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 FeII 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)A is considerably shorter than the Fe–O(OH2) distance of 2.141(5)A 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(dyz) and O(Py) 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 µeff 1.75 µB per iron atom.