Synthesis, characterization, and preliminary host-guest binding studies of porphyrinic molecular squares featuring fac-tricarbonylrhenium(I) chloro corners

The stoichiometric combination of rigid or semirigid difunctional ligands with appropriate cia-coordinating metal centers has been shown to produce macrocyclic tetrametallic assemblies (''molecular squares'') in remarkably high yield. The 30 or so available squares have been constructed from platinum, palladium, nickel, tungsten, and rhenium precursors and have been assembled in bath homometallic and heterometallic form.(1-3) Depending on cavity size and overall charge, selected molecular squares are capable of functioning as solution-phase hosts for either complex anionic guests(3a) or neutral aromatic guests where binding is achieved, respectively, via coulombic or hydrophobic interactions. While host-guest binding is typically monitored via NMR spectroscopy, detection based on host luminescence has also been demonstrated.(3) Particularly versatile in this respect are molecular squares derived from Re(C0)(5)Cl. These emit from comparatively long-lived metal-to-bridging-ligand charge-transfer (MLCT) excited states. The available square cavity sizes (i.e., minimum van der Waals diameters) range from ca. 3 Angstrom (pyrazine bridged species)(3a) to ca. 16 Angstrom (1,4-phenyl-bis(picolinoate)-bridged species).(3c) Pie reasoned that even larger cavity sizes could be accessed by utilizing 5,15-meso-substituted porphyrins as bridging ligands. If tricarbonylrhenium(I) chloro moieties are used as corners, the resulting squares should remain luminescent but with emission coming from porphyrin-localized excited slates. We additionally reasoned that porphyrin metalation would provide a basis for single or multipoint guest binding via axial ligation. We note that squares based on 5,10- and 5,15-substituted porphyrins and Pd(HI) corners have been reported by Lehn and Drain.(4) In addition, flexible alkyne-linked tetraporphyrin assemblies have been reported by Anderson.(5a) In the presence of appropriate guests, these fan adopt complementary tub-shaped geometries.