Inorg. Chem. 1992, 31, 4864-4873
4864
Electrocatalytic Reduction of Carbon Dioxide Based on 2,2'-Bipyridyl Complexes of Osmium Mitchell R. M. Bruce,' Elise Megehee,+B. Patrick Sullivan,'?$H. Holden Thorp,ll Terrence R. OTooIe,l Allison DOwnard,@J. Richard Pug4# and Thomas J. Meyer' Department of Chemistry, The University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received February 7, 1992
The complex cis-[Os(bpy)2(CO)H]+(bpy is 2,2'-bipyridine) has been found to be an electrocatalyst for the reduction of C02 in CH$N containing 0.1 M tetra-n-butylammoniumhexafluorophosphateat glassy carbon or Pt electrodes. Under anhydrous conditions CO is the dominant product, but addition of water results in up to 22% formate anion. Kinetic parameters derived by digital simulation of cyclic voltammograms under electrocatalytic conditions are consistent with a rate-determining associative step (kl)in which C02 undergoes a reaction with the direduced complex, [Os(bpy)z(CO)H]-, to form an intermediate. This intermediate either gives CO or a second intermediate which is a source of CO or formate by competitive pathways that follow the rate-determining step(s). On the basis of the results of isotopic labeling studies, neither the bound CO nor H- groups are involved in the reduction of CO1. The kinetics of reduction of C02 by the twice-reduced complexes ~is-[M(bpy)~(CO)R](M = Os, R = H, Me, Ph; M = Ru, R = CHZPh) have also been investigated. From these data, it has been found that a linear correlation exists between In kl and the cone angle of the group, -R.
Introduction
A number of group VI11 transition metal complexes have been reported to be electrocatalysts for the reduction of carbon dioxide.I.2 Some information concerning the key redox steps is available,'-' but in the main, the detailed mechanistic insight required to design new catalysts that operate at low overvoltages at high rates and with product selectivity remains to be acquired. We report here the results of a kinetic and mechanistic investigation on the reactions between the twice-reduced forms of cis-[Os(bpy)2(CO)H]+ (bpyis2,2'-bipyidine) and somerelated complexeswith C02 in acetonitrile. Theseresults,which appeared in preliminary form as a comm~nication,~ reveal a, heretofore, unidentified pathway for the reductive activation of carbon dioxide. In this pathway C02adds to the twice-reduced complexes by an associative step apparently involving expansion of the coordinationsphere. Thisgivesan intermediatethat subsequently + Current address: University of Maine, Orono, ME 04412. f Current address: Barnard College, New York, NY 10027. 1 Current address: University of Wyoming, Laramie, WY 82071. 1 Current address: North Carolina State University, Raleigh, NC 27695. I Current address: Eastman Kodak Corp., Rochester, NY 14650.
* Current address: University of Canterbury, Chriitchurch 1,New Zealand.
