyridyl)imidazole in CD2Cl2 gave sharp signals (Fig. 1). The compound has been characterized previously by NMR spectroscopy in CDCl3 [11], but gave only quite broad signals, which generated uncertainty about the multiplicity of the signals. Only the NeH group gave a slightly broadened singlet at 11.48 ppm, whose shift displacement indicates that the NeH group is not involved in strong NeH/N hydrogen bonds in solution [35]. 2,4,5-tris(2pyridyl)imidazole contains a total of 12 aromatic hydrogens attached to three pyridyl substituents, which gave a set of 10 wellresolved signals in the range of 7.00 and 8.90 ppm (Fig. 1). Of these, 5 signals are displaced downfield (8.10e8.90 ppm) and can be assigned to the hydrogens labeled g/g0 /g00 and d/d0 /d00 since their multiplicities correspond either to a doublet, doublet of doublet or triplet of doublets (d, dd or td). The remaining five NMR signals corresponding to the remaining six hydrogens are upfield shifted and display dd or ddd patterns, as expected for the hydrogens at the 4- and 5-position of the pyridyl rings. The division of the 1H NMR spectrum in two sections (d/g and e/f sets of hydrogens) is illustrated also from the homonuclear 1He1H gCOSY experiment (Figure S1, Supporting Information), which exhibits cross peaks only between the hydrogens in the high-field region (e/f set), but not between those in the downfield region (d/g set). On the contrary, for each of the hydrogens in the d/g set of signals, there is a cross peak with the e/f series of signals. This is illustrated most clearly by the connectivity of the signal centered at 7.71 ppm with those at 7.21 (dd) and 8.85 ppm, respectively. Of these, the peak at 7.21 ppm correlates also with the signal at 8.62, evidencing that the four signals comprise the hydrogens of one of the three pyridyl rings. From the 1He1H gNOESY experiment the signals for the d, e, f and g sets of hydrogens atoms in the pyridyl rings could be located. In Figure S2 of Supporting Information, reveals the correlation of hydrogens (d0 /d00 ), (e0 /e00 ) and (f0 /f00 ) of the pyridine groups attached at positions 4 and 5 of the imidazole ring. Finally, the assignment of the hydrogens was completed by a 1He13C gHMBC experiment (see, Figure S3). 1He13C HSQC and 13C{1H} NMR experiments accomplish the information of the 1D and 2D 1H NMR spectroscopic analysis further. The 13C{1H} NMR spectrum (Figure S4) displays one signal

for each of the 18 carbon atoms in the molecule with chemical shifts ranging from 120 to 156 ppm. This confirms the absence of tautomers in solution and indicates an asymmetric molecular structure. The assignment of the quaternary carbon atoms was performed with the aid of the 13C{1H} NMR attached proton test (APT), giving signals with chemical shifts of 155.1 (a), 149.3 (h”), 148.7 (b), 145.9 (h), 140.6 (h0 ) and 130.8 (c) ppm. The signals corresponding to the CH aromatic carbon atoms can be divided in three sections, which with the aid of the 1He13C gHSQC spectrum given in Figure S5 can be assigned to the d/g0 /g00 (148e150 ppm), f/ e0 /e00 (136e138 ppm) and e/g/d0 /f0 /d00 /f00 (120e124 ppm) sets of carbon atoms. These results are consistent with the crystal structure (vide infra) and a summary of the analysis is provided in Table S1. An important result of the NMR spectroscopic analysis is that there is an apparent outliner for one of the d hydrogen atoms (d00 ), which shows a significant low-field shift when compared to the remaining hydrogens of the d set (8.85 versus 8.67 and 8.17 ppm). Fortunately, the single-crystal structure analysis and the computational study provided additional information on the molecular conformation of the title compound, which helped to accomplish the structural characterization in solution (vide infra). The spectroscopic characterization of 2,4,5-tris(2-pyridyl)imidazole was accomplished by FT-IR spectroscopy. Fig. 2 illustrates the experimental and a spectrum simulated at the M06/6-31G(d) level of theory. Comparison of the most intense bands shows an overall good agreement, but a significant shift of the bands to larger wavenumbers in the calculated spectrum, particularly in the 12003600 cm1 range. This can in part be attributed to the anharmonicity of the vibrations and in part to the circumstance that the experimental spectrum was recorded in the solid state (KBr matrix), while for the simulated spectrum a continuum solvation model (with ethanol) was employed (IEF-PCM). Despite of this, the overall variations were in the range of 0.06e4.41%, giving a good correlation between the experimental and computational data (y ¼ 1.0614x 79.333, R2 ¼ 0.9988, Figure S6(a), supplementary material).

