Effects of electron-donating ability of binding sites on coordination number: the interactions of a cyclic Schiff...

The stepwise addition of Cu²⁺ ions to the nonplanar cyclic Schiff base 5,9,14,18-tetramethyl-1,4,10,13-tetraazacyclooctadeca-5,8,14,17-tetraene-7,16-dione (H₄daaden, C₁₈H₂₈N₄O₂), yields a one-end-open dinuclear copper chelate. The pyridine adduct of the dinuclear copper chelate, namely, [μ-6,11-dimethyl-7,10-diazahexadeca-5,11-diene-2,4,13,15-tetraolato(4-)](pyridine)dicopper(II), [Cu₂(C₁₆H₂₀N₂O₄)(C₅H₅N)], was characterized by single-crystal X-ray crystallography. The two Cu^II atoms of the copper chelate display different coordination modes, i.e. inner-N₂O₂ and outer-O₂O₂. The Cu atom which is bonded in the outer-O₂O₂ mode is axially bonded to a pyridine molecule, which suggests that the electron-donating ability of the O₂O₂ site to the Cu atom is poor. As a result, the O₂O₂-bonded Cu atom has a coordination number of five, showing square-bipyramidal geometry around the Cu atom. The N₂O₂-coordinated site provides sufficient electron density to the other Cu atom to be stabilized with a coordination number of four, showing square-planar geometry around the Cu atom. The electron-donating ability of the ligand coordination sites plays a key role in determining the coordination number of the Cu atoms of the dicopper chelate.