Langmuir 1990,6, 51-56
51
Applications of Thin-Layer FTIR, UV-vis, and ESR Spectroelectrochemistry for Evaluating (TPP)Ru(CO) Redox Reactions in Nonaqueous Media? X. H. Mu and K. M. Kadish* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received June 26, 1989.In Final Form: September 8,1989 In situ FTIR, UV-vis, and ESR spectroelectrochemistry were combined with microvoltammetry and classical electrochemical techniques in order to elucidate the prevailing electron-transfer mechanism for the oxidation and reduction of (TPP)Ru(CO) (where TPP is the dianion of tetraphenylporphyrin) in six different nonaqueous solvents. (TPP)Ru(CO) undergoes four reversible oxidation/reduction reactions at a 25-pm microelectrode. All four electrode reactions were investigated with respect to the site of electron transfer, the stability of the electrooxidation/reduction product, the fate of the axially bound CO ligand, and the presence or absence of other axial ligands. A CO ligand remains coordinated to the singly oxidized, singly reduced, and doubly reduced forms of (TPP)Ru(CO)(L) (where L is a solvent molecule). The doubly oxidized species is only stable on the cyclic voltammetr time scale, and a rapid chemical reaction occurs after electrogeneration of [ (TPP)Ru(CO)(L)]'+. The CO vibration frequencies of [(TPP)Ru(CO)(L)]+,[(TPP)Ru(CO)]+,(TPP)Ru(CO),(TPP)Ru(CO)(L),[(TPP)Ru(CO)(L)]-, and [(TPP)RU(CO)(L)]~vary between 1985 and 1853 cm-l. The CO vibration frequency of [(TPP)Ru(CO)(L)]"(where n varies from +1 to -2) depends upon the overall charge of the complex and shifts by 27-52 cm-' per unit change in n. The bindin of a solvent molecule to [(TPP)Ru(CO)]+shifts the CO vibration to a higher frequency by 2-25 cm- f? depending upon the ligand, but no systematic trend in the CO vibration was observed upon ligand binding by neutral (TPP)Ru(CO).
Introduction Thin-layer spectroelectrochemistry techniques are now widely utilized for spectrally monitoring redox reactions of various inorganic and organometallic One advantage of these combined electrochemical/spectroscopic methods is that rapid and complete electrolysis can be achieved in a very short time domain. Diffusion of the electrogenerated species in the thin-layer chamber is negligible, and a spectral characterization of the final electrode reaction products or reaction intermediates can be easily and rapidly carried out. Our own laboratory has concentrated in large part on the use of thin-layer spectroelectrochemistry for evaluating metalloporphyrin redox reactions. Numerous potentials of porphyrin redox reactions have been obtained over the last 25 years,' but it is only recently that one has been able t o routinely identify t h e spectra of each electrooxidation/reduction product. In many cases, these spectroscopic data are crucial for understanding the specific electron-transfer pathways of a given complex as well as for evaluating the fate and/or stability of the electrogenerated species. Several examples are illustrated in the present paper, which demonstrates the combined use of thin-layer FTIR, UV-vis, and ESR spectroelectrochemistry to characterize the electroreduction and electroox-
'
Presented at the symposium entitled "Photoelectrochemical and Electrochemical Surface Science: Microstructural Probes of Electrode Processes", sponsored jointly by the Divisions of Analytical Chemistry and Colloid and Surface Chemistry, 197th National Meeting of the American Chemical Society, Dallas, April 9-14, 1989. (1) Heineman, W. R. J. Chem. Educ. 1983,60, 305. (2) Heineman, W. R.; Hawkridge, F. M.; Blount, H. N. Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, Basel, 1984; Vol. 13, p 1. (3) Beden, B.; Lamy, C. Spectroelectrochemistry Theory and Practice; Gale, R. J., Ed.; Plenum Press: New York, 1988. (4) Bagchi, B. N.; Bond, A. M.; Scholz, F. Electroanalysis 1989, I , 1.
(5) Foley, J. K.; Pons, S. Anal. Chem. 1985,57, 945A. (6) Kadish, K. M. Prog. Inorg. Chem. 1986, 34, 435.
