J . Am. Chem. SOC.1983, 105, 5617-5623
r d
1.0-
PY
Figure 11. Electronic absorption spectra of (TPP)Fe(NO) in various solvents containing 0.1 M TBAP: (a) CH2CI2,(b) PhCN, (c) Me2S0,
(4 PY. remain almost invariant in the nondissociating solvents of classes I and 11. Also, it is of interest to note from Tables 11 and I11 that a larger solvent effect is noted for (TPP)Fe(NO)(S) than for (OEP) Fe( NO) (S). Potentials for the first reduction of (TPP)Fe(NO) and (OEP)Fe(NO) can be compared to the potentials for (TPP)Fe and (0EP)Fe reduction in a number of solvents. An earlier study of solvent effects on the redox reactions of (TPP)Fe(S) and
5617
(TPP)Fe(S), showed that E l / , (when corrected for liquid junction potential) was virtually independent of s01vent.l~ This is not the case for (TPP)Fe(NO)(S) and (OEP)Fe(NO)(S). Binding of a solvent molecule results in an anodic shift of potential from that of (TPP)Fe(NO) or (OEP)Fe(NO) with the magnitude of the shift being dependent upon the solvent DN. The maximum potential difference of this shift is 150 mV between (TPP)Fe(NO) and (TPP)Fe(NO)(S) (where S = DMA or Me,SO). A similar anodic shift is also observed for (OEP)Fe(NO)(S), but in this case the anodic shift was smaller in magnitude. In conclusion, we have shown that the binding of one or two nitrosyl ligands to (TPP)Fe and (0EP)Fe generate dramatic shifts of redox potentials for oxidation and reduction of the stable Fe(I1) complexes. For the case of (TPP)Fe(NO), the magnitude of shift in the Fe(III)/Fe(II) reaction with respect to (TPP)FeCl is approximately 1.O V, independent of solvent. Similar values of L W , , ~ are also observed between (OEP)Fe(NO) and (OEP)FeCl, but an even greater potential shift is observed after formation of (OEP)Fe(NO),, which is oxidized at a potential 200 mV anodic of (OEP)Fe(NO). These shifts of potential are comparable to those induced by the binding of CO to F ~ ( I I ) , ' but v ~ ~with NO as an axial ligand, stable oxidized complexes may be obtained. This is not true with CO, which does not bind to Fe(II1). In this paper we have also spectrally identified the first reduced complex of Fe(I1) with a bound axial ligand, [PorFe(NO)]-'. Similar spectral identifications have been obtained for [PorFe(NO)]+and [PorFe(NO),]+. These spectral and electrochemical characterizations of model Fe(I1) complexes that have been oxidized or reduced are of importance in understanding the redox reactions of heme proteins with small diatomic molecules such as NO, CO, or 0,. Acknowledgment. We are grateful for the financial support of this work from the National Institutes of Health (Grant G M 25172). We also with to thank Dr. S. Kelly for helpful discussions and R. Wilkins for making speciality electrochemical cells. Registry No. (TPP)Fe(NO),52674-29-0;(OEP)Fe(NO),5591 7-58-3; (TPP)Fe(N0)2, 53637-75-5; (OEP)Fe(NO),, 68879-28-7. (39) Buchler, W. J.; Kokisch, W.; Smith, P. D. Slruct. Bonding (Berlin) 1978, 34, 79.
Reduction Potentials for 2,2'-Bipyridine and 1,lO-Phenanthroline Couples in Aqueous Solutions C. V. Krishnan, Carol Creutz,* Harold A. Schwarz, and Norman Sutin Contribution from the Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973. Received February 7, 1983 Abstract: From pulse-radiolysis studies of 2,2'-bipyridine (bpy), 1,lO-phenanthroline(phen), and 4,4'-dimethyL2,2'-bipyridine ((CH,),bpy) and an analysis of the pH and free-energy dependence of the rate constants for quenching of poly(pyridine)ruthenium(I1) excited states by the above aromatic amines (L) and their protonated counterparts (LH', LH22+),the following parameters concerning L and its one-electron reduction product L-. have been deduced at 25 OC: for L = bpy, P K , ( L H ~ ~ + ) = 0.05, pKa(LH+) = 4.4, pKa(LH2+.)= 8.0, pKa(LH.) N 24, E0(LH2*+-LH2+.)= -0.50 V, Eo(LH+-LH.) = -0.97 V; for L = phen, PK,(LH,~+) = -0.2, pKa(LH+) = 4.9, pKa(LH.) e 24, Eo(LH22+-LH2+.)= -0.47 V, Eo(LH+-LH.) = -0.85 V; and for L = (CH,),bpy, PK,(LH~~+) = 0.68, pK,(LH,+.) = 9.3, pKa(LH.) 2 5 , Eo(LH22f-LH2C.)= -0.54 V, EO(LH+-LH.) = -1.05 V. The reduction potentials and other electron-transfer parameters of these and related couples are discussed.
