On the structure of Zn(II) and Cu(II) cyanin complexes in aqueous solution

Abstract

B3LYP/6-31++G(d,p) optimizations on models for the metal cyanin, Cy, complexes [MCy(H2O)n]+, (M = Zn(II), Cu(II); n = 2, 3, 4) in aqueous solution indicate that 4 is the most favoured coordination number in both cases. SP-4 and T-4 geometries are nearly isoenergetic for the former, while SP-4 is the only one obtained for the latter. Anionic cyanin displays higher affinity for Cu(II) than for Zn(II) or Mg(II). The electron density reorganization of cyanin model accompanying the complexation process was analyzed by means of the quantum theory of atoms in molecules. This analysis reveals that: (1) the O4′–M bond is stronger than O3′–M; (2) anionic cyanin displays a dual character between 4′-keto-quinoidal and 3′,4′-dienolate resonance forms; (3) Cu(II) takes more electron density than Zn(II) from Cy− and water ligands; (4) when the coordination number increases, each ligand (Cy− or water) transfers less electron density; (5) complex formation modifies the electron density in all the atoms of the ligands, but the largest modifications are displayed within the AC bicycle of Cy−; and (6) a third part of density lost by the Cy− ligand is removed from hydrogens.