Calixarene Metalloreceptors. Synthesis and Molecular Recognition Properties of Upper-Rim Functionalized Calix[4]arenes Containing an Organopalladium Binding Site

The compound 5,17-bis(2-chloroacetamido)-25,26,27,28-tetrapropoxycalix[4]arene, 4, was prepared in the cone conformation by the published method. Compound 4 was reacted with α,α‘- m -xylenedithiol and sodium ethoxide under high-dilution conditions in ethanol solution. The resulting macrobicyclic calix[4]arene, HL 1, 5, contains a symmetrically disposed macrocyclic loop across the upper rim of the calix[4]arene. Palladation of 5 yielded [Pd(L 1 )(CH 3 CN)][BF 4 ], 6 . Similarly 5,17-bis[2-(4-(chloromethyl)phenoxy)acetamido]-25,26,27,28-tetrapropoxycalix[4]arene, 8, was used to prepare HL 2, 9 . Compound 9 is similar to 5 but contains an aromatic spacer group separating the xylyl fragment from the calixarene unit. Palladation of 9 yielded [Pd(L 2 )(CH 3 CN)][BF 4 ], 11 . All new compounds and complexes were characterized in solution by 1 H NMR spectroscopy, and the molecular structure of 5 was verified by a single-crystal X-ray diffraction study. Compound 5 crystallized in the space group P 2 1 / c with a = 23.5095(1) Å, b = 9.9090(1) Å, c = 23.3586(1) Å, β = 91.53(1)°, V = 5439.59(8) Å 3, and Z = 4. The structure was refined to R ( F ) = 10.13% and R w ( F 2 ) = 21.91% for 5799 reflections with F o 2 > 2σ( F o 2 ). Compounds 6 and 11 are calix[4]arene-based metalloreceptors containing an organopalladium binding site and a hydrophobic cavity provided by the calix[4]arene. Binding of a substrate through σ-bonding to the palladium center and interaction within the hydrophobic site were demonstrated in solution by 1 H NMR spectroscopy. These multiple receptor−substrate interactions are used by 11 for the molecular recognition of 4-phenylpyridine in the presence of 2-phenylpyridine or 3-phenylpyridine.