Abstract study the relative importance of steric hindrance

AbstractAdduct formation constant of nickel(II)di(o-chlorophenyl)carbazonate with different monodentate &bidentate nitrogen bases were determined spectrophotometrically using Job’scontinuous variation method at 25±0.10C in chloroform medium.

Theinfluence of electron withdrawing substituents on the adduct formation constantof the nickel(II) di(o-chlorophenyl)carbazonate has been studied. The resultshave been discussed in terms of the basicity, steric factors of various nitrogenbases & ring structure.Keywords:Steric hindrance, N ligands (nitrogenbases), spectrophotometer, nickel(II) di(o-chlorophenyl)carbazonate, adductformation constant.

                     1.     IntroductionNickelis one of the several metal ions that play important role in biological system1,2 andit forms stable complexes with N, O & S donor ligands3. Diphenylcarbazone, di(o-chlorophenyl)carbazone and its analogs are toxic towardsbacteria and fungi activities4. Diphenyl carbazone and itssubstituents form highly colored metal complexes4 with Ni (II), Co (II),Cu (II) that offer the basis for their use as sensitive reagents for detection& determination of metal ions spectrophotometrically. Di(o-chlorophenyl)carbazoneis a bidentate neutral ligand, which coordinates through the ketonic oxygen andazo nitrogen to bivalent metal ions viz., Ni (II), Co (II), Cu (II) and formscorresponding metal chelates4.The determination of stabilityconstants of adducts of metal chelates of diphenyl carbazones with nitrogenbases forms the paramount importance of the knowledge of chelates in analyticalchemistry. From the literature survey5-13 it is found that much workhas been done in the evaluation of factors such as ligand basicity, solventeffect & steric effects in the study of adduct formation from metalchelates.

Although it has been found that there were very few referencesregarding the adduct formation constants of nickel (II) chelates of di(o-chlorophenyl)carbazone with nitrogen bases and also little is known about the factorsaffecting the formation of the adduct.The present investigation is anattempt to study the relative importance of steric hindrance with pyridine,substituted pyridines, aniline, substituted anilines and other monodentate andbidentate nitrogen bases with nickel(II)di-o-chlorophenylcarbazonatespectrophotometrically using Job’s method of continuous variation in chloroformmedium at 25±0.10C.2.     Materialsand methods2.1 Apparatus:            Bausch and Lomb spectronic 2000 spectrophotometerwas used to determine the composition of the complex at 25±0.10C.

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2.2 Reagents:All the reagentswere of analytical grade. Di(o-chlorophenyl)carbazonewas prepared by the method described elsewhere14, involvingpersulphate oxidation of di(o-chlorophenyl)carbazide. Di(o-chlorophenyl)carbazonewas synthesized by heating o-chlorophenylhydrazine and urea for 3 hrs at 1550C.

The melting point of di(o-chlorophenyl)carbazone was found to be 1340C. Aniline, 2-methylaniline, 3-methylaniline(B.D.H), 2,4-dimethylaniline, 2,6-dimethylaniline, 2-chloroaniline,3-chloroaniline, 4-chloroaniline, 4-bromoaniline (Merck),  pyridine(Fisher),  2,4-lutidine(dimethyl analogue of pyridine), 3,4-lutidine (dimethyl analogue of pyridine),  morpholine, piperidine, 2-chloropyridine (Merck), 2-picoline (methyl analogue ofpyridine), 3-picoline (methyl analogue of pyridine), 4-picoline (methylanalogue of pyridine), 2,4,6-collidine (trimethyl analogue of pyridine),ethylenediamine(B.D.H) were dried over potassium hydroxide and distilled.

Theconstant boiling fraction was collected and used. 4-Methyl aniline (Aldrich), neocuproine, 2,2′-bipyridyl, 1,10-phenanthroline,ethylenediamine (Merck), 2-aminopyridine, 4-aminopyridine, 2-amino-4-methylpyridine, 2-nitroaniline, 3-nitroaniline, 4-nitroaniline (B.D.H) and nickel chloride hexahydrate (Merck) were usedwithout purification. Chloroform, 1,4-dioxane, DMSO, DMF used were of Fisher ARgrade.2.3 Preparation ofNickel Di(o-chlorophenyl)carbazonate:About1g of nickel chloride hexahydrate was dissolved in 50ml of acetate buffer of pH6.

