March, 1959
ELECTRON TRANSFER IN COBALT SYSTEMS
371
ELECTRON TRANSFER IN THE SYSTEM, TRIS-(1,lO-PHENANTHROL1NE)COBALT(I1)-TRIS-( 1,lO-PHENANTHR0LINE)-COBALT(II1) BY B. R. BAKER,’ FRED BASOLO AND H. M. NEUMANN Contribution from the Department of Chemislry, Northwestern University, Evanston, Illinois Received October 4 #1068
The rates of electron transfer in the system tris-(l,l0-phenanthroline)-cobalt(II)-tris-(l,lO-phenanthroline)-cobalt(III) and analogous 2,2‘-bipyridine and 2,2‘,2‘’-tripyridine systems have been studied by a tracer technique. The rate laws as well as rate constants depend upon the anion present and the results are discussed in terms of ion-association. The rate is the same in DzO as in water but decreases in acetonewater solutions with increasing acetone concentration. The latter effect is discussed in terms of the barrier tunneling model for electron transfer. The rates and energy of activation me compared to those of other cobalt systems and the results are discussed in terms of the field strength of the ligand, metal ion electron configuration and shielding or conducting nature of the ligand. product from above was refluxed for 20 min. with 10 ml. of saturated bromine water and 20 ml. of water. Five mi. .of 70% HC104 was then added to the hot solution. This serves not only to precipit,ate the cobalt(II1) compound but destroys any cobalt(I1) remaining. Both compounds were characterized by nitrogen and water of hydration determination and by the absorption spectra from 340-250 mp as compared with the spectra of the ions as oreoared in situ. Anal. dalcd. for C O ( C ~ Z H ~ N ~ ) ~ ( C ~N, O ~10.30; )~~~H~O: HzO, 2.20. Found: N, 10.05; HzO, 2.22. Calcd. for Co( C ~ Z H ~ N Z ) ~ ( C ~ O ~ )N, ~ . ~9.00; H Z O :HzO, 3.85. Found: N, 8.80; Hz0, 3.98. Tris-(2,2’-bipyridine)-cobalt(III) perchlorate was prepared by the method of Ellis, Wilkins and Williams.* In contrast to the monohydrate reported this compound appeared to contain 3 molecules of water. The ultraviolet spectra of both the hydrated and dehydrated salts confirmed this and show that no decomposition of the compound occurred during dehydration at 156’ and 5 mm. pressure. Both spectra were unchanged in 0.03 IIf HC1. Anal. Calcd. for Co(c0H8N2)3(C104)3.3H20:HpO, 6.14. Found: HpO. - .5.88. Bis-(2,2’,2’’-tripyridine)-cobalt(II) perchlorate was prepared by dissolving cobaltous chloride hexahydrate (0.16 9.)and 2,2’,2”-tripyridine (0.4 .) in 5 ml. of boiled, cooled water. To this solution was afded 7 ml. of NaCIOa solution (2 to 3 g. per 10 ml.) yielding red crystals. After cooling in ice these were filtered, washed first with 75% ethanol then finally with absolute ethanol. They were dried in air; yield 53% of theoretical. Tripyridine was determined spectrophotometrically by comparing the s ectrum of the complex in 4 M HC1 with that of a standarfsolution of tripyridine in 4 M HC1. tripy, 62.9; Preparation of Compounds.-The compound [ C ~ ( p h e n ) ~ ] Anal. Calcd. for Co(tripy)z(C104)~~1H20: ( C ~ O ~ ) Z . ~ was HZO prepared by the method of Pfeiffer and HzO, 2.42. Found: tnpy, 63.0; H20,2.16. Werdelmann.’ To prepare [Co(phen)a](C104)3.2Hz0, the Bis“2,2’,2‘’-tripyridine)-cobalt(III) perchlorate wa8 prepared by refluxing cobaltous chloride (0.24 9.) and tripyri(1) Based upon a portion of the thesis submitted by B. R . Baker dine (0.5 g.) in 40 ml. of water with 22 ml. of bromine water. to Northwestern University in June, 1958, in partial fulfillment of the On the addition of 10 ml. of 70% HClO4, yellow crystale requirements for the degree of Doctor of Philosophy. Abbott Fellow, separated. These were filtered, washed with ice-water and 1955-1957. dried over CaCl2. (2) The ligand field theory of metal complexes, developed by Anal. Calcd. for CO(C,,H,IN,),(C~O&.~H~O: N, 9.98; H. Bethe, Ann. Physik, [5] 5, 133 (1929) and L. E. Orgel and coHzO, 2.14. Found: N, 10.03; HzO, 2.16. workers in numerous papers, is allied closely with the theories of electron transfer and has received consideration in this respect by L. E. Exchange Studies.