Structural characterization of a square planar Ni(II) complex and its application as a catalyst for the transfer hydrogenation of ketones

Abstract

A new 2-hydroxy-1-naphthaldehyde Schiff base derived nickel(II) complex (4) was synthesized and fully characterized. Analysis of the structure of 4 by single-crystal X-ray diffraction shows two chelating Schiff base ligands bound to nickel in a trans [O^∧N(Ni²⁺)N^∧O] fashion. Hence, in a molecule of 4, two ligands are four coordinate to a Ni(II) center through the imine nitrogen and naphthyloxyl oxygen atoms. This coordination mode resulted in a square planar complex that is stabilized in the solid-state by a network of intermolecular O-H···N hydrogen bonds between neighboring molecules. The ¹H NMR data showed the loss of the hydroxyl (OH) proton signal and an upfield shift of the metal-bound imine (-NH) proton signal, while the IR data also showed a lower energy shift in the absorption frequency of the imine (C = N) bond due to back donation from the coordinated Ni(II) center. As a catalyst for the transfer hydrogenation of a range of ketones, 4 showed good catalytic activity at a very low concentration of 0.1 mol% with 2-propanol as the model substrate. The catalyst is also effective for various related ketone substrates bearing a range of steric and electronic regulating groups.

Graphical abstract

Keywords:

• Transfer hydrogenation
• Ni(II) complex
• Schiff base
• square planar

Acknowledgements

We thank c* change (PAR-08) and ESKOM (TESP) for financial support and Sizwe Zamisa for XRD data collection and refinement.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by Eskom South Africa..