Electrometric study on La(III), Ce(III), Pr(III), Sm(III) complexes of methyl-3-mercaptopropionate

309

Notes REFERENCES

1. A. Werner and H. Miiller, Z. anorg, allg. Chem. 22, 91 (1899). 2. W. C. Waggener, J. A. Mattern and G. H. Cartledge, J. Am. chem. Soc. 81, 2958 (1959). 3. M. M. Chamberlain and J. C. Bailar, J. Am. chem. Soc. 81, 6412 (1959). 4. D. N. Purohit and Jannik Bjerrum, J. inorg, nucl. Chem. 33, 2067 (1971). 5. (a) G. C. Lalor and D. S. Rustad, J. inorg, nucl. Chem. 31, 3219 (1969). (b) C. St. E. Boyce, Ph.D. Thesis,

University of the West Indies (1969). 6. Analyses by Dr. Franz Pascher, Mikroanalytiches Laboratorium, Germany. 7. D. Gay and G. C. Lalor, J. chem. Soc. (A), 1179 (1966). 8. (a) K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, 2nd Edn. Wiley, New York (1970). (b) C. Pecile, G. Giacometti and A. Turco, Atti Accad. naz. Lincei, Rend. Classe Sci. fis mat. Nat. 28, 189 (1960). 9. F. A. Cotton, In Modern Coordination Compounds (Edited by J. Lewis and R. G. Wilkins). Interscience, New York (1960).

J. inorg, nucl. Chem., 1975, Vol. 37, pp. 309-311. Pergamon Press. Printed in Great Britain,

Electrometric

study

on L a ( I I I ) , C e ( I I I ) , P r ( I I I ) , S m ( I I I ) c o m p l e x e s o f methyl-3-mercaptopropionate

(First received 19 January 1974; in revised form 4 March 1974) COMPLEXformation of methyl-3-mercaptopropionate, CH 3OOCCH2CH2SH, with La 3+, Ce a+, Pr 3+ and Sm 3+ has been studied in 25 per cent ethanolic media by potentiometric and conductometric techniques. Formation curves indicate the formation of 1:1 and 1:3 complexes in each case and their stabilities follow the order Sm 3+ > Pr 3+ > C e 3 + > La 3÷. The overall changes in thermodynamic functions AG, AH, AS for the complexation reactions have also been reported. Thiols, containing an active-SH group, have a wide range of applications in biological, pharmaceutical, industrial and other chemical fields and are well known for their tendency to form complexes with metals. Several metal complexes of thiols have already been studied by Saxena et al.[1-3]. The present communication reports the composition and stabilities of Methyl-3-mercaptopropionate complexes of some metals of lanthanide series, viz. La 3+, Ce 3+, Pr 3+, Sm 3+ and the overall changes in free energy, enthalpy, entropy accompanying their complexation reactions. EXPERIMENTAL

Methyl-3-mercaptopropionate (referred to herein as RSH) was obtained from Evan's Chemetics, New York and

II

9

7

5

a~

i

I

J

0-8

L

1.6

i

[

2.4

I

I

3.2

I

I

4"0

ml of NoOH/mole of ligar~l

Fig. 1. pH titrations of the solutions (1) 4"0mM RSH; (2) 4.0mM RSH + 1.0mM Ce(III); (3) 4.0mM RSH + 2.0 mM Ce(llI); (4) 4.0 mM RSH + 4.0 mM Ce(llI).

Table 1. Stabilities of La(III), Ce(III), Pr(III), Sm(III) complexes of Methyl-3-mercaptopropionate Metal complexes Lanthanum Cerium Praseodymium Samarium

log K1

25°C log K 3

log/~

log K 1

35°C log g 3

log ]~

log K1

45°C log K 3

log fl

2.44 2.32 2-52 2.80

1.22 1.28 1.43 1.52

3.46 3.60 3.95 4.32

2"18 2.24 2.46 2.72

I. 12 1.18 1.33 1.42

3.30 3.42 3.79 4.14

2"12 2.14 2.40 2.64

1.02 1-08 1.23 1.32

3.14 3.22 3.63 3.96

310

Notes

the other chemicals were of Anal-R grade, pH measurements were made on a Cambridge bench pattern pH meter with glass calomel electrode assembly. Conductance was measured on an electronic eye-type conductometer. The experimental procedure, as described earlier[l], involved a series of pH and conductometric titrations of R S H in absence and presence of metal ion against standard NaOH.

