Stability constants of zinc 3-hydroxypropionato and 2-, 3-,
and 4-hydroxybutyrato complexes have been determined by
potentiometric titration. The constants and their standard errors
have been calculated with a digital computer. On the basis of the
results obtained, the stability of these complexes is discussed with
respect to the ligand basicity and the position of the hydroxyl
group within the ligand molecule

B. Bach-Dragutinović

B. Grabarić

I. Filipović

I. Kruhak

I. Piljac

Stability constants of zinc 3-hydroxypropionato and 2-, 3-,

and 4-hydroxybutyrato complexes have been determined by

potentiometric titration. The constants and their standard errors

have been calculated with a digital computer. On the basis of the

results obtained, the stability of these complexes is discussed with

respect to the ligand basicity and the position of the hydroxyl

group within the ligand molecule

Filipović, I.

Piljac, I.

Bach-Dragutinović, B.

Kruhak, I.

Grabarić, B.

Hrčak - Portal of scientific journals of Croatia

CR 0 AT I CA CHE MIC A A CT A 45 (1973) 447 CCA-783 541.122 :546.4 7 :5~ 7 .4 7 Original Scientific Paper Stability Constants of Zinc Complexes with 3-Hydroxypropionate and 2-, 3-, and 4-Hydroxybutyrates I. Filipovic, I. Piljac, B. Bach-Dragutinovic, I. Kruhak , and B. Grab.aric D epartment of I norganic Chemistry, Faculty of Technology, and Institute of Inorganic and Analytical Chemistry, University of Zagreb, 41000 Zagreb, Croatia, Yugoslavia Received December 6, 1972 Stability constants of zinc 3-hydroxypropionato and 2-, 3-, and 4-hydroxybutyrato complexes have been determined by potentiometric titration. The constants and their standard errors h ave been calculated with a digital computer. On the basis of the results obtained, the stability of these complexes is discussed with respect to the ligand basicity and the position of the hydroxyl group within the ligand molecule. This paper describes results obtained in a study of zinc hydroxymono-carboxylato complexes, continuing a systematic investigation of metal com-plexes with these ligands1•2. The present study aims to afford an insight into the influences exerted by size and electron configuration of the central metal ions upon the stability of their hydroxymonocarboxylato complexes at constant ionic strength. Such investigations may also serve to study the relationship of complex stability and ligand basicity, as well as the relation-ship between stability and hydroxyl group position within the ligand molecule. The literaturea gives only scant information on zinc complex stabilities; moreover, stability constants reported in the literature were obtained at different ionic strengths, in solutions containing »inert« salts, which can themselves enter into complexes with zinc (e . g. KCl). EXPERIMENTAL Potentiometric measurements with buffer solutions containing 2- , 3- , and 4-hydr oxybutyrato, as well as n-butyrato zinc complexes, were carried out according to H. Thun, F. Verbeek, and W. Wanderleen•. The titration vessel consisted of three compartments. Compartment 1 contained the reference solutton, 20.07 mM HClO~ , adjusted to ionic strength 2 with NaC104• Compartment 2 held 5 ml (initial volume) 50 or 100 mM zinc perchlorate, adjusted to the same ionic strength with NaCl04. A buffer solution, 1 M in both NaL and HL, was simultaneously delivered in portions to the solutions in _ compartments 2 and 3, using a precision microburet (Metrohm E 457, reproducibility 0.001 ml). Contact between compartments was established by means of a three-branched salt bridge filled with 2 M NaCl01. The bridge endings were made of special porous glass. Mixing of solution was performed by bubbling nitrogen through the liquids. During measurements mixing . was interrupted, and the gas was allowed to stream over the liquids surfaces. Potential differences between compartments 1 and 2 (E), and compartments 1 and 3 (EJ were measured with a