Distortion isomerism and plasticity of the coordination sphere of binuclear Cu(II) complexes: Crystal structure of the monoclinic isomer of [Cu₂(2-bromopropionato)₄(caffeine)₂]

Abstract

The crystal and molecular structure of monoclinic [di(caffeine)tetrakis(2-bromopropionato)dicopper(II)] · 0.8 water was determined using direct methods and Fourier techniques. The complex crystallizes in space group C2/c and is dimeric with square pyramidal geometry at each copper(II) center. The Cu ··· Cu distance is 2.674(1) Å. Experimental data are compared with those found in a similar triclinic [Cu₂(2-bromopropionato)₄(caffeine)₂] complex. The isomers differ in the geometry of copper(II) and the inter-aromatic interaction of caffeine molecules. The correlation of Cu ··· Cu distances vs. apical Cu–O bond length for dimeric copper(II) carboxylate complexes were interpreted by the bond-valence model. Minimum lengths of apical Cu–O bond and Cu ··· Cu separation were predicted.

Keywords:

• Copper(II)
• Halogenopropionate
• Caffeine
• Crystal structure
• Distortion isomers

Acknowledgements

This work was sponsored by Grant 1/3572/06 of the Slovak Grant Agency for Science (VEGA). The authors thank the Structural Funds Interreg IIIA for financial support in purchasing the diffractometer. The authors gratefully acknowledge Professors Ernst Egert and Katharina M. Fromm and Dr. Gèrald Bernardinelli for their helpful discussions.