-The rate of exchange was followed Orgel, Tenth Solvay Conference, Brussels, May 1956; and F. Basolo by a tracer method using cobalt-60, the activity being introand R . G . Pearson, “Mechanisms of Inorganic Reactions.” John Wiley duced either as cobalt(I1) or cobalt(II1). In order to oband Sons, Inc., New York, N . Y . , 1958. tain the greatest precision possible, the concentration of one oxidation state was always at least twice that of the other. (3) W. B. Lewis, C . D. Coryell and J. W. Irvine, J. Cliem. Soc., 5386 The activity then was introduced in the form of the oxidation (1 949). state of lowest concentration. (4) A. W. Adamson and K . S. Vorres, J . Inorg. Nuc. Chem., a, 206 (1956). Reaction solutions were prepared assuming additivity of (5) N. A. Bonner and J. P. Hunt, J . A m . Chem. Soc., 74, 1866 volumes except in the case of the water-acetone solutions in which the volumes of acetone and water were experi(1952). mentally determined to give a predetermined volume of re( 6 ) The rates of electron transfer in the phenanthroline and bipyridine systems have been reported by P. Ellis, R. G . Wilkins and M. J. G . action solution. The solutions were prepared to contain Williams, J . Chem. Soc., 4456 (1957); but this was an indirect meas[Co(phen)a]SO4(prepared in situ by mixing stock solutions urement, being obtained from the rate of exchange of radio-phenanof 1,lo-phenanthroline and CoSO,), excess phenanthroline throline and radio-bipyridine with the cobalt(II1) ions via the electron (1 X 10-3 M to 0.85 X loe3in all runs), and buffer solution. transfer with the labile cobalt(I1) ions. The reaction vessels were 10-ml. volumetric flasks or 15ml. glass stoppered test-tubes. (7) Von P. Pfeiffer and B. Werdelrnann, Z . anow. Chem., 261, 197 The reactions were started by pipetting 0.5 ml. of [Co(1950).
A number of electron transfer reactions have been studied in which the primary coordination spheres of the metal ion remain unaltered. These are believed t o proceed by a direct electron transfer rather than by an atom transfer mechanism. With new developments in the theory of electron transfer and in the theory of metal-ligand bonding in metal complexes it became imperative that the theories be subjected t o additional experimental tests.2 The systems chosen for this purpose were the tris (1,lO - phenanthroline) - cobalt(I1)-tris - (1,lOphenanthroline-cobalt(II1) and related 2,2‘-bipyridine and 2,2’2”-tripyridine systems. There are several reasons for investigating these systems. In the first place, because of the nature of the complexes, these systems would seem t o require electron transfer with little possibility of atom transfer. Secondly, some work has been done on cobalt(I1)-cobalt (111) systems containing ligands of weak and moderately weak ligand field strength, but no direct studies had been reported for ligands of high field strength such as 1,lO-phenanthroline.a-6 Finally it was desired t o study the effect of solvent on an electron-transfer system in which little possibility of a bridge-transfer mechanism exists. Experimental
-
B. R. BAKER, FREDBASOLO AND H. M. NEUMANN
372
Vol. 63
(phen)sl(C104)~solution (a proximately 1 X 10-8 M ) into surrounding the detector tube (Radiation Counter Laborathe reaction vessel followei by rapid stirring either by re- tories, Mark 1, Model 71, jacketed Geiger tube, shielded peated inversion of the stoppered vessel or by means of a with 0.5 inch of lead). The tube was connected to a model stream of nitrogen passed into the solution. Six 1-ml. ali- 182A scaling unit, Nuclear Instrument and Chemical Corp. quots were withdrawn at intervals of 2 to 5 min. and The sample was counted for a total of at least 5000 counts treated as described below. The solution then was allowed and the appropriate background correction was made. It to warm to room temperature for a time to ensure that iso- was shown that no coincidence corrections were necessary. topic equilibrium had been established. Two 1 4 . infiniteThe other two techniques involved pre aration of solid time samples then were withdrawn and treated exactly as samples. An aliquot (0.5-1.0 ml.) of the agohol phase from the other samples. the sodium acetate separation was measured onto an alumiThe experimental procedure for the 2,2'-bipyridine and num counting tray containing a piece of tightly fitting filter 2,2',2''-tripyridine systems was similar to that of the phen- paper. The solvent then was evaporated under 8 heat anthroline system except that no excess of tripyridine could lamp. For counting the tray was ositioned about one be used due to its insolubility in water. Instead a weighed inch from the window of a Tracerla& Geiger tube, model quantity of [ Co(tripy )a](C!O& lH2O was dissolved In a TGC 2/11384, connected to the same scaling unit described measured volume of water just rior to the exchange run. above. The reaction was started by adfing a measured volume of T o count samples from the triiodide separation the preactive [Co(tripy)~]( C104)s solution. Under the low con- cipitated cobalt(II1) triiodide salt first was dissolved in 2 centration conditions