4736
J. Phys. Chem. 1991, 95,4736-4741
Pulse Radiolysis of Cr(CO), and W(CO), In Alkane Solution Marsha M. Glezent and Charles D. Jonah* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: October 23, 1990)
Pulse radiolysis of Cr(C0)6 and W(CO)6in alkane solution is described. The first reaction is identified as electron capture and yields the transients Cr(CO)5' and W(CO)5' (X,= 720 nm). Formation rates of Cr(CO)$Sand W(CO)& are measured and the main mechanism is identified as geminate reaction between the anion formed and a cation of the solvent. Behavior in the presence and absence of electron scavengers is described as well. The absorption spectrum of Cr(CO)S'- in ethanol is also recorded ((Amx > 1000 nm).
Introduction Metal carbonyls have been of interest due to their catalytic and photochemical characteristics. They are among the most photosensitive organometallicsknown,and their photoactivity has been studied in great detail.' The photochemical pathways of metal d6 hexacarbonyls are fairly well understood and are dominated by neutral reactions (see reactions 1 and 2 below). The major final product is an uncharged pentacarbonyl species M(CO)$ (S = a solvent molecule or an unsaturated ligand). There are few radiolysis studies of metal and these studies have not fully exploited the unique ability to produce charged metal complexes in a nonpolar environment. Some pulse radiolysis studies of metal carbonyls have postulated the formation of charged 17-electron species as intermediate^.^' Ni(CO)c, Fe(CO),-, and Cr(CO)5'- have been observed at 10 K in the infrared region? Also, pulse radiolysis studies at room temperature of Fe(CO)5 in tetrahydrofuran showed a species that absorbs at 310 nm. This absorbing species was assigned as the 17-electron species Fe(CO),'-.6 Seventeen-electron species have been important as intermediates in both catalysis and synthetic organometallic chemistry. However, as seen in some photochemical studies, these intermediates are often very reactive and difficult to i ~ o l a t e . Seventeen-electron ~ pentacarbonyl intermediates of chromium and tungsten have been postulated in electrochemical experiments.'0.'' However, to our knowledge, there is no spectrascopic evidence for these species at room temperature. In this study we report the radiolysis behavior of Cr(C0)6 and W(CO)6 in alkane solution. We also identify the primary reaction involved as one with the electron leading to the formation of M(CO)$S (where M = Cr or W; S = solvent molecule). The absorption spectrum of the novel transient species M(CO)5'- is identified. This is the first absorption spectrum recorded for a 17-electron species for a metal carbonyl species of either Cr or W. These experiments also have the potential to help us understand the mechanisms of hydrocarbon radiolysis. The mechanisms of hydrocarbon radiolysis have been a subject of study for many years and are, in many ways, far more complicated than aqueous systems. The number of products is considerably larger and the time scales of the reactions are considerably shorter in hydrocarbon radiolysis. To assign the mechanistic pathways, one needs to understand the origin of the original species and their spatial distribution and the conversion of these species on time scales shorter than nanoseconds. Previous experimental studies have used many techniques for studying processes in hydrocarbons. We have chosen to add metal carbonyls (M-CO),in particular Cr(C0)6 and W(CO)6, as a probe of the chemical reactions in hydrocarbons. Because the metal carbonyl reacts quickly with the electron, it will intercept the electron at early times and thus provide a probe of the early-time events. When an M-CO molecule reacts with an electron, the resulting ion is much less mobile. Thus, the recombination reaction is slowed down, and one can directly observe the geminate recombination kinetics. Similar experiments have been carried out using aromatic molecules;12in much of that work the number Present address: Dow Chemical, Bldg 1897, Midland, MI 48667.
of reaction pathways are large because both the positive and negatiive ion can react with the added aromatic molecule and so one is less able to separate the chemistry. Other probes, such as N 2 0 , do not give an observable spectrum. There is a need to develop new and better chemical probes. With the carbonyl compounds the number of possible reactions are fewer and the product can be determined. The present limitation in time in these studies is the solubility of the M-COS. The maximum solubility is only 0.02 M so the M-COs intercept less than half of the electrons.
