J . Am. Chem. SOC.1990, 112, 3420-3426
3420
only one product 3e was obtained from the photolysis of (ditert-butoxydisilany1)iron complex 2e, in contrast to the formation of two geometric isomers, 3b and 3b', from the photolysis of 2b. The structure of 3e is assignable to the less sterically hindered isomer shown in eq 2. Me R'
Cp"
Me Me 1
\
1
F@ - sI -S I -o ~2 OC'I I I OC Me R1
1:
hU
- co
oc'
,."Fe\\
':O-R2 ,I'
(2)
SI' $\
Me Me
F
2t
G
I ~e
I %,
Me Me
3f
The mechanism for the photoreactions of 2a, 2c, 2d, and 2e is considered to be similar to that of 2b (Scheme I). However, for the formation of 3f from 2f, migration of a methyl group from one silicon atom to another is required to yield two SiMe2moieties. Such migration of substituents on silicon atoms has already been noted in the photochemical conversion of CpFe(C0)2SiR2SiR3 (R = alkyl, aryl) to CpFe(CO)2SiR3and it has been proposed that such migration occurs in silyl(si1yene)iron intermediate^.^^' Thus, 3f is formed from 2f via methyl group migration in the silyl(sily1ene)iron intermediate (eq 3) in the following manner:
(i) formation of a silyl(methoxysilylene)iron intermediate F; (ii) methyl group migration from the silyl group to the silylene ligand; formation of a methoxysilyl(sily1ene)iron intermediate G; and (iii) cyclization to form 3f. The bis(si1ylene)iron complexes, 3a, 3c, 3d, 3e, and 3f, as well as 3b and 3b', show 29SiN M R signals at very low field (90-1 25 ppm; see Table I). Particularly, each of the photolytic products, 3a, 3c, 3d, and 3f, which were derived from (monoalkoxydisilany1)iron complexes, gives only one signal at low field, exemplified by the spectrum of 3a in Figure 3. The result indicates that the two silicon atoms in each starting complex become equivalent in the corresponding product and is consistent with the structure of the alkoxy-stabilized bis(sily1ene)iron complex. There are some other characteristic spectral differences between the starting materials and the products (Table I). As stated above for 2b, 3b, and 3b', the 'HN M R signals assigned to the bridged alkoxy groups in the bis(sily1ene)iron complexes appear at appreciably high field compared to those for the terminal alkoxy1 groups of the starting materials. The chemical shift difference amounts to approximately 0.8 ppm for the methoxy-bridged complexes, 3a and 3f, and 0.2 ppm for the tert-butoxy-bridged complexes, 3c, 3d, and 3e. The IR spectrum of each of the bis(sily1ene)iron complexes shows only one terminal vc0 band below 1900 cm-' consistent with the structure with a single CO ligand.
Acknowledgment. This work has been supported by Nippon Sheet Glass Foundation for Material Science and Grants-in-Aid for Special Project Research No. 63740329 and Scientific Research No. 01740348 from the Ministry of Education, Science and Culture. We thank Toray Silicone Co., Ltd., for a gift of silicon compounds. Note Added in Proof. After this paper was submitted for publication, there appeared a paper by K. H. Pannell et al. ( J . Am. Chem. SOC.1989, 111, 4482) that is closely related to the work of refs 6 and 7. Supplementary Material Available: Tables of fractional coordinates and thermal parameters for 3b (2 pages); observed and calculated structure factors for 3b (9 pages). Ordering information is given on any current masthead page.
Effects of Redox Potential, Steric Configuration, Solvent, and Alkali Metal Cations on the Binding of Carbon Dioxide to Cobalt( I) and Nickel( I) Macrocycles Michael H. Schmidt, Gordon M. Miskelly, and Nathan S. Lewis* Contribution No. 791 6 from the Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125. Received September 14, 1989
Abstract: The binding of C 0 2 to metal macrocycles has been determined electrochemically by using cyclic voltammetry or differential pulse polarography. The C 0 2binding constants, K for a series of Co(1) tetraammacrocyclecomplexes in (CH3)#0 showed a strong correlation with the Co(II/I) redox potential. 3;hough meso and d,l stereoisomers of [Co1(Me6[14]4,1l-diene)]+ had identical redox potentials, Kco2 differed by a factor of IO2 for these stereoisomers, suggesting a large steric effect on C 0 2 binding. Binding of C 0 2 to Ni(1) tetraazamacrocycles in (CH3)2S0yielded a redox potential dependence similar to that of Co(1) macrocycles. A weak correlation between Kco and the solvent dielectric constant was found for d,l-[Co(Me6[ 1414,ll -diene)]+. In tetrahydrofuran (THF), d,l-[CO(Me6il4]4,1I-diene)]' was found to bind COPmore strongly in the presence of 0.1 M Li+ than in the presence of 0.1 M tetrabutylammonium (TBA'). Electrochemically determined Kco;s for Co(salen)and Co(Me2salen)- in THF indicated a strong dependence on redox potential and electrolyte cation.
