Electrochemical reduction of some transition metal cryptates: (222, M)2+ and (221, M)2+

J. Electroanal. Chem., 96 (1979) 81--86 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

81

ELECTROCHEMICAL REDUCTION OF SOME TRANSITION METAL CRYPTATES: (222, M) 2+ AND (221, M) 2÷

J.P. GISSELBRECHT, F. PETER and M. GROSS Laboratoire d'Electrochimie et de Chimie Physique du Corps Solide, E.R.A. au C.N.R.S. No. 468, Universit~ Louis Pasteur, 4, rue Blaise Pascal, 67070 Strasbourg Cedex (France)

(Received 20th April 1978; in revised form 6th June 1978)

ABSTRACT

The electrochemical reduction of the solvated cations M 2+ (M = Fe, Co, Ni) and of their cryptates (221, M) 2+ and (222, M) 2+ has been studied on a mercury electrode, in propylene carbonate, hy normal and reverse pulse polarography, by cyclic voltammetry and by potentiostatic coulometry. The in situ complexation of the cations M 2+ by the macrobicyclic ligands (221) and (222) has been observed. An unusual polarographic behaviour is observed with the cryptate (221, Ni) 2+.

The reduction of the cryptates of metallic cations at mercury electrode has been the subject of several previous studies [1--4]. These studies are extended here to the complexes (221, M) 2÷ and (222, M) 2÷ formed between the macrobicyclic ligands (221} and (222) [5] and the metallic cations Fe 2÷, Co 2÷, Ni 2+ of group VIII, two of which being of considerable biological interest. (I) EXPERIMENTAL

The measurements have been carried out in propylene carbonate (PC) at 25 ° C, with 0.1 M tetra-n-hexylammonium perchlorate (THAP) as supporting electrolyte. Pulse polarographic and cyclic voltammetry measurements were carried out with a Solea-Tacussel (Model PRG 4) device. In the normal pulse polarographic experiments, the imposed drop time was T = 1.000 S, the delay time before polarization was to = 0.950 s, and the pulse duration was ti = 0.040 s. The polarographic current was measured between 80 and 90% of the polarization pulse, i.e. during 4 ms. The mercury flow rate of the capillary, in solution and in open circuit, was m = 0.48 mg s- 1 with a mercury column height of 35 cm. The reported potentials are measured versus an aqueous saturated calomel electrode (SCE) connected to the cell solution by a salt bridge which has been previously described [4]. The junction potential, which corresponds to (H20 , sat KC1//PC, 0.1 M THAP) has not been determined but we have verified that it was constant during several series of experiments. In every case, the number of electrons exchanged during each elementary reduction step has been determined by potentiostatic coulometry with the aid

82 TABLE 1 Wave characteristics for t h e p o l a r o g r a p h i c r e d u c t i o n o f t h e solvated c a t i o n s in p r o p y l e n e carbonate

Fe2+ Co2÷ Ni2+

E 1 / 2 / V SCE

~/ A log(l/I d - - I)/ mV

n

10 6 × D~ c m 2 s--1 a

N a t u r e of t h e limiting c u r r e n t

--1.18 --1.29 --1.30

--85 --237 --225

2 2 2

1.3 0.9 0.65

Diffusion Diffusion Diffusion

a Calculated f r o m t h e C o t t r e l l e q u a t i o n [16,17 ] applied to pulse p o l a r o g r a p h y .

of a three electrode apparatus in which the working electrode was a mercury pool with an area of approximately 25 a m 2. The potentiostat used for coulometry was a Solea Model PRT 100-1 X. (II) P O L A R O G R A P H I C R E D U C T I O N O F T H E S O L V A T E D C A T I O N S Fe 2+, Co 2+, Ni 2+

