Complexes of phosphonate and phosphinate derivatives of dipicolylamine

Abstract

Four dipicolylamine (DPA) derivatives bearing methylphosphonic or methylphosphinic acid (P–R; R = H, Me, CH₂PO₂H₂) groups were synthesized. Their acid–base and coordination properties were studied by potentiometry, UV-Vis and NMR measurements. The phosphonate derivative shows increased basicity (log K₁ = 8.39), whereas the phosphinate derivatives show decreased basicity (log K₁ = 6–7) compared to the parent dipicolylamine. Consequently, the stability constants of the phosphonate complexes are 3 to 4 orders of magnitude higher than those of the phosphinate complexes. All ligands show excellent selectivity for Zn(II) over Ca(II) and Mg(II) ions. The structures of several Cu(II) and Ni(II) complexes in the solid state were determined by X-ray diffraction analysis. The complexes mostly show dimeric or polymeric structures and the two metal ions induce a different coordination geometry of the DPA group. The coordination geometry is always (pseudo)octahedral. The DPA fragment is bound in mer geometry in the Cu(II) complexes, whereas the Ni(II) complexes have fac geometry. In conclusion, the phosphonate and phosphinate derivatives of DPA are efficient complexing agents for divalent transition metal ions and the DPA-phosphinate grouping is a suitable fragment for sensing Zn(II) ions.