Indian Journal of Chemistry Vol.26A, June 1987, pp....
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Indian Journal of Chemistry Vol. 26A, June 1987, pp. 523-525 Studies on Barbituric Acid Complexes of Lanthanide Ions S TABASSUM, K S SIDDIQI·, N H KHAN, R I KURESHY & S A A ZAIDI Department of Chemistry, Aligarh Muslim University, Aligarh 202001 Received 28 April 1986; revised and accepted 20 November 1986 Complexes of barbituric acid with La(III), Ce(IlI), Pr(IlI), Ndilll), Sm(Ill), Eu(In), Gd(m), Tb(lll), Dy(ID), Ho(ID), Er\m) and Yb(ID) have been isolated in the solid state and characterized on the basis of elemental analysis, molecular weight determination, UY/visible and IR spectroscopy. The nephelauxetic parameter (~) indicates covalent bonding between lanthanides and the ligand, although molar con- ductance data show values close to those of I:I electrolytes. The pH-metric study carried out at 25°C, 30°C, 35°C and 0.1 M ionic strength indicates that only one proton dissociates and the com- plexes formed ill the solution are essentiallyionic in nature. Some unusual characteristics of barbituric acid, as reported in our previous communication I, prompted us to undertake a systematic study of its complexes with lanthanide ions in solution and the solid state. We report here the preparation and characteri- zation of some lanthanide (III) complexes of barbi turic acid, In addition, the stability constants of the complexes have been studied pH-metrically employing Irving-Rossetti technique", The rare earth chlorides (Reagent grade, May and Baker), NaOH, NaCl0 4 (Riedel), HCl0 4 (E. Merck) and barbituric acid (Koch light) were used as such. The IR spectra (600-4000 em - 1) were recorded on a Perkin Elmer 621 spectrophotometer in K Br and UVjvisible spectra were recorded on a Pye Unicam PU 8800 spectrophotometer. pH-metric titrations were done on an Elico pH-meter model LI-IOT. The molecular weights were determined by viscosity measurement method using an Ostwald viscometer. All the calculations were done on a computer model VAX;1l-780. Synthesis of complexes A mixture of the aqueous solution of barbituric acid and ethanolic solution of metal chloride in 1:3 (M:L) ratio was stirred for 4 to 5 hr on a water-bath. It was kept overnight at room temperature when it yielded amorphous powder which was washed with water, ethanol and dried at 100'JC. For the determination of stability constants Bjerrum-Calvin pH-metric titration technique as modified by Irving and Rossetti- was employed. Barbituric acid, NaOH, NaCl0 4 and HCl0 4 solutions were prepared in carbon dioxide-free doubly distilled water. In order to' prevent hydrolysis, the metal(IIl) chloride (0.005 M) solution was prepared in HCI0 4 . The experiments were carried out at 25°C, 30°C, 35°C and at O.l M ionic strength. The following solutions were titrated against 0.2128 M sodium hydroxide solution: (i) 5 ml HCI0 4 (0.1 M), (ii) 5 ml HCl0 4 (0.1 M) + 25 ml barbituric acid solution (0.00; M), and (iii) 5 ml HCI0 4 + (0.005 M) metal ion + 25 ml barbituric acid (0.005 M). The total volume was maintained at 50 ml. The site of proton dissociation in barbituric acid (BA) (2,4,6-trihydroxypyrimidine) is uncertain owing to its existence in tautomeric forms (A, B). OH ~N HO~NJlOH I H (A) (B) However, on the basis of halogenation+" reactions it has been shown to exist predominantly in the form (A). The analytical data (Table 1) indicate that the complexes isolated in the solid states are of the type M(BAHh.2H z O where M = lanthanide (Ill) ion and BA.= barbituric acid. Deprotonation of barbituric acid was verified by testing for the absence of chloride ions in the complexes. The .nolecular weights of the complexes determined by viscosity measurements are very close to those calculated (Taole I). The molar conductances of millimolar solutions of these complexes in DMF, however, correspond to that of a univalent electrolyte", It may probably be due to solvolysis in DMF. The solution IR spectrum of barbituric acid is only slightly different from that of the solid. Barbituric acid has been reported to exhibit three intense bands in the range 1690-1760 cm- I assignable to C = 0 stretching frequency. However, we have observed two bands at 1690 and 1720 ern ·-1 of which only the latter shows a negative shift of 5-10 ern -I. But a distinction between a coordinated and non-coordinated CO could not made. The insignificant shift in vCO is probably owing to the coupling of frequencies due to non-bonded CO groups with those of the bonded ones". Of the two I,N - H) modes appearing at 3480, 3500 cm -I in case of the 523
Transcript of Indian Journal of Chemistry Vol.26A, June 1987, pp....