By application of the pulse polarographic criteria [6], the limiting current for the reduction of these ions was shown to be diffusion controlled. Each of these three cations is reduced in a single bielectronic step, as demonstrated by potentiostatic coulometry. The characteristics of the corresponding polarographic waves are given in Table 1. One observes that the sequence of the half-wave potentials of M 2÷ is identical in both PC and water [7--9], but there is a shift of the waves towards negative potentials in PC. In cyclic voltammetry, the value of Ip¢/v 1/2 (scan rates: 2 ~< v/V s-1 ~< 100), on mercury electrode, is constant and therefore the limiting process is either diffusion or electron transfer. Furthermore, the cathodic peak potential (Ep¢) varies linearily towards more negative potentials with the logarithm of the scan rate: AEp/A log v is --125, --100 and --78 mV for Fe 2+, C o 2+ and N i 2+, respectively. Both these results and those obtained by logarithmic analysis of the polarographic waves show that the rate of reduction of each of these three cations is limited by its charge transfer, i.e. polarographically irreversible, in propylene carbonate. (III) E L E C T R O C H E M I C A L R E D U C T I O N O F T H E C A T I O N S Fe 2+, Co 2+ A N D Ni 2+ IN T H E P R E S E N C E O F T H E L I G A N D (222}

By comparing the size of the macrocyclic cavities (r¢) with the ionic radii (ri) of the cations studied: re (222) = 1.40 £ r~ (221) = 1.15 £

r i Fe 2+ = 0.74 £ r i Co 2+ = 0.72 h r i N i 2+ = 0.69 £

it appears that: r i F e 2+

r i C o 2+

r i N i 2+

r c (222)

rc (222)

r¢ (222)

m

~

0.5

83

From the observation that all the three ratios are approximately the same and quite different from unity, it is expected that the ligand (222) is a weaker complexing agent than (221), and that (222) exhibits a poor selectivity towards the group VIII cations. It has been actually demonstrated in water [15] that {222, M) 2+ (M = Co, Ni) have a smaller stability constant than their homologous (221, M) 2+. The present measurements on the electroreduction of (222, M) 2+ (M = Fe, Co, Ni) have been carried out to complement the previous studies of cryptates [1--4] in the same experimental conditions. The addition of ligand (222) to solutions of the cations M 2÷ (Fe 2+, Co 2+, Ni 2+) in such amounts that [222]/[M 2+] ~> 1 results in the total disappearance of the wave corresponding to the reduction of the solvated cations, together with the appearance of a more cathodic bielectronic wave which is attributed to the reduction of the cryptate (222, M) 2+. The half-wave potentials of the cryptates are:

E1/2(222, Fe) 2 = --1.70 V/SCE

and AEl/2(Fe2+/(222, Fe) 2+) = 0.52 V

El/2(222, Co) 2+ = --1.70 V/SCE

and AEl/2(Co2+/(222, Co) 2+) = 0.41 V

El/2(222, Ni) 2+ = --1.73 V/SCE

and AEl/2(Ni2+/(222, Ni) 2÷) = 0.43 V

For each of the three waves, studied in a solution where [222]/[M 2+] = 1, the limiting current was controlled by diffusion [6]. It should also be noted that just after addition of the (222) ligand to a solution of Ni 2+ or Co 2+ ions in propylene carbonate, only a transient wave is first observed. The characteristics of this transient, additional wave, i.e. half-wave potentials (E1/2 = --1.27 V/SCE for the cobalt and E1/2 = --0.95 V/SCE for the nickel), time of existence of the wave, are reproducible in our experimental conditions. This wave differs from both the wave described above and from that of the solvated cation. The height of this transient wave decreases with time and vanishes after about 3 or 4 h, together with simultaneous appearance and increase of the more cathodic wave due to the cryptate. This transient behaviour is actually not explained. However we think that it is related to the transition metal character of the cations, and also to the small ionic radii (ri) of the studied cations compared to the size (re) of the cavity of 222. Thus, the probability of a direct solvation of the cation M u+ by either solvent or residual water close to the face of the bicycle increases as the ratio ri/rc diverges from unity. (IV) REDUCTION OF THE CRYPTATES (221, M) 2+ (M = Fe 2+, Co 2+, Ni 2+) -- Fig. 1

As the ligand (221) is added progressively to a solution of M 2÷ ions, the reduction wave of M 2÷ decreases proportionally to the concentration of the added ligand. Simultaneously a new wave is observed which is attributed to the reduction of the cryptate {221, M) 2÷. The relative changes of the two limiting currents with the concentration of the dissolved ligand correspond to a 1/1 stoechiometry of the complex formed. For values of [221]/[M 2÷ ] >~ 1, only the wave of the cryptate is observed, whose limiting current is diffusion controlled [6]. This wave corresponds to a two-electron charge transfer process and appears polarographically irreversible

84

I/,A

¢lth 0.( 0.'~ OL~

/ ~'A

o

0

q

I

~

'

/

-2

0,~ (1{ Gt

/

/' -1

Mm(

-1.5

cMh.t

OA 11,6

//

,/

/

1 c~h. I G2

C

C o

/ ?