# Current address: National Renewable Energy Laboratory, Golden, CO 80401. (1) (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) Takahashi, K.; Hiratsuka, K.; Sasaki,H.;Toshima,S. Chem.Lett. 1979,305. (d) Eisenberg,R.; Fisher, B. J. Am. Chem. SOC.1980, 102, 7363. (e) Tezuka, M.; Yajima, T.; Tsuchiya, A. J. Am. Chem. SOC.1982, 104, 6834. (f) Kapusta, S.; Hackerman, N. J. Electrochem. SOC.1984, 231, 1511. (g) Becker, J. Y.; Vainas, B.; Egar, R.; Kaufman, L. J. Chem. SOC.,Chem. Commun. 1984, 328. (h) Lieber, C. M.; Lewis, N. S.J. Am. Chem. SOC.1984, 106, 5033. (i) Beley, M.; Collin, J.-P.; Ruppert, R.; Sauvage, J.-P. J. Chem. SOC.,Chem. Commun. 1984, 1315. u) Wagenknecht, J. H.; Slater, S.J. Am. Chem. Soc. 1984, 106, 5367. (k) Ishida, H.; Tanaka, K.; Tanaka, T. Chem. Lett. 1985,405. (1) O'Toole, T. R.; Margerum, L. D.; Westmoreland, T. D.; Vining, W. J.; Murray, R. W.; Meyer, T. J. J. Chem. SOC.,Chem. Commun. 1985, 1416. (m) Sullivan, B. P.; Bolinger, C. M.; Conrad, D.; Vining, W. J.; Meyer, T. J. J. Chem. Soc., Chem. Commun. 1985, 1414. (n) Bolinger, C. M.; Sullivan, B. P.; Conrad, D.;Gilbert, J. A.; Story, N.; Meyer, T. J. J. Chem. SOC.,Chem. Commun. 1985,796. ( 0 ) Andre, J.-F.; Wrighton, M. S.Inorg. Chem. 1986,34,67. (p) Surridge, N. A.; Meyer, T. J. Anal. Chem. 1986,58, 1576. (9)Breikas,A. I.; Abruna, H. D. J.Electroanal. Chem. Interfacial Electrochem. 1986, 201, 347. (r) Pearce, D. J.; Pletcher, D. J. Electroanal. Chem. Interfacial Chem. 1986,201,317. (s) Kusuda, K.; Ishihara, R.; Yamaguchi, H. Electrochim. Acta 1986, 31, 657. (t) Hawecker, J.;Lehn, J.-M.; Ziessel, R. Helv. Chem.Acta 1986,69,1990. (u) Sullivan, B. P.; Meyer, T. J. Organometallics 1986, 5 , 1500.
forms carbon monoxide or formate anion, the ratio of which depends on the water content of the solution.
Experimental Section Materials. Salts of the type cis- or trans-[M(bpy)~(CO)R][PF6] (M
= Ru, Os) were prepared by procedures described previously.s Acetonitrile (Burdick and Jackson) used in the electrochemical experiments was either used as received or distilled over CaH2 depending on the level of HzO desired. The supporting electrolyte, tetra-N-butylammonium hexafluorophosphate (TBAH), was prepared by metathesis of tetra& butylammonium bromide and HPF6 in HzO. The salt was purified by reprecipitation from CHzClz/ether and dried in vacuo at 80 OC. Molecular sieves were Linde 4-A 8 X 12 pellets. The COz gas was anaerobic or bonedry grade (Matheson), and theCOz/N2gasmixtures werecertified standard grades (Matheson). Electrochemical Measurements. Cyclic voltammetric (CV) experiments were conducted by using a Princeton Applied Research 173 potentiostat with a home-built sweep generator and an H P 7015B XY recorder. Solutions for CV measurements in 0.1 M TBAH in CHJCN (0.5-4.2 mM in metal complex) were made up in a 5-mL volumetric flask. Approximately 3-mL aliquots of the solutions were transferred to a single compartment electrochemical cell with a Teflon top that had provisions for a gas inlet line in addition to the working, auxiliary, and reference electrodes. All solutions were initially purged for a minimum of 10 min with Ar or COz/Nz gas mixtures that were presaturated with CHJCN (to minimize volume changes). In all experiments a Pt wire auxiliary electrode and a NaCl-saturated calomel reference electrode (SSCE) were employed with either a Pt (0.03 cm2) or glassy-carbon (0.03 cm2) working electrode. The E112values reported were calculated from E 1 p = l/z(Ev + E p ) where Ev and E p are the anodic and cathodic peak potentials. The electrodes were polished daily before use with l-Nm diamond polish (Metadi) and were wiped clean prior to each CV measurement. Addition of distilled H z 0 (18 M a grade) was made by syringe and the solution degassed for several minutes before measurements were made. In general, CV's were recorded from -0.6 to -1.8 V at the sweep rate of interest. Background CV's (Le., 0.1 M TBAH/CH,CN) (2) (a) O'Toole, T. R.; Sullivan, B. P.; Bruce, M. R. M.; Margerum, L. D.; Murray, R. W.; Meyer, T. J. J. Electroanal. Chem. Interfacial Electrochem. 1989, 259, 217. (b) Hurrell, H. C.; Mogstad, A.-L.; Usifer, D. A.; Potts, K. T.; Aburfia, H. D. Inorg. Chem. 1989, 28, 1080. (c) Tanaka, K.; Matsui, T.; Tanaka, T. J. Am. Chem.Soc. 1989,111,3765. (d) Tanaka, K.;Wakita, R.; Tanaka, T. J. Am. Chem. SOC.1989,111, 2428. (e) Sugimura, K.; Kuwabata, S.;Yoneyama, H. J . Am. Chem. SOC.1989, 111, 2361. (f) Rasmussen, S. C.; Richter, M. M.;Yi, E.; Place, H.; Brewer, K. J. Inorg. Chem. 1990,29,3926. (8) Schmidt, M. H.; Miskelly, G. M.; Lewis, N. S. J. Am. Chem. Soc. 1990,112,3420. (h) DuBois, D. L.; Miedaner, A.; Haltiwanger, R. C. J. Am. Chem. Soc. 1991, 113, 8753.
