J . Phys. Chem. 1986, 90, 4913-4915
Circular Dichroism Spectra of A-[ Ru(bpy),]
4913
2+/+/0/-
Paul S. Braterman, Brian C. Noble, and Robert D. Peacock* Department of Chemistry, The University of Glasgow, Glasgow G12 8QQ, U.K. (Received: June 12, 1986)
e
The circular dichroism (CD) spectra of A-[Ru(bpy),lZ+ and of the related singly, doubly, and triply reduced species A-[Ru(bpy)J+/O/- are reported. The singly and doubly reduced complexes are optically stable in dry CHJN solution while A-[R~(bpy)~]racemizes slowly in CH$N solution but is optically stable in dry DMF. The singly and doubly reduced complexes show exciton CD in the region of ligand absorption which are best interpreted by the localized electron model in which the complexes contain both bpy and bpy- ligands.
Introduction It is now generally accepted' that the one-, two-, and threeelectron reduction products of [ R ~ ( b p y ) ~ are ] ~ +complexes in which the ruthenium is in the formal 2+ oxidation state and the electrons are localized on the bpy ligands. Electrochemical measurements,2 ESR,, absorption: and Raman spectroscopyS all suggest that the reduced compounds contain both bpy and bpyligands so that, for example, [Ru(bpy),]+ must be formulated [R~(bpy)~(bpy-)]+. The activation energy for electron exchange between bpy and bpy- ligands in the partially reduced complexes bpy- charge-transfer band can be is 1000 cm-' and a bpy detected in the absorption spectrum at around 4000 cm-1.6 While the ruthenium is undoubtedly in the 2+ oxidation state there is some doubt about the extent of metal-ligand mixing in the reduced species. [Ru(bpy)J2+, being a low-spin d6 ion, can be resolved into A and A isomers which are optically stable. The singly oxidized form, [ R ~ ( b p y ) ~ ] ,is+ (low-spin ds), racemizes slowly in solution at room temperature. If there is any substantial metal-ligand mixing in the reduced species we might expect to see racemization here also (for example, [Rh(bpy),12+, a d7 ion, is labile, rapidly losing a bpy ligand in ~ o l u t i o n ) . ~ A number of tris-bipyridyl complexes have been shown to exhibit chiral discrimination in electron-transfer reactions; for example, A- [Ru(bpy),13+ preferentially oxidizes A- [Co(edta)]*and photoexcited A-'[Ru(bpy)J2+ preferentially reduces A[ C ~ ( p d ) , ] ~ + .It~ *has ~ recently been shown that A- and A-[Ruhen)^]^+ specifically bind to different sites in DNA.l0 Optically may have considerable potential stable A- and A-[R~(bpy),]+/~/as chiral one-, two-, and three-electron reducing agents, as chiral electron-transfer catalysts, or in chiral electrode systems. In this Letter we report and discuss the C D spectra of A[R~(bpy),]+/~/-.The three complexes have quite different and characteristic exciton CD patterns in the ligand absorption region which are best interpreted by formulating the complexes as A[Ru(bPY)2(bPY-)l+, A-[Ru(bPY)(bPY-)21, and A-[Ru(bPY-),l-, respectively.
-
-
Experimental Part [Ru(bpy)J2+ was resolved by literature methods" and the A (1) DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. Coord. Chem. Rev. 1985, 64, 65.
(2) Ohsawa, Y.; DeArmond, M. K.; Hanck, K. W.; Morns, D. E.; Whitten, D. G.; Neveux, P. E., Jr. J . Am. Chem. SOC.1983, 105, 6522. (3) Motten, A,; Hanck, K. W.; DeArmond, M. K. Chem. Phys. Lett. 1981, 79, 541. (4) Braterman, P. S.; Heath, G. A,; Yellowlees, L. J. J. Chem. Soc., Chem. Commun. 1981, 287. ( 5 ) Donohoe, R. J.; Angel, S.M.; Hanck, K. W.; Wertz, D. W.; DeArmond, M. K. J . Am. Chem. SOC.1984, 106, 3688. (6) Braterman, P. S.; Heath, G. A,; Yellowlees, L. J. Chem. Phys. Leu. 1982, 92, 646. (7) Kew, G.; DeArmond, M. K.; Hanck, K. W. J . Phys. Chem. 1974,78, 727. (8) Geselowitz, D. A,; Taube, H. J . Am. Chem. SOC.1980, 102, 4525. (9) Kaisu, Y.; Mori, T.;Kobayashi, H. J. P h p . Chem. 1985, 89, 332. (10) Barton, J. K.; Goldberg, J. M.; Kumar, C. V.; Turro, N. J. J . Am. Chem. SOC.1986, 108, 2081.
