J . Am. Chem. Soc. 1985, 107, 2005-2015 Second, the sterically hindered catalyst Mn(TMP)CI exhibits substantial shape selectivity in the catalytic epoxidation of olefins, and this is shown to be directly attributable to the differences in binding energies for formation of the intermediate. Our observations suggest a strategy for the development of other shapeselective, as well as asymmetric, porphyrin catalysts for olefin epoxidation. These findings in turn argue that the oxo-olefin complex does not contain carbon-based cationic or radicaloid centers. The formation of this complex is rather insensitive to electronic effects of the olefin, but it is markedly influenced by steric interactions when the more sterically demanding catalyst, Mn(TMP)Cl, is used. These conclusions are consistent with our postulate that the intermediate is a metallaoxetane, which we speculate is formed On ~ the basis by a concerted antarafacial 2 + 2 c y ~ l o a d d i t i o n . ~ of the evidence presented here, it appears the normal mode of decomposition of this species is a concerted reductive elimination. Complete substantiation of these postulates awaits direct structural and spectroscopic analysis of the intermediate. Such studies are currently being pursued in our laboratories. Finally, these studies have shed light on the general reactivity of high-valent manganese oxo porphyrins. At this point, the possible extension of this mechanism to other metals is not proven, (28) Halpern, J. Science 1982, 217, 401-407 and references therein. (29) The metallaoxetane species has also been considered by Groves as an intermediate in the FeTPPCl catalyzed oxygenation of cyclohexene with iodosylbenzene.8b
2005
but it seems plausible. In view of the similar reactivities of cytochrome P-450 and other model systems as evidenced by N the N I H shift,31and alkane hydroxylaand 0 demethylati~n,)~ tion,)* it would not be suprising to find other similar stereoselective olefin epoxidations occurring by this general route.
Acknowledgment. Support from the National Institutes of Health (Grant NIH GM17880-13,14) is gratefully acknowledged. The Nicolet NMC-300 spectrometer was purchased with funds from the National Science Foundation (Grant NSF CHE8 1-09064 to Stanford University). Support for Teruyuki Hayashi from the Science and Technology Agency of Japan and for Scott Raybuck from the Franklin Veatch Memorial Fellowship is also gratefully acknowledged. Registry No. Mn(TPP)CI, 32195-55-4; Mn(TMP)CI, 85939-49-7; lithium hypochlorite, 13840-33-0;cytochrome P-450, 9035-51-2;monooxygenase, 9038-14-6; cyclooctene, 93 1-88-4; (Z)-2-octene,13389-42-9; indene, 95-13-6; 1-ethenylbenzene,100-42-5; 1-methylcyclohex-I-ene, 591-49-1;( E ) -1-propenylbenzene,873-66-5;trans,trans,cis- 1,5,9-cyclododecatriene, 706-31-0; cis-stilbene, 645-49-8;( Z ) -1-propenylbenzene,
766-90-5; (E)-2-octene, 7642-04-8; (E)-4-octene, 14850-23-8; 1,3cyclohexadiene, 592-57-4; 1,3-cycloheptadiene,4054-38-0; 1,3-cyclooctadiene, 1700-10-3. (30) Shannon, P.; Bruice, T. C. J. Am. Chem. SOC. 1981,103,4580-4582. (31) Lindsay-Smith, J. R.; Piggott, R. E.; Steath, P. R. J . Chem. Soc., Chem. Commun. 1982, 55-56. (32) Groves, J. T.; Subramanian, D. V. J . Am. Chem. SOC.1984, 106, 2177-218 1.
