Co(III) complexes of Me-salpn and Me-salbn and the ring size effect on the coordination modes and electrochemical properties: The crystal structures of trans-[CoIII(Me-salpn)(py)2]PF6 and cis-α-[CoIII(Me-salbn)(4-Mepy)2]BPh4 · 4-Mepy

The synthesis and characterization of a series of cobalt(III) complexes of the general type [Co(N2O2)(L2)]+ are described. The N2O2 Schiff base ligands used are Me-salpn (H2Me-salpn = N,N′-bis(methylsalicylidene)-1,3-propylenediamine) (1–3) and Me-salbn (H2Me-salbn = N,N′-bis(methylsalicylidene)-1,4-butylenediamine) (4–5). The two ancillary ligands L include: pyridine (py) 1, 3-metheylpyridine (3-Mepy) 2, 1-methylimidazole (1-MeIm) 3, 4-methylpyridine (4-Mepy) 4 and pyridine (py) 5. These complexes have been characterized by elemental analyses, IR, UV–Vis, and 1H NMR spectroscopy. The crystal structures of trans-[CoIII(Me-salpn)(py)2]PF6, 1, and cis-α-[CoIII(Me-salbn)(4-Mepy)2]BPh4 · 4-Mepy, 4, have been determined by X-ray diffraction. Examination of the solution and crystalline structures revealed that the outer coordination sphere of the complexes exerts a noticeable influence on the inner coordination sphere of the Co(III) ion. The electrochemical reduction of these complexes at a glassy carbon electrode in acetonitrile solution indicates that the first reduction process corresponding to CoIII–CoII is electrochemically irreversible, which is accompanied by the dissociation of the axial (R-py)–cobalt bonds. It has also been observed that the Co(III) state is stabilized with increasing the flexibility of the ligand environment.

Graphical abstractCo(III) complexes of two tetradentate Schiff base ligands Me-salpn and Me-salbn, with two R-py ancillary ligands have been synthesized and structurally characterized. The core structures and the electrochemical properties have been studied in view of the chelate ring size. The Me-salpn forms trans-complexes, whereas the Me-salbn adopts a cis-α-configuration about Co. The Me-salpn complexes undergo sovolysis in acetonitrile and reduce at a more positive potential relative to those of Me-salbn due to the steric hindrance from the methylenic group in Me-salpn ligand.Figure optionsDownload full-size imageDownload as PowerPoint slide