(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 [(C5Me5)RhCl2](2) afford the complexes (calixPR(2))(C5Me5)Rh(Cl)(2) (8a,b), which can be converted into their corresponding dihydrides (calixPR(2))(C5Me5)Rh(H)(2) (9a,b) using NaBH4. 6a also reacts with (C5Me5)Rh(CO)(2) to form (calixPPh(2))(C5Me5)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 H-1 and P-31 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))(C5Me5)Rh complex exhibits the Rh atom well located at the opening of the free cavity.