Carbocations and organic radical cations inside zeolite matrices. Generation, characterization, stability and properties

The ability of acid zeolites to generate and stabilize positively charged carbenium ions and organic radical cations as persistent (sometimes indefinitely stable) species have been shown choosing examples of our own group. It has been found that this remarkable stability arises not only from the high polarity of the zeolite internal voids and the low nucleophilicity of the zeolite negative oxygens, but also from the protection of the rigid framework that imposes geometrical restrictions to the attack of external nucleophiles. Generation of the positive organic species was accomplished either by functional group transformation (alpha, omega-diphenylallyl, xanthylium, tropylium cations) or by ship-in-a-bottle in those cases where the target ions are too big to be adsorbed from the exterior to the interior of the particles (trityl, pyrylium, 9-phenylxanthylium cations). The key step of the ship-in-a-bottle strategies relies in the formation of new C-C bonds. It is also shown which in situ spectroscopic techniques can be applied to establish the molecular structure of the guest. Finally, some remarkable examples of how the confinement and immobilization influences the molecular properties of the organic guests are discussed.