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Cyano-bridged complexes of bis(2,2′-bipyridine)(pyridine)ruthenium(II)
Polyhedron Vol. 12, No. 8, pp. 955960, Printed in Great Britain
1993
0
0211-5387193 S6.oO+.lM 1993 Pergamon Press Ltd
CYANO-BRIDGED COMPLEXES OF BIS(2,2’-BIPYRIDINE)(IDINE)RUTHENIUM(II.)~ EDGARDO
H. CUTIN
and Nk3TOR
E. KATZS
Instituto de Quimica Fisica, Facultad de Bioquimica, Q&mica y Farmacia, Universidad National de Tucuman, Ayacucho 491,400O San Miguel de Tucumbn, Argentina (Received 30 September 1992 ; accepted 20 November 1992) Abstract-New complexes with the Ru(bpy)2(py)2’ moiety (bpy = 2,2’-bipyridine, py = pyridine) connected through a cyano group to Ru(NH,):’ and Fe(CN):- as electron acceptors have been prepared and their spectroscopic, electrochemical and photophysical properties investigated. Cyano-bridging is disclosed by changes in the shape and position of the cyanide stretching vibration, v(C=N), in the IR spectrum of the dinuclear ruthenium species, as compared with the mononuclear parent complex. Blue shifts in the lowest energy d, + z* (Ru + bpy) metal-to-ligand charge transfer (MLCT) transition occur when going from [Ru(bpy),(py)(CN)]+ (A) to [(bpy)2(py)Ru11-CN-Ru1u(NH3)5]4’ (B) and to (C), thus pointing to the existence of nitrile-bound -CN-Fe”‘(CN)5][(bpy)&y)Ru” pentaammineruthenium(II1) and pentacyanoferrate(I1) capping groups in the mixed-valence species B and C. Besides, new intense and broad absorptions at 697 (in HClO.01 M) and 700 nm (in H20/Me,C0, 1 : 1 v/v) appear in B and C, respectively, and can be assigned to metal-to-metal charge transfer (MMCT) or “intervalence” transitions. The luminescence of A is completely quenched in B, even at 77 K, a fact which can be explained on the basis of efficient excited-state electron transfer to form the electronic mixed-valence isomer of B. The strong asymmetric nature of B, as deduced from cyclic voltammetry data (the difference in redox potentials between both ruthenium sites amounts to 1.30 V), together with a strong electronic coupling [HAB= 2200 cn- ‘, calculated from the “intervalence” absorption data] indicate that the back electron transfer (or charge recombination) from the MMCT excited state of B probably lies in the “inverted” region.
Cyano-bridged transition metal complexes are interesting systems in the field of mixed-valence chemistry,’ because of the relatively strong metalmetal interaction mediated by the short cyanide group, which gives rise to intense metal-to-metal charge transfer (MMCT) or “intervalence” transitions in the visible or near-IR spectra. We have recently reported2-5 a new series of asymmetric mixed-valence complexes with Ru(bpy)2(py)2+ and (bpy = 2,2’-bipyridine, trpy = Ru(trpy)(bpy)‘+
TPresented, in part, at the XIX Congreso Latinoamericano de Quimica, Buenos Aires, Argentina, 1990. $ Author to whom correspondence should he addressed. Dedicated to Dr Pedro J. Aymonino on the occasion of his 65th birthday. 955
2,2’ : 6’,2”-terpyridine) as photosensitizing moieties connected through different bridging ligands to Ru(NH&+j3+ and Fe(CN):-‘2- groups that can act as electron “relays”. These dinuclear compounds can be useful as models for energy conversion devices and molecular electronics. 6 We report here the syntheses and spectroscopic, electrochemical and photophysical properties of new dinuclear complexes derived from the mononuclear species [Ru(bpy),(py)(CN)]+ (A), first reported by Calvert and Meyer,7 and which has been separated in this work from its main impurity, proRu(bpy) ,(CN) 2, using a chromatographic cedure. The new species, [(bpy)2(py)Ru11-CNRu”‘(NH~)~]~+ (B) and Kbpy)2(py)Ru”-CNFe”‘(CN)5]- (C), have been characterized in the solid state and in solution. Intense MMCT tran-
956
E. H. CUTIN
sitions have been detected in the visible region for both mixed-valence ions. In contrast to A complex B does not emit, even at 77 K, thus evidencing an efficient oxidative quenching process, probably due to the presence of low-lying MMCT excited states. EXPERIlW@NTAL, Syntheses [Ru(bpy)z(Py)(CNIl(PF,)
- 11HzO. DWmMpy)
Cl]Cl (0.60 g, 1.l mmol), prepared as in ref. 8, and KCN (0.07 g, 1.1 mmol) were refluxed for 2 h in a solution of water (8 cm3) and ethanol (2 cm3). After filtering and evaporating to a volume of 5 cm3 the solution wasp sorbed onto a CMSephadex C-25 column. El&ion with pure Hz0 was performed first in order to remove the undesired complex Ru(bpy),(CN)z. Then, 0.1 M LiCl was used to elute a second orange fraction containing the ion A. This fraction was rotoevaporated to a minimum volume and pr+ipitation was achieved by adding a concentrated aqueous solution of NH4PF6. The solid was filjered, washed with cold water and ether and dried in uacuo over KOH. Yield : 15%. Found : C, 36.4 ; H, 3.3 ; N, 9.1. Calc. : C, 36.5; H, 4.1; N, 9.8%. [O~(~Y>R~“~~*~~)~I~F~)~.
