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

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.