J . Phys. Chem. 1991, 95, 289-291
289
Electrochemistry of cis- and trans-114,8,11-Tetraazacyclotetradecane Complexes of Cobalt( I I I) at Gold Electrodes in Hydroxide Solutions Costas Tsintavis, Hu-lin Li, James Q. Chambers,* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996
and David T. Hobbs Westinghouse Corporation, Savannah River Laboratory, Aiken, South Carolina 29808 (Received: May 7, 1990)
The electrochemistry of trans- and cis-dihydroxycobalt(II1) 1,4,8,11-tetraazacyclotetradecane([ 14laneN4) complexes has been studied in 3 M NaOH at gold electrodes. A square electron-transfer scheme is found to be operative that incorporates electron-induced cis/trans isomerization via CoIIcyclam intermediates. Attention is focused on the role of gold surface interactions with the Co%yclam complexes and on the electron-induced isomerization of cis-[Co"'( [ 14]aneN4)(OH)2]+ complex. A consistent set of thermodynamic and kinetic parameters is obtained by a combination of cyclic voltammetric and chronoamperometric experiments. The analysis indicates that the cis- and rrans-Co"cyclam isomers have nearly equal energies, isomer is more stable than the trans isomer being more stable by 0.6 kJ/mol, while the truns-[C0~~'([14]aneN4)(OH)~]+ the cis-[Coiil([ 14]aneN4)(OH)2]+isomer by 21.3 kJ/mol. At rmm temperature the lifetime of cis-[Col'([14]aneN4)(OH2)2]3+ ( 7 = 0.693/k-,, where k-2 is the first-order rate constant for isomerization) is 2.6 s, 1500 times more short-lived than the cis-[Co"'( [ 14]aneN4)(OH)2]+complex.
Perusal of the vast literature on the electrode reactions of Col*l/li couples' reveals that they are usually kinetically sluggish and poorly defined both in aqueous and nonaqueous solvents. In the situations where they have been studied in some detail, seldom has fully reversible electrochemical behavior been reported. In part this behavior is probably related to the observations of Bond and co-workers,2who have used several CO~~'/" couples to illustrate the effects of multiple equilibria coupled to electron-transfer steps in ladder (or multisquare) schemes. Mechanistic complexities of this type can lead to apparent quasi-reversible or superNernstian behavior where the phenomenological heterogeneous rate constants extracted from the data have little meaning.3 Macrocyclic amine complexes of Co"' and ColI have attracted the attention of inorganic chemists for several years.4 In the 111 oxidation state the substitutionally "inert" cobalt(II1) tetraammine complexes provide a wide range of stable structures for the study of substitution (hydrolysis)s and electron-transfer chemistry? In the I I oxidation state, the axial bonds lengthen considerably and the ligands become much more labile, which opens the door for complications of the type envisioned by Bond and Oldham3 For the well-studied Col%yclam complex, specifically, trans-[Co"'([ 1 4]aneN4)X2]"+,' quasi-reversible electrochemistry has been observed in acetonitrile (e.g., X = CI, n = and aqueous HC104 (X = H 2 0 , n = 3): KCI (X = CI, n = and NaOH (X = OH, n = I ) " solutions. (In this paper, use of the term "cobalt (1) Maki, N.; Tanaka, N . In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1975; Vol. 111, Chapter 111-2. (2) la) . Bond. A. M.: Hamblev. T. W.: Snow. M. R. Inorp. Chem. 1985. 24, 1920. (b) Bond, A'. M.; Hakbley, T.W.; Mann, D. R, Snow, M. R: Inorg. Chem. 1987, 26, 2257. (3) (a) Bond, A. M.; Oldham, K. B. J. Phys. Chem. 1983,87,2492. (b) Bond, A. M.;Oldham, K. B. J. Phys. Chem. 1985,89, 3739. (c) Bond, A. M.; Keene, F. R.; Rumble. N. W.; Searle, G.H.; Snow, M. R. Inora. Chem. 1978. 17. 2847. (4) Melson, G. A., Ed. In Coordination Chemistry of Macrocyclic Comaounds: Plenum Press: New York. 1979. ( 5 ) Tobe, M. L. In Advances in Inorganic and Bioinorganic Mechanisms; Sykes. A. G., Ed.; Academic Press: New York, 1983; pp 1-94. (6) Endicott, J. F.; Durham, B. In ref 4, pp 393-460. (7) For a summary of nomenclature see: Melson, G. A. In ref 4, pp 6-15. (8) Hung. Y.; Martin, L. Y.; Jackels, S.C.; Tait, A. M.; Busch, D. H. J. Am. Chem. Soc. 1977, 99, 4029. (9) Geiger. T.;Anson, F. C. J. Am. Chem. SOC.1981, 103, 7489. (IO) (a) Taniguchi, 1.; Nakashima, N.; Yasukoushi, K. J . Chem. Soc., Chem. Commun. 1986, 1814. (b) Taniguchi, I.; Nakashima, N.; Matsushita, K.; Yasukoushi. K. J. Electroanal. Chem. Interfacial Electrochem. 1987, 224,
.-,
I
199.
0022-3654/91/2095-0289$02.50/0
SCHEME I: (L = [14]aneN4)
mas-reaction
cyclam" is meant to imply generic cobalt complexes of the [ 14laneN4 ligand without exact specification of the conformation or ligands occupying the other coordination sites in an octahedral geometry.) Electrochemical studies on the pH dependence of Co"'/" couples are conspicuously absent in the literature. This is probably related to the lability of the Co" species and the resulting interactions with buffer components, which would give rise to complex voltammetric behavior. Previously we found that, in dilute to concentrated NaOH solutions, quasi-reversible cyclic voltammograms were obtained for the Colll/llcyclam couple," which were in agreement with a Pourbaix diagram based on the Eo' value in acid solution9 and the pK, values of the trans-[Coil'( [ 141aneN4)(0H2)J3+ cation.12 Furthermore, in concentrated base, the cyclic voltammetric behavior at gold electrodes is indicative of a square scheme, which we have interpreted to involve electron-transfer-induced trans/& i~omerization.'~ In the work described below the kinetics of the reactions embodied in Scheme I were studied in 3 M NaOH. Attention is focused on the role of gold surface interactions with the Co'I'cyclam complexes and on the electron-induced isomerization of the cis-[ColI1([ 1_4]a_neN4)(0H),]+complex. The latter process is shown to be an ECE mechanism similar to the classical example of Feldberg and Jeftic.14 Experimental Section Chemicals. Reagent grade chemicals and doubly distilled water that had been passed through a Millipore purification column, (11) Li, H. L.; Chambers, J. Q.; Anderson, W. C.; Hobbs, D. T. Inorg. Chem. 1989, 28, 863. (12) Poon, C . K.; Tobe, M.L.Inorg. Chem. 1968, 7, 2398. (13) Tsintavis, C.; Li, H. L.; Chambers, J. Q.; Hobbs, D. T. Inorg. Chim. Acta 1990, 171, 1. (14) Feldberg, S.W.; Jeftic, L. J. J . Phys. Chem. 1972, 76, 2439.
0 199 1 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991
Tsintavis et al.
