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The synthesis of a new macrocyclic chelator incorporating a benzimidazole heterocycle is reported. Lanthanide complexes with macrocyclic chelators based on 1,4,7,10-tetra(carboxymethyl)-1,4,7,10-tetraazacyclododecane (DOTA) and 1,4,7-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane (DO3A) are of interest in luminescent, radiopharmaceutical and magnetic resonance (MR) biomedical imaging applications. The benzimidazole DO3A chelator allows for sensitisation of europium(iii), terbium(iii) and ytterbium(iii) luminescence by the heterocycle and also shows a pH dependent coordination change due to protonation of the chelator (pKa = 4.1 for the europium(iii) complex). The thermodynamic stability of the complexes has been investigated by potentiometric titration with the gadolinium(iii) complex showing significantly higher stability than the zinc(ii) complex, where log βZnLH = 28.1 and log βGdLH = 32.1.

Key indicatorsSingle-crystal X-ray study T = 150 K Mean (C-C) = 0.002 Å R factor = 0.059 wR factor = 0.194 Data-to-parameter ratio = 28.3For details of how these key indicators were automatically derived from the article, see

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At 150 K, the title compound, C10H12ClN2OS+·Cl−·C3H6O, exists as discrete hydrogen‐bonded [2 + 2]‐dimers of cations and chloride anions. Of the two possible thia­zolidin‐2‐yl­idene­ammonium cations that may be formed by the reaction of 1,3‐di­chloro­acetone with N‐phenyl­thio­urea, the reported isomer is unable to eliminate water to form the corresponding thia­zole.