Mixed-Valent Dicobalt and Iron–Cobalt Complexes with High-Spin Configurations and Short Metal–Metal Bonds

Cobalt–cobalt
and iron–cobalt bonds are investigated in coordination complexes
with formally mixed-valent [M2]3+ cores. The
trigonal dicobalt tris­(diphenylformamidinate) compound, Co2(DPhF)3, which was previously reported by Cotton, Murillo,
and co-workers (Inorg. Chim. Acta 1996, 249, 9), is shown to have an energetically isolated,
high-spin sextet ground-state by magnetic susceptibility and electron
paramagnetic resonance (EPR) spectroscopy. A new tris­(amidinato)­amine
ligand platform is introduced. By tethering three amidinate donors
to an apical amine, this platform offers two distinct metal-binding
sites. Using the phenyl-substituted variant (abbreviated as LPh), the isolation of a dicobalt homobimetallic and an iron–cobalt
heterobimetallic are demonstrated. The new [Co2]3+ and [FeCo]3+ cores have high-spin sextet and septet ground
states, respectively. Their solid-state structures reveal short metal–metal
bond distances of 2.29 Å for Co–Co and 2.18 Å for
Fe–Co; the latter is the shortest distance for an iron–cobalt
bond to date. To assign the positions of iron and cobalt atoms as
well as to determine if Fe/Co mixing is occurring, X-ray anomalous
scattering experiments were performed, spanning the Fe and Co absorption
energies. These studies show only a minor amount of metal-site mixing
in this complex, and that FeCoLPh is more precisely described
as (Fe0.94(1)Co0.06(1))­(Co0.95(1)Fe0.05(1))­LPh. The iron–cobalt heterobimetallic
has been further characterized by Mössbauer spectroscopy. Its
isomer shift of 0.65 mm/s and quadrupole splitting of 0.64 mm/s are
comparable to the related diiron complex, Fe2(DPhF)3. On the basis of spectroscopic data and theoretical calculations,
it is proposed that the formal [M2]3+ cores
are fully delocalized.