Redox Pairs of Diiron and Iron-Cobalt Complexes with High-Spin Ground States.

Abstract

A series of iron and iron-cobalt bimetallic complexes were isolated: LFe2Cl (1), LFe2 (2), Li(THF)3[LFe2Cl](Li(THF)3[2-Cl]), LFeCoCl (3), and LFeCo (4), where L is a trianionic tris(phosphineamido)amine ligand. As elucidated by single-crystal X-ray diffraction studies and quantum-chemical calculations, the FeIIFeII and FeIICoII complexes, 1 and 3, respectively, have weak metal-metal interactions (the metal-metal distances are 2.63 and 2.59 Å, respectively) with a partial bond order of 0.5. The formally mixed-valent complexes, FeIIFeI (3) and FeIICoI (4), have short metal-metal bonds (2.32 and 2.26 Å, respectively) with a formal bond order of 1.5. On the basis of magnetic susceptibility measurements, complexes 1-4 are all paramagnetic with high-spin ground states, S = 3-4, which are proposed to arise from ferromagnetic coupling of the two metals' spins through a direct exchange mechanism. Zero- and applied-field Mössbauer spectra corroborate the presence of distinct oxidation and spin states for the iron sites. The reduction potentials of 1 and 3 are -1.48 and -1.60 V (vs Fc+/Fc), respectively. Other characterization data are also reported for this series of complexes, electronic absorption spectra and anomalous X-ray scattering data.