Thermodynamic metal-ligand stability constants of rare earths with N-p-chlorophenyl-m-substituted benzohydroxamic acids

Journal of Radioanalytieal and Nuclear Chemistry, Articles, Vot 102, No. 2 (1986) 385-397

THERMODYNAMIC METAL-LIGAND STABILITY CONSTANTS OF RARE EARTHS WITH N-p -CHLO RO PHENYL-m-S UB ST IT UTED

BENZOHYDROXAMIC ACIDS

Y. K. AGRAWAL, N. SINHA, S. K. MENON

Analytical Laboratories, Pharmacy Department, Faculty of Technology and Engineering,

M.S. University of Baroda, Kalabhavan, Baroda-390 001 (India)

(Received March 14, 1986)

The thermodynamic metal ligand stability constants of rare earths, La s§ PrS*," Nd 3§ Sms§ i~u s+, Gd s§ and Tb 3+, with N-p-chlorophenyl-m-substituted benzohydroxamic acids in dioxan-water (60-70%) media at 25 ~ have been determined by the potentiometric method. The effect of basieity of the ligand, central metal ion and the order of stability constants are discussed. The order of stability constants of rareearths with the hydroxamie acids is La<Pr<Nd< Sm<Eu<Gd>Tb.

Introduction

A knowledge of stability constants is essential to rationalize the understanding and behaviour of rare earth complexes in solution. A qualitative measurement of the equilibria involved in chelate formation offers a fundamental knowledge of the structure of the chelates. It is necessary to correlate equilibrium data with important factors such as (1) basicity of the ligand, (2) electronegativity of rare earths ions, (3) steric requirements of chelating agents, and (4) the size and number of the chelating rings. The results of these studies find applications in a systematic and rational search for new analytical reagents showing better specific and selective reaction with rare earths.

Hydroxamic acids are versatile analytical reagents and form stable chelates with several metal ions. 1 -7 However, very little work has been reported on rare earth complexes except for benzo-, N-phenylbenzo-, N - o - , m- and p-tolyl-m-nitrobenzo- hydroxamic acids which have been studied by AGRAWAL et al.7 In the present in- vestigation the metal-ligand stability constants of La(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), and Tb(III) have been determined with N-p-chlorophenyl-m-substi-

Elsevier Sequoia S. A., Lausanne Akaddmiai Kiad6, Budapest

Y. K. AGRAWAL et al.: THERMODYNAMIC METAL-LIGAND STABILITY

tuted benzohydroxamic acids at 60 and 70% (v/v) dioxan-water media at 25 ~ in order to have more selective and sensitive analytical reagents for separation and determination of rare earths.

Experimental Reagents

The hydroxamic acids were synthesized as described elsewhere s and recrystallized twice before use. n-Bu4NOH (0.!M) was used as titrant. Dioxan ~r purified by the method of WEISSBERGER, 9 Doubly distilled water was redistilled over alkaline

potassium permanganate and tested for the absence of the carbonate. ~ 0 The rare earth

perchlorates were prepared from their oxides. Concentrations of rare earths were determined volumetrically. 11

Apparatus

A Radiometer pH meter PHM84, equipped with a combined electrode GK 2401C, was used for pH measurments.

Procedure

The titration procedure followed for determination of ionization constants and stability constants was similar to that described earlier.7,12- ~ s The thermodynamic

ionization constants of N-p-chlorophenyl-m- substituted benzohydroxamic acids are reported elsewhere. 17

For the metal ligand stability constant measurements a weighed quantity of hydroxamic acid (corresponding to 0.1M in a final volume of 50 cm 3) was transferred into a- dry titration vessel and then dioxan was (x crn 3) added (correspond to 50, 60 or 70% dioxan). Rare earth perchlorate (5 cm 3 of 0.01M), HC104 (5 cm 3 of 0.05M)

and I-I20 [50 - (x + 1(3) cm a ] were -then added. Due allowance was made for the cor- rection in volume upon solvent mixing. ~ 8,~ 9 Nitrogen gas, presaturated with thermostated solvent of appropiate composition, was passed through the solution and titrated against n-BuaNOH (0.1M), which was prepared so as to contain the requisite percent dioxan by adding small aliquots. The highest appropiate drift-free reading on pH meter was recorded.

