SbCl3-catalyzed one-pot synthesis of 4,4′-diaminotriarylmethanes under solvent-free conditions: Synthesis, characterization, and DFT studies

A simple, efficient, and mild procedure for a solvent-free one-step synthesis of various 4,4′-diaminotriarylmethane derivatives in the presence of antimony trichloride as catalyst is described. Triarylmethane derivatives were prepared in good to excellent yields and characterized by elemental analysis, FTIR, 1H and 13C NMR spectroscopic techniques. The structural and vibrational analysis were investigated by performing theoretical calculations at the HF and DFT levels of theory by standard 6-31G*, 6-31G*/B3LYP, and B3LYP/cc-pVDZ methods and good agreement was obtained between experimental and theoretical results.


Introduction
The leuco forms of triarylmethine dyes are compounds with numerous diverse industrial, biological, and analytical applications. They have a broad spectrum of technological applications. For instance, they have been used not only in textile industry to dye wool, nylon, silk, leather, cotton, and polyacrylonitrile fibers, but they also have applications for coloring of plastics, varnishes, waxes, and oils. They have been used in novel types of colorless copying papers, pressure and heatsensitive materials, light-sensitive papers, ultrasonic recording papers, electrothermic heat-sensitive recording papers, inks, crayons, typewritten ribbons, and photoimaging systems [1][2][3]. These compounds have been employed as dye precursors in nanocomposite preparations [4], in photoresponsive polymers [5], and as the thermal iniferter (initiator-transfer-terminator agent) in pseudo-living radical polymerizations [6]. They are also used in low-dose dosimeters [7], in chemical radiochromic dosimeters [8] and as sensitizers for photoconductivity [9].
Diaminotriphenylmethine (DTM) dyes are the most important group of triarylmethine dyes and were selected for the present study due to their brilliance, high pictorial strength, low cost, and wide variety of applications. This group includes a broad range of dyes such as Cresol Red, Bromocresol Green, Light Green SF Yellowish, Victoria Blue BO, Ethyl Green, Brilliant Green, Diaminotriphenylmethane, Fast Green FCF, Green S, Fuchsine Acid, Chlorophenol Red, Crystal Violet Lactone, Fuchsine, Pararosaniline, Water Blue, Thymolphthalein, Bromocresol Purple, and Aurin. These compounds are usually soluble in non-polar organic solvents and are insoluble in water. Because of the wide range of applications of DTMs, the development of new and more efficient synthetic methods for their preparation is of importance.
The reaction of arylaldehydes with N,N-dimethylaniline is one of the most efficient methods for the synthesis of DTMs. This reaction is usually carried out in the presence of Brønsted acids such as sulfuric acid, hydrochloric acid, methanesulfonic acid, or p-TSA, as well as Lewis acids such as zinc chloride, zeolites, montmorillonite K-10, and polymer-supported sulfonic acid (NKC-9) [35][36][37][38][39][40]. Microwave-assisted synthesis of DTMs in the presence of aniline hydrochloride has also been described [41]. Recently, bismuth(III) nitrate and zirconium(IV) dichloride oxide octahydrate have been successfully used for the preparation of DTMs in our group [42,43].
The reported procedures are associated with certain limitations, such as low yields, the use of corrosive acids and excess solvent, severe reaction conditions, long reaction times, costly reagents, as well as the inconvenience in handling the reagents. Considering these restrictions, the development of new and simple synthetic methods for the efficient preparation of DTMs is therefore an interesting challenge. In this contribution, we describe a new route for the preparation of DTM derivatives under solvent-free conditions by the use of SbCl 3 as a versatile catalyst. In addition, structural and vibrational studies of leuco compounds by DFT methods were investigated for the first time.

Synthesis of DTM
Our investigations showed that N,N-dimethylaniline reacts smoothly with arylaldehydes and heterocyclic aldehydes in the presence of SbCl 3 to produce the corresponding DTMs in good to excellent yields. At first we focused on the reaction of benzaldehyde and N,N-dimethylaniline as a model reaction under various reaction conditions (with solvent, solvent-free, and microwave-assisted). Various protic and aprotic solvents such as dimethyl sulfoxide, N,N-dimethylformamide, dichloromethane, ethanol, diethyl ether, and n-hexane were examined: The best results were obtained under solvent-free conditions without microwave irradiation. The general route for the synthesis of these compounds is shown in Scheme 1. In a typical procedure, the reaction of benzaldehyde (0.50 mmol, 1 equiv) and N,N-dimethylaniline (1.25 mmol, 2.5 equiv) in the presence of SbCl 3 (30 mol %) under solventfree conditions at 120 °C for 4 h afforded compound 1a in 80% yield ( Table 1, entry 1). To determine the influence of different substituents on the generality of this reaction, a variety of aromatic aldehydes was examined ( Table 1, entries 1-16). Furthermore, the number, nature, and position of these substituents affect the hue or color of the resulting dyes and the type of their applications.
The dominant mechanism for the reaction can be summarized as a tandem regioselective electrophilic aromatic substitution reaction of N,N-dimethylaniline and aldehydes, in which SbCl 3 (as a Lewis acid catalyst) activates the carbonyl group of the aldehydes. If we accept this mechanism, one can expect a general influence of electron-donating and electron-with-drawing groups on the feasibility of the reaction. Electron-withdrawing groups such as halo and nitro substituents at the paraposition of the arylaldehydes increase the electrophilic strength of the carbonyl group and subsequently increase the yield of the reaction (Table 1, entries 4-7). An electron-withdrawing nitrosubstituent at the orthoor metaposition of benzaldehyde also gave good product yields (Table 1, entries 2-3). The presence of a nitro group in the ortho-position of benzaldehyde increases the inductive effect of NO 2 but at the same time decreases its resonance effect due to its steric effect. The resonance effect of a meta-nitro group is relatively low and leads to a slightly lower yield of product in comparison to ortho-nitrobenzaldehyde. Thus, in these two cases, the yields are lower than with the para-nitro substituted benzaldehyde. On the other hand, electron-rich groups such as para-methoxy decrease the product yield (Table 1, entries [10][11]. Surprisingly, benzaldehydes with two or even three deactivating methoxy groups react with N,N- dimethylaniline to produce the corresponding products in moderately good yields (Table 1, entries [13][14] which highlights the application of SbCl 3 as useful catalyst in this methodology. Furthermore, the application range of the reaction was expanded to fused aromatic rings such as 9H-fluorene and anthracene: 9H-Fluorene-2-carbaldehyde and anthracene-9carbaldehyde react with N,N-dimethylaniline under the above reaction conditions to afford the desired products 1o and 1p in yields of 67% and 36% yields, respectively (Table 1, entries [15][16]. In the next step, the application of this procedure was further expanded to heterocyclic aldehydes for the synthesis of diaryl heteroaryl methane compounds (Scheme 1). These heterocyclic derivatives have received increasing attention due to their biological activities such as antibacterial and antitumor properties [44,45].

