Isocyano analogues of [Co(CO)₄]ⁿ: AA tetraisocyanide of cobalt isolated in three states of charge

The encumbering m-terphenyl isocyanide ligand, CNAr^Mes2 (Mes = 2,4,6-Me³C⁶H²), is used to stabilize homoleptic tetraisocyanide complexes of cobalt in the 1?, 0, and 1+ charge state. Most importantly, these complexes serve as isolable analogues of the binary carbonyl complexes [Co(CO)⁴]^?, Co(CO)⁴, and [Co(CO) ⁴]⁺. Sodium amalgam reduction of CoCl² in the presence of CNAr^Mes2 provides the salt Na[Co(CNAr^Mes2) ⁴], which can be oxidized with 1 equiv of ferrocenium triflate (FcOTf) to the neutral complex, Co(CNAr^Mes2)⁴. X-ray diffraction, FTIR spectroscopy, and low-temperature EPR spectroscopy reveal that Co(CNAr^Mes2)⁴ modulates between D^2d- and C^2v-symmetric forms. DFT calculations are used to rationalize this structural modulation in terms of thermal access to low-energy b ²-symmetric C?Co?C bending modes. Treatment of Na[Co(CNAr ^Mes2)⁴] with 2 equiv of FcOTf, followed by addition of Na[BAr^F4], provides the salt [Co(CNAr^Mes2) ⁴]BAr^F4, which contains a diamagnetic, square planar monovalent cobalt center. The molecular and electronic structures of [Co(CNAr^Mes2)⁴]BAr^F4 are compared and contrasted to the reported properties of the carbonyl cation, [Co(CO) ