Electrochemical behaviour of dicyanobis(2,2′-bipyridine) ruthenium(II) and dicyanobis(1,10-phenanthroline) ruthenium(II) complexes

J. Electroanal. Chem., 77 (1977) 349--359 © Elsevier Sequoia S.A., Lausanne - - P r i n t e d in The Netherlands

349

ELECTROCHEMICAL BEHAVIOUR OF DICYANOBIS(2,2'-BIPYRIDINE) RUTHENIUM(II) AND DICYANOBIS(1,10-PHENANTHROLINE) RUTHENIUM(II) COMPLEXES S. R O F F I A

Centro di Studio di Elettrochimica Teorica e Preparativa, Istituto Chimico "G. Ciamician ", Universitd di Bologna, Bologna (Italy) M. CIANO

C.N.R., Laboratorio di Fotochimica e Radiazioni d 'Alta Energia, Bologna (Italy) (Received l l t h May 1976)

ABSTRACT The electrochemical behaviour of Ru(bipy)2(CN)2 and Ru(phen)2(CN) 2 (bipy = 2,2'-bipyridine; phen = 1,10-phenanthroline) has been investigated in dimethylformamide. Both complexes exhibit one oxidation wave and three reduction waves. In the case of Ru(bipy)2(CN)2 the anodic process and the first two cathodic processes involve one electron and are reversible in the time scale of polarographic and cyclic voltammetric experiments. The third reduction step is irreversible and has been attributed to the addition of three electrons to Ru(bipy)2(CN)2 followed by liberation of one or more ligands and reduction of liberated bipyridine. The features of the redox processes for the Ru(phen)2(CN)2 are similar to those found for t h e bipy complex except for the first reduction wave, which is complicated by adsorption phenomena. A qualitative MO discussion of the redox processes is also reported.

INTRODUCTION

In the last few years the electrochemical study of the 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen) transition metal complexes has been of increasing interest both to investigate the properties of unusual oxidation states and to gain understanding of the molecular levels involved in the redox processes [1--71. Furthermore, recent studies [8--13] have shown the possibility of using bipy and phen complexes to convert light energy, including solar energy, into chemical energy. The approach consists in utilizing the unusual redox properties of the electronically excited states of these complexes. Especially studied from this point of view has been Ru(bipy)~ +, whose lower excited state can act as both a strong reducing and a strong oxidizing agent [8--13]. As for characterizing the redox properties of the excited states it is necessary to know the redox behaviour of the molecule in its ground state [8,11--13], the

350

electrochemical studies of these transition metal complexes are of primary importance. This paper reports the study of the voltammetric behaviour of Ru(dipy)2(CN)2 and Ru(phen)2(CN)2 in dimethylformamide (DMF). REAGENTS AND A P P A R A T U S

All compounds used were reagent grade chemicals. The DMF was further purified by distillation at reduced pressure after drying it for 48 h over BaO and storage over 4A molecular sieves. The water c o n t e n t of the purified solvent determined by Karl-Fischer titration, was about 2 × 10 - 4 M. 0.1 M tetraethyla m m o n i u m perchlorate (TEAP) was used as supporting electrolyte. Pure samples of Ru(bipy)2(CN)2 • 2 H20 have been kindly supplied by prof. V. Balzani of our Institute. All measurements, unless otherwise stated, have been performed at 25 + 0.1°C. A saturated calomel electrode (SCE) separated from test solution by 0.1 M TEAP solution in DMF sandwiched between two fritted disks, was used as a reference electrode and all potentials are referred to it. A dropping mercury electrode (DME) and a platinum electrode with periodical renewal of the diffusion layer (PRPE) were used in the polarographic measurements. Hanging mercury electrode (HME) and stationary platinum electrode were used in the potential sweep voltammetry (p.s.v.). Before each experiment, platinum electrodes have been conditioned according to the procedure previously described [14]. Polarographic measurement and controlled potential electrolysis experiments were carried out with a three electrode multipurpose unit Model 563 manufactured by Amel, Milano. Potential sweep voltammetric curves were recorded by means of an Amel 448 three-electrode oscillographic polarograph. The compensation of ohmic drop was achieved with positive feed-back network of the same instrument. RESULTS AND DISCUSSION

Ru(bipy)2(CN)2 Figure 1 shows the polarographic curves recorded with a PRPE (a) and with a DME (b) for l~u(bipy)2(CN)2. In the potential range examined, the complex exhibits one oxidation and three reduction waves. The corresponding redox couples will be designated by A and by roman numerals I through III, increasing towards more negative potentials. As one can see while il j and i1,ii are equal, the i~,Hi is more than twice as high as i1,~. The recording of the anodic wave together with the first two cathodic ones with the same PRPE electrode shows identical limiting currents for the three waves. The usual logarithmic analysis of the four waves results in straight lines for the E vs. log(i 1 -- i)/i plot with slope equal to 62 mV for the wave A and 60, 57 and 62 mV for the waves I, II and III respectively. The dependence of the limiting currents on complex concentration c is linear for