12I-W.
[Ru(NH,),Cl]Cl, (20 mg, 0.07 mmol) dissolved in 0.01 M HCl (6 cm3) was added to [Ru(bpy)z (py)(CN)](PF,)* llHzO ($0 mg, 0.05 mmol) in an H20/Me2C0 (5 cm3; 1: 1 v/v) solution under argon. After 0.5 h Zn(Hg) /was added and the mixture was stirred for 5 h with continuous argon bubbling at room temperature. After removal of the Zn(Hg) a few drops of H& 35% were added. The colour of the solution changed from red to green after a few minutes. After evaporating to a minimum volume, the solution1 was sorbed onto an SPSephadex C-25 column (3 x 50 cm). After eluting with 0.3 M HCl to removejtmreacted species a dark green solution was collected with 0.5 M HCl as an eluant. This solution was rctoevaporated to a small volume, and excess NH&F6 was added to precipitate the product, which was redissolved in Hz0 and reprecipitated with NH14PFs, washed with cold water and ether and dried in vacua. Yield: 39%. Found.C, 19.1;H,3.3;N, ll.O.Calc.:C,20.9;H, 3.5 ; N, 10.3%. [(bpy) ,(py)Ru”-CN-Fe”‘(CN) J. This complex was prepared in solution by mixing stoichiometric amounts of [Ru(bpy),(py)(CN)](PFJ llH*O and Na3[Fe(CN)k(NH3)]*3Hz0 in HzO/ Me2C0 (10 cm3; 1: 1 vjv). After stirring under argon for 1 h Brz vapoulr was added and visible spectra were recorded.
and N. E. KATZ Instrumentation and techniques
Analytical reagent chemicals were used throughout this work. IR spectra were recorded (as KBr pellets) on a Perkin-Elmer 983G spectrophotometer (wave-number precision : + 2 cm- I). W-vis spectra were obtained with Perkin-Elmer Coleman 124 and Shimadzu W-160A spectrophotometers at 22°C (errors in J,,,,=,and hax were +2 nm and + 5%, respectively). Cyclic voltammetry data were obtained as described before.3 Luminescence measurements were performed on a Perkin-Elmer LS5 spectrophotometer at room temperature and 77 K. Microanalyses were performed at UMYMFOR, Universidad de Buenos Aires, Argentina.
RESULTS
AND DISCUSSION
Syntheses
The mononuclear complex A can be separated from its main impurity, Ru(bpy),(CN), (cf. ref. 7), by a chromatographic exchange on a Sephadex C25 column. The dinuclear mixed-valence species B has been prepared as a solid PF; salt using a method similar to that reported9 for the dinuclear compound [(NH3)5Ru111-NC-Ru11(bpy)2(CN)] (PF,),. The corresponding dinuclear iron species C can be detected in an H20/Me2C0 (1: 1 v/v) solution. IR spectra
Figure 1 shows the IR spectra (as KBr pellets) of the PF; salts of A and B. Assignments can be made by comparison to analogous complexes.* For the mononuclear species A, the sharp absorption at 2071 cm- ’ can be attributed to the cyanide stretching vibration v(C=N). In the dinuclear mixedvalence complex B this band is much broader and appears at 2024 cm- ‘. The broadening effect in v(&N) has already been observed in the mixedvalence species (NH 3),Ru-NC-Ru(CN) 5 when compared to Ru(CN)-.‘O The negative shift in v(C=N) of B with respect to A contrasts to the increase observed in [(trpy)(bpy)Ru”CN-Ru”(NH3) 5](PF6)3- 3Me2C0,4 when compared with the mononuclear parent complex. This difference can be accounted for by the different inductive effects of a capping pentaammineruthenium group when the oxidation state of ruthenium is changed from (II) to (III). The wave-numbers of the ammonia stretching and deformation vibrations in the PF; salt of B [v(N-H) = 3300-3150 cm- ’
Cyano-bridged
complexes of Ru(bpy),(py)‘+
t %T
1200 -_wavcnvmk
Fig.
1. IR spectra
(as KBr pellets) of: () [Ru(bpy)&y)(CNIIPFA* [(~PYL~Y)R~-CN-R~~NH,)~I(PF&~ 12I-W.