Fig. 1. 1H NMR spectrum of 2,4,5-tris(2-pyridyl)imidazole in CD2Cl2.


129

Table 1 Experimental and scaled IR frequencies for 2,4,5-tris(2-pyridyl)imidazole. Experimental frequencies

Scaled frequencies

3440 3054 1588 1531 1479 1453 1423 1386 1292 1252 1152 1077 994 796 778 738 660

3512 3081 1619 1528 1488 1455 1428 1350 1309 1267 1163 1037 998 809 798 698 632

0.9422, the calculated frequencies are very close to the experimental values (Table 1). 4.2. X-ray crystallographic study

Fig. 2. Comparison of experimental (KBr matrix) and simulated (M06/6-31G(d) using a continuum solvation model) IR spectra for 2,4,5-tris(2-pyridyl)imidazole.

The vibrational bands with wavenumbers greater than 3000 cm1 correspond to the stretching modes of the Cpyr-H and NeH bonds. Of these, the intense band at 3440 cm1 in the experimental spectrum is assigned to the stretching vibration of the imidazole NeH group, which was calculated at 3592 cm1. The observed red shifting by 152 cm1 may be due to an intramolecular NeH/N interactions in the solid state, which weakens the NeH bond due to elongation of the NeH bond distance. The absorptions at 3054, 3072 and 3088 cm1 are characteristic of CeH aromatic stretching vibrations. In the computed spectrum these bands are observed at 3158, 3195 and 3211 cm1, respectively. The vibration bands with wavenumbers in the range of 1600e1300 cm1 represent mainly the CeC and CeN bond stretching within the heterocyclic rings and ring deformation modes. Among these vibrations, the bands in the region of 1479e1423 cm1 may be assigned to the C]N functions and the band at 1588 cm1 should correspond to the C]C stretching vibrations of the pyridyl substituents. The band typical for the C]N vibration of the central imidazole ring is located at 1531 cm1 [11,36,37]. Accordingly, the computed spectrum displays these bands at 1479e1455, 1656 and 1528 cm1, respectively. In order to take into account the anharmoncity of the vibrations, theoretical calculations combined with Pulay's methodology [38e40] (scaling factor for frequencies) were carried out with the goal to obtain a better comparison with the experimental data. Table 1 shows the experimental and scaled frequencies, and the values exhibit excellent agreement between them. The relation between the scaled calculated and experimental wavenumbers is described by the equation: y ¼ 1.0252x 31.106 (R2 ¼ 0.9992, Figure S6(b), supplementary material). Employing a scale factor of

The compound examined with detail herein was also characterized by low-temperature single-crystal X-ray diffraction analysis. The crystallographic study was carried out at T ¼ 100 K and revealed that the compound crystallized in the monoclinic crystal system with the space group P21/c, giving the following crystal lattice parameters: a ¼ 16.2464(17) Å, b ¼ 11.5056(12) Å, c ¼ 7.4294(8) Å, a, g ¼ 90 and b ¼ 93.555(2) . The data are in good agreement with previously reported data, which were collected at T ¼ 296 K [11]. The most relevant crystallographic data are summarized in Table S2 (see Supplementary Data). The molecular structure is given in Fig. 3a. Table 2 comprises the experimental and calculated values of selected bond lengths and bond angles. The geometry of the compound was also optimized in the ground state by DFT calculations at the M06/6-31G(d) level of theory with the observation that the deviations are less than 0.02 Å and 1.3 , respectively. The molecular structure of 2,4,5-tris(2-pyridyl)imidazole is nonplanar, as shown by the dihedral angles of 13.4, 22.7 and 5.4 between the planes of the central imidazole ring and the pyridyl substituents in the 2-, 4- and 5-positions. This is confirmed by the following torsion angles: N1eC1eC4eC5 [12.7(5) ], N2eC1eC4eN3 [13.3(4) ] and N1eC2eC9-10 [21.1(4) ]. A closer inspection of the factors that influence the molecular conformation of the title compound shows a series of close contacts, which indicate a total of 5 intramolecular interactions of the NeH/N and CeH/N type, see Fig. 3a. The geometric parameters for these contacts are summarized in Table S3 (Supplementary Data) and indicate that the strongest intramolecular interaction is probably the CeH/N contact formed between carbon C15 and N4, giving a 7-membered H-bonded cyclic ring (motif I). The corresponding geometric parameters are C15$$$N4, 3.102(4) Å; H15/N4, 2.28 Å and the C15eH15/N4 angle of 143.53 (Table S3). For motifs IIeV, particularly the D-H∙∙∙A angles deviate significantly from the ideal value of 180 and the H/N distances are longer (2.75e2.28 Å). The molecular structure established by X-ray diffraction analysis is consistent with the NMR characterization in solution. A particular observation was the low-field shift of one of the pyridyl hydrogens in position d0 . This low-field shift can be attributed to the Cpyr-H hydrogen involved in the intramolecular CeH/N contact giving rise to motif I.

