4838
J . Phys. Chem. 1985,89, 4838-4843
Reduction of Uranium(1V) and Its Mixtures with an Olefin or an Alkyne in Tetrahydrofuran Solutions by Solvated Electrons A. M. KoulkCs-Pujo,* J. F. Le Marechal, B. Le Motais, and G. Folcber U.A.-CNRS- CEA-331, Dzpartement de Physico- Chimie, CEN-Saclay 91 191 Gif sur Yvette Cedex, France (Received: April 8, 1985)
The reduction of UC14 and its mixtures with different olefins (stilbene, St, diphenylethylene, DPE, acenaphtylene, Ac) or with diphenylacetylene (DPA) was studied by pulse radiolysis of tetrahydrofuran (THF) solutions. U(II1) was formed by U(1V) reaction either with the solvated electrons created by THF radiolysis or with the transitory anions St- and DPA-. In the latter case, the reaction proceeds via a first step leading to [St-U(IV)]- or [DPA-U(IV)]-. In the case of DPE- the first species, [DPE-U(IV)]-, does not lead to U(II1) but is destroyed by THF(H)+ giving DPE(H). and U(IV). Ac- does not react with U(1V). A mechanistic scheme of this electron attachment is discussed as well as its implication in catalytic hydrogenation of olefins in LiA1H4-UCl4 solutions. It is concluded that the catalytic effect observed is rather the result of a hydride transfer from a uranium transient compound to the alkenes.
Introduction The use of U(1V) organometallic compounds as initiators of different reactions' has been reported in the literature. Recently, Folcher et a1.* observed a new catalytic activity for a U(II1) compound in tetrahydrofuran (THF) solutions: it was found that ethylene is hydrogenated in a solution of LiA1H4-UCl4. This reaction does not occur in the absence of U(II1) (the presence of the strong reducing agent LiAlH4 in the solution transforms U(1V) into U(II1)). The mechanism for this hydrogenation is not yet fully established, the situation being complicated by the presence of a l ~ m i n u m . The ~ first step involving the double bond activation by U could be either an hydride or an electron transfer leading to an unstable complex. It is reasonable to suggest that in the latter case a complex U(II1)-olefin may be transitorily formed by analogy with the case of Cu(I)-olefinic c o m p o ~ n d s . ~ In order to answer this question, we replaced LiA1H4 by the elementary reducing species, the solvated electron, created by radiolysis of THF and then searched for a possible U(III)-olefin transition complex formed during the radiolysis of U(1V)-olefin solutions. Such a species, if it exists, should have a relatively short lifetime and, if it has an absorption spectrum, it ought to be seen by pulse radiolysis, a technique well suited for detecting short-lived species. Several metal compounds have been reduced in protic media (water or alcohols) in this way,5 but very few in aprotic organic solvents,6 THF,7 or methyltetrahydrofuran.8 For the ethylenic compounds, we selected different aromatic alkenes, trans-stilbene (St), diphenylethylene (DPE), and acenaphthylene (Ac). We assumed that if U(III)-olefin complexes are formed, they should possess an absorption spectrum shifted toward the red as a function of group substitution at the double bond, a feature which has been noted before for other compound^.^ An acetylenic compound was added to this series, diphenylacetylene (DPA), which differs from S t or DPE by the absence of H atoms on the triple bond, giving a linear geometry and a shorter bond length. (1) Fagan, P.J.; Manriquez, J. M.; Marks, T. J.; Day, V. W.; Vollmer, S. H.;Day, C. S . J. Am. Chem. Soc. 1980, 102, 5394. (2) Folcher, G.; Le Markhal, J. F.; Marquet-Ellis, H. J. Chem. SOC., Chem. Commun. 1982, 324. (3) Le Markhal, J. F.; Folcher, G. Inorg. Chim. Acta 1984, 94, 105. (4) Buxton, G. V.; Green, J. C.; Sellers, R. M. J. Chem. SOC.,Dalfon Trans. 2 1976, 2160. ( 5 ) Hart, E. J.; Anbar, M. "The Hydrated Electron"; Wiley-Interscience: New York, 1970. (6) Koulkb-F'ujo, A. M.; Moreau, M.; Sutton, J. J. Phys. Chem. 1982,86, 1421. (7) Baxendale, J. H.; Beaumond, D.; Rodgers, M. A J. Trans. Faraday SOC.1970, 66, 1996. ( 8 ) Miller, J. R.; Calcaterra, L. T.; Closs, G. L. J . Am. Chem. SOC.1984, 106, 3047. (9) Hurst, S . K.; Lane, R. H. J. Am. Chem. SOC.1973, 95, 1703.
