Interactions between metal cations and the ionophore lasalocid. Part 6.—Potentiometric and electron spin resonance study of the complexation of Gd3+ in methanol by lasalocid and simpler carboxylic acids

Complexes formed in methanol between the ionophore lasalocid and the Gd3+ ion have been studied using pHmetric, e.s.r. and u.v.–visible titration against tetrabutylammonium methoxide solution. The same has been done for salicylic and benzoic acids. In all cases the formation of the successive complexes AGd2+, A2Gd+ and A3Gd between the acid AH and the Gd3+ion has been observed and the corresponding formation constants were determined. More or less strong variation of the height, width and intensity of the e.s.r. line has been observed with the formation of the complexes; individual e.s.r. parameters could be had for most of the complexes. Comparison of data obtained for lasalocid and salicylic or benzoic acid suggests that in lasalocid anion–gadolinium cation complexes in methanol, the carboxylate group is the main and possibly the sole binding site of the gadolinium ion. Other complexes are formed in more basic media in methanol; these may involve the salicylate dianion, as shown for salicylic acid–gadolinium complexes, or, more probably, result from successive substitution of a methoxide ion for a lasalocid anion in A3Gd.