Computational details
To predict the molecular structure of the title compounds and to assign their vibrational spectra, we performed theoretical calculations with compound 11 as the model compound ( Figure 1). All theoretical calculations were carried out with the GAMESS program [46].
The first stage for geometry optimization of this molecule was completed by the standard HF/6-31G* method. The resulted HF geometry was then employed in further DFT calculations and re-optimization was carried out by 6-31G*/B3LYP and B3LYP/ cc-pVDZ methods. The optimized structures were then used in the vibrational frequency calculations. The vibrational frequencies for these species were calculated and scaled by 0.899, 0.960, and 0.970 for HF/6-31G*, B3LYP/6-31G*, and B3LYP/ cc-pVDZ methods, respectively. No imaginary frequency modes were obtained for the optimized structure of compound 11, proving that a true minimum on the potential energy surface was found.

The molecular structure of leuco compounds
The optimized structural parameters of compound 11, calculated by HF and DFT levels with the HF/6-31G*, 6-31G*/ B3LYP, and B3LYP/cc-pVDZ methods, are listed in Table 2.
The optimized configuration is shown in Figure 2. Because of unavailability of X-ray crystal structures for these compounds, the optimized structure is compared with X-ray structures of similar compounds. This comparison shows good agreement between optimized and actual molecular structures. Here For the C23-O26 bond, the optimized bond lengths are 135.0 pm for the HF/6-31G* method, 136.7 pm for the B3LYP/ 6-31G* method, and 136.7 pm for the B3LYP/cc-pVDZ calculations which is in good agreement with a bond length of 137.0 pm found in similar compounds [48].
The bond lengths of C11-N14 and C6-N15 are 139.5 pm and 141.6 pm for the HF\6-31G*, 139.5 pm and 141.3 pm for the   compounds where the bond lengths are 137.8 and 140.0 pm [49]. Although there are some differences between our results and those from X-ray structure analysis of similar compounds, the optimized structural parameters are very close to the X-ray values and are good enough for further vibrational calculations.

Assignments of vibrational frequencies
To our best knowledge, no vibrational data and assignments of the modes have been reported for leuco compounds. The experimental infrared spectrum and the calculated infrared spectra of 11 are shown in Figure 3. A comparison between the infrared spectra of this compound can be made by establishing a correlation between the observed and the theoretically calculated wavenumbers. Linear correlations between the experimental and calculated wave numbers have been found (Figure 4). The correlation coefficients indicate a good linearity between the calculated and experimental wavenumbers. These correlation coefficients are 0.998, 0.999, and 0.999 for HF/6-31G*, 6-31G*/B3LYP, and B3LYP/cc-pVDZ methods, respectively.
Based on this comparison between the calculated and experimental IR spectrum, assignments of the fundamental modes were carried out on the basis of the B3LYP/cc-pVDZ calculations.
The resulting vibrational wavenumbers for the optimized geometry and the proposed assignments are given in Table 3. The observed bands at = 3067, 3003, 2945, 2924, 2890, 2861, 2829, and 2803 cm −1 are assigned to the aromatic CH and aliphatic CH 3 stretching modes, whereas the C-H stretching vibrations in aromatic rings occur above = 3000 cm −1 . The principal bands in the 1600-1000 cm −1 region are the C-O and C-N stretching vibrations modes as well as the bending vibrations of C-H bonds (Table 3).
Some vibrational frequencies over 2900 cm −1 are not observed clearly in the experimental spectra whereas they are obtained from DFT calculations. For instance, the peaks at 2951, 2955 and 3042 cm −1 are for [C-H of N(CH 3 ) 2 ] vibrations in the theoretical spectra (there is also a peak in 2976 cm −1 for [C-H of N(CH 3 ) 2 ] in the calculated infrared spectra). Other peaks at 2971, 3039 cm −1 (for C-H of OCH 3 ), 3130 and 3134 cm −1 (C-H symmetric stretching vibrations) are also not discernable in the experimental IR spectrum.