130


Fig. 3. Molecular structure (a) and intermolecular interactions (b,c) in the crystal structure of the title compound.

Analysis of the intermolecular interactions of the crystal structure of the compound reveals the presence of one-dimensional (1D) chains along bc, which are formed through intermolecular CeH/Ni contacts (H6/N5, 2.71 Å; C6/N5, 3.579(4) Å; Symmetry code: (i) x,y 1/2,z þ 1/2) between one of the pyridyl hydrogen atoms (H6) and the nitrogen atom of the pyridine in 4-position (Fig. 3b). Additionally, there are intermolecular p/p interactions [Cg/Cgi ¼ 3.645(1) Å; Cg centroid of the N3, C4eC8 ring; symmetry code (i) x, 1 y, 1 z] between the pyridyl substituents in 2-position of adjacent molecules, which gives rise to dimeric units (Fig. 3c), thus giving overall 2D layers parallel to ab. 4.3. HOMOeLUMO band gap by DFT method and photophysical properties The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) has been used to prove the bioactivity resulting from intramolecular charge transfer (ICT) and also reflects the chemical activity of the molecule [41]. According to the frontier molecular orbitals theory, the HOMO, LUMO and related orbitals are most important for the electronic properties of organic compounds. HOMO orbitals usually act as electron donors and LUMO orbitals as electron acceptors. Fig. 4 shows the isodensity plots of some of the molecular orbitals at the M06/6-31G(d) level of calculation, in which it is possible to observe the charge transfer (HOMO / LUMO) in all the system and the p-molecular orbital characteristics. The HOMO orbitals are localized mainly on the nitrogen atoms, while the LUMO orbitals are distributed over the remaining atoms. The HOMOeLUMO energy gap value was found to be 4.385 eV (see Figure S7). The experimental and theoretical UVeVis abpsortion spectra are shown in Fig. 5, of which the experimental spectrum was obtained for a 1.0 104 M solution in EtOH. The results of the TD-DFT calculations reveal four major bands in the UV region: two intense bands at 334 and 304 nm due to the transitions HOMO / LUMO and HOMO / LUMOþ1, respectively. These bands are consistent with the broad band centered at 320 nm found experimentally (ε ¼ 29,000). The theoretical spectrum displays two additional bands at 268 and 254 nm. The band at 268 nm was assigned to the transitions HOMO / LUMOþ3 and HOMO / LUMOþ4, while the band at 254 nm is due to the transition HOMO-1 / LUMO, which is consistent with the broad band

