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-Bu^tcalix[4](ONa)₂(OMe)₂] 1 was performed using [CrCl₃(THF)₃] and the resulting complex [Cr{p-Bu^tcalix[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^tcalix[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^tcalix[4](O)₂(OMe)₂}(Mes)] 4 [Mes = 2,4,6-Me₃C₆H₂] and [Cr{p-Bu^tcalix[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-Bu^tcalix[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-Bu^tcalix[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.