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.

Co(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 as PowerPoint slide