J. Phys. Chem. 1988, 92, 2449-2451
2449
Pulse Radiolysis of Solutions of Sodium Tetraphenylborate Ke Jian Liu, John R. Langan, G. Arthur Salmon,* Cookridge Radiation Research Centre, University of Leeds, Leeds, LSl6 6QB, England
Dolores M. Holton, and Peter P. Edwards University Chemical Laboratory, Cambridge, CB2 1 E W, England (Received: June 10, 1987; In Final Form: November 13, 1987)
Pulse radiolysis of solutions of sodium tetraphenylborate (NaBPh4) indicates that the absorption observed in organic amides with A- in the range 650-725 nm is not due to Na-, an electron adduct to BPh4-, a triplet excited state, or a proton-donating solvent cation. Experiments in aqueous solution are described in which the reactions of selected radicals with NaBPh, are studied. One-electron oxidation of NaBPh, by N3*-radicals yields a species, assumed to be BPh4', with absorption maxima at 335 and 800 nm. A similar spectrum is observed on pulse radiolysis of solutions of NaBPh4 in tetramethylurea (TMU), but the long-wavelength absorption is shifted to 725 nm. The formation of an oxidizing radical in irradiated TMU was confirmed by the observation of 12'- on pulse radiolysis of solutions of KI in this solvent. Pulse radiolysis of solution of NaBPh, and KI in TMU demonstrated that these solutes compete for the oxidizing intermediate.
Introduction In a previous report' we have described absorption spectra generated on pulse radiolysis of solutions of sodium tetraphenylborate (NaBPh4) in diethylacetamide (DEA), dipropylacetamide (DPA), dimethylpropanamide (DMP), and tetraranging methylurea (TMU). These absorptions, which have A, from 650 nm for DMP to 725 nm for TMU, were ascribed to the sodium anion (Na-) because this species is known2 to absorb in this region in blue solutions of sodium in some of these solvents, on solvent. In addition, and because of the dependence of A, 23NaN M R experiments on the metal solution1revealed the existence of the sodium anion. We report here experiments which negate this assignment.
TABLE I: Effect of Saturation with N 2 0 on G C ,for ~ ~Various Solutions of NaBPh, in TMU [NaBPh,]/M N 2 0 saturn 10SGc725/m2 J-' 0 3.8" 0 0.1" 0.03 8.9b 0.03 5.7' 0.10 13.8b 11.Y 0.10
+ +
+
"0.2-/rs, 60-Gy pulse.
50-ns, 8.4-Gy pulse.
50-ns, 24-Gy pulse.
pairs are formed on pulse radiolysis of solutions of NaBPh, in tetrahydrofuran. For a solution of 0.1 M NaBPh, in TMU saturated with N 2 0 , Experimental Section the absorption at 725 nm was formed over a period of about 400 The purification of solvents, preparation of solutions, and the ns following a 50-ns pulse, but reducing the concentration of techniques of pulse radiolysis used were as described previo~sly.'~~ NaBPh, to 0.03 M had little influence on the rate of growth of the absorption. Saturation of a 0.053 M NaBPh, in TMU solution Results and Discussion with air caused a 45% reduction in the end-of-pulse absorption In Figure 1 we show the effects on the end-of-pulse absorption at 725 nm following a 0.2-ps pulse (fwha), but the decay rate was spectrum observed using 0.2-ps, -35-Gy (3.5 krad) pulses of increased to 3.8 X lo6 s-I, which accounts for essentially all the saturating a 0.068 M solution of sodium tetraphenylborate in