Synthesis, spectral correlations and antimicrobial activities of 2-pyrimidine Schiff’s bases

. In the present study, a series of substituted ( E )- N -benzylidene-2-aminopyrimidine compounds have been synthesized by condensation reaction with 2-aminopyrimidine and meta - and para - substituted benzaldehydes. These synthesized Schiff’s base compounds are confirmed by their physical constants, UV, IR and NMR spectral data. All the observed UV absorption maximum λ max (nm), IR frequencies νC=N(cm -1 ), NMR δ(ppm) of C-H & C=N chemical shifts have been correlated with Hammett substituent constants and F and R parameters using single and multi-linear regression analyses in order to study the effect of substituents on these spectral data has been studied. The antimicrobial activities of all the synthesized Schiff’s base compounds have been studied using Bauer-Kirby disc diffusion method.


INTRODUCTION
Schiff's bases are an important class of organic compounds possessing biological activities and structural significance. It has been proved that the incorporation of this molecule into the biologically active azomethine linkage (-CH=N-) produces compounds with high pharmacological activity [1]. Schiff's bases are represented by the general formula RCH=NR' where >C=N is the azomethine group [2]. The colour of the Schiff's bases is due to the presence of this azomethine (>C=N) linkage and can vary by introducing other auxochromic groups. When functional groups, such as -OH, -SH, -COOH are present in these ligands in suitably tailored positions, they enable them to be powerful chelating, biological and analytical reagents. On this basis of such groups present, the Schiff's bases may be di, tri, tetra or polydentate in nature. Compounds containing azomethine group are basic in nature, as N atom of this group has a lone pare of electrons and the double bond has electron donating character. Schiff's bases displayed the phenomenon of tautomeris [3], thermochromy and photochromy [4] and geometrical isomerism [5]. They possess both types of hydrogen bonding i.e. intra-molecular as well as inter-molecular [6]. Schiff's bases are widely studied and used in the fields of organic synthesis and metal ion complexation [7] for a number of reasons: their physiological and pharmacological activities [8] their use in ionselective electrodes [9] in the determination of heavy metals ions in environmental samples [10] and in the extraction of metals ions [11,12] and their many catalytic applications (e.g. for epoxidation of olefins, alkene cyclopropanation [13,14] trimethylsilylcyanation of ketones [15] asymmetric oxidation of methyl phenyl sulfide enantioselective epoxidation of silylenol [16] and ring-opening polymerization of lactide [17]. In recent years, correlation analysis has been applied by chemists for solving the effect of substituents on Schiff's bases through spectral data. Literature survey shows that there is no information available regarding the study of UV, IR and NMR spectral correlation and antimicrobial activities of substituted (E)-N-benzylidene-2-aminopyrimidine compounds. Hence the authors have taken efforts for synthesizing substituted (E)-N-benzylidene-2aminopyrimidine compounds and studying the effect of substituents through the spectral data and their antimicrobial activities.

EXPERIMENTAL 2.1. General
All the chemicals used have been purchased from Sigma-Aldrich and E-Merck chemical companies. Melting points of all of substituted (E)-N-benzylidene-2-aminopyrimidine compounds have been observed in open glass capillaries on a Mettler FP51 melting point apparatus and are uncorrected. The UV spectra of all synthesized (E)-N-benzylidene-2-aminopyrimidine compounds have been recorded using SHIMADZU-1650 SPECTROMETER in spectral grade methanol. IR spectra (KBr, 4000-400 cm -1 ) have been recorded on AVATAR-300 Fourier transform spectrophotometer. The NMR spectra of all the substituted (E)-N-benzylidene-2-aminopyrimidine compounds have been recorded in BRUKER 400 spectrometer operating at 400 MHz for 1 H NMR spectra and 100 MHz for 13 C NMR spectra in CDCl 3 solvent using TMS as internal standard.

Synthesis of substituted (E)-N-benzylidene-2-aminopyrimidine compounds
Equimolar quantities of 2-aminopyrimidine (0.01mol), substituted benzaldehyde (0.01mol) and 0.5ml acetic acid were refluxed for 3h with 20 mL of absolute ethanol [18,19]. The completion of the reaction was monitored by TLC continuously. The resultant mixture was cooled at room temperature. Then the obtained precipitate was filtered and washed several times with cold water. A yellowish solid was obtained as the product. This crude product was recrystallized from ethanol to get glittering colorless solid, and their melting points were been observed. The general reaction is shown in Scheme 1. All the compounds synthesized in the present investigation have been identified by their physical constants, UV, IR and NMR spectral data and are shown in Table 1. The spectral data of all the synthesized substituted (E)-N-benzylidene-2-aminopyrimidine compounds have been correlated with Hammett substituent constants and F and R parameter and are shown in Table 2.