Indian Journal of ChemistryVol. 26A, June 1987, pp. 523-525
Studies on Barbituric AcidComplexes of Lanthanide Ions
S TABASSUM, K S SIDDIQI·, N H KHAN,R I KURESHY & S A A ZAIDI
Department of Chemistry, Aligarh Muslim University, Aligarh202001
Received 28 April 1986; revised and accepted 20 November 1986
Complexes of barbituric acid with La(III), Ce(IlI), Pr(IlI), Ndilll),Sm(Ill), Eu(In), Gd(m), Tb(lll), Dy(ID), Ho(ID), Er\m) and Yb(ID)have been isolated in the solid state and characterized on the basis ofelemental analysis, molecular weight determination, UY/visible andIR spectroscopy. The nephelauxetic parameter (~) indicates covalentbonding between lanthanides and the ligand, although molar con-ductance data show values close to those of I: I electrolytes. ThepH-metric study carried out at 25°C, 30°C, 35°C and 0.1M ionicstrength indicates that only one proton dissociates and the com-plexes formed ill the solution are essentiallyionic in nature.
Some unusual characteristics of barbituric acid, asreported in our previous communication I, promptedus to undertake a systematic study of its complexeswith lanthanide ions in solution and the solid state.
We report here the preparation and characteri-zation of some lanthanide (III) complexes of barbi turicacid, In addition, the stability constants of thecomplexes have been studied pH-metrically employingIrving-Rossetti technique",
The rare earth chlorides (Reagent grade, May andBaker), NaOH, NaCl04 (Riedel), HCl04 (E. Merck)and barbituric acid (Koch light) were used as such.
The IR spectra (600-4000 em - 1) were recorded on aPerkin Elmer 621 spectrophotometer in K Br andUVjvisible spectra were recorded on a Pye Unicam PU8800 spectrophotometer. pH-metric titrations weredone on an Elico pH-meter model LI-IOT. Themolecular weights were determined by viscositymeasurement method using an Ostwald viscometer.All the calculations were done on a computer modelVAX;1l-780.
Synthesis of complexesA mixture of the aqueous solution of barbituric acid
and ethanolic solution of metal chloride in 1:3 (M:L)ratio was stirred for 4 to 5 hr on a water-bath. It waskept overnight at room temperature when it yieldedamorphous powder which was washed with water,ethanol and dried at 100'JC.
For the determination of stability constantsBjerrum-Calvin pH-metric titration technique asmodified by Irving and Rossetti- was employed.
Barbituric acid, NaOH, NaCl04 and HCl04 solutionswere prepared in carbon dioxide-free doubly distilledwater. In order to' prevent hydrolysis, the metal(IIl)chloride (0.005 M) solution was prepared in HCI04.
The experiments were carried out at 25°C, 30°C, 35°Cand at O.l M ionic strength.
The following solutions were titrated against0.2128 M sodium hydroxide solution: (i) 5 ml HCI04
(0.1 M), (ii) 5 ml HCl04 (0.1 M) + 25 ml barbituric acidsolution (0.00; M), and (iii) 5 ml HCI04 + (0.005 M)metal ion + 25 ml barbituric acid (0.005 M). The totalvolume was maintained at 50 ml.
The site of proton dissociation in barbituric acid(BA) (2,4,6-trihydroxypyrimidine) is uncertain owingto its existence in tautomeric forms (A, B).