0 G2

,f

,

f

-2,__

0.4

m~L~ -1

E/V[SCE)

E/V(SCE

Fig. 1. Normal pulse polarographic reduction of solvated cations of group VIIIb (dotted line) and of the corresponding cryptate (solid line) in PC and 0.1 M THAP. (A) S o l u t i o n of Fe 2+ and of cryptate (221, Fe) 2+ at the c o n c e n t r a t i o n 1.0 × 10 - 3 M. (B) S o l u t i o n of Co 2+ and of c r y p t a t e (221, Co) 2+ at the c o n c e n t r a t i o n 1.4 x 10 - 3 M. (C) Solution of Ni 2+ and of cryptate (221, Ni) 2+ at the c o n c e n t r a t i o n 1.8 × 10 - 3 M. Fig. 2. The polarographic o x i d a t i o n due to the ligand (221) at E]~2 = +0.08 V / S C E , by reverse pulse polarography, in PC and 0.1 M THAP, of: (A) (221, Fe) 2+ = 5.8 x 10 - 4 M, (B) (221, Co) 2+ = 7.7 X 10 - 4 M, (C) (221, Ni) 2+ = 1.5 X 10 - 3 M.

from logarithmic analysis. The corresponding electrochemical parameters are given in Table 2. The cyclic voltammetry on a mercury electrode (scan rates: 2 ~< v / V s - i < TABLE 2 Characteristic parameters for the polarographic reduction of the cryptates (221, M) 2+ (M = Fe, Co, Ni) . . . . . . . . . . . . . . . . . . . . . . . .

E1/2/V SCE

n

AE/ A log(L'/d -- I)/ mV

(221, Fe) 2+ (221, Co) 2+ (221, Ni) 2+

--2.04 --1.74 --1.05

2 2 2

--90 --58 --55

106 X D/ cm 2 s--1 a

Nature of the limiting current

1.1 0.55 0.2

Diffusion Diffusion Diffusion

a Calculated f r o m the Cottrell e q u a t i o n [16,17 ] applied to pulse polarography.

85 100) shows that the electrochemical reduction of (221, M2+), involves a slow charge transfer: Ipc/V 1/2 is constant and, for the cryptates of Fe 2÷ and Ni 2÷, the variation of Epc with log v is linear (AEp~/A log v = --200 mV log -1 for (221 Fe) 2÷ and --32 mV log for Ni2÷). The shift of Ep¢ with log v is not linear for (221 Co)2÷: Epc = f(log v) is a curve whose shape is in agreement with an e.c. mechanism. For all these three cryptates (221, M) 2÷, the presence of free ligand (221) in solution after reduction of the cryptates is demonstrated by use of reverse pulse polarography [10 ], which exhibits the w a v e ( E l / 2 = +0.08 V/SCE) corresponding to the ligand-assisted oxidation of the mercury electrode [11] (Fig. 2). This wave allows the quantitative analysis of a solution containing the free ligand [12]. The existence of (221) in solution as a final product of the reductions reveals the absence of stable interactions between the metal and the ligand and demonstrates the existence of a chemical step (c) following the charge transfer (e), in the reduction mechanism of these cryptates. However, the nickel cryptate (221, Ni) 2÷ exhibits a peculiar behaviour in reduction: by addition of known quantities of ligand (221) to a solution of Ni 2÷ ions, the disappearance of the reduction wave of Ni 2÷ ( E l l 2 = --1.30 V/SCE) is observed as soon as [221]/[Ni 2÷ ] = 0.25. For values of this ratio larger than 0.25, a single wave is observed at El/2 = --1.05 V/SCE. Thus, in PC, the reduction of the (221, Ni) 2÷ cryptate is easier than that of the uncomplexed cation Ni 2÷. This is a remarkable exception among all the cryptates studied up to now [1--4], the only other known exception being the reduction of (221, Eu) 3÷ [13]. It may be recalled, however, that many other examples are known of a facilitated electroreduction of Ni(II), resulting from complexation by other mono- and polydentate ligands [18]. The occurrence of a single polarographic wave in the reduction of the cryptate (221, Ni) 2÷, already when [221]/[Ni 2÷ ]/> 0.25, may be explained by assuming that the Ni 2÷ ions in excess in the solution do not react with the electrode unless they have been previously complexed by the ligand released at the interface in the reduction of (221, Ni) 2÷. This is made possible by the fact that the reduction of the cryptate (221, Ni) 2+ occurs at potentials less negative than that of the solvated cation. The results obtained with these three cryptates indicate that the association equilibrium of the cation M 2÷ with the ligand (221) is not mobile and it cannot be analyzed by the usual corresponding procedure [14]. That confirms the relatively high stability of the (221 M) 2+ complexes in PC. A quantitative thermodynamic interpretation of the differences between the half-wave potentials of the complexed and of the uncomplexed cations is not possible, due to the irreversible character of the electrochemical reductions. The above results show that compounds of stoechiometry 1/1 are obtained with Fe 2+, Co 2÷, N i 2÷ cations complexed by (221) or (222) ligands. However, due to the covalent character of the metal-ligand bond [15] with these transition metals, the possibility remains of obtaining external complexes in solution. Also, the very different values obtained for the polarographic diffusion coefficients of the three cryptates (221, M) 2÷ studied (Table 2) support this hypothesis. Indeed, when different cations form internal complexes with the same macrobicyclic ligand, close values of the diffusion coefficients of the cryptates