Experimental Section Standard fast kinetic absorption techniques were used for measuring the optical transmission as a function of wavelength. The initial species were formed using a 30-ps 20-MeV pulse from the Argonne low-energy accelerator. A Hamamatsu flash lamp was used for the detection light. The wavelengths studied were chosen by using a series of approximately 10-nm fwhm interference filters. For detection of the light, either a Hamamatsu R1328 biplanar vacuum photodiode detector or a solid-state photodiode detector CD-10 from Optical Electronics Limited was used. Care was taken by using the solid-state photodiode detector to avoid problems with the wavelength-dependent rise time. The electronic apparatus and light path have been described previously as we11.16 The transient data were taken by using either 50- or 200-11s full scale. All results were reproducible. The data were acquired with a Tektronix 7250 waveform digitizer. From four to eight radiolysis traces were averaged for most of the experiments; for the experiments in ethanol one to two measurements were sufficient. Cr(C0)6 and W(CO)6 were purchased from Aldrich Chemical and sublimed twice before using. Cyclohexane and n-pentane were passed through an activated silica gel column prior to experimental runs. Ethanol (absolute) was of the highest grade commercially available. Solutions were degassed with Ar prior to radiolysis. All gases used were of highest purity available. Dilutions were ( I ) (a) Adamson, A. W., Fleischauer, P. D., Eds. Concepts of inorganic Photochemistry; Wiley: New York, 1975. (b) Geoffroy, G.L.; Wrighton, M. S . Organometallic Photochemistry; Academic Press: New York, 1979. (2) Flamingi, L. Radial. Phys. Chem. 1979, 13, 133. (3) Kang, Y. S.; Holroyd. R. A. Radial. Phys. Chem. 1982, 20. 231. (4) Castiglioni, M.; Volpe, P. fnorg. Chim. Acta 1982, 59, 1 1 7. (5) Meckstroth, W. K.; Walters, R. T.; Waltz, W. L.; Wojcicki, A.; Dorfman, L. M. J . Am. Chem. SOC.1982, 104, 1842. (6) Reed, D. T.; Meckstroth, W. K.; Ridge, D. P. J . Phys. Chem. 1985. 89, 4578. (7) Waltz, W. L.; Hackelberg, 0.;Dorfman, L. M.; Wojcicki, A. J . Am. Chem. Soc. 1978, 100, 7259. (8) Breeze, P. A.; Burdett, J. K.; Turner, J. J. Inorg. Chem. 1981,N, 3369. (9) Baird, M. C. Chem. Rev. 1988, 88, 1217. (IO) (a) Pickett, C. J.; Pletcher, D. J . Chem. Soc., Chem. Commun. 1974, 660. (b) Chum, H. L.; Koran, D.; Osteryoung, R. A. J . Organomet. Chem. 1977, 140, 349. ( I 1) Pickctt, C. J.; Pletcher. D. J . Chem. Soc., Dalton Trans. 1976.749. (12) (a) Katsumura, Y.; Tagawa, S.; Tabata, Y. J . Phys. Chem. 1980,84, 833. (b) Sauer, Jr., M. C.; Romero, C.; Schmidt, K. H. Radiat. Phys. Chem. 1987, 29,261. (c) Beck, G.;Thomas, J. K. J . Phys. Chem. 1972.76, 3856. (d) Jonah, C. D.; Sauer, Jr., M. C.; Cooper, R.; Trifunac, A. D. Chem. Phys. Lett. 1979, 63, 5335. (e) Tagawa, S.; Katsumura, Y.; Tabata, Y. Radial. Phys. Chem. 1982, 19, 125.
This article not subject to US.Copyright. Published 1991 by the American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4737
Pulse Radiolysis of Cr(C0)6 and W(CO)6 ' 2 0 ' 0
0.004
550 Wavelength nm
350
450
650
Figure 1. Absorption spectra after radiolysis of 0.02 M Cr(CO)6 in n-pentane; ( 0 )at the p u k , (A)10 ns after the pulse; (B) 40 ns after the
pulse. D
,
I
'
o
I
~
0.080012
-
0.08
a
10
-
0.04
40
Figure 3. Growth curves of species B monitored at 500 nm for various concentrationsof Cr(C0)6: (A) 0.02 M, (B) 0.01 M, (C) 0.005 M, (D)
W -O 550
0'021 0.004 350
30
Time ns
'
I
450
850
750
860
950
L -1 '
I
550
'
1
650
'
I
750
'
!
850
'
I
950
WAVELENGTH NM
Figure 2. Absorption spectra after radiolysis of 0.02 M W(CO)6 in n-pentane; (a) at the pulse; (A) 10 ns after the pulse; (B) 40 ns after the pulse. done by using a two-syringe technique that has been described previ~usly.~~ Samples (5 mL) were irradiated with a 30-ps pulse of highenergy (20 MeV) electrons from the linear accelerator at Argonne National Laboratory. Dosimetry was done using the absorption from the hydrated electron a t 590 nm. The G value for the electron was assumed to be 4 molecules/l00 eV at 1 nsI4 and the extinction coefficient was taken as 11 240 M-' cm-'.lS The dose was approximately 50 Gy, which was sufficient to create approximately 4 r M of the product pentacarbonyl compound.