Homogeneous Catalysis of the Photoreduction of Water. 6. Mediation by Polypyridine Complexes of Ruthenium( 11) and Cobalt( 11) in Alkaline Media' C. V. Krishnan, Bruce S. Brunschwig, Carol Creutz,* and Norman Sutin* Contribution from the Department of Chemistry, Brookhaven National Laboratory, Upton. New York 1 1 973. Received August 13, 1984
Abstract: The emission of (polypyridine)ruthenium(II) complexes (S) is quenched by (polypyridine)cobalt(II) (COL~~') complexes + respectively. via parallel oxidative, reductive, and energy-transfer paths, giving CoL3+ + S+, COLJ" + S-, and * C O L ~+~ S, The oxidative route provides the basis for a new water photoreduction sequence: in mixed acetonitrile-water solvents relatively high cage-escape yields of R~(4,7-(CH~)~phen))~+ and Co(bpy),+ (S = R~(4,7-(CH))~phen))~+, phen = 1,IO-phenanthroline, bpy = 2,2'-bipyridine) are obtained. The Ru(II1) complex is reduced by triethanolamine (TEOA), and the Co(bpy))+ reacts with water and/or TEOAH' to give HZ.The maximum H2 quantum yield obtained is 0.29 in 50% acetonitrile-water. Unusual features of the system are the fact that, at low TEOA, C ~ ( b p y ) , ~scavenging + of Ru(II1) reduces the H2yield and that the cage-escape and limiting H2 yields are strongly solvent dependent: for the R~(4,7-(CH~)~phen)~~+-Co(bpy),~+-TEOA system the H2 yield increases from -0.02 in H 2 0 to 0.29 in the mixed solvent. Reduction potentials for c0Lj3+/2+and COL,~+/+ couples are also reported.
Cobalt-polypyridine complexes are of interest in a number of contexts [L = polypyridine, Le., 1,lo-phenanthroline (phen) or 2,2'-bipyridine (bpy)]: the cobalt(1) complexes reduce water to Hz24 and COz to COS depending upon conditions. Since the (1) Part 5 in the series is ref 9a. (21 Krishnan. C. V.: Sutin. N. J . Am.
Chem. SOC.1981. 103. 2141 (3j Krishnan; C. V.;Creutz, C.; Mahajan, D.; Schwarz, H. A,'; Sutin, N. Isr. J . Chem. 1982, 22, 98. (4) Kirch, M.; Lehn, J.-M.; Sauvage, J. P. Helu. Chim. Acta 1979, 62, 1345. (5) Lehn, J.-M.; Ziessel, R. Proc. Nufl.Acud. Sci. U.S.A. 1982, 79, 701. 0002-7863/85/1507-2005$01.50/0
cobalt(1) species may be generated by the reduction of the corresponding cobalt(I1) or cobalt(II1) species, catalytic systems incorporating this chemistry are possible. When RuLJ2+complexes6-* are used as sensitizers, cobalt(1) complexes may be generated either via reductive (eq 2) or oxidative (eq 3) quenching of the excited state in the presence of an electron donor (D). (6) Lin, C.-T.; Bottcher, W.; Chou, M.; Creutz, C.; Sutin, N. J . Am. Chem. SOC.1976, 98, 6536. (7) Sutin, N.; Creutz, C. Adu. Chem. Ser. 1978, No. 168, 1. (8) Sutin, N. J . Photochem. 1979, 10, 19.
0 1985 American Chemical Society
Krishnan et al.
