Organometallics 1995, 14, 5093-5098
5093
Multistep COz Reduction Catalyzed by [R~(bpy)z(qu)(CO)]~+ (bpy = 2,2'-Bipyridine, qu = Quinoline). Double Methylation of the Carbonyl Moiety Resulting from Reductive Disproportionation of COS Hiroshi Nakajima, Yoshinori Kushi, Hirotaka Nagao, and Koji Tanaka" Institute for Molecular Science, Department of Structural Molecular Science, The Graduate University for Advanced Studies, Myodaiji, Okazaki 444, Japan Received March 21, 1995@ The title complexes catalyze the reductive disproportionation of C02 forming CO and Cos2in the electrochemical C02 reduction in CH3CN with LiBF4. The same reduction in the presence of Me4NBF4 instead of LiBF4 produces CH3C(O)CH3 and CH3C(O)CH2COO- in addition to HCOO- and CO in CH&N/DMSO (l:l,v/v). I n this reaction Me4N+ functions as the methylation agent of the carbonyl moiety resulting from the reductive disproportionation of COZ. The produced CH&(O)CH3 undergoes the abstraction of its a-proton and subsequent carboxylation to afford CH&(O)CH2COO-.
Introduction
A number of transition metal complexes have been shown to be active for generation of CO and/or HCOOin electro-l and photochemicallnp2C02 reduction. The mechanistic studies on those reactions have been also e ~ a m i n e d , l ~ and - ~ ~some ~ ~ ' of them have suggested the participation of metal-carbonyl complexes ([M-C01(fl+2)+) in the C02 r e d ~ c t i o n . l l - ~In, ~protic media, [M-C01(fl+2)+ may be formed through an acid-base equilibrium with [M-COOHl("+l)+ and [M-q1(C)-C02Ifl+ (eq 1),4while H+ [M-q'(C)-COJ"+
OH'
[M-COOH]("+')+
H+ e [M-C0]("+2>C(1) OH'
oxide transfer reactions from [M(C02)1"+ to C02 or another [M(COz)P+become predominant pathways for Abstract published in Advance ACS Abstracts, September 15,1995. (1)(a)Arana, C.; Keshavarz, M.; Potts, K. T.; Abruiia, H. D. Inorg. Chim. Acta 1994,225,285. (b) Steffey, B. d.; Miedaner, A.; Maciejewski-Farmer, M. L.; Bernatis, P. R.; Herring, A. M.; Allured, V. S.; Carperos, V.; DuBois, D. L. Organometallics 1994, 13, 4844. (c) Szymaszek, A.; Pruchnik, F. Rhodium Express 1994,5,18.(d) Fujita, E.; Haff, J.; Sanzenbacher, R.; Elias, H. Inorg. Chem. 1994,33,4627. (e) Ogura, K.; Sugihara, H.; Yano, J.; Higasa, M. J . Electrochem. SOC. 1994,141,419.(0 Chardon-Noblat, S.;Collomb-Dunand-Sauthier, M.N.; Deronzier, A.; Ziessel, R.; Zsoldos, D. Inorg. Chem. 1994,33,4410. (g) Christensen, P.; Hamnett, A,; Muir, A. V. G.; Timney, J . A,; Higgins, S. J . Chem. Soc., Faraday Trans. 1994,90,459.(h) Kimura, E.; Wada, S.; Shionoya, M.; Okazaki, Y. Inorg. Chem. 1994,33,770.(i)Kolodsick, K. J.; Schrier, P. W.; Walton, R. A. Polyhedron 1994, 13, 457. (j) Collomb-Dunand-Sauthier, M.-N.; Deronzier, A,; Ziessel, R. J . Chem. Soc., Chem. Commun. 1994, 189. (k) Haines, R. J.; Wittrig, R. E.; Kubiak, C. P. Inorg. Chem. 1994, 33, 4723. (1) Collomb-DunandSauthier, M.-N.;Deronzier, A,; Ziessel, R. Inorg. Chem. 1994,33,2961. (m) Nagao, H.; Mizukawa, T.; Tanaka, K. Inorg. Chem. 1994,33,3415. (n) Halmann, M.M., Ed. Chemical Fixation of Carbon Dioxide; CRC Press: London, 1993. (01 Sullivan, B. P., Ed. Electrochemical and Electrocatalytic Reduction of Carbon Dioxide; Elsevier: Amsterdam, 1993;and references therein. (2)(a) Ishitani, 0.; George, M. W.; Ibusuki, T.; Johnson, F. P. A,; Koike, K.; Nozaki, K.; Pac, C.; Turner, J. J.; Westwell, J. R. Inorg. Chem. 1994,33,4712. (b) Matsuoka, S.; Yamamoto, K.; Ogata, T.; Kusaba, M.; Nakashima, N.; Fujita, E.; Yanagida, S. J . Am. Chem. Soc. 1993,115,601. (c) Calzaferri, G.; Hadener, K; Li, J. J . Photochem. Photobiol. A 1992,64,259. (d) Kimura, E.; Bu, X.; Shinomihya, M.; Wada, S.; Maruyama, S. Inorg. Chem. 1992,31,4542and references therein. @
