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

Abstract

The metallation of [p-Butcalix[4](ONa)2(OMe)2] 1 was performed using [CrCl3(THF)3] and the resulting complex [Cr{p-Butcalix[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-Butcalix[4](O)3(OMe)}Cr(py)(Cl)] 3. The alkylation of 2 was successful only when the concurrent reduction to CrII was irrelevant, in which case the two well characterized organometallic derivatives [Cr{p-Butcalix[4](O)2(OMe)2}(Mes)] 4 [Mes = 2,4,6-Me3C6H2] and [Cr{p-Butcalix[4](O)2(OMe)2}(η5-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 d3 configuration. Complex 2 was easily reduced to the chromium(II) derivative [Cr{p-Butcalix[4](O)2(OMe)2}(THF)] 6, which underwent facile oxidative functionalization. The reaction with O2 led to an unprecedented di-µ-oxo chromium(IV) derivative, [Cr2{p-Butcalix[4](O)2(OMe)2}2(µ-O2)] 7, which displays a ferromagnetic coupling between the two d2 centres. The proposed structures are supported by X-ray analyses of complexes 24, 6 and 7.