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Langmuir 1993,9,874-876 0.7
Spontaneous Carbocation Generation on Clays
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F. L. Cozens, F. Gessner, and J. C. Scaiano’ Department of Chemistry, University of Ottawa, Ottawa, Canada K1N 6N5 Received June 29, 1992. In Final Form: November 6, 1992
Introduction Carbocations are common intermediates in catalytic processes employing clays and related materials. With this in mind we tried to incorporate on clays several precursors that are known to lead to photoinduced carbocation formation in acidic solvents, such as 2,2,2trifluoroethanol and 1,1,1,3,3,3-hexafluoro-2-propanol, which normally confer great stability to carbocations.’ To our surprise, we found that many of these precursors lead to the spontaneous formation of the cations that, on clays and protected from ambient moisture, are actually stable species. While it was our aim to use laser techniques to study these reaction intermediates, it soon become clear that more conventional diffuse reflectance spectroscopy would be adequate given the stability that the clays provide to the cations. Our studies employthe smectite montmorillonite;along with cation generation, we explore the reversibilityof the reaction, the fatigue resistance of the process, and the products of carbocation quenching by typical scavengers such as alcohols.
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Figure 1. Reflectance spectrum of the 9-(4’-methylphenyl)-9fluorenyl cation generated upon the addition of 9-(4’-methylphenyl)-9-fluorenol to (top) (A) SWy-1 clay; (B)absorption spectrum in concentrated HzS04; (bottom) SWy-1 clay after exposureto the atmosphere for 1week and redrying with nitrogen.
Experimental Section General Considerations. The substrates 9-(4’-methylphenand g-phenyl-9-fluoreno1,were yl)-,9-(2’,4’,6’-trimethylphenyl)-, prepared according to literature procedures.2 9-Phenyl-g-xanthenol and (triphenylmethy1)amine(Aldrich)were commercially available and used as received. Crude samples of smectite montmorillonite clays STx-1 and SWy-1 were purchased from Clay Minerals Society, University of Missouri,Columbia, MO, and purified according to the procedure outlined below. STx-1 and SWy-1 originate from Texas and Wyoming, respectively. Both have similar chemical compositions, differing mainly in their iron content, with STx-1and SWy-1containing about 0.65 % and 3.35% Fe2O3, re~pectively.~ Spectroscopic grade dichloromethane (BDH, ~ thus identifying the material produced in the clay as the same carbocation. The samples such as that of Figure 1 (top, A) were prepared under nitrogen. While the use of nitrogen does not appear to be important, handling of the samples in a dry environment proved to be essential. For example, when the sample shown in Figure 1(top, A) was exposed to the laboratory atmosphere, the orange color was discharged within 10 min. This process was largely reversible, and passing dry nitrogen or oxygen over a sample that had been exposed to ambient conditions for 1 week regenerated the characteristic carbocation absorption, Figure 1 (bottom). This process could be repeated several times, although there were indications that the cation yield was somewhat reduced after several cycles. The lower yield is apparent by the reduced top reflectance at ,A,, Figure 1 (bottom), and the appearance of some new absorption in the region of 300 nm. We attribute this behavior to reaction 1 (R = CH$,with the cation formed in the clay under dry conditions reverting back to starting material upon exposure of the sample to moisture.
methanol. Thus, extraction with ether following quenching by methanol led to a 3:l ratio of I to 111, suggesting that in spite of the intense color much of the precursor is present as the alcohol. Presumably, this ratio could favor the carbocation under appropriate drying conditions.
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Similar observationswere made with other precursors. Figures 2A and 3A showthe spectra recorded upon addition of 9-phenyl-and 9-mesityl-9-fluorenolto SWy-1clay;these spectra are in good agreement with those of the corresponding 9-substituted 9-fluorenyl cations generated in concentrated HzS04, Figures 2B and 3B.297 The 9-aryl-9-fluorenylcations produced in the clays can also be generated from their 9-aryl-9-fluorenolprecursors and observed as stable species in solution, but only under highly acidic conditions,such as in >60 % aqueoussulfuric a ~ i d These . ~ ~ ~conditions are suitable because the high acid content increases the rate constant for the loss of the OH group from the starting alcohol and the low water content reduces the rate constant for the regeneration of the alcohol from the cation, thus displacing the cationalcohol equilibrium in favor of the cation. Spectra under these conditions have been included in Figures 1-3. Our observationsthat the same cations are thermally stable in dry montmorillonite clays indicate that conditions mimicking concentrated sulfuric acid are present in the clay. Thus, the amount of nucleophilichydroxyl groups available for reaction with the cation is sufficiently low relative to the acid content of the clays to drive the cation-alcohol equilibrium toward the side of the cation. The spontaneous cation generation seems to be a rather general phenomenon on montmorillonite. Substrates IV and V yielded their readily detectable carbocations (Am= 440 and 380 nm, respectively) when incorporated using the same procedure described in the ExperimentalSection. An independent study has reported on the incorporation of V on silica ge1;8 under ambient conditions only traces (4) Theng, B. K. G. Clays Clay Miner. 1971, 19, 383. (5) Hendricks, S. B.; Alenxander, L. T. J. Am. SOC. Agron. 1940,32, 455. (6) Gessner, F.; Scaiano, J. C. Unpublished results. (7) Chandross, E. A.; Curtis,F. S., Jr. J. Am. Chem. SOC.1968,90,4345. (8)Berger, R. M.; Weir, D. Chem. Phys. Lett. 1990, 169,213.
Notes
876 Langmuir, Vol. 9, No. 3, 1993
of the cation are formed; efficient formation was however achieved by treatment at 130 OC for 12 h under vacuum.
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Numerous photochemical studies on clays have been reported during the last few years;gJO in the case of the ~
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(9) Thomas, J.
K.Acc. Chem. Res. 1988,21, 275.
precursors in this study it is clear that the clay provides a sufficiently acidic environment that carbocation formation does not require photoactivation. It is possible that other more "reluctant" carbocation precursors will indeed lead to these intermediates upon photoexcitation on clays.
Acknowledgment. We are grateful to Professor C. Detellier for his advice on the purification and handling of clays. This work has been supported by an operating grant (J.C.S.) from the Natural Sciencesand Engineering Research Council of Canada. (10) Cenens, J.; Schoonheydt, R. A.; De Schryver, F. C. ACS Symp. Ser. 1989,415, 378.
