Generation and Direct Observation of the 9-Fluorenyl Cation in Non-acidic Zeolites
Introduction
Zeolites are aluminosilicate host materials and catalysts constructed of [SiO4]4− and [AlO4]5− tetrahedra linked via oxygen bridges creating an open framework structure of molecular-sized pores, channels and cavities. Over the past several years, there has been considerable interest in the chemistry of carbocations generated within the cavities of the proton exchanged, acidic forms of these materials, especially with respect to direct spectroscopic detection and characterisation.1., 2., 3., 4., 5., 6., 7., 8., 9., 10., 11., 12., 13., 14., 15. Results from numerous studies in this area have led to tremendous advances in our understanding of the nature of stable carbocations within acidic zeolites and the reactivity of carbocation intermediates in zeolite catalysis. In particular, such research has contributed to the emerging picture of Brønsted zeolites not as superacid solid materials as once envisioned, but as strong acids whose ability to stabilise electrophilic species and influence their reactivity is largely tied to the dynamic role of the zeolite framework in carbocation chemistry.12 Thus, although several types of relatively stabilised carbocations such as triarylmethyl,4., 9. xanthylium,3., 5., 16. dibenzotropylium,16 indanyl10 and cyclopentenyl11 cations can be detected as stable ions in acidic zeolites, other more reactive carbocations such as phenethyl6., 10. cations do not persist as stable ions within these environments. The reactivity of such species is consistent with current predictions regarding the stability of reactive carbocations within these acidic environments.12., 14. However, these studies do not preclude these carbocations as transient intermediates within acidic zeolite catalysis, but do suggest that the catalytic role of zeolite properties other than acidity warrants further investigation.
In recent years, we have been studying the chemistry of carbocations generated within non-proton exchanged zeolites. Examining the behaviour of carbocations under these conditions provides information not available from studies using acid zeolites in which the high acidity of the environment dominates over other possible roles that the solid-state material might play. To date, we have generated as transient species several different kinds of carbocations such as the xanthylium cation,17 cumyl cations,18., 19. and 1,1-diarylmethyl cations.20 While each of these are reactive carbocations, they all possess a significant degree of stabilisation that allows them to be sufficiently long-lived to observe using nanosecond diffuse reflectance techniques. These studies have led us to examine carbocations that are less thermodynamically stable to determine if the properties of non-acid zeolites are suitable to support the formation and enhance the lifetime of these kinds of highly reactive electrophilic intermediates. The highly reactive carbocations we chose to examine in the present work are 9-alkyl-9-fluorenyl cations and especially the parent 9-fluorenyl cation. These carbocations possess unusually high reactivity and low thermodynamic stability in solution.21., 22., 23. In addition, they are readily generated in solution from 9-fluorenols via a photoheterolysis mechanism that is sensitive to the presence and ionising ability of the protic media.24., 25., 26. This system therefore also provides an opportunity to examine the effect of zeolite structure on a photochemical reaction characterised by photoheterolysis and significant charge separation in the rate determining step.
Section snippets
Laser photolysis of 9-alkyl-9-fluorenol in alkali metal zeolites
Laser irradiation of 9-methyl-9-fluorenol in oxygen-saturated, dry NaY (Si/Al=2.4) gives a transient diffuse reflectance spectrum with a number of distinct absorption bands, Fig. 1. One of these absorption bands is centred at 485 nm. The transient responsible for this band is not stable, and decays completely within approximately 2 μs after the laser pulse. The decay fits well to a first-order expression to give a rate constant of 3.5×106 s−1, Table 1. The rate constant for the disappearance of
Reactivity of 9-fluorenyl cations in non-protic zeolites
Evidence that illustrates the low thermodynamic stability of the 9-fluorenyl cation includes the lack of success in generating and observing the carbocation under strong or superacid conditions,33 and the unusually low pKR value of −17.3 for 9-fluorenol34 compared to the pKR of −13.3 for the diphenylmethanol.35 The kinetic instability of the fluorenyl cation has also been well documented. For example, in wholly aqueous solution, the rate constant for the decay of the carbocation is about 1011 s−1
Materials
9-Fluorenol is commercially available (Aldrich) and was used as received. 9-Methyl-, 9-ethyl- and 9-isopropyl-9-fluorenol were prepared form 9-fluorenone and the appropriate Grignard reagent and purified by recrystallization from ethanol. 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) and 1,1,1,3,3,3-hexafluoro-2-propanol-d(HFIP-OD) are commercially available from Aldrich. NaY(Si/Al=2.4) was obtained from Aldrich while Naβ (Si/Al=18) was purchased from the P.Q. Corporation. All zeolites were used as
Acknowledgements
We gratefully acknowledge the Natural Science and Engineering Research Council of Canada (NSERC) and Dalhousie University for financial support of this research. M. A. O. is the recipient of an NSERC graduate scholarship and F. L. C. is the recipient of an NSERC-WFA.
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