Results and Discussion Absorption Spectra. Figure'l displays the absorption spectrum of pulse radiolyzed 0.02 M Cr(C0)6 in n-pentane directly after the pulse, and 10 and 40 ns after the pulse. An initial spectrum at the pulse indicates a broad absorption in the UV region. This absorption decays rapidly to yield a spectrum with an absorption maximum at 510 nm, which is still growing 40 ns after the pulse. The latter absorption has been found to be stable up to at least 200 ns. Clearly, there are at least two different species involved. We will hereafter refer to the transient in the UV region as species A (although as is discussed later, there may be multiple species) and the species with a maximum at 510 nm as species B. Because of the absorption of the parent compound, data could only be obtained above 380 nm. Experiments under identical conditions in cyclohexane yielded similar results; i.e., two species were observed. (13) Hart, E.J.; Anbar, M. The Hydrared Electron; Interscience: New York. 1970 Chapter IX. (14) Jonah, C. D.;Matheson, M. S.;Miller, J. R.; Hart, E. J. J . Phys. Chem. 1976,80, 1267. (15) Fielden, E. M.;Hart, E. J. Tram Faraday Soc. 1967, 83, 2975. (16) Closs. G. L.; Calcaterra, L. T.; Green, N. J.; Penfield, K. W.; Miller, J. R.J . Phys. Chem. 1986, 90,3673.
0.0025 M.
Figure 2 shows similar spectra for 0.02 M W(CO)6 in n-pentane. Again an initial species, which we shall refer to as A, is formed which then is converted into B. The inset in the figure displays the behavior in the red region. The data at low energies were obtained by using a silicon photodiode instead of the vacuum photodiode. The final absorption spectrum in both the tungsten and chromium species is identical with that observed for M(CO).$ in time-resolved flash photolysis where S is a solvent m ~ l e c u l e . ~ ~ * ~ * The primary reactions in the photochemical studies are
M(CO)6
+ hv, S
M(C0)S
M(C0)S + CO
S + M(C0)SS
(1)
(2)
where the equilibrium lies strongly to the right in reaction 2. The absorption maximum for the solvent pentacarbonyl complex is extremely sensitive to the solvent ligand.I9 The spectrum of species B is also identical with that observed in a previous radiolysis study in cyclohexane by Flamingi? However, those results were recorded in a much longer time regime. They identified the absorption at 500 nm as Cr(C0)5S and measured decay rates for this species on the order of lo3 s-I. Kinetics. The formation mechaniism for M(CO&3 has not been previously assigned. One expects that the primary species would be ions because the primary reactions in radiolysis are
--
e- 20 MeV
S So+
+ e-
S'+
S*,
+ e-
(subpicosecond)
H2,olefin, etc.
(10-100 ps)
(3) (4)
where the approximate time scale for the reaction is given in parentheses. The dominant reaction in alkanes is the geminate recombination between the electron and the solvent radical cation (17) (a) Simon, J. D.; Xie, Xiaoliang J . Phys. Chem. 1989, 93.291. (b) Bonneau, R.;Kelly, J. M. J . Am. Chem. Soc. 1980, 102, 1220. (c) Joly, A. G.;Nelson. K. A. J. Phys. Chem. 1989, 93,2876. (d) Kelly, J. M.; Hcrmann, H.;Von Gusforf, E. K. J. Chem. Soc., Chem. Commun. 1973, 105. (18) (a) Langford, C. H.; Moralejo, C.; Sharma, D. K. Inorg. Chim. Acra 1987, 126, L11. (b) Lees, A. J.; Adamson, A. W . Inorg. Chem. 1981, 20,
438 I . (19) (a) Burdett, J. K.; Grzybowski, J. M.; Perutz, R. N.; Poliakoff, M.; Turner, J. J.; Turner, R. F. Inorg. Chem. 1978, 18, 147. (b) Perutz, R. N.; Turner, J. J. J. Am. Chem. SOC.1975, 97, 4791.
4138
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991
4
Glezen and Jonah
I
0.030
,
V.""
350
550
650
Wavelength nm
J 0.005
450
Figure 5. Absorption spectra after radiolysis of 0.02 M Cr(C0)6 in at the pulse, (A)40 ns after the n-pentane, without saturated N20:(0) pulse, with N 2 0(m) at the pulse, (A) 40 ns after the pulse. 70
I
I
60 50 -
40 0.000
I
10
I
20 TIME NS
I
I
30
40
30
-
20 -
Figure 4. Rate of formation of M(CO)$ for both Cr and W in n-pentane and Cr in cyclohexane. All curves have been normalized.
