The Chemistry of Vanadium-Carbon Bond Functionalities over an Oxo-Surface Defined by the Calix[4]arene Skeleton: the Redox Relationship between Vanadium(III) and Vanadium(IV) Assisted by Carbon-Oxygen Bond Cleavage

This report deals with the chemistry of V-C functionalities anchored to a quasi-planar O-4 matrix represented here by the [p-Bu-t-calix[4]-(OMe)(2)(O)(2)](2-) macrocycle. The starting material [p-Bu-t-calix[4]-(OMe)(2)(O)(2)V-Cl], 2, has been converted into the corresponding alkyl and aryl derivatives [p-Bu-t-calix[4]-(OMe)(2)(O)(2)V-R] (R = Me, 3; R = CH2Ph, 4; R = p-MeC6H4, 5) by conventional procedures. The structure of 5 reveals the self-assembling of the monomeric unit into a columnar arrangement with the p-tolyl substituent functioning as a guest moiety in a neighboring unit. Complexes 3-5 undergo migratory insertion reactions with CO and (BuNC)-N-t to the corresponding eta(2)-acyls [p-Bu-t-calix[4]-(OMe)(2)(O)(2)V(eta(2)-COR)] (R = Me, 6; R = CH2Ph, 7) and eta(2)-iminoacyls [p-Bu-t-calix[4]-(OMe)(2)(O)(2)V{eta(2)-C(NBut)R}] (R = Me, 8; R = CH2Ph, 9; R = p-MeC6H4, 10), the reaction with CO being strongly dependent on the R substituent. The structural model for the hexacoordinate intermediate leading to the inserted product is exemplified by the d(2) high-spin [p-Bu-t-calix[4]-(OMe)(2)(O)(2)V(Cl)(CNBut], 11, and proves the presence of four available orbitals at the metal for driving the reactivity in the [p-Bu-t-calix[4]-(OMe)(2)(O)(2)V] fragment. The bonding mode of the organic functionalities at the metal and the conversion of the alkyl substituent into the eta(2)-acyl have been inspected by extended Huckel calculations. The oxidative demethylation of 3 and 5 by a controlled amount of iodine opened an interesting synthetic route to the corresponding vanadium(IV) organometallic derivatives [p-Bu-t-calix[4]-(OMe)(O)(3)V-X] (X = Cl, 12; X = p-MeC6H4, 13). A structural report on 2, 5, and 9 is included.