Synthesis and magnetic relaxation properties of paramagnetic Gd-complexes of new DTPA-bis-amides. The X-ray crystal structure of [Gd(L)(H ₂O)]·3H₂O (L = DTPA-bis(4-carboxylicphenyl)amide)

A new type of DTPA-bis-amides (L1-L4) and their Gd(III)-complexes of the type [Gd(L)(H₂O)]·nH₂O (5: L1; 6: L2; 7: L3; 8: L4) have been prepared and characterized by analytical and spectroscopic techniques. The X-ray crystal structure of 8 has been determined for structural confirmation. The coordination geometry adopts a tricapped trigonal prism geometry with L4 acting as a chelate octadentate and a water molecule in the coordination sphere. Crystals are monoclinic, P2₁, a = 14.468(3), b = 19.235(4), c = 13.527(2) Å, β = 107.245(3)°, V = 3595.2(11) ų, Z = 4, D_calc = 1.570. Significant increases in relaxivities are observed with 6 and 7 as compared with that of Omniscan®, a commercial MR agent: R1 = 12.46 mM^-1 s^-1, R2 = 8.76 mM^-1 s^-1 for 6; R1 = 12.77 nm^-1 s^-1, R2 = 7.60 mM^-1 s^-1 for 7; R1 = 4.9 mM^-1 s ^-1, R2 = 4.8 mm^-1 s^-1 for Omniscan®. In the case of 5, however, both R1 and R2 are found to be lower to show 2.09 mM ^-1 s^-1 and 1.82 mM^-1 s^-1, respectively.