Water-Involved Hydrogen Bonds in Dimeric Supramolecular Structures of Magnesium and Zinc Phthalocyaninato Complexes

Two dimeric supramolecular structures {MgPc(H2O)center dot 4-methylmorpholine}(2)-(1) and {ZnPc( H2O)center dot 4-methylmorpholine}(2)-(2) formed from two molecules of magnesium and zinc aquaphthalocyanines in 4-methylmorpholine solution were revealed by single-crystal Xray diffraction. Coordinated H2O molecules to the metal center of respective Mg or Zn phthalocyanines oriented face-to-face to form two hydrogen bonds, one of them connecting via O-H center dot center dot center dot N-azamethine atom to form a dimeric structure in the crystals with ring-to-ring distances of 3.412(3) angstrom in 1 and 3.403(5) angstrom in 2, whereas the second hydrogen bonds link the solvent 4-methylpholine molecules to form supramolecular dimeric {MgPc(H2O)center dot 4-methylmorpholine}(2)-(1) and {ZnPc(H2O)center dot 4methylmorpholine}(2)-(2) structures. The interaction of the metal center of MgPc or ZnPc with O atom of the water molecule results its displacement of similar to 0.5 angstrom from the N-4-isonodole plane of the phthalocyaninate(2-) macrocycle that adopts a saucer-shape form. To better understand the interactions leading to the existence of the dimeric structures in the crystals, theoretical calculations of dimer stabilization energy composed of two monomers (MgPcH2O and ZnPcH2O), as well as three-dimensional molecular electrostatic potentials, have been performed using the density functional theoretical method. The calculated absorption spectra of both supramolecular monomers and dimers were compared with the experimental ones in 4-methylmorpholine solution. The water axial ligation of MgPc and ZnPc and the formation of the monomeric and dimeric supramolecular structures only slightly change the energy gap between highest occupied molecular orbital and lowest unoccupied molecular orbital levels that is compared with those of the parent MgPc and ZnPc pigments.