Model Milli-Q, were used to prepare all solutions. Literature procedures were employed for the synthesis of trans-[Co"'( [ 1 4]aneN4)C12]C115and cis-[Co"'( [ 14]aneN4)C12]C116from the free ligand, 1,4,8,11-tetraazacyclotetradecane (Aldrich). The trans- and cis-[Co"'( [ 14]aneN4)(0H),]+ complexes were prepared by base hydrolysis of the dichloro complexes by direct dissolution of the complex in 3 M NaOH or, in the case of the trans complex, by passage through an anion-exchange column (BioRad AG 1 -X8, 20-50 mesh) in the hydroxide form in order to eliminate the chloride ions. The structures of the compounds were established by elemental analysis and their UV/visible and IR spectra. Electrodes and Cells. Most of the voltammetric work employed a gold disk as the working electrode: Pine Instruments Co. Model AFDT27; area = 0.196 cm2. The reference was a double bridge SCE (Corning). A large area (6 cm2) Pt grid was used as a counter electrode. The gold and platinum electrodes were occasionally cleaned by immersion in fuming nitric acid, followed by copious rinsing with water and by IO-min sonication in distilled water. The gold electrode was polished with 0.3- and 0.05-Fm alumina lapping compound before sonication. It was pretreated electrochemically by stepping the potential to 0.5 V vs SCE for 60-1 20 s followed by a brief open-circuit rotation at 900 rpm before each measurement. A typical three-compartment cell was used. Oxygen was removed by flushing with argon through a glass frit for 15 min prior to each series of experiments, and an argon blanket was maintained over the solution at all times. "High-purity" argon was purified by passage through a 3-ft column containing Mn" dispersed on vermiculite." All experiments were conducted at ambient laboratory temperature, 21 f 3 OC. Instrumentation. Electrochemistry was carried out with a Bioanalytical Systems Electrochemical Analyzer, Model BAS 100. UV/vis absorption spectra were obtained on a Hewlett-Packard Model 8254A diode array spectrophotometer driven by a MICOMP computer. Infrared spectra were obtained on a BioRad Model FTS-7 spectrometer driven by a Model SPC 3200 microcomputer. The simulated cyclic voltammograms were generated by using software of Evans and Lerkel* using a IBM personal computer, Model 5 1 50. Procedures. Since the cis-[C20"'( [ 14]aneN4)(OH)2]+complex isomerizes in 3 M NaOH with a half-life of ca. 1 h, all the experiments were done with newly prepared solutions such that the initial measurements took place with an elapsed time of 15-60 s from the dissolution of the compound in the NaOH solution. Generally solutions of the trans-[Co"'( [ 14]aneN4)(OH)2]+ complex, which was stable in 3 M NaOH, were used for several days. The peak currents for the waves in the cyclic voltammograms (CVs) were measured as the difference between the peak current given by the BAS-IO0 and the decaying current base line obtained by holding the potential at the foot of the wave. This method, which permits one to estimate the peak current of reverse waves in CVs, eliminates any Faradaic current due to an earlier peak but does not compensate for charging currents. The Ohmic losses generally were not compensated electronically since oscillation noise was often observed with the low-resistance solutions employed in this study. It was observed that the potential separation between peaks was not decreased by compensation for the solutions less than IO mM in the complex. The peak potential values were obtained from the BAS-100, and the error limits are standard deviations of at least five measurements on different solutions.
TABLE I: Wavelengths a d Molar Absorptivity Coefficients from the * UV/vis Absorption Peaks of the cis- and trans-[C0~'([14]PneN4)(X)~I"+ Complexes with Different Coordinated Ligands in Aqueous Solutions
290
Results
Isomerization of cis-[Co"'( [ 14]aneN4)(OH),]' in 3 M NaOH Solution. The isomerization of cis-[Col"( [ 14]aneN4)] complexes, (15) Bosnich, B.; Poon, C. K.; Tobe, M. L. Inorg. Chem. 1965, 4, 1102. (16) (a) Pwn, C. K.: Tobe, M. L. J . CfiemSoc.1968, 1549. (b) Bauer, B. F.; Drinkard, W. C. J . Am. Cfiem. SOC.1960, 82, 5031. (17) Brown, T.L.; Dickerhoot, C. W.;Bafus, D. A,; Morgan, G . L. Rev. Sei. Instrum. 1962, 22, 49 1 . (18) Evans, 0.H.: Lerke, S. A. Personal communication.
a,
comdex trans- [Co"'( i4]aneN4)(C1)2]+
trans-[Co"'( 14]aneN4)(OH)2]+
A,..
nm
220 250 310
a a
432
40
632 270 387
31
472 (sh) 536
trans-[Co"'( 1 4]aneN4)(H2O),l3+ 245 430 570 cis-[Co"'( [! 4]aneN4)(C1)2]t 220 250 310
340 (sh) 407 (sh) 556 cis-[Co"'( [ 141aneN4)(OH),]+ cis-[Co"'( [ 14]aneN4)(H20),]'+
M-l-cm-' a
a
14000 53 31 a a a 1I 7 34
9
19b
101 a
377
1 IO 1I 7
367 505
this work
57 9 47
297 532 25 1
ref 15
a 100 Ill
16a 16a
"The molar absorptivity is estimated at extremely high values a >> 10000. It is not possible to achieve very low concentrations by multiple dilutions since the products isomerize rapidly in 3 M NaOH. TABLE 11: Rate Constants k , for the Isomerization of the cis- to trsns-[Con*([14]aneN4)(OH),1+ Complex in 3 M NaOH Solutions; Calculation of Absorptivity by Extrapolation at Time t = 0 concn,mM k , , s-' linearity ( n 2 ) a, M-'-cm-' 0.22 1.60 x 10-4 0.9993 103 2.04 x 10-4 0.9980 125 2.64 7.29 1.87 x 10-4 0.9997 114 av (1.80 f 0.2) x 10-4 l14& 11
which has been studied in depth by Tobe and co-workers, and others, several years ag0,5-12.16,19 was reexamined in NaOH solutions under the experimental conditions of the present study because of its important role in the above square scheme (Scheme I). The dihydroxy and diaquo complexes were readily obtained by base hydrolysis and aquation of the dichloro and dihydroxy complexes, respectively. Thus addition of up to 3 M NaOH to the emerald-green aqueous solution of trans-[Co1I1([ 141aneN4)C12]+ resulted in the quantitative formation of a dark-pink solution of trans-[ColT1([ 14]aneN4)(OH)2]+. Acidification of this solution with HCIO4 produced a pale-green solution of the trans-[Co"'( [ 14]aneN4)(H20)2]3+complex. These processes are known to proceed with complete retention of the ligand configuration,12 R,S,S,R in this case, by way of the c h l o r o h y d r ~ x y ' ~ ~ and hydroxyaquo cations,I2 respectively. Since identical spectra were obtained when the trans-dihydroxy complex was passed through an anion-exchange column in the hydroxy form in order to exchange the chloride counterions, the results reported below were obtained on solutions obtained by directly dissolving the chloro complexes in base (i.e., in the presence of ca. 0.01 M CI-). Spectral data are collected in Table I and compared to the literature values. In a similar manner addition of 3 M NaOH to the deep-purple solution of cis-[Co"'( [ 14]aneN4)C12]+resulted in the quantitative formation of a pink-purple solution of the ~is-[Co'~'([ 141ar1eN4)(OH)~]+ complex in the R,R,R,R (or S,S,S,S) configu(19) (a) Poon, C. K.; Tobe, M. L. J . Chem. Soc. ( A ) 1967, 2069. (b) Cooksey, C. J.; Tobe, M. L. Inorg. Chem. 1978, 17, 1558. (c) Lichtig, J.; Tobe,M. L. Inorg. Chem. 1978, 17,2442. (d) Pwn, C. K. Inorg. Chim. Acta 1971,5,322. (e) Hung, Y . ;Busch, D. H. J . Am. Chem. Soc. 1977. 99,4911.
Electrochemistry of [ 14]aneN4-Co(III) Complexes
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 291
I
trod
0.w 335
I
I
I
I
1
395
455
515
575
635
WAVELENGTH (iin nm)
k
0
I
-1.4-
;
-:
-1.8
-2.2 -2.4
-
-
-2.6
-3
c0
8
40
8
,
8
80
120 t
(t"
,
, 160
,
, 200
,
# 240
,
, 280
In minuted
Figure 1. (a) UV/vis illustration of the isomerization reaction of cis[Coili( [ I 41aneN4)(OH) complex to trans- [co"'( [141aneN4)(OH)']2 complex in 3 M NaOH solution at a concentration of 15.23 mM in a IO-" cell (to = 30 s; A? = 900 s). (b) Chronoabsorptometriccurve of the normalized absorption versus time at 536 nm. The data are collected from the UV/vis spectra corresponding to the isomerization of 0.22mM cis-[Coll'([14]aneN4)(OH)2]+in 3 M NaOH.