reduction in yield occurring during the pulse. Thus, the effect T M U with nitrous oxide and oxygen. Both of these gases are of oxygen seems to be mainly on the observed species rather than known to be efficient scavengers for solvated electrons (e;) and on its precursor. would be expected to prevent the formation of Na- under these Besides being an efficient scavenger for e;, oxygen is known conditions. Whereas oxygen completely eliminates the peak at to be an effective quencher of triplet excited states and a free725 nm, saturation with N 2 0 leads to a reduction of about 22% radical scavenger, and its effect on the absorption due to the in its magnitude. Experiments on a 0.06 M solution of NaBPh4 NaBPh4 species would be explicable if this species was either a in DMP gave similar results except that saturation with N 2 0 triplet excited state or a free-radical species. To investigate produced a 57% reduction in the 650-nm peak. The effects of whether triplet excited states of solutes can be generated by pulse saturation with N 2 0on Ge725(Ge is the product of the radiolytic radiolysis of solutions in amides, the end-of-pulse spectrum was yield of the species in units of mol J-' and its molar absorptivity recorded for a solution of 0.05 M anthracene in T M U by using in units of mz mol-') for various solutions of NaBPh, in T M U 0.6-ps,70-Gy pulses. This spectrum showed features attributableS are summarized in Table I. The relatively minor effect of N 2 0 to triplet excited anthracene with peaks at 410 and 430 nm suon the absorption at 725 nm for these solutions indicates that these perimposed on a background absorption mainly due to the anabsorptions are not due to Na-. The small differences in the thracene radical anion. By observing the absorption of anthracene change of GqZ5on saturation with N 2 0 a t the various concenradical anion at 720 nm and using its known spectrum: its contrations of NaBPhl may be due to the influence of Na+ ions on tribution to the absorption in the 40C-450-nm region was assessed. the absorption spectrum of e;. Applying this correction gave Ge430for anthracene triplet state The small effect of N 2 0also proves that the absorption at 725 of 8.1 X m2 J-' which, taking c430 = 8570 m2 mol-',' yields nm is not due to a species formed by reduction of the BPh, moiety G(3anth*) = 9.5 nmol J-' (0.09 molecules (100 eV)-l). by e;. This is in keeping with the observation4 that (Na+,e;) ion This demonstrates that small yields of triplet excited states of solutes are formed on pulse radiolysis of solutions of aromatic (1) Holton, D. M.; Edwards, P. P.; Salmon, G. A. J . Phys. Chem. 1984, solutes in amides but does not prove that the NaBPh, species is 88, 3855. (2) Young, C. A.; Dewald, R.R. J . Chem. SOC.,Chem. Commun. 1977, 188. ( 3 ) Logan, S. R.; Salmon, G. A. J . Chem. SOC.,Perkin Trans. 2 1983, 1781. (4) Bockrath, B.; Dorfman, L. M. J . Phys. Chem. 1973, 77, 1002.
0022-3654/88/2092-2449$01.50/0
(5) Porter, G.; Windsor, M. Proc. R. SOC.(London) 1958, A245, 238. (6) Balk, P.; Hoijtink, G . J.; Schreurs, J. W. H. Receuil 1957, 76, 813. (7) Ledger, M. B.; Salmon, G. A. J . Chem. SOC.,Faraday Trans. 2 1976, 72. 883.
0 1988 American Chemical Society
5 A
Liu et al.