86

are expected, compared to those of the solvated ions. Such a leveling effect on the D values has actually been observed with alkaline cryptates (222, M) ÷, which are internal complexes [4]. CONCLUSION

The electroreduction of the cryptates (221, M) 2+ (M = Fe, Co, Ni) occurs on mercury in a bielectronic, polarographically irreversible, reaction. The reduction half-wave potentials of the (221, Fe) 2+ and (221, Co) 2+ complexes are clearly more negative than the corresponding uncomplexed cations, in agreement with the electrochemical behaviour of the cryptates previously studied [1--4], but the inverse situation is observed for the nickel. As already observed for the a alkaline and earth alkaline cryptates, none of the three cryptation equilibria is mobile. For the three complexes (221, M) 2+, the free ligand {221) is one of the final products of the reduction and the experimental results may be interpreted in terms of an e.c. mechanism, the overall kinetics being determined b y the rate of the electron transfer. Based on these first results on transition metal cryptates, work is in progress in this laboratory, involving macrocyclic ligands whose complexes of transition metals exhibit unusual reversible redox behaviour. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

F. Peter and M. Gross, J. Electroanal. Chem., 53 (1974) 307. J.P. Gisselbrecht, F. Peter and M. Gross, J. ElectroanaL Chem., 74 (1976) 315. J.P. Gisselbrecht and M. Gross, J. Electroanal. Chem., 75 (1977) 637. F. Peter, J.P. Gisselbrecht and M. Gross, J. Electroanal. Chem., 86 (1978) 115. J.M. Lehn and J.P. Sauvage, J. Amer. Chem. Soc., 97 (1975) 6700. M. Gross and J. Jordan, J. Electroanal. Chem., 75 (1977) 163. J.N. Gaur and N.K. Goswani, Electrochim. Acta, 15 (1970) 519. G.P. Kumari and D.A. Pantony, J. Polarogr. Soc., 14 (1968) 84. R. Parsons, H a n d b o o k of Electrochemical Constants, Butterworths, 1959. K.B. Oldham and E.P. Parry, Anal. Chem., 42 (1970) 229. F. Peter, L. Pospisil, J. Kuta and M. Gross, J. Electroanal. Chem., 90 (1978) 251. F. Peter, unpublished results. O.A. Gansow and M.J. Weaver, J. Amer. Chem. Soc., 99 (1977) 7087. J. Heyrovsk¥ and J. Kuta, Principlesof Polarography, Academic Press,1966. F. Arnaud-Neu, B. Spiess and M.J. Schwing-Weill, Helv. Chim. Acta, 60 (1977) 2633. K.B. O 1 d h a m and E.P. Parry, Anal. Chem., 40 (1968) 65. M. Gross, Bull. Soc. Chim. Fr., 11--12 (1976) 1803. A.J. Atria and D. Posadas in A.J. Bard (Ed.), Encyclopeaia of Electrochemistry of the Elements, Vol. Ill,Marcel Dekker, 1975, Ch. III,p. 212.