the metal-carbonyl formation in aprotic media (eq 2).5325 Recently we have reported the first multi-electron reduction of CO2 affording HCHO, CH30H, HOOCCHO, and HOOCCHzOH under homogeneous protic conditions,lmin which the smooth conversion from [Ru(bpy)(trpy)(q1(C)-C02)l(bpy = 2,2'-bipyridine, trpy = 2,2': 6,2"-terpyridine) to [Ru(bpy)(trpy)(CO)12+ (eq 1)serves the following reduction of the latter to [Ru(bpy)(trpy)(CHO)I+and [Ru(bpy)(trpy)(CHzOH)I+.From the viewpoint of CO2 as a potential C1 source for organic compounds, using organic electrophiles instead of protons in multi-electron reduction of C02 may provide more versatile routes for the catalytic carbon-carbon bond formation.6 The oxide transfer from [M(C02)Ifl+ t o C 0 2 in aprotic media (eq 21, therefore, is expected t o be useful for multi-electron reduction of CO2 in the presence of organic electrophiles. A well-characterized [Ru(bpy)z(CO)(yl(C)-CO~)l is reversibly converted t o [Ru(bpy)z(C0)2l2+via [Ru(bpy)z(CO)(COOH)]+(pK, = 9.5) in aqueous solutions (eq 1). No oxide transfer from [Ru(bpy)z(CO)(y1(C)-COz)I to C02, however, occurs in organic solvent^.^ Taking into (3)(a) Ratliff, K. S.; Lentz, R. E.; Kubiak, C. P. Organometallics 1992 11, 1986. (b) Jeget, C.; Fouassier, M.; Mascetti, J. Inorg. Chem. 1991,30,1521. (c) Jeget, C.; Fouassier, M.; Tranquille, M.; Mascetti, J . Inorg. Chem. 1991,30, 1529. (d) Pugh, J. R.; Bruce, M. R. M.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1991,30,86. (e) Sullivan, B.P.; Meyer, T. J. J. Chem. SOC.,Chem. Commun. 1984,1244. (4)(a) Ishida, H.; Katsuyuki, F.; Ohba, T.; Ohkubo, K.; Tanaka, T.; Terada, T.; Tanaka, T. J . Chem. SOC.,Dalton Trans. 1990,2155. (b) Ishida, H.; Tanaka, K.; Morimoto, M.; Tanaka, T. Organometallics 1986,5,724. (c) Tanaka, K.; Morimoto, M.; Tanaka, T. Chem. Lett. 1983,901. (d) Choudhury, D.; Cole-Hamilton, D. J. J . Chem. Soc., Dalton Trans. 1982,1885. (5) (a) Lee, G. R.; Maher, J. M.; Cooper, N. J . J . Am. Chem. SOC. 1987,109,2956. (b) Maher, J. M.; Lee, G. R.; Cooper, N. J. J . Am. 1982,104,6792. (c) Maher, J. M.; Cooper, N. J . J . Am. Chem. SOC. Chem. SOC. 1980,102,7605. (d) Lee, G.R.; Cooper, N. J. Organometallics 1985,4,1467. (d) Guggenberger, L. J.; Herskovitz, T. J . Am. Chem. SOC.1976,98,1615.(0 Belmore, K.A,; Vanderpool, R. A.; Tsai, J.-C.;Khan, M. A,; Nicholas, K. M. J . Am. Chem. SOC.1988,110,2004. (6)Another carbon-carbon bond formation has been achieved through electrocarboxylation of [RPd(PPh&lo and [RW(CO)& to afford RCOO- (R = methyl, aryl, and benzyl). See the following references: (a) Amatore, C.; Jutand, A,; Khalil, F.; Nielsen, M. F. J . Am. Chem. SOC. 1992,114,7076. (b) Amatore, C.; Jutand, A. J . Am. Chem. SOC. 1991,113,2819. (c) Fauvarque, J. F.; de Zelicourt, Y.; Amatore, C.; Jutand, A. J . Appl. Electrochem. 1990,20,338. (d) Torii, S.;Tanaka, H.; Hamatani, T.; Morisaki, K.; Jutand, A,; Plfuger, F.; Fauvarque, J.-F. Chem. Lett. 1986,169. (e) Kudaroski, R.; Darensbourg, D. J . J . Am. Chem. SOC.1984,106,3672.
0276-733319512314-5093$09.00/00 1995 American Chemical Society
5094 Organometallics, Vol. 14, No. 11, 1995
Nakajima et al.
