NMR Characterization of Lanthanide(3+) Complexes of Tetraazatetrakisphosphinato and Tetraazatetrakisphosphonato Ligands

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 m M 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.