2006 J . Am. Chem. SOC.,Vol. 107, No. 7, 1985
-+ -
+ RuL3’+ *RuL3’+ *RUL,~++ D RuL3+ + D+ RuL~+ ~ +Co(1) RuL3+ + Co(I1) + Co(I1) R u L ~ +~ +Co(1) *RUL,~+ RuL~+ ~ +D RuL~+ ~ +D+ hu
(1) (2a) (2b) (3a) (3b)
In each case the net reaction is
hv
+ Co(I1)
D
Co(1)
+ D+
In earlier work, we found that a homogeneous system employing cobalt(I1)- and ruthenium(I1)-polypyridine complexes effected the photoreduction of water to hydrogen at pH 4-5 with visible light.2,3 Ascorbate ion was the electron donor and reduced *RuL:+ to RuL3+ (eq 2a); Co(bpy)?+ was reduced to Co(bpy),+ by RuL3+ (eq 2b) and Co(1) reacted with water or H30+via a cobalt(II1)-hydride9 to give H2. In the present work we have evaluated the rate constants for quenching of different (polypyridine)ruthenium(II) complexes by CoL2+ and C O L ~ ”as a function of L and found evidence that, depending on the sensitizer-quencher combination, reductive, oxidative, or energytransfer quenching may predominate. For the combination Ru(4,7-(CH3)2phen)32+-Co(bpy)32+ a new photoreduction system based on oxidative quenching of the ruthenium(I1) excited state has been characterized. In basic 50% aqueous acetonitrile with triethanolamine the electron donor in eq 3b, H2 production is limited only by the efficiency of cage escape in the quenching reaction (eq 3a). Experimental Section Materials. The (polypyridine)ruthenium(II) complexes were prepared as in an earlier study.6 All the ligands were from G. F. Smith and Fisher and were used without further purification. Cobalt(I1) sulfate was puratronic grade from Johnson Matthey. The solvents were of spectronic grade. The organic electron donors were from Aldrich and were used without further purification. The diquat dibromide and the tetraalkylammonium salts were from earlier Emission Intensity Measurement. The emission from the (polypyridine)ruthenium(II) complexes were measured as described previously.6 Solutions in 1 X 1 cm cells were deaerated for about 20 min with argon and then excited at the absorption maximum of the ruthenium(I1) complex around 450 nm. The emission intensities were measured at -600-630 nm in the energy mode. Neutral density filters were used to reduce the incident light intensity. Stern-Volmer constants (Ksv = [ I o / l - l]/[Q]) were obtained from plots of the emission intensity data as a function of the concentration of the quencher, Q. For quenching by the cobalt-bipyridine complexes, the emission intensities at 610 nm were corrected for absorption of the incident excitation light at 450 nm. Quenching rate constants were calculated from k, = Ksv/ro,where ro is the excited-state lifetime in the absence of added quencher.6 Cyclic Voltammetry. A Princeton Applied Research system consisting of a Model 173 potentiostat and a Model 175 universal programmer was employed in these studies. Sweep rates of 20-500 mV s-I were used and the cyclic voltammograms were recorded on an x-y recorder. Most of the determinations were carried out in acetonitrile in a cell containing a platinum working electrode and a platinum wire auxiliary electrode. A saturated calomel electrode was used as the reference electrode. The solutions were deaerated with argon. For cobalt-bipyridine and related complexes a Co(I1):ligand ratio of 1:5 was used. Some measurements on cobalt(II1) complexes were also performed. In addition, several of the COL,~+ or CoL3’+ complexes were studied in aqueous media. In these experiments a glassy carbon electrode (PAR) was the working electrode and the medium was 0.16 M Na2S04containing 0.02 M 2-aminoethanol buffer (pH 9.6). Continuous Photolysis. The photolysis system consisted of a 450-W xenon lamp, focusing lenses, UV cutoff filters, and a thermostated bath for the photolysis vessel. In the hydrogen generation studies 25-mL solutions were stirred continuously and irradiated in 2 X 2 cm square cells after deaerating for about 30 min with argon. The volume of gas pro(9) (a) Creutz, C.; Schwarz, H. A,; Sutin, N. J . Am. Chem. SOC.1984, Schwarz, H. A.; Creutz, C.; Sutin, N. Inorg. Chem. 1985, 24,
106, 3036. (b) 433.
(10) Creutz, C.; Keller, A. D.; Sutin, N.; Zipp, A. P. J . Am. Chem. SOC.
1982, 104, 3618.
(11) Krishnan, C. V.;Friedman, H. J . Phys. Chem. 1969, 73, 3934.
duced was measured as a function of time by a volumeter attachment or analyzed on a Varian 1400 gas chromatograph. Light intensities were determined by Ru(bpy)32+/Co(NH3)5C12+ actinometry3and were typically 3 x einstein min-’. Flash Photolysis. The yield of separated electron-transfer products Ye,was determined by using a frequency-doubled Nd laser as excitation source.6 The absorbance changes occurring during and after the 25-11s 530-nm pulse were monitored at 420-450 nm and comparedI2with those F ~ ~ +in 0.5 M H2S04, which was asproduced in the R U L ~ ~ + - system sumed to produce separated RuL,~+and Fe2+in 100% yield. Rate constants for R~L~~*-Co(bpy)?+ and RuLp3+-TEOAreactions were determined by using a Phase-R Model DL-1100 dye laser as excitation source.12 For both reactions the quenching of *RuL?+ (0.1 mM) by C~(bpy;?~+ (1 mM) was used to generate RuL,~+(and C~(bpy),~+). The RuL, reduction was studied at 420-450 nm under pseudo-firstorder conditions, with sufficient Co(bpy),Z+(0.246 mM) or TEOA (2-6 mM) being added so that the rate of the “primary”R~L~~+-Co(bpy),*+ back-reaction was negligible. Results Reduction potentials determined for COL33f/2+couples in either acetonitrile or water are summarized in Table I. Peak separations E,, - E , were 70-110 mV, depending upon the sweep rate (20-200 mV/s) and the couple. Since all the systems were chemically reversible, the average of E , and E,, is reported as Data for CoL,3’+/+ couples in the two solvents are also included in the table. These couples were electrochemically well-behaved in acetonitrile, exhibiting equal currents for cathodic and anodic peaks and peak separations of 60-100 mV. In water, however, large anodic spikes 40-130-mV positive of E , were observed for all five complexes investigated. In addition, for the phen derivatives, the presence of excess L caused coating of the electrode during the -1 to -1.5-V portion of the sweep. Thus the measurements were performed with 3:l L:Co(II) ratios or with the C0L33f complex. Frequent polishing of the carbon working electrode was essential. The numbers reported are obtained from E, (at 50 mV/s scan rate) but are not regarded as true values since the couples are not well-behaved in water and since the values (especially for the phen derivatives) do not change sufficiently with L. Thus the acetonitrile data are used in subsequent discussions. Rate constants for quenching of * R u L ~ emission ~+ by CoL2+ and C O L ~ ”complexes are presented in Tables I1 and 111, respectively. In Table IV rate constants for the quenching of * R u L ~ and ~ + * O S L ~ emission ~+ by C O L ~ ”complexes are sum+,COL~~+ marized. The dependence of k , for CoL2+,C O L ~ ~and on the nature of supporting electrolyte anion is shown in Table V. Rate constants for quenching * R ~ ( 4 , 4 ’ - ( C H ~ ) ~ b p y ) ~ ~ + emission by diquat dibromide as a function of supporting electrolyte cation are listed in the supplementary material, Table I. Triethanolamine does not detectably quench R ~ ( b p y ) , ~or+ Ru(4,7-(CH,),~hen),~+ emission; Le., for these sensitizers k , < lo5 M-1 s-l Rate constants for reduction of R u L , ~ +by C ~ ( b p y ) , ~(de+ termined by flash photolysis) are presented in Table VI. The rate constant for reduction of R ~ ( 4 , 7 - ( C H ~ ) ~ p h e nby) ~tri~+ ethanolamine is 5.2 X lo6 M-I s-I at 25 OC in 50% aqueous acetonitrile containing 0.25 M LiCl. Water Photoreduction. In preliminary experiments (supplementary material, Tables 11-VIII) it was established that H 2 is produced from R U L ~ ~ + / C O Lmixtures ?+ at pH > 7 only in the presence of rather high concentrations of Co(I1) (>5 X M) and reducing agent. Among organic amines investigated as electron donors, triethanolamine (TEOA) was superior. The sensitizer R~(4,7-(CH,),phen)~~+ was effective in purely aqueous media (and the best in 50% aqueous acetonitrile) and found to be extremely stable under irradiation under both H,-generating and nongenerating conditions. Addition of acetonitrile to mixtures such as those used in the above experiments considerably increased the H2 production rates (% CH3CN by volume, relative H2 production rate with 2.5 X M R~(4,7-(CH,),phen)~~+, 0.01 (12) Mok, C.-Y.;Zanella,A. W.; Creutz, C.; Sutin, N. Inorg. Chem. 1984, 23, 2891.