Prazisions Kurbelkompensator (VEB Messtechnik), connected to a Lange Multiflex Galvanometer Model MG-3. All measurements were carried out with a quinhydrone electrode in each compartment, at 25.0 ± 0.1 °c. Good agreement between formation curves obtained at different metal ion concen-448 I. FILIPOVIC ET AL. trations suggested the absence of polynuclear complexes from the systems measured and the absence of interaction between the metal ion and the quinhydrone electrode; if such interaction took place, it must have been negligible. Zinc 3-hydroxypropionato complexes were determined by continuous potentio-metric titration with an E 436 Metrohm potentiograph. 10 ml of a solution (S1) containing 20 mM NaL, 20 a mM HL, and enough NaC104 to obtain an ionic strength of 2, was placed in to the titration vessel; another solution (S2), containing 425 mM NaL, 425 a mM HL, adjusted to ionic strength 2 with NaC104, was used as Utrant and delivered continuously from an automatic buret. Potential differences, E 0 , between the solution in the vessel and a reference solution (20 mM HCl04, adjusted to ionic strength 2 with NaC104) were monitored during the titrant addition. Next, S2 was placed into the titration vessel, and S1 was used &s the titrant. In another series, 10 ml of a solution (S3) containing either 50 or 100 mM zinc perchlorate, in addition to 20 a mM HL, and NaC104 to give an ionic strength of 2, was conti-nuously titrated with a solution (S4) composed of 425 mM NaL, 425 a mM HL, and NaC104 up to ionic strength 2, and potential differences (E) against the above mentioned reference solution monitored during titration. Subsequently, again, the titration was repeated in reverse order of sample and titrant. The acid-to-salt (HL to NaL) ratio in one set of buffers was 1 : 1 (i.e. a = 1), and in another set 3 : 1 (a = 3). Here, too, good agreement of formation curves (Fig. 1) , recorded with different metal ion concentrations, and different acid-to-salt ratios, indicated the absence of polynuclear and hydroxo coplexe!l. Reproducibility of added volumes in these titrations was 0.01 ml, and that of potential measurements 0.2 mV. The solutions were mixed either magnetically, or by bubbling nitrogen. All chemicals were analytical grade, except the sodium salts of 2-, 3-, and 4-hydroxybutyric and of 3-hydroxypropionic acids. These salts were purified before use; sodium hydroxybutyrates were recrystallized twice from ethanol, and sodium 3-hydroxypropionate from a mixture of ethanol and methanol. Zinc perchlorate was prepared from zinc oxide and perchloric acid, and purified by three recrystal-lizations. Sodium perchlorate was also purified by three recrystalilizations. RESULTS AND DISCUSSION Table I shows a typical set of results for different buffer compositions. Zinc concentration in these buffers was 100 mM, and the acid-to-salt ratio was 1 : 1. Experimental data were treated with an IBM 1130 computer programmed to calculate average numbers, n, of ligands bound to the central atom, as well as values of free-ligand concentrations, [L] , according to well-known relationships4 • In addition, a numerical treatment was applied to Fronaeus' N graphical method5, to give values for the function F 0 ([L]) = 1 + L (Ji [L] i. By i=l solving these polynominals, using a weighted least-squares procedure, the stability constants and their standard errors were obtained. The weighted least-squares routine was the same as in a previous work6• Other details of the numerical treatment and the computer program will be discussed elsewhere. Table II shows the stability constants obtained and their standard errors. Taken together with reported data7,8, the results from Table II and Fig. 1 demonstrate that the central atom binds a maximum of three ligands in the hydroxypropionato, and a maximum of four in the hydroxybutyrato complex series, provided the same zinc concentration and approximately the same ligand concentrations