Direct observation of the benzyl radical and the benzyl anion within cation-exchanged zeolites. A nanosecond laser study [16]

Full text of DOI:10.1021/ja982892x

J. Am. Chem. Soc. 1998, 120, 13543-13544
13543
dence is also presented for the simultaneous detection of the
benzyl anion.
Direct Observation of the Benzyl Radical and the
Benzyl Anion within Cation-Exchanged Zeolites. A
Nanosecond Laser Study
Our belief was that the ill-defined spectra generated upon
photolysis of dibenzyl ketone within the cavities of zeolites was
due, at least in part, to the initial generation of a radical pair within
the supercage instead of a single “free” benzyl radical.23,24 Thus,
our approach was to utilize a precursor, phenylacetic acid,20,25-27
that upon 266-nm laser photolysis would generate a zeolite
supercage with only one radical species.
Frances L. Cozens,* Wendy Ortiz, and Norman P. Schepp
Department of Chemistry, Dalhousie UniVersity
Halifax, NoVa Scotia, Canada B3H 4J3
Figure 1 shows the transient diffuse reflectance spectrum
generated upon 266-nm laser28 excitation of phenylacetic acid
incorporated into dry NaY under reduced pressure (10-3 Torr).29
The spectrum shows a strong absorption band centered at 315-
nm and a shoulder at 305-nm that coincide nicely with the known
absorption spectrum for the benzyl radical in solution.26 The
transient species at 315-nm is completely quenched by the addition
of oxygen to the sample, which is characteristic of radical
species.30,31 Consistent with the formation of the benzyl radical
is that one of major products upon steady-state photolysis in NaY32
in the absence of oxygen is bibenzyl,33 the product from radical-
radical coupling. The proposed mechanism for the generation of
the benzyl radical is shown in eq 1, path a. Upon incorporation
into the nonacidic Y zeolite, phenylacetic acid deprotonates to
the phenylacetate anion,34 which undergoes photoionization20,25,35
upon 266-nm laser excitation to give the acyloxy radical that then
rapidly loses CO2 to yield the benzyl radical.
ReceiVed August 12, 1998
In recent years, there has been considerable interest in the
generation and reactivity of simple monoaryl benzylic radicals
within the cavities of zeolites.1-5 The majority of these studies
have determined the ratio of products upon steady-state photolysis
of dibenzyl ketone and its methylated derivatives. Both the
reactivity and mobility of the monoaryl radicals within the zeolite
framework have been inferred from the yield and distribution of
the product mixtures, with the conclusion that simple monoaryl
benzyl radicals are highly mobile within the zeolite framework
and that coupling reactions are very fast.4
The direct detection of reactive intermediates within the cavities
of zeolites has given important information about the reactivity
of transients species in the unique environment provided by the
zeolite.6-21 Surprisingly, however, no time-resolved studies of the
benzyl radical produced upon photolysis of dibenzyl ketone within
the cavities of zeolites have been reported. Presumably, the lack
of time-resolved results is due to the ill-defined transient diffuse
reflectance spectra generated upon laser excitation of dibenzyl
ketone within zeolites.22 We now report the direct observation of
photogenerated benzyl radicals within cation-exchanged Y fau-
jasites using nanosecond diffuse reflectance spectroscopy. Evi-
(1) Turro, N. J.; Lei, X.; McDermott, A.; Abrams, L.; Ottaviani, M. F.;
Beard, H. S. Chem. Commun. 1998, 695-696.
(2) Turro, N. J.; McDermott, A.; Lei, X.; Li, W.; Abrams, L.; Ottaviani,
M. F.; Beard, H. S.; Houk, K. N.; Beno, B. R.; Lee, P. S. Chem. Commun.
1998, 697-698.
A second weaker absorption band at 350 nm is also clearly
observed, (Figure 1). The decay rate constant for the 350-nm band,
k350 ) 5 × 105 s-1, is significantly faster than that for the 315-
nm band,36 k315 ) 2 × 105 s-1, indicating that these absorption
bands do not belong to the same transient species. The 350-nm
(3) Garcia-Garibay, M. A.; Lei, X. G.; Turro, N. J. J. Am. Chem. Soc.
