Enantiopure Tetranuclear Iron(III) Complexes Using Chiral Reduced Schiff Base Ligands: Synthesis, Structure, Spectroscopy, Magnetic Properties, and DFT Studies

Four new tetranuclear iron(III) complexes of formula [{Fe(L)2}3Fe], 1−4, have been prepared by reacting [Fe(ClO4)3]·6H2O with H2L in methanol. Here, L2− is the deprotonated form of N-(2-hyrdoxybenzyl)-l-valinol (H2L1), N-(2-hyrdoxybenzyl)-l-leucinol (H2L2), N-(5-chloro-2-hyrdoxybenzyl)-l-leucinol (H2L3), and N-(2-hyrdoxybenzyl)-l-phenylalaninol (H2L4). The complexes are prepared in an enantiomeric pure form. The complexes have been characterized with the help of IR, UV−vis, circular dichroism (CD), 1H, and elemental analyses. The complex [{Fe(L2)2}3Fe]·CH3OH·2H2O, 2·CH3OH·2H2O, crystallizes in enantiomeric pure form containing a propeller-like Fe4O6 core. 1H and CD spectral studies of the four species are consistent with the structural similarities of the complexes in solution. Variable-temperature magnetic susceptibility of one case shows an intramolecular antiferromagnetic coupling between the Fe(III) ions. Magnetic measurements are in accord with the S = 5 ground state and suggest single molecular magnet behavior. The magnetic exchange coupling constant between the iron centers within the molecule is interpreted using broken-symmetry density functional theory calculation.