Synthesis, crystal structure, spectral properties, and charge density analysis of 4-coordinate Bis[1-(4-N,N-dimethylaminophenyl)-1,3-butadionato-O,O′]copper(II)1

The reaction of 1-[4-(dimethylamino)phenyl]-1,3-butanedione (Hdapb) with copper(II) chloride produces bis[1-(4-N,N-dimethylaminophenyl)-1,3-butadionato-O,O′]copper(II), Cu(dapb)₂ (I), with high yields. The title compound crystallizes exceptionally well, forming suitable crystals for X-ray crystallography and detailed electron distribution studies for copper. The copper atom can be treated as an isolated 4-coordinate atom with a single unpaired 3d electron. Contrary to expectations, the two dapb ligands bonded to the copper are inequivalent, characterized by unequal CuO bond lengths. A suitable model confirming the correct d-electron splitting and distortion of this d⁹-distorted square planar complex and establishing its spectral properties is proposed. R-free calculations were carried out to determine the optimal weighting parameters for the chemical equivalence and local symmetry restraints on the charge density. The charge density confirms the appropriate d-electron splitting for a distorted d⁹-square planar complex and establishes its compatibility with the complex's spectral properties. The UV–visible absorption spectra show a charge transfer band in addition to the expected d-d bands, whose energies deviate from the trend previously documented in the literature.