A square-pyramidal organochromium(v) compound

Abstract

The mononuclear, five-coordinate organochromium(V) compound [NBu₄][CrO(C₆F₅)₄] (1) has been obtained as a dark red solid in moderate yield by treatment of the homoleptic organochromium(III) derivative [NBu₄]₂[Cr(C₆F₅)₅] with NO[BF₄] in CH₂Cl₂ solution under an oxygen atmosphere. The Cr^V centre in the [CrO(C₆F₅)₄]⁻ anion shows a square-pyramidal geometry with the four C₆F₅ groups in the basal positions and the oxo ligand in the apical one (X-ray). The short Cr–O distance (153.8(2) pm) suggests a high degree of triple bond character for the chromyl unit. The EPR spectrum of 1 in solution shows an isotropic signal with g_iso = 1.995(1) and a rich hyperfine structure due to coupling with the ⁵³Cr isotope [a_Cr = 46.95(4) MHz] as well as with the ortho-F and meta-F substituents of the C₆F₅ rings [a_F = 4.20(2) MHz and a′_F = 2.12(2) MHz] in keeping with the presence of non-interacting, fast tumbling, paramagnetic [CrO(C₆F₅)₄]⁻ units (d¹). In the solid state, however, both the microscopic (EPR) and macroscopic magnetic properties (isothermal magnetisation and thermal dependence of the magnetic susceptibility) suggest the existence of weak ferromagnetic interactions with T_C = 0.20(2) K. Such magnetic interactions may probably be favoured by π interactions between C₆F₅ rings of neighbour [CrO(C₆F₅)₄]⁻ units in the crystal.