4686
J . Phys. Chem. 1987, 91, 4686-4689
ESR Study of Reduced Mixed Ligand Complexes of Ru(I1): 4,4'- and Ei,S'-Diester Bipyridines with Ru( II) J. N. Gex, J. B. Cooper, K. W. Hanck, and M. K. DeArmond* Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 (Received: December 22, 1986; I n Final Form: May 19, 1987)
The electron spin resonance spectra for a series of reduced tris [ML3(2-")+]and mixed ligand complexes [ML2L'(2-")t],where L and L' are 2,2'-bipyridine (bpy), 4,4'-dicarbethoxy-2,2'-bipyridine (4-COOEt), and 5,5'-dicarbethoxy2,2'-bipyridine (SCOOEt), exhibit a range of behavior. The one-electron ( n = 1) spectra of [R~(bpy)~L-]+ where L = 4-COOEt and 5-COOEt give hyperfine structure with no temperature-dependent line broadening. While the n = 1 and n = 2 reduction products of the tris RuL,~' complexes exhibit a temperature-dependentS = signal from which an activation energy for intramolecular electron hopping can be obtained, the n = 3 product does not broaden with temperature. These results are consistent with the spatially isolated localized orbital model. The ESR for the n = 4 and n = 5 reduction products of [Ru(5-COOEt),]*+ signal that is not temperature dependent. The n = 6 species is diamagnetic, consistent with expectation. give only an S = The hyperfine splitting constants (ai)obtained from the mono complexes indicate a small spin density at the Ru2+metal center.
Introduction Electrochemical'~2and spectroscopic results3-" including ESR"' indicate that RUL,~' complexes (L = A electron diimine complexes) are readily reduced to produce stable multielectron products with unusual optical and magnetic properties. For example, the near-infrared absorption and the resonance Raman of the one, two, and three reduction products give data that indicate that the one- and two-electron reduction products contain L- and L chromophores rather than L-lj3 and L-2/3for the oneand two-electron products, respectively. The ESR spectra for the one-, two-, and three-electron products can all be fit by a simple S = I / , spin Hamiltonian with a g factor = 2.00 (at room temperature and 77 K), indicating that the radical is predominantly ligand localized. All of these results can be explained by the concept of a "spatially isolated redox orbital" in which the redox orbitals for at least the first three electrons added are localized in the A* orbital associated with an individual L chelate (eq 1 and 2) ring rather than in a delocalized orbital encompassing all three chelate rings.
The ability of the ESR method to empirically determine the wave function for an organic radical species from hyperfine splitting (hfs) is well-known.I2 Since these paramagnetic species can be viewed as metal-containing heterocycles, the measurement (1) Ohsawa, Y.; DeArmond, M. K.; Hanck, K. W.; Morris, D. W.; Whitten, D. G.;Neveux, P. E., Jr. J . Am. Chem. Soc. 1983, 105, 6522. (2) Ohsawa, Y.; Hanck, K. W.; DeArmond, M. K. J . Electroanal. Chem. Interfacial Electrochem. 1984, 175, 229. (3) DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. Coord. Chem. Rec. 1985, 64, 6 5 .
(4) Heath, G. A,; Yellowlees, L. J.; Braterman, P. S. J . Chem. Soc., Chem. Commun. 1981, 287. ( 5 ) Elliot, C. M.; Hershenhart, E. J . Am. Chem. Soc. 1982, 104, 7579. (6) Braterman, P. S.; Heath, G. A.; Yellowlees, L. J. J . Chem. Soc., Dalton Trans. 1985, 108 I . (7) Donohoe, R. J.; Angel, S. M.; DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. J. Am. Chem. Soc. 1984, 106, 3688. (8) Donohoe, R. J.; Tait, C. D.; DeArmond, M. K.; Wertz, D. W. Spectrochim. Acta, Part A 1986, 42A, 233. (9) Motten, A. G.; Hanck, K. W.; DeArmond, M. K. Chem. Phys. Lett. 1981, 79, 541. (10) Morris, D. E.; DeArmond, M. K.; Hanck, K. W. J. Am. Chem. Soc.
