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The synthesis of three triazamacrocycles containing the pyridine moiety and three acetate pendant arms (PCTA) is reported. The three systems differ due to the number of carbon atoms in the macrocyclic ring forming ligands PCTA-[12], -[13], and -[14], endowed with different coordination capabilities toward lanthanide(III) ions. Microscopic protonation sequences for the three ligands have been investigated by (1)H NMR spectroscopy. Complexes of PCTA-[12], -[13], and -[14] with La(III), Gd(III), and Lu(III) have been prepared. Relaxometric measurements on the aqueous solutions of the paramagnetic Gd(III) complexes in the presence of competitive ligands gave the following stability constants: log K(f) = 20.8 for Gd-PCTA-[12], log K(f) = 19.3 for GdPCTA-[13], and log K(f) = 12.5 for GdPCTA-[14]. The measurement of water relaxation rates indicated a tendency to decrease the degree of hydration upon increasing the ring size. The VT (1)H and (13)C-NMR spectra of the diamagnetic La(III) and Lu(III) complexes exhibit a large variability of the solution structures dictated by the matching of the size of the lanthanide ion and the macrocyclic cavity. This results in noticeable differences in their stereochemical nonrigidity, hydration state, and thermodynamic stability. To some extent the changes observed in continuing from the 12-14-membered ring macrocyclic complexes parallels the behaviors shown by the octacoordinated lanthanide(III) complexes with DOTA and TETA. GdPCTA-[12] and -[13] feature promising properties in view of their possible use as contrast agents for magnetic resonance imaging.

Lanthanide(iii1 complexes of a novel heptadentate chelating ligand display structural and dynamic properties that are particularly interesting in view of potential applications in magnetic resonance imaging (MRI).The search for new contrast agents (CA) for MRI is currently orientated towards the synthesis of paramagnetic complexes, possibly neutral, of increasing ability to enhance the water proton relaxation rates, while maintaining the favourable complexing properties of dota and dtpa-like ligands.'? We report here the synthesis and solution state NMR characterisation of a novel macrocyclic ligand 3 and its complexes with EuIII, YbIII and Gd"' ions which satisfy both requirements. Ligand 3 was synthesised as shown in Scheme 1.3,6,10,16-Tetraazabicyclo[ 10.3.llhexadeca-l( 16),12,14-triene 1 was prepared according to the procedure of Costa and Delgado.2 The three carboxymethyl moieties on N3, N6 and N10 were introduced by the reaction of compound 1 with 3 equiv. of methyl bromoacetate in the presence of silver(1) carbonate in THF at room temperature. The reaction mixture was filtered through Celite, the filtrate evaporated to dryness and the residue acidified with hydrochloric acid. After addition of sodium sulfide, the silver(1) sulfide precipitated was filtered off. The solution was made basic to pH 11 and extracted with dichloromethane. The resulting triester 2$ was obtained in 80% yield.By treating compound 2 with 3 equiv. of KOH in methanol, the potassium salt of the triacid 35 was isolated in quantitative yield and used for the preparation of the Ln"' complexes. EuIII, YbIII and Gd"1 complexes of ligand 3 were prepared by mixing stoichiometric amounts of the ligand and the lanthanoid(II1) chloride at neutral pH.The high-temperature-limit 400 MHz 1H NMR spectrum of the EuIII-3 complex (363 K) shows 23 resonances [ Fig. 1 (A)] spread over a range of ca. 30 ppm. The resonances could be assigned to the 23 different protons by use of homo-and heterocorrelated 2D experiments carried out at 2.1 T in order to minimize the magnetic-field-induced line-broadening3 On cooling to room temperature and below, all resonances markedly broaden, indicating that a dynamic process is slowing down although it was not possible to obtain the spectrum corresponding to the 'frozen' structures. Me02C, N-CHp

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