used, the cobalt(I1) ion may be about ml. of acetone and then transferred quantitatively to an 10% dissociated at equilibrium; however the dissociation of aluminum counting tray lined with a piece of filter paper. the ion is reported to be slows ( t i / , = 2.8 hr..at 15'): The solvent was evaporated under a heat lamp. PreparaThus it may be assumed that all of the cobalt(I1) ion exists tion of a uniform sample by this procedure is difficult. For in the bis form. this reason the weak @-activity of the cobalt could not be Temperature control was provided by an ice-water mix- measured precisely and was absorbed by an aluminum abture in a Dewar flask for the 0' runs and by an ordinary sorber leaving only the ?-radiation to be counted. thermofltated water-bath for the runs at higher temperature. Preparation of Radioactive Solutions.-Radioactive coAll reactions were run in the dark even though there was no balt-60 was obtained in the form of an HCl solution from the evidence for a light catalyzed reaction. Prior deaeration by Oak Ridge National Laboratory, .Union Carbide Nuclear nitrogen had no effect on the rate; however this precaution Company. It was found to contain iron and nickel as imwas taken for the tripyridine system because of the apparent purities. Nickel was removed by evaporating the solution greater ease of oxidation of Co(tripy)Zz+ than C~(phen)~Z+. to dryness and extracting the residue with an ether solution Methods of Separation.-Three methods were used for of HC1. The cobalt is removed in such a procedure as a the separation of cobalt(I1) from cobalt(II1). I n the first, solution of H2CoClh along with iron. Following evaporation an ali uot of the reaction mixture was pipetted into a mix- of the ether, iron was removed by precipitation as the hyture 4 ml. of n-hexyl alcohol and 4 ml. of a nearly satu- drous oxide. The solution then was diluted to give a conrated solution of sodium acetate (440 g. of anhydrous so- venient volume of stock solution. Stock solutions prepared dium acetate per liter of water) a t 0". When this mixture by further dilution of this solution, on standing, frequently was shaken for 35 sec. about 70-80'% of the cobalt(I1) was led to exchange rates much greater than normal. It was extracted from aqueous solution, or about 90% from ace- found that di estion with HaSO4, followed by removal of tone-water solution, into the alcohol phase, leaving all of the the excess H&36 by repeated evaporation and ignition over a cobalt(II1) in the aqueous phase. The phases were sepa- bunsen flame, led to normal rates again. Therefore all rated by centrifugation and when all the samples of a par- subsequent runs were made with CoS04 solutions prepared ticular run had been collected the alcohol phase was with- in this manner, the treatment with HpSO, being repeated drawn and repared for counting. The degree of separation whenever necessary. This unusual behavior is possibly was reprofucible as shown by the linear kinetic plots. due to catalysis by H2On formed by radiation induced deSince exchange occurred during the separation, an attempt composition of the water. was made to carry out each se aration in exactly the same Radioactive [C0(phen)~](C10~)8 solutions were prepared manner so that the fraction of exchange occurring during in the followin manner. A 0.001 M solution of [Cose aration was the same for each sample. (phen)8](C104)a710 ml.) was heated on a steam-bath with %he second separation method was carried out by ipet- 0.2 ml. of 0.01 M phenanthroline solution and 0.1 ml. of ting a reaction mixture aliquot into 1 ml. of 4 M H d c o n - active CoCl2 solution. This treatment served to convert tained in a 50-ml. separatory funnel. This serves to quench about 90% of the active cobalt to the cobalt(II1) form by the reaction due to dissociation of the cobalt(I1) complex electron transfer. About 0.5 ml. of 70% HClO4 solution ion. The cobalt(II1) ion is not affected by this treatment. then was added and the solution was chilled in ice. The T o this solution was added 4 ml. of 30% NH4SCN solution precipitate of [ C ~ ( p h e n ) ~ClO,), ]( was washed with several and the mixture was shaken with 15 ml. of 1:1 ether-n- portions of ice-water, the supernatant liquid being removed hexyl alcohol solution. Extraction of Co*+ was about each time with a dropping pipet. The solid then was dis85-90% complete and was reproducible. This rocedure solved in 9 ml. of water, the solution passed through a sincan be used only when the concentration of Co(pKen)P+ 18 tered glass filter and its concentration determined spectro
