Synthesis and structural and spectroscopic characterization of a complex between Co(II) and imino-bis(methylphosphonic acid): Gaining insight into biologically relevant metal-ion phosphonate interactions or looking at a new Co(II)-organophosphonate material?

Abstract

Cobalt is an essential metal ion involved in key biomolecules, regulating processes in human physiology. As a metal ion, Co(II) assumes forms, which are dictated by the nature of organic binders in biological fluids, and the conditions under which metal ion ligand interactions arise. Among the various low molecular mass metal ion binders in biological fluids are amino acids, organic acids, as well as their variably phosphorylated forms. As a representative metal ion binder, the organophosphonate ligand H2O3P-CH2-NH2 + -CH2-PO3H- was employed in aqueous reactions with Co(II), ultimately leading to the isolation of complex [Co(C2H8O6NP2)2 (H2O)2] (1) at pH 2. The complex was characterized analytically, spectroscopically (FTIR, UV-vis, EPR), and magnetically. Compound 1 crystallizes in the monoclinic space group P21/n, with a = 7.361(3) Å, β = 8.133(3) Å, c = 14.078(5) Å, β = 104.40(1)°, V = 816.3(5) Å3, and Z = 2. X-ray crystallography reveals that 1 is a compound with a molecular type of lattice. In it, there exist mononuclear octahedral sites of Co(II) surrounded by oxygens, belonging to terminal phosphonates and bound water molecules. Both ends of the ligand zwitterionic form are involved in binding to adjacent Co(II) ions, thus creating tetranuclear 32-membered rings, with cavities extending in two dimensions and holes in the third dimension throughout the lattice of 1. Similar structural features were observed in other metal organophosphonate lattices of potential catalytic and chemical reactivity. The magnetic and EPR data on 1 support the presence of a high-spin octahedral CO(II) in an oxygen environment, having a ground state with an effective spin S = 1/2. The solution UV-vis and EPR data suggest retention of the high-spin octahedral CO(II) ion, consistent with the magnetization measurements on 1. Collectively, the data reflect the existence of a soluble Co(II)-iminodiphosphonate species not unlike those expected in biological fluids containing the specific ligand or ligands similar to that. Both biologically relevant perspectives and a synthetic outlook into Co(Il)-organophosphonate materials are discussed.