Investigation of group 12 metal complexes with a tridentate SNS ligand by x-ray crystallography and ¹H NMR spectroscopy

Two series of zinc triad complexes containing the ligand 2,6-bis(methylthiomethyl)pyridine (L¹) were synthesized and characterized by X-ray crystallography and solution-state ¹H NMR spectroscopy. The distorted meridional octahedral M(L¹) ₂(ClO₄)₂ series includes the first structurally characterized Zn(II) and Cd(II) complexes with N₂(SR ₂)₄ coordination spheres. Coordination of HgCl₂ and ZnCl₂ with 1 equiv of ligand afforded mononuclear, five-coordinate species Hg(L¹)Cl₂ and Zn(L ¹)Cl₂, respectively, with distorted square-pyramidal and trigonal-bipyramidal geometries. With CdCl₂, the dimeric [Cd(L ¹)Cl(μ-Cl)]₂ complex was obtained. The distorted octahedral coordination geometry of each Cd(II) center in this complex is formed by one tridentate ligand, two bridging chloride ions, and one terminal chloride ion. NMR spectra indicate that the intermolecular ligand-exchange rate of [M(L¹)₂]²⁺ decreased in the order Cd(II) > Zn(II) > Hg(II). Slow intermolecular ligand-exchange conditions on the chemical-shift time scale were found for 1:2 metal-to-ligand complexes of L ¹ with Hg(II) and Zn(II) but not Cd(II). Slow intermolecular ligand-exchange conditions in acetonitrile-d₃ solutions permitting detection of ^3-5J(¹⁹⁹Hg¹H) were found for 1:1 and 1:2 Hg(ClO₄)₂/L¹ complexes, but not for the related Cd(ClO₄)₂ complexes. The magnitudes of J( ¹⁹⁹Hg¹H) for equivalent protons were smaller in [Hg(L ¹)₂]²⁺ than in [Hg(L¹)(NCCH ₃)_x]²⁺. The relative intermolecular ligand-exchange rates of the zinc triad complexes investigated here suggest that the toxicity of Hg(II) is accentuated by the relative difficulty of displacing it from the coordination sites encountered.