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all waves. As regards the limiting current dependence on rest or drop time ta, the log il vs. log td is linear in all cases, the slopes of the straight lines being 0.2, 0.2 and 0.3 for the cathodic waves and 0.5 for the anodic one. The half wave potentials E A 1/2, E~/2 and E ~ 2 are independent on c and on td. The E,~2 does n o t vary with c, but shows a dependence on td, shifting towards more positive potentials with increasing t d. The data reported are consistent with the hypothesis that under our experimental conditions the anodic process and the two first cathodic ones are oneelectron diffusion controlled reversible processes, while the third reduction process is irreversible. In order to have a better insight into the nature of the above redox phenomena, the complex has been examined by single and cyclic potential sweep voltammetry (p.s.v.). Figure 2 shows cyclic voltammograms for Ru(bipy)2(CN)2. The arrows indicate the direction of the potential scanning. As one can see, while the A, I and II processes show reversal currents, the III reduction wave shows no oxidation wave at 25°C up to the m a x i m u m scan rate utilised, i.e. 60 V s-1 . Table 1 summarises the p.s.v, data obtained for Ru(bipy)2(CN)2. The data reported confirm that the anqdic and the two first cathodic processes are one-electron reversible processes. They can be described in terms of the following reactions: Ru(bipy)2(CN)2 ~ Ru(bipy)2(CN)~ + e -

(A)

Ru(bipy)2(CN)2 + e- ~ Ru(bipy)2(CN)2

(I)

Ru(bipy)2(CN)2 + e - ~- Ru(bipy)2(CN)2-

(If)

The irreversibility of the third reduction step is confirmed by the p.s.v, data.

352 III

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.

-

.

-

.

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a)

+•6

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A

Fig. 2. Cyclic voltammograms for 1 mM Ru(dipy)2(CN)2 in DMF solution of 0.1 M TEAP. Working electrodes: (a) Pt and (b) HME. Scan rate: 1 V s- 1 .

These d a t a also s h o w t h a t t h e irreversibility is n o t d u e to a low rate o f e l e c t r o n transfer. In fact, while the absence o f an a n o d i c p e a k in the suitable p o t e n t i a l range and the d e p e n d e n c e o f E~ H o n scan rate v c o u l d be c o n s i s t e n t w i t h an irreversibility due t o the low rate o f the e l e c t r o n transfer, t h e decrease w i t h v o f t h e r a t i o '~p,¢/v m 1/2 is n o t explicable o n this ground. F u r t h e r m o r e , a slow e l e c t r o n transfer w o u l d n o t explain the d i f f e r e n c e E~ II ---p/2vm f o u n d in p.s.v, o n the basis o f the a n value d e d u c e d f r o m the slope o f t h e straight line o f the E vs. log(il - i)/i plot. In fact f o r c o n s i s t e n c y b e t w e e n t h e d.c. and p.s.v, e x p e r i m e n t s t h e Em _ J~H~ should have b e e n a b o u t twice as high as t h a t e x p e r i m e n t a l l y f o u n d . p -~ p/2 T h e w h o l e o f the e x p e r i m e n t a l d a t a just described lead us t o t h i n k t h a t some kinetic c o m p l i c a t i o n s are p r e s e n t in the t h i r d r e d u c t i o n process. In particular, by taking into a c c o u n t t h a t : .-m ~:i at t h e l o w e s t scan (a) t h e ~"mp,cc o r r e s p o n d s , o n t h e basis o f t h e ratio ,p,c/~p,c, rate, t o a 2.4 e l e c t r o n process; (b) the b i p y m o l e c u l e is r e d u c e d at these p o t e n t i a l s with an overall process involving at least t w o electrons. T h e first step is a o n e - e l e c t r o n reversible step with Ep,c = - - 2 . 1 2 V, whereas t h e s e c o n d o n e is irreversible with an Ep,c = - - 2 . 5 0 V, at v = 15 V s - l ; (c) free b i p y has b e e n f o u n d in solutions e l e c t r o l y s e d at c o n t r o l l e d p o t e n t i a l s c o r r e s p o n d i n g t o t h e plateau o f wave III.