of hfs could be invaluable in understanding the absence of spinspin interaction for these reduced species. However, the temperature-dependent line broadening that occurs for the one- and two-electron-reduced species indicates a dynamic process (identified as ring-to-ring hopping of the electron) that eliminates hfs. An ESR spectrum" obtained for [Ru(bpz-)(bpy),]' did produce a well-resolved hfs for the electron localized in the bpz ring system since no electron hopping can occur for this complex. Further, the temperature-dependent spectra for the [Ru(bpz-)bpy,]+ and the reduced [ R u ( b p ~ ) ~ b p y and ] ~ + [ R ~ ( b p z ) ~ complexes ]~+ do verify that the origin of the line broadening is the intramolecular electron hopping. Since the cyclic voltammogram of a mixed ligand complex can be used as a qualitative estimate of the barrier to hopping, a first criterion for the observation of hfs is possible. The absence of hfs for most of the three-electron-reduction products (for which no electron hopping process is occurring) apparently is the result of inhomogeneous line broadening and spectral density. Indeed a recent high-temperature (70 "C) ESR spectrumI3 for [Ru(bpy-),]- does show hfs. The room temperature spectra for the n = 1, n = 2, and n = 3 ester complexes have near-Lorentzian shapes that become Gaussian just above the solvent freezing point. The electrochemistry of the tris complexes of 5,S-dicarbethoxy-2,2'-bipyridine (5-COOEt) and 4,4'-dicarbethoxy-2,2'-bipyridine (4-COOEt) is quite interesting with both complexes reduced] at much less negative potentials than [ R ~ ( b p y ) ~ ] * + . Indeed, the voltammetry pattern for [ R U ( ~ - C O O E ~ ) ,is] ~re+ markable with ten one-electron waves at -54 OC while that of the 5-COOEt analogue indicates six one-electron products under the same conditions. The optical and resonance Raman (RR) spectra5$*for the reduction products of these two complexes have been examined, and both series indicate that the spatially isolated orbital model is appropriate although the R R data indicates that back-bonding is a factor for the reduced 4-COOEt complexes. A quantum-mechanical MNDO calculation for the free ligands14 is consistent with these data and rationalizes the different voltammetry patterns for these two series. Since the voltammetry of these tris ester complexes is similar to that of [ R u ( b p ~ ) ~ ] * + , the possibility exists that the mixed ligand ester complexes can produce well-resolved hfs as for the mixed ligand bpz complexes. The determination of hfs for such mixed ligand complexes is valuable not only because of the comparison between the different mono complexes but it should also provide a very good estimate of the wave function for the parent tris complexes for which hfs
1983, 105, 3032.
(11) Gex, J. N.; DeArmond, M. K.; Hanck, K. W. J. Phys. Chem. 1987, 91. 251. (12) Wertz, J. E.; Bolton, J. R. Electron Spin Resonance; Chapman & Hall: New York, 1986.
0022-3654/87/2091-4686$01.50/0
(13) Gex, J. N.; DeArmond, M. K.; Hanck, K. W., unpublished results. (14) Ohsawa, Y.; Whangbo, M. H.; Hanck, K. W.; DeArmond, M. K. Inorg. Chem. 1984, 23, 3426.
0 1987 American Chemical Society
ESR of Ciester Bipyridines with Ru(I1)
-5
-10
The Journal of Physical Chemistry, Vol. 91, No. 18, 1987 4687
-15
I
-. 5
.
.
.
.
.
.
I
- 1.0 - 1.5 E (Volt vs SCE)
Figure 1. Cyclic voltammograms of (a) [R11(4CoOEt)(bpy)~]~+ and (b) R ~ ( 5 C O 0 E t ) ( b p y ) , ] ~in + 0.1 M TEAP/DMF at 298 K.