The solution was heated to 600C and to this hot solutionalcoholic solution of di(o-chlorophenyl)carbazone was added dropwise. The mixturewas diluted with water and the precipitate obtained was digested on low heatfor an hour. It was filtered under suction and washed several times with waterand finally twice with alcohol to remove unreacted metal and ligand residues.The product obtained was dried at room temperature over phosphorus pentoxideunder vacuum for 12 hrs.

The nickel content of the complex was found to be 8.70%by phenanthroline-dithizone method15 (Calculated 8.68%).

  2.4 Selectionof analytical wavelength:Vosburghand Cooper method16 was used for the selection of analytical wavelength.A series of solutions were made by mixing different volume of stock solution ofreactants in the same solvent. The absorbance was scanned between 400-800nm inchloroform as a reference to determine the ?maxusing a UV-visible spectrophotometer. The wavelength of maximum absorption wasfound to be 520nm (Fig.1).              Figure 1: Absorption spectra of  Ni(OClPC)2 & aniline mixture inchloroform: Ni(OClPC)2=1.5X10-5M.

AnilineX10-1M= (a) 0.00, (b) 0.05, (c) 0.10, (d) 0.20,(e) 0.40, (f) 0.80, (g) 1.

20, (h) 2.00, (i) 3.00, (j) 5.00, (k) 9.00.

 2.5 Determination of AdductFormation Constants:Adduct formation constants, Kf of nickel(II)di(o-chlorophenyl)carbazonateNi(OClPC)2 with nitrogen bases have been determined by Job’smethod of continuous variation a reliable method for investigation of adductformation,  after selecting thewavelength of maximum absorption by Vosburgh and Cooper method16. 0,1.0, 2.0, 3.0, 4.0, 5.0, 6.

0, 7.0, 8.0, 9.

0, 10 ml of  5.9X10-5M Ni(OClPC)2  were pipetted in 25 ml volumetric flask andmixed with 10, 9.0, 8.0, 7.0, 6.0, 5.0, 4.

0, 3.0, 2.0, 1.0, 0 ml 5.9X10-5Mnitrogen base was added keeping both the number of moles of Ni(OClPC)2and nitrogen base constant but varying either mole fractions of Ni(OClPC)2or ligand (whereas in case of most of the monodentate bases 5.9X10-1MNi(OClPC)2  and 5.9X10-1Mnitrogen bases were used).

The absorption for all the compositions was recordedat a constant wavelength (?max,520nm). The data of absorption and percentage composition of Ni(OClPC)2and nitrogen base solution at the constant temperature (25±0.10C) wereused and curves were constructed. A typical plot is shown in Figure.2.                   Figure 2. : Job’s curves of equimolar solutions of Ni(OClPC)2 – nitrogenbases at25±0.10C in chloroforma)      Job’splot for Ni(OClPC)2 (5.

9X10-5M) and 1,10-phenanthroline(5.9X10-5M)at 270C in chloroform medium.b)      Job’splot for Ni(OClPC)2 (5.9X10-5M) and 2,2′-bipyridyl(5.

9X10-5M) at270C in chloroform medium.c)      Job’splot for Ni(OClPC)2 (5.9X10-3M) and pyridine (5.9X10-3M)at 270C inchloroform medium.

d)      Job’splot for Ni(OClPC)2 (5.9X10-1M) and 2,4-dimethyl aniline(5.9X10-1M)at 270C in chloroform medium. 3.     CalculationsBy constructing a graph ofabsorbance vs mole fraction of a metal chelate, the ratio of metal chelate to thebase was determined.

Formation constants of the adducts of Ni(OClPC)2 withnitrogen bases were determined by Job’s method of continuous variation by usingthe following equation2,17,                                            —————- (1)Where,  Kf: adduct formation constant             A1: absorbance at breakpoint             A2: actual absorbance             CM: concentration of Ni(OClPC)2             CL: concentration of base (ligand)4.     Resultsand discussionThe literature survey revealsthat few workers attempted to study the stability constant values of divalentmetals with different organic bases3,17-22. Thepresent study of the determination of stability constant of Ni(OClPC)2with N-bases considered as the most accurate method for evaluation of stabilityconstant of complexes.Ni(OClPC)2 isparamagnetic23, air stable, non-hygroscopic and soluble in polar& non-polar solvents.