0020-1669/92/133 1-4864%03.00/0 0 1992 American Chemical Society
Inorganic Chemistry, Vol. 31, No. 23, 1992 4865
Electrocatalytic Reduction of COZ were run prior to each set of experiments. All experiments were run at ambient temperatures (20-23 "C). The diffusion coefficient for cis-[Os(bpy)2(CO)H] [PFf,] in CH3CN was found to be 2.4 (h1.0) X 1od c d / s by CV measurements by employing the RandlesSevcik equation (eq 1).6
D = [i,/(-2.69
X 105)(n3~2)(C,")(v'/2)]2
(1)
In eq 1, i, is the peak value of the current amplitude in amperes per cm2, n is the number of electrons transferred, CO'is the bulk concentration of the electroactive species in moles per cm2, and v is the scan rate in V/s. The peak current was measured at the first reduction wave and n was assumed to equal 1. The results from 30 CV experiments with 4.2 mM cis-[Os(bpy)2(CO)H][PF6]at scan rates from 0.01 to 0.2 V/s were averaged. The experimentally determined diffusion coefficient was in good agreement with values found elsewhere for related complexes (e.g., D = 4.0 X 10" cm2/s for R~(typ)(bpy)(OH#+).~ The diffusion coefficient of C02 in aqueous solution has been estimated to be 1 X 10-5 cm2/s by Kapusta and Hackerman and 3.05 X cm2/s in CH3C N by Eggins and McNeill.* Constant Potential Electrolysis. A three-compartment cell designed for handling air-sensitive materials as well as sampling the gas above the reaction solution was used for all constant potential electrolysis experi m e n t ~ .The ~ reference electrode was a saturated sodium chloride calomel electrode (SSCE) brought into the main compartment via a lugin capillary. The main compartment contained a cylindrical F't mesh electrode and a Teflon stir bar centered within the Pt mesh. The auxiliary Pt mesh electrode was separated from the main compartment by a fine glass frit. In order to equalize pressure differences between the main compartment and auxiliary compartments, a small hole was placed above the surface of the solution in the glass wall between the compartments. This allowed the gases to mix but not the solutions. In addition, the cell was connected to an oil bubbler via an 18-gauge stainless steel needle and a short piece of Tygon tubing. These pressure-equalizing features prevented mixing of the solutions in the main and auxiliary compartments as the cell became pressurized, due to the production of CO. The total gas volume of the main and auxiliary compartments was estimated to be 41.5 mL when 25 mL of solution was present in the main and auxiliary
-