is not routinely available. The comparison of complex and free ligand hfs can provide a good estimate of the role of the metal ion in the complex as was evident for the [Ru(b~z-)(bpy)~]+ complex. The unique properties of these reduced species may correlate with those of the lowest excited state of the parent [Ru( b ~ y ) ~ ]l5-I' * + since this excited state can involve a spatially isolated orbital (optical). Recently, we have used a time-resolved photoselection result to observe the polarization decay at 77 K.18 The qualitative indication is that the rate of the polarization decay (due to spin-lattice relaxation) in this excited state in frozen solution (77 K) is roughly comparable to the rate of electron hopping of the electron in the reduced species at 223 K in fluid solution. To date, an activation energy for the intramolecular exciton hopping has not been measured. Finally, Hirota and Yamauchi have recently reported the excited-state ESR19 (at liquid He temperatures) of the luminescent state for [ R ~ ( b p y ) ~ ] ~The +. zero field splitting parameter (D)has a value of 0.1 cm-l, comparable in magnitude to the D for the analogous free ligand l,l0-phenanthroline (phen)% and unexpected for this state assigned as a d-?r* state. Such a zero field splitting parameter implying little metal character is consistent with the existence of the localized emitting states required by the photoselection spectroscopy1>l8 results. The measurement of the hfs for this excited state would provide details of the wave function and perhaps a rationale of the absence of the delocalization. Unfortunately, hyperfine structure is not normally available for S = 1 states. However, the ability of the reduced species to simulate the excited state might provide a useful first approximation of the excited-state wave function. This paper reports ESR results for the reduction products of the mono ester and tris ester complexes of Ru(I1) in an attempt to rationalize the unusual electronic structure of the reduced RuL3"+ complexes.
Experimental Section Materials. The 2,2'-bipyridine (bpy) was purchased from Aldrich and was purified by recrystallization from petroleum ether or methanol, while the diester ligands (5-COOEt and 4-COOEt) were prepared by a previously published p r o c e d ~ r e .Complexes ~ were synthesized as described in the literature. Purification was done by column chromatography on Sephadex LH-20 in acetone. Tetrabutylammonium hexafluorophosphate (TBAH) was prepared (15) Carlin, C. M.; DeArmond, M. K. Chem. Phys. Lett. 1982, 89, 297. (16) Carlin, C. M.; DeArmond, M. K. J . Am. Chem. Soc. 1985, 207,53. (17) DeArmond, M. K.; Carlin, C. M. Coord. Chem. Rev. 1981, 36, 325. (1 8) Blakley, R. L.; Myrick, M. L.; DeArmond, M. K. J . Am. Chem. Soc., 1987, 209, 2841.
(19) Yamauchi, S.; Komada, Y.; Hirota, N . Chem. Phys. Lett. 1986, 129, 197. (20) Rabold, G . P.; Piette, L. H. Photochem. Photobiol. 1966, 5 , 733.
Figure 2. ESR spectra of (a) 4COOEt- radical anion and (b) SCOOEtradical anion in 0.1 M TEAP/DMF at 298 K.
Figure 3. ESR spectra of (a) [R~(4COOEt-)(bpy)~l+ and (b) [Ru(SCOOEt-)(bpy),]+ in 0.1 M TEAP/DMF at 298 K.
by metathesis of the chloride salt (Kodak) using recrystallized KPF6. The TBAH was recrystallized twice from methanol. NJV-Dimethylformamide (DMF) (ACS-grade Fisher Scientific) was purified by storage over AW-500 molecular sieves (Alfa Products) and vacuum distilled prior to use. Cyclic voltammetric measurements and total electrolysis were carried out by using a three-compartment H-cell under a nitrogen atmosphere. Electrochemical apparatus and procedure have been reported previously.z1 ESR spectra (X-band) were measured with an IBM-Bruker Series 200 spectrometer equipped with a Model 5500 temperature controller and an LTR-3 cryostat from Air Products. The fluid solution spectra were obtained in the -60 to 25 O C range. Data were obtained on a A T & T 6300 computer with E P R D A S . ~ ~ Simulation of spectra were done with SIMESR.23 Results The cyclic voltammetry of the tris ester complexes has been reported previously as has that of [ R ~ ( b p y ) ~ ] * +The . ' voltam(21) Morris, D. E.; Hanck, K. W.; DeArmond, M. K. J . Electroanal. Chem. Interfacial Electrochem. 1983, 149, 1 1 5 . (22) Adaptable Laboratory Software, Inc., Rochester, NY. (23) Daul, C. University of Fribourg (Switzerland).