0022-3654/86/2090-4913$01 .50/0
isomer converted to the [BF4]- salt by anion-exchange chromatography. The reduced species A-[Ru(bpy),]+, A-[Ru(bpy),], and A- [Ru(bpy),]- were produced by controlled potential electrolysis (at -1.78, -2.00, and -2.30 V vs. Ag/Ag+, respectively) of mol L-' solutions of A-[Ru(bpy),] [BF4I2in purified acetonitrile in a 1-mm pathlength quartz cell equipped with a platinum gauze working electrode. The supporting electrolyte was 0.1 mol L-' [Bun4][BF,], the auxiliary electrode was Pt wire, and the reference and auxiliary electrodes were separated from the bulk solution by porous Vycor glass frits. Electrolyses were performed both at room temparature and at -40 OC and the solutions were checked for decomposition and racemization at each stage by regenerating the 2+ species. A-[Ru(bpy)J was also synthesized by chemical reduction of A-[Ru(bpy),I2+ by lithium in D M F solution using vacuum line techniques.
Results and Discussion The reduced species A-[Ru(bpy)J+iO/- were synthesized by electrochemical reduction of the divalent parent complex in acetonitrile solution at an optically transparent electrode. The first two reduction products, A - [ R ~ ( b p y ) ~and ] + A-[Ru(bpy),], are completely optically stable; the triply reduced species racemizes slowly in dry CH3CN with a half-life of -1 h as shown by a reduction in optical activity of the regenerated 2+ species. We have also prepared A-[Ru(bpy),]- by chemical reduction in DMF solution. Prepared this way the complex is optically stable in both the solid state and in D M F but racemizes (with a half-life of 1 h) in acetonitrile. There are three possibilities why the triply reduced complex is more labile than A-[Ru(bpy),I2+ itself there is substantial electron delocalization from ligand to metal producing a partial d7 configuration, the ligand field of bpy- is considerably less than that of bpy, or repulsion between the negatively charged bulky bpy- ligands leads to much easier dissocation. The fact that the racemization is solvent dependent suggests that the last possibility is the most likely, with the acetonitrile coordinating to the ruthenium and stabilizing the intermediate. The absorption and CD spectra of the four complexes are shown in Figures 1 and 2. They are presented as pairs for comparison; Figure 1 shows the spectra of the two complexes with equivalent ligands, A-[Ru(bpy),12+ and A-[Ru(bpy-),]-, and Figure 2 shows those of the mixed ligand complexes. The absorption spectra of all the complexes are identical with those reported previ~usly.~ Bipyridine has twelve Huckel MOs: six filled bonding orbitals and six empty antibonding orbitals. The lowest energy absorption x7, which is long-axis (at around 300 nm) is therefore 7r6 polarized.'* In bpy- the electron enters x7, the 7 6 1r7 transition moves to somewhat lower energy (- 380 nm) and is still long-axis p01arized.I~ The absorption spectrum of bpy- shows two further transitions; the one at -480 nm has been assigned as 7r7 7r10
-
-
-+
+
(1 1) Dwyer, F. P.; Gyarfas, E. C. J. Proc. R. SOC.N . S . W. 1949,83, 174. (12) McCaffery, A. J.; Mason, S. F.; Norman, B. J. J . Chem. SOC.A 1969, 1428. (13) Konig, E.; Kremer, S. Chem. Phys. Lett. 1970, 5 , 87.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
4914
Letters 1
- 6
E'lOL
11
-L
200
X/nm
400
- 2
20
100
10
A€
A€ I
0
1 I
- 10
-100
-20
300 90 700 i/nm Figure 1. Absorption and CD spectra of A-[R~(bpy)~]*+ (full line) and A-[R~(bpy-)~](dashed line) both in acetonitrile solution. Note the scale change in the CD spectra between the ligand and charge-transferregions.