Much recent work has been focussed on the catalytic activation of C02,1*2including many electrocatalytic schemes involving
macrocyclic complexes of cobalt and ni~ke1.h~Although several mechanistic studies have been performed on thermal COP acti-
0002-7863/90/ 15 12-3420%02.50/0 0 1990 American Chemical Society
J . Am. Chem. Soc., Vol. 112, No. 9, 1990 3421
Binding of C02 on Co and Ni Macrocycles
n
has received much relatively little quantitative information is available on the factors influencing the binding of COz to transition-metal complexes.I0*" Trends in C 0 2 binding strength as a function of the metal basicity,"+I2the steric properties of the coordinated ligands: and the presence of additional Lewis acid sites in the are of particular interest, since these factors have been invoked to explain the wide variety of reaction pathways observed in C 0 2 activation processes. To address these issues in a quantitative fashion, we have determined the C 0 2 2 1 3 binding constants (Kc%) for a series of structurally similar Co(1) and Ni(1) macrocycles and have also examined the effects of solvent and electrolyte on the binding of C 0 2 to Co(1) macrocycles and salen complexes. Our work has focussed on tetraazamacrocycle complexes of Ni and Co because these complexes have been used in schemes for the catalytic reduction of C 0 2 to CO.',, C02binding by [Co(5)]+ (5 = Me6[14]4,11-diene, see Figure 1) has been observed previously.los" A Kco, of 7 X IO4 M-l for COz binding to this complex was obtained electrochemically in 0.1 M tetrabutyl4 5 6 ammonium perchlorate (TBACIO,) in (CH3)2S0,'owhile a KCO, of 1.2 X lo4 M-' was measured in acetonitrile by using spectroscopic methods." Additionally, pulse radiolysis studies of [Co(S)]+ in aqueous solution have suggested a binding constant of 1.6 X lo8 M-' for this complex.6 The strong binding of COz to this Co macrocycle complex and the existence of many similar macrocycle complexes provided an excellent opportunity to explore C02 N' "N binding as a function of redox potential, solvent properties, and ligand structure. Studies by Floriani and co-workers of Co(sa1en)- and related comolexes indicated that the stability of CO, adducts of these 9 7 8 " complexes was influenced by the natuie of the alkali metal cation Figure 1. Macrocyclic ligands used in this study: 1 = Me,[14]present.* The Co(7,7'-diethylsalen)Li(C02) complex was found 1,3,8,10-tetraene, 2 = Me2[14]1,3-diene, 3 = Me6[14]1,4,8,1 I-tetraene, 4 = Me,[l4]4,1 I-diene, 5 = Me,[l4]4,1 I-diene, 6 = ~ ~ , [ 1 4 ] 1 , 8 - d i e ~ ~ , to be the most stable in the solid state, since it did not lose COZ in vacuo or in the air. This was in contrast to the [Co(7,7'-di7 = Me2[141I-ene, 8 = Me6[ I4]ane, 9 = [ 141ane or cyclam. propylsalen)K(C02)(THF)], complex, which had to be crystallized and studied while under a C 0 2 atmosphere. The exceptional vation processes,'V2there have been only a few detailed studies stability of the Li+ complexes, however, seemed to be limited to of possible mechanistic steps in the electrocatalytic activation of the solid state; in solution, comp~exation seemed to be favored by COz.5,6 In particular, although C 0 2 binding to metal complexes