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354 The third reduction process could be attributed to the addition of three electrons to Ru(bipy)2(CN)2 followed by liberation of one or more ligands and reduction of liberated bipyridine, i.e. to an e.c.e, mechanism. Both polarographic and voltammetric data reported above tend to support this t y p e of mechanism. The slope of 0.059 V obtained for E vs. log(il -- i)/i plot and the shift of E 1/2 towards more positive potentials with increasing t d a r e in agreement with the proposed mechanism [ 15]. Also in p.s.v, experiments the variation of i~II/v 112 and E~H with log v are of the direction and of the order of magnitude expected for such mechanism [16]. Indications of the presence of a chemical reaction between two electron transfers are also provided by some p.s.v, experiments carried out at --25°C. In these conditions, in fact, at higher scan rates the process tends to become a one-electron reversible process. Since C N - ions are f o u n d in the solutions electrolyzed at a potential corresponding to the third reduction peak, a chemical reaction involving detach ment of C N - from [ Ru(dipy ) 2(CN) 2 ] a and reduction of di-negative species thus formed, according to an e.c.e, mechanism, should also be taken into account. With regard to the stability of the intermediates involved in the oxidation and in the first two reduction processes, controlled potential electrolysis has shown that chemical reactions are probably present in these processes too. In most cases in fact coulometric experiments gave non-integer values of napp much higher than unity. Evidently the rates of such reactions are n o t sufficiently high to produce appreciable effects in the time scale of polarographic and voltammetric experiments. In particular, calculations carried out for the case of reversible charge transfer followed by an irreversible chemical reaction give an upper value of about 0.02 s-1 for the rate constants of the chemical reactions possibly involved in these processes. Finally, in order to ascertain the role of the water present in our system as the main impurity and as crystallization water of the solid complex, cyclic voltammograms have been recorded, with increasing amounts of added water. The data obtained show no perturbation below a water concentration of 3 × 1 0 - 2 M. Furthermore no difference has been observed in the behaviour of the Complex crystallized from CH3OH. All these results seem to indicate t h a t water is n o t involved to any significant extent in the electrode process.

Ru(phen)2(CN)2 Polarographic investigation of the Ru(phen)2(CN)2 shows that this complex, like the bipy one, presents one oxidation and three reduction waves (see Fig. 3). The redox couples will thus be designated with the same symbols used previously. Also in this case the recording of the anodic and of the first two cathodic waves with the same PRPE electrode shows identical limiting currents for the three waves. Each plot of E vs. log(il -- i)/i gives for all waves straight lines with slopes equal to 60, 54, 57 and 62 mV, respectively for the waves A, I, II and III The dependence of limiting curves on depolarizer concentration is linear for all the waves, except for the first cathodic one, for which the plot is shown in Fig.

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4. Figure 5 s h o w s typical v o l t a m m o g r a m s recorded for R u ( p h e n ) 2 ( C N ) 2 at different scan rates. As o n e can see, the general behaviour is similar to t h a t f o u n d for the bipy c o m p l e x e x c e p t for the first r e d u c t i o n wave. It is clear t h a t while at l o w scan rate i p,c~ is practically equal to i H p , C , by increasing the scan rate it b e c o m e s m u c h higher than i pH, c " Tabulations of voltammetric parameters are m a d e in Table 2.

The above data show that, like the bipy complex, the anodic and the second cathodic process are one-electron reversible processes, while the third r e d u c t i o n process appears t o be irreversible. Also in this case we are inclined t o consider 1.5

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356

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that this last step is governed by an e.c.e, mechanism, in which one or more phenanthroline molecules, liberated by addition of three electrons to Ru(phen)2(CN)2 are reduced at the electrode surface. The phenanthroline molecule is reducible at these potentials. Indeed its behaviour closely resembles the one observed in bipy and reported above. The Ep of the first reversible process is in this case --2.10 V; the one of the irreversible one is --2.45 V at v = 15 V s- 1 . As to the first reduction process, the morphology of p.s.v, curves, which becomes more triangular in shape with increasing scan rates, suggests that adsorption phenomena are present. Figures 6 and 7 show respectively the dependence of i~,¢/cv 1/2 and of lp,a/lp, c ' I "I on log(v 1/2). Using the diagnostic criteria previously suggested [17], the plots of Figs. 6 and 7 confirm that the first reduction process is complicated b y adsorption phenomena. In particular the behaviour is what would be expected for weak adsorption of the reactant. REDOX ORBITALS

In order to gain understanding u p o n the molecular levels involved in the redox processes for the complexes investigated it is helpful to compare the behaviour