E / Volt vs S.C.E.
ration.Igb Immediate addition of 1 .O M HCIO4 to this solution Figure 2. Cyclic voltammetric curves of gold electrodes (area = 0.196 produced a pale-green solution attributed to the ci~-[Co'~'( [ 141cm2) in carefully deaerated 3 M NaOH solutions of the [C0~~'([14]aneN4) (H20)J 3+ complex. a11eN4)(0H)~]+complex at different concentrations: (a) 3.08 mM, S Unlike the stable trans isomer of the complex, the cis isomer = 20 pA; (b) 5.0mM, S = 25 pA; (c) 10.0mM, S = 50 pA; mM. Initial was unstable and isomerized to trans-[C0~~'([14]aneN4)(OH)~]+negative scan from +OS00 to -1.200 V vs SCE at a sweep rate of 100 mV s-l. Panel a shows the CV of a blank 3 M NaOH solution. Two full in quantitative fashion. The isomerization reaction, which proceeds cycles are shown for (b) and (c). by way of the labile ~is-[Co~~'([l4]aneN4)(0H)(H~O)]~+ over a wide pH range, is slower than the hydrolysis steps and is acthan -0.5 V vs S C E the voltammetric pattern of the [CO~~'/"companied by exchange of two of the four ammine protons and ([14]aneN4)] redox couple appeared, and at potentials more inversion at nitrogen to give the trans complex in the R,S,S,R positive than -0.5 V, surface oxide formation and stripping waves configuration.IZ were present. In this latter region current due to the ligand-based The slow cis to trans isomerization was followed in 3 M NaOH by UV/vis spectroscopy at different concentrations. A typical oxidation of the Co-cyclam complex was also evident. The voltammetric waves labeled Ird, I,,, IIrd, and II,,, which set of absorption spectra illustrating this conversion is shown in Figure 1 and kinetic data are collected in Table 11. for conare discussed extensively below, involve the [Co"'/"( [14]aneN4)] redox couple. However, these voltammograms were reproducible centrations less than 0.01 M the absorbance plots exhibited good only if the potential sweep upper limit is set at 0.5 f 0.1 V, and linearity for first-order kinetics. After ca. 30 s, sharp isosbestic they depended on the delay or quiet time at the initial potential. points were observed at 427 and 469 nm, which were maintained Upon consecutive cycles between -0.5 and -1.2 V the potential throughout the course of the reaction. (The initial spectrum peak separations increased, suggesting that the efficiency of the obtained after dissolution of the cis complex probably included contributions from incompletely hydrolyzed or neutralized comelectron exchange with the substrate decreased. The reproducibility of the voltammograms could be restored by stepping the plexes.) The rate constant for the first-order reaction was found potential to 0.5 V for a short period of time and rotating the to be (1.8 f 0.2) X IO4 s-I at 21 f 3 OC corresponding to a electrode briefly to remove any soluble oxidatively desorbed half-life of 64 min. products from the electrode surface. This process increased the Voltammetric Behavior of Gold Electrodes in the Presence of efficiency of electron exchange of the Colll/llcyclam species with [Co"'([ 14]aneN4)(0H),]+ in 3 M NaOH. Cyclic voltammograms the gold electrode as gauged by the subsequent CVs which disof a gold electrode in carefully deaerated 3 M NaOH with conplayed the relatively narrow peak separations reported in Tables centrations of trans-[Co"'( [ 14]aneN4)(0H)J+ ranging from 0 111 and IV. to 10 mM are shown in Figure 2. These CVs, which were Information relating to this electrode activation phenomenon perfectly reproducible, can be divided at ca. -0.5 V vs S C E into can be gleaned from the CVs of Figure 2. In the absence of the two general electroactive regions. At potentials more negative
292
Tsintavis et al.
The Journal of Physical Chemistry, Vol. 95, No. 1 , 1991
TABLE 111: Cyclic Voltammetric Peak Potential and Peak Current Values for trmns-[C0*~'([14]aneN4)(OH)~]+ in 3 M NaOH at a Gold Electrode at Different Scan Rates and Concentration at Room Temperaturea
5
IO I .o
50
IO0 500
IO00 5000 5
IO 50 2.5
IO0 500 1000 5000
5 IO 5.0
50 100 500 1000 5000
5 IO
so 10.0
IO0 500
IO00 5000
-918 -921 -925 -929 -934 -939 -949 -919 -920 -925 -929 -933 -939 -951 -921 -923 -931 -937 -946 -950 -975 -922 -925 -931 -936 -949 -960 -991
-849 -849 -851 -855 -852 -846 -837 -848 -848 -852 -855 -850 -842 -830 -847 -848 -855 -852 -844 -840 -820 -848 -848 -848 -847 -839 -831 -805
-884 -885 -888 -892 -893 -893 -893 -884 -884 -889 -892 -892 -89 1 -89 1 -885 -886 -893 -895 -895 -895 -898 -885 -887 -890 -892 -894 -896 -898
69 72 74 74 82 93 112 71 72 73 74 83 97 121 74 75 76 85 102 1 IO 155 74 77 83 89 I IO 129 I86
NIP
NIP
N/M
-737 -742 -746
-676 -619 -681
-707 -711 -714
N/M N/M N/P N/P N/P N/P N/P N/P
N/M N/M N/M N/M
N/M N/M N/P N/P N/P N/P NIP NIP
N/M N/M N/M N/M
N/M N/M N/P N/P N/P N/P N/P N/P
N/M N/M N/M N/M
N/M N/M N/P N/P NIP N/P
N/M N/M N/M
-740 -742 -746
-744 -748 -750
-749 -752 -755
-677 -678 -678
-677 -679 -680
-680 -681 -684
-708 -710 -712
-710 -713 -715
-714 -716 -719
N/M
6.0 8.5 17.8 23.9 51.0 77.9 149.1 16.6 22.6 47.4 64.2 142.6 204.3 487.1 32.2 44.8 88.9 114.9 264.0 319.1 574.0 61.6 83.0 175.5 226.0 437.1 557.9 958.1
4.6 6.4 14.1 21.1 45.2 73.5 147.8 12.3 15.6 33.2 53.7 127.7 200.0 330.1 24.8 31.3 67.4 100.0 257.1 318.4 573.1 45.2 57.0 127.7 184.8 422.1 557.5 957.4
N/P 0.2 1 .I 1 .5
N/M N/P N/P NIP 0.6 1.5 3.1
N/M NIP NIP NIP 1.7 3.7 5.3
N/M NIP N/P NIP 2.3 8.5 12.7
N/M N/P N/P
N/P
1.3 1.3 1.3 1.1
1.1
1.o 1 .o
1.3 1.4 1.4 1.2 1.1 I .o 1 .o I .3 1.4 1.3 1.1 1 .o 1.o 1 .o 1.4 1 .5 1.4 1.2 1.o 1.o 1.o
'NIP, not present; N/M, not measurable.
