A series of bulky and neutral gadolinium complexes of [bis(R-amide)dtpaH3] (where R = Pri,, Bui,, Bz and phenylethyl, L1--L4)) with potential application as contrast agents for magnetic resonance imaging has been prepared and characterized ; the relaxivity of these complexes is comparable to the contrast agent [Gd(dtpa)(H2OO)]; which is which is currently used in clinics; the X-ray structure analysis of the gadolinium(III) complex of L3 reveals a neutral nine-coordinate complex featuring a water molecule in the tricapped trigonal prismatic metal coordination sphere.
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Acknowleulyc~ments-We thank the S.E.R.C. for financial support (I.J.S.) and access to the Chemical Database Service at Daresbury. and Dr Orla Ni Dhubhghaill (Insti-tut de Chimie Minerale et Analytique. Lausanne) for the relaxivity measurements.