(a) Sullivan, B. P.; Bruce, M. R. M.; O'Toole, T. R.; Bolinger, C. M.; Megehee, E.; Thorp, H.; Meyer, T. J. Adv. Chem. Ser. 1988, 363,52 and references therein. (b) Behr, A. Carbon Dioxide Activation by Metal Complexes;VCH: Weinheim, Germany, 1988;see also references therein. (c) Maher, J. M.; Cooper, N. J. J . Am. Chem. Soc. 1980,102, 7604. (d) Keene, F. R.; Creutz, C.; Sutin, N. Coord. Chem. Rev. 1985, 64, 247. (e) Darensbourg, D. J.; Kudarowski, R. J . Am. Chem. SOC. 1984,106,3672. (f)Darensbourg, D. J.; Hanckel, R. K.; Bauch, C. G.; Pala, M.; Simmons, D.; White, J. N. J . Am. Chem. SOC.1985, 107, 7463. (8) Sullivan, B. P.; Meyer, T. J. Organometallics 1986,5, 1500. (h) Fujita, E.; Szalda, D. J.; Creutz, C.; Sutin, N. J. Am. Chem. SOC. 1988, 110,4870. (i) Sakaki, S.;Ohkubo, K. Organometallics 1989,8, 2970. (j)Tsai, J.; Khan, M.; Nicholas, K. M. Organometallics 1989, 8,2967. (k) Eisenschmid, T. C.; Eisenberg, R. Organometallics 1989, 8,1822. (1) Alvarez, R.; Carmona,E.; Galindo, A.; Gutierrez,E.; Marin, J.; Monge, A.; Poveda, M. L.; Ruiz, C.; Savariault, J. M. Organometallics 1989,8,2430. (m) Sakaki, S.; Ohkubo,K. Znorg. Chem. 1989, 28, 2583. (n) Silavwe, N. D.; Goldman, A. S.;Ritter, R.; Tyler, D. R. Inorg. Chem. 1989,28,1231. ( 0 ) Bo, C.; Dedieu, A. Inorg. Chem. 1989, 28,304. (p) Sakaki,S. J . Am. Chem. SOC.1990,112,7813. (9) Lundquist, E. G.; Huffman, J. C.; Folting, K.; Mann, B. E.; Caulton, K. G. Znorg. Chem. 1990,29,128. (r) Amatore, C.; Jutand, A. J . Am. Chem. SOC.1991,113,2819. (s) Vites, J. C.; Steffey, B. D.; Giuseppetti-Dery, M. E.; Culter, A. R. Organometallics 1991,10, 2827. (t) Alvarez, R. A.; Atwood, J. L.; Carmona, E.; Perez, P. J.; Poveda, M. L.; Rogers, R. D. Znorg. Chem. 1991, 30, 1493. (u) Darensbourg, D. J.; Mueller, B. L.; Bischoff, C. J.; Chojnacki, S.S.;Reibenspies, J. H. Znorg. Chem. 1991,30,2418. (v) Creutz, C.; Schwarz, H. A.; Wishart, J. F.; Fujita, E.; Sutin,N. J . Am. Chem. SOC.1991,113,3361. (w) Sakaki, S.J . Am. Chem. SOC.1992, 114, 2055. (x) Tanaka, H.; Nagao, H.; Peng, S.; Tanaka, K. Znorg. Chem. 1992, 31, 1450. Bruce, M. R. M.; Megehee, E.; Sullivan, B. P.; Thorp, H.; OToole, T. R.; Downard, A.; Meyer, T. J. Organometallics 1988, 7 , 238. (a) Sullivan, B. P.; Casper, J. V.; Johnson, S.R.; Meyer, T. J. Organometallics 1984,3,1241. (b) Megehee, E. Ph.D. Thesis, The University of North Carolina, Chapel Hill, NC, 1985. Greef, R.; Peat, R.; Peter, L. M.; Pletcher, D.; Robinson, J. Instrumental Methods in Electrochemistry;Halsted Press and John Wiley And Sons: New York, 1985; Chapter 6. McHatton, R. C.; Anion, F. C. Inorg. Chem. 1984, 23, 3935. (a) Kapusta, S.;Hackerman, N. J . Electrochem. Soc. 1984,131,1511. (b) Eggins, B. R.; McNeill, J. J . Electroanal. Chem. 1983, 148, 17. Bolinger, C. M.; Story, N.; Sullivan, B. P.; Meyer, T. J. Znorg. Chem. 1988, 27, 4582.