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988
2450
I
1
I
TABLE II: The Effect of Various Additives on the Visible Absorptions Observed in Aqueous Solutions of NaBPh4
I
I
[NaBPhd / M 1
10-2
2 3
10-2 10-2
700
600
h/nm
e..,
an excited state. To further test this possibility, a solution of 0.05 M NaBPh, in acetone was studied by using 0 . 6 - ~-,.50-Gy pulses. Acetone was chosen as a solvent since it is knowns to produce large yields of triplet excited states of solutes on pulse radiolysis and also readily dissolves NaBPh,. A weak background absorption was observed extending to the limit of observation at 800 nm, but the 725-nm absorption was absent. Addition of 0.01 M anthracene to the solution gave a large absorption at 425 nm due to triplet excited anthracene which was 2 12.5 times the background absorption at 725 nm. Thus it seems evident that the 725-nm absorption in NaBPh,/TMU is not due to a triplet excited state. Addition of the cation scavengers ethanol (0.09 M) or triethylamine (0.1 M) to a solution of 0.068 M NaBPh, in TMU caused no reduction in the 725-nm peak and similar results were obtained in DMP. Thus it seems unlikely that the absorptions in the visible can be attributed to reaction of NaBPh, with a proton-donating cationic species. Since the species absorbing in the visible on pulse radiolysis of NaBPh4 solutions in amides is not due to Na-, an electron adduct to BPh,, a triplet species or a cation which is able to donate protons, it seems that the precursor of the absorption must be a free radical derived from the solvent. To investigate further the nature of this precursor some pulse radiolysis experiments were carried out on aqueous solutions of NaBPh, containing various additives, which were chosen so that reaction of selected radicals with the salt could be studied; the results of these studies, are summarized in Table 11. Solutions 1-6 were saturated with N 2 0 so that the hydrated electron is converted rapidly ( t l j z 3 ns) to 'OH radical by reaction 1. For solution 1, where the main N 2 0 + ea;
H+
N,
-
+ 'OH
(1)
species reacting with the NaBPh, is the 'OH radical, the largest absorption observed is at 300 nm, which is expected for the radical derived by 'OH addition to the phenyl groups. However, a small absorption with A,, at 750 nm grew in over a period of -2 p s , although no decay of the 300-nm absorption was observed over this period. The small yield of H', which is generated in N 2 0 saturated water, is expected to behave similarly to the *OH and add to the phenyl groups. Thus, the origin of the weak red absorption in this system is not apparent. In solution 2, the high concentration of TMU is expected to scavenge both the 'OH and H' generated on radiolysis of water according to reaction 2.
(8) Arai, S.; Dorfman, L. M. J . Phys. Chem. 1965, 69, 2239.
1.0 M TMU 1.0 M TMU
750 (300) N2O 800 N 2 0 / 0 2= 800
4:1 N2O N2O N2O
N2O
5
1.0 X
6
1.0 X
0.05 M NaN,
7
1.0 X
8
1 x IO-,
1 X lo-' M Na2S208;Ar 0.05 M Z-BUOH 5 x lo-' M Na2S208; Ar 0.05 M Z-BUOH
800
Figure 1. End-of-pulse absorption spectra for a 0.068 mol dm-, NaBPh, O2 saturated. solution in TMU; -, degassed;---, N 2 0 saturated; A is the absorbance per unit dose in arbitrary units. Pulse: 0.2 ps. -56 GY.
additives
1.0 M Z-BUOH 0.02 M NaN,
4 10-2
500
saturating A,,/ gas nm
700 335 800 335 800 335 800 335 800
105 x Gc(X,,,)/
m2 J-' 1.5O
(25.2) 2.6" l.04 0.55" 41"' 39 56"s' 46 26"' 46 11b.c 5
"0.2-ps pulse. b25-ns pulse. 'Recorded after growth of absorption.
DO
Figure 2. Spectrum observed 5 ps after pulse radiolysis of an N,O-saturated aqueous solution with [NaBPh4]= 1.0 mM and [NaN,] = 0.02 M. A is the absorbance per unit dose in arbitrary units. Pulse: 0.2 ps, -12 Gy.
