15N, 1H and 13C NMR. A study of the complexation of binuclear lanthanide(III)-silver(I) shift or relaxation reagents with substrates containing multiply bonded nitrogen

Binuclear lanthanide(III)‐silver(I) β‐diketonate chelates are effective NMR shift and relaxation reagents for substrates containing one or two multiply bonded nitrogen atoms, despite the fact that some of the substrates are able to react with lanthanide chelates alone. The silver cation is bonded to the nitrogen—nitrogen or nitrogen—carbon multiple bond. The lanthanide(III) chelate tetrakis anion is held in proximity to the substrate, without delocalization of spin density from the lanthanide to the studied nuclei (15N, 13C, 1H) of the substrate. Greater spectral simplification is achieved with a binuclear shift reagent than with a single lanthanide chelate in two ways. First, the complexation ability of the π electron pair may be greater than that of the heteroatom lone pair when that of the latter is lowered owing to steric (azobenzene, trans‐aldimine derivatives) or conjugative effects (benzonitrile is similar in this respect to heteroaryl substrates in which the heteroatom is devoid of basicity). Second, resolution may be far better preserved in cases where very strong binding to the lanthanide chelates results in extensive broadening of the NMR signals (imidazole, thiazole and oxazole derivatives).