10
-
0
due to the strong Coulomb attraction between them (see reaction 4). The growth of species B in our measurements a t various concentrations is shown in Figure 3. These formation curves are corrected for the decay in absorbance in the neat solvent. The rates for M(CO)$ formation are in the nanosecond regime rather than the picusecond regime and thus are clearly not commensurate with any of the rates given in eqs 3 and 4. One of the unusual characteristics shown by the kinetics is the independence of the rate of the formation of M(CO)$ as a function of the starting product M(CO)6 or the solvent. This is illustrated in Figure 4 which shows the growth curves for both Cr(CO)$ and W(C0)$3 in various solvents at their absorption maxima. One possible reaction mechanism to explain this would be a series of reactions in which the M(CO)6 reacts with the electron to form an unstable species (which could be an excited state). This species would then relax to form the final product M(CO)$. However, the formation of M(CO)$ is not first order but continues to grow at longer times. Also inconsistent with this type of mechanism is the identical kinetics in the tungsten or chromium system shown in Figure 4. The strength of the absorption also shows that these data are not due to the formation of the excited states. Excited states of M(CO)6 might be formed either by energy transfer from an excited solvent molecule or by absorption of Cerenkov light generated in the solution. Excited-state energy transfer from a primary solvent species S* to M(CO)6 will not occur significantly at these concentrations of M(CO)6 because the lifetime of the primary excited state is approximately 0.3 ns in n-pentane." If excited state due to absorption of Cerenkov light from the electron beam were solely responsible for the formation of species B, the extinction coefficient for species B would have to be larger than 9.0 X lo' M-I cm-l. This is unreasonably high for metal carbonyls involving either ligand field or low-lying metal to ligand charge-transfer excited states.l The extinction coefficient for W(C0)&3 has been reported as 7.5 X lo3M-I cm-' at 425 nm.IEb We suggest the following mechanism for the formation of M(CO)SS: M(CO)6 + eM(C0)5'- + C O (5) M(C0);-
-+ So+
M(CO)$
(20) Sauer, M. C.; Jonah, C. D.; Le Motais, Phys. Chcm. 1988, 92,4099.
(6)
B. C.; Chernovitz, A. C. J .
1 .o
2.0 11
k x10, sec
-1
Figure 6. Effective first-order rate for the reaction of the electron with three electron scavengers versus the percent of the absorption at 400 nm that has been removed by that amount of an electron scavenger in 0.02 M W(CO)6 at 400 nm in n-pentane solution.
In this mechanism compound A is M(CO)s'- and compound B is M(CO)$. The discussion of these assignments is given below in a section on spectral assignments. Such a mechanism suggests that there are two geminate reactions occurring, one where there is a competition between M(CO)6 and So+for reaction with the e-, and the second is the reaction between S'+ and M(CO)S'. To confirm the first reaction, we have measured the yield of M(CO)$ as a function of an added electron scavenger. The data for N 2 0 are displayed to show that the species are not changed but the yield of the final product is changed. The spectra with and without N 2 0 are shown in Figure 5. N 2 0 is a very efficient electron scavenger?I Clearly the species formed are not changed, however, the yield of the final product is changed. In addition, the formation kinetics were not altered in the presence of N20. Similar results were obtained in the presence of CCll and C02. The percent decreases of the final absorption versus the pseudo-first-order rate (the product of k for reaction of the scavenger with the electron and the scavenger concentration) are displayed in Figure 6. Saturated solutions were used for the gases and the concentration of CCI4 was 0.1 M. The solubilities of the gases were approximated from published measurements in n-hexaneZ5and the rate constants estimated from results in hexane.% The weak inhibition (21) Luthjens, L. H.; Codee, H.D. K.;de Leng, H.C.; Hummel, A. Chem. Phys. Lett. 1981, 79, 444. (22) Warman, J. M. "The Dynamics of Electrons and Ions in Non-polar Liquids." Report of the Interuniversity Reactor Institute, Mekelweg 15,2629 JB Delft, The Netherlands. (23) Wrighton, M.; Hammond, G. S.;Gray, H.B. J . Am. Chem. Soc. 1971, 93, 4336. (24) (a) Sherman,
W. V. J . Chem. Soc. A 1966, 599. (b) R a d . S . J.; Schuler, R. H.J . Phys. Chem. 1968, 72, 228. (c) Rzad, S . J.; Warman, J. M. J . Chem. Phys. 1968, 49, 2861. (d) Warman, J. M.; Asmus, K.-D.; Schuler, R. H. J . Phys. Chem. 1969, 73,931. (e) Warman, J. M.; Rzad, S . J. J . Chem. Phys. 1970, 50,485. (25) Horseman-van den Dool, L.; Warman, John "The Solubilities, in a Variety of Organic Liquids, of Some Gaseous Compounds Commonly Used as Scavengers in Radiation Chemical Studies." Report of the Interuniversity Reactor Institute, Mekelweg 15, 2629 JB Delft, The Netherlands.
The Journal of Physical Chemistry, Vol. 95, No. 12, I991 4739
Pulse Radiolysis of Cr(C0)6 and W(CO)6
0
0 . 0 1 Y .