compartments. There was some loss of gaseous products from the apparatus during the electrolyses. This led to a systematic error in the analysis for CO that underestimated its production by =5-20% in a "typical" experiment, depending somewhat on the duration of the electrolysis. This resulted in material balances that were less than 100%. No attempt was made to acwunt for this error by correcting the results of individual experiments. In each constant potential electrolysis experiment, the apparatus was assembled, after oven drying, and cooled under an atmosphere of Ar or C02. The electrolytic solution was introduced into the cell and degassed with the appropriate gas or gas mixture for a minimum of 10 min with stirring. If molecular sieves were added, they were added just prior to purging. The rotation rate of the magnetic spin bar was kept approximately constant throughout the experiment. The solution was potentiostated at -1.40 V, and after the current had reached a steady state, typically within 5-10 min, the background current (ibl;e) was measured. This measurement was used to estimate the water concentration, since the rate of hydrogen evolution at the electrode was shown to be linearly proportional to [H20] at -1.40 V under our experimental conditions. In the experiments with added water, a known amount of water was added and the background current was recorded. The catalyst was introduced, the solution purged for an additional 10 min, and the electrolysis initiated at -1.40 V. The steady-state current in the presence of CO2 (ico,) was measured after 10-15 min. Theelectrolysispotentialof-1.40Vison the foot ofthesewndreduction wave for the catalyst, but with stirring gave significant catalytic current levels. This potential was chosen to minimize H2 evolution at the Pt electrode. Similar results were obtained at more negative potentials (to -1.6 V) but there was an increase of 20-3095 in the background current. Electrolyses at the first wave with stirring failed to give catalytic currents. Gas samples were analyzed for CO by means of a Hewlett Packard 5890A gas chromatograph with a 5-8, molecular sieve column attached to the thermal conductivity detection channel by using He as the carrier gas. UV-vis measurements were made in 1-cm quartz cuvettes with Bausch and Lomb Spectronic 2000 or Hewlett-Packard 8451A diode array spectrophotometers. FTIR measurements were made in 0. l-mmpathlength NaCl solution cells by means of a Nicolet 20 DX spectrcphotometer. Analysis of anionic products was carried out with a Dionex 2000i ionchromatograph. TBAH was removed from the reaction solutions prior to analyzing for anionic products by the following procedure: distilled water was added to thereaction solution in a 2:l ratio (HlO/total reaction volume), and the resulting solution was concentrated and filtered through Q5 (Fisher) filter paper to remove TBAH (the solubility of TBAH in H2O is lo6 M-' s-', the self-reaction in equation 15 would besufficientlyrapid that it would not interfere and koh = kl. At kZ = lo4 M-l s-l, an increase of 0.1 in the current ratio, id/iCOl, is predicted for [COz] = 0.14 M, u = 0.2 V/s, and kl = 45 M-' s-l when cis-[Os(bpy)~(CO)H][PF~] isdecreased from 4.5 to 1.2 mM. A small increase in the current ratio was observed experimentallyunder these conditions; compare experiments 1 with 25, 2 with 26, and 3 with 27 in Table III.23 Constant Potential Electrolysis. Labeling Experiments. As a further test of mechanism, two different isotopic labeling experiments were conducted. When the deuterium labeled complex, cis-[Os(bpy)z(CO)D] [PFs], was used as a catalyst under conditions where formate anion was a significant product, (Table I, experiment3, >0.2 M HzO, 20% formate), there was no proton incorporation into the osmium catalyst after 1.8 turnovers with respect to formate anion production. This was shown by monitoring ~ ( 0 s - H )in the infrared at 2005 cm-I (see Figure 6A). In theinverseexperiment (Table I, experiment 1l), 325 p L of D20 was added to 26 mL of an electrolysis solution making [DzO] 0.7 M. Upon electrolysis to 100 C passed, no decrease in the hydride stretch at 2005 cm-I was observed (see Figure 6B). When I3COz was used as the source of COz, there was no incorporation of 13Cinto the CO group of the catalyst after 5.5 turnovers of the catalyst with respect to CO formation as shown by IR measurements in the v ( C 0 ) region (Table I, experiment 9; 39 mg of cis-[Os(bpy)z(CO)H][PF6].24 Kinetics: Added Water. The effect of added HzOwas studied by CV at glassy-carbon and Pt electrodes ( u = 0.2 V/s; cis[Os(bpy)z(CO)H]+ = 3.8 mM). At the glassy carbon electrode, there was an initial decrease in the current ratio, id/icoz as the water concentration was increased, followed by a leveling off as [HzO] approached 300 mM (Table IV). At a Pt electrodeunder identical conditions, the current ratio, id/icol, showed a similar dependence on [HzO] with a slightly larger decrease in id/iC02, presumably due to hydrogen evolution.