RESULTS AND D I S C U S S I O N

Stoichiometry For establishing the stoichiometry of the complex species formed during the interaction of metal ion and RSH, the magnitude of the proton displacement was determined by titrating the solution containing RSH and the metal ion in different molar ratios against standard alkali. On the addition of NaOH to the ligand, a sudden rise in pH was observed (Fig. 1, curve 1) indicating the non-titrability of the proton o f - S H group under the experimental conditions. The addition of an equimolar concentration of metal ion greatly alters the shape of the free ligand titration curve (Fig. l, curve 4) indicating the complex formation which results in the lowering of buffer region due to the proton displacement. M 3+ + R S H ~ - M ( R S ) 2+ + H +

(1)

where M stands for La, Ce, Pr, Sm. Since the extent of proton displacement depends on the relative affinity of ligand for H + and the metal ion, it is obvious from the curve that the interaction of M 3+ ion with RSH is sufficient to compete with a relatively high concentration of hydrogen ions. The inflection at m = 3 (m being the moles of NaOH per mole of ligand) may suggest the formation of either M(RS)(OH)2 or M(RS)3 and M(OH) 3 in accordance with the following equations: M 3+ + RSH + 3 O H - ~- M(RS)(OH)2 + H20,

(2)

M 3 + RSH + 3 O H - ~-IM(RS)3 + H 2 0 + ~M(OH) 3.

(3)

Stability constants Calvin and Melchior's[5] extension of Bjerrum's[6] method has been employed for the determination of stability constants of the complexes. The pH titrations of RSH solution with ionic strength ~ = 0'1 M (KNO3) in absence and presence of metal ion were carried out at 25° , 35* and 45°C against 0'1 M NaOH and the concentration of the bound ligand, calculated from the horizontal distance between the corresponding curves, was divided by the total metal ion concentration to obtain formation function (~). At any pH, the value of free ligand concentration, [A], was calculated from the relation

[RS H]rotal

[A] =

-

[R S H]bound

-

(4)

[H+]/K, + 1

Where K,, the dissociation constant of RSH determined polarographically[7], is 1'12 x 10- lo The formation curves obtained at different temperatures by plotting fi vs - log [A] reveal the formation of 1:1 and 1:3 complexes for all the four metals La 3+, Ce 3+, Pr 3+ and Sm 3+. The stabilities of the complexes formed follow the order (Table 1). S m > Pr > Ce > La. A possible explanation for the trend is that the lanthanide metal ions differ from each other in the number of electrons in the 4f orbitals which are effectively shielded from interaction with ligand orbitals by electrons in the 5s and 5p orbitals. Hybridization would involve normally unoccupied higher energy orbitals (e.g. 5d, 6s, 6p) and this may be expected to occur with the most strongly coordinated ligands. Lanthanides, therefore, normally form ionic compounds. The possibility of covalent interaction, however, cannot be completely excluded as reported in the case of acetylacetone chelates of lanthanides[8]. If the bonds are ionic, the Born relation E = e2(1 - 1/D)/2r should hold for the energy change on complexation of a gaseous ion of charge e and radius r in a medium of dielectric constant D. Since the stability constant is related directly to this energy, the log K values should increase linearly with e2/r. The plots of e2/r vs log K 1 and log K 3 values of the metal complex studied (Fig. 2, curve 1, 2) do not however,

The appearance of precipitate during the titration rules out the possibility of formation of hydroxo-metal complex [4] as indicated by Eqn (2) and supports the formation of M(RS)3 and M(OH)3 [Eqn (3)]. With the metal and the ligand in the ratio 1:2 and 1:4, the inflection is obtained at rn = 1.5 and 0.75 (Fig. 1, curve 3, 2) which confirm the above conclusions. Conductometric titrations of RSH in absence and presence of M 3+ ion, mixed in different ratios, against standard NaOH yield breaks in the curves corresponding to the formation of M(RS)3 as obtained from pH titrations.

2-S

1"8

2-7

1"6

2"~

I "4

2'~

1"2

~ o

Table 2. Thermodynamic functions for La(lll), Ce(Ill), Pr(III), Sm(IIl)complexes of Methyl-3-mercaptopropionate Metal complexes Lanthanum Cerium Praseodymium Samarium

AG AH AS (Kcal/mole) (Kcal/mole) (Cal/deg/mole) - 4-65 - 4-82 -5.34 - 5.84

- 7.06 - 7.95 -7.15 - 7.97

- 7.8 - 10.2 -5.8 - 6.9

2.,

l •4

1 8"6

l

I

i

8"8

I

9"0

I

I

9"2

I 9'4

,.o

e2/r

Fig. 2. Curve 1 plot of logK1 vs e2/r; Curve 2 plot of log K3 vs e2/r.