Ru: C, 42.62; H, 3.01; N, 9.62. Found: C, 42.36; H, 3.20; N, 9.45. FAB-mass (mlz): 729 (M - PF6). Preparation of [Ru(bpy)2(qu)(CO)I(PFs)z (2). A 2-methoxyethanol solution (35 mL) containing [Ru(bpy),(qu)ClI(PF6) (100 mg) and equimolar AgPF6 (35 mg) was heated at 90 "C for 2 h with vigorous CO bubbling, during which time the solution changed from dark red to orange in color. Precipitated AgCl was removed by filtration with Celite, and the filtrate was evaporated under reduced pressure. An addition of excess NH4PF6to the aqueous solution of the product gave [Ru(bpy)Z(qU)(CO)](PF6)2as a yellowish orange precipitate which was recrystallized from ethanol. Yield: ca. 60% (based on Ru(bpy)2C12*2H20).IR (KBr): v ( C ~ 0 1996 ) cm-l. Anal. Calcd for C30H230N5F12P2Ru: C, 41.86; H, 2.70; N, 8.13. Found: C, 41.82; H, 3.02; N, 8.32. FAB-mass (mh): 716 (M - PF6). Preparation of [Ru(bpy)2(i-qu)(CO)I(PF6)2(i-qu= Isoquinoline). [Ru(bpy)2(i-qu)(CO)](PFs)2 was prepared in analogy with the method for 2 except that isoquinoline (1mL) was used for the preparation of ERu(bpy)~(i-qu)Cl](PF6).Yield: ca. 50% (based on Ru(bpy)zCly2H20). IR (KBr): v ( C ~ 0 1995 ) cm-l. Anal. Calcd for C ~ O H ~ ~ O N ~ FC, ~ ~41.86; P ~ RH, U2.70; : N, 8.13. Found: C, 41.48; H, 3.01; N, 8.29. FAB-mass (mi 2): 716 (M - PF6). Physical Measurements. Cyclic voltammetric experiments were performed in CH3CN containing Me4NBF4 (5 x M) and the complexes (1 x M) with a Hokuto Denko HA-151 potentiostatlfunction generator and a Riken Denshi Co. F-35 X-Y recorder. One component cell using a glassy carbon working electrode (0.07 cm2),a platinum wire auxiliary electrode, and an Ag(AgC1reference electrode was employed. The working electrode was polished with an alumina paste and washed with distilled water before use. The Eli2 values reported here were determined from average of the oxidative and reductive peak potentials. IR spectra were obtained on a Shimadzu FTIR-8100 spectrophotometer. IH-NMR spectra were measured on a JEOL EX270 (270 MHz) spectrometer. Experimental Section FAB-mass and GC-mass spectra were obtained on ShiMaterials. Tetramethylammonium tetrafluoroborate and madz-atos Concept 1s and Shimadzu GC-mass QPlithium tetrafluoroborate were purchased and recrystallized 1000EX, respectively. Elemental analyses were carried out from a methanol. CH3CN and DMSO were distilled over CaH2 at the Chemical Materials Center of the Institute for Molecular before use.l0 Tetrabutylammonium tetrafluoroborate for elecScience. trochemical use and all other chemicals were used as received. IR Measurement under Electrolysis Condition. The Ru(bpy)zClz*2H~0 was prepared according to the previously approximate cell constitution is depicted in Figure 1. The cell described method.ll is constructed by plane KBr windows, spacers made of Novix Preparation of [Ru(bpy)~(qu)(CHsCN)l(PFs)2 (qu = files (purchased from Iwaki Co.), and a three-electrode system Quinoline) (1). An ethanol solution (30 mL) containing Ru(an Au mesh for a working electrode, a Pt wire for an auxiliary (bpy),C12*2H20(200 mg) and an excess amount of quinoline electrode, and an AgiAgCl reference electrode separated from (1 mL) was refluxed for 4 h. The solution was concentrated the working electrode by a luggin capillary). The thickness to ca. 5 cm3 under reduced pressure. An addition of 50 mL of of the cell is 0.3 mm, and the total cell volume is about 0.1 aqueous HPF6 (12%)to the solution gave [Ru(bpy),(qu)ClI(PF6) mL. The CD3CN solution used for the measurement contains as a brown precipitate in a 70% yield. A 2-methoxyethanol the complex (1 x M) and LiBF4 (5 x M) as an solution (35 mL) containing [Ru(bpy),(qu)Cl](PFs) (100 mg), electrolyte. To prevent evaporation of the solution the cell was equimolar AgPF6 (35 mg), and CH3CN (0.5 mL) was heated continually cooled by refrigerants and only exposed to an at 70 "C for 1h. Precipitated AgCl was removed by filtration infrared lay on measuring. with Celite, and the filtrate was evaporated under reduced Electrochemical Reduction of COZ. Electrochemical pressure. An addition of excess NH4PF6 to the aqueous reduction of COZwas performed in CO2-saturated CH3CN or solution of the product gave [Ru(bpy)z(qu)(CH3CN)I(PFs)2 as CH&N/DMSO ( l : l , v/v) solutions containing the complex (1.0 an orange precipitate. Each product was purified by column x M) and an appropriate electrolyte (Bun4NBF4or Me4chromatography on neutral alumina using a CsHdCH3CN (1: M) under controlled potential electrolysis at a NBFd, 5 x 1, v/v) eluent, and the final product was further purified by range of -1.50 to -1.60 V (us AglAgCl). The electrolysis cell recrystallization from an ethanol-acetone. Yield: ca. 60% of three components, one for a glassy carbon working (based on Ru(bpy)~C1~.2H20). Anal. Calcd for C ~ I H ~ ~ N S F I ~ P Zconsisted electrode, the second for a Pt plate auxiliary electrode which was separated from the working electrode cell by a Nafion (7) Ishida, H.; Tzeng, B.-C.; Nagao, H.; Peng, S.-M.; Tanaka, K. membrane, and the third for an AglAgCl reference electrode. Inorg. Chem. 1993,32,1508. (8) (a) Katz, N. E.; Szalda, D. J.; Chou, M. H.; Creutz, C.; Sutin, N. The first two were connected to volumetric flasks filled with J . Am. Chem. SOC.1989,111, 6591. (b) Creutz, C.; Schwarz, H. A,; C02 gas with a stainless-steel tube. At an appropriate interval Wishart,J. F.; Fujita, E.; Sutin, N. J.Am. Chem. SOC.1989,111,1153. of coulombs consumed in the reduction, a 0.1 mL portion of (c) Sweet, J. R.; Graham, W. A. G. Organometallics 1982,1, 982. (d) gas was sampled from the gas phase of the working electrode Bercaw, J.E.; Goh, L.-Y.; Halpen, J. J . Am. Chem. SOC.1972,94,6534. (9)Nakajima, H.; Mizukawa, T.; Nagao, H.; Tanaka, K. Chem. Lett., component with a pressure-locked syringe. Characterization in press. and quantification of gaseous products such as CO and C2H6 (10)GC analysis revealed that DMSO and CH&N contained 1.7 x were performed on a gas chromatograph. The analysis of the and 5.6 x M HzO, respectively, even after the purification. solution was carried out by sampling each 0.1 mL portion from (11)Sullivan,B. P.; Salmon, D. J.;Meyer, T. J. Znorg. Chem. 1978, 17,3334. the liquid phase of the working electrode component at an
account that the smooth conversion from anionic [W(CO)5(r1(C)-C02)l2-and [CpFe(CO)2(r1(C)-C02)1-to [W(CO)61and [CpFe(CO)al+ under COz atmosphere at low t e m p e r a t ~ r e , no ~ ~reactivity -~ of [Ru(bpy)~(CO)(rl(C)-CO2)1toward C02 may be correlated with the weak compared with basicity of [Ru(bpy)2(C0)(r1(C)-CO2)1 those anionic r1(C)-C02 complexes. The basicity of rl(C)-COz complexes is reasonably estimated from the equilibrium constant (Ka) of the hydroxycarbonyl complexes ([M-COOH]("+l)+)in eq 1. The reported pKa values of hydroxycarbonyl complexes range widely from ca. 14 to 2.5,4b,8 which are quite different from those of organic carboxylic acids. On the basis of such a wide range of pKa values, replacement of the carbonyl ligand in [Ru(~~~)~(CO)(~~'(C)-CO~)I with an electron-donating group would greatly enhance the basicity of the rl(C)C02 moiety, which will be a promising gateway to the multi-electron reduction of C02 in the presence of organic electrophiles in aprotic media. This paper describes the first catalytic formation of acetone and acetoacetic acid in electrochemical COZ reduction by [Ru(bpy)z(qu)(C0)l2+(qu = quinoline) under aprotic conditions, which is composed of two key reactions. The first is the double methylation of [RuCola to afford CH3C(O)CH3where [Ru-C0I2+ is regenerated through the oxide transfer from [Ru-r1(C)-C02I0 to COZ (eq 2). The second is the abstraction of the a-proton from resulting CH3C(O)CH3by [Ru-q1(C)-C02l to give [Ru-COOHI+ and CH3C(O)CH2-, the latter of which further undergoes carboxylation to produce CH3C(O)CHzCOO-. A part of this study has been reported e l ~ e w h e r e . ~
C02 Reduction Catalyzed by [Ru(bpy)~qu)(CO)~+
Organometallics, Vol. 14, No. 11, 1995 5095
A
I
I
four pieces of Novix files (pressed by heating)
auxiliary electrode
Gi-++
)
Figure 1. Approximate cell constitution employed for the IR measurements under the electrolysis condition. appropriate interval of coulombs consumed. The amounts of the formic acid, acetone, and acetoacetic acid produced during the electrolysis were determined with an isotachophoretic analyzer, gas chromatograph, and HPLC, respectively.