0022-365418512089-4838$01SO10
Experimental Section THF and UC14 were respectively purified7 and synthesized'O as previously published. The best quality olefins and DPA from Fluka were used without further purification, except for DPE, a yellow liquid, which was distilled after being dried over P205. The solutions, which are oxygen- and water-sensitive, were prepared and handled under argon. The pulse experiments were carried out with a Febetron 707 that delivered 30-11s base pulses of approximately 1.8-MeV electrons with doses from 50 to 77 h a d . The analyzing light came from a high-voltage pulsed xenon source (XBO 450). Either a photomultiplier or one of two different diodes (a Si photodiode, BPY 13A, or a Ge photodiode, J16LD) was used depending upon the wavelengths explored. The setup has already been described." The electrical signal was sent through a 50-ohm load resistor into a Tektronix 7912 AD oscilloscope and then reproduced on the screen of a Tektronix 4052 console where it was analytically treated with an adapted program. The values of the optical densities were obtained by extrapolation to time t = 0, corresponding to the end of the pulse, or to a given time, t. Results and Discussion 1 . U(ZZZ)Formation. The portion of the spectrum displayed (300 to 1550 nm) confirmed that the formation of eTHF- in pure THF, which disappeared with slightly varying rate constants around 7 X IO6 s-I, depended on the purification or the degassing of the liquid. Adding UC14 in various concentrations (5 X to 10-lM) leads to the total disappearance of eTHF- and to the formation of two peaks at 475 and 550 nm (Figure 1). The ratio of the absorbances (or optical densities (OD)) at these wavelengths is OD475/OD550= 1.1 f 0.05. At low concentrations M) of U(IV), we followed the eTH,?- disappearance at 1550 nm to establish the rate constant for the reaction U(1V) + eTHF-. This reaction is pseudo first order and the rate constant is (4 f 0.2) X 1Olo M-I s-], corrected for electron disappearance within the matrix. In this solvent, only reactions with high rate constants can be studied, the rate constant for the *HF- recombination with the counterion T H P or its derivatives being very high (1.7 X 10l2 M-' s-'). When N 2 0 is initially bubbled through the solution, *HF- is not formed and the two peaks at 475 and 550 nm do not appear. Thus we confirmed that these two peaks belong to U(II1). Even under argon, the U(II1) lifetime is short. By multipulsing a solution 2.5 X lo-* M in U(IV), we obtained an increase of the U(II1) absorbance as well as its lifetime. In Figure 2 the absorbance at 475 nm is plotted for different times and number of pulses. One sees that the immediate absorbance is constant (10) Hermann, J. A.; Suttle, J. F. Inorg. Syn. 1957, 5 , 143. (1 1) Tran Thi, T. H.; Koulkb-Pujo, A. M. J. Phys. Chem. 1983,87, 1166.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985 4839
Reduction of U(1V) by Solvated Electrons
I
+
immediatly after the pulse
i
o
9ms after the pulse
I
=O
II
x
t
I
o
800 ns
7
+
180 P
I
after the pulse 0,
0'
I I
i
0.
0-0-0-0
I
I
0 500 700 h(nm) Figure 1. Spectrum obtained by pulse radiolysis of a 4.15 X solution in THF at different times after the pulse.