are used. This difference is obviously unrelated to the order of stabilities, since the 2-hydroxybutyrato complexes are considerably more stable than the 3- and 4-hydroxybutyra to complexes. However, the latter complexes containing three ligands, ML3-, are \'ery unstable (K0 values n 3 2 n 3 2 n 3 2 >---0 >->--->->-STABILITY CONSTANTS OF ZINC COMPLEXES 449 2- hydroxybutyrate • n 3-hydroxybutyrate • I 3- • ~ • • 0 :• .p •• 2- I • • • ~ l. ./ 0 ~ of; 1-0 .o 0 g" 1 .o 2 1 2 0 eO I I q • I 4-hydroxybutyrate butyrate n 3 >- 0 • 0 e J 0 0 0 ,1 2- 8 00 00 /~ 1- l rJ>c9 0 00 2 ._ oe 1 00 0 0 I • 0 ,p I 10 0 I 3- hydr oxypropionate 2 1 0 0.1 M zn 2• NaL:HL=1:1 • 0.05 M zn2• Nol :HL=1 :1 • • o. 1 M zn 2• Nal : HL 1 :3 / <J~~ , ' Io '~· I 2 -log [ L l fig. 1. Formation curves of zinc 3-hydroxypropionate, n -butyrate, 2-hydroxybutyrate, 3-hydroxy-b utyrate and 4-hydroxybutyrate complex systems >!:>-LJ1 0 TABLE I Zinc Solutions 2-Hydroxybutyrate I 3-Hydroxybutyrate I 4-Hydroxybutyrate I Butyrate I 3-Hydroxypropionate SoLn. S1 Eo E I Soln. S1 Eo E I Soln. S1 Eo E ) So1n. S1 E0 E IS_ Eo E --ml mV mV ml mV mV ml mV mV ml mV mV mM mV mV 0.100 117.7 73.7 0.100 158.7 140.8 0.200 171.2 155.3 I 0.110 179.7 162.9 20.00 169.8 156.5 0.200 116.5 73.1 0.200 158.0 140 .8 0.300 171.0 155.3 0.200 179.2 162.8 39.29 169.8 156.8 0.300 116.2 73.9 0.300 157.8 141.0 0.400 170.8 155.4 0.300 179.1 163.1 56.82 169.8 156.9 0.400 116.2 75.0 0.400 157.7 141.4 0.500 170.8 155.9 0.400 179.2 163.5 72.83 169.8 157.0 0.500 116.1 76.5 0.500 157.7 141.7 0.700 170.7 156.6 0.500 179.3 163.9 87.50 169.8 157.2 0.600 116.1 77.9 0.700 157.7 142.5 0.800 170.7 156.9 0.600 179.5 164.4 101.00 169.8 157.4 !"" 0.700 116.1 79.3 0 .800 157.7 142.9 0.900 170.7 157.3 0.700 179.7 164.8 113.46 169.8 157.5 '-=:! 0.800 116.1 80.8 0.905 157.7 143.3 1.000 170.6 157.6 0.800 179.8 165.2 125.00 169.8 157 .7 ""' t-< 0.900 116.2 82.0 1.000 157.7 143.4 1.100 170.6 157 .7 0.901 179.9 165.7 135.71 169.8 157.9 H 1.000 116.2 83.6 1.100 157.7 143.9 1.200 170.6 157.8 1.001 180.0 166.0 146.55 169.8 158.0 '\j 0 1.100 116.3 84.9 1.200 157.7 144.1 1.500 170.8 158.9 1.100 180.1 166.2 155.00 169.8 158.1 <: 1.200 116.3 85.8 1.350 157 .7 144.6 1.650 170.8 159.0 1.200 130.2 166.7 171.38 169.8 158.3 H 0 1.800 116.6 90.3 1.500 157.8 145.2 2.200 170.9 160.1 1.300 180.4 167.1 186.76 169 .8 158.5 t"I 2.000 116.6 93.1 1.650 157.8 145.5 2.400 170.9 160.4 1.400 180.6 167.5 200.00 169.8 158.6 "-3 2.200 116.7 95.0 1.802 157.8 146.0 3.000 171.0 161.7 1.500 180.7 167.8 211.84 169.8 158.8 ~ 2.400 116.8 97.6 2.000 157.8 146.3 3.250 171.1 162.1 1.650 181.0 168.4 222.50 169.8 158 .9 r 2.600 116.8 99.3 2.200 157.9 146.9 3.500 171.1 162.2 1.801 181.2 168.9 233.16 169.8 158.9 2.800 116.9 100.8 2.400 157.9 147.1 3.750 171.2 162.8 2.000 181.5 169.6 245.00 169.8 159.0 3.000 117.0 101.7 2.600 158.0 147.8 4.099 171.2 163.2 2.200 181.6 170.3 258.24 169.8 159.2 3.250 117.1 102.9 2.800 158.0 148.2 4.252 171.3 163.5 2.400 181.9 170.9 273.13 169.8 159.4 3.500 117.2 104.1 3.001 158.0 148.5 4.500 171.3 163.8 2.600 182.1 171.5 290.00 168.8 159 .5 3.750 117.3 105.l 3.206 158.1 148.9 5.500 171.4 164.7 2.800 182.5 172 .1 309.29 169.8 159.7 4.000 117.4 106.0 3.400 158.1 149.1 6.500 171.4 165.5 3.000. 182.6 172.6 331.54 169.8 159.9 4.250 117.4 106.8 3.620 158.2 149.4 3.250 183.0 173.0 357.50 169.8 160.1 4.500 117.5 107.4 3.800 158.2 149.8 3.502 183.3 173.9 388.18 169.8 160.4 5.500 117.7 109.5 4.000 158.2 150.0 3.750 183.7 174.3 425.00 169.8 160.9 6.500 117.9 111.0 4.250 158.2 150.3 4.000 183.9 174.9 4.500 158.2 150.6 4.250 184.2 175.4 5.500 158.4 151.6 4.501 184.4 176.0 6.500 158.4 152.4 5.500 185.2 177.7 6.500 186.1 179.3 STABILITY CONiSTAINT:S OF ZIINC C01VI!PiLEXES Ligand 3-Hydroxypropionate 2-Hydroxybutyrate 3-Hydroxybutyrate 4-Hydroxybutyrate n-Butyrate TABLE II Cumulative Stability Constants f11 f12 7.2 ± 0.1 13 ± 1 52 ± 1 1 059 ± 16 9.8 ± 0.1 51 ± 1 9.2 ± 0.1 36 ± 1 9.4 ± 0.1 45 ± 2 (13 27 ± 1 6 974 ± 