6(NHJ = 1312 cm-‘] are typical of pentaammineruthenium(III) moieties. ’ ’
and
UV-vis spectra Table 1 shows the observed absorption maxima in the W-vis region for complexes A (in CH$N) and B (in 0.01 M HCl). For the former ion the lowest-energy band appears at J,,,,, = 470 nm, in agreement with the value previously reported,’ and can be assigned to the metal-to-ligand charge transfer (MLCT) transition I + xt(bpy). The following absorption band at J,,,= = 346 nm has con-
400
800 (cm”1
llH@
and (---)
tributions from &(Ru) + w:(bpy) and I --) I* MLCT transitions, as discussed before. I2 These two low-energy bands are shifted in Hz0 to 442 and 330 nm, respectively, as shown in Fig. 2. This strong solvatochromic effect can be explained by considering the higher electron-accepting abilities of H20 with respect to CH3CN.13 When comparing the lowest-energy absorption maximum of this ion in aqueous solution to the corresponding values of Ru(bpy),(CN)2 (J_, = 431 nm)14 and [Ru(bpy)2@y)J2+ (&_ = 455 nm),14 we infer that increasing the number of CN- ligands attached to a ruthenium bipyridil moiety causes a noticeable
Table 1. Electronic absorption spectra Complex
&v,, (mn) (10-4~~,,,
DWm9&y)(CW1+ (A> [(bpy),(py)Ru”-CN-Ru”‘(NH,),]4+ [(bpy),(py)Ru”l~N-Fe’ll(CN),]QIn CH,CN. ‘In 0.01 M HCl. ‘In H,O/Me,CO (1 : 1 v/v).
(B)b (C)
470 244 697 700
(0.9), (2.3), (0.2), (0.2),
346 208 423 440
M-’ cm-‘)
(1. l), 288 (5.3), (2.9) (0.6), 285 (4.0), 243 (2.2) (1.2)
958
E. H. CUTIN
and N. E. KATZ
X (nm)-
Fig. 2. Visible spectra in aqueous solutions of: (-) [Ru(bpy),(py)(CN)]+ (C = 9 x lo-’ (---) [(bpy),(py)Ru”-CN-Ru”‘(NH,)J4+ (C = 4.8 x lo-’ M).
decrease in Ru + bpy rc-backbonding, with a concomitant increase in the energy of the MLCT band. The bands at 288 and 244~1 in A are assigned to A + n* intraligand transitions, while the band at 208 nm is due to a d,(Ru) + n*(CN-) MLCT transition. The W-vis absorption pectrum of B in aqueous solution is also shown in E ig. 2. The bands at 285 and 243 ~1 are again assi ed to K + x* intraligand transitions. The d-t x* “R ( u” + bpy) CT band at 442 nm in A is shifted to 423 MI in B, as expected for dinuclear complexes with metal binding through cyanide bridges.9 A new intense and broad absorption band at J,,- = 697 nm can be confidently assigned to an MMCT transition, Rug + Ruf’ (Rub = Ru bonded to bpy, Ru, = Ru bonded to NH& since it disappears in the corresponding Ru!---CN-Rut’ and Ru~‘---CN-Ruk” species (obtained by adding solid S20:- and Ce’” salts, respectively, to aqueous solutions of B). This band is shifted to 690 mn in CH3CN, typical of this type of asymmetric system9 Besides, it compares reasonably well in shape and position to the MMCT bands of the complexes: [(trpy)(bpy)Ru”-CNRu”‘(NH~)~]~+ (a, = 7ob nm in CH3CN)4 and [(NH 3),Ru”‘-NC-Ru”(bpy) z(CN)l 3+ (&lax= 694 mn in Hz0).9 For the Run--CN-Fe”’ complex C the MMCT band is observed at 700 nm in H20/Me2C0 (1 : 1 v/v) solution.
M) and
Electrochemistry The redox potentials for the ruthenium-based and two bpy-based couples have already been determined for complex A : 1.04, - 1.48 and - 1.72 V, respectively, in CH,CN, vs S.C.E. ’ 5 In complex B ELjZ (Ru~“/Ru~~), as determined by cyclic voltammetry, appears at 1.28 V (in CH,CN, vs S.C.E.), as expected for its higher charge with respect to A. The value for E1,2 (Ruin/Ru$) = -0.02 V (in CH&!N, vs S.C.E.) is consistent with those values observed for nitrile-bound pentaammineruthenium complexes.’ The energy difference between both mixed-valence isomers of B thus amounts to 1.30 eV. The reduction waves were somewhat irreversible, with peak potentials at - 1.46 and - 1.74 V due to bpy reduction processes. Due to the narrow electrochemical window for aqueous solutions, El,* values for both metallic couples could not be determined for the ion C, but we expect them to be very similar to B, the ruthenium analogue.
