Synthesis and Complexation Properties of a New Macrocyclic Polyaza Polyphosphinate Ligand, DOTEP (1,4,7,10-Tetraazacy clododecane-1,4,7,10-tetrakis (methyleneethylphosphinate))

A new macrocycle, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methyleneethylphosphinate) (DOTEP), has been synthesized, and its protonation constants and metal ion binding properties have been examined. The first two chelate protonations occur at nitrogens (log K₁ and log K₂: 10.94 and 8.24, respectively) while the pendant phosphinate groups are protonated only below pH 4. DOTEP forms less stable complexes with the alkaline-earth-metal cations, Cu²⁺, Zn²⁺, and Cd²⁺, and the trivalent lanthanides (Ln³⁺) than does the tetraacetate analogue, DOTA, largely due to the lower basicity of the nitrogen and oxygen donor atoms of DOTEP. Like DOTA, DOTEP forms complexes with the trivalent lanthanide ions rather slowly in the pH range 6-7 but, once formed, the Ln(DOTEP)^- complexes release free Ln³⁺ slowly in strong acid.