(η5-Pentamethylcyclopentadienyl)rhodium Complexes of Upper-Rim Monophosphinated Calix[4]arene

A new and convenient synthesis of the upper-rim monobrominated calix[4]arene Br-calixPR 2 is reported and has been used as the key precursor for the preparation of both the diphenyl- ( 6a ) and diisopropylphosphine ligands ( 6b ). Reactions of these new ligands with [(C 5 Me 5 )RhCl 2 ] 2 afford the complexes (calixPR 2 )(C 5 Me 5 )Rh(Cl) 2 ( 8a, b ), which can be converted into their corresponding dihydrides (calixPR 2 )(C 5 Me 5 )Rh(H) 2 ( 9a, b ) using NaBH 4 . 6a also reacts with (C 5 Me 5 )Rh(CO) 2 to form (calixPPh 2 )(C 5 Me 5 )Rh(CO) ( 10 ). The X-ray structures for both 8a and 8b confirm the cone geometry of the calix[4]arene bowl and reveal the presence of the phenyl and isopropyl groups over the cavity. With respect to the Rh−P bond, a gauche conformation is depicted in the solid state. Complexes 8a, b and 9a, b appear to be fluxional in solution, as demonstrated from VT 1 H and 31 P NMR measurements for 8a, b . Molecular modelings confirm the presence of 13 conformers associated with the rotation around the C(calix)−P and P−Rh bonds and the cavity locking groups above it. The minimum energy conformation for the unsaturated (calixPPh 2 )(C 5 Me 5 )Rh complex exhibits the Rh atom well located at the opening of the free cavity.