I
0.06
,
,
,
,
,
I
"
0.10
0.08
I
'
0 12
1
I
'
,
014
0.16
1/2
conc, M
Figure 7. Final absorption of Cr(CO)5S in n-pentane at 500 nm as a function of the [Cr(CO)6]1/2.
0.035
by C 0 2 may be due to the reaction of the product COT molecule with the M-CO. Both CC14 and N 2 0 dissociate to form stable species after the electron capture; C02-does not dissociate quickly, and the donation of the electron has been observed in other systems.2' The kinetic behavior of M(CO)$ can also be explained as a geminate competitive reaction of M(CO)6 with the positive ion (Sa+). However, a positive carbonyl species is ruled out below in the section on the assignment of spectra. Also, the behavior expected in the presence of electron scavengers would be the direct inverse of what was found experimentally (see Figure 6). Spectra of 0.02 M solutions were determined in the presence of solvent positive ion scavengers. Cyclopropane is an effective positive ion scavenger and reacts according to reaction 7.'" In
0.030
-
CnH2n+2*+ c-C3H6
CnH2,,'-
+ n-C3H8
(7)
saturated solutions of cyclopropane [ -2 M] very little effect on the formation of either species A or B was observed. Obviously M(CO)6 competing with So+is not the primary reaction involved in the kinetics (see reaction 5). The amount of both A and B is nearly the same with or without the presence of cyclopropane. Our results with triethylamine (TEA) are not as clear, however. Neither species A nor B was found in the presence of high concentrations of TEA t0.l MI. However, the absorption of the final products showed a low-lying absorption similar to that seen for W(CO)5L, where L is a saturated nitrogen-donor ligand.23 This leads one to believe that the large reduction of species A and B in the presence of TEA is due primarily to its coordinating ability as a ligand and not its scavenger characteristics. This is likely due to the very poor ligand characteristics of the saturated solvent molecule. If the mechanism is as shown above (reactions 5 and 6 ) , one would expect the yield to be described by the following expression
Y = c~(CY[M(c0)6])'/~ where CY is a proportionality constant and Y is the yield of M(C0)5'-.24 However, since all of the M(CO)S'- will go on and form M(CO)& Y will be also the yield of M(CO)SS. This is a good description of the results as can be seen in Figure 7. This expression can also be used to give an estimate for the rate constant of the electron with the metal carbonyl. For W(CO)6 where the extinction constant is known (see above), the G value of the W(CO)JS can be estimated to be about 1.3 molecules/lOO eV in 0.02 M W(CO)6. For these results one can estimate that act is approximately 85. This is consisteqt with the value for act for CH,CI. For methyl chloride, the reaction rate of the electron is approximately 1.5 X 10l2 M-I From the data in Figure 6, one can also estimate the rate of the reaction of the electron with W(CO)6. Making use of the square root law as discussed above and the rates of the electron (26) Allen, A. 0.;Gangwer, T. E.; Holroyd, R. A. J . Phys. Chem. 1975,
0.025
1 I
0.1
I
I
I
I
I
0.2
0.3
0.4
0.5
0.6
t"'
I
I
0.9
(nsecr''
Figure 8. Rate of formation of Cr(CO)$ recorded on a 50 and 200 ns time scale in n-pentane as a function of l/tlj2 to display the geminate character of the reaction (the left tick is 100 ns the right tick mark is I .23 ns) .
with the known scavengers,we can estimate a value of 2.5 X 1OI2 M-' s-l from the data on the CC14 and N 2 0scavenging experiments. The independenceof the kinetics as seen in Figure 3 on M(CO)6 concentration is expected for the geminate reactions 5 and 6. For those ionization events where there is a M(CO)6 molecule sufficiently close to react with the electron, the formed anion will be attracted to the solvent cation by the Coulombic charge and the recombination time will be of a typical geminate.reaction. Of course the recombination reaction will be slower than shown in eq 3 because the mobility of the anion will be considerably smaller than that of the electron. The time dependence for geminate reactions is of the form l/r112for a geminate-type reaction in low dielectric fluids.22g28Figure 8 shows the kinetics plotted for the chromium carbonyls as a function of l/t'12. This figure shows that this is a good approximation over much of the time region. Only at early times (right-hand side of Figure 8), where it is difficult to separate the solvent absorption, do the data fail to follow the l/t112behavior. The rate of the recombination process will be dominated by the mobilities of the positive solvent ion and the negative M(C0); ion. Since the cation is the same and the anions are very similar, one would expect the recombination process to be nearly independent of the particular M-CO molecule. Figure 4 clearly shows that this is the case. Also illustrated in Figure 4 are the results in cyclohexane. The rates are slightly slower, consistent with the lower mobility expected in the more viscous solvent. The rate of reaction of M(CO)6 with the e;, in ethanol has also been determined by measuring the decay of the electron absorption at 600 nm. The rate constant obtained was 5.7 X lo9 M-' S-I, approximately diffusion controlled, which is consistent with a very fast reaction of the M(CO)6 with the electron. Assignment of Spectra. As mentioned above, species B was assigned to the M(CO)$ from the similarity in the absorption maxima between species B and the previously measured pentacarbonyl species observed both for tungsten and chromium. This is also supported by the stability of species B in solution. The electron as well as the So+scavenger results support that the M(C0)sS compound is formed from species A which itself is the direct result of a reaction with the electron. The geminate com-
79, 25-3 I .
