Synthesis, Crystal Structure, Vibration Spectral, and DFT Studies of 4-Aminoantipyrine and Its Derivatives

Three compounds derived from 4-aminoantipyrine (AA) were synthesized and their structures confirmed by melting point, elemental analysis, FT-IR, and 1H-NMR. The molecular structures of the four compounds were characterized by single-crystal X-ray diffraction and calculated by using the density functional theory (DFT) method with 6-31G (d) basis set. The calculated molecular geometries and the vibration frequencies of the AA derivatives in the ground state have been compared with the experimental data. The results show that the optimized geometries can reproduce well the crystal structural parameters, and the theoretical vibration frequencies show good agreement with the experimental data, although the experimental data are different from the theoretical ones due to the intermolecular forces. Besides, the molecular electrostatic potential (MEP) and the frontier molecular orbital (FMO) analysis of the compounds were investigated by theoretical calculations.


Introduction
4-Aminoantipyrine (AA) and its derivatives ( Figure 1) have potential biological activities [1][2][3][4][5][6][7], such as analgesic, anti-inflammatory, antimicrobial, and anticancer properties. Recently, AA and 4-methylantipyrine (MAA) were found to correlate with the analgesic effect of dipyrone [8]. A study demonstrated for the first time that dipyrone and some AA derivatives have a high potential to attenuate or prevent the anti-platelet effects of aspirin [9]. This was confirmed by docking studies, which revealed that MAA forms a strong hydrogen bond with serine 530 within the COX-1 enzyme, thereby preventing enzyme acetylation by aspirin. The three-dimensional structures of COX-1 and COX-2 have been solved by X-ray crystallography [10,11].
Although there have been many studies of the synthesis and biological activities of AA and its derivatives, there are only a few articles concerning the structures of these compounds. To our knowledge, there are no articles describing their complete structural analysis. Since AA and its derivatives are biologically active compounds, information about their 3-dimensional structures, especially their crystal structures, may be of great interest for rational drug design. On the other hand, we also aimed to obtain and analyze the electronic structures of AA and its derivatives. B3lyp theory with 6-31G* basis set was used since it is known to be quite a reliable method [12]. In this study, we present results of a detailed investigation of the structural characterization of AA and its three derivatives (FAA, MMAA, MCAA) using single crystal X-ray diffraction, IR spectroscopy, and quantum chemical methods. The geometrical parameters, fundamental frequencies of the three derivatives in the ground state have been calculated by using the DFT (B3LYP) method with 6-31G (d) basis set. This calculation is valuable for providing insight into molecular parameters and the vibration spectrum. The aim of this work was to explore the molecular dynamics and the structural parameters that govern the chemical behavior, and to compare predictions made from theory with experimental observations.

Vibration Spectra
Vibration spectroscopy is used extensively in organic chemistry for the identification of functional groups of organic compounds, the study of molecular conformations, reaction kinetics, etc. The vibration spectral data obtained from the solid phase FT-IR spectra are assigned based on the results of the normal coordinate calculations. The experimental and the simulated infrared spectra, where the intensity (km/mol) is plotted against the vibration frequencies, are shown in Figure 6. The resulting vibration wave numbers for the optimized geometry and the proposed assignments are given in Table 7. As seen from Table 7, the observed and the calculated spectra are in good agreement with each other.    (Table 7).

Theoretical Structures
The optimized parameters (bond lengths and bond angles) of AA, FAA, MMAA, and MCAA were obtained by using B3LYP/6-31G (d) method and listed in Tables 2-5 to compare with the X-ray experimental data.
As seen from Table 2, the biggest difference between the X-ray and calculated values of the bond lengths of AA is at N3-C8. The calculated value is 0.0291 Å longer than the X-ray value. The biggest difference between the X-ray and calculated values of the bond angles of AA is at C9-N2-C10. The calculated value is 2.2356 smaller than the X-ray value. It is because there are intermolecular N3-H3•••O hydrogen bond and the C-H•••π interactions (Figure 2), there are such differences between the X-ray and calculated values.
As seen from Table 3, for the same reason, the biggest differences between the X-ray and the calculated values of the bond length and angles of FAA is at N3-C12 and at C7-N2-C10, respectively. The differences are 0.0773 Å and 6.2166, respectively.
As seen from Table 4, the biggest difference between the X-ray and the calculated values of the bond lengths and angles of MMAA is at N1-C2 and at N1-C3-C4, respectively. The differences are 0.0278 Å and 4.4298, respectively. The reasons could be that the carbonyl oxygen atom of the pyrazole ring is involved not only in the intermolecular hydrogen bond (N1-H1•••O3) but also in the intramolecular hydrogen bond (C9-H9···O3), and that there are C-H•••π interactions between the molecules.
As seen from Table 5, the biggest difference between the X-ray and the calculated values of the bond lengths and angles of MCAA is at N3-C6 and at C5-N2-C8, respectively. The differences are 0.0285 Å and 6.0658, respectively. When the X-ray structures of the compounds are compared with their optimized counterparts (Figure 7), conformational discrepancies are observed between them. We noted that the experimental results correspond to the solid phase of the compounds and that the theoretical calculations are for the gas phase. In the solid state, there are intermolecular hydrogen bonds between molecules, and the experimental results are related to molecular packing, while isolated molecules are considered in the theoretical calculations. In spite of these small differences, calculated geometric parameters represent a good approximation and they are the basis for calculating other parameters, such as frontier orbitals and energy, and molecular electrostatic potential, as we describe later.

