Synthesis of new self-assembled PdII and PtII rectangular metallomacrocycles: a comparative study of their inclusion complexes

New palladium and platinum metallocycles have been synthesized by reacting 4,4'-bipyridinium-based ligands with PdII and PtII complexes. Strict thermodynamic self-assembly of 1 and [M(en)(NO3)2] (M=Pd, Pt) 6 a,b afforded metallocycles 7 a,b. However, the synthesis of 8 a,b and 9 a,b required a self-assembly process that used sodium p-phenylenediacetate (12) as a template. Finally, metallocycles 10 a,b were synthesized under high dilution conditions from ligand 4. The formation of inclusion complexes between metallocycles 7-10 and substrates 13 and 14 were studied by low-temperature 1H NMR, and the association constants were determined in nitromethane and water by following the characteristic charge-transfer band that these metallomacrocycles show in their UV-visible absorption spectra. A clear correlation between the affinity for a substrate and the dimensions of the metallocycle was observed. Metallocycles 8 b and 9 b exhibited the highest binding constants in water and nitromethane. This observation is in agreement with the DFT (B3LYP)-optimized geometries obtained for the different metallomacrocycles, which indicate that only macrocycles 8 and 9 possess a cavity with a width larger than 3.5 A. The insertion of hydroquinone or diol 13 into the cavity of metallocycle 11 a was confirmed by single-crystal X-ray crystallography.