Molecular Structure, Hirshfeld Surface Analysis, Optical and Electrochemical Studies of Stable Solid Diazonium Salt of p-Sulphophenyl-3-methyl-5-pyrazolone

The title compound is a new pyrazolone derivative which was synthesized starting from p-sulphophenyl-3-methyl-5-pyrazolone (1) by nitrosation at low temperature to afford the corresponding p-sulphophenyl-3-methyl-4-nitroso-5-pyrazolone which can exist both in nitroso (2a) and oxime tautomeric forms (2b). Reduction of the latter using zinc with hydrochloric acid furnished the 4-amino-p-sulphophenyl-3-methyl-5-pyrazolone (3). The diazotization of (3) under careful control of temperature and pH afforded the p-Sulphophenyl − 3-methyl-5-pyrazolone diazonium salt (4) which was re-crystallized from acidied ethanol to afford crystal suitable for X-ray studies. UV-visible spectrum and cyclic voltammetric studies were also carried out indicating λ max at 420 nm and HOMO-LUMMO energy gap was also calculated (E g ) of 2.95 eV. The molecular and crystal structures of the compound were claried by single crystal X-ray diffraction indicated that it crystallizes as the sodium salt in the triclinic space group P -1, with the 4-azo-pyrazolone and the sulphophenyl groups being nearly coplanar. To get an insight to the intermolecular interactions in the crystal a Hirshfeld surface analysis was also carried out.


Introduction
Aryl diazonium salts are versatile intermediates having a variety of applications in organic syntheses [1].
In addition, aryl diazonium salts are surrogate to aryl halides, which are mostly used in transition metal mediated cross coupling reactions for carbon-carbon and carbon-hetero atom bond formation [2]. Good leaving nature of the stable N 2 molecule found in diazonium salts makes them versatile electrophiles which do not interfere with the reaction mixture and allows the use of ambient reaction conditions. The chemistry of diazonium salts has been studied for long starting around 1858. There are several named reactions associated with aryl diazonium salts including the Sandmeyer reaction (1884), the Pschorr cyclization (1896), the Gomberg-Bachmann reaction (1924), and the Meerwein arylation (1939) [3]. Meerwein rst reported the arylation of coumarin, cinnamic acid, and acrylic acid with aryl diazonium salts catalyzed by copper (II) salts [4]. Later, the scope of this reaction was further extended to electron rich ole ns [5]. New improved variants of Meerwein and Pschorr reactions have been developed by several researchers to achieve the syntheses of complex organic molecules [6]. The syntheses of benzothiophenes from the corresponding o-methylthio aryl diazonium salts have been investigated by Zanradi [7]. Heinrich and coworkers reported different types of Meerwein arylation reactions by employing TiCl 3 and FeSO 4 as chemical reductants [8][9][10]. Schiesser reported the syntheses of benzoselenophene and benzothiophene through a radical cyclization process involving the addition of aryl radicals to alkynes [11]. Alternatively, aryl radicals can also be generated from diazonium salts using organic reducing agents such as tetrakis(dimethylamino)ethylene (TDAE) [12]. The aryl diazonium salt generates an aryl radical and dinitrogen by taking up an electron from the reducing agent; in the classical reactions the catalytic or stoichiometric amounts of transition-metal salts have been used. Visible light can also provide the required redox energy and it is an ideal reagent for organic syntheses [13]. Many research groups focus on the heterocyclic azole compounds, especially pyrazolone derivatives which have a wide range of pharmacological applications such as analgesic [14], anti-in ammatory, anti-cancer [15] and antimicrobial activities [16]. The pyrazolone scaffold was widely used in treating neurodegenerative disorder [17], cardiovascular diseases, rheumatoid arthritis (RA), osteoarthritis (OA) and Crohn's disease.
Though the pyrazolone derivatives exhibit widespread pharmacological properties, the literature review shows that they are highly related to the treatment of in ammatory diseases [19][20][21][22].
In this work, a new diazonium pyrazolone derivative, namely p-sulphophenyl-3-methyl-5-pyrazolone diazonium, is synthesized by diazotization of 4-amino-p-sulphophenyl-3-methyl-5-pyrazolone under careful control of temperature and pH. The molecular and crystal structures of the sodium salt were determined by X-ray diffraction technique. The electronic and electrochemical properties were determined in aqueous media by UV-visible and cyclic voltammetric studies.

