Low-spin iron(III) Schiff-base complexes with symmetric hexadentate ligands: Synthesis, crystal structure, spectroscopic and magnetic properties

Abstract

The synthesis and characterization is reported of four new iron(III) complexes, [Fe(L 1)](ClO 4) ( 1), [Fe(L 1)](BF 4) ( 2), [Fe(L 2)](ClO 4) ( 3), [Fe(L 2)](BF 4) ( 4) with the N 4O 2-donor hexadentate Schiff-base ligands (L 1 = N, N′, N″, N‴-1,5,8,12-tetraazadodecane-bis(salicylaldiminato), L 2 = N, N′, N″, N‴-1,5,8,12-tetraazadodecane-bis(3-methoxysalicylaldiminato)) obtained from salicylaldehyde, or a 3-methoxysalicylaldehyde. The iron(III) complexes have been prepared with different counter ions, i.e., ClO 4 - and BF 4 - , and characterized by different experimental techniques: X-ray crystallography, IR, ligand field spectroscopy, EPR, cyclic voltammetry, and magnetic susceptibility measurements. The molecular structure of the complexes is built up by complex cations [Fe(L 1)] + or [Fe(L 2)] + and counter anions ClO 4 - or BF 4 - , and has been proven by X-ray diffraction on single crystals for 1, 3 and 4. In all cases, the coordination geometry around the iron(III) ion is distorted octahedral with the iron to phenoxo oxygen bonds being trans to each other. In the crystal lattice, the mononuclear entities are organized into infinite 1D chains due to the hydrogen-bonding interactions involving the secondary amine functions and one oxygen/fluorine atom of a counter anion. In acetonitrile, all complexes exhibit a quasi-reversible one-electron reduction process Fe(III) → Fe(II), with half-wave potential E 1/2 = −0.5 V. The EPR spectra of 1– 4 clearly indicate a low-spin d 5 ( S = 1/2) electronic configuration with a rhombic symmetry. In all cases, the temperature dependent magnetic properties (5–300 K) agree with the presence of one unpaired electron ( S = 1/2) and with moderate orbital contribution due to the orbitally degenerate electronic ground state ( 2T 2g) of the octahedral iron(III) complexes.