C o h y c l a m complex, Figure 2a, the voltammetric pattern a t potentials more positive than -0.5 V is due to the formation of surface oxide species. In the region of wave 2 a porous gold(II1) oxide film is formed which is then stripped on the reverse sweep in wave 1 . In addition, formation of surface hydroxide layers, Le., Au-OH species, which takes place presumably on Au( 100) microcrystal faces,20gives rise to the current plateaus labeled 4 and 5 in Figure 2a. This voltammetric behavior, which is in accord with the extensive literature on the behavior of gold in alkaline solutions,2' is indicative of high purity of the solvent/electrolyte and electrode material. Upon addition of the trans-[Co"'( [ 14]aneN4)CIz]+complex, marked changes in the voltammetric pattern were noted in the 0.0 f 0.5 V region. A new oxidation wave, wave 3, appeared which was approximately independent of concentration and electrode rotation rate, but was linearly dependent on the voltage sweep rate. On the negative-going potential sweep a sharp decrease in the current was seen near the end of the oxide stripping wave. The current actually changed direction and became anodic at a potential between 0.0 and -0.1 V. This decrease was directly proportional to the Co"'cyclam concentration, and even at 0.48 mM a reversal of the direction of current flow was observed. This behavior has been observed by others for a variety of organic substrates and can be explained by an 'electrocatalytic" oxidation of the [Coil1([14]aneN4)(OH)2]+complex at the 'clean" gold surface exposed by the oxide stripping process.** Thus the current (20) Adzic, R. R.; Markovic, N . M.; Vesovic, V. B. J . Electrwwl. Chem. 1984, 165, 105.
(21) (a) Woods, R. in Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1976; Vol. 9, pp 1-162. (b) Burke, L. D. In Electrodes of Conductive Metallic Oxides Part A; Trasatti, S., Ed.;Elsevier: Amsterdam, 1980; pp 141-181. (c) Burke, L. D.; Lyons, M. E.; Whelan, D. P. J . Electroanal. Chem. 1982, 139, 131. (d) Burke, L. D.; McCarthy, M. M.; Roche. M. B. C. J . Electroanal. Chem. 1984, 167,291. (e) Anastasijevic, N . A.; Strbac, S.; Adzic, R. R. J . Eleetroanal. Chem. 1988, 240, 239. (22) (a) Burke, D. B.; Cunnane, V. J. J . Electroanal. Chem. 1986,210, 69. (b) Paliteiro, C.; Hamnett, A.; Goodenough, J. B. J. Elecfrwnal. Chem. 1987. 234. 193. (c) Welch, L. E.; La Course, W. R.; Mead, D. A.; Johnson, D. C.; Hu, T. Anal. Chem. 1989, 61, 555. (d) De Mele, M. L. F.; Videla, H. A.; Arvia, A. J. Bioelectrochem. Bioenerg. 1986, 16, 213. (e) Vitt, J. E.; Larew, L. A.; Johnson, D. C. Electroanalysis 1990, 2, 21.
observed in this potential region was the net of the reduction current for the oxide film removal and the oxidation current of the complex. Several experiments were performed in order to confirm the origin of wave 3 in the CV of Figure 2. Voltammograms of the free cyclam ligand as a saturated solution in 3 M NaOH, while exhibiting a shoulder on the oxide formation wave in the region of wave 3, were distinctly different on the negative-going sweep from those in Figure 2. Specifically, the current reversal phenomenon was not observed on the negative-going potential sweep and markedly increased peak potential separations were observed for the Colli/llcyclam waves. In separate control experiments, addition of NaCl at these concentration levels had no effect on the height or shape of wave 3, although minor effects on the oxide formation and stripping wave were seen in accord with the report of Desilvestro and Weaver.23 Thus the voltammetric behavior is ascribed to the electrochemical oxidation of the coordinated ligand of the [Co"'([ 14]aneN4)(OH)2]+complex. Of pertinence to this result is the observation that the voltammetric behavior of the [ C O ~ ~ '[/14]aneN4)] '~( redox couple described below, Le., the peak potential separations of waves I and 11, was dependent on the above activation of the gold electrode in the presence of the Co-cyclam complex. As discussed below, this behavior is believed to be due to weak adsorption of CoII'cyclam complexes at the clean Au or Au-O-/OH surface exposed by the electrochemical treatment. The gold-Co%yclam interaction and the oxidation of the complexes at gold electrodes under these conditions are under investigati~n.~~ Voltammetric Behavior of the trans-[Co1''([14]aneN4)(OH)z]+ Complex in 3 M NaOH. The cyclic voltammetry of the transCo"'( [ 14]aneN4)(OH)2]+complex has been studied over a wide range of concentration (0.48-10 mM) and sweep rate (0.01-10.24 V/s). Voltammetric data are collected in Table 111, and representative CVs at three different sweep rates are shown in Figure 3. (23) Desilvestro, J.; Weaver, M. J. J . Electroanal. Chem. 1986, 209, 377. Hobbs, D. T. To be pub(24) Tsintavis, C.; Li, H. L.; Chambers, J. Q.; lished.
Electrochemistry of [ 14]aneN4-Co(III) Complexes
The Journal of Physical Chemistry, Vol. 95, No. I. 1991 293
TABLE IV: Cyclic Voltammetric Peak Potential and Peak Current Values for cis-[C0~~~([14]aneN4)(OH)~]+ in 3 M NaOH Solutions at a Cold Electrode at Different Scan Rates and Concentration at Room Temperature'
[cis], V, m M mV*s-' 5 1.01
100
3.01
100
5.00
100
5
5 5 10.0 "/A,
IO0
mV
E,, mV
Ird
Io,
-916 -916 -918 -920 -922 -928 -922 -934
-846 -845 -846 -844 -843 -846 -833 -834
E1/2(1) -88 1 -881 -881 -882 -882 -887 -878 -884
Ird
- Io,
70 71 75 76 79 82 89 100
-676 -706 -680 -713 -682 -717 -688 -730
110, N/A -654 N/A -641 N/A -636 N/A -627
E , mV EI/AII) 11re.j - 110,
N/A -680 N/A -677 N/A -677 N/A -478
N/A 52 N/A 72 N/A 81 N/A 103
rp, A Ird
Io,
5.3 8.9 15.9 26.8 26.7 43.7 52.5 78.7
4.7 11.7 14.1 35.2 23.5 59.7 47.5 117.0
IIrd 2.1 16.7 6.3 50.1 10.1 82.7 21.7 159.6
110,
IredIoa
N/A 8.3 N/A 24.7 N/A
1.13 0.76 1.12 0.76
41.5
N/A 76.6
IIdIIL
1.14 0.73 1.11 0.67
not available.
E / Volt
YO
S.C.E.
Figure 3. Cyclic voltammetric curves of gold electrodes (area = 0.196 cm2) in carefully deaerated 3 M NaOH solutions of 3.0 mM [ColI1([ 14]aneN4)(OH)2]+complex at different scan rates: (a) 5 mV s-l, S = 5 @A: (b) 100 mV s-l, S = 20 PA: (c) 5120 mV s-I, S = 250 PA. Initial negative scan from +0.500 to -1.200 V vs SCE. The first, second, fifth, and tenth cycles are shown.
