Syntheses, characterization, density functional theory calculations, and activity of tridentate SNS zinc pincer complexes based on bis-imidazole or bis-triazole precursors

A series of tridentate pincer ligands, each possessing two sulfur- and one nitrogen-donor functionalities (SNS), based on bis-imidazole or bis-triazole salts were metallated with ZnCl2 to give new tridentate SNS pincer zinc(II) complexes [(SNS)ZnCl]+. The zinc complexes serve as models for the zinc active site in liver alcohol dehydrogenase (LADH) and were characterized with single crystal X-ray diffraction, 1H, 13C, and HSQC NMR spectroscopies, electrospray mass spectrometry, and elemental analysis. The zinc complexes feature SNS donor atoms and pseudotetrahedral geometry about the zinc center, as is seen for liver alcohol dehydrogenase. The bond lengths and bond angles of the zinc complexes correlate well to those in horse LADH. The SNS ligand precursors were characterized with 1H, 13C, and HSQC NMR spectroscopies, elemental analysis, and cyclic voltammetry, and were found to be redox active. Gaussian calculations were performed and agree with the experimentally observed oxidation potentials for the pincer ligand precursors. The zinc complexes were 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 electron-poor aldehydes. The SNS zinc pincer complexes with bis-triazole ligand precursors exhibit higher activity for the reduction of 4-nitrobenzaldehyde than do SNS zinc pincer complexes with bis-imidazole ligand precursors. Quantitative stoichiometric conversion was seen for the reduction of pyridine-2-carboxaldehyde via SNS zinc pincer complexes with either bis-imidazole or bis-triazole ligand precursors.

Graphical abstractA series of tridentate SNS ligand precursors, based on bis-imidazole and bis-triazole precursors, were metallated with ZnCl2 to give new tridentate SNS pincer zinc complexes that serve as models for the zinc active site in liver alcohol dehydrogenase (LADH). These systems were characterized with single crystal X-ray diffraction, 1H, 13C, and HSQC NMR spectroscopies, and electrospray mass spectrometry, and were studied by density functional theory. The ability of these compounds to catalyze the reduction of electron-poor aldehydes was examined.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Synthesized and characterized a series of SNS Zn(II) pincer compounds. ► Zn(II) complexes enhance reduction of aldehydes in presence of hydrogen donor. ► Correlated cyclic voltammetry results with Gaussian calculations.