* M UC14
whatever the pulse number, having a normalized value around s after the 0.67. But the absorbance (0.054) measured 9 X first pulse increases rapidly after several pulses, finally reaching the same value as the immediate one. This seems to show that (i) the U(II1) formed initially corresponds only to the U(1V) reaction with *HF- and then (ii) when U(II1) is already present, it protects the U(II1) being formed either by the oxidizing species coming from the parent positive ion, THF', or by O2traces still present. It is well-known5 that multipulsing "cleans up" the solutions from 02. Nevertheless, this last assumption is less probable, the solutions being carefully degassed because of the O2 sensitivity of U(IV). The UC13 spectrum in THF is knownI3 to contain several weak absorption bands in the visible and near-IR but with our setup, we can only see the two most intense bands. As far as we know, the corresponding molar extinction coefficients c have never been determined. From our results, it is possible to calculate these extinction coefficients, if we assume that U(1V) scavenges eTHF-9 giving U(II1) with a yield G(U(II1)) = G(eTHF-). (The symbol G represents the yield in molecules of a product formed for a 100-eV energy absorbed by the medium.) An important problem is the determination of the yield of solvated electrons, G(eTHF-),that escape recombination with the parent positive ions in the spurs. Many studies have been devoted to this question, because in nonpolar liquids an appreciable fraction of the electrons reacts with these geminate ions and the question arises: what is the effect of increasing the solute concentrations, e.q., U(IV), on this fraction? An extended discussion of the determination of the *HF- yield as a function of solute concentration was recently published by (12) Bockrath, B.; Dorfman, L. M. J . Phys. Chem. 1973, 77, 1002. (13) Moody, D. C.; Odom, J. D. J . Inorg. Nucl. Chem. 1979, 41, 533.
i I
I
5
IO
pulse number
Figure 2. Multipulsing effect: X = 475 nm; [U(IV)] = 2.5 X
c
M.
Kadhum and Sa1m0n.l~ We have used several semiempirical approaches and the best agreement with our results was given by the Warman f0rmu1a.l~
G(P) = G,,,
+ Gg,[(aC)'/z/l+ (aC)1/2]
(1)
where P is the reduced product, C is the electron scavenger concentration, G,,, is the yield of the solvated electrons that normally escape spur recombination, G,, is the yield of ion'pairs undergoing geminate recombination in the spurs, and a is an adjustable parameter related to the scavenger efficiency. This formula allowed us to calculate G(U(II1)) for different U(1V) concentrations after having determined a. To this effect, we previously calculated the empirical coefficient a for St (as will be described latter) and corrected it as a function of the relative electron scavenging efficiencies of S t and U(1V). The results of these calculations, as well as the extinction coefficient c estimated from them, for U(II1) bands are given in Table I. These coefficients were calculated from the usual Lambert-Beer formula: OD = cC1 where 1 = 2.5 cm and C = 0.92GD X M, D being the normalized dose, 100 krad. The coefficient (0.92) is introduced to take into account the relative number of electrons per gram of T H F compared to that of the aqueous dosimeter (the introduction of UC14 in solution is negligible for the dose correction, less than 1%). Nevertheless, these c values (41 10 and 3740 M-l cm-') are higher than expected for f-f transitions. The f-f transitions of actinides are Laporte forbidden (AI = 0) and their oscillator strength are generally low. Typical values of c in the range 30-1000 are measured for many uranium (111) compounds.16 The optical spectrum of trivalent uranium, whose electronic configuration is 5f3, presents many transitions in the investigated (14) Kadhum, A. A. H.; Salmon, G. A. Radial. Phys. Chem. 1984,23,67. (15) Warman, J. M.; Asmus, K. D.; Schuler, R. H. J . Phys. Chem. 1969, 73, 931. (16) Shiloh, M.; Marcus, V. Isr. J . Chem. Cohen, A. J.; Carnall, W. T. J . Chem. Phys. 1960, 64, 1933.
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985
4840
TABLE I: C ( P ) = C,
+ Gpl(aC)1'2/1 + (c&)"*' @-),
k(e;+P), P St DPE Ac DPA
U(IV)
Koulkes-Pujo et al.