The one-electron reduction of 2,2'-bipyridine (bpy) to bpy-. is Polarographic data have been reported for the one-electron process 2.13 V vs. aqueous SCE) electrochemically reversible ( E , in water,6 but the interpretation of these data is complicated by in acetonitrileLand N,N-dimethylfiorlaiide,2 and the two-electron reduction process has been characterized in aqueous n ~ e d i a . ~ - ~ (3) Erhard, H.; Jaenicke, W. J . Electroanal. Chem. 1975, 65, 675. (1) Tokel-Takvoryan, N. E.; Hemingway, R. E.; Bard, A. Sot. 1973, 95, 6582.
J. J . Am. Chem.
( 2 ) Saji, T.; Aoyagui, S. J . Electroanal. Chem. 1975, 63, 3 1 .
0002-7863/83/1505-5617$01.50/0
(4) Erhard, H.; Jaenicke, W. J . Electroanal. Chem. 1977, 81, 79. (5) Erhard, H.; Jaenicke, W. J . Electroanal. Chem. 1977, 81, 89. (6) Gurtler, 0.;Dietz, K. P.; Thomas, P. Z . Anorg. Allg. Chem. 1973, 398, 217.
0 1983 American Chemical Society
5618 J. Am. Chem. SOC.,Vol. 105, No. 17, 1983
Krishnan et al. l
200
F?] -
.
NoCI/HCI
i
I
800
i
t
rI
NotiSOq/ Na2S04
I
2
0
6
4
8
PH
Stern-Volmer constants for quenching of *Ru(4,4’(CH3)2bpy)32+ emission by diquat as a function of pH in chloride and sulfate media at 0.5 M ionic strength and 25 O C . Figure 2.
with a Perkin-Elmer Hitachi spectrofluorimeter as a function of quencher Q (bpy, phen, etc.) concentration (excitation wavelength 430-470 nm, emission wavelength 600-630 nm, [ R U L ~ ~=+ ]3 X 10” M, [Q] = (0.5-5) X M, p = 0.5 M with NaCl/HCI or Na2SO4/NaHSO4). The compositions of the buffered solutions used are given in supplementary Table 1. Quenching rate constants k, were calculated from k , = K s v / i o where T~ is the excited state lifetime in the absence of added quencher (see ref 10). Flash-photolysis measurements were made as I I i 0 described in ref IO and pulse-radiolysis experiments as described in ref 0 2.0 40 60 14. pH measurements were made with glass combination electrodes on PH Metrohm or Beckman pH meters; commercial buffers were used in Figure 1. Stern-Volmer constant for quenching of *Ru(4,7calibrating the electrodes. Absorbances were measured with Cary 17, (CH3)2phen)?+(circles, i o = 1.74 X 10” s) and *R~(5,6-(CH,),phen)~~+ 210, and 219 spectrophotometers. (squares, T~ = 1.8 1 X IO” s) emission by 2,2’-bipyridine as a function of pH in 0.5 M ionic strength sulfate-bisulfate media. Results pH Dependence of the Emission Quenching. This study was the fact that the one-electron reduction product bpyH. (or bpyH2+. undertaken in order to gain information about the properties of depending upon the pH) undergoes a rapid disproportionation the bpyH+-bpyH. and other LH+-LH. couples in aqueous solureaction.’ Equations 1 and 2 describe the sequence in basic tion. Interpretation of the quenching measurements made under solution. This disproportionation (and the low reduction potentials a given set of conditions is, however, complicated by the fact that the diprotonated species, e.g., b ~ y H ~are ~ +intrinsically , better + H+ bpy ebPYH. quenchers than the corresponding monoprotonated species and may be responsible for most of the quenching even when LH+ 2bpyH. bpyH2 + bpy (2) is the dominant form of L present. This effect is illustrated in Figure 1 where the pH dependence of the quenching of two of the one-electron couples) renders direct measurement of the different sensitizers is contrasted for L = bpy. The pH dependence reduction potentials of the bpyH+-bpyH. and related couples very arises through protonation equilibria and the parallel quenching difficult in aqueous media. An understanding of these thermopaths shown in eq 3-7. (The quenching mechanism written is dynamics is, however, important in water photoreduction systems * ~ ~ bpy reduction can occur at in which