0743-7463/90/2406-0051$02.50/0
idation of Ru(11) porphyrins complexed with carbon monoxide and a sixth axial ligand. The investigated comp o u n d s a r e r e p r e s e n t e d by ( T P P ) R u ( C O ) or (TPP)Ru(CO)(L),where TPP = the dianion of tetraphenylporphyrin and L = a coordinated solvent molecule. This present work is an extension of our earlier electrochemical and spectroelectrochemical studies on synthetic iron and cobalt metalloporphyrins containing bound No7-11 or CO'2p'3 axial ligands. The reactivity of ruthen i u m p o r p h y r i n s h a s b e e n c o m p a r e d t o t h a t of the iron porphyrins,14 and numerous s t r u ~ t u r a l , ~and ~ *e~l e~c*t r~o~~ h e m i c a l ~investigations ~-~~ (7) Olson, L. W.; Schaeper, D.; Lancon, D.; Kadish, K. M. J. Am. Chem. SOC.1982,104, 2042.(8) Lancon, D.; Kadish, K. M. J . Am. Chem. SOC.1983, 105, 5610. (9) Kelly, S.; Lancon, D.; Kadish, K. M. Inorg. Chem. 1984,23,1451. (10) (a) Mu, X. H.; Kadish, K. M. Inorg. Chem. 1988,27,4720. (b) Mu, X. H., Kudish, K. M. Inorg. Chem., in press. (11) Kadish, K. M.; Mu, X. H.; Lin, X. Q. Inorg. Chem. 1988, 27, 1489. (12) Swistak, C.; Kadish, K. M. Inorg. Chem. 1987, 26, 405. (13) Mu, X. H.; Kadish, K. M. Inorg. Chem. 1989,28,3743. (14) James, B. R. The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. V, p 286. (15) Collman, J. P.; Brauman, J. I.; Fitzgerald, J. P.; Sparapany, J. W.; Ibers, J. A. J. Am. Chem. SOC.1988, 110, 3486. (16) Tsutsui, M.; Ostfeld, D.; Hoffman, L. M. J. Am. Chem. SOC. 1971, 93, 1820. (17) Antipas, A.; Buchler, J. W.; Gouterman, M.; Smith, P. D. J. Am. Chem. SOC.1978,100,3015. (18) Barley, M.; Dolphin, D.; James, B. R.; Kirmaier, C.; Holten, D. J. Am. Chem. SOC.1984,106, 3937. (19) Bonnet, J. J.; Eaton, S. S.; Eaton, G. R.; Holm, R. H.; Ibers, J. A. J. Am. Chem. SOC.1973,95, 2141. (20) Eaton, G. R.; Eaton, S. S. J. Am. Chem. SOC.1975, 97, 235. (21) Collman, J. P.; Brauman, J. I.; Fitzgerald, J. P.; Hampton, P. D.; Naruta, Y.; Sparapmy, J. W.; Ibers, J. A. J. Am. Chem. SOC.1988, 110,3417. (22) Little, R. G.; Ibers, J. A. J . Am. Chem. SOC.1973, 95, 8583. (23) Kadish, K. M.; Chang, D. Inorg. Chem. 1982,21, 3614. (24) Kadish, K. M.; Leggett, D. J.; Chang, D. Inorg. Chem. 1982, 21, 3618. (25) Malinski, T.; Chang, D.; Bottomley, L. A.; Kadish, K. M. Inorg. Chem. 1982,21, 4248.
0 1990 American Chemical Society
52 Langmuir, Vol. 6, No. I , 1990
M u and Kadish
of (P)Ru(CO)and (P)Ru(CO)(L) (where P = a given porphyrin ring) have been reported in the literature. However, the electrochemical studies of these compounds have been limited in large part to electrooxidations, and very little voltammetric information is available with respect to the electroreduced complexes. (TPP)Ru(CO) is known to undergo two reductions in Me,SO and one reduction in other nonaqueous solvent^,'^ but very little is known as to the nature of the electroreduced products. The addition of one electron t o (TPP)Ru(CO)(L)has been assigned to occur a t the porphyrin K ring system to produce a porphyrin 7~ anion r a d i ~ a l ,but ~ ~spectral . ~ ~ data of the anion radical have not been published nor are there any data available in the literature as to the fate of the axially complexed CO after electrogeneration of [ (TPP)Ru(CO)(L)]- or [(TPP)RU(CO)(L)]~-.