Materials and Method
p-Sulphophenyl-3-methyl-5-pyrazolone was obtained from Sigma-Aldrich, sodium nitrite and hydrochloric acid were purchased from Daejing Korea. Zinc dust, sulfamic acid and sodium hydroxide were common laboratory grade chemicals.
A previously cooled solution of NaNO 2 (6.9 g, 0.1 mol) in H 2 O (25 ml) at 0°C, was then added to the reaction mixture over a period of 35 min with stirring which was continued for 1 h maintaining the same temperature, until a positive test for nitrous acid formation. The excess of nitrous acid was destroyed with required amount of sulphamic acid till the (nitroso/oxime) (2) was ltered after salting out. The nitroso was reduced by stirring in water (200 ml) containing HCl (85 ml) and Zinc metal (23 g) at re ux for 4 h. On completion of the reaction, the pH was raised to 9 using 6N NaOH, to precipitate the 1-(pmethylphenyl-3-methyl − 4-aminopyrazolone (3).

X-ray crystallography
The crystallographic data of the diazonium salts were collected on a Bruker AXS SMART APEX diffractometer using Mo K α radiation (λ = 0.71073 Å) [23]. The multi-scan absorption correction [SADABS] [24] applied data were processed by SHELX97 program package [SHELXS97 and SHELXL97] [25] for solving and re ning the structures, and ORTEP-3 [26] and PLATON [37] programs were used in drawings. All non H-atoms were re ned anisotropically, water hydrogen atoms were clearly located from difference Fourier maps. Remaining hydrogen atoms were geometrically positioned and re ned by riding on the carbon atoms with isotropic displacement parameters U iso (H) = 1.2 U eq (C) or 1.5 U eq (CH 3 Table 1.

Synthesis and X-Ray Crystal Study
Synthesis of diazonium salt of (4) was achieved according to the route sketched in Scheme 1.
1-(4-sulphophenyl) 3-methyl-2-pyrazolin-5-one (1) was nitrosated at -2-0 o C using NaNO 2 and HCl as described by Knorr 1 to afford the nitroso compound (2) which was ltered to remove some terry material. The nitroso derivative (2a), usually exists in its tautomeric oxime (2b) form as indicated by its FTIR spectrum, was salted out by common salt, and dried after ltration. Reduction of (2) was achieved and zinc metal in the presence of HCl added in small portions at re ux to afford a colorless solution. A small amount of additional zinc was added, and the resultant amine hydrochloride was quenched to -7 ºC. The excessive un-reacted zinc was removed by ltration. The amine hydrochloride (3) was diazotized very carefully by using an aqueous solution of NaNO 2 and HCl at -5 to -3 ºC to provide the title compound (4).
The careful control of temperature is essential to avoid the formation of rubazoic acid, which is routinely formed during this reaction on increasing temperature due to oxidizing action of nitrous acid formed in situ.
In the molecular structure of diazonium salt the phenyl-as well as the pyrazolone-rings lie almost in plane, the relevant torsion angle C1-N1-C5-C6 The crystal structure (Fig. 2) shows various hydrogen bonding patterns with the solvent water molecules.