-
(23) The predicted increase in the current ratio is based on computer simulations at the highest levels of CO2 in Table 111. At lower COz levels,it ismuch harder todetect experimentally. Thebackgroundcurrent is a larger fraction of the overall current, and the predicted current increment is difficult to observe. (24) By comparison, WO2 electrolysis experiments in the presence of cis[Ru(bpy)z(CO)H][PFs] resulted in the formation of significantamounts (see of [Ru(bpy)z(C0)(NCCH,)l2+ and [Ru(bpy)~(C0)(0C(O)H)]~+ ref 12).
4810 Inorganic Chemistry, Vol. 31, No. 23, 1992
Bruce et al.
A.
I
I 2000
I
2100
k'
I
I
0.4
1
Wavenumber (cm-7)
I
200
300
[H201,m M Figure 7. Plot of id/iCOI vs [H20] with CO2 = 45, 91, and 140 mM as indicated. The simulations (-) were based on eqs 9,10, 14,15, 18, and 20 with kl = 37 M-l s-I, k2 = 1 X lo6 M-I s-l, ks = 5 X 102 M-1 s-1, k7 = 1 X lo5 M-I s-I, [COz] = 45, 91, and 140 mM, and scan rate = 200 mV SKI.Experimental points were taken from Table IV.
B. .32
I
100
I
1800
1900
I
! 0
4
self-reaction in eq 15 and a reaction with H20. A possible sequence for the H2O-dependent branch is shown in eqs 18-20. The experimental variation of (id/iCOz) with [HzO] at a glassy carbon electrode is shown in Figure 7 (boxes).
I,
+ H,O
OH-
-
-. + ks
+ CO,
I,,
-
OH-
(18)
HC03-
(19)
rapid
kb
2100
2000
1900
Wavenumber
I,
1000
cis-[Os(bpy),(CO)H]+
+ HC(0)O-
(20)
(cm-1)
Figure6. Infrared spectra acquired during constant potential electrolysis. (A) Before (1) and after (2) electrolysis of cis-[Os(bpy)2(CO)D] [PF,], to 40 C, Table I, experiment 3. The arrow at 2005 cm-I shows where (B) As in part A ~ ( 0 s - H )is observed in cis-[Os(bpy)2(CO)H][PF6]. but for cis-[Os(bpy)2(CO)H][PF6] D20 (325 pL) to 100 C,Table I, experiment 11. The arrow indicates 2005 cm-I. The original FT-IR spectra were digitized by using un-Plot-It (Silk Scientific) and plotted from Spectra Calc (Galactic Industries).
