Solution and Solid-State Characterization of Highly Rigid, Eight-Coordinate Lanthanide(III) Complexes of a Macrocyclic Tetrabenzylphosphinate

The ligand 1,4,7,10-tetraazacyclododecanetetrakis(methylenebenzylphosphinic acid), 1, forms kinetically stable complexes with lanthanide ions (Ln = Eu, Gd, Tb, Yb, and Y) which are eight-coordinate (no bound water), with single diastereoisomers being preferentially formed (RRRR or SSSS at each stereogenic phosphorus centre). The NMR spectra for the Eu and Yb complexes have been assigned and no significant fluxional behavior is observed in the temperature range 5–80 °C at 400 MHz. The gadolinium complex is a promising outer-sphere contrast agent for magnetic resonance imaging, and analyses of variable temperature NMRD profiles suggest that the nearest water molecule is 4.25 Å distant from Gd (cf. 3.82 Å for the P-Me analogue). A relatively strong complex is formed with bovine serum albumin (K_d = 2.8 × 10⁻⁴ dm³ mol⁻¹) leading to a significantly enhanced relaxivity in which a major contribution may arise from the exchange of mobile protons on the protein which undergo rapid dipolar relaxation caused by the proximity of the paramagnetic Gd ion. The crystal structures of the monohydrochloride salt of the free ligand and of the anionic yttrium(III) complex (as its oxonium salt) are reported. In the structure of the yttrium complex there is no metal-bound water molecule, with the nearest water being 5.62 Å from the yttrium atom. Crystal data for [(1)H]Cl: monoclinic, C2/c, a = 48.21(3) Å,b= 11.455(4) Å, c = 18.474(11) Å, β= 103.88(4)°, V= 9905(9) ų, Z= 8, 2484 reflections, R = 0.078. Crystal data for H₃O[Y(1)]: orthorhombic, Pbcn, a = 22.332(5) Å,b = 23.052(9) Å, c = 21.301(8) Å, V = 10 965 ų, Z = 8,1625 reflections, R = 0.089.