Bis-tridentate chelates of an asymmetric ligand: X-ray structures and solution NMR characterization of divalent zinc triad metal ion complexes of N-(2-pyridylmethyl)-N-(2-(methylthio)ethyl)amine

Divalent zinc triad metal ion complexes of type M(L)₂(ClO₄)₂ (L = N-(2-pyridylmethyl)-N-(2-(methylthio)ethyl)amine) with N₄S₂ metal coordination spheres were isolated and characterized by X-ray crystallography and variable temperature proton NMR. Although bis-tridentate chelates have nine geometric isomers, the crystallographically characterized complexes of all three metal ions had trans facial octahedral coordination geometry with C_i symmetry. Despite the low coordination number and geometric preferences of d¹⁰ metal ions, which facilitate inter- and intramolecular exchange processes, dilute solutions of these bis-tridentate chelates exhibited slow geometric isomerization. Symmetry, sterics and shielding arguments supported specific isomeric assignments for the major and minor chemical shift environments observed at low temperature. At elevated temperature, rapid intramolecular exchange occurred for all three complexes but slow intermolecular exchange on the coupling constant time scale was evidenced through detection of J_HgH interactions for Hg (L)₂^{2 +}. These unusual observations are discussed in the context of the zinc triad metal ion coordination chemistry of related bis-tridentate chelates.