OH
~N
HO~NJlOHI
H
(A)(B )
However, on the basis of halogenation+" reactionsit has been shown to exist predominantly in the form(A). The analytical data (Table 1) indicate that thecomplexes isolated in the solid states are of the typeM(BAHh.2HzO where M = lanthanide (Ill) ion andBA.= barbituric acid. Deprotonation of barbituricacid was verified by testing for the absence of chlorideions in the complexes. The .nolecular weights of thecomplexes determined by viscosity measurements arevery close to those calculated (Taole I). The molarconductances of millimolar solutions of thesecomplexes in DMF, however, correspond to that of aunivalent electrolyte", It may probably be due tosolvolysis in DMF.
The solution IR spectrum of barbituric acid is onlyslightly different from that of the solid. Barbituric acidhas been reported to exhibit three intense bands in therange 1690-1760 cm-I assignable to C = 0 stretchingfrequency. However, we have observed two bands at1690 and 1720 ern ·-1 of which only the latter shows anegative shift of 5-10 ern -I. But a distinction between acoordinated and non-coordinated CO could not made.The insignificant shift in vCO is probably owing to thecoupling of frequencies due to non-bonded CO groupswith those of the bonded ones". Of the two I,N - H)modes appearing at 3480, 3500 cm -I in case of the
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INDIAN 1. CHEM., VOL. 26A, JUNE 1987
Table I-Characterization Data of Barbituric Acid Complexes
Complex Found (Calc.), % m.p. Molar Molec. wt.(Colour) CO conductance Found (Calc.)
C H
La(BA - Hh. 2H zO 26.53 2.24275 68
556.04Light Yellow (25.87) (2.35) (530.00)Ce(BA -H)J.2HzO 26.25 2.25
260 68557.25
Light Yellow (25.86) (2.35) (550.00)Pr(BA - Hh. 2H zO 25.70 2.28
265 67.5 558.03Yellow (25.82) (2.34) (552.00)Nd(BA -Hh.2HzO 25.75 2.29 270 80
561.37
Yellow (25.67) (2.33) (540.00)Sm(BA-HlJ·2HzO 25.55 2.20
280 85567.48
White (25.39) (2.30) (548.00)Eu(BA - HlJ. 2H zO 25.50 2.25
265 80569.09
Light Yellow (25.32) (2.30) (545.00)Gd(BA - H), .2HzO 25.10 2.30
265 90574.38
Light Yellow (25.09) (2.28) (555.00)Tb(BA -H)J.2HzO 25.15 2.22
290 75576.05
Light Yellow (25.01) (2.27) (55400)Dy(BA - Hh. 2H 20 25.20 2.23
260 85579.63
Yellow (24.86) (2.26) (559.00)Ho(BA - Hh· 2H zO 24.82 2.20
290 85582.06
Yellow (24.76) (2.25) (572.00)Er(BA - Hh· 2H zO 24.70 2.19
280 85584.39
Yellow (24.66) (2.24) (570.00)Yb(BA-HlJ·2HzO 24.40 2.15 285 90
590.76Yellow (24.39) (2.21) (571.00)
---~-. - - --.----.-.----~---
Table 2-Nephelauxetic Effect (I - f1) and NephelauxeticRatio (fJ) for Barbituric Acid Complexes
Complexes (I-PJ (/fJCe(BA-HlJ.2HzO O.90xIO-z 0.999Sm(BA-Hh.2HzO 0.69 x 10-2 0.993Dy(BA-Hh.2HzO 0.70xI0-z 0.993
ligand, only one band disappears after complexationwhile the other band remains unchanged showing thereplacement of hydrogen atom by metal. Similarly6NH and I{C - N) modes could also not bedistinguished for they appear nearly in the same region
Complex
La(1I1)Ce(III)Pr(llI)Nd(Ill)Sm(III)Eu(lll)Gd(lll)rtJ(llI)1)ytlll)Ho(lIl)Er(llI)Yb(III)
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as has also been observed in the case of amides 7. Inanalogy to the behaviour of urea and trihydroxy-pyrimidines, it is concluded that barbituric acidcoordinates through a nitrogen and carbonyl oxygen;the presence of two water molecules makes thelanthanide eight coordinated.