-1.0
-2.0
0
Volts vs. AglAgCl
Results and Discussion Reaction of Two-ElectronReduced Form of Ill2+ With C02. A cyclic voltammogram of [Ru(bpy)Z(quKC&CN)I2+([112+)shows two successive one-electron redox waves at E1/z = -1.37 ([1I2+I1+) and -1.57 V ([1I1+/O) (us AglAgCl), while [Ru(bpy)~(qu)(CO)l~+ ([212+)shows three reversible one-electron redox waves at -1.11 ([2I2+I1+), -1.37 ([2I1+/O),and -1.65 V ([21°'1-) (solid lines in Figure 2). Introduction of COZ into the CH3CN solutions of ill2+and [212+by bubbling results in an appearance of strong cathodic currents at potentials more negative than the cathodic waves of the [112+/'+and [2]l+/Ocouples suggesting high reactivity of both reduced form of Ill2+ and [212+with COZ(dotted lines in Figure 2). In fact, pouring a dark red CH3CN solution of [11° l 2 upon dry ice13 followed by warming the mixture t o ambient temperature gave a small amount of a white precipitate of (Me4NIzC03 (characterized by comparison of the IR spectrum with an authentic sample). Moreover, the IR spectrum of the crude product obtained by evaporation of the solution under reduced pressure displayed a v(CzO) band a t 1999 cm-I assignable to [212+. The identification of [212+and cos2- species in the reaction mixture suggests that the reaction of [11° with COz initially produces [Ru(bpy)z(qu)(y1(C)-C0~)I0 (eq 3a), which then undergoes the oxide transfer by another COZ (eq 3b). (12)Doubly reduced [112+ ([130) was prepared by the controlled potential electrolysis of an orange solution of [lI(PF& (2 x M) at -1.60 V in the presence of Me4NBF4(5 x M) at 0 "C under Nz. (13)Dry ice used in this experiment was prepared by freezing a purified COz gas with liquid Nz.
Figure 2. Cyclic voltammograms of CH3CN solutions containing Me4NBF4(5 x M) and [lI(PF& (a)and [23M) (b). Solid and dotted lines are under (PF& (1 x Nz and COz atmospheres, respectively. dE/dt = 100 mV/s.
+
[Ru(bpy)Z(qu)(CH3CN)Io CO,
[11°
-
~Ru~bpy~z~qu~~~'~C~-COz~lo (3a)
-
[R~(bpy)z(q~)(~~(C>-C +OCOZ ,)1~
+ COZ-
[Ru(bpy)z(cu)(C0)12f [212+
(3b)
Electrochemical C02 Reduction by [212+. As expected from Figure 2b, the two-electron reduced form of [212+ ([21°) also has an ability to catalyze COZ reduction. The controlled potential electrolysis of 123(PF& (1 x M) at -1.50 V in COz-saturated CH3CN using Bun4NBF4 ( 5 x M) as an electrolyte predominantly produced HCOO- and CO with current efficiencies (17) of 75 and 2%, respectively, after 70 C passed. Concomitant formation of CHZ=CHCZH~ and Bun3N without the generation of c03'- species in the electrolyte solution reveals the function of Bun4N+as a proton donor for HCOO- formation (eq 4).14 On the
HCOO-
+ CHz=CHCzH5 + (C4H,),N
(4)
other hand, the similar electrochemical COz reduction
5096 Organometallics, Vol. 14,No.11, 1995
Nakajima et al.
/
20
60
40
Coulomb consumed / C
Figure 3. Plots of the amounts of CO (O), CH3C(O)CH3 (01, HCOO- (A), and CH3C(O)CH2COO- (A)against the electricity consumed in the C02 reduction by [21(PF& (1 x M) in CH&N/DMSO (1:l v/v) with Me4NBF4 ( 5 x M). by [21(PF& (1 x M) using LiBF4 (5 x M) instead of Bun4NBF4 under otherwise the same conditions gave Li2CO3 and CO (17 = 78% after 10 C passed),lkJ5and HCOO- was not detected (eq 5). Thus, the main product in the electrochemical C02 reduction by [212+is changed from HCOO- to CO (eqs 4 and 5 ) in the presence and the absence of the proton donor.
2c02+ 2e-
[Ru(bpy)z(q~)(CO)l~+ -1.50 V in CH&N
co+co,2-
(5)
The reductive disproportionation of CO2 (eq 5) is of interest from the viewpoint of carbonyl generation as the starting material for organic synthesis in aprotic media. Deposition of Li2CO3 on the glassy carbon electrode, however, gradually inhibited the progress of the C02 reduction in CH3CN. On the other hand, the electrochemical COSreduction by [2](PF6)2(1x M) M) in CH3CN/DMSO (1:l using (Me4N)BFd (5 x v/v) smoothly proceeded without forming a ( M e & & 0 3 layer on the electrode, and CH3C(O)CH3 and CH3C(O)CH2COO- (7 = 16 and 6%, respectively) were unexpectedly produced together with CO and HCOO- (17 = 42 and 7%, respectively) after 60 C passed in the electrolysis (Figure 3). Furthermore, [112+was obtained in a relatively high yield ('70%) from the reaction residue (confirmed by NMR and IR spectra). The electrochemical C02 reduction conducted in CH3CN/ DMSO-& under otherwise the same reaction conditions gave the same products, and neither CD3C(O)CD3nor CD3C(O)CH3was identified by GC-mass. This observation16 clearly indicates that CH3C(O)CH3 is formed through double methylation of the carbonyl moiety resulting from the reductive disproportionation of CO2 by [212+(eq 51, in which Me4N+ plays a role as the methylation agent (eq 6). It is worthy of note that [212+ (14) (a) Bolinger, C. M.; Sullivan, B. P.; Conrad, D.; Gilbert, J. A,; Story, N.; Meyer, T. J. J. Chem. SOC.,Chem. Commun. 1986,796. (b) Tezuka, M.; Yajima, T.; Tsuchiya, A.; Matsumoto, Y.; Uchida, Y.; Hidai, M. J . Am. Chem. SOC.1982, 104, 6834. (15) (a) Christensen, P.; Hamnett, A.; Muir, A. V. G.; Timney, J. A. J.Chem. SOC.,Dalton Trans. 1992, 1455. (b) Sullivan, B. P.; Bolinger, C. M.; Conrad, D.; Vining, W. J.; Meyer, T. J. J. Chem. SOC.,Chem. Commun. 1985. 1414. (16) The similar electrolysis in CH3CN/DMSO using Et4NBF4in the place of Me4NBF4 afforded (CzH&(O)(CzHs) without generation of CH3C(O)CH3 or CH~C(O)CZH~.