x:Ux
U
c: :a
&
n x"" y
NUN
n
c:;.:X
U
n (Ny) U
( I ) (a) Darensbourg, D. J.; Kudaroski, R. A. Ado. Organometallic Chem. 1983, 22, 129. (b) Palmer, D. A,; Van Eldik, R. Chem. Reo. 1983.83, 651. (c) Walther, D. Coord. Chem. Reu. 1987, 79, 135. (d) Behr, A. Angew. Chem., Int. Eng. Ed. 1988, 27,661. (e) Ayers, W. M. ACSSymp. Ser. 1988, 363. (f) Behr, A. Carbon Dioxide Activation by Metal Complexes; VCH Publishers: Weinheim, Federal Republic of Germany, 1988; and references therein. (2) (a) Becker, J. Y.; Vainas, B.; Eger, R.; Kaufman, L. J . Chem. SOC., Chem. Commun. 1985,1471. (b) Hammouche, M.; Lexa, D.; Saveant, J.-M.; Momenteau, M. J . Electroanal. Chem. 1988, 249, 347. (c) Cabrera, C. R.; Abmna, H. D. J . Elecrroanal. Chem. 1986,209, 101. (d) Hawmker, J.; Lehn, J.-M.; Ziessel, R. Helu. Chim. Acta 1986,69. 1990. (e) Tezuka, M.; Yajima, T.; Tsuchiya, A. J . Am. Chem. SOC.1982, 104, 6834. (f) Slater, S.; Wagenknecht, J. H. J. Am. Chem. SOC.1984, 106, 5367. (9) Ishida, H.; Tanaka, K.; Tanaka, T. Organometallics 1987, 6, 181. (h) Ishida, H.; Tanaka, H.; Tanaka, K.; Tanaka, T. J. Chem. Soc., Chem. Commun. 1987, 131. (i) DuBois, D. L.; Miedaner, A. J. Am. Chem. SOC.1987, 109, 113. (j) Ziessel, R.; Hawecker, J.; Lehn, J. M. Helu. Chim. Acta 1986,69, 1065. (k) Keene, F. R.; Creutz, C.; Sutin, N. Coord. Chem. Reu. 1985, 64, 247. (I) Maidan, R.; Willner, I. J . Am. Chem. Soc. 1986, 108, 8100. (m) Willner, I.; Maidan, R.; Mandler, D.; Diirr, H.; DBrr, G.; Zengerle, K.J . Am. Chem. Soc. 1987, 109, 6080. ( 0 ) Daniele, S.; Ugo,P.; Bontempelli, G.; Fiorani, M. J. Electroanal. Chem. 1987. 219. 259. (3) (a) Meshitsuka, S.; Ichikawa, M.; Tamaru, K. J . Chem. Soc., Chem. Commun. 1974, 158. (b) Hiratsuka, K.; Takahashi, K.; Sasaki, H.; Toshima, S. Chem. Lett. 1977, 1137. (c) Kapusta, S.; Hackerman, N. J. Electrochem. SOC.1984. 131, 1511. (d) Lieber, C. M.; Lewis, N. S. J . Am. Chem. SOC. 1984, 106, 5033. (e) Takahashi, K.; Hiratsuka, K.; Sasaki, H.; Toshima, S. Chem. Lett. 1979. 305. (f) Fisher, B.; Eisenberg, R. J. Am. Chem. Soc. 1980, 102,7361. (8) Tinnemans, A. H. A.; Koster, T. P. M.; Thewissen, D. H. M. W.; Mackor, A. Red. Trav. Chim. Pays-Bas 1984, 103,288. (h) Grant, J. L.;Goswami, K.; Spreer, L. 0.; Otvos, J. W.; Calvin, M. J . Chem. SOC., Dalton Trans. 1987, 2105. (4) (a) Bradley, M. G.; Tysak, T.; Graves, D. J.; Vlachopulos, N. A. J. Chem. Soc., Chem. Commun. 1983, 349. (b) Beley, M.; Collin, J.-P.; Ruppert, R.; Sauvage. J.-P. J. Chem. SOC.,Chem. Commun. 1984, 1315. (c) Beley, M.; Collin, J.-P.; Ruppert, R.; Sauvage, J.-P. J. Am. Chem. Soc. 1986, 108, 7461. (d) Beley, M.; Collin, J.-P.; Sauvage, J.-P.; Petit, J.-P.; Chartier, J.-P. J. Electroanal. Chem. 1986, 206, 333.