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359 of complexes having different ligands around the same central metal, or complexes having equal ligands and different central metals. The comparison between Ru(bipy) 2+ [1] and Ru(bipy)2(CN)2 shows that the number of reversible reduction steps is equal to the number of bipy molecules. On the other hand the E1/2 of the first reduction wave of Fe(CN)2(bipy)2 (--1.59 V) [18] is almost the same as the corresponding E1/2 (--1.54) of the analogous ruthenium complex. These features suggest that each added electron in the first two reduction waves of these bis-bipy complex occupies a molecular orbital having a large ligand II* orbital character. The comparison of the difference in the first reduction peak potentials of free ligands phen and bipy, and of the relative ruthenium complexes shows that only a minor change is taking place upon complexation. Again this kind of behaviour is that expected if the ligands only are involved in the reduction step. It remains to be seen which orbital is involved in the irreversible reduction step III of Ru(bipy)2(CN)2. This process could be interpreted as the electron entering into an essentially metal centered orbital, thus bringing the metal to a d 7 configuration from which irreversible ligand detachment ensues. Finally the oxidation of Ku(bipy)u(CN)2 can be compared with the corresponding Fe(bipy)2(CN)2 oxidation [18]. The considerable difference in the half wave potentials ( 2 0 . 4 V) seems to indicate that the removed electron comes from a metal centered orbital t2g in octahedral symmetry. ACKNOWLEDGEMENTS The authors are indebted to Prof. G. Semerano for his interest in this work. They also want to thank Prof. V. Balzani and his coworkers for valuable suggestions and stimulating discussions. This work has been partly supported by the National Research Council of Italy, which is gratefully acknowledged. REFERENCES

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

N.E. T o k e l - T a k v o r y a n , R . E . H e m m i n g w a y a n d A . J . B a r d , J. A m e r . C h e m . Soe., 9 5 ( 1 9 7 3 ) 6 5 8 2 . T. Saji a n d S. A o y a g u i , J. E l e c t r o a n a l . C h e m . , 5 8 ( 1 9 7 5 ) 4 0 1 ; 6 0 ( 1 9 7 5 ) 1; 6 3 ( 1 9 7 5 ) 4 0 5 . T. Sail, T. Y a m a d a a n d S. A o y a g u i , J. E l e e t r o a n a l . C h e m . , 61 ( 1 9 7 5 ) 1 4 7 . T. Saji, T. F u k a i a n d S. A o y a g u i , J. E l e e t r o a n a l . C h e m . , 6 6 ( 1 9 7 5 ) 8 1 . T. Saji a n d S. A o y a g u i , Bull. C h e m . S o c . J a p . , 4 6 ( 1 9 7 3 ) 2 1 0 1 ; 4 7 ( 1 9 7 4 ) 3 8 9 . T. Sail, T. Y a m a d a a n d S. A o y a g u i , Bull. C h e m . S o c . J a p . , 4 8 ( 1 9 7 5 ) 1 6 4 1 . G. K e w , K. De A r m o n d a n d K. H a n e k , J. P h y s . C h e m . , 7 8 ( 1 9 7 4 ) 7 2 7 . A. J u r i s , M.T. G a n d o l f i , M . F . M a n f r i n a n d V. B a l z a n i , J. A m e r . C h e m . Soe., 9 8 ( 1 9 7 6 ) 1 0 4 7 . R . C . Y o u n g , T.M. M e y e r a n d D . G . W h i t t e n , J. A m e r . C h e m . S o c . , 97 ( 1 9 7 5 ) 4 7 8 1 . C. C r e u t z a n d N. S u t i n , P r o c . N a t . A c a d . Sci. U S A , 7 2 ( 1 9 7 5 ) 2 8 5 8 . F. B o l l e t t a , M. M a e s t r i , L. M o g g i a n d V. Balzani, J. C h e m . S o c . C h e m . C o m m u n . , 9 0 1 ( 1 9 7 5 ) . C. C r e u t z a n d N. S u t i n , I n o r g . C h e m . , 1 5 ( 1 9 7 6 ) 0 0 0 . V. Balzani, L. Moggi, M . F . M a n f r i n , F. B o l l e t t a a n d G.S. L a u r e n c e , C o o r d . C h e m . R e v . , 1 5 ( 1 9 7 5 ) 3 2 1 . G. K e w , K. D e A r m o n d a n d K . H a n c k , J. P h y s . C h e m . , 7 8 ( 1 9 7 4 ) 7. B. K a s t e n i n g , P r o g r e s s in P o l a r o g r a p h y , Vol. 3~ W i l e y - I n t e r s c i e n c e , N e w Y o r k , 1 9 7 2 , C h . I V , p. 1 9 5 . M. M a s t a g o s t i n o , L. N a d j o a n d J . M . S a v e a n t , E l e e t r o c h i m . A c t a , 1 3 ( 1 9 6 8 ) 7 2 1 . R . H . W o p s c h a l l a n d I. S h a i n , A n a l . C h e m . , 3 9 ( 1 9 6 7 ) 1 5 1 4 . T. Saji, T. Y a m a d a a n d S. A o y a g u i , Bull. C h e m . Soe. J a p . , 4 8 ( 1 9 7 5 ) 1 6 4 1 .