Calculations

The thermodynamic rare earths stability constants were calculated as described elsewhere.7,12-16 The data were fed into a CDC-3600 computer in order to calculate log K~, log I(2 and log K3.2 o

386


Results and discussion

A large number o f pH ti trat ions were performed at different concentrations o f

reactants and by varying the metal to ligand ratio. N-p-chlorophenyl-m- substi tuted

benzohydroxamic acid complexes were so insoluble that they precipitated immediately

on mixing dioxan with water (50% dioxan water) and hence their stability constants

were determined by increasing the dioxan content to 60 or 70% (v/v). The average

values o f stabili ty constants are given in Tables 1 - 8 .

Hydroxamic acids are bidentate ligands and form chelates with metal ions. The

metal to ligand ratio in these complexes is 1 : 3, showing a coordination number o f

6 for the metal ion. The t i t rat ion data and formation curves indicate that in solution

too the maximum complexat ion involves the six-coordinate species, and no hepta- or

Table 1 Thermodynamic metal tigand stability constants of N-p-Chlorophenyl-m-substituted

benzohydroxamie acids in 60% (v/v) dioxan-water media at 25 +- 0.1 ~

K 1 Complexing log K x log K2 log K~ log/~3 log

ion K~

Compound I : X = H

H + 11.47 La 3+ 11.0 9.6 8.3 28.9 1.4 PP+ 11.7 10.3 9.1 31.1 1.4 Nd 3+ 12.0 10.5 9.2 31.7 1.5

Sm 3+ 12.3 10.8 9.3 32.4 1.5

Eu 3+ 12.7 11.3 10.0 34.0 1.4 Gd 3+ 12.5 11.1 99.8 33.4 1.4

Tb 3+ 13.0 11.5 10.2 34.7 1.5

Compound II : X = CH 3

H § 11.54 La 3+ 11-5 10.1 8.8 30.4 1.4 Pr 3+ 12.1 10.6 9.3 32.0 1.5 Nd 3§ 12.3 10.9 99.7 32.9 1.4 Sm 3+ 12.6 11.2 9.9 33.7 1.4 Eu 3§ 12.9 11.4 10.2 34.5 1.5 Gd s§ 12.7 11.2 10.0 33.9 1.4 Tb s+ 13.2 11.8 I0.5 35.5 1.4

387

Y. K. AGRAWAL et al.: THERMODYNAM/C METAL-LIGAND STABILITY

Table 2 Thermodynamic metal ligand stability constants of N-p-Chlorophenyl-m-substituted-

benzohydroxamie acids in 60% (v/v) dioxan-water media at 25 -+ 0.1 ~

K 1 Complexing log K1 log K 2 log Ka log f13 log - -

ion Kz

Compound III: X = OCH~

H § 11.36

La 3+ 10.9 9.4 8.2 28.5 1.5

Pr 3+ 11.5 10.0 8.8 30.3 1.5

Nd 3§ 11.8 10.4 9.1 31.3 1.4

Sm 3+ 12.1 10.7 9.4 32.2 1.4

Eu a+ 12.4 10.9 9~ 33.0 1.5

Gd 3+ t2.2 10.8 9 -5 32.5 1.4

Tb s+ 12.6 11.2 10.0 33.8 1.4

Compound IV: X = F

H + 11.12 La 3+ 10.8 9.3 8.0 28.1 1.4

Pr a+ 11.2 9.7 8.4 29.3 1.5 Nd 3+ 11.6 10.2 9.0 30.8 1.4

Sm 3§ 11.9 10.5 9.3 31.7 1.4

Eu ~§ 12.1 10.7 9.5 32.3 1.4

Gd s§ 12.0 10.6 9.4 32.0 1.4

Tb 3§ 12.4 10.9 9.6 32.9 1.5

octa-coordinate species is formed. The three species which are fo rmed stepwise can be

explained by the reactions~

M 3. § 3 c, @ y o\

I an M 3H*" ~ C 0 . " + ~ C 0/" 3 "