1992, 114, 2749-2751.
(4) Garcia-Garibay, M. A.; Zhang, Z.; Turro, N. J. J. Am. Chem. Soc. 1991,
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(23) The generation of radical pairs within the same cavity could lead to
the rapid formation of coupling products with absorption near 330-350-nm.
(24) Langhals, H.; Fischer, H. Chem. Ber. 1978, 111, 543-553.
(25) Meiggs, T. O.; Grossweiner, L. I.; Miller, S. I. J. Am. Chem. Soc.
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(27) Meiggs, T. O.; Miller, S. I. J. Am. Chem. Soc. 1972, 94, 1989-1994.
(28) Continuum Nd: YAG laser 266-nm, <10 mJ/pulse, <8 ns/pulse.
(29) The sample was prepared in a glovebox and was permanently sealed
under vacuum prior to photolysis to ensure that no water would be coadsorbed
during the course of the experiment.
(30) Kelly, G.; Willsher, C. J.; Wilkinson, F.; Netto-Ferreira, J. C.; Olea,
A.; Weir, D.; Johnston, L. J.; Scaiano, J. C. Can. J. Chem. 1990, 68, 812-
819.
(31) Cozens, F. L.; Garcia, H.; Scaiano, J. C. J. Am. Chem. Soc. 1993,
115, 11134-11140.
(32) Product analyses of phenylacetic acid within dry NaY after steady-
state photolysis using a mercury lamp equipped with a quartz filter were carried
out under a variety of conditions. Products were extracted from the zeolite
framework using continuous liquid-solid extraction with dichloromethane
as the solvent.
(33) Photolysis of phenylacetic acid in NaY (loading level of 1 molecule/2
cavities) under dry conditions with a stream of dry nitrogen purged over the
sample throughout the photolysis time gave toluene (60%) and bibenzyl (20%)
after 50% conversion of starting materials.
(34) IR spectra of phenylacetic acid within the cation-exchanged Y zeolite
showed a strong carbonyl band centered at 1620 cm-1 and no signal above
1700 cm-1, which is consistent with the carboxylate ion being the predominate
species in the zeolite environment.
7884.
(7) Yonemoto, E. H.; Kim, Y. I.; Schmehl, R. H.; Wallin, J. O.; Shoulders,
B. A.; Richardson, B. R.; Haw, J. F.; Mallouk, T. E. J. Am. Chem. Soc. 1994,
116, 10557-10563.
(8) Haw, J. F.; Nicholas, J. B.; Xu, T.; Beck, L. W.; Ferguson, D. B. Acc.
Chem. Res. 1995, 29, 259-267.
(9) Xu, T.; Haw, J. F. J. Am. Chem. Soc. 1994, 116, 10188-10195.
(10) Kim, Y. I.; Mallouk, T. E. J. Phys. Chem. 1991, 96, 2879-2885.
(11) Yoon, K. B.; Hubig, S. M.; Kochi, J. K. J. Phys. Chem. 1994, 98,
3865-3871.
(12) Yoon, K. B.; Park, Y. S.; Kochu, J. K. J. Am. Chem. Soc. 1996, 118,
12710-12718.
(13) Sun, H.; Frei, H. J. Phys. Chem. B 1997, 101, 205-209.
(14) Vasenkov, S.; Frei, H. J. Am. Chem. Soc. 1998, 120, 4031-4032.
(15) Pitchumani, K.; Lakshminarasimhan, P. H.; Prevost, N.; Corbin, D.
R.; Ramamurthy, V. J. Chem. Soc., Chem. Commun. 1997, 181-182.
(16) Brancaleon, L.; Brousmiche, D.; Rao, V. J.; Johnston, L. J.; Rama-
murthy, V. J. Am. Chem. Soc. 1998, 120, 4926-4933.
(17) Cano, M. L.; Corma, A.; Forne´s, V.; Garc´ıa, H. J. Phys. Chem. 1995,
99, 4241-4246.