It forms red colored solution in organic solventslike chloroform & carbon tetrachloride(non-coordinating solvents) and formspink color in anilines, substituted anilines, pyridines and substitutedpyridines (N-bases), this may be probably due to the formation of the Ni(OClPC)2– N-base adduct of the following type:Ni(OClPC)2  + n Base         ————-> Ni(OClPC)2 . n Base ……………(1)The equilibrium constant of thenickel(II)chelate-base adduct formed from the Ni(OClPC)2 andnitrogen bases can be studied spectrophotometrically. Since Ni(OClPC)2  and nitrogen base adduct of Ni(OClPC)2  absorbs at different wavelengths. A chloroformsolution of Ni(OClPC)2 exhibits a ?maxat 580 nm, the addition of nitrogen base to   Ni(OClPC)2  shifts the band to around 520 nm. The changein the spectra of Ni(OClPC)2  on the addition of nitrogen base could be used to determine theformation constant of adducts24 (Figure.1).The stability constants of theNi(OClPC)2 with nitrogen bases at 25±0.

10C are listedalong with their pKa values in Table1.The stabilities ofnickel(II)di-o-chlorophenylcarbazonate adducts were found to increase in thefollowing order of bases:2-chloro aniline < 2,6-dimethyl aniline,2-chloropyridine < 3-chloro aniline < piperidine < 4-bromo aniline<  morpholine < 2-methyl aniline< 2,4-dimethyl aniline < 4-chloro aniline < quinoline < 2-picoline< 2,4-lutidine < 2,4,6-collidine < aniline < 3-methyl aniline <4-methyl aniline < 2-aminopyridine < 2-amino-4-methylpyridine < 3-picoline < pyridine < 4-picoline,  3,4-lutidine < 4-aminopyridine

44a 2   1.31 2     3-Me aniline 4.73a 2   2.39 2     4-Me aniline 5.08a 2   2.64 2     2,4-di Me aniline 5.00a 2   1.

33 2     2,6-di Me aniline 4.10a 2   0.92 2     2-chloroaniline 2.65a 2   0.

84 2     3-chloroaniline 3.46a 2   1.04 2     4-chloroaniline 4.15a 2   1.50 2     4-bromoaniline 3.86a 2   1.18 2     Piperidine 11.

12a 2   1.17 2     Morpholine 8.35b 2   1.

30 2     Quinoline 4.90a 2   1.79 2     Pyridine 5.20b 2   3.21 2     2-picoline 5.90b 2   1.83 2     3-picoline 5.86b 2   3.

17 2     4-picoline 6.08b 2   3.41 2     2,4-lutidine 6.99b 2   2.

11 2     2,6-lutidine   2     2     3,4-lutidine 6.52b 2   3.41 2     2,4,6-collidine 7.43b 2   2.21 2     2-chloropyridine – 2   0.92 2     2-aminopyridine 6.82c 2   3.

03 2     4-aminopyridine 9.25c 2   3.66 2     2-amino-4-methylpyridine – 2   3.15 2     2,2′-bipyridyl 4.40b 1   5.28 1     1,10-phenanthroline 4.

95b 1   5.30 1     2,9-neocuproin 5.85b 1   5.19 1     Ethylenediamine 6.84b 1   5.26 1     aT.Suresh, S.

S.Hippargi, V.H.

Kulkarni, Natl. Acad. Sci.

Lett. (India), 1994, 17(7),141-144.bA.H.M.Siddalingaiah, A.F.Doddamani, G.

Sunil Naik, Spectrochim. Acta, Part A, 2001, 57,2721-2727.cC.W.Robert, Hand Book of Chemistry and Physics. Cleveland, Ohio. 1976.

The stability constant values foradducts of  Ni(OClPC)2  withanilines, substituted anilines, pyridines and substituted pyridines increasessteadily with the increase in pKa valuesas expected. In case of 2-methylaniline,2-chloroaniline & 2-aminopyridine there is a decrease in the stability ofadducts compared to aniline and pyridine respectively, even though their pKa values are higher than aniline andpyridine, which is indicative of steric hindrance due to the methyl, chloro andamino groups in the ortho position. A similar effect was also seen in the caseof 2,4-dimethylaniline, 2,6-dimethylaniline and hence these bases did not formstable adducts as expected by observing their pKa values.The stability constant of4-chloroaniline was found to be higher than 4-bromoaniline. Since in case of4-substituted aniline and pyridine stability constant strongly depends on theelectron withdrawing or donating character of the substituent25,therefore stability constant of 4-chloro adduct relative to the 4-bromo adductreflects the increased donor power of nitrogen atom due to substituent effect.Similarly, stability constant of 2-chloropyridine was found to be lower thanthat of 2-aminopyridine this can also be attributed to the electron withdrawingor donating character of the substituent in nitrogen base.The smaller formation constantvalues of the adduct of Ni(OClPC)2  with morpholine may be because of thedecreased basicity of the nitrogen atom in the morpholine ring. The formationconstant values for the piperidine are not so high, this may be attributed tothe saturity of the ring.

