STEPWISE ASSEMBLY OF CHARGE-TRANSFER COMPLEXES WITHIN ZEOLITE SUPERCAGES AS VISUAL PROBES FOR SHAPE SELECTIVITY

The shape-selective formation of various aromatic charge-transfer complexes occurs within zeolite supercages doped with methylviologen (MV2+) and other large mono- and dipyridinium acceptors. For example, ion exchange leads to a series of acceptor-doped zeolite Y such as MV(x)Y, typically with x = 1 and 2 MV2+ per supercage. Mere exposure of a hexane slurry of the colorless MVY to various substituted benzenes, naphthalenes, and anthracenes leads immediately to brilliant, distinctively colored yellow, orange, and purple zeolites, while the supernatant solution remains singularly colorless. The diffuse reflectance spectra of the colored zeolites show characteristic charge-transfer bands that are the same as those obtained in solution with MV2+ and the corresponding arene. Thus, the successful isolation of the bright orange, single crystal of a naphthalene complex with MV2+ allows X-ray crystallography to establish the relevant cofacial donor-acceptor arrangement within the zeolite cavity. Steric restrictions of the supercage are indicated in these zeolite experiments by the size and shape of the arene donors-all the methylbenzenes including pentamethylbenzene rapidly forming brilliant yellow zeolites, the single striking exception being hexamethylbenzene. Shape selectivity is also indicated by the exclusion of 1,4-dimethoxynaphthalene (but not the 2,6-isomer) as well as 9-phenyl- and 9,10-dimethylanthracene (but not 9-methylanthracene). A van der Waals "width" of roughly 8 angstrom is sufficient to inhibit an arene from complex formation, and it represents the maximum value for the kinetic diameter sigma of arenes in zeolite Y catalysis. Mass transport through zeolite Y is quantitatively evaluated by measuring the solvent effect on the rates of intercalation and extraction of various arene substrates.