Facile high-yield synthesis of unsymmetric end-off compartmental double Schiff-base ligands: easy access to mononuclear precursor and unsymmetric dinuclear complexes

A straightforward and easy to handle two-step synthetic route for unsymmetric double Schiff-base ligands is presented. The isolated intermediate single Schiff-base precursor ligands H2sc-difo and H2tsc-difo were derived by condensation of 2,6-diformyl-4-methylphenol with semicarbazide and thiosemicarbazide, respectively. Further reaction of the precursor ligands with different amine components, including both aliphatic and aromatic examples, allows the synthesis of ditopic unsymmetric double Schiff-base ligands in high yields and purity. As aliphatic cases we used (2-aminoethyl)bis(2-pyridylmethyl)-amine (H2sc-hydra and H2tsc-hydra) and 2-(aminomethyl)-pyridine (H2sc-ampy and H2tsc-ampy), whereas 2-aminophenol was used as an aromatic sample (H3sc-amph and H3tsc-amph). The overall synthetic route allows for the preparation of the employed ligands in large scale. To explore the coordination capabilities of the reported ligand systems a mononuclear nickel(II) complex [Ni(tsc-difo)PPh3] and a homodinuclear zinc(II) complex [Zn2(tsc-hydra)(OAc)2] were synthesized with the single Schiff-base precursor ligand H2tsc-difo and the double Schiff-base ligands H2tsc-hydra, respectively. Both complexes crystallized in the monoclinic space group P21/n. For [Ni(tsc-difo)PPh3] a square-planar geometry is found for nickel(II) ion with H2tsc-difo acting as a tridentate ligand. Whereas the structure of complex [Zn2(tsc-hydra)(OAc)2] reveals two zinc(II) ions in distinctly different coordination geometry, one in distorted octahedral coordination located in the bispyridine based binding pocket with [N4O2] donor set and the other zinc(II) ion in a distorted square-pyramidal coordination given by the thiosemicarbazone based binding pocket with [NO3S] donor set.