observed in the experimental spectrum at 236 nm. Of these, the HOMO / LUMO transition implies that intramolecular charge transfer takes place[42,43]. A detailed assignment of the TD-DFT calculations in terms of FMO is included in the Supplementary Data (Table S4). The emission spectra of the compound were measured in solution (EtOH) and in the solid state at room temperature. The solid state spectrum (Figure S8, Supplementary Data) displays two blue emission peaks at 405 and 423 nm, respectively, attributed to the p / p* and n / p* transitions [44]. An additional red-shifted peak at 502 nm suggests the formation of an excimer [45], which is in agreement with the supramolecular pep stacked dimer described above (vide supra) [46e48]. Using the formula reported by Berezin and Achilefu, the mean lifetime of the excited state is 18.57 ms [49]. A similar phenomenon was observed in solution. At C ¼ 1 105 M (Fig. 6), the emission spectrum shows an additional peak at ~517 nm [45]. To analyze if it can be attributed to excimer formation, the emission spectra were measured at different concentrations (1.0 103, 1.0 104, 1.0 105 and 1.0 106 M), observing that the normalized intensity of the corresponding band decays when the concentration is diminished (Fig. 5). This behavior is in agreement with excimer formation, since increasing concentrations favor the p/p stacking interactions [50]. In addition, when comparing the absorption and emission spectra in solution (Figure S9, Supplementary Data), it is evident that the fluorescence spectrum is approximately the mirror image of the absorption spectrum. The mirror image relation indicates similar torsional potentials in the ground (S0) and excited (S1) states [51]. This is also evidenced by a small value for the Stoke displacement (~65 nm). 4.4. Natural and Mulliken atomic charges To provide a more complete description of the charge distribution in the structure of 2,4,5-tris(2-pyridyl)imidazole, two different computational schemes were employed in order to represent the atomic charges in the compound, namely, Mulliken population analysis and NPA (natural population analysis). The charges are indicated in Figure S10 (Supplementary material) and a list of the calculated atomic charges is given in Table 3. The charge distribution is qualitatively similar using the Mulliken and NPA scheme. As expected, the N-atoms are the most electronegative atom


131

Table 2 Experimental and calculated values of selected bond lengths [Å] and bond angles [ ] for the title compound. Parameter

X-ray data T ¼ 100 Ka

X-ray data T ¼ 298 Kb

Computed data

N(1)-C(1) N(1)-C(2) N(2)-C(1) N(2)-C(3) N(3)-C(8) N(3)-C(4) N(4)-C(13) N(4)-C(9) N(5)-C(18) N(5)-C(14) C(1)-C(4) C(2)-C(3) C(2)-C(9) C(3)-C(14) C(4)-C(5) C(5)-C(6) C(6)-C(7) C(7)-C(8) C(9)-C(10) C(10)-C(11) C(11)-C(12) C(12)-C(13) C(14)-C(15) C(15)-C(16) C(16)-C(17) C(17)-C(18) C(1)-N(1)-C(2) C(1)-N(2)-C(3) C(8)-N(3)-C(4) C(13)-N(4)-C(9) C(18)-N(5)-C(14) N(1)-C(1)-N(2) N(1)-C(1)-C(4) N(2)-C(1)-C(4) N(1)-C(2)-C(3) N(1)-C(2)-C(9) C(3)-C(2)-C(9) N(2)-C(3)-C(2) N(2)-C(3)-C(14) C(2)-C(3)-C(14) N(3)-C(4)-C(5) N(3)-C(4)-C(1) C(5)-C(4)-C(1) C(6)-C(5)-C(4) C(5)-C(6)-C(7) C(8)-C(7)-C(6) N(3)-C(8)-C(7) N(4)-C(9)-C(10) N(4)-C(9)-C(2) C(10)-C(9)-C(2) C(11)-C(10)-C(9) C(10)-C(11)-C(12) C(13)-C(12)-C(11) N(4)-C(13)-C(12) N(5)-C(14)-C(15) N(5)-C(14)-C(3) C(15)-C(14)-C(3) C(16)-C(15)-C(14) C(15)-C(16)-C(17) C(18)-C(17)-C(16) N(5)-C(18)-C(17)

1.321(4) 1.385(4) 1.346(4) 1.364(4) 1.340(4) 1.345(4) 1.342(4) 1.345(3) 1.327(4) 1.351(4) 1.468(4) 1.397(4) 1.477(4) 1.470(4) 1.388(4) 1.380(4) 1.381(4) 1.373(4) 1.401(4) 1.375(4) 1.386(4) 1.378(4) 1.392(4) 1.373(4) 1.383(4) 1.381(4) 105.4(2) 109.0(2) 116.7(2) 117.3(2) 118.3(2) 111.5(2) 127.2(3) 121.2(3) 109.7(2) 117.0(2) 133.3(3) 104.5(2) 116.6(3) 138.7(3) 123.3(3) 115.2(2) 121.5(3) 118.4(3) 119.1(3) 118.6(3) 123.9(3) 122.0(3) 119.6(2) 118.4(2) 119.4(3) 119.0(3) 118.1(3) 124.2(3) 121.4(3) 113.7(2) 124.9(3) 119.2(3) 119.5(3) 117.8(3) 123.7(3)