Experimental Section Instrumentation and Procedure. Solution infrared measurements were carried out using an IBM 32 FTIR spectrometer and an FTIR spectroelectrochemical cell whose con~ t r u c t i o nand ~ ~application'0*" have been described in the literature. Difference spectra were obtained by subtraction of the spectrum before and after each electron-transfer step. Positive absorbance peaks in the difference spectra correspond to a product generated during electrolysis, while negative absorbance peaks correspond to a disappearance of the reactants. Thin-layer UV-vis spectroelectrochemical experiments were performed with a vacuum-tight thin-layer spectroelectrochemA ical cell whose design has been described in the literat~re.~' Tracor Northern 1710 holographic optical spectrometer/ multichannel analyzer was used to obtain time- or potentialresolved UV-vis spectra. In situ thin-layer ESR measurements were performed with a column-shapedthin-layer ESR spectroelectrochemicalcell which has a 9.4 X 4.0 mm expanded platinum sheet working electrode with an opening of 0.25 X 0.51 mm. A complete description of the cell construction and operation is given el~ewhere.~' Controlled potential electrolysis was carried out with a PAR Model 174A polarographic analyzer or an IBM 225A voltammetric analyzercoupled with a Princeton Applied Research Model RE 0074 X-Y recorder. Microvoltammetric experiments were carried out inside a well-grounded Faraday cage using a homemade potentiostat which was driven by a PAR Model 175 universal programmer. A Tetronix Model 5111 storage oscilloscope and a Tetronix Model CS-A camera were used to record current-voltage curves. A homemade three-electrode electrochemical cell was utilized and consisted of a 25-pm-diameter platinum working electrode, a large surface area auxiliary electrode, and a saturated calomel reference electrode (SCE). Except when otherwise noted, all solutions contained 0.1 M M tetrabutylammonium perchlorate (TBAP) and 1.0 X porphyrin. Deaeration was performed by passing a stream of solvent-saturated high-purity nitrogen or argon through the solution for 5-10 min and maintaining a positive pressure of the inert gas over the solution while making the measurements. Chemicals. Six different nonaqueous solvents were used in this study. Each solvent was either freshly distilled under highpurity nitrogen or argon or vacuum distilled before use. Spectroanalyzed grade methylene chloride (CH,Cl,) was distilled from P,O,. Pyridine (Py),dimethyl sulfoxide (Me,SO), and tetrahydrofuran (THF) were freshly distilled from CaH, under argon
-
(26) Barley, M.; Becker, J. Y.; Domazetis, G.; Dolphin, D.; James, B. R. Can. J. Chem. 1983,61, 2389. (27) Barley, M.; Becker, J. Y.; Domazetis, G.; Dolphin, D.; James, B. R. J. Chem. SOC., Chem. Commun. 1981, 982. (28) Rillema, D. P.; Nagle, J. K.; Barringer, Jr., L. F.; Meyer, T. J. J . Am. Chem. SOC.1981, 103,56. (29) Brown, G. M.; Hoft, F. R.; Ferguson, J. A,; Meyer, T. J.; Whitten, D. G. J . Am. Chem. SOC.1973,95, 5939. (30) Kadish, K. M.; Mu, X. H.; Lin, X. Q. Electroanalysis 1989, 1, 35.
(31) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985,57, 1498. (32) Mu, X. H.; Kadish, K. M. Electroanalysis, in press.
b
L---
-1-
-1 0
-1 5
~
--..L_-_j
-~
-2 0
-2 5
POTENTIAL, V vs SCE
Figure 1. Cyclic voltammogram of 1.5mM (TPP)Ru(CO)(scan rate = 10 V/s) at a 25-pm-diametermicroelectrodein THF containing 0.1 M TBAP. before use. Benzonitrile (PhCN)and dimethylformamide(DMF) were freshly distilled from activated 4-A molecular sieves. (TPP)Ru(CO)was prepared according to literature methods.33
Results and Discussion Electroreduction of (TPP)Ru(CO) at a Microelectrode. (TPP)Ru(CO) undergoes two reversible (Nernstian) one-electron reductions at a conventional electrode in Me,SO. These reactions occur at E , = -1.35 and -1.78 V to generate [ (TPP)Ru(CO)(Me,dO)]and [ (TPP)Ru(C0)l2- or [(TPP)Ru(CO)(Me SO)]'- as shown in eq 1 and 223J8 (where L = Me,S0)34
,
(TPP)Ru(CO)(L)+ e
+
[ (TPP)Ru(CO)(L)]- e
[(TPP)Ru(CO)(L)]- (1) t
[(TPP)Ru(CO)(L)]~-(2)
Figure 1shows acyclicvoltammogramof (TPP)Ru(CO)(THF) in THF containing 0.1 M TBAP at a 25-pm-diameter microelectrode. Under these conditions, the compound undergoes two Nernstian reductions at El,, = -1.46 and -2.03 V, both of which are well defined. Two reductions of (TPP)Ru(CO)(L) (where L = a solvent molecule) are also observed at a microelectrode in PhCN, Py, Me,SO, and DMF, and half-wave potentials for each of these reactions are listed in Table I. The potential separation between the first and second reduction of (TPP)Ru(CO) ranges between 0.47 V in Me,SO and 0.59 V in DMF and P y (see Table I). The values of AEl in PhCN (0.56 V), THF (0.57 V), DMF (0.59 V), and b y (0.59 V) are larger than the separation of 0.42 f 0.05 V generally observed for other porphyrin K ring centered reduction^.^'^^ This might suggest that the second electron addition occurs at the central Ru ion, but there are a number of exceptions to the trend in AE, ,6 and this diagnostic criteria is not so strong so as t o definitely support the assignment of a metal-centered reaction. Thin-Layer UV-vis Monitoring of (TPP)Ru(CO) Reductions. The UV-vis spectral changes observed during the stepwise controlled potential electroreduction of (TPP)Ru(CO)(THF) in THF a t -1.7 and -2.2 V are shown in Figure 2. Isosbestic points are observed at 383, 421, 514, and 540 nm during the first reduction (see Figure Za), indicating the presence of only two spectroscopically detectable species in solution. The final spectrum of [(TPP)Ru(CO)(THF)]-has a split Soret band at,,,A = 421 and 445 nm and broad visible bands over the range 500-700 nm. The spectral changes in Figure 2a are con-
,
(33) Adler, A.; Longo, F.; Finarelli, J.; Goldmacher, J.; Assour, J.; Kaasakoff, L. J. Org. Chem. 1967, 32, 476. (34) The binding of a solvent molecule to the singly reduced complex has been e s t a b l i ~ h e dbut , ~ ~the data in the present paper are not clear with respect tp solvent binding by the doubly reduced [ (TPP)Ru(CO)]'- species. ( 3 5 ) Fuhrhop, J.-H.; Kadish, K. M.; Davis, P. G. J . Am. Chem. SOC. 1973, 9Ei, 5140.