173 31 ± 5 34 ± 3 54 ± 9 451 (14 17 415 ± 262 313 ± 6 239 ± 4 373 ± 14 are 0.6 and 0.9, respectively), while type ML/- complexes have much greater stabilities (K4 values: 10 and 7, respectively). The relative stability of ML/-type complexes is in accordance with the zinc coordination number of 4. The effects of presence and position of a hydroxy group in the mono-carboxylate anion, and the effect of the latter's basicity, upon zinc complex stabilities can be seen from data presented in Table III. This Table shows TABLE III Stability Constants of Monoligand Complexes at Ionic Strength 2 (NaCW4) Ligand Acetate7 2-Hydroxyacetate7 Propionate7 2-Hydroxypropionaten 3-Hydroxypropionate n-Butyrate 2-Hydroxybutyrate 3-Hydroxybutyrate 4-Hydroxybu tyrate • Our measurements. 7.0 ± 0.6 52 ± 7 9.9 ± 0.7 40.7 7.2 ± 0.1 9.4 ± 0.1 52 ± 1 9.8 ± 0.1 9.2 ± 0.1 pK~ 4.74 3.77 4.89 3.82 4.56 4.86 3.80 4.53 4.85 values of stability constants for monoligand complexes ((31 ), along with pK.\ values for the corresponding monocarboxylic acids, characterizing ligand basicity. It is clear that a 2-hydroxy group stabilizes the corresponding zinc monoligand complex quite considerably, despite a low ligand basicity. Vvhen, however, the hydroxyl group occupies positions 3 or 4, it does not influence complex stability, the hydroxymonocarboxylate ions thus acting as mono-dentate ligands, and bind the metal ion only via the carboxylic group. Such circumstances are in good accordance with the well-known property of ring formation to contribute to complex stability, since a stable ring can be formed only with the hydroxy group in position 29. Ring stabilization depends, however, on the size of the central metal atom2, e.g. there is no difference in stability between cadmium complexes with unsubstituted and substituted monocarboxy-late anions. Acknowledgement. The authors are indebted to the Federal Fund for Scientific Work and to the Fund for Scientific Work of the Socialist Republic of Croatia for financial support. 452 I. FILIPOVIC ET AL. REFERENCES 1. J. Savic, M. Savic, and I. Fi 1 i po vi c, Croat. Chem. Acta 44 (1972). 2. S. N u s h i, I. P i 1 j a c, B. G r a b a r i c, and I. F i 1 i p o v i c, Croat. Chem. Acta 45 (1973) 453. 3. Stabiiity Constants, Compiled by L. G. Si 11 en and A. E. Marte 11, The Chemical Society, London, 1964. - Stability Constants, Supplement No. 1, Compiled by L. G. S i 11 e n and A. E. Ma rte 11. The Chemical Society, London, 1971. 4. H. Thu n , F. Verb eek and W. Van de r 1 e en, J . Inorg. Nucl. Chem. 27 (1965) 1813. 5. S. Fron a e us, Acta Chem. Scand. 4 (1950) 72. 6. I. Pi 1 j a c, B . Grab a r i c and I. Fi 1 i po vi c, J. Electroanal. Chem. 42 (1973) 433. 7. H. Thu n, W. Guns and F. Verb eek, Anal. Chim. Acta 37 (1967) 332. 8. I. Fi 1 i po vi c, T. Matus in o vi c, B. Mayer, I. Pi 1 j a c, B. Bach-- D rag u ti n o vi c, and A. B u j a k, Croat. Chem. Acta 42 (1970) 541. 9. B. Larsson, Acta Chem. Scand. 19 (1965) 783. IZVOD Konstante stabilnosti cinkovih kompleksa s 3-hidroksipropionatom i 2-, 3- i 4-hidroksibutiratom I. Filipovic, I. Piljac, B. Bach-Dragutinovic, I. Kruhak i B. Grabaric Odredene su konstante stabilnosti 3-hidroksipropionato i 2-, 3- .i 4-hidroksi-butirato-kompleksa cinka potenciometrijskom titracijom. Konstante stabilnosti i njihove standardne pogreske dobivene su pomocu elektronickog racunala. Na osnovu rezultata diskutirana je stabilnost kompleksa s obzirom na bazicnost liganda i polofaj hidroksilne skupine u ligandu. ZA VOD ZA ANORGANSKU KEMIJU TEHNOLOSKOG FAKULTETA U ZAGREBU I INSTITUT ZA ANORGANSKU I ANALITICKU KEMIJU SVEUCILISTA U ZAGREBU Primljeno 6. prosinca 1972. 

Stability Constants of Zinc Complexes with 3-Hydroxypropionate and 2-, 3-, and 4-Hydroxybutyrates

Stability Constants of Zinc Complexes with 3-Hydroxypropionate and 2-, 3-, and 4-Hydroxybutyrates

Abstract

Similar works

Full text

Available Versions

Hrčak - Portal of scientific journals of Croatia