Homogeneous Catalysis of the Photoreduction of Water
J . Am. Chem. SOC.,Vol. 107, No. 7, 1985 2007
Table I. Reduction Potentials E1,,(V) for c0L,3++/2+ and COL,~+/+ Couples at -22 OC C0L33+12c COL,~+/+ CH,CNO~~ H2O' CH3CN'sb H20" 0.30 0.30,' 0.304: 0.309 -0.98 -0.89,'-0.95g bPY -1.10 -1.0: -1.039 4,4'-(CHd,bp~ 0.18 0.17,c 0.169 5-Clphen 0.48' 0.509 -0.86' -0.828 phen 0.37 0.36: 0.38,d 0.378 -0.96 --0.8,d -0.849 4,7-(CH3),phen 0.18 0.19,c 0.26,d 0.178 -1.10 -0.829 -1.16 2,9-(CH,),phed 0.6e 0.6C3e OThe average of E,, and E,, determined at 20-200 mV s-l sweep rates. 'Measured in acetonitrile containing 0.1 M tetraethylammonium or tetrabutylammonium perchlorate; Pt working electrode, SCE reference electrode. 'Measured in aqueous 0.166 M Na2S04 (0.01 M H2S04) with a glassy carbon working electrode. Value vs. N H E . dFor 0.5 M H,SO,, reported by: Chen, Y . W.; Santhanam, K. S. V.; Bard, A. J. J . Electrochem. SOC.1982, 229, 61. e E , for CoL,,'; irreversible. /Reference 3, determined in carbonate buffer, vs. N H E . SMeasured with a glassy carbon working electrode in 0.16 M N a 2 S 0 4 containing 0.02 M 2-aminoethanol buffer, pH 9.6; vs. N H E . hLarge anodic current spikes are observed for all L reported. The number reported is calculated from E, measured at 50 mV s-I by using El,, = E , + 0.03 + 0.242 where 0.03 V assumes electrochemical reversibility (AEp = 60 mV) and 0.242 is the correction for the SCE reference electrode. 'Mahajan, D.; Zanella, A. W., unpublished observations. 'For steric reasons, these complexes may have the formula CoL,"' rather than CoLS"+. L
Table 11. Rate Constants k, (XIOB,M-I s-I) for the Quenching of (Polypyridine)ruthenium(II) Emission by CoL2+ at 25 O C , 0.05 M Ionic Strength, and pH 6.0' sensitizer
L 5-Brphen 5-Clphen phen 5-(CH3)phen 5,6-(CH3),phen 4,7-(CH3),phen 2,9-(CH3),phen bPY 4,4'-(CH,hb~~
RU(~PY)~~+
Ru(S-Clphen),*+
Ru(phen),,+
0.23 0.27
1.7 0.64
1.9 0.9
0.7
2.4
2.7
0.55
0.27
0.34
R~(4,7-(CH~),(phen)~~+ 3.4 2.8 1.6 2.0 3.0 4.3 5.4 0.29 0.74
"Co(1I):L is 4.54:l; 0.05 M phosphate buffer.
Table 111. Rate Constants k, (X108, M-l s-]) for the Quenching of (Polypyridine)ruthenium(II) Emission by CoL3,' at 25 "C, -0.075 M Ionic Strength, and pH 8.0' sensitizer L
R~(~PY),~+
R~(5-Clphen),~+
0.28 bPY 0.63 5-Clphen 0.30 phen 0.74 4.7-(CH2Lphen "Co(1I):L is 1:5; 0.05 M phosphate buffer.
R~(4,7-(CH,),(phen),~+ 1.oo 1.36 1.14 1.39
0.63 1.24 0.77 1.05
Table V. Rate Constants k , (X109, M-'
Table IV. *ML,,+ Redox Potentials and Rate Constants for Quenching of ML,,+ Emission by Co(bpy),2* at 0.5 M Ionic Strength and 25 O C " 10-9k,, M-1 s21 *E2,1°qbV *E,,,',' 1.32 1.oo -0.77 0.63 0.84 -0.84 0.55 0.72 -0.85 0.97 0.79 -0.87 1.15 0.89 -0.90 1.22 0.69 -0.94 0.57 -0.96 0.74 0.67 -1.01 1.63 1.48 0.50 -1.03 "Conditions: 0.16 M Na,SO,, pH 7.8, phosphate buffer. CoSO, is 5 : l . *Data taken from ref 6 and 7.