bpy is a ~ o m p o n e n tsince bpyHz2+ bpyHf Ht K,, (3) the expense of H 2 0 reduction. Here we have used the pulseradiolysis technique and the well-characterized R u L ~ ~ + - * R u L ~ ~ + bpyH+ ==bpy H+ Ka2 (4) couple^'^'^ (L is a 2,2’-bipyridine or 1,lo-phenanthroline deriv* R u L ~ ~ +bpyH22+ R u L ~ ~ ’ bpyH2+. k , (5) ative, * R u L ~ denotes ~+ the luminescent excited state of R u L ~ ~ + ) to estimate the reduction potential and self-exchange rate of the *RuL~+ ~ +bpyH+ R u L ~+ ~ +bpyH* kb (6) bpyH+-bpyH. couple. The latter, taken with pK, data we have determined and literature Eo data, have in turn been used to *RuL,~++ bpy kc (7) estimate the Eo values for the bpyH22+-bpyH2+. and bpyH+justified under Discussion.) For quenching of the *Ru(4,7bpyH. couples. In addition, data pertinent to the related 4,4’(CH,),~hen),~+ emission (circles) kb and k , are comparable, but dimethyl-2,2’-bipyridine ((CH,),bpy) and 1,lO-phenantholine for * R ~ ( 5 , 6 ( C H ~ ) ~ p h e(squares) n ) ~ ~ + k , >> kb. Even at pH 2.7 (phen) systems are also reported. where >99% of the total bpy is present as bpyH+ about 20% of Experimental Section the quenching arises through eq 5 . At the onset of this work the quenching measurements were The R u L , ~ +samples were prepared as in ref 10; 2,2’-bipyridine and made in sulfate media because the quenchers and sensitizers are 1,IO-phenanthroline were obtained from Fischer and 4,4’-dimethyl2,2’-bipyridine from G. F. Smith. Stern-Volmer constants (Ksv= [ ( I o / I ) both highly soluble in S04-/HS04-. However, because of the need - l]/[Q]) were obtained from emission intensity (0 datal0 determined for p H variations below pH 2 the HS04-/S04*-equilibrium (pK,(HSO,-) = -1.33 at 0.5 M ionic strength, 25 O C L S ) was recognized as a problem: the rate constants for the quenching (7) Mulazzani, Q. G.; Emmi, S.; Fuwhi, P. G.; Venturi, M.; Hoffman, M. reactions, which involve the encounter of two cations (eq 5 and 2.;Simic, M. G. J . Phys. Chem. 1979, 83, 1582. (8) Chan, S. F.; Chou, M.; Creutz, C.; Matsubara, T.; Sutin, N. J . Am. 6), are susceptible to specific anion effects. This is illustrated in Chem. SOC.1981, 103, 369. Figure 2 where the experimental Stern-Volmer constants for (9) Krishnan, C. V.; Creutz, C.; Mahajan, D.; Schwarz, H . A,; Sutin, N. quenching of the * R ~ ( ( c H ~ ) ~ b p yemission ) , ~ + by diquat are Isr. J . Chem. 1982, 22, 98. plotted as a function of pH for both chloride and sulfate media. (10) Lin, C-T.; Bottcher, W.; Chou, M.; Creutz, C.; Sutin, N . J . Am. Chem. SOC.1976, 98, 6536. The chloride values are p H independent, as is expected since (11) Sutin, N.; Creutz, C. Ado. Chem. Ser. 1978, No. 168, 1.
+
-
+
+
+
-
+
-
+
(12) Sutin, N. J . Photochem. 1979, 10, 19. (13) Creutz, C.; Keller, A. D.; Sutin, N.; Zipp, A. P. J . Am. Chem. SOC. 1982, 104, 3618.
(14) Schwarz, H. A,; Creutz, C. Inorg. Chem. 1983, 22, 707. (15) Dunsmore, H. S ; Nancollas, G. H. J . Phys. Chem. 1964, 68, 1579.
J . A m . Chem. SOC.,Vol. 105, No. 17, 1983 5619
Reduction Potentials for bpy and phen
Q-Q \
/
CHz-CH2
2+
?Oo o z
neither diquat nor the sensitizer undergoes protonation in this pH range. By contrast the sulfate/bisulfate data are pH independent above pH -2 but K,, increases 15% at low pH. This increase is due to the change of medium from to HS04-. Indeed, the pH dependence of K,, suggests that the pKa for HSO, is 1.4 f 0.1, in rather good agreement with the literature value 1.33.15 In order to simplify the data analysis the p H dependences were carried out in chloride media as well, and Figure 3 contrasts the low pH behavior of some typical sensitizer-quencher combinations. For data treatment eq 8, which arises from eq 3-7, was used.