Langmuir, Vol. 6, No. 1, 1990 53
Redox Reactions of (TPP)Ru(CO):FTIR, UV-vis, ESR
Table I. Half-Wave Potentials for Oxidation and Reduction of (TPP)Ru(CO)* E l l z , V vs SCE
reduction
oxidation solvents
2nd
CH,Cl, PhCN PY Me,SO DMF THF
1.43 1.35 1.37
1st 0.87
0.93 1.02
1.33 1.33 1.46
LiE
1st
2nd
Ai3
0.56
-1.59 -1.44 -1.43 -1.38 -1.41 -1.46
-2.08 -2.00
0.49
0.42 0.35 0.33 0.33 0.34
1.00 1.00 1.12
-2.02 -1.85 -2.00
-2.03
0.56 0.59
0.47 0.59 0.57
'A t a 25-pm-diameter Pt microelectrode
in selected solvents containing 0.1 M TBAP. In all solvents except for CH,Cl,, the actual species in solution is (TPP)Ru(CO)(L)where L = a solvent molecule (see ref 23). I
1
L
1
1
300
400
500
600
700
800
WAVELENGTH, nm
Figure 2. Time-resolved thin-layer UV-vis spectra obtained during stepwise controlled potential reduction of (TPP)Ru(CO) in THF containing 0.1 M TBAP at (a) -1.7 and (b) -2.2 V.
sistent with a one-electron addition to the porphyrin ir ring system and the generation of a porphyrin ir anion radical. Spectral changes which occur upon controlled potential reduction of [(TPP)Ru(CO)(THF)]- a t -2.2 V are shown in Figure 2b. Isosbestic points are located a t 450, 644, and 674 nm as the Soret peak a t 445 nm shifts to 455 nm during controlled potential reduction. The final spectrum of [ (TPP)Ru(CO)(THF)]'- in Figure 2b has peaks a t 421 and 455 nm and is featureless between 500 and 800 nm. This spectrum is very similar to the spectrum of doubly reduced (TPP)Rh(02).36 In the latter case, the first electron addition is to the Rh central metal and the second electron addition is to the porphyrin a ring system of an electrogenerated rhodium porphyrin dimer. The spectral data shown in Figure 2 are consist e n t w i t h a p o r p h y r i n d i a n i o n , a n d t h e second electroreduction product can thus be assigned as [(TPP)Ru(CO)(THF)]'- or [(TPP)Ru(CO)]'-. FTIR Monitoring of (TPP)Ru(CO)Reductions. Figure 3 shows the changes which occur in the FTIR spectrum of (TPP)Ru(CO)in TI-IF during the controlled potential reduction a t -1.7 and -2.2 V. The spectrum of (TPP)Ru(CO)(THF) before reduction is shown in Figu r e 3 a . T h e s t r o n g a b s o r p t i o n a t 1 9 4 1 cm-' is d u e t o t h e CO vibration of t h e complex a n d is well defined in T H F containing 0.1 M TBAP. This peak d i s a p p e a r s a s ( T P P ) R u ( C O ) ( T H F ) is converted t o [ ( T P P ) R u ( C O ) ( T H F ) ] - , a n d a t t h e same time, a positive CO vibration peak is observed a t 1898 cm-'. This conversion of (TPP)Ru(CO)(THF) to [ (TPP)Ru(CO)(THF)]-is shown by the difference FTIR (36) Anderson, J. E.; Yao, C.-L.; Kadish, K. M. Inorg. Chem. 1986,
25, 3224.