At very slow sweep rates, u I 0.01 V/s, Figure 3a, or at sweep rates greater than 1 V/s, Figure 3c, a single cathodic peak (Id) and a single anodic peak (lox) dominated the CVs in the potential region between -0.5 and -1.2 V vs SCE. The peak potential separation, AEp = E Ox - Epd, the peak current ratio, Ip""/Ipd, and the dependence oP the I p values on u1l2indicate electrochemical quasi-reversibility and chemical reversibility on the voltammetric time scale. The electrochemical quasi-reversible criteria are based on the influence of the sweep rate on the values of AEp and I while the chemical reversibility criterion is the peak current ratio.$; At intermediate sweep rates, 0.01 5 u 5 0 . 5 V/s, Figure 3b, an additional redox couple, peaks lIrd and II,,, clearly appeared in the CVs of the tmn~-[Co'~'([ 14]aneN4)(0H)2]Ccomplex at a potential ca. 0.2 V more positive than the of the original couple. The reduction wave of this couple, Urd, was not present on the initial negative-going sweep, but persisted upon subsequent cycles. The peak current of wave II,, ( I p / u 1 / 2 increased ) with sweep rate at the expense of peak I,,. Concomitantly the appearance of wave II,, on the reverse sweep was coupled to the decrease of the peak current ratio, Ip""/Ipd,for the wave. This indicates that a chemical reaction involving the [ColI([ 14]aneN4)] species generated in the diffusion layer took place at these sweep rates. In contrast to the behavior for the lox/rdcouple, the peak current ratio for the II, red wave was close to unity, indicating that these electrogenerated species were chemically reversible. The sum of the currents for the oxidation peaks, I, and II,,, was equal to the and I&&, indication that the sum for the reduction peaks, Ired overall electrode reaction is chemically reversible. For both redox couples the separation between anodic and cathodic peaks increased with scan rate, suggesting that the ( 2 5 ) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods Fundamental and Applications; Wiley: New York, 1980; Chapter 6.
electrode reactions were not perfectly Nernstian. It was also noted that E I l 2for both waves was independent of the scan rate and the concentration. To assess the possibility that the IIox/IIrd wave was due to strongly adsorbed cyclic voltammograms were obtained over a concentration range from 0.5 to 10 mM in 3 M NaOH. Each of the four peaks in the voltammograms increased linearly with concentration, and the ratio of the peak current for wave 11,, to that for wave I,, was independent of concentration. Furthermore, although all peaks shifted slightly with concentration, no shift of the peak potential for wave II,, relative to the El12for the Iox/Ird couple was seen, nor did the IIr~/I1,,, wave become symmetrical as the sweep rate or the concentration was increased. These observations indicate that the currents that give rise to the IIo,/IId couple arise from freely diffusing species formed during the reduction of the trans-[Co"'( [ 14]aneN4)(OH)2]+complex in wave Ired. Since the effect of uncompensated resistance on the voltammetric behavior is almost the same as that of kinetic effects, it cannot be easily ruled out as a contributor to the quasi-reversible behavior of the Io,/1rd14wave.*' This is especially true for the peak potential values obtained at 10 mM reported in Table 111. At the lower concentrations, however, use of the positive feedback IR correction function of the potentiostat did not significantly decrease the values of AEpreported in the table. Moreover, cyclic voltammograms obtained with electrodes of much smaller geometrical areas, and consequential lower peak currents, gave similar peak separations. With a further increase of scan rate, u 1 0.5 V/s, Figure 3c, the wave for the IIox/rdcouple decreased, and above 1.0 V/s the voltammograms were again dominated by the wave for the couple. However, upon consecutive cycling between the -0.5 to -1.2 V limits, a shoulderlike wave was seen to develop in the potential region where the IIox/rd wave was found at the intermediate sweep rates. Voltammetric Behavior of the cis-[C0~~~([14]aneN4)(OH)~]+ Complex in 3 M NaOH. Cyclic voltammograms of freshly prepared and carefully deaerated 3 M NaOH solutions of cis[Co"'( [ 14]aneN4)(OH)2]+obtained at two sweep rates are shown in Figure 4. A complete set of voltammetric data is collected in Table IV. For simplification and convenience all the voltammetric waves in Figure 4 are labeled as in the voltammograms of the trans-[Co"'([14]a11eN4)(0H)~]+complex in Figure 3. At very slow sweep rates, u 5 0.01 V/s, two cathodic peaks (IId and Ired)and one anodic peak (lox)were observed, a pattern that is maintained on subsequent cycles. The peak current ratio of I, to Ired was significantly less than unity, indicating that a chemical reaction involving the cis- [Co"( [ 14]aneN4)] species occurred in the diffusion layer on the voltammetric time scale. On the second and subsequent cycles, the peak current ratio of wave 1Ird to wave Ird decreased relative to the initial potential sweep. As was the case for the voltammetry of the trans[CoI1'([ 14]ar1eN4)(OH)~]+ complex, the Iox/Id wave displayed quasi-reversible behavior. (26) Wopshall, R. H.; Shain, I. Anal. Chem. 1967, 39, 1514. (27) Nicholson, R. S. Anal. Chem. 1965, 37, 1351.
Tsintavis et al.
294 The Journal of Physical Chemistry, Vol. 95,No. I, 1991 ./-\
//
Q
, b
60
1
50
-
40-
\
c
30
-
i
0
2
4
6
a
1 0
i
i
l/i t l m e / s e c l " 2
Figure 5. Plot of the Cottrell equation, current vs the inverse of the square rmt of time, from the chronoamperometriccurves of gold electrodes (area = 0.196 cm2) in carefully deaerated 3 M NaOH solutions of 5.0 mM of ~ i s - [ C o "14]aneN4)(OH),]+ ~([ complex: (a) potential step at V = -1 100 mV; (b) potential step at V = -800 mV.
cis complex relative to the trans complex between acidic and basic solutions is calculated from eq 1 using the literature pK, values. . - E O ! trans = (2*3RT/F)[pKatCis- P K ~ , ' " ~ + I 2(2.3RT/F)[pKa,C'S - PK,,'""~] (1)
EO' CIS
E / Volt
V I
S.C.E.
Figure 4. Cyclic voltammetric curves of gold electrodes (area = 0.196 cm2) in carefully deaerated 3 M NaOH solutions of 3.0 mM cis-
[Co'Il([14]aneN4)(OH)2]+complex at different scan rates: (a) 5 mV S = 5 FA; (b) 100 mV s-I, S = 20 FA. Initial negative scan from +0.500 to -1.200 V vs SCE. For (a) the first and second cycles are shown; for (b) the first, second, fifth, and tenth cycles are shown. s-',
This value is exactly equal to the difference in the E'" values obtained from the analysis of the voltammetric and chronoamperometric data described below. As for the trans complex, cyclic voltammograms of the cis[C0'~'([14]aneN4)(0H)~]+ complex in 3 M NaOH were obtained over the concentration range from 0.45 to 9.7mM. The behavior paralleled that found for the trans complex: no evidence of strong adsorption was detected. Chronoamperometric Behauior of the cis-[ColIi( [ 141ar1eN4)(OH)~]+Complex in 3 M NaOH. Further analysis of the Co"'/"cyclam redox process was obtained from chronoamperometric studies a t potentials between the Iox/d and IIox,d waves. At these potentials for solutions containing predominantly the c i ~ - [ C o ~[ '14]aneN4)(OH)2]+ ~( complex, marked negative deviations from Cottrellian behavior are expected due to the electron-transfer-indy+ isomerization. This behavior is characteristic of the classical ECE mechanism of Feldberg and Jeftic.14 Typical results obtained at a concentration of 3.34 mM are shown in Figure 5. Curve A in this figure was obtained by stepping the potential to -1.1 V where both the cis- and the trans-[Coil'( [ 14]aneN4)(0H)z]+complexes were reduced under diffusion control as indicated by the voltammetric results. For this experiment agreement with the simple Cottrell equation was obtained in the 2-15-s time regime for experiments at OS-10.0 mM. The value of nD1/2calculated from the Cottrell slopes of all the chronoamperometric and chronocoulometric experiments was 1.60 X equiv cm mol-' S - I / ~ (s = *8.8%; n = lo), corresponding to a diffusion coefficient of ca. 2.6 X 10" cm2 s-]. This value is in reasonable agreement with the literatureg when the high viscosity of 3 M NaOH v / v 0 = 2.04 a t 20 OC, is taken into account.28 Several experiments were performed to detect adsorbed species on pretreated electrodes that had been transferred to background electrolyte solution after rinsing with 3 M NaOH. Upon scanning the electrode potential between -0.5 and -1 $2V at high sensitivity, no peaks that could be unambiguously assigned to the Coii1/%yclam couples were evident in the CVs. Furthermore, Anson plots29 of chronocoulometric data gave no indication of adsorption. These plots were linear with Cottrell slopes in exact agreement with the corresponding chronoamperometric values but gave intercepts on the Q axis that were negative with respect to the intercepts for background trials. These experiments indicate that electroactive CoII'cyclam species do not adsorb on the gold
