Effective Coordination Control by Utilizing the Cavity of Calixarenes

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Calixarenes are known to serve as a versatile ligand not only with two distinct types of coordination sites, phenolic oxygens and arene rings, but also with an inclusion site (cavity). However, coordination chemistry of transition-metal complexes inside the cavity of calixarenes still remains to be explored. In this study we have investigated two series of calix[4]arene complexes in which the metal centers are located inside the cavity of calix[4]arenes.The reaction of calix[4]arene with [CpRu(CH_3CN)_3]PF_6 in the presence of Et_3N has been found to give the ruthenium-arene type complex [CpRu{calix[4]-(0)(OH)_3}] (1)in good yield. An X-ray diffraction study of complex 1 has revealed that the CpRu+ fragment resides in the cavity of the calix[4]arene ligand, and the Cp protons of 1 exhibit characteristic high-field shift in the ^1H NMR. This fact makes a sharp contrast to the complexation of a Cp*Ru+ fragment with calix[4]arene where the metal center is bound at the outside face of a ph … More enolic ring(complex 2), and 1 can be viewed as a "self-inclusion" product. Further introduction of a Cp*Ru+ fragment into complex 1 gives rise to the inside-outside isomer of [(CpRu)(Cp*Ru){calix[4]-(0)(OH)_3}]+ (3), while introduction of a CpRu+ into 2 leads to its outside-outside isomer. These results indicates that the CpRu+ moiety is first coordinates at the outside face of a calixarene molecule, and then the ring inversion of the calixarene skeleton affords the inside coordination product.On the other hand, when [Ph_4P][ReO_2{p-tBu-calix[4]-(0)_4}] (4)is treated with a variety of cationic transition metal complex ML+, the phenoxo-bridged Re-M complexes [ReO_2{p-tBu-calix[4]-(0)_4}ML] (5a, ML = Cu(NCMe); 5b, Ag(Me2C0); 5c, Au(Ppha); 5d, Rh(cod)) can be obtained in good yield. The heterodinuclear core of complex 5a is located inside the cavity and have revealed to show unique spectral and chemical behaviors. For example, complex 5a undergoes selective alkyne incorporation, while complex 5d undergoes solvent-dependent isomerization to π-arene type regioisomers. Less