1841%
I 2500 2300 2100 le00 1700 1500
WAVENUMBER, cm-'
Figure 3. (a) In situ FTIR spectrum of (TPP)Ru(CO)in THF containing 0.1 M TBAP. (b) Difference FTIR spectrum after controlled potential reduction at -1.7 V. (c) Difference FTIR spectrum after controlled potential reduction at -2.2 V.
spectrum in Figure 3b and unambiguously demonstrates that the CO ligand remains bound to the complex after formation of a porphyrin ir anion radical. The difference FTIR spectrum obtained after the addition of two electrons to (TPP)Ru(CO)(THF)is shown in Figure 3c. This spectrum has a negative peak a t 1941 cm-l, consistent with the disappearance of (TPP)Ru(CO)(THF) as [(TPP)Ru(CO)(THF)]'- or [(TPP)Ru(CO)]'is formed in solution. The positive peak a t 1853 cm-l is assigned as the CO vibration of either [(TPP)Ru(CO)(THF)]'- or [(TPP)Ru(CO)]'-, while the positive peak a t 1898 cm-' in Figure 3c is due to the singly reduced [ (TPP)Ru(CO)(THF)]- intermediate which is present before total conversion to the doubly reduced product. The presence of a peak at 1853 cm-' clearly indicates that a CO ligand remains coordinated to the doubly reduced species in THF. Similar FTIR measurements were carried out during the electroreduction of (TPPIRu(C0) in Me'SO, Py, DMF, or PhCN, and the resulting CO vibration frequencies of (TPP)Ru(CO)(L),[(TPP)Ru(CO)(L)]-, and [(TPP)Ru(C0)(L)l2- are summarized in Table 11. Each one-electron addition to (TPP)Ru(CO)(L) shifts the CO vibration to lower frequencies by 27-52 cm-l, and this can be explained by the a-ir interaction between the central Ru metal and the axially coordinated CO group. This T-ir interaction results in a delocalization of the electron den-
M u and Kadish
54 Langmuir, Vol. 6 , No. I, 1990 Table 11. CO Vibration Frequencies (Wavenumber, em-')* of (TPP)Ru(CO),(TPP)Ru(CO)(L),[(TPP)Ru(CO)(L)]+,* [ (TPP)Ru(CO)(L)J-, and [(TPP)RU(CO)(L)]~in Nonaqueous Solvents Containing 0.1 M TBAP ligand
[(TPP)Ru-
(TPP)Ru-
[(TPPIRu-
[(TPP)Ru-
(L)
iCO)(L)l+
(CO)(L)
(Co)(L)l-
(c0)(L)l2-
none MezSO
PY DMF PhCN THF
1960' 1964 1985 1962 1985 1971
d 1896 1905 1896 1896 1898
1937' 1935 1942 1923 1946 1941
d 1853
L!--
1853
"Values of uc0 good to f 2 cm-'. * D a t a obtained in CH,Cl, containing 0.1 M TBAP and Py (0.06 M), Me,SO (0.06 M), DMF (0.06 M), THF (0.61 M), or PhCN (0.98 M), respectively. Neither the CH,Clz solvent nor the C10,- supporting electrolyte anion are coordinated to the complex. The reduction of (TPP)Ru(CO) in CH,Clz is irreversiblez3 and involves a reaction of the reduced porphyrin with the solvent.41
I_-12
14
I
10
i-
08
i
J
06
POTENTIAL V vs SCE
Figure 5. Conventional cyclic v o l t a m m o g r a m of 0.5 m M ( T P P ) R u ( C O ) in CH,Cl, containing 0.1 M TBAP (scan rate = 0.1 VIS).
W
I
I
I
~
ii 300
400
500
600
700
WAVELENGTH, nm
Figure 6. Time-resolved thin-layer UV-vis spectra during controlled potential oxidation of ( T P P ) R u ( C O ) a t 1.2 V in CH,Cl, containing 0.1 M T B A P .
(TPP)Ru(CO) G [(TPP)Ru(CO)]++ e I ,
v
Figure 4. In situ thin-layer ESR spectrum of electrogenerated [(TPP)Ru(CO)(THF)]-at -1.7 V in THF containing 0.1 M TBAP.
sity from the central metal d orbitals to the CO group antibonding orbitals. This will weaken the C-0 bond and be reflected by a lower CO vibration frequency and a longer C-0 bond.37 Similar shifts in CO vibration frequency have been reported for other systems where each one-electron addition causes a shift of 12-20 cm-' toward a lower f r e q ~ e n c y . ~ ' ESR Monitoring of (TPP)Ru(CO) Electroreduction. In situ thin-layer ESR spectroelectrochemistry was also used to monitor the electron transfer reaction shown in eq 1, a n d t h e ESR spectrum of t h e first electron reduction product a t -1.7 V in T H F is shown in Figure 4. The spectrum of [(TPP)Ru(CO)(THF)]-is well defined and has g = 2.00 and a peak to peak separation, AH,of 5.4 G, both of which are characteristic of a porphyrin P anion radical.39 Thus, the ESR data are also consistent with the UV-vis data in regard to the site of electron transfer for the reduction of (TPP)Ru(CO)(L). Electrooxidation of (TPP)Ru(CO). Figure 5 shows a cyclic voltammogram for the oxidation of (TPP)Ru(CO) in CH,Cl, a t a conventional Pt electrode. Two reversible oxidations are observed a t 0.87 and 1.43 V, and these correspond to the electrode reactions given by eq 3 and 4.40 (37) Cotton, F. A.; Wilkinson, G. Advanced Inorg. Chem., 5th ed.; Willey: New York, 1988; Chapter 2. (38) Shu, C. F.; Wrighton, M. S. Inorg. Chem. 1988, 27, 4326. (39) Fajer, J.; Davis, M. S. The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. IV, pp 198-256. (40) Neither the CH,CI, solvent nor the C10,- supporting electrolyte anion is coordinated.