[PI, M 10-2 10-2 10-2 10-2 4.5 x 10-2 2.5 X 10-3
normalized OD calcd
M-I cm-l
expt
G(F)
1.13 X 10" 2.35 X 1O1O 5.6 X 1O'O 9.5 x 10'0
a, M-' 1.54 0.32 0.76 1.29
6.1 X 1.2 x 104 2 x 103 7.2 x 104
10.5' 1 .6d 0.3e
4 X lolo
0.54,
I
0.745 0.58 0.64 0.72 0.82 0.72 0.42
M-1
s-I
11.91 0.789 0.679 0.388
0.69 0.4
C(U(III)), M'I cm-'
41 10 at 475 nm 3740 at 550 nm
"G,, = 0.338 f 0.017 [ref 14); Ggi = 3.69 f 0.35. bReference 14. 'This work, 500 nm. dThis work, 400 nm. 'This work, 560 nm. /This work, 445 nm. gThis work, 475 nm.
range: 300-1550 nm assigned to f-f transitions. But two assumptions can be made concerning our high c values found at 550 and 475 nm in an attempt to explain them: (i) 5f2 6dI transitions have been found in the case of uranium(II1) hexamethylphosphorotriamide solution and are allowed;17(ii) hypersensitivity in electronic transitions of lanthanide and actinide compounds can also be inferred.l* In this case the transitions 419/2 4Fs12and 419,2 4G512 have been reported to be hypersensitive but are expected a t slightly higher energies. In any case we cannot clearly understand why such large c values are calculated from our pulse radiolysis results except by a solvent effect, a consideration already suggested by Kamenskaya et a1.19 A question still arises as to the nature of the U(II1) compound obtained by pulse radiolysis reduction: it is UC13 or UCl;? We followed the spectrum evolution from to 10 s after the pulse, the latter being a limit for our signal detection. The absorbances decrease, but no spectrum change occurs during this lapse of time, so we consider that the spectrum obtained at the end of the pulse corresponds to only one species, probably UC13, because there is no effect of C1- addition to the medium (LiCl in the presence of 15-crown-5). 2. Olefin Anions in THF. 2.1. trans-Stilbene (St):l,2-Diphenylethylene. The absorption spectrum of the radical anion St- formed by the reaction eTHF- + St St- in the radiolysis of trans-stilbene solution has been described in the literature. It presents several bands, the most intense located at 500 and 720 nm. The extinction coefficient at 500 nm is known, t = 6.1 X lo4 M-1cm-1.20 We obtained similar bands and from this value of t we calculated a G(St-) = 0.745, in agreement with that previously f o ~ n d . ~As ! * ~already noted, this G value is higher than the yield of electrons escaping spur recombination, G,, because St scavenges part of the geminate reacting electrons. From this value, we calculated the coefficient a in eq 1 reported in Table I. We determined the formation rate of this anion, measuring the electron disappearance a t 1500 nm. This reaction is pseudo first order. The rate constant is k = 1.3 X 10" M-ls-', in very good agreement with Langan and Salmon's valueZo obtained directly from the St- formation rate measured at 550 nm. The ratio of this rate constant to those previously determined for U(1V) and the other solutes employed was taken as the relative electron scavenging efficiencies of these solutes compared to S t (see Table I). St- disappears by a second-order reaction with a rate constant equal to 1.75 X 1O'O M-' s-l. The spectral intensity decreases with time but its form remains constant. Under N 2 0 , which reacts with solvated electrons, the spectrum has the same profile, with optical densities corresponding to 10% of those determined for argon-saturated solutions. 2.2. 1 ,I-Diphenylethylene (DPE). DPE, an isomer of stilbene, reacts with entF-with a rate constant of 2.35 X 1D'O M-' s-l. DPEhas several bands in its optical spectrum, the most intense located
-
-
-
-
(17) Ryan, J. L.; Jorgensen, C. K. Mol. Phys. 1963,7, 17. (18) Henrie, D. C. Fellows, R. L.; Choppin, G. R. Coord. Chem. Rev. 1976,18, 199. (19) Kamenskaya, A. N.; Drozhdzhinskii, Ya.; Mikheev, N . B. Radiats. Khim. 1981,23, 264. (20) Langan, J. R.; Salmon, G. A. J . Chem. Soc., Furaday Trans. I 1982, 78. 3645.
at 400, 525, and 600 nmeZ1 DPE is about five times less reactive with eTHF- than is St. With the same approach used to determine G(DPE-) as for U(III), we calculate cW = 1.2 X lo4 M-lcm-'. DPE-disappears by a second-order reaction with a rate constant of 1.8 X 10" M-ls-'. Contrary to what happens in the case of St-, the spectral profile changes with time. After 800 ns, only one peak remains at 525 nm, whose assignment is equivocal. It may be done to either a radical coming from THF' as an oxidant or a radical formed by reaction of DPE- with H', the latter coming from the reaction of THF+ with the solvent. The following reactions may be suggested: THF e-
-
THF+
+ nTHF
---+
eTHF- + DPE THF'
+ THF THF'
THF' DPE-.