Therefore, the absorption at 800 nm is apparently formed by reaction of radical I with NaBPh,. Addition of 20% O2 to the nitrous oxide reduces this absorption to 38%. In this solution, the ratio of [N,O]:[O,] -. 77 and G('0H) is, therefore, expected to be. reduced by only 0.6%. Hence we can conclude that in solution 3, the effect of the oxygen is either due to its reaction with radical I or on the product resulting from reaction of radical I with NaBPh4. In solution 4, 'OH and 'H are scavenged by tert-butyl alcohol in a reaction analogous to (2) and yield tert-butyl alcohol radicals which react with NaBPh, to yield a weak absorption with A,, at 700 nm. Horii and Taniguchig have observed an absorption with A,, = 335 nm which is formed on pulse radiolysis of nitrous oxide saturated aqueous solutions of NaBPh, and azide ion and which they attribute to the oxidation of BPh,- by the azide radical (reaction 3). To test whether the absorption in the visible region BPh,
+ N3*
-+
BPh4'
+ N3-
(3)
formed on pulse radiolysis of solutions of NaBPh, in organic amides could have as its precursor an oxidizing species, we have extended the wavelength range for measurements similar to those of Horii and Taniguchi to cover the range 300-1000 nm. The spectrum observed 5 ps after pulse radiolysis of an N20-saturated solution with [NaBPh,] = 1.0 mM and [NaN,] = 0.02 M is shown at 335 and 800 nm. in Figure 2 and shows two peaks with A, Both peaks decayed by first-order kinetics with k = (3.95 f 0.2) X lo4 s-l and we conclude that they result from the same transient species, but, as found by Horii and Taniguchi, the 335-nm peak (9) Horii, H.; Taniguchi, S. J . Chem. SOC.,Chem. Commun. 1986, 915.
Pulse Radiolysis of Sodium Tetraphenylborate
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2451
decayed to give an absorption which decayed on a longer time scale. The rate constant for the first-order decay reported here is to be compared with that of 2.0 X lo4 s-I found by Horii and Taniguchi and which they attributed to reactions 4 or 5. BPh4' BPh4'
-
-
+ Phi BPh3 + Ph' BPh2
IO. 0
+-+-
--t-t-
7
8.0,
(4) (5)
In a solution with [NaBPh4] = 1.06 mM and [NaN3] = 0.05 M, both absorptions were observed to be. formed with a first-order rate constant of (7.5 f 0.1) X lo5 s-I, indicating that k3 7.1 X lo* dm3 mol-' s-I, which is somewhat lower than the value of 1.4 X lo9 dm3 mol-' s-l measured by Horii and T a n i g ~ c h i . ~ We have also tested whether the SO4'- radical anion, which is known to be a strong electrophile and one-electron oxidant,I0 is able to oxidize NaBPh4. Solutions 7 and 8 contained sodium persulfate, which reacts with hydrated electrons (reaction 6) with
-
ea;
-
+ S20g2-
S042- + SO4*-
(6)
a rate constant of 1.1 X 1O'O dm3 mol-' s-I,l1 and also 0.05 M tert-butyl alcohol to scavenge H' and 'OH. The absorption of was observed to decay over -0.75 ps to yield a spectrum which also showed peaks at 335 and 800 nm as with the azide solutions. However, the ratio of the absorptions at 335 and 800 nm was different from that in the azide solutions and also varied with the persulphate concentration (see Table 11). Also, whereas the two peaks again decayed at about the same rate, the decay was dependent on persulfate concentration indicating that the transient reacted with persulfate with a rate constant of (5.0 f 1.O) X lo7 dm3 mol-' s-I. It seems that the reaction of SO4'- with BPh4' does not only lead to oxidation to BPh,' but that some other process also occurs. In view of the above results with the aqueous azide and persulfate solutions, it was decided to investigate whether the 335-nm absorption was present in the solutions of NaBPh, in the amid& An N,O-saturated solution with [NaBPh,] = 0.1 M in TMU was irradiated with 25-ns pulses and peaks were observed with A,, = 335 and 725 nm. Irradiation of the neat degassed solvent is known to yield transient absorptions at X < 400 nmI3 and the transient spectrum in the NaBPh4/TMU/N20 solution was therefore corrected for the underlying absorption in N20-saturated TMU. Allowing for this correction, the Gt values at 335 and 725 nm were evaluated to be 1.4 X and 1.1 X m2 J-I, respectively, the correction at 335 nm amounting to 42% of the total absorption. After allowing for the decay of the background absorption at 335 nm, both absorptions decayed with a first-order rate constant of (6.0 f 0.3) X lo4 s-l. The close similarity of the spectra observed in the aqueous azide solutions and in TMU solutions suggests that the transient species formed in the amides solutions results from oxidation of BPh4by a solvent-derived species. To further test this possibility, an N20-saturated solution of 0.0362 M potassium iodide in T M U was irradiated with 0.2-ps, 33-Gy pulses. The end-of-pulse absorption spectrum showed unequivocal evidence for the formation (10) Neta, P.; Madhavan, V.; Zemel, H.; Fessenden, R. W. J . Am. Chem. SOC.1977, 90, 163.
