Vanadium calixarene complexes as molecular models for supported vanadia

Reaction of dimethylated tert-butyl calix[4]arene, H2AMe2, with OVCl2(thf)2 unexpectedly led to the isolation of ClVIVAMe, 2, whose crystal structure was determined. Hence, one of the MeO–C bonds present in H2AMe2 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function to coordinate at the V centre. 2 smoothly reacts with O2 to give OVVAMe, B, i.e. the chloride ligand has been replaced by a terminal oxo ligand with concomitant oxidation of the vanadium atom to the oxidation state +5.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 H2AMe2, 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–VIVAMe, 3, has been synthesised and investigated with respect to its behaviour in the presence of O2: it reacts the same way as 2.

The calixarene vanadium (IV) chloro complex, 2, has been synthesised via a surprising reaction sequence and shown to react with dioxygen to give the vanadium(V)oxo complex B which is conformationally flexible and crystallises in the cone or partial cone conformation. The first step leading to the formation of B certainly represents the binding of O2 to the VIV centre. The OO bond cleavage probably involves a second molecule of 2. This type of reactivity is not restricted to 2: replacement of chloride by triphenylsiloxide leads to the same results. Figure optionsDownload as PowerPoint slide