(3)
[(TPP)Ru(CO)]+G [(TPP)Ru(C0)l2++ e (4) The UV-vis spectral changes monitored during the first one-electron oxidation of (TPP)Ru(CO) in CH,Cl, are shown in Figure 6. Well-defined isosbestic points are observed a t 398,432,510, and 544 nm. The final oxidation product after the abstraction of one electron has a UV-vis spectrum with split Soret peaks a t 396 and 439 nm and a relatively low molar absorptivity compared with the spectrum of neutral (TPP)Ru(CO). The final UVvis spectrum of [ (TPP)Ru(CO)]+ is characteristic of a metalloporphyrin P cation radical and clearly shows that the first electron abstraction is from the porphyrin x ring system. Isosbestic points are not observed during the second one-electron abstraction from (TPP)Ru(CO),and the final UV-vis spectrum of the product generated a t 1.6 V is featureless. This indicates that the ultimate species produced in the second oxidation is not the one shown by eq 4 but that a chemical reaction most likely has occurred after electrogeneration of [ (TPP)Ru(C0)l2' in solution. [ (TPP)Ru(C0)l2+has been reported to undergo a decomposition reaction which leads to loss of the axial CO group.'' This appears to be the case as shown by the UV-vis data as well as by in situ generated FTIR spectral data, which are discussed in the following paragraphs. Figure 7a shows t h e F T I R spectrum of neutral (TPP)Ru(CO)in CH,Cl, and 0.1 M TBAP. The CO vibration a t 1937 cm-' disappears when (TPP)Ru(CO)is converted to [ (TPP)Ru(CO)]+,and this process is accompanied by the appearance of a new CO vibration a t 1960 cm-' (see Figure 7b). These data unambiguously show that the CO group remains ligated to electrogenerated [(TPP)Ru(CO)]+. The CO vibration of [(TPP)Ru(CO)]' is shifted by 23 cm-' from that of the unoxidized (41) Deng, Y. J.; Mu, X. H.; Kadish, K. M., manuscript in preparation.
Redox Reactions of (TPP)Ru(CO):FTIR, UV-vis,ESR
Langmuir, Vol. 6, No. I, 1990 55
I1
\L
1937
I
2300
II
I
IS00
1700
1500
WAVENUMBER. cm-'
u
Figure 8. (a) FTIR spectrum of (TPP)Ru(CO) in CH,Cl, containing 0.98 M PhCN and 0.1 M TBAP before electrooxidation. (b) Difference FTIR spectrum after controlled potential oxidation a t 1.2 V.
2200 21M) 2000 Is00 1800 1700 WAVENUMBER. Cm-'
Figure 7. (a) In situ FTIR spectrum of (TPP)Ru(CO) in CH,Cl, containing 0.1 M TBAP. (b) Difference FTIR spectrum after controlled potential oxidation a t 1.2 V.
(TPP)Ru(CO),and this is comparable to differences in the CO vibration of (OEP)Ir(CO)Cl (2056 cm-') and [ (OEP)Ir(CO)Cl]+ (2081 ~ m - ' ) ~ in ' CH,Cl, (where OEP is the dianion of octaethylporphyrin and the oxidation occurs a t the porphyrin 7~ ring system). A characteristic CO vibration of [(TPP)Ru(C0)l2+was n o t observed a f t e r t h e second electrooxidation. T h i s is c o n s i s t e n t with l i t e r a t u r e data,,' which indicates a decomposition of t h e doubly oxidized porphyrin product. The loss of CO or NO from doubly oxidized [ (TPP)CO(CO)]~+,'~ [ (TPP)CO(NO)]~+,"or [(P)Fe(N0)l2+lo has been reported in the literature under similar experimental conditions. FTIR Monitoring of Axial Ligand Binding. The b i n d i n g of a x i a l l i g a n d s t o ( T P P ) R u ( C O ) a n d electrogenerated [ (TPP)Ru(CO)]+in CH,Cl, was monitored by FTIR spectroelectrochemistry. Figure 8a shows the FTIR spectrum of (TPP)Ru(CO)in CH2C1, containing 0.98 M PhCN. A CO vibration of (TPP)Ru(CO)(PhCN) is observed a t 1946 cm-' before electrooxidation, and this value can be compared to 1937 cm-' for the neutral five-coordinate (TPP)Ru(CO) species