Coordination properties of cyclam (1,4,8,11-tetraazacyclotetradecane) endowed with two methylphosphonic acid pendant arms in the 1,4-positions

Abstract

The title ligand, 1,4,8,11-tetraazacyclotetradecane-1,4-diyl-bis(methylphosphonic acid) (H₄te2p^1,4, H₄L), was prepared by an optimized synthetic approach and its complexing properties towards selected metal ions were studied by means of potentiometry. The ligand forms a very stable complex with copper(II) (log β(CuL) = 27.21), with a high selectivity over binding of other metal ions (e.g. log β(ZnL) = 20.16, log β(NiL) = 21.92). The crystal structures of two intermediates in the ligand synthesis and two forms of the nickel(II) complex (obtained by crystallization at different pH) were determined. From acid solution, the crystals of trans-O,O-[Ni(H₃L)]Cl·H₂O were isolated. In such complex species, one phosphonate pendant arm is double- and the second arm is monoprotonated. The isolation of such species demonstrates a high kinetic inertness of the complex. The central metal ion is surrounded by four in-plane nitrogen atoms (in the ring configuration III) and two oxygen atoms of pendant moieties in the apical positions of octahedral coordination sphere. From neutral solution, the crystals of {trans-O,O-[Ni(H₂L)]}₃·5H₂O were isolated. The molecular structures of the complex units found in this structure are analogous to that found in trans-O,O-[Ni(H₃L)]Cl·H₂O.