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

• Published in: RSC advances Vol. 6; no. 79; pp. 75844 - 75854
• Main Authors: Schmidt, Markus, Görls, Helmar, Plass, Winfried
• Format: Journal Article
• Language: English
• Published: 01.01.2016
• Subjects: Aliphatic compounds; Binding; Condensation; Distortion; Ligands; Pocket; Precursors; Synthesis
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/C6RA16870A

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 H 2 sc-difo and H 2 tsc-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 (H 2 sc-hydra and H 2 tsc-hydra) and 2-(aminomethyl)-pyridine (H 2 sc-ampy and H 2 tsc-ampy), whereas 2-aminophenol was used as an aromatic sample (H 3 sc-amph and H 3 tsc-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)PPh 3 ] and a homodinuclear zinc( ii ) complex [Zn 2 (tsc-hydra)(OAc) 2 ] were synthesized with the single Schiff-base precursor ligand H 2 tsc-difo and the double Schiff-base ligands H 2 tsc-hydra, respectively. Both complexes crystallized in the monoclinic space group P 2 1 / n . For [Ni(tsc-difo)PPh 3 ] a square-planar geometry is found for nickel( ii ) ion with H 2 tsc-difo acting as a tridentate ligand. Whereas the structure of complex [Zn 2 (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 [N 4 O 2 ] donor set and the other zinc( ii ) ion in a distorted square-pyramidal coordination given by the thiosemicarbazone based binding pocket with [NO 3 S] donor set.