-
k, =
ka
+ kd(a\/[H+I + kcKa\Ka2/[H+12 + Ka&a2/[H+l2
1 + Ka,/[H+I
i
(8) 5 0 0 L b
With bpy as quencher the best fits were obtained with pKaz = 4.4, which is in good agreement with the recommended value 4.33 f 0.05.16 Problems arose, however, with pKal: reported values range from -0.2 to -0).516while we obtained the best fits (five sensitizers) with pKal = 0.05. With pKal = 0.05, 100% bpyH?+ cannot be produced in the 0.5 M ionic strength media used since the minimum pH obtainable is -0.3. Thus direct confirmation of the bpy pKal value proved impossible. The validity of the bpy pKal value extracted from quenching measurements is, however, supported by results for the dimethylbipyridine system in which pKal is greater and therefore more readily measured (pK,, = 5.516). For (CH,),bpyH?+ near identical pKa, values were obtained from fits to the quenching data (pKal = 0.68) and from UV spectral measurements (pKal = 0.64 f 0.15; for LH+ €288 = 1.20 X lo4 M-' cm-' and €299 = 1.53 X lo4 M-'cm-I; for LH22+€288 = 1.53 X lo4 M-I cm-I and €299 = 1.10 X 1O4 M-' cm-I ). (No literature pKal value is reported for this amine.) In fitting the pH dependences, pKa2= 4.4 and pKal = 0.05 were used for bpy as quencher. The rate constants obtained are summarized in Tables I and 11. With R ~ ( 4 , 7 - ( C H , ) ~ p h e n ) , ~k,+ , = 4.3 X lo7 M-' s-l; for the other sensitizers k, < 1 X lo7 M-I s-I. Also included in Table I1 are rate constants for quenching by diquat and by (CH3)2bpyH2Z+ calculated with pKa, = 0.68. Except in the case of R ~ ( 4 , 7 - ( C H , ) ~ p h e n ) ,and ~ + (CH3)2bpyH+ for which kb = 1.2 X lo8 M-Is-I (pKaz = 5 . 5 9 , no quenching by (CH3)2bpyH+or by (CH,),bpy was detected (kb, k, -2 V),I and the protonated species are presumably even more difficult (18) Elliott, C. M.; Hersenhart, E. J. J . A m . Cfiem.SOC.1982, 104, 7519. (19) Creutz, C. Comments Inorg. Cfiem. 1982, I , 293.
5622 J . Am. Chem. SOC.,Vol. 105, No. 17, 1983
Krishnan et ai. I
Scheme 111
R~L:+
t
a
RUL~'+(C)
to oxidize; the bpy-bpy+. potential when bpy is bound to +3 metal centers is -+2.8 V.20 Since the * R U L ~ ~ + - R U L reduction ~+ potentials relevant to eq 15 range only up to -+1 V," it is apparent that eq 15 is likely to be unfavorable by 2 1 eV for the systems studied here. In addition, energy-transfer quenching, eq 16, is expected to be slow because of the high excited-state energies of the aromatic amines2' By contrast, oxidative quenching, eq 17, is thermodynamically feasible for many of the sensitizerquencher pairs studied. The RuL~~+-*RuL?+ reduction potentials range from -0.77 to -1.1 V and are listed as a function of L in Tables I and 11. Thus quenching by bpyHZ2+should proceed spontaneously for all of the sensitizers used. In accord with this, close to diffusion-controlled rate constants are found with Q = bpyH?+ and the other dipositive quenchers in Table 11. Quenching by bpy (EO = -2.1 V) is not expected by this or any other mechanism and is not observed. Since the reduction potential of bpyH+ (-0.97 V) is intermediate in value, reaction should be rapid with some, but not all, of the sensitizers. This prediction is borne out in the rate constants for bpyH+ quenching listed in Table I. The k, values range from less than lo7 M-' s-I for the poor reducing agents (top of table) to greater than lo9 M-l s-l for the strongest reducing agents. Since either endergonic (*E03,25 4 . 9 7 V) or exergonic electron transfer may occur depending upon the nature of * R u L ~ ~the +, detailed model for oxidative quenching given in Scheme 111 will be used to treat the bpyH+ quenching data. Scheme I11 leads to eq 18 for kqo the diffusion-corrected quenching rate constant (l/kq = l / k q c + l/kdiff). kqc =
K12k23
+ k32/(k30 -k k34) In order to treat the free-energy dependence of the quenching rate constants I 3 s z 1 we use eq 19 K12v23K23
kqc =
1+
V23K23
1
I
exp(-AG23*/RT)
exp((AG23' - AG23*)/RT)/(k30 + k34)
(19)
where
In the above equations r is the separation of the centers of the two reactants in the precursor complex, 6r is the thickness of the reaction layer (-0.8 A), w is the work required to bring the two 3 a nuclear frequency factor, and ~ 2 is 3 reactants together, ~ 2 is the electronic factor for the reactions (in the normal free energy region K = 1 for an adiabatic reaction; K