( 5 ) (a) Sullivan, B.; Meyer, T. J. J . Chem. Soc., Chem. Comm. 1984, 1244. (b) Sullivan, B. P.;Bolinger, C. M.;Conrad, D.; Vining, W. J.; Meyer, T. J. J . Chem. SOC.,Chem. Commun. 1985, 1414. (c) Bolinger, C. M.; Sullivan, B. P.; Conrad, D.; Gilbert, J. A.; Story, N.; Meyer, T. J. J . Chem. Soc., Chem. Commun. 1985, 796. (d) Bolinger, C. M.; Story, N.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1988, 27, 4582. (e) Sullivan, B. P.; Bruce, M. R. M.; OToole, T. R.; Bolinger, C. M.; Megchee, E.; Thorp, H.; Meyer, T. J. ACS Symp. Ser. 1988, 363, 52. (6) Creutz, C.; Schwarz, H. A.; Wishart, J. F.; Fujita, E.; Sutin, N. J. Am. Chem. SOC.1989, 111, 1153. (7) (a) Herskovitz, T.; Guggenberger, L. J. J . Am. Chem. SOC.1976, 98, 1615. (b) Herskovitz, T. J . Am. Chem. SOC.1977, 99, 2391. (c) Bristow, G. R.; Hitchcock, P. B.; Lappert, M. F. J. Chem. Soc., Chem. Commun. 1981, 1145. (d) Calabrese, J. C.; Herskovitz, T.; Kinney, J. B. J. Am. Chem. Soc. 1983, 105, 5914. (e) Gambarotta, S.; Strologo, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J. Am. Chem. SOC.1985, 107, 2985. (f) Aresta, M.; Ballivet-Tkatchenko, D.; Bonnet, M. C.; Faure, R.; Loiseleur, H. J . Am. Chem. Soc. 1985, 107, 2994. (g) Alvarez, R.; Carmona, E.; Marin, J. M.; Poveda, M. L.; Gutierrez-hebla, R.; Monge, A. J . Am. Chem. Soc. 1986, 108,2286. (h) Henary, M.; Zink, J. J . Am. Chem. Soc. 1988, 110, 5582. (i) Lundquist, E. G.; Huffman, J. C.; Caulton, K. G. J . Am. Chem. SOC.1986, 108, 8309. (8) (a) Facinetti, G.; Floriani, C.; Zanazzi, P. F. J . Am. Chem. Soc. 1978, 100, 7405. (b) Gambarotta, S.; Arena, F.; Floriani, C.; Zanazzi, P. F. J. Am. Chem. SOC.1982, 104, 5082. (9) Ibers, J. A. Chem. SOC.Rev. 1982, 1 1 , 57. (IO) Gangi, D. A.; Durand, R. R., Jr. J. Chem. SOC.,Chem. Commun. 1986, 697. ( I 1) Fujita, E.; Szalda, D. J.; Creutz, C.; Sutin, N. J . Am. Chem. SOC. 1988, 110, 4870. ( I 2) Evans, G. 0.;Walter, W. F.; Mills, D. R.; Streit, C. A. J . Organomet. Chem. 1978, 144, C34. ( I 3) (a) Dennard, A. E.; Williams, R. J. P. Tramition Metal Chemistry; Carlin, R. L., Ed.; Marcel Dekker: New York, 1966; Vol. 2, p 123. (b) Allen, L. C. ACS Symp. Ser. 1988, 363,91. (c) Lee, G. R.; Maher, J. M.; Cooper, N. J. J . Am. Chem. SOC.1987, 109, 2956. (d) Aresta, M.; Ballivct-Tkatchenko, D.; Bonnet, M. C.; Faure, R.; Loiseleur, H. J. Am. Chem. Soc. 1985, 107, 2994. (e) DeLaet, D. L.; Del Rosario, R.; Fanwick, P. E.; Kubiak, C. P. J. Am. Chem. Soc. 1987, 109, 754. (f) Forschner, T.; Menard, K.;Cutler, A. J. Chem. Soc.. Chem. Commun. 1984, 121. (g) Giuseppetti, M. E.; Cutler, A. R. Organometallics 1987, 6. 970.
Schmidt et al.
3422 J . Am. Chem. SOC.,Vol. 112, No. 9, 1990 Na+ and K+ ions. These observations have prompted our attempts to quantify the effects of electrolyte cation on Kco, in T H F SOlutions.