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The Journal of Physical Chemistry, Vol. 91, No. 18, 1987
TABLE I: ESR Parameters of Free Radicals Anions 4COOEt- and SCOOEt- and of [R~(4COOEt-)(bpy)~p~
Gex et al.
I i
a)
A
l‘i
I
R1
Rl
4COOEt: R, a H. R 2 COOEt 5COOEt: R1 = C O O E t , Rz * H
a
nX
u
0.31 0.31 0.081 0.037 0.27
1,l’
0.21
2N 2H 2H 2H 2N 2H 2H 2H 2N 2H
0.45
1Ru
0.165 0.12 0.045
[R~(4COOEt-)(bpy)2]’, g = 2.001
0.44
5.5’ 3,3’ 6,6’ 1,l’ 3,3’
4,4’ 6.6’ 1,l’
5,5’
‘Coupling constants are given in milliteslas ( T = 298 K; solvent = 0.1 M T B A H / D M F ) . b 9 9 R ~I := 5/2; natural abundance = 12.72%; p = -0.6430. ’O’Ru: I = J/2;natural abundance = 17.07%; p = -0.7207. TABLE 11: Activation Energies for Electron Hopping” compound AE, cm-’ [Ru(4COOEt-)(4COOEt)2]’ 670 f 50 [ R u ( ~ C O O E ~ - ) ( ~ C O O E ~ ) ~ ] + 860 f 50 [ R u ( ~ C O O E ~ - ) ~ ( ~ C O O E ~ ) ] ~380 f 50 ‘All measurements in DMF. Temperature range, -55 to 25 ‘C deg.
mograms for the mono 4,4’-ester and mono 5,5’-ester complexes are given in Figure 1. The ESR spectra of the reduced free ligands are shown in Figure 2 while the spectra of the n = 1, [ R ~ ( 5 - C 0 0 E t ) ( b p y ) ~ l I + and [R~(4-COOEt)(bpy)~]l+, are shown in Figure 3 (n = number of redox electrons). The hfs constants determined from the simulation are given in Table I. The degree of resolution of the hfs for the complex clearly correlates with that of the free ligand, with the absence of hfs for the 5,5’ complex likely a consequence of the spectral density for this species. The ESR spectra of the tris complexes of the 4,4’-COOEt ligand ( [Ru(4-COOEt),J2+)and the 5-COOEt ligand ([Ru(5-CO0Et),l2+) are broad, and no hfs can be observed in the fluid solution region for these solvents due to the intramolecular electron hopping. The Arrhenius plot of the temperaturedependent line widths produces activation energies for the n = 1 and n = 2 tris complexes of 4-COOEt and 5-COOEt given in Table 11. The line width of the n = 3 products for the 5-COOEt complex is temperature independent while the spectrum of the n = 3, 4-COOEt complex could not be obtained in fluid solution. The frozen solution spectra (77 K) for the n = 1, n = 2, and n = 3 species does indicate axial distortion for the tris 4,4’-ester complexes and the tris 5,5’-ester complexes. The spectra of the multielectron species (n > 3) are available only for the n = 4 and n = 5 reduction products of the [Ru(5COOEt)J2+ complex (Figure 4). The n = 6 species gave no signal a t any temperature from 77 to 298 K, consistent with the expectation that the complex is diamagnetic. The ESR line widths are not temperature dependent in the -55 to 25 OC range for these species (n = 4 and n = 5); however, the ESR line shape does change with an increase in temperature, indicating the Occurrence of underlying structure in these -30-G peak-to-peak lines. Detailed examination of these and other n > 3 electron species are planned to rationalize the complex line shapes and the absence of hfs.