+

where ~n - the first, second and third stepwise meta l chelate stabil i ty constants

(e,g. K1 + K2 + K3) , M 3+ - La 3+, Pr 3+, Nd 3+, Sm 3+, Eu 3+, Gd 3+ or Tb 3+,

X - H, CH3, C H 3 0 , F , C1, Br, 1 or NO2

388


Table 3 Thermodynamic metal-ligand stability constants of N-p-Chlorophenyl-m-substituted-

benzohydroxamic acids in 60% (v/v) dioxan-water media at 25 + 0.1 ~

K 1 Complexing log K 1 log K S log K S log #3 log - -

ion Kz

Compound V: X = C1

H + i 1.07 La 3* 9.9 8.5 7.2 25.6 1.4 Pr S* 10.7 9.3 8.0 28.0 1.4 Nd 3+ 11.0 9.6 8.4 29.0 1.4 Sm 3+ 11.3 9,8 8.7 29.8 1.5

Eu S+ 11.6 10.2 9.0 30.8 1.4

Gd S+ 11,4 9.9 8.7 30.0 1.5

Tb S* 11.8 10.4 9.2 31.4 1.4

Compound VI; X = Br

H + 11,04 La S* 9.8 8.4 7.1 25.3 1.4

Pr S* 10.6 9.1 7.9 27.6 1.5

Nd ~+ 10.9 9.4 8.2 28.5 1.5

Sm 3+ 11.1 9.6 8.4 29.1 1.5 Eu S+ 11.4 9.9 8.6 29.9 1.5

Gd 3+ 11.2 9.8 8.5 29.5 1.4 Tb S+ 11.6 10.2 9.0 30.8 1.4

The unionized and ionized forms of the N-p-Chlorophenyl-m- substituted benzo-

hydroxamic acid can be represented as under

C[ _@7 o.:. c, o +~--C = 0"" + ~ C = 0

HA A-

The dotted line in the formula for molecular species shows intramolecular hydrogen

bonding between the hydroxylamino hydrogen and oxygen of the carbonyl.

The simple picture of complex formation shows the similarity between HA and the

first metal ligand complex formed MA. In other words, HA can be considered as the

complex formed between proton and the ligand ion A-, This can be further explained

389

Y. K. AGRAWAL et aL: THERMODYNAMIC METAL-LIGAND STABILITY

Table 4 Thermodynamic metalqigand stability constants of N-p-Chlorophenyl-m-substituted-

benzohydroxamie acids in+ 60% (v/v) dioxan-watet media at 25 -+ 0.1 ~

Complexing K1 ;,on log K~ log K2 log I( s log 3s log - -

K~

Compound VII; X = I

It* 11.10 La s§ 10.3 8.8 7.5 26.6 1.5 PP+ 11.0 9.5 8.3 28.8 1.5 Nd s+ 11.3 9.9 8.6 29.8 1.4 Sm s+ 11.7 10.3 9.0 31.0 1.4 Eu s * 11.9 10.4 9.2 31.5 1.5 Gd s§ 11.8 10.3 9.0 31.1 1.5 Tb s+ 12.1 10.7 9.4 32.1 1.4

Compound VIII; X = NO~

I-l* 10.65 La s+ 9.0 7.5 6.2 22.7 1.5 Pr ~+ 9.6 8.2 7.0 24.8 1.4 Nd a+ 9 ~9 8.5 7 .3 25.7 1.4 Sm s+ 10.2 8.8 7.5 26.5 1.4 Eu s+ 10.7 9.2 8.0 27.9 1.5 Gd s+ 10.4 8.9 7.6 26.9 1.5 Tb s+ 10.9 9.4 8.2 28.5 1.5

by the order in which the stability o f metal complexes and pKa of the ligand vary.

The pKa values give a measure o f the stabilities of the ligand proton complexes.

