Interactions of some nitrogen, oxygen, and sulphur heterocycles with gadolinium and nickel chelates. A carbon-13 nuclear magnetic resonance spin–lattice relaxation study

Abstract

Electron–nuclear spin–lattice relaxation times for 13C in natural abundance are determined for six saturated and aromatic heterocyclic compounds containing nitrogen, oxygen, and sulphur in the presence of the metal chelates [Gd(tmhd)3] and [Ni(tmhd)2](tmhd = 2,2,6,6-tetramethylheptane-3,5-dionate). The measurements provide evidence of complex formation between [Gd(tmhd)3] and pyrrolidine and tetrahydrofuran, and between [Ni-(tmhd)2] and pyrrolidine, tetrahydrofuran, and tetrahydrothiophen. The relative affinities of the metal chelates for different ligands have been studied in a series of competition experiments, yielding the affinity order N > O > S for [Gd(tmhd)3] and N > O ≈ S for [Ni(tmhd)2]. For the case of [Ni(tmhd)2] complexes it is found that the relaxation rates deviate largely from the predictions of the Solomon–Bloembergen equation. Comparisons with contact shifts are employed in order to rationalize these deviations.