The Relationship between the Structure and Magnetic Properties of Bioinspired Iron(ii/iii) Complexes with Schiff‐Base‐Like Chelate Ligands, Part I: Complexes with Dianionic [N4] Macrocycles

Abstract

AbstractThe molecular structures and the spin ground‐state of the iron(ii/iii) complexes Fe1, Fe2 and Fe3, which contain dianionic macrocyclic [N4] ligands derived from substituted acetylacetaldehydes and 1,2‐diamines, have been investigated by X‐ray analysis and temperature‐dependent susceptibility measurements, respectively. The iron(II) complexes FeII1 (which is dimeric by intermolecular coordination of one peripheral carbonyl group), FeII1MeOH, (FeIIL)2dabco (L = 1, 2, 3; dabco = 1,4‐diazabicyclo[2.2.2]octane) and the iron(III) complex FeIII1Cl are pentacoordinate and have an intermediate‐spin ground‐state at room temperature (S = 1 for FeII; S = 3/2 for FeIII). The intermediate‐spin state was confirmed by DFT‐MO calculations for FeII1MeOH and FeIII1Cl and is also in agreement with the Mössbauer data of FeII1 and ESR measurements of FeIII1Cl. The iron centre in FeII2 is nearly square‐planar and shows a strong decrease of the magnetic moment below T ≈ 250 K. This is probably due to an S = 0 to S = 1 spin‐crossover or, more likely, to intermolecular antiferromagnetic interactions (shortest Fe–Fe distance of 3.4 Å). A new pair of octahedral iron(ii/iii) complexes with an N‐heterocyclic axial ligand, FeII3(Py)2/[FeIII3(Py)2]PF6, could be crystallised. In contrast to the previously described pairs FeII1(Him)2/[FeIII1(Him)2]PF6 and FeII2Py2/[FeIII2Py2]ClO4, the orientation of the planes of the axial ligands is nearly independent of the oxidation step of the central atom. Octahedral derivatives with biologically relevant anions as axial ligands have been crystallised for the first time, namely [BzEt3N][FeIII1(NCS)2], [Et4N][FeIII2(CN)2](H2O)0.5, Na[FeIII2(NO2)2](MeOH)2(H2O)0.5 and [FeIII2(NO2)OH2](MeOH). The latter shows a magnetic moment of μeff ≈ 2.85 μB at room temperature, which decreases to 1.85 μB at lower temperature. This indicates an incomplete S = 1/2 to S = 3/2 spin‐crossover, which is probably due to the presence of H2O as a weak axial ligand. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)