-
+
+ e-
*HF-
DPE-*
THF(H+) + THF(-H).
+ *HF-
+ DPE
+ THF(H')
---+
DPE+*+ T H F
-
-+
THF
DPE(H)- + T H F
As indicated later the last reaction is the most probable and this suggests that DPE(H). is the species absorbing at 525 nm. 2.3. Acenaphthylene (Ac). This olefin contains a naphthyl group instead of two separated phenyl groups: CH=CH
Its anion is not described in the literature. We have established its spectrum which presents several bands and shoulders, the best resolved peak being at 560 nm. The Ac- formation rate constant is 5.6 X 1O'O M-ls-'. We calculated G(Ac-) = 0.64 and tsb0= 2 X lo3 M-l cm-'. The transient spectrum disappears without any change by a second-order reaction with a rate constant of 3.66 x 1Olo M-I s-]. 2.4. Diphenylacetylene (DPA). DPA, reacts with eHFgiving DPA-. The spectrum of this anion was already described in methyltetrahydrofuran at low temperatures.*I It has a main band at 445 nm and several shoulders. Our spectrum is identical and we determined c445 = 7.2 X lo4 M-' cm-'. The formation rate constant of the anion is 9.5 X 1O'O M-ls-l; it disappears, without any change with a rate constant k = 4.5 X 10" M-l s-l. We assume that all these different anions react with THF(H+) or their counterion, which explains the second-order reaction. The spectra of these anions are drawn in Figure 3. We shall now examine what happens in the presence of U(1V). 3. Radiolysis of UCl.,-Olefin Mixtures. The U(II1) spectrum and those of the different olefin anions having been separately established. We prepared two kinds of mixtures containing either (21) Zabolotny, E. R.; Garst, J. F. J . Am. Chem. SOC.1964,86, 1645. (22) Habersbergerova,A.; Janovsky, I.; Teply, Radiat. Res. Rev. 1968,I , 172.
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985 4841
Reduction of U(1V) by Solvated Electrons
G(St-I'
i
+ I
I
1
I
41
I
I
I
I
45
P
R = [U IV]
/ [St]
Figure 4. Reciprocal of G(St) vs. R = [U(IV)]/[St] corresponding to
the calculated equations and experimental values.
. I
-
I
I
I
I
I
LOO
500
600
700
800
hlnml
-
Figure 3. Alcene and alcyne anion spectra.
an excess of U(1V) or an excess of one of these olefins or the alkyne. Knowing the rate constants of their formation, we chose the relative concentrations of the solutes so that +HF- could react mainly with one of them. When the ratio, R, of [U(IV)]/[olefin] is greater than 1, we always obtained, immediately after the pulse, the U(II1) spectrum, with its two peaks at 475 and 550 nm, decreasing slowly with time, without any change in the spectral shape. We concluded that either no complex U(II1)-alkene formed, as was the case for Cu(1)-olefin in aqueous medium? or its lifetime was too short for any detection with our set up. When this ratio R is less than 1, the results are more interesting and depend on the nature and relative concentrations of the solute added to U(1V). 3.1. St-U(W) Mixtures. In this case, immediately after the pulse, St- is the only species present, but the absorbances were always smaller than in the absence of U(IV), probably because, during the pulse some erHF- reacts with U(1V). If eTHF- reacts only with U(1V) and St, a simple competition ought to be established between the two solutes. The following formula then alloys calculation of G(St-) as a function of the [U(IV)]/[St] ratio: G(St-)-' = G(eTHF-)-I[l -k k,[U(IV)]/k2[St]]
(11)
We maintained the St concentration at M, which corresponds to G(e,,-&) = 0.74. Taking for kl and k2 the values that we have previously determined, we may draw the straight line representing G(St-)-l vs. [U(IV)]/[St] (Figure 4). The experimental values follow relation I1 up to R = [U(IV)]/[St] = 0.15. Beyond this value, G(St-) is lower than that calculated. St- disappears during the pulse probably by reaction with U(1V). To check the validity of this hypothesis, we increased the U(1V) concentration to 1.25 X M for R = 0.083. The corresponding value of G(St-) is
