Mixed-ligand complexes of zinc(II), cobalt(II) and cadmium(II) with sulfur, nitrogen and oxygen ligands. Analysis of the solid state structure and solution behavior. Implications for metal ion substitution in alcohol dehydrogenase

In this paper we characterize new, mixed ligand complexes of zinc(II), cobalt(II) and cadmium(II) with tri-tert-butoxysilanethiolate and 2-(2′-hydroxyethyl)pyridine ligands. Due to the chelating versus non-chelating behavior of 2-(2′-hydroxyethyl)pyridine ligand we have obtained an interesting structural variety in the studied system. The presented coordination patterns together with the results of NMR studies have been used to illustrate a rapid chemical exchange undergoing in methanolic solutions of zinc(II) and cadmium(II) complexes. UV–Vis spectra of cobalt(II) species have also evidenced an exchange in the case of cobalt(II) complex. The relative strength of hydrogen bond formed by hydroxyl group bonded to Zn(II), Co(II) or Cd(II) was evaluated by analysis of structural parameters and position of the OH stretching vibrations in the FT-IR spectra of the complexes in solid state. The data were compared with the activity of zinc (native) alcohol dehydrogenase and alcohol dehydrogenase substituted with cobalt and cadmium ions. The enthalpies of proton abstraction in zinc and cobalt complexes were calculated and found to be very similar. The attempt to apply zinc tri-tert-butoxysilanethiolate as a catalyst in the biomimetic reaction of reduction of N-benzylnicotinamide chloride by ethanol was unsuccessful.

Zinc(II), cobalt(II) and cadmium(II) complexes with silanethiolate and 2-(2′-hydroxyethyl)pyridine ligands were synthesized and structurally characterized. The following sequence of hydroxyl group activation in the complexes was elucidated: Co ≥ Zn > Cd.Figure optionsDownload as PowerPoint slideHighlights
► Hydrogen bonds in Zn(II), Cd(II) and Co(II) complexes were analyzed by X-ray diffraction and FT-IR.
► There is a fast exchange of N-ligands in mixed S,N-ligand zinc(II) and cadmium(II) complexes.
► DFT calculations indicate similar reactivity of Zn(II) and Co(II) complexes as catalysts in dehydrogenation.