-2.0
0
-1.0 Volts vs. AglAgCl
Figure 4. (a)Cyclic voltammograms Of [21(PFs)z(1x M) in the CH3CN solution with Me4NBF4 (5 x 10-2 M) under Nz in the presence (a solid line) and the absence (a dotted line) of CH31(1 x M). (b)Cyclic voltammogram of [1](PFs)z (1 x M) in a CH3CN solution under Nz containing Me4NBF4 (5 x M) and CH31(1 x 10-2 M). dE/dt = 100 mV/s.
+
CH3C(0)CH3 COS2-
+ 2(CH3),N
(6)
also catalyzes the carboxylation of the resulting CH3C(O)CH3 producing CH&(O)CH2COO- and HCOO- (eq 7) under the electrolysis conditions, since the electro-
CH3C(0)CH2COO-
+ HCOO-
(7)
chemical reduction of COSby [21(PF& (1 x M) in the presence of CH3C(O)CH3 (1 x M) in CH3CN/ DMSO (1:l v/v) predominantly produced those produ c t ~ Although .~~ electrochemical carboxylation of PhC(O)CH3 affording PhC(O)CH2COO- and HCOO- by [Fe4S4(SPh)4I2- and [Ru(bpy)2(C0)2l2+ in dry CH3CN has been reported,ls the present CH&(O)CH2COO- is formed by the double methylation of the carbonyl ligand resulting from the reductive disproportionation of COZ followed by the subsequent carboxylation, where three carbon-carbon bonds are newly formed. Reactivity of the Carbonyl Ligand on [21°. From the preceding discussion, [21° possibly undergoes double methylation in the presence of Me4N+ to afford CH3C(O)CH3 (eq 6). Although such unusual methylation of [21° by M e a + was not detected in the cyclic voltammogram of [2I(PFs)z in CH3CN (a solid line of Figure 2b), [212+shows an additional cathodic wave a t -1.53 V in the presence of CH31(1 x MI (Figure 4a). On the other hand, CH3I (1 x lop2M) gave no significant effect (17) In the absence of [2lZc,the carboxylation of CH&(O)CH3 by , (CH&N did not take place in COz-saturated CH3CNDMSO ( l : l v/v).
Organometallics, Vol. 14, No. 11, 1995 5097
COz Reduction Catalyzed by [Ru(bpy)dqu)(C0)l2+
molar concentration of the complex). While, similarly prepared [21° reacted with CH31(equimolar concentration of the complex) t o bring about an appearance of a strong band at 1568 cm-l in the IR spectrum. In addition, the lH-NMR spectrum of the reaction mixture in DMSO-& showed two singlet signals a t 6 = 1.71 and 1.67, and the prolonged reaction in the presence of three times excess of CH3I at 40 "C for 4 h resulted in disappearance of the latter signal with the generation of a singlet methyl signal of CH&(O)CH3 at 6 = 2.01.2l Accordingly, the 1568 cm-' band in the IR spectrum and the singlet signal a t 6 = 1.67 in the lH-NMR are reasonably assigned to the v(C=O) band and the methyl signal22 of [Ru(bpy)z(qu)(C(O)CH3)]+formed by the reaction of [21° with CH3I (eq 8). Unfortunately, several attempts to separate the acyl complex from an unidentified product with a singlet signal at 6 = 1.71 have been unsuccessful so far.