(27) Sauer, M. C., Jr. Private communication.
I
0.7 0.8
(28) Hummel, A. J . Chem. Phys. 1968,49,4840.
Glezen and Jonah
4740 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 1
0.020,
I
j
,
0.000 600
700
800
900
1000
Wavelength nm Figure 9. Absorption spectrum 210 ns after radiolysis of 0.02 M Cr(CO), in ethanol.
petition between M(CO), and the S+ for the electron is also consistent with the lack of a concentration dependence on the formation rate of M(CO)5S. Since there is a good understanding of the primary reaction as well as the final product formed, one must now consider the assignment of species A. The results from the W(CO)6 data (see Figure 2) show a = 720 nm) which decays rapidly within low-lying absorption (A, 40 ns. To our knowledge, this low-energy absorption has not been seen previously for either photolysis or radiolysis results of chromium or tungsten hexacarbonyl. However, on the basis of electrochemical evidence1h*" which indicates the instability of W(CO)6' as well as ideas concerning the covalency of the ligand field levels,lbone would not expect W(CO)6' to be very long lived. The electron occupation of a ligand field orbital (e,) which has a significant amount of us character would lead to the loss of CO and the formation of W(CO)5'- (see reaction 5 ) . After the formation of W(C0);- there must be a following neutralization reaction with a positive solvent species to form the final product W(CO)5S (reaction 6 ) . Specifically what this positive solvent species is that is involved in the neutralization process will be discussed in greater detail below. W(CO)5'- is isoelectronicwith Re(CO)5'. Radiolysis experiments as well as photolysis data5J9 have determined the low-lying absorption maximum of Re(CO)5' in the 535-540-nm region in either methanol or isooctane solution. Because of the lower oxidation state of tungsten, one would expect this same transition to occur at lower energy. This is what was found experimentally. No low-lying absorption was found for Cr(C0)6 in n-pentane to correspond to the tungsten results. The low-lying absorption of isoelectronic Mn(CO)5' is in the 850-nm region.30 Similar to the tungsten arguments (vide supra), one would expect the absorption of the Cr(C0)5'- radical to be even lower in energy and thus not detectable with the present experimental system. Because Cr(CO)S'- is a charged species, one would also expect the low-lying absorption to shift to higher energy with more polar solvents. This charge could be stabilized to a large degree in alcohol solution and would also be expected to be longer lived. Experiments under identical conditions in ethanol indicate this to be the case. Figure 9 shows that the low-lying absorption is present after radiolysis of 0.02 M Cr(C0)6 in ethanol 200 ns after the pulse. This indicates that species A for the chromium data is most likely Cr(CO),'. Spectra at earlier times were not possible due to the high extinction coefficient and stability of the solvated electron in the same spectral region.31 This low-lying absorption effectively rules out the possibility of Cr(C0)6'+* The ionization potentials (IP) of metal carbonyls are almost 1-1.5 eV lower than the ionization potentials of the solvent radical cations of alkanes. The possibility of hole transfer from So+ to M(CO)6 exists. Also electrochemical data have
indicated that Cr(C0)6'+ is a very stable spaCies.'O However, this positive species is also isoelectronicwith V(CO)6. The absorption spectrum of vanadium hexacarbonyl consists of broad bands in the UV region32and no low-lying absorption bands have been recorded in the visible region. Also, electrochemical evidence has indicated thta W(CO)6'+ is Since the radiolysis results for the W and Cr are so similar, it is unlikely that the transient species present (species A) for the Cr data is Cr(C0);'. The UV absorption for species A can also be due in small part to M(C0)5' in solution for both the tungsten and chromium data (Figures 1 and 2). However, in the case of Cr(C0)6 there must also be some absorption due to the solvent radical cation.)'OS( One indication of this is the increase of the early absorption at 400 nm as one increases the concentration of hexacarbonyl. This is typical behavior for So+in the presence of an electron scavenger. When one scavenges the electron, geminate recombination becomes much slower (see reaction 4) and the S'+ will be longer lived. Second, the absorption spectrum at the pulse (see Figure 1) for 0.02 M Cr(C0)6 is identical in shape with the radiolysis spectrum in the absence of Cr(CO)6. However, if the absorption at the pulse (species A) were entirely So+, one would expect that upon the addition of N 2 0 (see Figure 5 ) the absorption