At intermediate sweep rates for this study, 0.01 I u I 0.5V/s, Figure 4b, a different voltammetric pattern was established with two cathodic peaks (Ild and Ird) and two anodic peaks (IIox and lox). It should be remembered that these CVs were obtained ca. 1 min after dissolution of the cis-Co"'cyclam complex before significant isomerization of the complex had taken place in the bulk of the solution (tlj2 = 64 min). Both waves IIox/d and IoXlrd displayed quasi-reversible electrochemical behavior, and the peak current ratios, Ip"/Ipd, for each were significantly less than unity. The latter observation again indicates that chemical steps follow the heterogeneous electron-transfer steps. With a further increase in sweep rate, u I ca. 0.5 V/s, wave 1Ird grew at the expense of wave Ired. The ratio of the peak currents for wave llrd to Ired was strongly dependent on time, rapidly decreasing as the cis-[Coiil([ 14]aneN4)(OH),]+ isomerized into the trans complex. When a solution of the cis complex was allowed to stand for a period of 10 half-lives of the isomerization, the cyclic voltammetry obtained was identical with that obtained on an authentic sample of the trans-[Co1''([l4]aneN4)(0H)2]+ complex. Also, addition of t r ~ n s - [ C o[~141~~( aneN4)(0H),]+ to solutions containing initially only the cis complex merely increased the peak currents for the Id& couple and did not influence the relative magnitude of the IId/IIox wave. This behavior is readily explained by electron-transfer-induced cis-to-trans isomerization of the CoIkyclam complex in the diffusion layer, Scheme I. We were not able to outrun this isomerization by fast sweep cyclic voltammetry under our experimental conditions even for CVs at 10-50 V/s obtained a few seconds after dissolution. (Experiments at lower temperature and/or using much smaller electrodes were not performed on solutions of the cis-[Coil'([ 14]aneN4)(OH)2]+complex.) The position of the IIre,,/Iox wave for the cis-Coiil/llcyclam couple relative to the ld/Iox wave for the trans couple is consistent with the Pourbaix diagram previously presented" and the pK, (28) CRC Handbook of Chemistry and Physics; CRC Press: Cleveland, values determined by Poon and Tobe for the [ C 0 ~ ~ ~ ( [ 1 4 ] - OH,1977;p D-256. aneN4)(OH2)J3+ cations.'* A shift of 0.21 V in the Eo' for the (29) Anson, F.C.Anal. Chem. 1966,38, 54.
.
Electrochemistry of [ 14]aneN4-Co(III) Complexes
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 295
-.;I .o
\\
-.2
-.34
-700
-500
-300
100
-100
300
-E
Figure 6. Simulation of CVs for the [Coel([ 14]aneN4)(OH)2]+complex; Xaxis vs EO' for Co"'/"cycIam; current function plotted on Yaxis. Input Simulation parameters: trans, = k , / ( m ~ D ) '=/ ~1.00; cis, = k,/ ( ~ a D ) l=/ 0.50; ~ 6 = k,u-' = 0.06; AEO' = 215 mV; T = 25 "C; Kq = 0.8; a = 38.9 u ; insert for $ J ~= k3& = IO. Output simulation parameters: for a scan rate of u = 100 mV d,the calculated rate constants are k,trsnr = 5.6 X cm s-l, kC's= 2.8 X IO-' cm S-I, k2 = 0.23 s-l, k-2 = 0.29 d,k-, = 1.8 X IO4 ls' (from Table 11), and k, = 3.3 X lo-* s-1.
electrodes under the conditions of the present study. For the chronoamperometric experiments, significant deviation from Cottrellian behavior was observed when the potential was stepped to -0.80 V between the voltammetric waves, curve B in Figure 5. The apparent n value, which was conveniently obtained from the ratio of the current measured at -0.80 V to that measured at -1.1 V for a given time
rapidly approached zero with increasing time. This behavior, which is expected for an electron-induced isomerization, is discussed in the next section. Simulation and Kinetic Analysis of the Results. Simulation of square scheme voltammetric and amperometric data for systems where a square scheme is operative is fraught with difficulty and uncertainty. Nonetheless, it is important in an analysis of data of this nature to demonstrate that a set of phenomenological kinetic parameters is consistent with the experimental results. Due to the large number of variables involved (two (Y values, two heterogeneous rate constants, two rate constants for the isomerization process, the separation of Eo' values, and the rate constants of the cross-reaction), it is not often feasible to produce a definitive set of kinetic parameters for a given process. Numerous simulated cyclic voltammograms were generated with the use of a program of Evans and Lerke,Is which had provision for including the cross-reaction in Scheme I. Very good, but not perfect, agreement with the experimental CVs was obtained. A typical simulation is shown in Figure 6 . The major difficulty we had in matching the experimental CVs at intermediate sweep rates with the simulations was in matching the currents in the region of the diffusion tail of the II,, wave. It can be noted that in this potential range background subtraction is made difficult by the presence of current due to formation of Au-OH species (see Figure 2 ) . Thus part of the difficulty is probably due to inaccurate background subtraction in this region. The following qualitative observations can be stated on the basis of the computer simulations (which were performed with both a values set equal to 0.5): (i) the agreement was best when K for the isomerization step ( = k l / k - , ) was approximately unity 48 and (ii) exclusion of the cross-reaction considerably improved the match of experimental and simulated CVs in the region of wave llrd for the voltammograms of the trans-[Co"'([ 14laneN4)(OH),]+complex. (Note that the insert in Figure 6 shows the (30). Under the conditions of the simulations, the magnitude of the 1Im& couple IS very sensitive to the magnitude of Kcq. It is also realized that this must be a sluggish equilibrium step on the voltammetric time scale; otherwise for a facile equilibrium, a single reversible one-electron wave would be observed where the reduced state would consist of an equilibrium mixture of cisand rrans-[CoIi([ 14]aneN4)] isomers.
.o
\
-2
-1
0 1OQ
1 (kt)
2
3
Figure 7. Curves of apparent n value vs logarithm of time. Experimental napp(open points) values are calculated as the ratio of (V = -800 mV) to Pan* ( V = -1 100 mV); the solid points are calculated from the ratio of Pisto the Cottrell line; see Figure 5 . Curves F and B are theoretical
curves for the models of Feldberg and Jeftic and Bond and Oldham, respectively. simulated CV wave pattern in this region when a rapid crossreaction is included in the calculation.) In support of this latter result, the voltammetric pattern was independent of concentration in the 0.5-5.0 mM range. Inclusion of the cross-reaction in the model renders the voltammetric response concentration dependent.14 A sluggish cross-reaction is in agreement with the work of Endicott and co-workers, who have reported slow homogeneous electron-exchange reactions for related macrocyclic c ~ m p l e x e s . ~ ~ The chronoamperometric data were analyzed according to existing theoretical treatments in the literature. Figure 7 displays experimental nappvalues as points and two theoretical curves as a function of log ( 1 ) . Curve F in this figure is simply a plot of napp= exp(-kt), which corresponds to the situation where the cross-reaction does not take place and the isomerization is irrever~ib1e.I~ Examination of the data reveals that the values of napp decreased more slowly than this function. Inclusion of the cross-reaction using the working curves of Feldberg and Jeftiel* lessened the agreement with the experimental values. In fact no indication of a negative n, value was ever seen in the numerous cyclic voltammetric or cironoamperometric experiments performed on solutions of the cis-[ColI1([ 14]aneN4)(OH)z]+complex. The above behavior of the experimental nap values at long time suggests the participation of the backward reaction in the isomerization step, a case of the ECE mechanism, which has been treated by Bond and Oldham.3 Working curves of nappvs log (kt) were calculated from eqs 3-7 (see eq 2.26 of ref 3) using several values of the equilibrium constant for isomerization of the CoIIcyclam species.