Luminescence Complex A emits at room temperature (A,,,,= 620 nm in H,0).7 Figure 3 shows the emission spectrum of A at 77 K as an EtOH/MeOH glass (4 : 1 v/v ; the solution had to be filtered). At 1,, = 466 mn there is an emission maximum at 585
Cyano-bridged
complexes of Ru(bpy) ;?(py)‘+
959
coupling in tlte mixed-valence species B from spectroscopic data. Indeed, from values of v,, =
t I (au)
1.45 x lo4 cm-‘, s,,,= = 3.4x lo3 M-i cm-’ and Av,/, = 6.0 x lo3 cm- ‘, obtained by analysing the MMCT band of B, a value of HAD= 2200 cm-’ (0.27 eV) results, typical for cyano-bridged mixedvalence complexes. ‘*1* The reorganization barrier for the optical electron transfer, Iz, can be calculated from the following equation : ” EoP = a+AG”$AEeX,
where Ea,, is the maxims of the MMCT band, AGOis the free energy difference between both metal sites and AEe, is a correction for excited state/ ground state differences. Since, for ion B, EoP = 1.80 eV, AGo = 1.30 eV and AE,, = 0.15 eV, ” a value of il = 0.35 eV is obtained. For the ion [(py)(NH,), I Ru”‘-NC-Ru”(bpy),(CN)]“, for which AG ’ is 600 700 lower by z 0.3 eV, a concomitant decrease of 2 0.2 wavekngthtnmt-r, eV in EoP is observed. 20 Values for Iz and AGO are very similar for analogous cyano-bridged comFig. 3. Emission spectrum (A, = 466 nm) of [Ru plexes4Fgand thus Eop appears at almost the same (bpy)&y)(CN)J+ in EtOH/MeOH (4 : 1 v/v) : (-) at 77 K and (- - -) at room temperature. value (r 1.8 eV). For the thermal back electron transfer RuF’ + Ruf;’ AGO = - 1.30 eV and 1 < -AGO, so that an “inverted” behaviour is predicted.4 A non-adiabatic treatment** gives the nm and a shoulder at 623 nm, characteristic of reverse rate constant kths E 7 x IO* s- ‘. However, Ru(bpy)~(L)~+ complexes which decay from an recent picosecond IR experiments2* performed on 3MLCT excited state.16 The excitation and absorpthe mixed-valence dimer [(NC)5Ru11-CN-Ru*11 tion spectra were almost identical. At room tem- (NH3J* have shown that an ultrafast (r < 0.5 perature this alcoholic solution emits at 636 mn, as ps) back electron transfer, followed by ~brational also shown in Fig. 3. Complex B does not emit at relaxation, occurs in this system from the MMCT room temperature or 77 K. This lack of emission excited state, suggesting that a theory considering may be due to an efficient oxidative quenching pro- vibrational equilibrium is not adequate for this process, such as those proposed” for similar cyano- cess. Besides, it has been pointed out that electronic bridged systems : coupling elements for optical electron transfers are not necessarily equal to those for thermal electron transfers. 23 ~(bpy)(bpy-)~y)Ru”‘CNRu”‘fNH,),j4+ To conclude, strong coupling in cyano-budged asymmetric mixed-valence complexes, such as B or [(bpy)(bpy-)(py)Ru111CNRu”‘(NH3),14+ k, C, gives rise to intense MMCT bands in the visible region. The complete lack of emission, even at 77 K, of dimer B points to efficient quenching of the MLCT (Ru --) bpy) excited state, probably through crossing to the MMCT excited state. Increasing the [(bpy)2(py)Ru11CNRu’11(NH,),]4+. metal-to-metal separation, such as in the complex The “unstable” isomer of B is involved in the [~t~y)(bpy)RuJ1(4,~-bpy)Ru1”(~ 3)s15+ (with quenching step k,, i.e. the MMCT excited state can 4,4’-bpy = 4,4’-bipyridine),’ causes a higher requench the radiative decay from the 3MLCT excited organization barrier 1, which may result in longerstate. lived MMCT excited states. Electronic coupling
By using the Marcus-Hush formalism’ it is possible to calculate the electronic metal-to-metal
Acknowledgements-We thank CONICET and CIUNT for fmaneial support, Dr C. Catalan for permission to use the W-vis s~ctrophotometers and Dr C. Colombano for helping in the luminescence experiments. The
960
E. H. CUTJN and N. E. KATZ
authors are Members of the Research Career, CONJCET, Argentina.