Experimental Section All compounds were synthesized according to literature procedures. Ligand 5 was synthesized as the HCI04 salt." The d,l Co complex of this ligand was prepared in methanol by using an excess of ligand to maintain solution acidity and prevent isomerization.15 The meso diastereomer was prepared in CH3CN.16 Product recovery from the CH,CN preparation was difficult, and recrystallization from CH,CN resulted in substantial product loss unless performed anaerobically. Metathesis of this complex with aqueous NH4PF6 yielded the meso[ C O ( S ) ] [ P F ~ ]This ~ product could be more easily recrystallized and was [CF3S0,] salt was synused in preliminary studies. The d,/-[Co(S)] thesized because it dissolved more rapidly in THF. This product was made by dissolving the d,l C104- salt in acetonitrile and adding an excess of K(CF,SO,). KC104 precipitated, and the filtrate was reduced to dryness under vacuum. Addition of T H F dissolved the [Co(5)][CF3S03] while leaving most of the K(CF,SO,) undissolved. Filtration, evaporation, and redissolution in T H F were repeated until orange crystals of the ICo(S)1 ICF$OJ2 could be isolated independently of the colorless K(C.F,SO,) cry&. 6. and 7,' were DreDared according to Comolexes of 4." 2.'' 3.19 publishdd procedures. Dihalo complexes of 2 and 7 were prepared and converted to the bis-acetonitrile complexes before performing electrochemical measurements on these complexes.22 Salen complexes were prepared according to literature procedure^.^^ Ni complexes of ligand 9 were obtained from Bryan Balazs of Caltech. All electrolytes were dried at > 80 OC in vacuo (C0.1 mmHg) for at least 12 h, except for LiC104, which was fused in a quartz tube at 270-300 OC in vacuo. TBA(C104) (Southwestern Analytical) was ground to a fine powder before drying. TBA(CF,SO,) was made by neutralization of dilute CF3SOIH with TBA(0H). Recrystallization of this complex was accomplished by dissolving the salt in a minimum of acetonitrile or acetone, adding a large excess of water (ca. 15 volumes), and boiling for several hours to remove the volatile organic solvent. After the solution cooled overnight, a crystalline product formed and was isolated by filtration. Alkali metal CF,SO< salts were synthesized by neutralization of dilute CF3S03Hby the appropriate hydroxide, followed by evaporation of water under vacuum on a rotary evaporator. These salts were dried without recrystallization. Li(CF,SO,) was obtained from Aldrich. All electrochemistry was performed in dried solvents. Dimethyl sulfoxide and dimethylformamide were stirred over CaH2 for ca. 12 h, vacuum distilled, and stored under dry N2. Due to the rapid decomposition of dimethylformamide, it was used within 48 h of distillation. Propylene carbonate was stirred and heated over KMnO, and Na2C03, vacuum distilled, and used shortly after distillation. T H F was distilled over Na/benzophenone under a dry N 2 atmosphere. The acetonitrile used in UV-visible spectroscopy was distilled under dry N, from CaH, or P205. Electrochemistry was performed on 5-mL samples in a two-necked, 50" pear-shaped flask. A reference electrode (0.1 M TBA(CI04), CH3CN, Ag wire, ca.0.001 M AgCF,SO,) with a ground glass joint and a Vycor frit was fitted to one neck; the other neck was fitted with a rubber septum accommodating working and counter electrodes and gas intake and exhaust needles. The working electrode in all cases was basal plane pyrolytic graphite (0.024.10 cm2) which was sealed in white epoxy (Hysol). A Pt mesh counter electrode was used. After binding constants were measured, dry, sublimed ferrocene was added to the solution, in order to determine values of Eo' relative to this internal reference by cyclic voltammetry of both the ferrocene and the complex of interest. Because the ferrocene/ferricinium couple appears near the Co(salen)o/+ and Co(dimethylsalen)o/+ couples, decamethylferrocene was used as a (14) Tait, A. M.; Busch, D. H.; Curtis, N. F. Inorg. Synth. 1978, 18, 2. (15) Gcedken, V. L.; Kildahl, N. K.; Busch, D. H. J. Coord. Chem. 1977,