Variation of stability constants with the nature of ligands

Since the reactions leading to the formation of proton-ligand and metal-ligand

complexes are similar, the constants pKa and log KI (or K1 1(2 or ~n) should be related linearly. It has been pointed out by several workers 7,t 3-16,1 s ,z 1 -2 s that an

approximately linear relationship exists between the logarithms of stability constants

of a series o f metal complexes derived from one metal ion with a set of closely related

ligands. On this basis it is expected that more basic ligand should form a more stable

complexes. In the present studies with the hydroxamic acids examined here, a linear

relationship between PKa and log K1 is observed for all the rare earths. Typical plots

tbr a few rare earths are given in Fig. 1, which show a linear relationship. This indi-

390



benzohydroxamie acids in 70% (v/v) dioxan-water media at 25 • 0.1 ~

Complexing K1 ion log K1 tog K s log K 3 log 33 log

K2

Compound I; X = H

H + 12.43

La 3. 12.0 10.5 9.2 31.7 1.5

Pr ~+ 12.6 11.1 10.0 33.7 1.5

Nd 3+ 13.2 11.7 10.5 35.4 1.5

Sm s+ 13.5 12.0 10.8 36.3 1.5

Eu 3+ 13.7 12.3 11.1 37.1 1.4

Gd 3+ 13.6 12.1 10.9 36.6 1.5

Tb 3§ 14.1 13.5 11.3 37.9 1.4

Compound II; X = CH9

H + 12.45

La 3§ 12.4 11.0 9.7 33.1 1.4

PP+ 13.0 11.5 10.4 34.9 1.5

Nd 3+ 13.4 11.9 10.6 35.9 1.5

Sm 3+ 13.7 12.3 11.1 37.1 1.4

Eu ~* 13.9 12.5 11.5 37.9 1.4

Gd 3+ 13.8 12.3 11.2 37.3 1.5

Tb 3§ 14.3 12.9 11.6 38.8 1.5

A J

12 -- A Pr

10

9

1 0.6 108 11.0 11.2 11.4 11.6

pK a of hydroxomic acids

Fig. 1. Variation of log K 1 of rare earths with pE a of hydzoxamic acids

391



benzohydroxamic acids in 70% (v/v) dioxan-water media at 25 -+ 0.1 ~

K 1 Complexing log Ka log K~ log K 3 log #s log

.ion K~

Compound III; X = -OCH3

H § 12.32 La s+ 11.8 10.3 9.0 31.1 1.5 Pr a+ 12.4 10.9 9.8 33.1 1.5 Nd 3+ 13.0 11.5 10.3 34.8 1.5 Sm 3+ 13.3 11.8 10.6 35.7 1.5 Eu 3+ 13.5 12.0 10.9 36.4 1.5 Gd 3+ 13.4 11.9 10.7 36,0 1.5 Tb a+ 13.9 12,5 11.3 37.7 1.4

Compound IV; X = F

I-I + 12.11 La 3+ 11.4 9.9 8.6 29.9 1 ~5 Pr 3+ 12.0 10,5 9.4 31.9 1.5 Nd 3+ 12.6 11.1 9.9 33.6 1.5

Sm ~* 12.9 11.4 10,2 34.5 1.5 Eu 3+ 13.2 11.7 10.5 35.4 1.5 Gd 3+ 13.0 11.5 10.3 34.8 1.5 Tb ~+ 13.5 11.9 10.7 36.1 1.4

cates that the increase i n free energy of the first step in complex formation with

ligand basicity is smaller than the corresponding increase in the proton ligand affinity.

Although A GI~I (partial molar free energy o f HA) and G~IA (partial molar free

energy o f MA) depend similarly upon one proper ty of the ligand, viz. ,the ~r donor

power, they must differ in their dependence o f its 7r donor (or acceptor) character.

It is evident from the pKa values of the hydroxamic acids and the logarithms

of the successive stabili ty constants of the corresponding metal ligand complexes

given in Tables 1 - 8 that the hydroxamic acid molecule has a parallel effect on the

proton-ligand ar_d metal-ligand stability constants as

p K a o r d e r C H 3 > H > O C H a > F > I > C I > B r > N O 2

and log KI order CH3 > H > OCHa > F > I > C1 > Br > NO2.

