SYNTHESIS, PROPERTIES AND CRYSTAL-STRUCTURES OF IRON-(II) AND IRON-(III) COMPLEXES OF 2,2'/6',2''/6'',2'''-QUATERPYRIDINE

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)(mu-O)]. 8.5H(2)O 2 . 8.5H(2)O and [{FeL(Cl)}(2)(mu-O)][ClO4](2) . 2H(2)O 3 . 2H(2)O, 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)degrees. In 3, the Fe-O-Fe unit is linear and the Fe-O distance of 1.779(1)Angstrom is considerably shorter than the Fe-O(OH2) distance of 2.141(5) Angstrom in 1. The results of molecular orbital calculations revealed that the unoccupied molecular orbitals of 2 and 3 mainly comprise the pi* 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 mu(eff) 1.75 mu(B) per iron atom.