Notes exhibit any linear increase of logK x and logK 3 with increase in e2/r.

Thermodynamic functions The values of overall changes in free energy (AG), enthalpy (AH) and entropy (AS) accompanying complexation reaction have been determined at 35°C with the help of standard equations[9]. The value of AG is obtained from the expression AG = - RT In fl where fl = K1K 3 is the overall stability constant. AH is determined with the help of an isobar (d In fl)/dT = AH/RT 2 which may be rewritten as d(log fl)

AH

d0/T)

4.57'

The values of log fl obtained at different temperatures are plotted as a function of 1/T. The gradient of the tangent drawn at the point corresponding to 350C is determined and equated to -AH/4.57: AS is then evaluated from the relation AH - AG

AS

T

311 The values of AG, AH and AS are summarized in Table 2.

Department of Chemistry R.S. SAXENA Malaviya Regional Engineering College S. K. BHATIA Jaipur India REFERENCES

1. R. S. Saxena, K. C. Gupta and M. L. Mittal, Can. J. Chem. 46, 311 (1968). 2. R. S. Saxena and Randhir Singh, Mh. Chem. 102, 1956 (1971). 3. R. S. Saxena and U. S. Chaturvedi, J. inorg, nucl. Chem. 34, 913 (1972). 4. S. Chamberek and A. E. Martell, Organic Sequestering Agents, p. 72. Wiley, New York (1959). 5. M. Calvin and N. C. Melchior, J. Am, chem. Soc. 70, 3270 (1948). 6. J. Bjerrum, Metal Amine Formation in Aqueous Solutions. Hasse, Copenhagen (1941). 7. R. S. Saxena and S. K. Bhatia, Electrochim. Acta 18, 933 (1973). 8. T. D. MoeUer, Chem. Rev. 65, 1 (1965). 9. K. B. Yatsimirskii and V. P. Vasil'ev, Instability Constants of Complex Compounds. Pergamon Press, Oxford (1960).

J. inorg, nucl. Chem., 1975, Vol. 37, pp. 311-313. Pergamon Press. Printed in Great Britain.

Investigations of bonding in the cyclic sulphur imides by X-ray photoelectron spectroscopy (Received 10 March 1974; in revised form 16 March 1974) WE nAVE examined the X-ray photoelectron spectra of cyclooetasulphur and all the known cyclic sulphur imides Ss-x(NH)x [Formulae (I)-(VlII)]. Our object was to determine from chemical shifts the charges on the sulphur and nitrogen atoms in compounds (II)-(VIII). This, we hoped, would lead to better understanding of the N--S bonding, in which a pn-dn contribution has been postulated[l, 2], and would provide a firmer basis for explaining the reactions of the imides, a subject of current interest[2, 3]. S--S--S

S--S--NH

S--S--NH

I

I

I

I

I

I

I

I

S

S

S

S

S

S

S

S

I

I

I

I

I

I

~l

I S--S--S

S--S--S

(I)

(II)

S--S--NH I I S S

I

I

HN--S--S

(v)

S--S--NH

S--S--NH

S--NH--S

(III)

(IV)

S--S--NH S--S--NH HN--S--NH I I I I I I HN S S S S S

I

I

HN--S--NH

(Vl)

I

I

I

[

S--S--NH HN--S--NH (VII)

(VIII)

EXPERIMENTAL

The imides were prepared by published methods[4, 5]. Fresh samples, shown by i.r. spectroscopy, thin-layer chromatography and elemental analysis to be over 99 per cent pure, were used. Finely-powdered specimens were pressed on to double-sided adhesive tape attached to the copper sample probe of the electron spectrometer, an AEI ES200. MgKg X-radiation (1253.6eV)was used to eject photoelectrons from the specimens, all of which were held at -60°C, and the spectrometer entrance and exit slits were adjusted to give a resolution of 1.3 eV (full width at half maximum) for the Ag 3ds/z peak. After a preliminary wide scan of each compound, a fresh specimen was used to study the N ls, C ls, and S 2p regions, omitting the intermediate energy ranges in order to minimize radiation-induced decomposition. A Du Pont 310 Curve Resolver was used for deconvolution of the unresolved S 2p peaks. RESULTS AND D I S C U S S I O N

Each imide spectrum contained, as its most prominent features, the N ls and S 2p peaks, both unresolved.