Copper(I) and copper(II) chelates containing lmidazole and thioether groups; synthesis of the ligand 1,2-bis(benzimidazol-2′-ylmethylthio)-benzene (bbtb) and the X-ray crystal structure at –52 °C of [Cu-(bbtb)(H2O)][ClO4]2·5EtOH

Abstract

Stable copper(I) and copper(II) complexes of the title ligand (bbtb) have been prepared. The design of the ligand prevents planar four-co-ordination of Cu^II. The X-ray structure of [Cu(bbtb)(H₂O)][ClO₄]₂·5EtOH shows that the co-ordination geometry around the metal ion is trigonal bipyramidal. Two benzimidazole nitrogen atoms occupy axial positions, the equatorial positions being occupied by the two thioether sulphur atoms and an oxygen atom of a co-ordinated water molecule. The structure is compared with recently solved structures of closely related imidazole–thioether chelates. The compound crystallizes in the monoclinic space group P2₁/n with a= 18.865(3), b= 12.652(2), c= 17.902(7)Å, β= 101.99(2)°, and Z= 4. The intensities of 5 095 independent reflections were measured on an automatic diffractometer at –52 °C (to prevent decomposition). The structure was solved by direct methods and refined using full-matrix least-squares techniques to a residual R of 0.069 for 2 869 reflections with I > 2σ(I). The e.s.r. spectrum of the solid compound [Cu(bbtb)][ClO₄]₂·2H₂O is ‘exchange narrowed’ and shows only one signal. The ligand-field spectrum is consistent with a trigonal-bipyramidal geometry. The e.s.r. and ligand-field data indicate a co-ordination geometry in solution which is different from the solid state, and in which the unpaired electron is mainly in a d_x²–y² orbital.