Jahn-Teller Effect in the [CuEn3]CrO4 Structure

A change in the coordination of the copper atom in the crystal structure of [CuEn(3)]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) , b = 13.335(3) , c = 13.339(3) , alpha = 71.77(3)A degrees, beta = 70.53(3)A degrees, gamma = 70.42(3)A degrees) composed of two crystallographically independent [CuEn(3)](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 ; two long ones are 2.415 and 2.470 . 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) , c = 9.7573(13) , V = 2082.6(6) (3), space group P c1 (No. 165), Z = 6) shows that chromate anions are disordered over three positions about the N axis, and Cu-N distances are 2.120-2.177 . 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) , c = 9.7715(5) , V = 697.4 (3), space group P c (No. 163), Z = 2) all Cu-N distances are 2.155 , and chromate anions are disordered over six positions about the N axis. A comparative crystal chemical analysis of the packing of the studied structures is carried out.