1.326(2) 1.383(2) 1.346(2) 1.370(2) 1.339(2) 1.343(2) 1.346(2) 1.342(2) 1.335(2) 1.350(2) 1.463(2) 1.395(2) 1.473(2) 1.471(2) 1.383(2) 1.379(2) 1.373(2) 1.373(2) 1.393(2) 1.381(2) 1.373(2) 1.370(2) 1.386(2) 1.382(2) 1.373(2) 1.375(2) 106.2 109.0 117.3 118.5 118.6 111.1 126.7 122.2 109.7 117.1 133.1 104.0 115.8 140.0 123.3 116.1 120.4 118.3 118.9 118.3 123.7 121.6 119.6 118.8 119.2 119.1 117.7 123.9 121.7 113.4 124.9 118.7 119.6 117.6 123.7

1.318 1.373 1.350 1.372 1.333 1.342 1.334 1.340 1.329 1.350 1.459 1.404 1.469 1.469 1.395 1.386 1.391 1.392 1.403 1.385 1.392 1.388 1.397 1.389 1.389 1.391 105.9(1) 109.4(1) 117.0(1) 117.5(1) 117.6(1) 110.8(1) 127.5(1) 121.7(1) 109.7(1) 117.0(1) 133.3(1) 104.2(1) 116.8(1) 138.8(1) 123.0(1) 115.3(1) 121.7(1) 118.5(1) 119.2(1) 118.7(1) 123.6(1) 121.6(1) 119.8(1) 118.6(1) 119.5(1) 119.0(1) 118.4(1) 123.9(1) 122.0(1) 113.2(1) 124.7(1) 118.9(1) 119.4(1) 118.3(1) 123.8(1)

a b

This work. Reference [11].

sites in the molecule and the calculated charge densities at the Natoms are in range of 0.53 to 0.72 (Mulliken) and 0.49 to 0.54 (NPA), respectively. All H-atoms are electropositive with the most electropositive H-sites being H20 , H5, H15 and H16. Of these, the NeH proton is more electropositive than the aromatic protons. On the other hand, some of the carbon atoms have positive charges, while others are negatively charged. The pyridine carbon atoms,

except for C4, C8, C9, C13, C14 and C18 are all electronegative. All carbon atoms of the imidazole ring are positively charged with C1 having the highest positive charge density, since this atom is embedded between two electronegative N-atoms. The Mulliken population analysis has been performed also for the dimer observed in the unit cell, which has been also geometryoptimized using the M06/6-31G(d) level. The charge distribution


132

Fig. 4. Plots of the frontier orbitals involved in the lowest-lying electronic absorption transitions of 2,4,5-tris(2-pyridyl)imidazole.

Exp. Calc.

Absrobance (a.u.)

3

2

1

0 200

300

400

Wavelength (nm) Fig. 5. Comparison of the experimental and simulated UVeVis absorption spectra of 2,4,5-tris(2-pyridyl)imidazole.

obtained in this study is similar to that of the single molecule, see Fig. 7 and S11. 4.5. Molecular electrostatic potential The analysis of the charge distribution indicates also that the above-mentioned intermolecular hydrogen bonding scheme (motifs IeV) is reasonable, since the corresponding H-donor atoms all carry positive charges and are close to negatively charged nitrogen atoms. Electrostatic potential maps are very useful threedimensional diagrams used to illustrate the charge distributions and charge-related properties of molecules. The MEP is typically visualized by mapping the values onto the molecular surface, thus reflecting the boundaries which allow to visualize the size and shape of the corresponding molecules. The MEP diagram has been also used to predict the reactive sites for electrophilic and nucleophilic attack, and for studies of molecular recognition and hydrogen

Fig. 6. Emission spectra of 2,4,5-tris(2-pyridyl)imidazole in EtOH at different concentrations. Excitation wavelength: lex ¼ 320 nm.