Ru( phen)
0.79 1.25 0.98 1.20
bpy:
M C0S04, 0.02 M bpy, 1.0 M TEOA-HC1, pH 8.1): 0%, 1; 30%, 5.6; 50%, 14. With 70%acetonitrile the mixture separated into two phases. Consequently 50% acetonitrile was used in subsequent experiments. Supplementary material Tables VI-VI11 summarize preliminary results bearing on the dependence of H, production rate on TEOA, Co(II), and bpy concentrations and on pH for aqueous and 50% aqueous acetonitrile solutions. In Tables VI1 and VI11 the dependence of the quantum yield for H, production on TEOA and C ~ ( b p y ) , ~concentration + and pH in 50% aqueous acetonitrile are presented. The dependence of the H, quantum M) yield on the nature of the R u L , ~ +sensitizer ((2-3) X in the presence of 5 X M C ~ ( b p y ) , ~and + 1.0 M tri-
SKI ) for Quenching of *Ru(bpy)32+Emission as a Function of Supporting Electrolyte at 25 OC and 0.5 M Ionic Strength" quencher
medium Co(bPY)2+ Co(bPY)32+ Co(bPY),3+ NaF 0.043 (0.096b) NaCl 0.047 (0.15') 1.01c 2.95 NaBr 0.052 (0.18') 1.35 3.64 Na2S04 0.042 (O.lOb) 0.68 2.08 'Solutions contained 0.5 M NaX (0.17 M Na,SO,) and 0.025 M phosphate buffer at pH 6.0 for Co(bpy)2+ and pH 8.0 for C ~ ( b p y ) , ~ + . Sensitizer is R~(4,7-(CH,),phen),~+. Measurements of the ionic strength dependence of k, with NaCl supporting electrolyte yielded the following data [ p (M), 10-9k, (M-' s-l)]: 0.03, 0.28; 0.28, 0.64; 0.78; 1.0; 1.3; 1.5, 1.2.
'
Table VI. Rate Constants k (X108, M-I
) for the Reduction of
SKI
RuL,j+ bv Co(buvl,2+ ~
~
~~
~
medium 50% aq CH,CN (0.25 M LiCI) water (0.25 M LiCI) water (0.166 M Na2S04)
phen 0.48
L 4,7(CH,),phen 0.22
1.9
3.1
2.5
5.1
3,4,7,8(CH,),phen 0.20
2008
Krishnan et al.
J . Am. Chem. Soc., Vol. 107, No. 7, 1985
z
Table VII. Variation of H 2 Production Yields with C ~ ( b p y ) , ~and + TEOA Concentrations (50% aq CH3CN, fi = 0.25 M LiCI) at 25 O c a [TEOAI, M [ C O ( ~ P Y ) ~M ~+I, bH, 0.10 0.04 0.0067 0.10 0.03 0.0086 0.10 0.02 0.021 0.25 0.03 0.036 0.25 0.01 0.092 0.50 0.03 0.080 0.50 0.01 0.17 0.50 0.0075 0.18 0.50 0.005 0.20 Irradiation of 2.5 X lo4 M R~(4,7-(CH,),phen),~+with visible light (A L 400 nm);pH 8.0 f 0.1. Values have been corrected for the extent of quenching by the Co(bpy),2t (KsV = 1390 M-I). H 2 evolution rates were 0.4-10 mL/h with 20 mL of solution. Typically a brief induction period due to residual O2 in the solution was followed by production of H 2 at a constant rate; bH2values tabulated are calculated from the slope of moles of H2 vs. time plots for the latter region. (1
0.0
'
I
-0.5
0.5
0 AEO(RU
( m ) + c o ( ~ v) ) ,
Figure 1. Plot of the driving force for reductive quenching (eq 5) vs. the driving force for oxidative quenching (eq 3) with RUL,~+as sensitizer and C ~ ( b p y ) , ~as + quencher.