392


Table 7 Thermodynamic metal-ligand stability constants of N-p-Chlorophenyl-m-substituted

benzohydroxamic acids in 70% (v/v) dioxan-water media at 25 -+ 0.1 ~

Complexing K1 ion log K1 log K2 log K~ log 33 log - -

K2

Compound V; X = C1

H § 12.08

La 3§ 10.8 9.3 8.1 28.2 1.5 Pr 3§ 11.4 9.9 8.8 30.1 1.5 Nd 3+ 12.0 10.5 9.3 31.8 1.5

Sm 3§ 12.3 10.8 9.6 32.9 1.5 Eu 3§ 12.5 11.1 9.9 33.5 1.4 Gd 3§ 12.4 10.9 9.7 33.0 1.5

Tb 3+ 13.0 11.5 10.3 34.8 1.5

Compound VI; X = Br

H § 12.06

La 3§ 10.6 9.1 7.9 27.6 1.5 Pr 3+ 11,2 9.7 8.6 29 .5 1.5

Nd 3+ 11.8 10.3 9.1 31.2 1.5

Sm 3§ 12.1 10.6 9.4 32.1 1.5

Eu 3+ 12.3 10.9 9.7 32.9 1.4 Gd 3 § 12.2 10.7 9.5 32.4 1.5

Tb 3+ 12.7 11.2 9.9 33.8 1.5

The hydrogen ion is never a n-donor or acceptor and only electron density around

the donor atoms affects the proton ligand bond. Further, as the metal ions are capable

of accepting n electrons in their vacant orbitals or donating 7r electrons to the vacant

orbitals of donor atoms, the electron density around the donor atoms can also affect the

metal ligand bond. In the absence of the steric effects, since the methyl group is located

so far from the coordination centre that it causes no steric hindrance to coordination,

the contribution of bonding to the metal-ligand bond can be assessed from a systematic

comparison between the pKa of closely related ligand and log Ka of the corresponding

1 : 1 complexes of the ligands with the same metal ion. IRVING and ROSOTTI z z gave

thermodynamic interpretations to the possible correlations existing between stability

constants of similar types of complex species in aqueous as welt as mixed solvent media.

8 393


Table 8 Thermodynamic metal ligand stability constants of N-p-Chlorophenyl-m-substituted-

benzohydroxamie acids in 70% (v/v) dioxan-water media at 25 -+ 0.1 ~

Complexing KI ion log KI log K: log K s log/3 s log - -

Ks

Compound VII; X = I

1-1" 12.10 La s* 11.1 9.6 8.3 29.0 1.5 Pr s§ 11.7 10.2 9.1 31.1 1.5 Nd ~§ 12.3 10.8 9.6 32.7 1.5 Sm a+ 12.6 11.1 9.9 33.6 1.5 Eu s§ 12.8 11.4 10.2 34.4 1.4

Gd s§ 12.7 11.2 10.0 33.9 1.5 Tb s+ 13.3 11.8 10.6 35.7 1.5

Compound VIII, X = NO2

H + 11.69 La s+ 9.8 8.3 7.1 25.2 1.5 Pr s§ 10.4 8.9 7.6 26.9 1.5 Nd s+ I 1.0 9 .5 8.3 28.8 1.5

Sm s+ 11.3 9.9 8.7 29.9 1.4 Eu s+ 11.5 10.1 8.8 30.4 1.5 Gd s+ 11.4 10.0 8.7 30.1 1.4 Tb s§ 12.0 10.5 9.2 31.7 1.5

Correlation of chelate stability with nature of metal ion

A number of empirical correlations of metal chelate stability with properties of

metal ion have been employed b y recent investigations. The properties selected for

correlation are (1) the reciprocal of the ionic radius and (2) the total (first, second

and third) ionization potentials of rare earths.