-
-
(long-axis polarized) and that at -900 nm to a7 r g and r7 r9 (short- and long-axis polarized, re~pectively).'~ The absorption and C D spectra of A - [ R ~ ( b p y ) ~ ]are ~ +wellknown and have been frequently discussed. The strong CD couplet centered at 300 nm is due to the exciton coupling of the long-axis polarized transitions of the three bpy molecules into A2 and E combinations which should, neglecting other perturbations, have equal and opposite rotational strengths.12 The absorption and CD bpy charge transfer; spectra in the visible region is due to Ru the origin of the C D in particular has been uncertainI4 until recently.15 The strongest feature in the absorption spectrum of r7transition the triply reduced species is assigned as the 7r6 bpy- charge transfer of bpy- with perhaps an underlying Ru to account for the breadth of the band.4 The accompanying C D is a beautiful exciton couplet which has the same sign and comparable magnitude to the exciton couplet of A-[Ru(bpy)JZ+ and so is fully compatible with the assignment of the transition; the couplet shows only slight asymmetry which could be due to weak underlying charge-transfer CD. The C D spectrum of the visible absorption is much more problematic. If the transition is the long-axis polarized r7 7cIDwe would expect to see a conservative exciton couplet with the same sign as the one to higher energy. The observed CD is clearly more complex than expected. We shall not discuss the visible spectra of any of our complexes further at
- -
-
(14) Peacock, R. D.; Stewart, B. Coord. Chem. Reu. 1982, 46, 129. (1 5) Ferguson, J.; Herren, F.; McLaughlin, G. M. Chem. Phys. Lett 1982, 89, 376.
I
1
300
5 00
-lo
J 700
b/nm
Figure 2. Absorption and CD spectra of A-[R~(bpy)~(bpy-)]+ (full line) and A-[Ru(bpy)(bpy-)J (dashed line) both in acetonitrile solution. Note the scale change in the CD spectra. The inserts show the calculated CD spectra (see text for details of the parameters used) for the singly and doubly reduced species (upper and lower diagrams, respectively). present; we are currently investigating various aspects of the spectrum of [Li+][bpy-] in the hope of clarifying the situation. The spectra of the partially reduced species are shown in Figure 2. The CD spectra in particular are quite distinct for each complex and illustrate graphically the individuality of the two species. This is further confirmation that the complexes contain bpy and bpy-. The exciton coupling of the ligand transitions in mixed tris-chelate complexes such as [ R ~ ( b p y ) ~ ( p h e n )and ] ~ +[Ru(bpy)(phen),12+ have been treated in some detail by Bosnich.I6 There is considerable similarity between the CD spectra of our mixed (bip~),(bpy-)~, species and the mixed (bpy),(phen),, complexes which suggests that we should be able to calculate the exciton patterns of the former in the same way. Taking average energies for the bpy and bpy- transitions from the absorption spectra of the respective tris-chelates and using exciton interaction energies of 970 cm-l for bpy with bpy, 1400 cm-' for bpy- with bpy-, and 1200 cm-I for bpy with bpy-,I7 we calculate the exciton splitting patterns shown in the insert to Figure 2. The energies and relative signs and intensities are quite well reproduced and completely confirm the localized electron model for the complexes. We are (16) Bosnich, B. Inorg. Chem. 1968, 7, 2374. (17) Parameters used were E(bpy) = 34480 cm-', E(bpy-) = 29850 cm-I, V(bpy-bpy) = 970 cm-', V(bpy--bpy-) = 1400 cm-', V(bpy-bpy-) = 1200 cm- . All values except V(bpy-bpy-) were taken from the experimental spectra; V(bpy-bpy-) is the rounded-up average of the bpy-bpy and bpy--bpy' exciton interaction energies.
J . Phys. Chem. 1986, 90, 4915-4916 currently completing our studies of the corresponding Os(I1) and Ir(II1) complexes and will be reporting these in the near future. The CD spectrum of the transient species A-*[R~(bpy)~]*+ has recently been reported.]* All the evidenceI8J9suggests that this (18) Gold, J. S.;Milder, S.J.; Lewis, J. w.; Kliger, D. s.J . Am. Chem. SOC.1985, 107, 8285. (19) Braterman, P. S.;Harriman, A,; Heath, G. A,; Yellowlees, L. S.J . Chem. Soc., Dalton, Trans. 1983, 1801.
4915
complex should be formulated A-[R~'~'(bpy)~(bpy-)]~+ and so its CD spectrum in the ligand region is expected to be similar to that of A-[Ru"(bpy),(bpy-)]+. The C D spectrum of the photoexcited species does show a qualitative resemblance to that of our singly reduced species (with the exception of a positive feature in the spectrum of the former complex around 370 nm). The transient CD spectrum appears to be weaker than ours. If this is a real effect it may be a consequence Of an charge-transfer associated with the Ru"'(bpy-) chromophore.
