Experimental Electron Density Study of Tetrakis-μ-(acetylsalicylate)dicopper(II): a Polymeric Structure with Cu···Cu Short Contacts

The electron density, its topological features, and the electrostatic potential of tetrakis-μ-(acetylsalicylate)dicopper(II), Cu[C9H7O4]2, have been derived from an accurate high-resolution diffraction experiment at 100 K. This complex exhibits a polymeric structure involving one acetyl oxygen atom as a bridge in the solid state. Only van der Waals interactions between the polymeric chains are observed. The copper cation is octahedrally coordinated with five oxygen atoms of the aspirinate ligands and one adjacent Cu with short Cu···Cu contact distances in the range of 2.6054(1) Å. The Cu−O bond lengths are equal to 1.96 Å except the apical one which is 2.2183(7) Å. The multipole refinements were carried out using the Hansen−Coppens model coded in the MOPRO computer program. Starting from the 3d104s1 copper electron configuration, the electron density analysis and Cu d-orbital populations reveal that the observed configuration is close to being [Ar]3d94s1. As expected from the ligand field theory, the most depopulated 3d-orbital is the dx2−y2 (1.17 e) one with lobes pointing toward the carboxylic oxygen atoms. Conversely, the dz2 is the most populated orbital for a z-axis directed along the Cu···Cu bond. The atomic charges were derived from a κ-refinement and yielded a metal net charge of +1.20(3) e. Deficits of +0.72(6) and +0.59(7) e are obtained for the acetyl carbon atoms of the aspirinate ligands, those involved in the drug activity of aspirin. Comparisons are made to the results of our previous work on the zinc-aspirinate complex.