UV-Visible spectral correlations
The assigned UV absorption maximum λ max (nm) values of all the substituted (E)-N-benzylidene-2-amino pyrimidine compounds are presented in Table 1. These UV absorption maximum λ max (nm) values are correlated with different Hammett substituent constants and F & R parameters using single and multi-linear regression analyses according to the approach of John Shorter [22]. Hammett equation employed, for the correlation analysis, involving the UV absorption maximum is shown in equation (1). λ = ρ σ + λ 0 (1) where λ 0 is the absorption maximum of the parent member of this series.

NMR spectral correlation
The observed chemical shift values (ppm) of all the substituted (E)-N-benzylidene-2-amino pyrimidine compounds are presented in Table 1. These chemical shift values (ppm) are correlated with different Hammett substituent constants and F & R parameters using single and multi-linear regression analyses according to the approach of Reynolds [32,33]. In this correlation, the structureparameter equation employed is shown in equation (7). δ = ρσ +δ 0 (7) where δ 0 is the chemical shift of the corresponding parent compound.

1 H NMR spectral correlation:
From the results of the statistical analyses [18,[23][24][25][26][27][28][29] in Table- The failure in correlation is due to the reason that has been stated earlier with resonance conjugative structure as shown in Figure 1. All the correlations with Hammett substituent constants F & R parameters have shown positive ρ values. It indicates the operation of normal substituent effect with respect to 1 H NMR spectral data of all the compounds.

13 C NMR spectral correlation
The results of the statistical analyses [18,[23][24][25][26][27][28][29] are presented in Table 2. From Table 2, it is evident that the 13  0.905). The remaining Hammett constants σ I , σ R and Swain-Lupton's [30] F & R parameters have shown poor correlations (r < 0.900). The failure in correlation is due to the reason that has been stated earlier with resonance conjugative structure as shown in Figure 1. All the correlations with Hammett substituent constants F & R parameters have shown negative ρ values. It indicates the operation of reverse substituent effect with respect to 13 C NMR spectral data of all the compounds. Some of the single regression analyses have shown poor correlations. Hence, it is decided to go for multi-regression analysis. While seeking the multi-correlation collectively the inductive, resonance and field effects [30] have shown satisfactory correlation as shown in the equations (10) and (11).

ANTIMICROBIAL ACTIVITIES 4.1. Antibacterial sensitivity assay
The antibacterial activities of all synthesized substituted (E)-N-benzylidene-2aminopyrimidine compounds have been studied against three gram positive pathogenic strains B. substilis, M. lutes, S. aureus and two gram negative strains E. coli and P. aeruginosa. The disc diffusion technique has been followed using the Kirby-Bauer method [34], at a concentration of 250 μg/mL and ciprofloxacin was used as standard. The antibacterial screening effect of prepared substituted (E)-N-benzylidene-2-aminopyrimidine compounds is shown in Figure 2 (Plates 1-10). The measured zone of inhibition values are given in Table 3 and the corresponding clustered column chart is shown in

Antifungal sensitivity assay
The antifungal activities of all synthesized substituted (E)-N-benzylidene-2aminopyrimidine compounds have been studied against three fungal species namely A. niger, M. species and T. species. The The disc diffusion technique has been followed using the Kirby-Bauer method [34], at a concentration of 250 μg/mL and Ciprofloxacin was used as standard. The antifungal screening effect of prepared substituted (E)-N-benzylidene-2-aminopyrimidine compounds is shown in Figure 4. (Plates 1-4). The measured zone of inhibition values are given in Table 4 and the corresponding Clustered column chart is shown in

CONCLUSIONS
A series of substituted (E)-N-benzylidene-2-aminopyrimidine compounds have been synthesized by condensation method. These compounds were confirmed by their physical constants and spectral data. The spectral data of these compounds have been correlated with Hammett sigma constants and F & R parameters using single and multi-linear regression analysis. Most of the single linear regression analyses have been satisfactory correlations for all the substituents with UV, IR and NMR spectral data. Some of the compounds have been shown good antibacterial and antifungal activities. Most of the compounds have been shown moderate activity.