Synthesis, properties and crystal structures of iron-(II) and -(III) complexes of 2,2′:6′,2″:6″,2‴-quaterpyridine

Abstract

Several iron complexes of 2,2′:6′,2″:6″,2‴-quaterpyridine (L) in di- and tri-valent oxidation states have been prepared, and the structures of [FeL(OH₂)₂][ClO₄]₂1, [{FeL(ClO₄)}₂(µ-O)]·8.5H₂O 2·8.5H₂O and [{FeL(Cl)}₂(µ-O)][ClO₄]₂·2H₂O 3·2H₂O, have been determined by X-ray crystal analyses. In each complex, the quaterpyridine ligand is planar. In 2, the Fe^II is seven-co-ordinate with the [ClO₄]^– acting as a bidentate ligand with Fe–O–Fe 155.2(4)°. In 3, the Fe–O–Fe unit is linear and the Fe–O distance of 1.779(1)Å is considerably shorter than the Fe–O(OH₂) distance of 2.141(5)Å in 1. The results of molecular orbitals calculations revealed that the unoccupied molecular orbitals of 2 and 3 mainly comprise the π* orbital of L. In 2, bending of the Fe–O–Fe unit gives rise to a decrease in the antibonding interaction between Fe(d_yz) and O(P_y) orbitals. Complex 3 is less stable in the spin-paired (S= 0) than in open-shell (S= 1) state by 49.3 kcal mol^–1. This coincides with magnetic susceptibility measurements which show that 3 is paramagnetic with µ_eff 1.75 µ_B per iron atom.