Synthesis, X-ray crystal structures and properties of chromium complexes with semiquinonate and catecholate

Abstract

The reaction of tris-tetrahalogeno-o-benzosemiquinonate chromium(III) complexes, [Cr^III(X₄SQ)₃] (X = Cl 1a or Br 1b), with bis(cyclopentadienyl) cobalt [Co^IICp₂], tetramethyltetraselenafulvalene (TMTSF) and tetrakis(methylsulfanyl)tetrathiafulvalene (TMT-TTF) afforded four charge-transfer compounds, [Co^IIICp₂][Cr^III(X₄SQ)₂(X₄Cat)] (X = Cl 2a or Br 2b), [TMT-TTF][Cr^III(Br₄SQ)₂(Br₄Cat)] 3b and [TMTSF][Cr^III(Br₄SQ)₂(Br₄Cat)] 4b, where Cat is catecholate. The paramagnetic [Cr^III(X₄SQ)₂(X₄Cat)]^– complexes are commonly formed by cocrystallization with a diamagnetic [Co^IIICp₂]⁺ cation for 2a·C₆H₆ and 2b, and paramagnetic TMT-TTF^˙+ and TMTSF^˙+ cations for 3b·C₆H₅CH₃ and 4b·2CH₂Cl₂, respectively. The one-electron reduced complexes, [Cr^III(X₄SQ)₂(X₄Cat)]^–, with two semiquinonate and one catecholate ligands were isolated and crystallographically characterized. The crystal structures of 2a·C₆H₆ and 3b·C₆H₅CH₃ consist of alternating stacks of cations and anionic complexes, which form one-dimensional column structures. On the other hand, the anionic complexes in 4b·2CH₂Cl₂ form a hexagonal honeycomb network, whose cavities are occupied by the dimerized cation molecules. The temperature dependence of the magnetic susceptibilities reveals that all the [Cr^III(X₄SQ)₂(X₄Cat)]^– complexes are in a ground state of S = 1/2, which results from the intramolecular antiferromagnetic interaction between Cr^III(d³) and two semiquinonates. In addition, 3b and 4b have a contribution of the paramagnetic TMT-TTF^˙+ (3b) and TMTSF^˙+ (4b) cations. In all compounds weak intermolecular magnetic interactions were recognized from the decrease of the χ_mT values at low temperature.