REFERENCES 1. C. Creutz,Prog. Znorg. C&em. 1983,30, I. 2. E. H. Cutin and N. E. Katz, Polyhedron 1991, 10, 653. 3. A. Ben Altabef, S. D. Ribotta de Gallo, M. E. Folquer and N. E. Katz; Znorg. Chim. Acfa 1991, 188,67. 4. A. Ben Ahabef, S. B. Ribotta de Gallo, M. E. Folquer and N. E. Katz, rt’rans.Met. Chem. 1993, in press. 5. F. Fagalde and N. E. Katz, J. Chem. So&., D&m Trans. 1993, in press. 6. V. Balzani and F. Scandola, Supramolecular Photochemistry. Ellis Horwoodi U.K. (1991). 7. J. M. Calvert and T. J. Meyer, Znorg. Chem. 1982, 21, 3978. 8. B. Bosnich and F. P. DHiyer, Au& J. Chem. 1966, 19, 2229. 9. C. A. Bignozzi, S. Raffia and F. Scandola, J. Am. Chem. Sot. 1985,107,16& 10. S. Siddiqui, W. W. Hend and R. E. Shepherd, Znorg. Chem. 1987,26,3 1
11. R. E. Clarke and P. C. Ford, Znorg. Chem. 1970,9, 227. 12. G. M. Bryant, J. E. Fergusson and H. K. J. Powell, Aust. J. Chem. 1971,24-257. 13. N. Kitamura, M. Sato, H.-B. Kim, R. Obata and S. Tazuke, Znorg. Chem. 1988,27,65 1. 14. A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and A. Von Zelewsky, Coord. Chem. Rev. 1988,84,85. 15. B. P. Sullivan, D. Conrad and T. J. Meyer, Znorg. Chem. 1985,24,3640. 16. N. E. Katz, C. Creutz and N. Sutin, Znorg. Chem. 1988,27, 1687. 17. V. Balzani, N. Sabbatini and F. Scandola, Chem. Rev. 1986,86,319. 18. A. Burewicz and A. Haim, Znorg. Chem. 1988, 27, 1611. 19. B. S. Brunschwig, S. Ehrenson and N. Sutin, J. Phys. Gem. 1986,!M, 3657. 20. C. A. Bignozzi, C. Paradisi, S. Roffia and F. Scandola, Znorg. Chem. 1988,27,408. 21. R. A. Marcus and N. Sutin, Biochim. Biophys. Acta 1985,811,265. 22. S. K. Doom, P. 0. Stoutland, R. B. Dyer and W. H. Woodruff, J. Am. Chem. Sot. 1992,114,3133. 23. M. H. Chou, C. Creutz and N. Sutin, Znorg. C&m. 1992,31,2318.
1993
0
0211-5387193 S6.oO+.lM 1993 Pergamon Press Ltd
CYANO-BRIDGED COMPLEXES OF BIS(2,2’-BIPYRIDINE)(IDINE)RUTHENIUM(II.)~ EDGARDO
H. CUTIN
and Nk3TOR
E. KATZS
Instituto de Quimica Fisica, Facultad de Bioquimica, Q&mica y Farmacia, Universidad National de Tucuman, Ayacucho 491,400O San Miguel de Tucumbn, Argentina (Received 30 September 1992 ; accepted 20 November 1992) Abstract-New complexes with the Ru(bpy)2(py)2’ moiety (bpy = 2,2’-bipyridine, py = pyridine) connected through a cyano group to Ru(NH,):’ and Fe(CN):- as electron acceptors have been prepared and their spectroscopic, electrochemical and photophysical properties investigated. Cyano-bridging is disclosed by changes in the shape and position of the cyanide stretching vibration, v(C=N), in the IR spectrum of the dinuclear ruthenium species, as compared with the mononuclear parent complex. Blue shifts in the lowest energy d, + z* (Ru + bpy) metal-to-ligand charge transfer (MLCT) transition occur when going from [Ru(bpy),(py)(CN)]+ (A) to [(bpy)2(py)Ru11-CN-Ru1u(NH3)5]4’ (B) and to (C), thus pointing to the existence of nitrile-bound -CN-Fe”‘(CN)5][(bpy)&y)Ru” pentaammineruthenium(II1) and pentacyanoferrate(I1) capping groups in the mixed-valence species B and C. Besides, new intense and broad absorptions at 697 (in HClO.01 M) and 700 nm (in H20/Me,C0, 1 : 1 v/v) appear in B and C, respectively, and can be assigned to metal-to-metal charge transfer (MMCT) or “intervalence” transitions. The luminescence of A is completely quenched in B, even at 77 K, a fact which can be explained on the basis of efficient excited-state electron transfer to form the electronic mixed-valence isomer of B. The strong asymmetric nature of B, as deduced from cyclic voltammetry data (the difference in redox potentials between both ruthenium sites amounts to 1.30 V), together with a strong electronic coupling [HAB= 2200 cn- ‘, calculated from the “intervalence” absorption data] indicate that the back electron transfer (or charge recombination) from the MMCT excited state of B probably lies in the “inverted” region.