Table VIII. H, Yields as a Function of oHQ PH [TEOA] /[Co(II)]* bHIC 7.46 12 0.11 7.80 21 0.14 29 0.17 8.1 1 8.38 36 0.19 8.75 43 0.16 'The solution contained 2.5 X M R ~ ( 4 , 7 - ( C H , ) ~ p h e n ? ~0.01 ~+, M C ~ ( b p y ) , ~ +and , 0.5 M total TEOA adjusted to the pH given with HCI before addition of acetonitrile (final 50% aqueous acetonitrile). "he pK, value 7.95 was used in calculating [TEOA]. CCorrectedfor + = 1390 M-l). *RuL,~+not quenched by C ~ ( b p y ) , ~(Ksv
Pable IX. COL,~+I~'/+ Potentials (vs. N H E ) Appropriate to Aqueous Solution -25 OC
L bPY (CH3)2bpy 5-Clphen phen 4,7-(CH3)2phen (H,O),
E3,2',
0.30 0.16 0.50 0.37 0.19 1.9
V
E2.i0, V
-0.95 -1.07 -0.83 -0.93 -1.07 -1.44
AEO, V 1.25 1.25 1.31 1.30 1.26 3.34
E,,,', V -0.33 -0.45 -0.18 -0.29 -0.44 +0.23
ethanolamine at pH 8.4 and 25 OC. The quantum yields determined are 0.12, 0.028, 0.23, and 0.23 for L = 5-Clphen, phen, 4,7-(CH3)2phen, and 3,4,7,8-(CH3)4phen, respectively. Discussion COL~"'Reduction Potentials. It is interesting that c0L33+/2+ potentials vs. N H E in water (0.16 M Na2S04)and vs. SCE in acetonitrile (0.1 M (R4N)C104) (Table I) are identical within experimental error (fO.O1 V). As mentioned earlier, none of the C O L ~ ~ +couples /+ studied was electrochemically well-behaved in aqueous media. To estimate the C O L ~ ~ +E" / +values in water we use the acetonitrile E , , , values corrected for liquid junction xmtributions as reflected in the of the ferrocene-ferrocinium zouple,I3 Le., Eaqo(vs.NHE) = EAN0(vs.SCE) 0.026 V. The Co(II1)-Co(I1) and Co(I1)-Co(1) Eo values thus obtained are compiled in Table IX, where calculated Co(III)-Co(I) reduction potentials are also presented. The values are in good agreement with previously reported values in comparable media.14 For the polypyridine complexes, E3,Z0 ranges between +O. 16 and +OS0 V, between -0.83 and -1.07 V, and E3,I0 between -0.18 and -0.45 V. In general the three potentials parallel each other with the difference between E3,Z0 and E2,10being 1.25 f 0.05 V
+
(13) Sahami, S.; Weaver, M. J. Electroanal. Chem. 1981, 122, 155. (14) (a) Chen, Y.W.; Santhanam, K. S.V.; Bard, A. J. J . Electrochem. SOC. 1982, 129, 61. (b) Rao, J. M.; Hughes, M. C.; Macero, D. J. Inorg. Chim. Acta 1979, 35, L369. (c) Margel, S.; Smith, W.; Anson, F. C . J. Electrochem. SOC.1978, 125, 241. (d) Prasad, R.; Sciafe, D. B. J . Elec'roanal. Chem. Interfacial Electrochem. 1977, 84, 873.
(for the ruthenium series, the corresponding potential difference is 2.52 f 0.04 V, but the 2+/+ reduction process involves ligand rather than metal reduction'). As is usually found for 1,lOphenanthroline and 2,2'-bipyridine, the thermodynamic properties of the complexes strongly correlate with the basicities of the free ligands (and therefore with each other). It is noteworthy that in the COL~"'series the polypyridines stabilize both Co(II1) and Co(1) with respect to Co(II), with the relative stabilization of Co(II1) being somewhat greater, e.g., for eq 4, KrII= 1.3 X M-3, K,, = 6.6 X 1015 M-3, and KI = 4.0 X M-3. CO,