A plot of the stabili ty o f various rare earth chelates (log Kx)Versus reciprocal of

the ionic radius (Fig. 2) is linear for all the hydroxamic acids studied here. A similar

relationship was obtained with EDTA chelates of rare earths. 2 s,2 6

IRVING and WILLIAMS pointed out that the strength of coordinative bond be-

tween a meal ion and a ligand is governed by two factors. For metal ions of a

definite charge, the ionic radius controls the contr ibution o f purely electrostatic

interaction between the metal and the ligand, and the overall ionization potential of

394

~-14

0

A

- -o x=OCH 3, Compound III / - A x=Cl, Compound V

13

12

Y. K. AGRAWAL et aL: THERMODYNAMIC METAL-LIGAND STABILITY

I , , I i i I I L i ~ I , . 10.900 098 1.06 1.14

l/r

Fig. 2. Correlation of stability constants of rare earths with ionic radii (III. N-p-chlorophenyl-m- methoxy benzo and V. N'p-chlorophenyl-m-chlorobenzohydroxamic acids)

the metal for the process

M(gas) ~ M z+ (gas) + Ze,

determines the afffmity of central ion to accept electrons from the tigand in covalent bond formation. 2 7 In the present study, since all the rare earth ions were in 3 + oxida- tion state and are more or less of the same size, the electrostatic contribution to metal ligand bond in the first complexes formed with the same ligand is assumed to be very nearly the same. Therefore, log K1 values for metal complexes with different ligands were plotted against the respective overall ionization potential of La 3§ Pr 3+, Nd 3+, Sm 3§ and Gd 3+ to see the effect of the electron affinity of the rare earth ions on the stability of the complexes they formed. 2 6 In each case a straight line was old'rained. This indicates that in each ligand system the stability of first complex formed is directly proportional to the total ionization potential of the central atom.

Order of stability constants

The log K1, log K2 and log K3 values (Tables 1-8) for rare eartffs metal complexes examined here on comparison show the following order irrespective of the substituent

8* 395


in hydroxamic acid

La 3§ < pr 3§ < Nd a+ < Sm a+ < Eu a+ ~, Gd a§ < Tb a+.

The trend noticed here is a regular increase in the stability constants from La 3* to Tb 3§ which can be attributed to the electrostatic force of attraction between the rare

earth ion and the ligand along the series. As the atomic number increases the cottlombic

A

_ l ~ X=H, Compound I ~ ,

ll

l o [ , I , I ( I ~ I , I ~36 58 60 62 64 66 ="

Atomic number

Fig. 3~ Order of stability constants of rare earths: variation of log K, values of rare earth hydroxamate with atomic numbers in 60% dioxan-water media at 25 ~

attraction for the ligand towards the rare earth increases and the stability constants (log K1, log K2, log K3 or/3n) increase from La 3§ to Tb a§ with all the N-p-Chloro-

phenyl-m- substituted benzohydroxamic acids. The only exception in this finding is noticed in the case of the stability constants

of Gd 3§ where the stability constants are less than that of Eu 3§ This may be due to

the change in crystal radii centering at the Gd 3§ Further variation in the enthalpy and entropy contribution to energy is also responsible for this change.

The log K1 values for rare earth complexes with N-p-Chlorophenyl-m-substituted benzohydroxamic acids plotted against the atomic number, shown in Fig. 3, exhibit more or less regularly increasing stability with increasing atomic number up to Eu 3§ there is a sudden decrease at Gd 3§ and then a further increase to Tb s§ Similar trends have been observed in other instances too. 2 8,2 9

Ratio of successive stability constants

As the tendency of a metal ion to take up ligand is proportional to the number of vacant sites, the ratio between consecutive constants, to a certain extent, is statisti- cally determined)0 For anionic ligands the coulombic attraction is more for M 3§

396


than for MA § Therefore log K1- log K2 is usually positive. 3~ Tables 1--8 show

that for all systems studied here, log K~ - l o g K2 is positive and lies within 1 .4-1.5

log units. ,

The authors are grateful to C.S.I.R. New Delhi, ONGC, Western Region and M.S. University of Baroda for financial assistance.

References

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1955, p. 139. 10. I. M. KOLTHOFF, et al., Textbook of Quantitative Inorganic Analysis, 3rd ed., MacMillan,

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381. 24. Y. K. AGRAWAL, Montatsch. Chem., 108 (1977) 713. 25. A. E. MARTELL et al., Chemistry of Metal Chelate Compounds, Prentice Hall, New York,

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Thermodynamic metal-ligand stability constants of rare earths with N-p-chlorophenyl-m-substituted benzohydroxamic acids