Stabilization of Reactive Organometallic Intermediates Inside a Self-Assembled Nanoscale Host

摘要

Molecular container compounds provide a new space for reaction chemistry, both literally and figuratively, through the encapsulation of smaller guest molecules.[1] The environment of the host interior may be very different from that of the exterior surroundings, resulting in novel patterns of reactivity. In effect, the energetic landscape within a host may be significantly altered from that of the bulk solution, so that what is unstable in solution may become stable and vice versa. The stabilization of reactive species within the narrow confines of a host cavity is an intriguing functional property of covalent and self-assembled hosts and has been observed in a number of different systems. Both kinetic and thermodynamic factors can contribute to this stabilization: the tight host portals may kinetically inhibit guest escape and subsequent reactivity, or strong host-guest binding may stabilize an otherwise reactive structure.[2] Cram and Warmuth have used hemicarcerands to isolate the highly unstable intermediates cyclobutadiene, o-benzyne, and cycloheptatetraene by preventing them from dimerization or reaction with bulk-phase species.[3]-[5] Fujita and co-workers observed the formation of an otherwise nonisolable cyclic silanol trimer within the cavity of their M6L4 supramolecular host.[6], [7] Rebek et al. were able to inhibit homolytic bond cleavage of benzoyl peroxide by encapsulation, even at elevated temperatures for prolonged periods of time.[8] All of these examples involved reactive organic species. Herein, we report the stabilization of reactive organometallic intermediates in aqueous solution through molecular encapsulation. This result augurs well for the further development of encapsulated organometallic chemistry.