Bis(1,2,6,7-tetracyano-3,5-dihydro-3,5-diiminopyrrolizinido) complexes of iron and cobalt: properties, crystal structures and interaction with dioxygen

Abstract

The synthesis and characterization of complexes of the ligand 1,2,6,7-tetracyano-3,5-dihydro-3,5-diiminopyrrolizinide, [ML₂]·nA (M = Fe 1 or Co 2; A = crystallization solvent) are described. For A = 1,2-dimethoxyethane (dme), n= 2, the compounds are isomorphous and isostructural as shown by crystal structure determinations. The central metal atom, which lies on a symmetry centre, co-ordinates four imino groups in a square-planar arrangement within the ligand equatorial plane, and completes its octahedral, tetragonally distorted MN^eq₄N^ax₂ co-ordination geometry with two axial cyano groups from two crystallographically equivalent complex molecules. The only significant difference between the two structures is confined to the M–N bond distances [Fe–N_eq 2.090(5); Fe–N_ax 2.295(5); Co–N_eq 2.017(5); Co–N_ax 2.369(5)Å]. Variable-temperature magnetic measurements on these two complexes reveal that they are both high-spin species (S= 2 and , respectively) with antiferromagnetic superexchange interactions, which are stronger when M = Co. The optical spectra of both sets of complexes are very similar to those of corresponding metal phthalocyanines. Also, both can be chemically oxidized by using O₂ as oxidant in MeCN solution to give insoluble polymeric species, corresponding analytically to the formulae [FeL₃(O₂)]·6MeCN and [CoL₂(O₂)]·4MeCN. These complexes are shown to be superoxo adducts in which the dioxygen is centrosymmetrically bridged between the metal atoms in the formal oxidation state Fe^IV and Co^III respectively.