Vanadium calixarene complexes as molecular models for supported vanadia

Reaction of dimethylated tert-butyl calix[4]arene, H(2)A(Me2), with O=VCl2(thf)(2) unexpectedly led to the isolation of Cl-V(IV)A(Me), 2, whose crystal structure was determined. Hence, one of the MeO-C bonds present in H(2)A(Me2) has been cleaved and one equivalent of HCl eliminated leaving three phenolate and one phenolether function to coordinate at the V centre. 2 smoothly reacts with O-2 to give O=V(V)A(Me), B, i.e. the chloride ligand has been replaced by a terminal oxo ligand with concomitant oxidation of the vanadium atom to the oxidation state +5. B crystallises isostructurally with 2; crystallisation of a mixture of 2 and B even leads to crystals that contain both compounds beside each other. However, when B was co-crystallised together with H(2)A(Me2), from acetonitrile a different structure with a partial cone conformation was obtained. The mechanism by which B is formed from 2 is discussed. To obtain information concerning the fate of the chloride ligand in 2, a derivative, Ph3SiO-V(IV)A(Me), 3, has been synthesised and investigated with respect to its behaviour in the presence of O-2: it reacts the same way as 2. (c) 2006 Elsevier B.V. All rights reserved.