Density functional theory investigation of Cu(I)- and Cu(II)-curcumin complexes.

Density functional theory is used to obtain the lowest energy geometries of bis-aqua curcumin complexes and bis-curcumin complexes of Cu(I) and Cu(II). Three conformations of curcumin, obtained by rotation of the substituted aromatic groups, were considered in each case. Steric repulsion, due to the methoxy–methoxy interactions, was found to be an important factor in determining the lowest energy conformer of Cu(II)(curcumin)2 but was less important for the Cu(I) analog. Using a sufficiently large basis set, the results show that the lowest energy Cu(II)(curcumin)2 geometry is square planar around the copper atom, in contrast to the results from a previous study (Shen et al. THEOCHEM-J Mol Struct 2005, 757, 199). In addition, other studies suggested that the formation of this complex is followed by the reduction of Cu(II) to Cu(I). We also examined the singly occupied molecular orbital, spin density, and natural bond orbitals of Cu(II)(curcumin)2. While the former two analyses show little evidence of electron transfer from curcumin into the Cu center, the latter indicates that Cu(II) is partially reduced to Cu(I) as a consequence of complexation. Finally, we consider the bis-aqua curcumin and bis-curcumin complexes on a reaction path involving progressive displacement of water molecules by curcumin ligands. The results show that the bis-curcumin complex is the most stable Cu(II) complex, showing consistently exothermic steps in the reaction path. However, for Cu(I), the final step in the reaction path is essentially thermoneutral, indicating that the 1:1 and 1:2 Cu(I) complexes are equally stable thermodynamically.