Chromium-(II) and -(III) over a planar oxo surface modelled by calix[4]arene anions: redox chemistry and formation of Cr-C functionalities

The metallation of [p-Bu(t)calix[4](ONa)(2)(OMe)(2)] 1 was performed using [CrCl3(THF)(3)] and the resulting complex [Cr{p-Bu(t)calix[4](O)(2)(OMe)(2)}(Cl)(THF)] 2 allowed entry into low valent and organometallic chemistry of chromium. In the presence of strong bases or nucleophiles the ligand underwent demethylation exemplified by the reaction of 2 with pyridine, which led to [C5H4NMe][Cr{p-Bu(t)calix[4](O)(3)(OMe)}Cr(py)(Cl)] 3. The alkylation of 2 was successful only when the concurrent reduction to Cr-II was irrelevant, in which case the two well characterized organometallic derivatives [Cr{p-Bu(t)calix[4](O)(2)(OMe)(2)}(Mes)] 4 [Mes=2,4,6-Me3C6H2] and [Cr{p-Bu(t)calix[4](O)(2)(OMe)(2)}(eta(5)-C5H5)] 5 were obtained. None of them showed, even under photochemical or thermal forced conditions, any sign of reactivity, due to the kinetic inertness of the d(3) configuration. Complex 2 was easily reduced to the chromium(II) derivative [Cr{p-Bu(t)calix[4](O)(2)(OMe)(2)}(THF)] 6, which underwent facile oxidative functionalization. The reaction with O-2 led to an unprecedented di-mu-oxo chromium(IV) derivative, [Cr-2{p-Bu(t)calix[4](O)(2)(OMe)(2)}(2)(mu-O-2)] 7, which displays a ferromagnetic coupling between the two d(2) centres. The proposed structures are supported by X-ray analyses of complexes 2-4, 6 and 7.