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]2 1, [{FeL(ClO4)}2(μ-O)]·8.5H2O 2.8.5H2O and [{FeL(Cl)}2(μ-O)][ClO4]2·2H 2O 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) 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(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.