Cyano-bridged transition metal complexes are interesting systems in the field of mixed-valence chemistry,’ because of the relatively strong metalmetal interaction mediated by the short cyanide group, which gives rise to intense metal-to-metal charge transfer (MMCT) or “intervalence” transitions in the visible or near-IR spectra. We have recently reported2-5 a new series of asymmetric mixed-valence complexes with Ru(bpy)2(py)2+ and (bpy = 2,2’-bipyridine, trpy = Ru(trpy)(bpy)‘+
TPresented, in part, at the XIX Congreso Latinoamericano de Quimica, Buenos Aires, Argentina, 1990. $ Author to whom correspondence should he addressed. Dedicated to Dr Pedro J. Aymonino on the occasion of his 65th birthday. 955
2,2’ : 6’,2”-terpyridine) as photosensitizing moieties connected through different bridging ligands to Ru(NH&+j3+ and Fe(CN):-‘2- groups that can act as electron “relays”. These dinuclear compounds can be useful as models for energy conversion devices and molecular electronics. 6 We report here the syntheses and spectroscopic, electrochemical and photophysical properties of new dinuclear complexes derived from the mononuclear species [Ru(bpy),(py)(CN)]+ (A), first reported by Calvert and Meyer,7 and which has been separated in this work from its main impurity, proRu(bpy) ,(CN) 2, using a chromatographic cedure. The new species, [(bpy)2(py)Ru11-CNRu”‘(NH~)~]~+ (B) and Kbpy)2(py)Ru”-CNFe”‘(CN)5]- (C), have been characterized in the solid state and in solution. Intense MMCT tran-
956
E. H. CUTIN
sitions have been detected in the visible region for both mixed-valence ions. In contrast to A complex B does not emit, even at 77 K, thus evidencing an efficient oxidative quenching process, probably due to the presence of low-lying MMCT excited states. EXPERIlW@NTAL, Syntheses [Ru(bpy)z(Py)(CNIl(PF,)
- 11HzO. DWmMpy)
Cl]Cl (0.60 g, 1.l mmol), prepared as in ref. 8, and KCN (0.07 g, 1.1 mmol) were refluxed for 2 h in a solution of water (8 cm3) and ethanol (2 cm3). After filtering and evaporating to a volume of 5 cm3 the solution wasp sorbed onto a CMSephadex C-25 column. El&ion with pure Hz0 was performed first in order to remove the undesired complex Ru(bpy),(CN)z. Then, 0.1 M LiCl was used to elute a second orange fraction containing the ion A. This fraction was rotoevaporated to a minimum volume and pr+ipitation was achieved by adding a concentrated aqueous solution of NH4PF6. The solid was filjered, washed with cold water and ether and dried in uacuo over KOH. Yield : 15%. Found : C, 36.4 ; H, 3.3 ; N, 9.1. Calc. : C, 36.5; H, 4.1; N, 9.8%. [O~(~Y>R~“~~*~~)~I~F~)~.
12I-W.
[Ru(NH,),Cl]Cl, (20 mg, 0.07 mmol) dissolved in 0.01 M HCl (6 cm3) was added to [Ru(bpy)z (py)(CN)](PF,)* llHzO ($0 mg, 0.05 mmol) in an H20/Me2C0 (5 cm3; 1: 1 v/v) solution under argon. After 0.5 h Zn(Hg) /was added and the mixture was stirred for 5 h with continuous argon bubbling at room temperature. After removal of the Zn(Hg) a few drops of H& 35% were added. The colour of the solution changed from red to green after a few minutes. After evaporating to a minimum volume, the solution1 was sorbed onto an SPSephadex C-25 column (3 x 50 cm). After eluting with 0.3 M HCl to removejtmreacted species a dark green solution was collected with 0.5 M HCl as an eluant. This solution was rctoevaporated to a small volume, and excess NH&F6 was added to precipitate the product, which was redissolved in Hz0 and reprecipitated with NH14PFs, washed with cold water and ether and dried in vacua. Yield: 39%. Found.C, 19.1;H,3.3;N, ll.O.Calc.:C,20.9;H, 3.5 ; N, 10.3%. [(bpy) ,(py)Ru”-CN-Fe”‘(CN) J. This complex was prepared in solution by mixing stoichiometric amounts of [Ru(bpy),(py)(CN)](PFJ llH*O and Na3[Fe(CN)k(NH3)]*3Hz0 in HzO/ Me2C0 (10 cm3; 1: 1 vjv). After stirring under argon for 1 h Brz vapoulr was added and visible spectra were recorded.
and N. E. KATZ Instrumentation and techniques
Analytical reagent chemicals were used throughout this work. IR spectra were recorded (as KBr pellets) on a Perkin-Elmer 983G spectrophotometer (wave-number precision : + 2 cm- I). W-vis spectra were obtained with Perkin-Elmer Coleman 124 and Shimadzu W-160A spectrophotometers at 22°C (errors in J,,,,=,and hax were +2 nm and + 5%, respectively). Cyclic voltammetry data were obtained as described before.3 Luminescence measurements were performed on a Perkin-Elmer LS5 spectrophotometer at room temperature and 77 K. Microanalyses were performed at UMYMFOR, Universidad de Buenos Aires, Argentina.
RESULTS
AND DISCUSSION
Syntheses
The mononuclear complex A can be separated from its main impurity, Ru(bpy),(CN), (cf. ref. 7), by a chromatographic exchange on a Sephadex C25 column. The dinuclear mixed-valence species B has been prepared as a solid PF; salt using a method similar to that reported9 for the dinuclear compound [(NH3)5Ru111-NC-Ru11(bpy)2(CN)] (PF,),. The corresponding dinuclear iron species C can be detected in an H20/Me2C0 (1: 1 v/v) solution. IR spectra
Figure 1 shows the IR spectra (as KBr pellets) of the PF; salts of A and B. Assignments can be made by comparison to analogous complexes.* For the mononuclear species A, the sharp absorption at 2071 cm- ’ can be attributed to the cyanide stretching vibration v(C=N). In the dinuclear mixedvalence complex B this band is much broader and appears at 2024 cm- ‘. The broadening effect in v(&N) has already been observed in the mixedvalence species (NH 3),Ru-NC-Ru(CN) 5 when compared to Ru(CN)-.‘O The negative shift in v(C=N) of B with respect to A contrasts to the increase observed in [(trpy)(bpy)Ru”CN-Ru”(NH3) 5](PF6)3- 3Me2C0,4 when compared with the mononuclear parent complex. This difference can be accounted for by the different inductive effects of a capping pentaammineruthenium group when the oxidation state of ruthenium is changed from (II) to (III). The wave-numbers of the ammonia stretching and deformation vibrations in the PF; salt of B [v(N-H) = 3300-3150 cm- ’
Cyano-bridged
complexes of Ru(bpy),(py)‘+
t %T
1200 -_wavcnvmk
Fig.
