HERBERT B. SILBER AND JAMESH. SWINEHART
4344
A Kinetic Investigation of Lanthanide Complexation. I.
Dysprosium(II1) and Anthranilic Acid
by Herbert B. Silber and James H. Swinehart Department of Chemistry, University of California, Davis, California 95616 (Received May 9, 1967)
The kinetics of the aqueous complexation reaction between dysprosium(II1) and anthranilic acid (HAn, o-aminobenzoic acid) was investigated by the temperature-jumprelaxation technique. The formation constant of the dysprosium(II1)-anthranilic acid complex was determined independently of the kinetic measurements by glass-electrode titrations. Assuming the mechanism HAn Dy3+
+ An-
H+
+ An-
kia
DyAn2+ kaz
IC23 and k32 were determined to be 1.8 X lo7 M-l see-' and 1.5 X lo4 sec-l a t 20". Values of AH* and AS* for k23 and k32 were 4.4 kcal/mole and -10 gibbs and 4.3 kcal/moleand -25 gibbs, respectively. These results are discussed in relation to other rate data on lanthanide complexation reactions.
Introduction I n many cases metal ion complexation occurs via a two-step mechanism
Yt"+(HzO),
+ L E RI
m+
(HzO),.L
kr
h!Im+(H20),*L
R!Im+(H20),-1L kb
+ H20
(1)
(2)
The first step (reaction 1) is outer-sphere ion-pair formation between the solvated metal ion and the ligand and is represented by the ion-pairing constant, K . The second step (reaction 2) consists of the substitution of the ligand for a solvent molecule in the inner solvation shell of the metal ion. I n the cases where the second step is rate determining, the rate constant for substitution, kr, is often correlated with the rate constant for water exchange. Determinations of the rate constants for substitution reactions of rare earth ions became possible only after the development of rapid reaction techniques. Geier studied the reactions of the lanthanide ions with murexide using the temperature-jump-relaxation technique.2 Geier found formation rate constants of the order of lo' M-1 sec-' (k&) for all of the lanthanides. The Journal of Physical Chemiatry
However, small but important differences were noted. The rate constants were approximately equal for lanthanum(II1) through samarium(II1) , decreased from samarium(II1) to erbium(II1) , and increased toward lutetium(II1). Because the rate constants for complex formation with these trivalent metal ions were significantly larger than with bivalent ions of similar ionic radius and electron configuration, Geier proposed a coordination number greater than 8 for the rare earth ions. Other workers have pointed out that the lanthanides could possess a coordination number of 8 or 9.3 Geier also suggests that a change in the number of water molecules in the first solvation sphere may occur near erbium(III), with the possibility of aquo complexes of differing coordination numbers being in equilibrium with each other. This study of the complexation of dysprosium(II1) with anthranilic acid (o-aminobenzoic acid) has been (1) ,M. Eigen and R. G. Wilkins in "Mechanism of Inorganic Reactions," American Chemical Society, Washington, D. C., 1965. (2) G. Geier, Ber. Bunaenges. Phyeik. Chem., 69,617 (1965). (3) For example: L. C. Thompson, Inorg. Chem., 2 , 89 (1963): L. 0. Margan, J . Chem. Phys., 38, 2788 (1963).
A KINETICINVESTIGATION OF LANTHANIDE COMPLEXATION
undertaken in an attempt to elucidate the value of the rate constant for water substitution of a rare earth ion. A temperature-dependence study of both the rate constants for complexation and complexation constant were made to determine the energy factors which are operative. The study is being extended to complexes of other rare earth ions.
Experimental Section
Chemicals. Deionized distilled water was used in the preparation of all solutions. Dysprosium perchlorate stock solutions were prepared from 99.9% pure dysprosium oxide (American Potash and Chemical Corp., Lindsay Chemical Division) by gently heating weighed amounts of the oxide with known quantities of dilute perchloric acid. Stock solutions of known M in Dy3+. concentration were of the order of The anthranilic acid (Eastman, practical grade) was twice recrystallized from ethanol solutions. The final crystals were air dried, mp 145-146.8". The stock solutions were prepared by weighing out sufficient M solutions. The concentraanthranilic acid for tion was checked by titration with standardized NaOH. Methyl red (Matheson Colman and Bell) was used withM, were out further purification. Stock solutions, made by weighing the solid and dissolving it in slightly basic solutions. Sodium perchlorate was prepared by neutralization of known HC10, and S a O H and by neutralization of weighed Sa2C03with HC104. These stock solutions, 1-2 Ai' NaC104, were boiled to remove Con. The T\ITaC104 was used to regulate the ionic strength of the final solutions containing dysprosium, anthranilic acid, and methyl red indicator to 0.2 M . Apparatus. 'The temperature-jump-relaxation instrumentation was manufactured by the Nessanlagen Studiengeschellschaft, G.m.b.H., Goettingen, Germany. The instrument was thermostated to *0.5". The relaxations were monitored by observing changes in the visible spectrum of the methyl red indicator. The signal to noise ratio was improved considerably by operating the light source supplied with the instrument at voltages higher than the rated value. The conditions for using the temperature-jump-relaxation method to investigate the kinetics of a system are the following. First, the system must be in equilibrium. Second, the temperature perturbation applied to the system must be small so tha: the rate equations can be linearized, that is, AG/RT
