Synthesis, characterization, density functional theory calculations, and activity of a thione-containing NNN-bound zinc pincer complex based on a bis-triazole precursor

A novel ambidentate tridentate pincer ligand based on a bis-triazole precursor, was prepared, characterized, and metallated with ZnCl2 to give a new tridentate NNN-bound pincer zinc(II) pincer complex: dichloro(η3-N,N,N)-[2,6-bis(3-[N-butyl]triazol-5-thione-1-yl)]pyridinezinc(II), [(NNN)ZnCl2]. This compound has pseudo-trigonal bipyramidal geometry at the zinc(II) center and exhibits metal–ligand binding that contrasts with our previously reported SNS-bound systems despite the availability of these same donor atoms in the current ligand set. The zinc complex was characterized with single crystal X-ray diffraction, 1H, 13C, and HSQC NMR spectroscopies, and electrospray mass spectrometry. The ligand precursors were characterized with 1H, 13C, and HSQC NMR spectroscopies, and cyclic voltammetry, and were found to be redox active. Density functional calculations, which investigate and support the nature of the NNN binding suggest that the experimentally observed oxidation and reduction waves are not the result of a simple one-electron process. The zinc complex was screened for the reduction of electron-poor aldehydes in the presence of a hydrogen donor, 1-benzyl-1,4-dihydronicotinamide (BNAH), and it was determined that they enhance the reduction of 4-nitrobenzaldehyde. Quantitative stoichiometric conversion was seen for the reduction of pyridine-2-carboxaldehyde.

In an effort to model the zinc active site in liver alcohol dehydrogenase (LADH), a novel tridentate pincer ligand was prepared, characterized, and metallated with ZnCl2 to give a new NNN-ligated, rather than SNS-bound, zinc(II) complex. The free ligands and metallated compounds were characterized via multiple spectroscopic techniques and were examined by DFT calculations to examine various structural and electronic features. The ability of the zinc compound to model LADH through the reduction of various electron-poor aldehydes was also studied.Figure optionsDownload as PowerPoint slideHighlights
► Synthesized and characterized a novel ambidentate ligand precursor.
► Metallated ligand precursor to form a novel NNN Zn(II) pincer complex.
► Zn(II) complexes enhance reduction of aldehydes in presence of hydrogen donor.
► Correlated structural features and cyclic voltammetric data with Gaussian calculations.