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 [{&mgr;-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 Bu(t)NC and PhCN, which bond to the metal inside the cavity of the calix[4]arene leading to [{&mgr;-p-Bu(t)-calix[4]-(OMe)(O)(3)}(V-L)](2) [L = Bu(t)NC, 3; L = PhCN, 4]. In contrast, the reaction of 2 with pyridine and 4,4'-dipyridyl cleaves the dimeric form into a monomeric complex [{&mgr;-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)(&mgr;-4,4'-dipyridyl)], 6. Complex 2 undergoes one electron oxidation by I(2) to the corresponding vanadium(IV) dimer [{&mgr;-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)(&mgr;-C(6)H(4)O(2)), 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&tbd1;N-N=CPh(2)], 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 Hückel 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.