spectrum at the pulse would increase similarly with higher concentrations of CI(CO)~. This is clearly not the case and is currently being investigated. Reactions with the Positive Ion. Figure 4 shows the formation of the M(CO)$. From the discussion above, this process has been assigned to the reaction of the metal carbonyl anion with a solvent positive ion. It is clear that this reaction is independent of the metal ion, which is to be expected. The similarity of the rates in n-pentane and cyclohexane indicates that the mobility of the positive ion in cyclohexane is similar to that in n-pentane. Therefore, these results indicate that the positive ion of cyclohexane involved in reaction 6 is not the high-mobility positive ion which has been observed in c y c l ~ h e x a n e . ~ ~ * ~ ~ Other results from this laboratory have suggested that the primary positive ion in cyclohexane is not stable" but fragments, either by proton transfer or by the expulsion of H2 to form an olefin radical cation.36 Because the hydrocarbon is a poor ligand and can be quite labile, it is not necessary that the S of M(COhS be the ion that was originally neutralized. An olefin radical cation would probably work as well. It seems unlikely, however, that the proton-transfer ion would react with the metal carbonyl anion. If the proton transferred, the ion would be neutralized; however, it would still be a radical. If an electron transferred to the proton, the anion would be neutralized and could coordinate a solvent molecule. The proton would then be converted to an H atom and would be free.
Conclusions We can conclude that in the radiolysis of M(CO)6 (M = Cr, W) the reaction responsible for the formation of M(CO)SS is primarily an electron reaction to form the transient M(CO);-. This species then combines with a positive solvent species (reaction 6) to form the neutral species M(CO)5S. The absorption at low energy (720 nm) in n-pentane with 0.02 M W(CO)6 is the 17electron species W(CO)s'-. Similarly the absorption in ethanol seen for the Cr experiments at low energy (Figure 9) is due to the transient Cr(C0)5*-. These are the first absorption spectra recorded for a 17-electron species of either W or Cr in solution at room temperature. Radiolysis provides a new means of generating 17-electron intermediates of metal carbonyls that have (32) Rubinson, K. A. J . Am. Chem. Soc. 1976, 98. 5188-5191. (33) Warman, J. M. In The Study of Fast Processes and Transient
(29) (a) Wrighton, M.; Bredsen, D. J. Orgonomet.Chem. 1973.50, C35. (b) Wrighton, M. S.; Ginley, D. S. J. Am. Chem. Soc. 1975, 97, 2065, (30) Kobayashi. T.; Ywufuku, K.; Iwai, J.; Yesaka, H.; Noda, H.; Ohtani, H.Coord. Chem. Rev. lWS,64, 1-19. (31) Kenney-Wallace, G. A.; Jonah, C. D. J . Phys. Chem. 1982, 86,
D. Species by Electron Pulse Radiolysis; Baxendale, J. H . , Busi, F., Us.; Reidcl: Boston, 1982; pp 433-537. (34) Sauer, M. C., Jr.; Trifunac, A. D.; McDonald, D. M.; Cooper, R. J . Phys. Chem. 1984,88.4096. (35) Sauer, M. C., Jr.; Jonah, C. D.; Naleway, C. A. J . Phys. Chem. 1991, 95, 730. (36) Wcrst, D.W.; Bakker, M. B.; Trifunac, A. D. J . Am. Chem. Soc.
2572-2586.
1990, 112,40.
4741
J. Phys. Chem. 1991, 95,4741-4748 not been seen previously. Because of the formation of a new species in solution, new chemical pathways are possible using radiolysis which have not been available photochemically or thermally. Acknowledgment. We thank Drs.J. R. Miller, P. Piotrowiak, and N. Liang for experimental assistance. These experiments
could not have been run without the assistance of G. L. COXand D. T. Ficht in tuning and maintaining the linac. Helpful discussions with Drs. M.c. Sauer, Jr., and A. D. Trifunac are gratefully acknowledged. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, US-DOE, under contract no. W-31-109ENG-38.
Kinetics of Comproportionation: A Spectroelectrochemical Approach Richard G.Compton,* Barry A. Coles, and R. Anthony Spackman Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ,U.K. (Received: October 29, 1990; In Final Form: January 30, 1991)
-.
A general spectroelectrochemical approach to the kinetics of the comproportionation reaction Y + Y2- 2Y- is presented which combines both rotating-disc double-step chronoamperometry and quantitative in situ electrochemical ESR spectrosc~py. It is shown that when the electroreduction of Y to Y2- proceeds via two separate one-electron waves and Y'-is ESR active, the combination of the two techniques allows the unambiguous determinationof the kinetics together with the diffusion coeffdents of the three species involved. The method is applied to the case where Y = p-chloranil (tetrachloro-p-benzoquinone),and good agreement between theory and experiment is found.
