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 L^Ph), 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 FeCoL ^Ph is more precisely described as (Fe_0.94(1)Co _0.06(1))(Co_0.95(1)Fe_0.05(1))L^Ph. 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.