(11) Anbar, M.; Bambenek, M.; Ross, A. B. "Selected Specific Rates of Reactions of Transients from Water in Aqueous Solution. 1. Hydrated electron"; NRSDS-NBS 43; U. S. Department of Commerce: Washington, DC, 1981. (12) Hug, G.L. "Optical Spectra of Nonmetallic Inorganic Transient Species in Aqueous Solution"; NSRDS-NBS 69; U. S. Department of Commerce: Washington, DC, I981. (13) Guy, S. C.; Edwards, P. P.; Salmon, G.A. J . Chem. SOC.,Chem. Commun. 1982, 1257.
0.0
a
450
500
550
600
650
700
750
800
X/nm Figure 3. End-of-pulse absorption spectra for N20-saturated solutions of NaI and NaBPh, in TMU: (a) [NaI] = 0.047 M (-O-), (b) [NaBPh,] = 0.045 M (-V-), and (c) [NaI] = 0.047 M and [NaBPh,] = 0.045 M (-A-). Spectrum (d) is the sum of (a) and (b). Pulse: 0.2 ps, 100 GY.
-
of 12'- with peaks at 385 and 750 nm.I2 12*-is the well established14 product of the oxidation of iodide by the reactions ox
-
+ I- o x - + I' + I- e 12*-
(7)
I'
(8) and hence this result establishes that an oxidizing intermediate is generated on radiolysis of TMU. In Figure 3 are shown the spectra produced on pulse radiolysis of N20-saturated solutions with [NaI] = 0.047 M and [NaBPh4] = 0.045 M and also that of a solution containing both solutes at these concentrations. The measurements were restricted to X > 450 nm because of the background absorption by the solvent at these wavelengths. Clearly the absorption in the solution containing both solutes is considerably less than the sum of the absorptions when the solutes are separate, thus indicating that the solutes are competing for a common oxidizing precursor. In summary, we conclude that the species absorbing in the visible on pulse radiolysis of NaBPh4 solutions in amides is not Na-, a triplet-state species, or a species formed by scavenging a proton-donating cation. The experiments in the aqueous azide solutions, taken together with these of Horii and T a n i g ~ c h i , ~ indicate that the one-electron oxidation of BPh4- gives rise to a species, presumably BPh,', which absorbs at 335 and 800 nm. This suggests that the species formed in the amide is also BPh4' formed by reaction 9, where Ox is an oxidizing species formed on the radiolysis of the amides. Ox' BPh,BPh,' Ox(9)
+
-+
+
It seems that the long-wavelength absorption of the BPh,' species is strongly solvent dependent. This may be associated with the electron-deficient nature of this radical.
Acknowledgment. We wish to acknowledge support from the S.E.R.C. by the award of a research grant and a Postdoctoral Fellowship to D.M.H., and the Ministry of Education of the People's Republic of China for the award of a studentship to K.J.L. We also wish to acknowledge Professor R. H. Schuler for helpful discussions and also a referee who drew our attention to the paper by Horii and Taniguchi. Registry No. TMU, 632-22-4; NaBPh4, 143-66-8; N 2 0 , 10024-97-2; t-BuOH, 75-65-0; NaN,, 26628-22-8; Ar, 7440-37-1; Na,S20,, 777527-1; KI, 7681-11-0.
(14) Thomas, J. K. Trans. Faraday SOC.1965, 61, 702.