Hydrogen Spillover and the Rate of Heterogeneous Catalytic Hydrogenation. Quantitatlve Model A. V. Filikov* and N. F. Myasoedov Institute of Molecular Genetics, USSR Academy of Sciences, Moscow U.S.S.R. (Received: March 4, 1986; In Final Form: August 7, 1986)
A model is proposed for the hydrogenation of organic compounds on supported metal catalysts by spilt-overhydrogen, presuming that the spilt-over atomic hydrogen distribution on the support surface is determined by the reaction of hydrogen atoms with the adsorbed organic compound. Analytical expressions are obtained for the rate of spillover-based hydrogenation, the radius of the reaction zone. and the minimum metal content in the catalyst ensuring the maximum reaction rate.
In heterogeneous catalytic hydrogenation where metal dispersed
on a support serves as catalyst, the support becomes chemically active1q2-the atomic3 hydrogen forming as a result of dissociative adsorption on the metal migrates onto the support (hydrogen spillover). The chemical reaction proceeds not only on the metal surface but also on the support: a reaction zone with a radius of up to 500 nm may form around the metal cry~tallite.~Thus, the rate of hydrogenation on a supported catalyst is defined by the formula v = Vo v,
+
where Vois the reaction rate on the metal surface and VI is the reaction rate on the support surface. Vl > Vois possible.' Hydrogen reactions on the metal surface and on the support surface probably yield different product spectra. Hence, hydrogenation by spillover should be allowed for while optimizing the reaction process both to minimize the metal content in the catalyst and to establish the conditions ensuring the necessary set of reaction products. The objective of this study was to construct a quantitative model for the hydrogenation by spilt-over hydrogen. The present knowledge obtained experimentally does not provide complete assurance in the applicability of our model suggestions. But the constructing of such a model at the current stage of research is,necessary-it will show new approaches to the process mechanism investigation. H atoms on the carrier surface may both react with the adsorbed reagent and enter the mutual recombination reaction. The disappearance of H atoms due to mutual recombination is likely to be negligible, for H atoms have been found6 to migrate (1) Bianchi, D.; Gardes, G. E. E.; Pajonk, G. M.; Teichner, S. J. J. Catal. 1975, 38, 135. (2) Bianchi, D.; Lacroix, M.; Pajonk, G.; Teichner, S.T. J. Catal. 1979,
59, 467. ( 3 ) Lobashina, N. E.; Sawin, N. N.; Myasnikov, I. A. Kine?. Karal. 1983, 24, 747. (4) Sancier, K. M. J. Catal. 1971, 23, 404. (5) Antonucci, P.; Truong, N.;Giordano, N.; Maggiore, R. J. Catal. 1982, 75, 140. (6) Minachev, H. M.; Dmitriev, R. B.; Steinberg, K.-G.; Bremer, G.; Detyuk, A. N . Izu. Akad. Nauk, SSSR, Ser. Khim. 1975, 12, 2670.
0022-3654/86/2090-4915$01.50/0
onto the support surface to a distance of about 1 mm in the absence of a reagent. That is why, for our model, we shall consider the distribution of the atomic hydrogen concentration C to be determined by hydrogenation with a rate constant k. Thus, the process may be described by the equation DAC = kCR
(1)
D is the coefficient of H atom diffusion on the support and R is the concentration of the reagent adsorbed on the support surface. It is necessary to mention that the atomic hydrogen concentration distribution may be determined by not only basic hydrogenation but also the following reactions: RH
+ H = R' + H2
R'+H=RH
(9 (ii)
This possibility is obvious from the data of Davydov et al.;' they have found that the rate of n-pentaneldeuterium isotope exchange considerably exceeds the rate of n-pentane hydrogenolysis on Pt/A1203 and Ni/A1203. Besides, it was proved that isotope exchange with tritium is also the fastest reaction in the solid-state catalytic hydrogenation for eight compounds studied8pg(the H atoms migrate from the metal catalyst surface into the crystalline organic compound). In studies,'oJ' it has been shown that the C-H bond hydrogen isotope exchange reaction with atomic hydrogen has two steps (i ii). In this case, the constant k is equal to the slow-step rate constant (i). Taking the concentration R as constant over r (the distance from the center of the metal crystallite), we get for the plane
+
(7) Davydov, E. M.; Gudkov, B. S.;Harson, M. S.;Kiperman, S.L. Kiner. Katal. 1973, 19, 650. (8) Filikov, A. V.; Myasoedov, N. F. J. Radioanal. Nucl. Chem. 1984, 85, 373. (9) Filikov, A. V.; Myasoedov, N. F.J . Radioanal. A'ucl. Chem. 1985, 93, 355.
(10) Dubinskaya, A. M. Usp. Khim. 1978, 47, 1169. (11) Filatov, A. S.;Simonov, E. F.; Orlova, M. A. Usp. Khirn. 1981, 50, 2167.
0 1986 American Chemical Society