Discussion The temperature-dependent line broadening for the tris complexes (n = 1 and 2) and the absence of temperature dependence for the n = 3 spectrum of the tris 5,5’ complex is consistent with that behavior observed for other RuL6”+one-, two- and three-
Figure 4. Frozen solution ESR spectra of [Ru(~COOE~),](~-”)+ in 0.1 M TEAH/DMF at 140 K. (a) n = 4; (b) n = 5. electron-reduction products.”’ The fact that nearly identical S = spectra are observed for the one-, two-, and three-electron products and that a temperature-dependent line broadening occurs for the n = 1 and n = 2 species but not for the n = 3 species is rationalized by the concept of the spatially isolated orbital. With such a model, each of the redox electrons is sequentially placed in accidentally degenerate orbitals localized on the three IT electron chelate rings. The electrons of n = 1 and n = 2 species are capable of hopping from ring to ring producing a TI broadening of the lines resulting in the loss of the hfs. The absence of broadening for the three-electron species is the result of the placement of one electron in each of the three orbitals with the net result that the barrier to hopping must be high. The disappearance of the fluid solution ESR signal for the n = 3 complex of the 4,4’-ester ligand is an unusual result. Indeed, the relative intensities of the successive reduction products are here a problem, since the two-electron-reduction products of the [Ru(4-COOEt),I2+ product does also give a weak signal relative to the n = 1 species. An obvious explanation is that the magnetic coupling has altered for these species and that S > ‘/2 states occur. However, no spectral evidence (ESR or optical) for such a result is evident. For example, examination of the half-field region at 77 K (Am, = 2 transitions) to measure S = 1 spectra for the n = 2 species gave no evidence of such transitions. Nor were the additional lines expected for an S = 1 species observed in the Ams = 1 region at 77 K. The occurrence of an orbital degeneracy (E state) could explain the disappearance of the three-electron signal in fluid solution. Indeed, a 2E state, if split by a vibronic interaction to give two close-lying Kramer doublets, would still produce a spin-lattice broadened signal above 77 K. However, the occurrence of an orbitally degenerate state does imply that “delocalization” over .the three rings is occurring. The resonance Raman datas and the UV-visible absorptions for the n = 3,4,4’-tris complex are not consistent with such a possibility. Ultimately, low-temperature ( T < 77 K) ESR data will be required to clarify this issue by determining the spin Hamiltonian parameters under conditions where short T , should not be a problem. The voltammograms of the mono complexes compared to those voltammograms of the three tris complexes indicate that, within the spatially isolated orbital model, the first redox electron is placed into an IT* orbital localized on the ester chelate ring and that the second and third electrons are placed in bpy ring orbitals. As for the one-electron case” of the [R~(bpz)(bpy)~]~’, the occurrence of hyperfine splitting simultaneous with the disappearance of the temperaturedependent line broadening is evidence that the electron hopping rate has been diminished for this material such that T I is adequately long to permit measurement of the hfs. The relative magnitude of the coupling constants for the reduced 4,4’-ester free ligand and the mono 4,4’-ester complex are as expected and in agreement with those results for the mono bpz reduction product. The polarizing effect of the metal ion increases the nitrogen spin density such that, for the complex, the nitrogen splitting constant is approximately equal to that of the carbon-5 whereas in the free ligand the carbon-5 splitting (and spin density) exceeds the nitrogen spin density. The hyperfine constants for + are comparable to the n = 1, [ R ~ ( 4 - C 0 O E t ) ( b p y ) ~ ]complex those observed for the [R~(bpz-)(bpy)~]+ complex with the small
ESR of Diester Bipyridines with Ru(I1)
The Journal of Physical Chemistry, Vol. 91, No. 18, 1987 4689