0
1
I
2000
1900
1
3
wavenumberlcm-'
Figure 6. IR spectra of [21(PF& (1 x
M) with the passage of time under constant potential electrolysis at -1.21 V (a) and subsequently at -1.50 V in a CD3CN solution with LiBF4 (5 x M) under N2 (b). Scheme 1 +e'
+e'
[212+ V(Cd):
2015
' L[21+ e [ZIO -e'
-e-
1980
1939 cm"
on the cyclic voltammogram of [1I(PF& (Figure 4b) up to -1.85 V. Thus, CH3I smoothly reacts with [21° but not with [11°, suggesting the formation of [Ru(bpyMqu)(C(O)CHdI+ rather than [Ru(bpy)2(qu)(CHdI+(vide infra). The solution IR spectra of [2](PF& in CD3CN under controlled potential electrolysis conditions well afford the information for the induced reactivity of the carbonyl ligand. The strong v(C=O) band of [212+at 2015 cm-' l9 shifts to 1980 and 1939 cm-l upon the formation of [21+ and [21° under electrolysis at -1.21 and -1.50 V, respectively (Figure 5).2O Reoxidation of the resulting [21+ and [21° solutions at 0 V essentially regenerated the original IR spectrum of [212+although the intensity of the v(C=O) band of [212+decreased to some degree due t o partial elimination of CO from [21° (formally 20 electron complex) during the reductionoxidation process. The red shift of the v ( C ~ 0band ) of [212+by about 40 cm-' per one electron reduction reflects the reactivity of the electrophilic attack of CH31on the carbonyl ligand (Scheme 1). The v(C10) band at 1980 cm-l of [21+ prepared electrochemically in CH3CN was hardly affected by the presence of CH31 (even 5 times (18)(a) Tanaka, K.;Wakita, R.; Tanaka, T. J.Am. Chem. SOC.1989, 1 I I, 2428. (b) Tanaka, K.; Miyamoto, H.; Tanaka, T. Chem. Lett. 1988, 2033. (19)The CO stretching band of [2Iz+observed at 1996 cm-l in a KBr pellet shifted to 2015 cm-' in CDsCN. (20)Electrochemical reductions of [Re(bpy)(CO)sCl] analogs have caused similar red shifts of v ( C ~ 0bands. ) See refs lg and 15a.
Electrochemical C02 Reduction in the Presence of CySI. The electrochemical C02 reduction by [21(PF6)2 using MeaBF4 in CH3CNDMSO affords CO, HCOO-, C&C(O)C&, and CH&(O)CH2COO- as described above. The similar electrochemical C02 reduction in the presence of CH31 (3 x M) also gave the latter three products, but CO evolution was completely depressed.23 Similarly, the electrochemical C02 reduction by [Ru(bpy)2(i-qu)(CO)I(PFs)2(i-qu = isoquinoline) (1 x lo-, M) in the presence of CH31(3 x M) and Me4NBF4 (5 x M) also generated essentially same amounts of those products without CO evolution under otherwise the same electrolysis conditions. The depression of CO evolution in those reactions probably results from the smooth reaction of CH3I with [Ru(bpy)2(L)(CO)lo(L = qu and i-qu) forming [Ru(bpy)2(L)(C(O)CH3)1+ (eq 8) as the precursor to CH3C(O)CH3prior t o the Ru-CO bond cleavage (eq 5). Small amounts of ethane evolution, however, were observed in the C02 reduction catalyzed by [Ru(bpy)2(i-qu)(CO)12+ in contrast to the COz reduction by [212+(Figure 6). In addition t o the recovery of [ R u ( ~ ~ ~ ) z ( L ) ( C H ~ C(L N )=] ~qu + and i-qu) after the electrochemical C02 reduction in the presence of CH3I (in ca. 70% yield after 60 C passed on the basis of the NMR spectra), the C2H6 formation depending on the substituent of [R~(bpy)2(L)(C0)]~+ may be related to the steric effects of those ligands and reasonably excludes the possibility of dissociation of the quinoline analogue ligands (L)and dechelation of the bpy ligands during the electrochemical C02 reduction. In the presence of CH31,[Ru(bpy)~(L)(solvent)1~ and/or [Ru(bpy)dL)l0would be expected to undergo a competitive electrophilic attack of CH31 and COS producing [Ru(bpy)z(L)(CH3)1+and [Ru(bpy)~(L)(17~(C)-CO~)l~ (eq 3a), respectively. Assum(21)Silverstein, R. M.,Bassler, G . C., Morril, T. C., Eds. Spectrometric Identification of Organic Compounds, 4th ed.; Wiley & Sons, Inc.: New York, 1981. (22)(a) Reference lo, p 40. (b) Belt, S. T.; Ryba, D. W.; Ford, P. C. J. Am. Chem. SOC. 1991,113, 9524. (c) Shafiq, F.;Kramarz, K. W.; Eisenberg, R. I n o g . Chim. Acta 1993,213, 111. (d) Brumbaugh, J. S.; Whittle, R. R.; Parvez, M.; Sen, A. Organometallics 1990,9, 1735. (23)The current efficiencies for CO, HCOO-, CHsC(O)CHs, and CH&(O)CH&OO- formation in the COz reduction in the presence of CH3I are not so improved compared with those in the absence of CHd because of large background currents of a n I d - redox reaction on the working and auxiliary electrodes.
5098 Organometallics, Vol. 14, No. 11, 1995
Nakajima et al. Scheme 2
15.