0.13 and its reciprocal is noticeably higher than that calculated from the straight line of Figure 4,thus confirming the suggested hypothesis. When a still higher U(1V) concentration was added to St, keeping the ratio of the two solute concentrations around 0.1,a new phenomenon appeared. In this case, the St--U(IV) reaction rate was so high than St- had completely disappeared during the pulse. With [U(IV)] = 5 X M and [St] = 5 X lo-' M the spectrum at the end of the pulse corresponded to neither St- nor U(II1) (Figure 5). It consisted of different bands, the most striking difference being the band at 475 nm and the shoulder at 550 nm, which also differed from those of U(II1) particularly by their intensity ratio (1.92instead of 1.1). The evolution with time of this spectrum gave the usual U(II1) spectrum. We may conclude that the U(1V) reduction by St- proceeds through an intermediate, the spectrum of which is occulted if St- is still present in the solution. 3.2. DPE-U(IV) Mixtures. As with St, we performed two kinds of experiments with the same ratio R = 0.1 and different M,the DPE and U(1V) concentrations. For DPE < 1.5 X spectrum at the end of the pulse corresponds to DPE-, with absorbances slightly lower than those of DPE alone. However, it is difficult to say whether the DPE- spectral evolution with time leads to U(II1) because DPE- has several peaks, some of them coinciding with those of U(II1). At 900 ns after the pulse, the peak at 525 nm is the same as when DPE was the only solute present. When the DPE and U(1V) concentrations are respectively increased in the same proportion, the DPE- bands disappear and, at the end of the pulse, two peaks appear at 415 and 550 nm, as in the U(II1) spectrum. It is, however, interesting to note that the absorbance ratio at 475 and 550 nm is reversed (0.93instead of 1.1) (Figure 6). The most important spectral change with time occurs at 525 nm where the absorbing species already observed in DPE solutions grows in. By conveniently selecting the detection time conditions, we were able to follow its formation. Its normalized absorbance (1.45)becomes 3.6 times greater than for DPE alone or DPE with U(1V) at low concentration. We attribute this absorption to DPE(H)., the initial one detected just after the pulse
Koulkes-Pujo et al.
A
A >
.-c
U CId 5 xlO-'M St 5xlO'M
VI
d
n 0.d.
P
-
[+ o
OOZM 02M
Y
0
+
0 a
L o
-8
U 0
.-c
UCI, DPE
h
" .H
at 64 ns after the pulse at 1801s
at 64 ns
after the pulse
at 1 5 0 1 s
0
.-
-?! E ij
z
b
2
2
+
1-
+\
7
-
being [DPE-U(IV)]- and disappearing by the reaction [DPEU(IV)]- + H+ DPE(H). U(1V). It is interesting to notice the different behavior of St and DPE, coming probably from the large affinity of Ph2C=CH2 for H atoms giving the radical Ph2C.-CH,. We have already shown that another differences was established, namely, the rate constants for anion formation and disappearance:
A
+
k(eTHF-+DPE)= 0.2k(emF-+St) and k(-DPE-) = 13k(-Sr-) 3.3. Ac-UCl., Mixtures. For the same ratio, R = 0.1 and at M and [Ac] = 1.45 low concentration ([U(IV)] = 1.34 X X lo-* M) the initial spectrum corresponds to Ac- (one peak at 550-560 nm and a shoulder at 600 nm). The absorbance at 550 nm is the same as that determined when Ac is the only solute, meaning that U(1V) did not react with Ac- during the pulse. This spectrum decreased with time leading to absorption in the same positions up to 3.2 hs, the limit of our experiment. For higher U C 4 and acenaphthalene concentrations (0.013 and 0.04 M or 0.13 and 0.4 M, respectively), the spectrum was always identical with that of Ac-, but the optical densities were greatly increased (Figure 7). Using formula I and the constants of Table I, we calculated the absorbance which an Ac concentration equal to 0.13 M should give. This is 0.57, compared with the experimental value (0.575), thus furnishing another argument that, after the pulse (at 64 ns), Ac- is the only species involved. It does not react with U(1V) present in the medium. This spectrum decreased slowly with time, the peaks remaining constant up to 3.3 ps and then slightly shifting to 560 and 630 nm (at 1.12 ms). Some absorption still remained 8 ms after the pulse, but the absorbances were very small and it was difficult to draw the corresponding spectrum, which, however, does not differ appreciably from the previous ones. This spectrum does not correspond to U(II1).