bonding interactions [52,53]. The MEP of 2,4,5-tris(2-pyridyl) imidazole was calculated using the DFT method with M06/631G(d) and the isodensity surfaces are shown in Fig. 8. Red areas indicate negative electron density, while blue areas denote positive electron density. Sites with negative atomic charge and electrostatic potential indicate possible sites for coordination with metal ions and allow to identify the atoms that may favor the acceptance of hydrogens in the formation of hydrogen bonding interactions. Thus, this analysis facilitates the purpose of designing supramolecular networks in combination with metal complexes and organic building blocks. The title compound has five N-atoms, which might act as proton acceptor sites and are colored in red (0.015 to 0.028 au). The most positive region (blue) is localized at H7 (þ0.023 au). Hence nitrogen N1 and hydrogen H7 are the most reactive sites for electrophilic and nucleophilic attacks, respectively [54]. As in the case


133

Table 3 Mulliken and natural atomic charges of 2,4,5-tris(2-pyridyl)imidazole calculated at the M06/6-31G(d) level. Atoma

N1 N2 H20 N3 N4 N5 C1 C2 C3 C4 C5 H5 C6 H6 C7 H7 C8 H8 a

Atoma

Charges NPA

Mulliken

0.51621 0.54072 0.48276 0.48695 0.50298 0.50802 0.38840 0.11074 0.14190 0.17001 0.24543 0.26302 0.19543 0.24667 0.28507 0.24583 0.03927 0.22559

0.597692 0.716836 0.384499 0.529165 0.526463 0.542443 0.524174 0.132799 0.291609 0.292749 0.171316 0.191576 0.134941 0.174533 0.175902 0.168543 0.034509 0.166312

C9 C10 H10 C11 H11 C12 H12 C13 H13 C14 C15 H15 C16 H16 C17 H17 C18 H18

Charges NPA

Mulliken

0.20560 0.24869 0.26568 0.19675 0.24396 0.28864 0.24355 0.03741 0.22072 0.17981 0.24562 0.27911 0.19612 0.24383 0.28590 0.24380 0.04142 0.22345

0.272269 0.176355 0.190112 0.12588 0.169101 0.183358 0.164703 0.03575 0.156297 0.261208 0.157089 0.213865 0.143741 0.169446 0.176624 0.165181 0.035723 0.162849

The atom numbering corresponds to that given in Fig. 3a.

of the atomic charges, the MEP diagram is in excellent agreement with the presence of CeH/N intramolecular interactions proposed based on the X-ray diffraction analysis (see red spots). 4.6. TG-DSC analysis The TGA and DSC curves were performed over the temperature range 25e600 C under a flowing nitrogen atmosphere (Fig. 9). The TGA graph indicates an initial mass loss of ~1.9%, starting at approximately 106 C (Tpeak ¼ 114 C), which is accompanied by an endothermic peak in the DSC plot and attributed to a melting process with decomposition. In the temperature range of 106e371 C, a mass loss of ~98.6% is observed, which is related to an endothermic event observed in the DSC trace at Tpeak ¼ 371 C. Since the temperature range for this event is relatively small and no residue remains, it seems that the compound entered the gasphase without decomposition.

Fig. 7. The Mulliken atomic charge distribution for a single molecule and the dimer observed in the solid-state structure of the title compound. Green bars represent the values for the single molecule, while red and blue bars represent the two molecules in the dimer. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8. Electrostatic potential mapped on the isodensity surface of 2,4,5-tris(2-pyridyl) imidazole. Color code thresholds are red 0.03 and blue þ0.03. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

5. Conclusions An easily accessible synthetic route for 2,4,5-tris(2-pyridyl) imidazole has been stablished. The title compound was characterized by NMR, FT-IR, UVeVis and fluorescence spectroscopy, highresolution mass spectrometry, TGA-DSC analysis and lowtemperature single-crystal X-ray diffraction analysis. The geometry of the compound was also optimized in singlet states using the DFT method at the M06/6-31G(d) level of calculation. Comparison with the experimental results (X-ray data) showed that the bond lengths and angles vary less than 0.02 Å and 1.3 , respectively. The combined NMR spectroscopic and structural analysis showed the presence of intramolecular CeH/N interactions that apparently stabilize a single molecular conformation and tautomer in solution and in the solid state. The crystallographic study revealed also a series of additional intermolecular interactions, in particular CeH/N and p/p contacts, that play a significant role in stabilizing the supramolecular network of the compound.