The absorption spectra of the complexes inmethanol showed absorptions largely in the UVregion. The complexes showed a negative shift inspectral bands when compared to those of thecorresponding aquo ions due to the well-knownncphelauxetic effect". The nephelauxetic parameter
Table 3-Log K, -6.G, 6.H and 6.S Values at 25'C, 30 C and 35"C
Log K, -ec -/lH -!:J.S.--.---- --- - .-. .- -- --_.- - .- --- ----- (kcal mol ') (kcal mol ') (kcal deg -I mol :")._---
25"C 30'C 35"C 25"('- 30 C 35C
3.47 ± .26 3.10 ± .24 2.88± .26 4.72 4.29 4.04 24.65 0.0673.49 ± .30 3.1 I± .28 2.91 ± .25 4.74 4.30 4.09 24.08 0.0653.54 ±.27 3.34 ± .26 3.00±.26 4.81 4.62 4.22 22.42 0.0593.77±.28 3.38 ± .25 3.11 ±.24 5.13 4.67 4.37 27.75 0.0764.21 ±.27 3.81 ± .26 3.37 ± .25 5.72 5.26 4.73 35.23 0.0994.26±.26 3.85 ± .27 3.41 ± .26 5.79 5.32 4.79 35.60 0.1003.93 ± .20 3.58 ± .24 3.13±.22 5.34 4.94 3.39 33.68 0.0954.32± .28 3.77 ± .28 3.54 ± .24 5.88 4.90 4.88 35.02 0.0944.31 ±.30 3.80 ± .28 3.44± .28 5.86 5.26 4.84 36.23 0.1024.27 ±.30 3.90± .26 3.44 ± .26 5.81 5.40 4.84 34.74 0.0974.15±25 3.76±.26 3.27 ± .24 5.64 5.19 4.59 35.23 0.0994.65 ±.4 4.02± .30 3.55 ± .25 6.32 5.53 4.92 40.60 0.124
.- --------- -- --- ---- "------ --- -.- ---_. __ ._- --.- -- - - --- -----. ~- ------ - ..--- -----.--- -- - -----
(/1),(1 -/1) for complexes of Ce3 + , Sm3 + and DyJ + aregiven in Table 2. In rest of the cases these parameterscould not be calculated owing to maximumoverlapping of band maxima.
Proton-ligand stability constantsThere is only one dissociable proton in BA9. The nA
values were calculated using Irving and Rossottiequation. The proton ligand stability constant pJ<Mwere obtained by pointwise calculation method. Theacid-ligand curves in all cases deviated from the pureacid curve at pH-meter reading B = 2.5. The highest nAvalue obtained was 0.93.
Metal-ligand stability constantsThe metal-ligand titration curves deviated from
pure ligand curve only at pH = 3.5. The n values werebelow unity and hence only log KJ was obtained(Table3) using the equation log KJ =pL+ log nil =h.It has been observed that log K is inversely
NOTES
proportional to temperature. The nature of thecomplexes formed in solution seems to be ionicnevertheless formation of a polymeric species can notbe ignored.
References
1 Siddiqi K S, Khan P, Khan S & Zaidi S A A, Synth React inorgmet-org Chern, 12 (1982) 68l.
2 Irving M & Rossotti N S, J chem S~, (1953) 3397; (1954) 2904.3 Franssen M C R & Vander H C Plas, Rec Tra» Chim Pays Bas
Belg, 103 (1984) 99.4 Brown D J & Mason S F, The pyrimidines (lnterscience-John
Wiley, New York) 1962, 172,494,300.5 Geary W J, Coord Chern Rev, 7 (1971) 81.6 Nakamoto K, Infrared & Raman spectra of inorganic &
coordination compounds (Interscience-John Wiley, NewYork) 1970, 217.
7 Bellamy L J, The infrared spectra of complex molecules (JohnWiley, New York) 1958,218.
8 Henrie D E & Choppin G R, J chem Phys, 49 (1968) 478.9 Briggs A G, Sawbridge J E & Tickle P, J chem Soc,(B) (1969) 802.
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