UCI' O O L M AC OLM
~
,E
5 .-e
+
at 64 ns after the pulse
w
a
{E
+
at l l 2 m s ., ( 0 d scale x 2.51
I
I
I I
I 1
pt
7;: I I
0,5 -
\!
L'
LOO
I
I
I
1
500
600
700
800
c
A Inml
Reduction of U(1V) by Solvated Electrons
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985 4843 with a rate constant of 8.5 X lo9 M' s-'. At 150 ~s after the pulse, the spectrum corresponds to U(II1). At higher UCl, and DPA concentrations, the spectrum 64 ns after the pulse has three bands at 425, 475, and 550 nm, the absorbance ratio of the last two being 2.08. The spectrum is different from that of DPA-. At 800 ns after the pulse, the spectrum is already very similar to that of U(I1I) with an absorbance for the two peaks of 1.16, which becomes 1.1 after 150 MS. The spectrum is that of U(II1) (Figure 8).
U C l r O.OLM DPA O . L M
I
,
I
+
6 L ns after the pulse
m
150ms
I)
*
7W hlnm) Figure 8. Spectra obtained with DPA and UC14 mixtures: [UC14] = 4 X lo-* M and [DPA] = 4 X lo-' M. 400
500
600
is 6 X lo3 s-l; however, if we compare the rate of disappearance of Ac-, in the absence and presence of UCl,, we find that it has considerably fallen from 3.6 X lo7to 6 X lo3 s-I. As the reaction is second order, we cannot propose a U(1V)-Ac- reaction but rather suggest that the UC14 protects Ac- against either an oxidizing species or H+, both having concentrations that are of the same order of magnitude. In conclusion, the acenaphtylene anion appears not to react with U(1V) at least under the conditions of our experiment. 3.4. DPA-U(ZV) Mixtures. This case is easier to interpret, since the results are very similar to those of St. At low concentrations of UCl, and DPA in the ratio 0.1, DPA- is present at the end of the pulse. It disappears by a pseudo-first-order reaction
Conclusion The results show that, in THF, electron transfer occurs in some cases between an olefinic anion and uranium (IV) (6F). They suggest that this electron transfer is mediated by a x-conjugated chain between metal with the olefin x-antibonding orbital. It is necessary that a vacant acceptor orbital of appropriate symmetry be provided for overlap with the x-chain and that restrictions arising from requirements of conservation of spin momenta are not significant. For both stilbene and diphenylacetylene, it is possible to show that their anions transfer the excess electron to U(1V). Disruption of the T conjugation may arise with decreasing energy of the a * orbitals which is probably the case for acenaphtylene. There is no electron transfer from Ac- to U(1V). As for diphenylethylene, it seems that, even if a transitory [DPE-U(IV)]-is formed, it does not lead to free U(II1) because of the large affinity of DPE- for H+ or DPE for H atoms. One of the principal objectives of this work was to determine whether a U(III)-olefin transient complex is responsible for any catalytic effect produced by U(II1). The answer is no, the electron transfer occurring in the opposite direction. However, in the transitory complex formed [olefin-U(IV)]-, the uranium formal oxidation state is 3. So the second hypothesis remains as the most probable one, Le., the catalytic effect is the result of hydride transfer from a uranium transient compound formed (for example, UC13 LiAlH4 LiCl Cl2U(p-H2)A1H2). It seems worthwhile to reiterate that this class of reactions studied at very short times, by the pulse radiolysis technique, offers considerable promise for resolution of the effects of reactant modification upon intrinsic electron processes.
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Acknowledgment. We are very grateful to P. Nectoux for his very important help to the radiation chemistry group in adapting the appropriate program to the automatic calculations of the data. We thank J. Potier for his technical assistance, J. Sutton for fruitful discussions, and one of the referees for having tried to improve our poor english sentence structure.