The Bi variables are the Nernst factors for the surface concentration ratios at the applied potential; the subscripts 1 and 2 refer to the trans- and cis- [ColI1/%yclam] couples, respectively. Bond and Oldham have supplied the necessary functions to calculate the +(z) function for both positive and negative, and large and small, arguments. As abbreviated here, this calculation of nappassumes that the concentration of trans-[Co"'([ 14]aneN4)(OH)z]+is zero at t = 0. This treatment also does not take into account the cross-reaction. (31) Endicott, J . F.; Durham, B.; Glick, M. D.; Anderson, T. J.; Kuszaj, J. M.; Schmonsees, W. G.; Balakrishnam, K. P.J. Am. Chem. SOC.1981,103, 1431.
296 The Journal of Physical Chemistry, Vol. 95, No. I , 1991
Tsintavis et al.
Values of naWwere calculated as a function of the dimensionless parameter kt = ( k , k-,)t, using the Eo' values and the equilibrium constant, Kq = kz/k..,, obtained from the above numerical simulation of the CVs. A typical fit with the experimental data is shown in Figure 7. Only the I-t data pairs obtained on the first E step to -0.800 V after dissolution of the cis-[Co"'([l4]aneN4)] salt were used in these analyses. For t greater than ca. 2 s the fit was excellent for individual experiments and markedly better than the fit with the working curve of Feldberg and Jeftic (curve F in Figure 7), which assumes an irreversible chemical reaction. The important influence of the backward reaction ( k - 2 )on the chronoamperometric response is clearly evident. There was quantitative agreement between the chronoamperometric and cyclic voltammetric fits to the square scheme model. For example, the values of k2 derived from the two analyses were 0.23 and 0.22 s-I, respectively. Of course, this was naturally a result of the approach taken to the data analysis: the value of K, obtained from the initial CV simulations was used to construct the chronoamperometric working curves; the derived value of k from the chronoamperometric data was then used to refine the CV simulation. After two iterations of this process, a consistent set of parameters was obtained; these are contained in the caption to Figure 6. This appears to be a good operational approach to extraction of the kinetic and thermodynamic values for systems of the type encountered here. These chronoamperometric results also showed that the cross-reaction is not an influencing factor under these conditions. Essentially superimposable nappvs log ( t ) plots were obtained in the concentration range 1-10 mM, indication that the isomerization step does not involve a second-order process such as the cross-reaction. As suspected in view of the deviation from the Cottrell slope evident in Figure Sa, the experimental nappvalues for t 5 2 s were less than the values calculated from the model. While correction for these deviations using the Cottrell slope from Figure 5 improved the fit with the model, this was generally not done in the data analyses because of the uncertainty in correcting both of the it1/2 values in eqn 2. (In Figure 7 the solid points were calculated from the Cottrell slope and not the actual values of the currents.) Finally, it can be noted that the deviation of the chronoamperometric nappvalues from the working curves at short times was consistent with the CV results for the cis complex at fast sweep rates where we were not able to outrun the isomerization step. This is a possible indication that the isomerization was catalyzed in some fashion as a result of the electrode pretreatment procedure.
( = R T In [K,]) in the 111 oxidation state and only 0.6 kJ/mol in the I1 oxidation state. Apparently, the increased Co-0 axial bond lengths in the I1 state, by ca. 50 pm, lessen the steric constraints for the isomerization process.6 In a similar fashion the isomerization process is more facile in the I1 state, relaxing to equilibrium with a time constant of ca. 1 s vs 1 h in the higher oxidation state. Several open questions remain concerning this very interesting system that have not been answered by the present The role of various configurational isomers related by inversion at nitrogen in the redox process has not been defined. Initially, it appeared that the values of E t - E,: Le., the reversibility of the electron-transfer steps in the square scheme, were smaller when the couple was generated in situ than when obtained from the Co"' complex in the bulk of the solution. This would result, for example, in the possible generation of the trans-[Co"'([ 14]aneN4)(OH),]+ species in the S,S,S,S (or R,R,R,R) configuration in the diffusion layer when starting with the corresponding cis-[Co"'( [ 141aneN4)CI2]CI compound for which this is the stable form. However, we were not able to establish a discernible pattern in the values of the peak separations when starting with the trans complex in either the R,S,S,R or S,S,S,S (or R,R,R,R) configurations or the cis complex in the latter configuration. The simulated CVs gave acceptable fits to the data when roughly equal heterogeneous rate constants ( k , values) were used for the trans and cis steps in the scheme. However, the caveat of Bond and Oldham3 regarding the lack of significance of apparent k, values certainly pertains to this system where there are both configurational and acid/base species that can exist in the diffusion layer. We find that the major factor in determining the values of the peak separations was the electrode pretreatment and history of the gold electrode. For concentrations above ca. 7 mM, the peak separations increased well beyond that which could be explained by uncompensated IR problems. The reason for this increase is not understood. Two possibilities are that at the higher concentration the electrode pretreatment process produced a surface that no longer efficiently exchanged electrons with the Coll'/llcyclam species. Alternatively, bimolecular species involving hydroxy bridges between cobalt center are known that could become important at the higher concentrations. Comments on the Role of the Electrode. The majority of the previous electrochemical studies on Co"'/" couples in aqueous solutions have employed mercury working electrodes. Thus the question arises as to the role of the electrode surface, and possible surface complexes (e.g., Au-O-Coll'cyclam), in the present study. Several literature precedents exist for surface complexes of this type, notably the report of Burke and Cunnane,,Ia who detected gold pyridine species under conditions similar to ours. Pretreatment of the gold electrode in the presence of the free ligand gives rise to voltammetric behavior similar to that observed by these researchers. It can also be noted that, in the potential region where wave 3 is seen in the CVs of the Co"kyclam complexes, Au-OH or possibly Au-0- sites exist on the electrode surface. Finally, there is ample evidence for the mediating role of surface adsorbed ligand species in electron-transfer reactions from the work of Hill and c o - w o r k e r ~ . ~ ~ In the present study, however, it is not clear that the adsorbed species actually catalyze the electron-transfer reactions that take place at surfaces modified with adsorbed cobalt complexes. It appears to be more likely that the heterogeneous electron-transfer reactions are hindered by adsorbates resulting from decomposition of Colkyclam species and that the pretreatment process is effective because of oxidative removal of these layers. In this regard it is significant that homogeneous electron-transfer reactions between Co"' and Co" are rather slow. For the conditions of the present study, quasi-reversible behavior is seen that becomes more irreversible as the concentration of Co%yclam is increased. This result is understandable if the modified electrode does not efficiently exchange electrons with the diffusing species in solution.
+
Discussion From an electrochemical viewpoint electrode reactions that conform to some variant of the square scheme are a fascination. They present the opportunity to obtain detailed chemical information on the kinetics of unstable intermediates generated in the diffusion layer in relatively straightforward experiments in many cases. The studies of Evans3, and Kochis3illustrate well the insight that can be obtained for organic and organometallic redox couples when square schemes are operative. The number of variables required to model a given response, on the other hand, presents a challenge to the experimentalist and can render conclusions ambiguous. An immediate conclusion of the present study is that on the voltammetric time scale (less than ca. 1 min) the Co"cyclam complex remains intact in 3 M NaOH. The most striking observation in the present study, however, is the markedly decreased preference for the trans vs the cis geometrical configuration in the Co" oxidation state compared to the Co"' oxidation state for the cyclam complexes in base. In both oxidation states the trans configuration is the more thermodynamically stable, 2 1.3 kJ/mol (32) Evans, D. H.; OConnell, K. M. In Elecrroanalyrical Chemistry; Bard, A . J., Ed.; Marcel Dekker: New York, 1986; Vol. 14, pp 157-189. (33) (a) Bockmann, T. M.; Kochi, J. K . J . Am. Chem. SOC.1987, 109,
7725. (b) For a review of applications see ref 34. (34) Astruc, D.Angew. Chem., Inr. Ed. Engl. 1988, 27, 643.
