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-[Co III(Me-salpn)(py) 2]PF 6 and cis- α-[Co III(Me-salbn)(4-Mepy) 2]BPh 4 · 4-Mepy

Abstract

The synthesis and characterization of a series of cobalt(III) complexes of the general type [Co(N 2O 2)(L 2)] + are described. The N 2O 2 Schiff base ligands used are Me-salpn (H 2Me-salpn = N, N′-bis(methylsalicylidene)-1,3-propylenediamine) ( 1– 3) and Me-salbn (H 2Me-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-[Co III(Me-salpn)(py) 2]PF 6, 1, and cis- α-[Co III(Me-salbn)(4-Mepy) 2]BPh 4 · 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 Co III–Co II 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.