reference for these complexes. The Co macrocycle and Co salen complexes were used in concentrations between 5 X IO-' and 5 X IO-, M. A flow of N, or CO, was maintained to the cell at all times. For determinations of Kco2, N2 and C 0 2 gases were mixed in a simple glass tee, with >0.5 m of tubing between the tee and the needle entering the cell. For some experiments, gases were passed through a deoxygenating column (Ridox, Fisher Scientific) and a drying column (3 A molecular sieves, Davison) before entering the cell. These columns had no observable effect on the electrochemistry. The columns were not used for most experiments, because the sieves retained CO, and long purges of the columns were necessary to reestablish a nitrogen atmosphere after using C 0 2 . For experiments involving THF, the gases were presaturated with T H F vapor in a gas-washing bottle to prevent evaporation of the solvent in the electrochemical cell. The vapor-saturated gas was conducted to the cell in I / , in. automobile fuel line, which was found to be more resistant to T H F vapor than Tygon tubing. After the C 0 2 / N 2 mixture was adjusted, the tip of a syringe needle was immersed in the solution, and the solution was sparged with the new gas mixture for at least 10 min before taking electrochemical measurements. The cell headspace was then sampled with a gastight syringe (Precision Scientific) to determine the CO, partial pressure. These samples were injected into a gas chromatograph designed for gas analysis (Carle). The integrated response ( H P 3390A integrator) was found to be linear for C02/N2 mixtures from 0.02 to 1.0 mol fraction CO,. Sparging all C 0 2 from the solution took much longer (approximately 1 h for most solvents) than it took to saturate the solutions with CO,. Because of the slow displacement of CO,, measurements with less than 1 atm of CO, were taken by increasing the C 0 2 partial pressure by steps until 1 atm was reached. The cell was then sparged with N2, and Eo' was remeasured. Solubility coefficients for C 0 2 in various solvents were obtained by titration of electrolyte solutions saturated wiih C02.24 One milliliter aliquots of the C02:saturated electrolyte solutions were added to IO or 20 mL of a standardized 0.05 M Ba(OH), solution to precipitate the C 0 2 as BaCO,. The solution was then back-titrated with a standardized 0.1 M HCI solution, by using m-cresol purple as the indicator. The saturation concentration of CO, in (CH3)2S0 was found to be higher than that reported in ref 25, which was the value used in previous determinations of Kco2 for [ C O ( ~ ) ] + . ' ~ Determination of the saturation concentration of [CO,] in propylene carbonate could not be performed as described above due to hydrolysis of the solvent by the Ba(OH),. The amount of C 0 2 dissolved in a saturated solution was determined by degassing a sample under vacuum and measuring the volume of gas liberated with a Toepler pump. The value achieved by this method was very close to the value obtained by removing the CO, under vacuum with several freeze-pump-thaw cycles and weighing the amount of liberated CO, by condensing it into a tared flask cooled with liquid nitrogen. Electrochemistry was performed using a PAR Model 174A polarographic analyzer or other standard electrochemical instrumentation. Voltammograms were recorded using Houston Instruments Model 2000 recorders. UV-visible spectra were taken by using a Hewlett-Packard Model 8452A diode array spectrophotometer. Infrared spectra were taken by using a Perkin-Elmer 1600 series fourier transform infrared spectrophotometer. IR spectra were obtained as mineral oil mulls on NaCl plates.
Results In this study, binding constants of COz to Co and Ni complexes, KCO2,were determined by measuring the shift in the half-wave potential, El,*, as a function of [CO,].This approach relies on the following relationship derived from the Nernst equation and the equilibrium constant expression for Kco2:10*26
+ q c o 2 t MI-L(CO~), RT RT = Eo' + - In (Kco2) + q z In [CO,] nF MI-L
E,,z
.7., 89.
(16) Vasilevskis, J.; Olson, D. C. Inorg. Chem. 1971, 10, 1228. (17) Rillema, D. P.; Endicott, J. F.; Papaconstantinou. E. Inorg. Chem. 1971, IO, 1739. (18) Tait, A. M.; Busch, D. H.; Cragel, J., Jr. Inorg. Synrh. 1978, 18.27. (19) Tait, A. M.; Busch, D. H. Inorg. Chem. 1976, IS, 197. (20) Tait, A. M.;Busch. D. H.;Khalifa, M. A.; Cragel, J., Jr. Inorg. Synth. 1978, 18. 22. (21) Tait, A. M.; Busch, D. H. Inorg. Chem. 1977, 16, 966. (22) Tait, A. M.; Lovecchio, F. V.; Busch, D. H. Inorg. Chem. 1977, 16, 2206. (23) Floriani, C.; Calderazzo, F. J. Chem. Soc. A 1969, 946.
where q is the number of COz molecules binding to each metal center. Cyclic voltammetry of the metal complexes indicated that the diffusion coefficients of the oxidized and reduced forms were sufficiently close so that E l l zin the absence of COz could be used (24) Pieters, H. A. J. Anal. Chim. Acfa 1948, 2, 263. (25) Haynes, L. V.; Sawyer, D. T. Anal. Chem. 1967, 39, 332. (26) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications; John Wiley 8c Sons, Inc.: New York, 1980 p 34.
J . Am. Chem. SOC.,Vol. 112, No. 9, 1990 3423
Binding of Cot on Co and Ni Macrocycles I
1
Table 1. Binding Constants, Kco of C02 to Co and Ni Tetraazamacrocycles in (CH3)2S&/0.1M TBA(CI0,) EO'
-
comolex ligand ( M I W Kc-o, M"