(35) Hill, H. A. 0.; Page, D. J.; Walton, N. J. J . Electtoanal. Chem. 1987, 217, 129.
J . Phys. Chem. 1991, 95, 297-302
Acknowledgment. This work was supported by a contract from
E. I. Du Pont de Nemours & Co., Inc., Savannah River Plant, Aiken*sc (now Operated by Westinghouse Savannah River cO*)* We are grateful to c* E* Barnes and G. Brown (University of Tennessee, Knoxville) for many helpful Suggestions and discussions. Professor Dennis Evans (University of Delaware) is thanked for supplying us with copies of his programs for digital
297
simulation of cyclic voltammograms. Registry No. trans-[Co([l4]aneN4)(OH)2]+, 128900-14-1:cis-[Co( [ 14]aneN4)(oH),]+,128900-16-3; Au, 7440-57-5;NaOH, 1310-73-2: trans-[Co([ 14]aneN4)CI2]C1,90529-42-3; cis-[Co([ I4]aneN,)Cl2]CI,
90580-83-9; rrans-[Co([ 14]aneN4)(H20)2]”+, 130466443; cis-[co( [ 14]aneN4)(H20)J3+, 130466-95-4; trans-[Co( [ 1 4]aneN4)(OH),], 128835-34-7; cis-[Co([ 14]aneN4)(OH)2],128900-18-5.
Structure Sensitivity, Selectivity, and Adsorbed Intermediates in the Reactions of Acetone and 2-Propanol on the Polar Surfaces of Zinc Oxide J. M. Vohst and M. A. Barteau* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 (Received: May IO, 1990)
The reactions of 2-propanol and acetone on the (0001)-Zn and (0OOf)-0 polar surfaces of zinc oxide were investigated by using temperature-programmed desorption and X-ray photoelectron spectroscopy. 2-Propanol adsorbed dissociatively on the (000 I)-Zn surface to form surface isopropoxide ((CH,),HCO) species. These alkoxide intermediates underwent both dehydration and dehydrogenation reactions to produce propylene and acetone. Acetone dissociated on the (0001)-Zn surface to form enolate intermediates, fingerprinted by a broad envelope in the C( 1s) spectrum. These enolates decomposed unselectively above 600 K to deposit carbon, which was oxidized to CO and C 0 2 at higher temperatures. Neither acetone nor 2-propanol reacted on the oxygen polar surface, and both were adsorbed molecularly and desorbed intact from this surface below 300 K.
Introduction The reaction of 2-propanol is one of the most common probes of the behavior of metal oxide catalysts. The focus of such experiments is generally on the dehydration (to propylene) vs dehydrogenation (to acetone) selectivity as a measure of surface acid-base properties.’+ Other side reactions are less important for this secondary alcohol than for primary alcohols, although such reactions reported for 2-propanol include both oxidation and formation of higher molecular weight product^.^*^ In considering the behavior of 2-propanol and its dehydrogenation product, acetone, on a single oxide, zinc oxide, it is apparent that a wide variety of observations have been reported in previous studies. Temperature-programmed desorption studies of 2propanol decomposition on ZnO powders by Bowker, Waugh, and Pett~’-~indicated that roughly equal amounts of acetone and propylene, the principal products, were produced. These workers concluded that stable isopropoxide species were formed upon 2-propanol adsorption; these decomposed to acetone (peak temperature = 468 K) and propylene (480 K). It was suggested that the two principal products might arise from alkoxides bound on different types of sites; these in turn might be located on different crystallographic planes. These workers reported minimal side reactions: no oxidation to C O or C 0 2 was observed, and it was further noted that acetone desorbed intact below 450 K in acetone adsorption/TPD experiments. Similar 2-propanol TPD experiments on ZnO powders by Chadwick and O’Malley’O yielded results differing in several notable respects from those above. First, the dehydration/dehydrogenation selectivity was approximately 9:l rather than 1 : l . Second, they detected oxidation products such as C02, H20, and CH4 at high temperatures (>700 K) in addition to the C3 products appearing at 400-600 K. They concluded that different crystallographic planes could account for the different product states observed, with propylene produced on both polar and nonpolar planes and acetone evolved only from the nonpolar surface.I0
* Author to whom correspondence should be addressed. ‘Present address: Department of Chemical Engineering, University of Pennsylvania, Philadelphia, PA 19104. 0022-3654/91/2095-0297%02.50/0
As we have noted previously,11J2the structural dependence of alcohol decomposition has been the subject of a number of conflicting reports. In contrast to the above powder studies which proposed structural effects on product distribution, Djega-Mariadassou et al.I3 have concluded that 2-propanol decomposition on ZnO is a structure-insensitive reaction. TPD and steady-state reaction studies on ZnO single crystals by Kung and co-workersI4-I6have found evidence for 2-propanol conversion to acetone plus lesser amounts of propylene on the (0001)-Zn and (0007)-0 polar and (5051) nonpolar surfaces. The zinc polar surface was the most active in both steady-state and temperature-programmed experiments.16 In contrast, Zwicker et al.” found that activity for the decomposition of 2-propanol was confined to the (0001)-Zn face. Previous results from this laboratory for primary alcohols and other Brwsted acids have also detected activity for dissociation and decomposition only on the zinc polar surface.11J2J8-20 (1) Tanabe, K. Solid Acids and Bases; Academic: New York, 1970. (2) Krylov, 0. V. Catalysis by Nonmerals; Academic: New York, 1970. (3) Ai, M. Bull. Chem. SOC. Jpn. 1976, 49, 1328. (4) Winterbottom, J. M. Catalysis; Royal Society of Chemistry: London, 1981, Vol. 4, p 141. ( 5 ) Volta, J. C.; Turlier, P.; Trambouze, Y. J . Catal. 1974, 34, 329. (6) Wheeler, D. J.; Darby, P. W.; Kemball, C. J. Chem. SOC.1960,332. (7) Bowker, M.; Petts, W.; Waugh, K. C. J. Chem. SOC.,Faraday Trans. I 1985, 81, 3073. (8) Waugh, K. C.; Bowker, M.; Petts, R. W.; Vandervill, H. D.; OMalley, P. J. R. J. Appl. Catal. 1986, 25, 121. (9) Bowker, M.; Petts, R. W.; Waugh, K. C. J. Catal. 1986, 99, 53. (10) Chadwick, D.; OMalley, P. J. R. J. Chem. SOC., Faraday Trans. I 1987, 83, 2227. ( 1 1 ) Vohs, J. M.; Barteau, M. A. Surf. Sci. 1986, 176, 91. (12) Vohs, J. M.; Barteau, M. A. Sur5 Sci. 1989, 221, 590. (13) DjBga-Mariadassou, G.; Davignon, L.; Marques, A. R. J . Chem. Soc., Faraday Trans. 1 1982, 78, 2447. (14) Akhter, S.; Lui, K.; Kung, H. H. J . Phys. Chem. 1985, 89, 1958. (15) Lui, K.; Vest, M.; Berlowitz, P.; Akhter, S.; Kung, H. H. J. Phys. Chem. 1986, 90, 3138. (16) Berlowitz, P.; Kung, H. H. J . Am. Chem. SOC. 1986, 108, 3532. (17) Zwicker, G.; Jacobi, K.; Cunningham, J. Int. J. Mass Spectrosc. Ion Phys. 1984, 60, 213. (18) Vohs, J. M.; Barteau, M. A. J. Phys. Chem. 1987, 91, 4766. (19) Vohs, J. M.; Barteau, M. A. Surf. Sci. 1988, 201, 481.
0 1991 American Chemical Society