differences attributable to the increased electronegativity of the extra nitrogen in the bipyrazine ring. As for the mono bpz complex, a metal hfs is evident. The magnitude of the splitting here is comparable to that of the ruthenium splitting in the mono bpz system. The small magnitude (-400 cm-') of AE measured here and for some other complexes is perplexing since simple electrontransfer theoryz4for two-center mixed valence systems does predict that the k for the electron hopping would have magnitude 1011-1012,much greater than the 109-10'0 time scale of X-band ESR. This discrepancy may indicate that the two-center model is not appropriate for these trimer-like systems. The simple electron-transfer theory typically presumes that intramolecular vibrations of -400 cm-I magnitude determine the dynamics. This may not be true for this case. The possibility that the preexponential transmission coefficient K is not 1 but is small also could rationalize the discrepancy. Finally, the mechanism of the hopping for the various reduced species may be different, perhaps involving more than a single step in some cases. Consequently, for example, the molecular parameters for the hopping of the [Ru(5COOEt-)(5-COOEt),]+ may not be transferable to the [Ru(5COOEt-),(5-COOEt)] species. Extraction of the rate constant data from the ESR results rather than only the activation energy would be useful in solving this dilemma. The spin densities for the reduced free ligand can be used to estimate the spin densities for carbon-1, -3, and -6, in the mono ester complex by using the simple McConnell relations.l2 An estimate of the Ru a spin density is not readily done,24however, and the fact that g factor anisotropy due to spin-orbit coupling is small implies that Ru a spin density must be small, perhaps only 0.01 or less. The resolution of hfs in these reduced diimine complexes, as we noted earlier," results not only from increasing the barrier to hopping for the n = 1 reduction products but also from a minimization of the spectral density. Thus, the absence of well-resolved hfs for the reduced free ligand 5,5'-ester does rationalize the absence of hfs for this mono 5,S-ester complex. The inability to measure spin at the bridging carbon (C-2) atom is a key problem for all the species. M N D O calculation^^^ for the reduced bpy ligand predict a mixing coefficient comparable to that of the C-5 atom. For the diester ligands, the substituents provide an inductive and a conjugative effect upon the bpy system.
To a first approximation the ESR data imply that only a small perturbation on the electronic distribution of bpy occurs for the 4-COOEt ligand. The total width of the spectrum for reduced bpy and 4-COOEt are comparable (2.5 and 2.1 mT, respectively), indicating a small electron density for the C-4 atom. In contrast, the reduced free ligand 5-COOEt has a total spread of 1.35 mT, indicating a large amount of spin density in the 2 and 4 positions undetectable by the use of ESR spectroscopy. In agreement with chemical intuition, the coordination of the metal to the ligand has as its primary effect a polarization of the electron cloud in the direction of the coordinating nitrogen; thus the coupling constant for the nitrogen increases and the C-5 coupling decreases in the [Ru(bpy),(4-COOEt-)]' complex as observed in the bipyrazine complex.' ENDOR measurements are planned and should provide more information on the spin density distribution as the transitions from different nuclei arise at different frequencies. The simplicity of the ESR signal (no hfs) for the paramagnetic diimine complexes produced by electrochemical reduction can now be rationalized. The single S = line observed for the n = 1 and n = 2 species is the result of a dynamic process (electron hopping) producing T 1broadening. The observation of hfs for those reduced species that do not broaden with temperature verifies this explanation. However, the absence of fine structure for those reduced species with n > 1 is still not understood. A small metal mixing coefficient or a node at the metal could, in part, rationalize the absence of coupling between rings for the n = 2 sample. Additional possibilities remain: the geometry of individual chelate rings may minimize the dipole-dipole interaction or the dynamic process may average (to zero) fine structure interactions. Efforts to generate two-electron-reduction products of trans Ru(I1) complexes to test the geometry effect have not been successful. Further, the weaker signals measured for the n = 2 and n = 3 ester complexes in solution cannot yet be understood but may be a clue to the perplexing absence of S = 1 and S = 3/z spectra. Quite obviously the unusual magnetic and dynamic properties of the Ru diimines are just beginning to be understood. Future direction for the magnetic studies will attempt to obtain hopping rate constants in fluid solution from line shape analysis. Liquid He spectra may prove enlightening if such will permit the magnetic interaction (for n = 2 and n = 3 species) to be measured in the absence of the dynamic process.
(24) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1 . (25) Kuska, H. A.; Rogers, M. T. In Radical Ions; Kaiser, E. T., Kevan, L., Eds.; Interscience: New York,1968; Chapter 13.
Acknowledgment. Support of this work by National Science Foundation Grant CHE-85-07901 is acknowledged. J.N.G. was supported by funds from the Swiss National Science Foundation.
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