Jahn–Teller Effect in the [CuEn3]CrO4 Structure

Abstract

A change in the coordination of the copper atom in the crystal structure of [CuEn₃]CrO₄ (En is ethylenediamine) in studied in the range 150–300 K. According to the single crystal X-ray diffraction (XRD) data at 150 K, the single crystal has a complicated twining character based on the triclinic cell (a = 9.027(2) Å, b = 13.335(3) Å, c = 13.339(3) Å, α = 71.77(3)°, β = 70.53(3)°, γ = 70.42(3)°) composed of two crystallographically independent [CuEn₃]²⁺ complex cations. The coordination of copper atoms is a distorted square bipyramid; four short Cu–N distances lie in the range 2.049-2.082 Å; two long ones are 2.415 Å and 2.470 Å. According to the differential scanning calorimetry (DSC) data, near 218 K there is a phase transition. The single crystal XRD experiment performed at 250 K (a = 15.6992(19) Å, c = 9.7573(13) Å, V = 2082.6(6) ų, space group P\(\bar 3\)c1 (No. 165), Z = 6) shows that chromate anions are disordered over three positions about the с axis, and Cu–N distances are 2.120-2.177 Å. According to the DSC data, on further heating the structure undergoes yet another two alterations (260 K and 270 K) due to the disordering of oxygen atoms of chromate anions and the subsequent equalization of Cu–N distances. At 300 K in the structure (a = 9.0778(6) Å, c = 9.7715(5) Å, V = 697.4 ų, space group P\(\bar 3\)c (No. 163), Z = 2) all Cu–N distances are 2.155 Å, and chromate anions are disordered over six positions about the с axis. A comparative crystal chemical analysis of the packing of the studied structures is carried out.