Introduction This paper is concerned with investigating and quantifying the effect of a homogeneous comproportionation reaction on an electrode reaction that proceeds stepwise via two single-electron transfers: A+e-+B
(i)
B+e-+C
(ii)
A+C,-2B
(iii)
where E0Ap is more positive than E O B p Hitherto, electrochemical measurements, largely cyclic voltammetry, have yielded thermodynamic information (notably comproportionation constants, K(mmp)= exp[-F/RT(EoBlc - EoAlB)])via the determination of the standard electrode potentials E O A p and EoB/C and numerous applications of this type have been Theoretical considerations of the kinetics of comproportionation (reaction iii) have been carried out by Smith, Feldberg, and Ruzic6.' in respect of ac voltammetry but otherwise are limited!*9 It will be shown below that when (i) and (ii) are electrochemically reversible and A, B,and C have equal diffusion coefficients, purely electrochemical experiments are blind to (iii), a conclusion reached by Guidclli' and also by Magno and Bontempelligin respect of potential step chronoamperometry, although the latter technique was shown to be viable when the first cathodic step (A to B) in the above mechanism is sufficiently hindered by overvoltage that (1) Polcyn. D. S.;Shain, 1. Anal. Chcm. 1966, 38, 370. (2) Heize, J. Angcw. Chcm., fnr. Ed. Engl. 1984, 23, 831. (3) Kaiffer, A. E.; Bard, A. J. J . Phys. Chcm. 1985,89, 4876. (4) Mohammed, M. Elecrrochlm. Acra 1988, 33, 417. (5) Richardson, D. E.; Taube, H.fnorg. Chcm. 1981, 20, 1278. (6) Ruzic, 1.; Smith, D. E.; Feldberg, S.W. J . Elecrroanal. Chem. 1974, 52, 157. (7) Ruzic, 1.; Smith, D. E. J . Elecrroanal. Chcm. 1975, 58, 145. (8) Guidelli, R. Anal. Chcm. 1971.13. 1715. (9) Magno, F.; Bontempelli. G. Anal. Chcm. 1981, 53, 599.
a single irreversible tweelectron process is observed. In this case, the homogeneous comproportionation kinetics are accessible because of the enhanced currents due to (iii). In other cases the Occurrence of irreversible chemical reactions coupled to the electrode reaction makes visible the effects of comproportionation in voltammetric experiments. Some other techniques have also been employed in studying comproportionation kinetics, including stopped flow,1° various optical techniques at transparent electrodes (OTE!S),~'-'~and ESR studies."J* In this paper, we present a general electrochemical approach to the study of comproportionation kinetics involving a combination of double potential step chronoamperometry at the rotating disc electrode (RDE) and quantitative in situ electrochemical ESR spectroscopy. Both these experiments are highly sensitive to the diffusion coefficients, D, of A, B, and C and the rate constant, khi,for (iii). Typically DB, Dc, and kiiiare unknown, and it is for this reason that the combination of techniques is essential so that all of these parameters may be unambiguously determined. Theory is developed for the chronoamperometric response to a potential step at a RDE for the particular case when a stepwise two-electron transfer is coupled to a comproportionation reaction as in (i)-(iii). The precise form of the potential step sequence to be examined can be understood by reference to Figure 1 and recognizing that the bulk solution contains only one electroactive species A. The initial potential El is one at which no current flows. (IO) Bennion, B. C.; Auborn, J. J.; Eyring, E. M. J . Phys. Chcm. 1972, 76, 701. (1 1) Kuwana. T.; Winograd, N. Anal. Chcm. 1966,38, 1810. (12) Winograd, N.; Kuwana, T. J . Am. Chcm. Soc. 1970, 92, 224. (13) Winograd, N.; Kuwana, T. J . Am. Chcm. Soc. 1971, 93, 1353. (14) Strojek, J.; Kuwana, T.; Feldberg, S. J. Am. Chcm. SOC.1968. 90, 4343. (15) Kuwana, T.; Strojek, J. Discuss. Furaday Soc. 1968. No. 15. 134. (16) Armstrong, N. R.; Vanderborgh, N. E. J . Phys. Chcm. 1976. 80, 2140. (17) Compton, R. G.; Monk, P. M. S.; Rosseinsky, D. R.; Waller, A. M. J . Elecrroanal. Chem. 1989, 267, 309. (1 8) Male, R.;Samotowka, M. A.; Allendoerfer, R. D. Elccrrocrnalysb 1989, I, 333.
0022-365419112095-4741S02.50/0 0 1991 American Chemical Society