(18) Cozens, F. L.; O’Neill, M.; Schepp, N. P. J. Am. Chem. Soc. 1997,
119, 7583-7584.
(19) Cozens, F. L.; Bogdanova, R.; Re´gimbald, M.; Garc´ıa, H.; Mart´ı, V.;
Scaiano, J. C. J. Phys. Chem. 1997, 101, 6927-6928.
(20) Cozens, F. L.; Cano, M. L.; Garc´ıa, H.; Schepp, N. P. J. Am. Chem.
Soc. 1998, 120, 5667-5673.
(21) Cano, M. L.; Cozens, F. L.; Vicente, F.; Garc´ıa, H.; Scaiano, J. C. J.
Phys. Chem. 1996, 18145-18151.
(22) Laser photolysis of dibenzyl ketone in NaY in the absence of oxygen
leads to a broad absorption located around 330-350-nm. This absorption
hinders the observation of the photogenerated benzyl radicals.
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10.1021/ja982892x CCC: $15.00 © 1998 American Chemical Society
Published on Web 12/30/1998

13544 J. Am. Chem. Soc., Vol. 120, No. 51, 1998
Communications to the Editor
Figure 2. Transient decay traces at 315 nm for the benzyl radical
generated upon 266-nm laser excitation of phenylacetic acid in (left) dry,
evacuated (10-3 Torr) LiY (9), NaY (4), KY ([), RbY (O), and CsY
(b) and (right) dry, evacuated (10-3 Torr) NaY with loading levels
(phenylacetic acid:number of NaY cavities) of 2:1 (b), 1:1 (O), 1:2 (9)
and 1:5 (0), 1:10 ([), 1:100 (]). The inset shows the amount (∆∆J )
∆Jt)16µs) of the fast-decaying benzyl radicals as a function of loading
level.
Figure 1. Transient diffuse reflectance spectrum generated upon 266-
nm laser photolysis of phenylacetic acid in evacuated (10-3 Torr) NaY
under dry conditions. The inset shows the transient spectrum after the
sample was exposed to the atmosphere for a period of 10 s and then
reevacuated (10-3 Torr). Spectra were recorded (2) 0.20, (4) 0.74, (O)
1.76, and (b) 6.36 µs after the laser pulse.
band is also completely quenched by oxygen, indicating that the
transient species is not due to a carbocation or radical cation
intermediate.37
the fast-decaying radicals are generated in open void spaces and
are sufficiently mobile to cavity-jump into neighboring cavities.
The differences observed in the decay kinetics are presumed
to be due to the variations in the mobility of the benzyl radical.
With the larger cations, such as Cs+, the mobility of the radicals
is considerably reduced due to the cations partially blocking the
movement of the radicals through the pores connecting neighbor-
ing cavities. In addition, the relatively high concentration of
precursor molecules produces a secondary cage that inhibits the
mobility of radicals,4 and this effect increases as a function of
cation size.
The intensity of the 350-nm band was found to be highly
dependent on the amount of water coadsorbed into the zeolite
framework. Exposure of the sample to the atmosphere for 10 s
and reevacuation to remove oxygen significantly enhanced the
production of the 350-nm band (Figure 1, inset). We assign the
350-nm transient species to the benzyl anion generated upon
photolysis of phenylacetate,27 eq 1, path b, on the basis of the
following information. The benzyl anion is known to be a
photoproduct upon photolysis of phenylacetate in polar environ-
ments.25,38 The benzyl anion is also known to have a strong
absorption in the 350-nm region and to be rapidly quenched by
oxygen.39,40 In addition, the second major product generated upon
steady-state photolysis of phenylacetic acid within NaY in the
absence of oxygen is toluene, which is the product expected from
the benzyl anion. Furthermore, when phenylacetic-R,R-d2 acid
was irradiated in NaY in the absence of oxygen, toluene-R,R-d2,
produced by protonation of the benzyl-R,R-d2 anion, was obtained
as a major product in addition to bibenzyl-R,R-d4. No toluene-
R,R,R-d3 was produced, indicating that toluene is not generated
by reaction of the benzyl radical with the precursor. In addition,
when unlabeled phenylacetic acid was photolyzed within NaY,
also in the absence of oxygen after the co-incorporation of
methanol-d (CH3OD), the products obtained were toluene,
toluene-R-d, and bibenzyl.