in pure CH,Cl,. This difference in uco indicates the binding of a PhCN ligand to (TPP)Ru(CO) to form the six-coordinate (TPP)Ru(CO)(PhCN)species, as shown by eq 5. (TPP)Ru(CO) + PhCN s (TPP)Ru(CO)(PhCN) (5) The conversion of (TPP)Ru(CO)(PhCN) to [ (TPP)Ru(CO)(PhCN)]+ was also monitored by FTIR spectroscopy, and this is shown in Figure 8b. A CO vibration of [ (TPP)Ru(CO)(PhCN)]+is observed a t 1985 cm-', and t h i s value can be compared to vco = 1960 cm-l for [(TPP)Ru(CO)]+in pure CH,Cl, (see Figure 7b). Similar shifts in uco are observed upon complexation of (TPP)Ru(CO) or [(TPP)Ru(CO)]+with Py, Me2S0, DMF, or T H F as shown by eq 6 and 7. The resulting FTIR data for the CO vibration of (TPP)- Ru(CO), [(TPP)Ru(CO)]+, and [(TPP)Ru(CO)(L)]+ in CH,Cl, and CH,Cl,/L mixtures are listed in Table 11. (TPP)Ru(CO)+ L .= (TPP)Ru(CO)(L)
1
2100
(6)
[(TPP)Ru(CO)]++ L
2
[(TPP)Ru(CO)(L)]+ (7)
In conclusion, the combination of in situ FTIR, ESR, and UV-vis spectroscopic techniques with conventional electrochemical methodologies can be used to elucidate a n d m o n i t o r t h e e l e c t r o n - t r a n s f e r r e a c t i o n s of (TPP)Ru(CO). The site of electron transfer, the fate of the axial bound CO, and the presence or absence of a sixth axial ligand can all be monitored on a time scale of less than 1 min. In the present study, the first electron addition and the first electron abstraction of (TPP)Ru(CO) involve the porphyrin K ring system, and the CO ligand remains coordinated to the singly oxidized, singly reduced, and doubly reduced forms of the complex. This is not the case for doubly oxidized [ (TPP)Ru(CO)(L)I2+,which is only stable on the cyclic voltammetric timescale. Each electron addition to [ (TPP)Ru(CO)(L)]+ shifts the CO vibration to a lower frequency by 23-52 cm-', but axial ligand binding to [(TPP)Ru(CO)]+ shifts the CO vibration to a higher frequency by 2-25 cm-l. No systematic trends in the CO vibration are noted upon the binding of an axial ligand by neutral (TPP)Ru(CO). The CO vibration frequencies of the electrogenerated complexes occur between 1985 and 1853 cm-l and decrease in the following order: [(TPP)Ru(CO)(L)]" > [(TPP)Ru(CO)]+ > (TPP)Ru(CO)- (L) > [(TPP)Ru (CO)(L)]> [(TPP)Ru(CO)(L)I2-.
Acknowledgment. The support of the National Science Foundation (Grant CHE-8822881) is gratefully acknowledged. Registry No. DMA, 68-12-2; T H F , 109-99-9; (TPP)Ru(C0),32073-84-0; [(TPP)Ru(CO)]+, 43145-32-0; [(TPP)Ru(C0)l2+,123332-72-9; [(TPP)Ru(CO)]-, 112490-263; [ (TPP)Ru(CO)]~-,123332-73-0; (TPP)Ru(CO)(PhCN),8261454-8; [ (TPP)Ru(CO)(PhCN)]+, 82614-61-7; [(TPP)Ru(CO)(PhCN)]'+, 123332-74-1; [(TPP)Ru(CO)(PhCN)]-,82614-69-5; [ (TPP)Ru(CO)(PhCN)]'-, 123332-79-6;(TPP)Ru(CO)(Py),4175182-0; [(TPP)Ru(CO)(Py)]+, 43070-17-3; [(TPP)Ru(CO)(Py)]'+, 123332-75-2; [(TPP)Ru(CO)(Py)]-, 82621-21-4; [(TPP)RU(CO)(PY)]~-, 123332-80-9; (TPP)Ru(CO)(Me,SO), 82621-20-3; [(TPP)Ru(CO)(Me,SO)]+, 82614-68-4; [(TPP)Ru(CO)(Me,S0)]2+, 123332-76-3; [(TPP)Ru(CO)(Me,SO)]-, 8261476-4; [(TPP)Ru(CO)(Me,SO)]'-, 123332-81-0; (TPP)Ru(CO)-
Langmuir 1990,6, 56-65
56 (DMF), 82614-58-2;
[(TPP)Ru(CO)(DMF)]+, 82614-66-2; [ (TPP)Ru(CO)(DMF)]'+ 123332-77-4; [(TPP)Ru(CO)(DMF)]-, 82614-74-2; [(TPP)Ru(CO)(DMF)]'-, 123332-82-1; ( T P P ) R u ( C O ) ( T H F ) , 82614-57-1; [ ( T P P ) R u ( C O ) n T H F ) ] + ,
82614-65-1; [(TPP)Ru(CO)(THF)]'+, 123332-78-5; [ ( T P P ) R u (CO)(THF)]-,82614-73-1; [(TPP)Ru(CO)(THF)]'-, 123332-832; CH,Cl,, 75-09-2; P h C N , 100-47-0; P y , 110-86-1;Me,SO, 6768-5; P t , 7440-06-4.