Optical and Electrochemical Studies
The optical and electrochemical studies of newly synthesized diazonium salt were conducted to get information regarding the absorption intensity and redox behavior.
The UV-visible absorption spectrum of diazonium salt of 4-amino-p-sulphophenyl-3 -methyl − 5pyrazolone in ethanol is shown in Fig. 8. The absorption maximum (λ max ) of diazonium was taken in ethanol (1x10 − 5 M solution) to observe the molar extinction coe cient of the compound. The molar extinction was found to be log = 6.5 and λ max of diazonium was found at 420 nm. This transition can be assigned as due to n-π* and π-π* transitions of the azo linkage. This value of the molar extinction coe cient is the evidence of the high absorption intensity. These transitions are bathochromic shifted to higher λ max due to the presence of o-hydroxy group to diazo linkage. The hydroxy group has the ability to donate the lone pair of electrons to ring which increases the electron density at the chromophoric motif and hence decreases the energy required for electronic transitions. Moreover, this compound undergoes tautomerism and keto form is formed which increases the stability of azo linkage through strengthening the carbon to nitrogen atom of azo linkage.
The electrochemical characterization of diazonium salt was made by cyclic voltammetry using DMSO containing 0.1 M TBAPF 6 as a supporting electrolyte with glassy carbon electrode. All redox potentials, HOMO (highest occupied molecular orbital), LUMO (lowest unoccupied molecular orbital) and band gap energies (E g ) were calculated from this technique.
The reduction potential of diazonium compound was observed from cyclic voltammogram (Fig. 9) and was found to be of valued 0.9 V. By utilizing the reduction potential of the compound, the HOMO and LUMO levels were determined which were found at levels of -6.45 and − 3.5 eV, respectively, with the help of Bredas Equations (Eqs. 1, 2 and 3). This diazonium has a high electron a nity (LUMO level − 3.5 eV) and may act as an acceptor material for organic heterojunction solar cells. The band gap energy (E g ) of the compound was 2.95 eV (

Application of Diazonium Salt in Acid Azo Dyes
The p-sulphophenyl − 3-methyl-5-pyrazolone diazonium salt was employed in the syntheses of a vast range of acid dyes and their Cr (III), Fe (II) and Cu (II) metal complexes with varied hues from yellow to dark brown, depending upon the nature of coupler component used (Scheme 2, Fig. 10-12). The absorption maxima of the synthesized complexes were found to be in the range of 475-530 nm. In order to observe the application properties, dyes were applied on leather and they exhibited high values of fastness properties (Tables 6 and 7).

Conclusion
In this work it has been found that the amine hydrochloride of (4) was stable at -7 C. The best diazotization temperature for this amine has been found to be -5 to -3 C. At a temperature, above this range, amine hydrochloride is oxidized by nitrous acid to form rubazoic acid. Similarly, the formation of rubazoic acid is formed at higher pH as well. The higher rate of nitrite addition forms greater amount of nitrous acid that favored the formation of rubazoic acid, while a slow addition of nitrite lead to more of diazotization. The title diazonium compound has been isolated and found to be stable in dry form. It The asymmetric unit of the title compound. Anisotropic displacement ellipsoids are drawn at the 50% probability level.

Figure 2
A partial crystal packing of p-Sulphophenyl -3-methyl-5-pyrazolone diazonium with hydrogen bonding patterns shown as dotted lines. H-atoms not involved are omitted for clarity.

Figure 3
Page 18/23 View of the three-dimensional Hirshfeld surface of the title compound plotted over dnorm in the range of -0.6911 to 1.2708 a.u.

Figure 4
View of the three-dimensional Hirshfeld surface of the title compound plotted over electrostatic potential energy in the range -0.0500 to 0.0500 a.u. using the STO-3 G basis set at the Hartree-Fock level of theory.
Hydrogen-bond donors and acceptors are shown as blue and red regions around the atoms corresponding to positive and negative potentials, respectively.

Figure 5
Hirshfeld surface of the title compound plotted over shape-index.   Cyclic voltammogram of diazonium salt (4) in DMSO.

Figure 10
Application of Cu (II) complex acid dyes 8a-g on leather at 2 and 5 % depth.

Figure 11
Application of unmetallized acid dyes 10a-f on leather at 2 and 5 % depth.