The decay kinetics at 315-nm for the benzyl radical in LiY,
NaY, KY, RbY, and CsY41,42 are shown in Figure 2 (left). The
decay of the benzyl radical is fastest in LiY and slowest in CsY.
This suggests that the benzyl radical decays in a fast manner by
a coupling reaction with a second benzyl radical and that the
mobility decreases along the series from LiY to CsY. In addition,
the end absorption, which indicates the number of radicals
remaining after the fast decay, increases significantly along the
series toward the larger counterions. These radicals associated
with the end absorption are very long-lived and do not decay
over 1 ms, the longest time scale of our laser system. The slow-
decaying radicals are thought to be generated within local
domains, where diffusion is restricted and less efficient, whereas
In contrast, the decay of the anion was found not to be
dependent on the zeolite counterion.43 In each case, the benzyl
anion decayed with closely similar kinetics, k ≈ 5 × 105 s-1
.
The decay traces return close to the baseline, indicating that
mobility of the anion is not an important factor in the reaction of
the benzyl anion. The coadsorption of water into the zeolite
framework had little influence on the decay kinetics.
The decay traces at 315-nm upon laser photolysis of phenyl-
acetate within NaY as a function of loading level are shown in
Figure 2 (right). Clearly, the fraction of radicals that decay rapidly
over the first 15 µs increases significantly as the total yield of
radical increases. Thus, as more radicals are produced, the
probability of finding a neighboring cavity containing another
benzyl radical is higher than that at low loading levels.
In conclusion, we have shown clearly that the benzyl radical
can be readily generated and observed within the cavities of alkali-
exchanged Y faujasites. The benzyl anion is also generated upon
photolysis of phenylacetate, and its yield is highly dependent on
the hydration state of the zeolite. Our investigations into the
behavior of photogenerated simple radicals and anions within the
three-dimensional lattice of zeolites continue.
Acknowledgment. This work was supported by the Natural Sciences
and Engineering Research Council of Canada (NSERC) and Dalhousie
University. F.L.C. is the recipient of an NSERC-WFA.
JA982892X
(41) The exchanged zeolites were prepared by stirring the NaY (Aldrich,
LZY52 molecular sieves, Si/Al ) 2.4) zeolite with 1 M aqueous solutions of
the corresponding chlorides (LiCl, KCl, RbCl, CsCl) at 80 °C for 1 h. The
zeolites were then washed until no chlorides appeared in the washing water
and dried under vacuum. This procedure was repeated three times, and the
zeolite was calcinated between washings. The percent exchange was 54% for
LiY, 97% for KY, 44% for RbY, and 47% for CsY.
(42) The samples were prepared in the glovebox and sealed under vacuum
to ensure dry conditions. The loading level was kept constant at 1 molecule/2
cavities.
(43) The benzyl anion was observed only upon laser photolysis in the
exchanged zeolite samples that were briefly exposed to atmospheric moisture
and then reevacuated at 0.001 Torr for 10 h. The amount of anion generated
under hydrated conditions decreased significantly along the series LiY to CsY.
(36) The decay of the 315-nm band over long time scales of 1 ms was
distinctly nonexponential. Over a short time scale of 10-20 µs, the decay fit
reasonably well to a first-order expression, but the calculated rate constant is
qualitative. The band at 350 nm decayed completely over 20 µs, with
reasonably good first-order kinetics.
(37) Cationic species typically do not react with oxygen.
(38) Epling, G. A.; Lopes, A. J. Am. Chem. Soc. 1977, 99, 2700-2704.
(39) Dorfman, L. M.; Sujdak, R. J.; Bockrath, B. Acc. Chem. Res. 1976,
9, 352-357.
(40) Hopton, F. J.; Hush, N. S. Mol. Phys. 1963, 6, 209-213.