Fig. 9. TGA and DSC curves of 2,4,5-tris(2-pyridyl)imidazole.

134


Moreover, vibrational band assignments were performed at the M06/6-31G(d) theory level and compared with the experimental data (FT-IR), showing that the calculated vibrational frequencies are in good agreement with the experimental data (R2 ¼ 0.9988), giving variations in the range of 0.06e4.41%. The molecular electrostatic potential distribution has been calculated using the same level of theory. The MEP results showed that the imine nitrogen N1 and the H7 atom are the most reactive sites for electrophilic and nucleophilic attack, respectively. The absorption UVeVis experiment in EtOH solution revealed two lmax peaks at 236 and 320 nm, which are consistent with the calculated electronic transitions. The photophysical properties study in the solid state and in solution (EtOH) revealed a monomer emission in the blue region and an excimer emission in the red region. The HOMOeLUMO energies and the energy gap reveal the existence of intramolecular charge transfer, which indicates that the compound may be used as fluorescent chemosensor for metal ions. Acknowledgments noma de Sinaloa, This work was supported by Universidad Auto xico (DGIP-PROFAPI-2012/031). The authors gratefully Me acknowledge access to the analytical facilities (X-ray diffraction laboratory and LANEM) at the Centro de Investigaciones Químicas, noma del Estado de Morelos (CIQ-UAEM). A. B. C. Universidad Auto thanks the support from Consejo Nacional de Ciencia y Tecnología (CONACYT) in the form of a graduate scholarship (248950). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.molstruc.2015.05.055. References [1] K. Shalini, P.K. Sharma, N. Kumar, Chem. Sin. 1 (2010) 36e47. ndez, Org. Lett. 13 (2011) 972e975. [2] A. Richaud, N. Barba-Behrens, F. Me [3] Y.S. Zhao, W. Yang, D. Xiao, X. Sheng, X. Yang, Z. Shuai, Y. Luo, J. Yao, Chem. Mater. 17 (2005) 6430e6435. [4] J. Gao, Z.-P. Wang, C.-L. Yuan, H.-S. Jia, K.-Z. Wang, Spectrochim. Acta. A. Mol. Biomol. Spectrosc. 79 (2011) 1815e1822. [5] S.-H. Fan, A.-G. Zhang, C.-C. Ju, L.-H. Gao, K.-Z. Wang, Inorg. Chem. 49 (2010) 3752e3763. [6] (a) G.A. Barclay, R.S. Vagg, E.C. Watton, Acta Cryst. Sec. B 33 (1977) 3487e3491; (b) S.R. Batten, A.R. Harris, K.S. Murray, J.P. Smith, Cryst. Growth Des. 2 (2002) 87e89; (c) T.J. Podesta, A.G. Orpen, Cryst. Growth Des. 5 (2005) 681e693. [7] Y. Liu, Z.-Y. Duan, H.-Y. Zhang, X.-L. Jiang, J.-R. Han, J. Org. Chem. 70 (2005) 1450e1455. [8] P. Lenaerts, A. Storms, J. Mullens, J. D'Haen, C. Gorller-Walrand, K. Binnemans, K. Driesen, Chem. Mater. 17 (2005) 5194e5201. [9] M.V. Proskurnina, N.A. Lozinskaya, S.E. Tkachenko, N.S. Zefirov, Russ. J. Org. Chem. 38 (2002) 1149e1154. [10] F. Bellina, R. Rossi, Adv. Synth. Catal. 352 (2010) 1223e1276. [11] V.K. Fulwa, R. Sahu, H.S. Jena, V. Manivannan, Tetrahedron Lett. 50 (2009) 6264e6267. [12] S. Kamijo, Y. Yamamoto, Chem.-Asian J. 2 (2007) 568e578. [13] A.R. Siamaki, B.A. Arndtsen, J. Am. Chem. Soc. 128 (2006) 6050e6051. [14] S.C. Zimmerman, K.D. Cramer, A.A. Galan, J. Org. Chem. 54 (1989) 1256e1264. [15] M.E. Campos, R. Jimenez, F. Martinez, H. Salgado, Heterocycles 40 (1995)

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