Frontier Molecular Orbital Analysis
Molecular orbital and their properties, like energy, are very useful for physicists and chemists and their frontier electron density used for predicting the most reactive position in -electron systems and also explained several types of reaction in conjugated system [14]. Moreover, eigenvalues of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) and their energy gap reflect the chemical activity of the molecule. Recently the energy gap between HOMO and LUMO has been used to prove the bioactivity from intramolecular charge transfer (ICT) [15,16]. The HOMO-LUMO energy gaps for the four compounds were calculated by B3LYP/6-31G (d). From the HOMO-LUMO orbital picture (Figure 8), it is found that the filled -orbital (HOMO) is mostly located on the pyrazole ring and -N(H) group of the compounds, while the unfilled anti-orbital (LUMO) is on the benzene ring. When electron transitions take place, electrons are mainly transferred from the pyrazole ring and -N(H) group to the phenyl ring. Therefore, introduction of an electron withdrawing group into the -N(H) group will reduce the energy of the HOMO. It can be seen from Figure 8 and Table 8 that the HOMO energy of AA is highest (−0.192 a. u.), and that the gap is the smallest (0.174 a. u.). It implies that the electronic transfer in AA is easier.

Molecular Electrostatic Potential
Molecular electrostatic potential (MEP) is related to the electronic density and is a very useful descriptor in understanding sites for electrophilic attack and nucleophilic reactions as well as hydrogen bonding interactions [17][18][19]. The electrostatic potential V(r) are also well suited for analyzing processes based on the "recognition" of one molecule by another, as in drug-receptor and enzyme-substrate interactions, because it is through their potentials that the two species first "see" each other [20,21]. Being a real physical property, V(r)s can be determined experimentally by diffraction or by computational methods [22].
Many researchers have used graphic models, especially MEP, as a tool in conformational analysis [23]. The fundamental application of this study is the analysis of noncovalent interactions [24][25][26][27], mainly by investigating the electronic distribution in the molecule. Thus, this methodology was used to evaluate the electronic distribution around molecular surface for the four compounds.
To visually consider the most probable sites of the molecules for an interaction with electrophilic and nucleophilic species, MEP was calculated at the B3LYP/6-31G (d) optimized geometry. While electrophilic reactivities are visualized by red color which indicates the negative regions of the molecule, the nucleophilic reactivities are colored in blue, indicating the positive regions of the molecule, as shown in Figure 9. The nitro and carbonyl oxygen atoms are surrounded by a greater negative charge surface, becoming these sites potentially more favorable for electrophilic attack. As can be seen from the results, the MEP map confirms the existence of intra-and intermolecular interactions observed in the solid state. In the MEP of AA, the main negative center includes the nitrogen atom attacked at C(4) of pyrazole ring and the pyrazole carbonyl group, which should be responsible for the interaction with the active drug-receptor sites [28]. It is clear in the MEPs that around the nitrogen atoms attached at C(4) of the pyrazole ring, FAA, MMAA and MCAA show an electronic density lower than those of AA. That is, there is a larger electronic concentration in the active sites of AA. It could be the reason for the preferential COX 2 -drug binding and in agreement with the activity observed in AA. A significant change in the molecular structure of the compounds is the presence of a different substituent attached to C(4). For AA, the substituent at C(4) is -NH 2 , while for FAA, MMAA, and MCAA, the substituents are -NHCHO, -NHCOOCH 3 , and -NH(CH 3 )COOCH 3 , respectively. The amide carbonyl substituents should increase the electronic delocalization in the molecules. The electronic density of the atoms is shown in Figure 10.

Crystallography
Single crystals of AA, MMAA, and MCAA were prepared by recrystallization from acetonitrile, acetone, and diethyl ether, respectively. The X-ray diffraction data were collected on an automated Enraf-Nonius CAD-4 diffractometer (Mo-Ka radiation, 0/20 scanning technique). The positions and thermal parameters of the non-hydrogen atoms were refined anisotropically. H atoms were positioned geometrically and refined as riding groups, with N-H = 0.86Å (for NH), C-H = 0.93, 0.93 and 0.96Å (for aromatic, aldehydic and methyl H), respectively, and constrained to ride on their parent atoms, with Uiso (H) = xUeq (C), where x = 1.2 for aromatic H, and x = 1.5 for other H. The positions of the hydrogen atoms were located according to the difference of electron density. All calculations were carried out with the use of the SHELXL-97 program package. Details of the parameters are given in Table 1. CCDC-660447, 801822, 801827, 801826 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44(0) 1222-336033; email: deposit@ccdc.cam.ac.uk].

Theoretical Calculation
The molecular structures of the compounds in the ground state (in vacuo) were optimized using DFT (B3LYP) method with the 6-31G (d) basis set with the Gaussian 03 software package. The initial configurations for calculation were constructed according to the X-ray data. Frequency calculations at the same levels of theory revealed no imaginary frequencies, indicate that the B3LYP/6-31G (d) method was the optimal one in our system.

Conclusions
In this study, three AA derivatives (FAA, MMAA, and MCAA) have been synthesized and characterized by elemental analysis, FT-IR, and 1 H-NMR spectroscopy. AA and the three derivatives were characterized by single-crystal X-ray diffraction techniques. The theoretical calculations of AA and the derivatives have been performed by using the density functional theory (DFT) method with the 6-31G(d) basis set. Although differences were observed in the geometric parameters, the general agreement is in a good range and the theoretical calculations support the solid-state structures. The experimental vibration frequencies are in a good agreement with the results of the B3LYP method. The calculated MEP map verifies the solid-state interactions.