A new family of low-coordinate Co complexes supported by three redox-noninnocent tridentate [OCO] pincer-type bis(phenolate) N-heterocyclic carbene (NHC) ligands are described. Combined experimental and computational data suggest that the charge-neutral four-coordinate complexes are best formulated as Co(II) centers bound to closed-shell [OCO] dianions, of the general formula [(OCO)CoL] (where L is a solvent-derived MeCN or THF). Cyclic voltammograms of the [(OCO)CoL] complexes reveal three oxidations accessible at potentials below 1.2 V vs Fc/Fc, corresponding to generation of formally Co(V) species, but the true physical/spectroscopic oxidation states are much lower. Chemical oxidations afford the mono- and dications of the imidazoline NHC-derived complex, which were examined by computational and magnetic and spectroscopic methods, including single-crystal X-ray diffraction. The metal and ligand oxidation states of the monocationic complex are ambiguous; data are consistent with formulation as either [(OCO)Co(THF)] containing a closed-shell [OCO] diphenolate ligand bound to a S = 1 Co(III) center, or [(OCO)Co(THF)] with a low-spin Co(II) ion ferromagnetically coupled to monoanionic [OCO] containing a single unpaired electron distributed across the [OCO] framework. The dication is best described as [(OCO)Co(THF)], with a single unpaired electron localized on the d Co(II) center and a doubly oxidized, charge-neutral, closed-shell OCO ligand. The combined data provide for the first time unequivocal and structural evidence for [OCO] ligand redox activity. Notably, varying the degree of unsaturation in the NHC backbone shifts the ligand-based oxidation potentials by up to 400 mV. The possible chemical origins of this unexpected shift, along with the potential utility of the [OCO] pincer ligands for base-metal-mediated organometallic coupling catalysis, are discussed.

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The unprecedented interdigitated and layered structures of 1,1′,3,3′-tetra­(2-methyl-2-nonyl)­ferrocene (DEC) and the oxoanion ferrocenium salts DEC+NO3 –, DEC+ClO4 –, and DEC+ReO4 – were determined by single-crystal X-ray diffraction. The four structures are similar except that the three DEC+ salts have layers of XO n – oxoanions stuffed between the layers of interdigitated ferrocenium ions. The perpendicular distances between layers of Fe atoms are 8.530, 9.108, 9.009, and 9.158 Å for DEC, DEC+NO3 –, DEC+ClO4 –, and DEC+ReO4 –, respectively. The structures also contain layers of rigorously coplanar Fe and X atoms that are tilted 65.4, 75.9, 61.9, and 61.1° from the aforementioned layers of Fe atoms for DEC, DEC+NO3 –, DEC+ClO4 –, and DEC+ReO4 –, respectively. The local environments of the XO n – oxoanions consist of networks of C–H···O hydrogen bonds, and the structures exhibit channels through which these anions could diffuse. Facile diffusion of these anions in thin films of DEC+XO n –, with structures that appear to resemble the crystal structures, has been demonstrated.