+
Table IV. Kinetic Parameters for the Reduction of C02 by (3.8 i 0.1 mM) in 0.1 M TBAH in MeCN ci~-[Os~~(bpy)2(CO)H]+ in the Presence of H2O
current ratio (id/iCOJn [HzOl (mM)
[C021 =
45mM
[COZI = 91 mM
[cod = 140 mM
0 27 54 81 108 135 162 189 216 243 270 297
0.854 0.833 0.820 0.807 0.808 0.798 0.197 0.784 0.78 1 0.764 0.755 0.755
0.741 0.714 0.687 0.671 0.656 0.647 0.639 0.636 0.627 0.623 0.624 0.618
0.650 0.631 0.608 0.599 0.584 0.571 0.571 0.560 0.556 0.554 0.556 0.557
As in Table I1 at a scan rate of 0.2 V/s at a glassy carbon electrode.
A variety of possible mechanistic cases were considered with added water with thereactions ineqs9-17 includedintheanalysis. The dependenceof (id/ico2)on [H20] was most satisfactorily fit by a mechanism involving reduction (eqs 9 and lo), followed by a second-order reaction between cis-[Os(bpy)2(CO)H]-and COZ to give I,, eq 14, followed by a competition for 1, between the
The conditions for the CV and constant potential electrolysis experimentsoverlap. In the absenceof H20, CO is the dominant product. At high concentration of added water, both CO and formate anion are major products of the reaction, but the ratio levelsoff above [HzO] = 400 mM. Thecatalytic current increases with [H20] but the effect levels off at [H20] 300 mM. By inference, CO is the product of the water-independent branch. From the kinetics and the leveling off in the amount of formate anion produced at high [H20],the water-dependentbranch must lead to both CO and formate. A reaction that would explain the appearance of CO as a product of the H2O-dependent branch is shown in eq 21. A catalytic cycle which incorporates all of the proposed steps is shown in Scheme I.
-
1,
+ CO,
ki
+
~i~-[0s(bpy),(CO)H]+ C O
+ HCO;
(21)
Kinetics: COz Reduction by Related Complexes. The kinetics of reduction of C02 by a series of related complexes were studied
by CV in dry CH3CN solution (Table V). In all cases current increases occurred in the presence of CO2 but only upon reduction at the second bpy wave. The complexes studied were cis[M(bpy)2(CO)R]+(M = Os, R = Me, Ph; M = Ru, R = CH2Ph), and truns-[Os(bpy)2(CO)R]+ (R = Me, Et, Ph). As reported in a parallel study, the reduction of C02 catalyzed by cis[R~(bpy)~(C0)H]+ occurs by a different mechanism involving insertion into the Ru-H bond after the addition of one electron.*2 Values of kl (Scheme I) in dry 0.1 M TBAH in CH3CN solution were estimated by assuming that the mechanism established for cis-[Os(bpy)2(CO)H][PFs] held throughout the series. Evaluation of kl from measurements of id/ico2 was based on the simulations approach. The rate constants are listed in Table V. For the cis complexes, a plot of In kl versus the cone angle of the
Inorganic Chemistry, Vol. 31, No. 23, 1992 4871
Pathway A 2 cis-[Os(bpy)z(CO)H]o + CO + C032 -
,==,
(15)
la +H20
It,
+
OH'
(IS)
Pathway B cis-[Os(bpy),(CO)H]+
+
COZ +
OH-
co2
,f a s t
+
HCO;
cis-[Os(bpy)z(CO)H]+ + CO
HC03'
+
(20)
HCOj
(21)
(19)
ligand cis to CO (as defined by Tolman25) is shown in Figure 8. Several features emerge from these data: (1) there is a linear relationship between In k, and the molecular volume of the cis ligands as measured by the cone angle, (2) the stereochemically more crowded cis isomers are more sensitive to steric effects than are the trans isomers and when twice-reduced are intrinsically more reactive toward C02 (e.g., compare cis- and trans[Os(bpy)z(CO)Me][PF6] in Table V), and (3) if isomeric intermediates exist for the cis and trans complexes, they are stereochemically rigid under the conditions that lead to C 0 2 reduction since from the CV experiments there is no interconversion between isomers. Discussion. The experimentsdescribed here establish that the complex cis-[O~(bpy)~(CO)H]+ is a relatively stable, facile electrocatalyst for the reduction of C02 to either CO or formate with oxalate forming at
