Iron, cobalt, nickel and ruthenium complexes of 2,6-bis(3,4-dihydro-2H-pyrrol-5-yl)pyridine, a pybox analogue

The coordination of 2,6-bis(3,4-dihydro-2H-pyrrol-5-yl)pyridine (L) to nickel, ruthenium, iron, and cobalt was studied. The reaction with nickel salts gives the nickel (II) complex [Ni(L)2]2+ (1). The reaction of ruthenium(III) chloride gives only the ruthenium(II) complex [Ru(L)2]2+ (2). In the case of iron, both [Fe(L)2]2+ (3) and [Fe(L)2]3+ (4) have been synthesised and characterised. In the case of cobalt, the cobalt(III) complex [Co(L)2]3+ (5) is obtained, even if a cobalt(II) salt is used. However, the cobalt(II) [Co(L)2]2+ (6) can be obtained under nitrogen. Molecular structures of 1–5 complexes have been determined by X-ray analysis of the corresponding perchlorate salts; the structure of 3 has been solved for both the perchlorate and the tetrachloroferrate(III) salts. All complexes show an octahedral coordination geometry with meridional arrangement of the two tridentate ligands. The electrochemical behaviour of 2, 3 and 6 has been studied by cyclic voltammetry. Quasi reversible electron transfer is observed for the redox pairs FeIII/FeII and CoII/CoI. The reaction shows somewhat weaker reversibility for CoIII/CoII, whereas the reaction of RuIII/RuII is not reversible. The measurements suggest 2,6-bis(3,4-dihydro-2H-pyrrol-5-yl)pyridine (L) to have a higher stabilisation of low-valent oxidation states of iron and cobalt than terpyridine.