Synthesis, Spectroscopy, and Magnetic Properties of Transition-Metal Complexes with the Diethyl 2-Quinolylmethylphosphonate (2-qmpe) Ligand − Crystal Structures of [Ni(2-qmpe)4(H2O)2](ClO4)2 and [Mn(2-qmpe)4(H2O)2](ClO4)2 Showing Unexpected O-Binding of the qmpe Ligands

Abstract

The synthesis and characterization of transition-metal coordination compounds containing the ligand diethyl 2-quinolylmethylphosphonate (2-qmpe) is described. Complexes of CoII, MnII, NiII, CdII, and ZnII were found to be mutually isomorphous according to their X-ray powder diffraction patterns and IR spectra. The new coordination compounds were identified and characterized by elemental analysis, magnetic measurements, infrared and ligand-field spectra. The crystal structures of the complexes [M(2-qmpe)4(H2O)2](ClO4)2 (M = Ni, 1; Mn, 2) reveal six-coordinate [M(2-qmpe)4(H2O)2] cations with the qmpe ligand coordinating via the oxygen rather than via the nitrogen atom, as well as lattice perchlorate anions. The metal ion is octahedrally surrounded by the four oxygen atoms of the four organic ligands. The non-participation of the pyridine nitrogen atom is unusual. The two water molecules occupy the fifth and the sixth coordination sites in a trans configuration. The coordinated water molecules are strongly hydrogen-bonded to the pyridine nitrogen atom of the 2-qmpe, further stabilizing the solid-state structure. The geometry of the metal ion can be described as distorted octahedral. For the CuII complex, a hydroxo-bridged dinuclear structure of the type [Cu(2-qmpe)2(OH)(H2O)2]2(ClO4)2 is proposed, based on its spectroscopic and magnetic properties. Ligand-field spectra of the Co and Ni compounds were found to be in agreement with the tetragonally distorted octahedral geometry.