1. IR spectra
(as KBr pellets) of: () [Ru(bpy)&y)(CNIIPFA* [(~PYL~Y)R~-CN-R~~NH,)~I(PF&~ 12I-W.
6(NHJ = 1312 cm-‘] are typical of pentaammineruthenium(III) moieties. ’ ’
and
UV-vis spectra Table 1 shows the observed absorption maxima in the W-vis region for complexes A (in CH$N) and B (in 0.01 M HCl). For the former ion the lowest-energy band appears at J,,,,, = 470 nm, in agreement with the value previously reported,’ and can be assigned to the metal-to-ligand charge transfer (MLCT) transition I + xt(bpy). The following absorption band at J,,,= = 346 nm has con-
400
800 (cm”1
llH@
and (---)
tributions from &(Ru) + w:(bpy) and I --) I* MLCT transitions, as discussed before. I2 These two low-energy bands are shifted in Hz0 to 442 and 330 nm, respectively, as shown in Fig. 2. This strong solvatochromic effect can be explained by considering the higher electron-accepting abilities of H20 with respect to CH3CN.13 When comparing the lowest-energy absorption maximum of this ion in aqueous solution to the corresponding values of Ru(bpy),(CN)2 (J_, = 431 nm)14 and [Ru(bpy)2@y)J2+ (&_ = 455 nm),14 we infer that increasing the number of CN- ligands attached to a ruthenium bipyridil moiety causes a noticeable
Table 1. Electronic absorption spectra Complex
&v,, (mn) (10-4~~,,,
DWm9&y)(CW1+ (A> [(bpy),(py)Ru”-CN-Ru”‘(NH,),]4+ [(bpy),(py)Ru”l~N-Fe’ll(CN),]QIn CH,CN. ‘In 0.01 M HCl. ‘In H,O/Me,CO (1 : 1 v/v).
(B)b (C)
470 244 697 700
(0.9), (2.3), (0.2), (0.2),
346 208 423 440
M-’ cm-‘)
(1. l), 288 (5.3), (2.9) (0.6), 285 (4.0), 243 (2.2) (1.2)
958
E. H. CUTIN
and N. E. KATZ
X (nm)-
Fig. 2. Visible spectra in aqueous solutions of: (-) [Ru(bpy),(py)(CN)]+ (C = 9 x lo-’ (---) [(bpy),(py)Ru”-CN-Ru”‘(NH,)J4+ (C = 4.8 x lo-’ M).
decrease in Ru + bpy rc-backbonding, with a concomitant increase in the energy of the MLCT band. The bands at 288 and 244~1 in A are assigned to A + n* intraligand transitions, while the band at 208 nm is due to a d,(Ru) + n*(CN-) MLCT transition. The W-vis absorption pectrum of B in aqueous solution is also shown in E ig. 2. The bands at 285 and 243 ~1 are again assi ed to K + x* intraligand transitions. The d-t x* “R ( u” + bpy) CT band at 442 nm in A is shifted to 423 MI in B, as expected for dinuclear complexes with metal binding through cyanide bridges.9 A new intense and broad absorption band at J,,- = 697 nm can be confidently assigned to an MMCT transition, Rug + Ruf’ (Rub = Ru bonded to bpy, Ru, = Ru bonded to NH& since it disappears in the corresponding Ru!---CN-Rut’ and Ru~‘---CN-Ruk” species (obtained by adding solid S20:- and Ce’” salts, respectively, to aqueous solutions of B). This band is shifted to 690 mn in CH3CN, typical of this type of asymmetric system9 Besides, it compares reasonably well in shape and position to the MMCT bands of the complexes: [(trpy)(bpy)Ru”-CNRu”‘(NH~)~]~+ (a, = 7ob nm in CH3CN)4 and [(NH 3),Ru”‘-NC-Ru”(bpy) z(CN)l 3+ (&lax= 694 mn in Hz0).9 For the Run--CN-Fe”’ complex C the MMCT band is observed at 700 nm in H20/Me2C0 (1 : 1 v/v) solution.
