Reactivity of a vanadium(III) center over an oxo surface modeled by calix[4]arene

Metalation of the monomethoxycalix[4]arene [p-Bu-t -calix[4]-(OMe)(OH)(3)], 1, using [VMes(3) . THF] led to the coordinatively unsaturated V-III-d(2) fragment [p-Bu-t-calix[4]-(OMe)(O)(3)V] collapsing to the dimer [{mu-p-Bu-t-calix[4]-(OMe)(O)(3)}V](2), 2, where each calix[4]arene shares a bridging oxygen donor atom. The dimeric complexity remains intact in the reaction with (BuNC)-N-t and PhCN, which bond to the metal inside the cavity of the calix[4]arene leading to [{mu-p-Bu-t-calix[4]-(OMe)(O)(3)}(V-L)](2) [L = (BuNC)-N-t, 3; L = PhCN, 4]. In contrast, the reaction of 2 with pyridine and 4,4'-dipyridyl cleaves the dimeric form into a monomeric complex [{mu-p-Bu-t-calix[4]-(OMe) (O)(3)}V(Py)], 5, or to a different dimer containing a bridging 4,4'-dipyridyl, [{p-Bu-t-calix [4] -(OMe)(O)(3)}(2)V-2(mu-4,4'-dipyridyl)], 6. Complex 2 undergoes one electron oxidation by I-2 to the corresponding vanadium(IV) dimer [{u-p-Bu-t-calix[4]-(O)(4)}(2)V-2], 7, and by p-benzoquinone to [{p-Bu-t-calix[4] -(OMe)(O)(3)}V](2)(mu-C6H4O2), 8. A two-electron oxidation of the V-III-d(2) to vanadium(V) derivatives has been achieved using styrene epoxide and diphenyldiazomethane. In the former case the oxovanadium(V) derivative was obtained [p-Bu-t-calix [4] -(OMe)-(O)(3)V=O], 9, and in the latter case a metallahydrazone complex [p-Bu-t-calix[4]-(OMe)(O)(3)V=N-N=CPh2], 10. The dimeric d(2) and d(1) derivatives display significant antiferromagnetic couplings between the two metal centers, namely as follows: 2, J = -74.6 cm(-1); 3, J = -17 cm(-1); 4, J = -33.4 cm(-1); 7, J = -131.7 cm(-1). The extended Huckel calculations gave an appropriate picture of the two-electron reduction of the diphenyldiazomethane substrate. The proposed structures have been supported by X-ray analyses on 3, 7, 9, and 10.