Abstract
The three‐dimensional structures in aqueous solution of the entire series of the Ln^3+^ complexes [Ln(DOTP*‐Et)]^−^ (formed from the free ligand P,P′,P″,P′′′‐[1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetrayltetrakis(methylene)]tetrakis[P‐ethylphosphinic acid] (H~4~DOTP*‐Et) were studied by NMR techniques to rationalize the parameters governing the relaxivity of the Gd^3+^ complex and evaluate its potential as MRI contrast agent. From the ^1^H‐ and ^31^P‐NMR lanthanide‐induced‐shift (LIS) values, especially of the [Yb(DOTP*‐Et)]^−^ complex, it was concluded that the [Ln(DOTP*‐Et)]^−^ complexes adopt in solution twisted square antiprismatic coordination geometries which change gradually their coordination‐cage structure along the lanthanide series. These complexes have no inner‐sphere‐H~2~O coordination, and preferentially have the (R,R,R,R) configuration of the P‐atoms in the pendant arms. Self‐association was observed in aqueous solution for the tetraazatetrakisphosphonic acid ester complexes [Ln(DOTP*‐OEt)]^−^ (=[Ln(DOTP‐Et)]^−^) and [Ln(DOTP*‐OBu)]^−^ (=[Ln(DOTP‐Bu)]^−^) at and above 5 mM concentration, through analysis of ^31^P‐NMR, EPR, vapor‐pressure‐osmometry, and luminescence‐spectroscopic data. The presence of the cationic detergent cetylpyridinium chloride (CPC; but not of neutral surfactants) shifts the isomer equilibrium of [Eu(DOTP*‐OBu)]^−^ to the (S,S,S,S) form which selectively binds to the cationic micelle surface.