A Rhenium Tricarbonyl 4-Oxo-terpy Trimer as a Luminescent Molecular Vessel with a Removable Silver Stopper

摘要

One of the great challenges for the synthetic chemist is the preparation of large, hollow, molecular species capable of hosting smaller molecules. In contrast to solid-state materials, which have no true existence in solution, there are only a very small number of truly discrete families of molecular species that provide a shell enclosing a void, such as calixarenes, cyclodextrins, and cucurbiturils. In the field of coordination chemistry, this area has been dominated by the self-assembly of cage complexes. Multidentate ligands that may bind more than one metal center provide a route to cage complexes, and the use of preformed complexes with additional binding sites (complex-as-ligand approach) has led to a variety of coordination networks with a range of complex topologies, including examples of cage complexes appropriate for host-guest applications. Coordination complexes containing cylindrical or psuedo-cylindrical cavities analogous to cyclodextrins or calixarenes are, however, very rare, with very few examples of metallacalixarenes, in which arene ligands mimic calixarene walls. These species potentially offer more modes of binding than the organic analogues: both organic-based ligand-guest interactions similar to those in pure organic systems, and contributions from metal-guest or charge-guest interactions. Furthermore, coordination complexes acting as hosts may, with judicious choice of metal and ligand, be capable of responding to the presence of the guest by modulation of a detectable property, such as luminescence. Amongst the most widely studied luminescent metal complexes are the rhenium tricarbonyl bisimine cations [Re(CO)3(bisim)X]~+ (bisim = 2,2'-bipyridine or derivative thereof; X = pyridine or derivative) owing to their high quantum yields, long luminescence lifetimes, and large Stokes shifts arising from emission from a ~3MLCT excited state. Recently these species have been applied in the fluorescence microscopy imaging of cells, and many have been proposed as sensors in the past. No examples of rhenium metallacalixarenes, and indeed no ~3MLCT emissive metallacalixarenes have been reported to date, and no rational design for such species exists.'31 Herein we report the development of a luminescent rhenium bisimine-based cylindrical trimer synthesized by the complex-as-ligand approach: the cavity has the correct dimensions for hosting linear diatomics, and a pendant tripyridine unit can bind metal ions, closing one end of the cylinder to form a cup and simultaneously triggering a modulation of the luminescence of the core trimer.