/
I
ea,
I I [Ru(~olvent)]~ or [Rul0
co
[Ru- CO]o
A
-1
/ A
Coulomb consumed / C Figure 6. Plots of the amounts of C2Hs evolved against the electricity consumed in the C02 reduction by [Ru(bpy)z-
M)in CH3(L)(CO)I(PF&(L = qu (0);i-qu (A)) (1 x CN/DMSO (1:l v/v) containing Me4NBF4(5 x 10+ M) and CH3I (3 x lo-' M). ing that the former is the precursor t o CZH6 no CZH6 evolution in the COS reduction by [212+ (Figure 6) implies a large steric hindrance of the qu ligand against the electrophilic attack of bulky CH31 on the metal center. Thus, the formation of CZH6 (Figure 6) is reasonably explained by the position of the phenyl groups of the qu and i-qu ligands in those complexes. Mechanism. The reaction of [11° with CO:! affording [212+and Cos2- (eq 3) possibly proceeds via [Ru(bpy)z(qu)(~,J(C)-C02)]and the subsequent oxide abstraction by COz. The reductive disproportionation of COZ affording CO and C032- (eq 5 ) in the electrochemical COZ reduction by [212+in the presence of LiBF4 also could be ascribed t o the participation of [11° and [Ru(bpy)z(qu)(r1(C)-C02)1in the catalytic cycle. It has been reported that two CO2 molecules react with Ir(C8H14)-
CH3C(O)CH3
+
2(CH3)3N or 21-
+
2e*
[Rul= [Ru(bpy)2(qu)l
BQN in the electrochemical COZreduction using BunD+ (eq 4) also reflects the strong basicity of [Ru(bpy)z(qu)(vl(C)-COz)l,and [Ru(bpy)~(qu)(C(O)OH)l+ may exist as the precursor for HCOO- formation, since [Ru(bpy)z(CO)(C(O)OH)I+formed by the protonation of [Ru(bpy)z(CO)(q1(C)-COz)1is known as the precursor for the HCOO- formation in photo- and electrochemical CO:! r e d u c t i ~ n s . ~ The * , ~ ~oxide transfer from [Ru(bpy)z(qu)(vl(C)-COz)lto COZregenerates [212+in the absence of a proton donor. The subsequent two-electron reduction of [212+under the electrolysis conditions induces the double methylation of the carbonyl ligand with M e a + and/or CH3I t o produce CH3C(O)CH3possibly via [Ru(bpy)z(qu)(C(O)CH3)1+(Scheme 2). As similar to the case for Bun4N+,the resulting CH3C(O)CH3 probably undergoes the abstraction of its a-proton by [Ru(bpy):!(qu>(q'(C)-COz)l,and consequently both CH~C(O)CHZCOO- and HCOO- would be formed under COZatmosphere (eq 9h2'
-
[Ru(bpy),(qu>(ql(C)-CO,)I + CH3C(O)CH3+ CO, [Ru(bpy)z(qu)(solvent)lz+ + HCOO- + I Cl(PMed3 and Mo(Nz):!(PMe3)4 to produce IrC(0)OCCH3C(0)CHzCOO- (9) 1 (O)O(Cl)(PMe& and MO(~~(O,O)-CO~)(CO)(PM~~)~, reThe present work describes the first catalytic formas p e c t i ~ e l y . Although ~ ~ ? ~ ~ the condensation and disprotion of CH3C(O)CH3 and CH3C(O)CHzCOO- by the portionation reactions of CO:! on those metal complexes double methylation of the carbonyl moiety resulting are feasible models for the catalytic reductive disprofrom the reductive disproportionation of COZfollowed portionation of COZ,at least two coordination sites are by the subsequent carboxylation in the electrochemical required for the attack of CO:! on the metal centers in COz reduction, which demonstrates the feasibility of those reactions. Taking into account the rigidity of the COz as a building block in organic synthesis. ambient ligands on the hexacoordinated [Ru(bpy)z(qu)(v1(C)-C02)land the strong basicity of the v1(C)-C0z Acknowledgment. This work is partially supported moiety compared with that of [Ru(bpy)~(C0)(~~(C)-COz)l, by JSPS Fellowships for Japanese Junior Scientists and the oxide transfer from [Ru(bpy):!(qu)(rl(C)-COz)lto a Grant-in-Aid for Scientific Research from the Ministry of Education of Japan. another CO2 may not involve an expansion of the coordination number of the complex. In accordance with OM950206M this, [w(cO)6]formation in the reaction of Liz[W(CO)5(vl(C)-CO:!)l with free C02 is explained in terms of a (26) (a) Ishida, H.; Tanaka, K.; Tanaka, T. Organometallics 1987, 6,181. (b) Ishida, H.; Terada, T.; Tanaka, T. Inorg. Chem. 1990,29, direct oxide transfer through a W-C(0)O-COzLi 905. intermediate.5a-c The formation of CZH~CH=CHZ and (27) On the basis of the ,preferential HCOO- formation in the (24) Herskkovitz, T.; Guggenberger, L.J. J.Am. Chem. SOC. 1978, 17, 1615. (25)Alvarez, R.;Atwood, J. L.; Carmona, E.; PBrez, P. J.; Poveda, M.L.;Rogers, R. D. Inorg. Chem. 1991,30, 1493.
electrochemical COz reduction by [212+in the presence of Bun4N+(eq 41, the discrepancy in the amount of CH&(O)CH&OO- and HCOOin Figure 3 may result from the participation of HzO involved in DMSO/ CHaCN (see ref 11) as well as the produced CH&(O)CH3 as proton sources for the formation of HCOO- in the COz reduction.
