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 [M-2](3+) cores. The trigonal dicobalt tris(diphenylformamidinate) compound, Co-2(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 [Co-2](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 angstrom for Co-Co and 2.18 angstrom 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))L-Ph. The iron-cobalt heterobimetallic has been further characterized by Mossbauer spectroscopy. Its isomer shift of 0.65 mm/s and quadrupole splitting of 0.64 mm/s are comparable to the related diiron complex, Fe-2(DPhF)(3). On the basis of spectroscopic data and theoretical calculations, it is proposed that the formal [M-2](3+) cores are fully delocalized.