Stability constant value for theNi(OClPC)2 – 4-picoline adduct is found to higher than that of Ni(OClPC)2– pyridine, this is due to the methyl substitution at position 4 is morereactive due to increased basicity and it does not possess any stericallyhindering group. Identical effects would be expected for substitution in the2-position, so that much lower stabilities observed for the adduct of2-picoline. Among lutidines, 2,6-lutidine forms weaker adducts, as both themethyl groups being adjacent to the nitrogen atom, and hence offers strongsteric hindrance. In case of 2,4-lutidine & 3,4-lutidine, there is adecrease in the stability of adducts. This is indicative of steric hindrancecaused by the methyl groups present adjacent to the nitrogen atom.The lower stabilities of2,4-lutidine, 2-picoline & 2,4,6-collidine compared to pyridine are due to thesteric hindrance offered by the methyl groups present at 2- & 6- positionseven though their pKa values are higher than pyridine.

The stability constant of2-aminopyridine & 2-amino-4-methyl pyridine are found to be lower than thatof 4-aminopyridine, a decrease in the stability of adducts is due to the sterichindrance caused by the amino group in the ortho position.Ni(OClPC)2 – anilineadducts exhibit low values of the formation constant (logKf), however, Ni(OClPC)2 – pyridine adductsshow comparatively higher values of the formationconstant this may be attributed to the weak basic character of aniline andsubstituted anilines compared to pyridine and substituted pyridines. Thus, itmay be anticipated that the stronger the basic character the stronger will bethe bonding in Ni(OClPC)2-L adducts.Whereas in case of 2,2′-bidentateligands like bipyridyl, 1,10-phenanthroline and ethylenediamineequally stable adducts are found to be formed, since these are not adversely affected by steric hindrance and also due to therearrangement of the chelate ring in order to form cis-position for bidentate adducting bases26. But 2,9-neocuproine, a bidentate ligand whichforms less stable adduct due to the steric hindrance caused by the methylgroups which are adjacent to nitrogen atom blocks the nitrogen atom and hencedecreases the stability of the adduct. It is surprising to find that the absorption spectrumof Ni(OClPC)2 does not change upon addition of 2-nitroaniline,3-nitroaniline & 4-nitroaniline.

This may be due to the fact that thesenitrogen bases are very weak and hence do not form adducts with the Ni(OClPC)2 under our experimentalconditions.Further calculated logKf values ofNi(OClPC)2 – nitrogen base adducts were compared with the   logKf values of Ni(DPC)2, log Kf values ofNi(DPC)2 are given Table 1. The comparative study reveals that logKfvalues Ni(OClPC)2 – nitrogen base adduct are lower than that of the logKf values of Ni(DPC)2 – nitrogen baseadduct. This can beattributed to the electron withdrawing chloro substitution in Ni(OClPC)2 chelate,since existance of chloro group in chelate decreases the strength ofinteraction between the chelate and nitrogen base and there by causescomparatively weaker bond in Ni(OClPC)2 – nitrogen base adducts.5.      ConclusionIn the reaction between aniline,pyridine, substituted anilines, substituted pyridines & other bidentatenitrogen bases with Ni(OClPC)2 chelate, adduct formation constantshave been evaluated by Job’s continuous variation method spectrophotometrically.Further influence of electron withdrawing substituents,  steric hindrance caused by substituent groups& basicity of nitrogen bases on the stability constant of Ni(OClPC)2 chelatewith different mono & bidentate nitrogen bases were studied. Further logKfvalues of Ni(OClPC)2 – nitrogen base adduct  were compared with the logKfvalues of Ni(DPC)2 – nitrogen base adducts, variation in thestability constant values have been discussed in terms of electron donar andacceptor  substituents present in chelatemolecule.

  AcknowledgementOne of the author (Shivaraj)wishes to express his sincere thanks to the Government of Karnataka, forproviding Devaraj Urs fellowship in carrying out this research work. Authoursalso thank the University authorities for providing facilities to carry outthis work. References1 M.B.Ibrahim,A.Moyosore, Arabian J. Chem., 2014, 5(1), 51-55.

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