Synthesis, Characterization and Single Crystal Structure of Carbanionic Sigma Complexes of Sodium and Potassium Salts

Barbituric acid derivatives are very much effective in anxiolytics, hypnotics and anticonvulsant agents. In this context in the present investigation, new molecular salts have been synthesized in the crystalline form. They have been characterized through UV-Vis, IR, 1H NMR, 13C NMR, and X-ray diffraction studies. In this anion, 1,3-dimethylbarbituric acid ring and substituted dinitrophenyl ring, linked via a C-C bond.


INTROdUCTION
Synthetic nitroaromatic compounds widely used in a variety of materials, including drugs 1-3 , pharmaceutical dyes 4-5 , explosives [6][7] , plastics, and pesticides. Several high energy density molecules are derived from nitroaromatic compounds [8][9][10] . Nitroaromatic compounds are electron-deficient and react readily with electron-rich species. With electron-rich organic and inorganic bases, they undergo many types of reactions [11][12][13][14] . Depends on the nature of electron-deficient nitroaromatic compounds and electron-rich species, they had three different types of interactions which occur, leading to the formation of Charge transfer complexes 15 , Radical anion 16 and Anionic sigma complexes 17 .
2, 4-dinitrochlorobenzene (1-chloro-2, 4-dinitrobenzene) is a unique functional group used in medicinal chemistry. Nitro group is associated with mutagenicity and genotoxicity and therefore is often used in the drug discovery process 18 . The nitroaromatic compound is also used to synthesize dyes and high energy materials. These compounds are very common environmental pollutants that are used as pesticides and explosives 19 . DNCB molecule used in the preparation of many diverse organic molecules and biologically active molecules in recent years.
Barbituric acid (1,3-barbituric acid) has an active methylene group, i.e., (a cyclic molecule). It is expected to form a carbanion in the presence of a base. The C-5hydrogen atom substitution gives the complexes. Dyes involving active methylene group of barbituric acid have been considered in recent years 20,21,22,23 .

Mechanism
Strong interaction between the base and the aromatic compound results in sigma adduct formation 24 . A particular transfer of electronic charge from the base to the aromatic nucleus depleted of π electron density gives rise to the π-complex known as donor-acceptor or charge-transfer complex 25 . The complete transfer of an electron from the base to the nitroaromatic compound results in the formation of radical anion 26 . If the nitroaromatic bears a substituted alkyl group, carbanion is formed due to proton abstraction 27 . The mechanism is given in scheme 1.
is dissolved in 30 ml of ethanol be mixed with 1,3-dimethyl barbituric acid in 30 ml of ethanol. Then added Sodium/potassium methoxide in 30 ml of ethanol and shaken for 2-3 hours. The excess ethanol be removed by distillation also the remaining solution was kept for one week when red colored crystals separated. These crystals recrystallized from distilled ethanol.

Thin Layer Chromatographic studies
The study was carried out to check the purity of the isolated molecules. Silica gel was used as an adsorbent. The slurry of silica gel was made with chloroform and coated uniformly on a TlC plate. The complex solution was prepared in pure absolute ethanol and spotted on the plate. The eluent used was the mixture of tertiary butyl alcohol and ethyl acetate.

RESULTS ANd dISCUSSION
In the present investigation, a red crystalline solid is obtained and the yield is 42% and 75%.

Crystal structure determination of complexes Crystal structure of sodium salt
The amorphous solid of sodium salt is crystallized from ethylene glycol. The sodium cations are bridged through an oxygen atom of 1-chloro-2, 4-dinitro benzene and 1, 3-dimethyl barbituric acid. In barbiturate residue bond lengths, bond angles are well-matched with that of barbiturate ion 42 and it shows the delocalization of negative charge. The crystal data represent selected bond angles, bond distances of sodium salt. According to X-ray crystallographic data, the isolated molecule is monoclinic system. The unit cell parameters having P121/n1 space group a=7.3269(5) Å, b=19.9241(13) Å, c=11.5431(9) Å, V=1665.1(2) Å 3 ; z=4.

Scheme 2
The carbanionic sigma complex can also be made as sensors in many fields such as industry, environment, and biotechnology.

Uv-visible Analysis
In the UV-Vis spectrum, the two peaks are formed with λ max values of 287.30nm, 484.34nm, and 245.93 nm, 446.86 nm (Figure 2, 2a).
Here, C-Cl band is absent in the synthesized molecule The broad band observed between ~3600-2100 cm -1 is characteristic of amine salt [43]. Carbonyl stretching frequency of 1,3-diethyl barbituric acid 1680 cm -1 and 1670 cm -1 during the formation of the complex.

NMR Analysis
1H NMR spectra are depicted in (Fig. 4, 4a). This shows two peaks characteristics of the ring protons, one at δ 8.1ppm (S, 2H) and another at δ 8.3 ppm (S, 1H) are observed. The peak corresponding to six methyl protons appears at δ 3.1 ppm. 13 C NMR spectra of molecular salts are presented in (Fig. 5,  5a). 13 C NMR spectrum indicate ten different carbon environments in the molecule. The peaks at 141.5 ppm and 87.1 ppm represents formation of C=C.

CONCLUSION
The present study focused on the characterization through various studies of carbanionic sigma complexes. Spectral data are inconsistent with the putative structure and further evidenced by single crystal XRD data. Various types of interactions possible between electron deficient compounds nitro aromatic compounds and bases are described. The cation and anion moiety are connected by hydrogen bonds. The complex for this work is stable in nanosize. Therefore, these may be changed into a potent drug in future.