Detection of Catecholamines in Brain Tissue: Surface-Modified Electrodes Enabling in Vivo Investigations of Dopamine Function' Ross F. Lane*?$and Charles D. Blahas Department of Chemistry and Departments of Psychiatry and Psychology, University of British Columbia, Vancouver, British Columbia, Canada V6T 1W5 Received J u n e 27, 1989. I n Final Form: August 17, 1989 Recent evidence has shown that graphite paste electrodes modified with stearic acid show high resolution for catecholamines, dopamine (DA), and norepinephrine (NE), when implanted into brain tissue. The extent of this resolution has now been examined in more detail both in vitro and in vivo. Voltammetry at the modified electrodes at physiological pH shows DA and NE can be resolved from their major metabolites and precursors: 5-hydroxytryptamine and its principal metabolite and precursors, ascorbic acid and uric acid. Measurements with electrodes chronically implanted in conscious, unrestrained rats give voltammograms for DA in brain regions rich in DA nerve terminals with a peak potential similar to that observed for DA in vitro. This voltammogram is completely eliminated by selective lesions of DA neurons by 6-hydroxydopamine, is abolished by y-butyrolactone (GBL), and is reduced to undetectable levels by the directly acting DA agonist apomorphine (APO). Combined, these findings demonstrate that the voltammograms represent DA efflux that originates from intact DA nerve terminals, depends on axonal conduction of nerve impulses (GBL), and is regulated by DA receptors controlling normal DA cell activity (APO). The electrodes exhibit reproducible, long-term stability in brain tissue, enabling continuous monitoring of DA efflux for periods of 2 months or more in individual animals. An example is provided demonstrating that chronic treatment with classical antipsychotic drugs decreases the basal efflux of DA in the striatum and nucleus accumbens, whereas "atypical" antipsychotic drugs decrease basal DA efflux only in the accumbens. Evidence is presented that the decreases, when observed, are due to the induction of depolarization block in DA neurons. These findings suggest that the inability of atypical antipsychotic drugs to decrease striatal DA efflux may be related to their low incidence of neurological side effects and that a decrease in limbic DA efflux may be involved in the delayed onset of therapeutic efficacy in man.
Introduction In recent years, there has been considerable interest in developing methods to measure the efflux of small molecules, known as neurotransmitters, from neurons in multiple brain areas of both anesthetized and freely moving animals (for reviews, see ref 1-4). The interaction of neuPresented at the symposium entitled "Photoelectrochemical and Electrochemical Surface Science: Microstructural Probes of Electrode Processes", sponsored jointly by the Divisions of Analytical Chemistry and Colloid and Surface Chemistry, 197th National Meeting of the American Chemical Society, Dallas, April 9-14, 1989. Department of Chemistry. 3 Departments of Psychiatry and Psychology. (1) Adams, R. N.; Marsden, C. A. In Handbook of Psychopharmacology; Iversen, L. L., Iversen, S. D., Snyder, S. H., Eds.; Plenum Press: New York, 1982; Vol. 15; p 1. (2) Measurement of Neurotransmitter Release I n Vivo; Marsden, C . A,, Ed.; IBRO Handbook Series; Wiley: New York, 1984. (3) Justice, J. B.; Michael, A. C.; Neill, D. B. In Neuromethods; Boulton, A. A,, Baker, G. B., Baker, J. M., Eds.; Humana Press: New Jersey, 1985; p 212. (4) Blaha, C. D.; Lane, R. F.; Phillips, A. G. In Advances in Behavioral Biology; Carpenter, M. B., Jayaraman, A,, Eds.; Plenum Press: New York, 1987; Vol. 32; p 115.
*
rotransmitters with specific receptors is one of the major modes of communication between neurons. The ability to detect neurotransmitters directly within the brain, but exterior to neurons, provides a direct method to understand this mode of chemical communication. The catecholamines (dopamine (DA) and norepinephrine (NE)) and their metabolites, as well as 5-hydroxytryptamine (5-HT, serotonin) and its metabolites, are easily oxidized. This enables voltammetric detection. Voltammetric probes of micrometer dimensions have been developed that cause minimal damage to tissue and are sufficiently large t h a t extracellular measurements a r e assured.'-* One aspect of particular interest to our investigations in this area is the monitoring of the neurotransmitter DA because of its established connection to Parkinson's disease and other movement impairments and its putative involvement in the pathology of several neuropsychiatric disorders. A necessary condition for the application of in vivo voltammetry to the study of the central nervous system (CNS) is the identification of neurochemicals responsible for changes in voltammetric oxidation currents. Oxidation currents measured in the brain extracellular fluid 0 1990 American Chemical Society