M) and
Electrochemistry The redox potentials for the ruthenium-based and two bpy-based couples have already been determined for complex A : 1.04, - 1.48 and - 1.72 V, respectively, in CH,CN, vs S.C.E. ’ 5 In complex B ELjZ (Ru~“/Ru~~), as determined by cyclic voltammetry, appears at 1.28 V (in CH,CN, vs S.C.E.), as expected for its higher charge with respect to A. The value for E1,2 (Ruin/Ru$) = -0.02 V (in CH&!N, vs S.C.E.) is consistent with those values observed for nitrile-bound pentaammineruthenium complexes.’ The energy difference between both mixed-valence isomers of B thus amounts to 1.30 eV. The reduction waves were somewhat irreversible, with peak potentials at - 1.46 and - 1.74 V due to bpy reduction processes. Due to the narrow electrochemical window for aqueous solutions, El,* values for both metallic couples could not be determined for the ion C, but we expect them to be very similar to B, the ruthenium analogue.
Luminescence Complex A emits at room temperature (A,,,,= 620 nm in H,0).7 Figure 3 shows the emission spectrum of A at 77 K as an EtOH/MeOH glass (4 : 1 v/v ; the solution had to be filtered). At 1,, = 466 mn there is an emission maximum at 585
Cyano-bridged
complexes of Ru(bpy) ;?(py)‘+
959
coupling in tlte mixed-valence species B from spectroscopic data. Indeed, from values of v,, =
t I (au)
1.45 x lo4 cm-‘, s,,,= = 3.4x lo3 M-i cm-’ and Av,/, = 6.0 x lo3 cm- ‘, obtained by analysing the MMCT band of B, a value of HAD= 2200 cm-’ (0.27 eV) results, typical for cyano-bridged mixedvalence complexes. ‘*1* The reorganization barrier for the optical electron transfer, Iz, can be calculated from the following equation : ” EoP = a+AG”$AEeX,
where Ea,, is the maxims of the MMCT band, AGOis the free energy difference between both metal sites and AEe, is a correction for excited state/ ground state differences. Since, for ion B, EoP = 1.80 eV, AGo = 1.30 eV and AE,, = 0.15 eV, ” a value of il = 0.35 eV is obtained. For the ion [(py)(NH,), I Ru”‘-NC-Ru”(bpy),(CN)]“, for which AG ’ is 600 700 lower by z 0.3 eV, a concomitant decrease of 2 0.2 wavekngthtnmt-r, eV in EoP is observed. 20 Values for Iz and AGO are very similar for analogous cyano-bridged comFig. 3. Emission spectrum (A, = 466 nm) of [Ru plexes4Fgand thus Eop appears at almost the same (bpy)&y)(CN)J+ in EtOH/MeOH (4 : 1 v/v) : (-) at 77 K and (- - -) at room temperature. value (r 1.8 eV). For the thermal back electron transfer RuF’ + Ruf;’ AGO = - 1.30 eV and 1 < -AGO, so that an “inverted” behaviour is predicted.4 A non-adiabatic treatment** gives the nm and a shoulder at 623 nm, characteristic of reverse rate constant kths E 7 x IO* s- ‘. However, Ru(bpy)~(L)~+ complexes which decay from an recent picosecond IR experiments2* performed on 3MLCT excited state.16 The excitation and absorpthe mixed-valence dimer [(NC)5Ru11-CN-Ru*11 tion spectra were almost identical. At room tem- (NH3J* have shown that an ultrafast (r < 0.5 perature this alcoholic solution emits at 636 mn, as ps) back electron transfer, followed by ~brational also shown in Fig. 3. Complex B does not emit at relaxation, occurs in this system from the MMCT room temperature or 77 K. This lack of emission excited state, suggesting that a theory considering may be due to an efficient oxidative quenching pro- vibrational equilibrium is not adequate for this process, such as those proposed” for similar cyano- cess. Besides, it has been pointed out that electronic bridged systems : coupling elements for optical electron transfers are not necessarily equal to those for thermal electron transfers. 23 ~(bpy)(bpy-)~y)Ru”‘CNRu”‘fNH,),j4+ To conclude, strong coupling in cyano-budged asymmetric mixed-valence complexes, such as B or [(bpy)(bpy-)(py)Ru111CNRu”‘(NH3),14+ k, C, gives rise to intense MMCT bands in the visible region. The complete lack of emission, even at 77 K, of dimer B points to efficient quenching of the MLCT (Ru --) bpy) excited state, probably through crossing to the MMCT excited state. Increasing the [(bpy)2(py)Ru11CNRu’11(NH,),]4+. metal-to-metal separation, such as in the complex The “unstable” isomer of B is involved in the [~t~y)(bpy)RuJ1(4,~-bpy)Ru1”(~ 3)s15+ (with quenching step k,, i.e. the MMCT excited state can 4,4’-bpy = 4,4’-bipyridine),’ causes a higher requench the radiative decay from the 3MLCT excited organization barrier 1, which may result in longerstate. lived MMCT excited states. Electronic coupling
By using the Marcus-Hush formalism’ it is possible to calculate the electronic metal-to-metal
Acknowledgements-We thank CONICET and CIUNT for fmaneial support, Dr C. Catalan for permission to use the W-vis s~ctrophotometers and Dr C. Colombano for helping in the luminescence experiments. The
960
E. H. CUTJN and N. E. KATZ
authors are Members of the Research Career, CONJCET, Argentina.
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