NMR characterization of lanthanide(3+) complexes of tetraazatetrakisphosphinato and tetraazatetrakisphosphonato ligands

The three-dimensional structures in aqueous solution of the entire series of the Ln³⁺ 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 ₄DOTP*-Et) were studied by NMR techniques to rationalize the parameters governing the relaxivity of the Gd³⁺ complex and evaluate its potential as MRI contrast agent. From the ¹H- and ³¹P-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-H₂O 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 ³¹P-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.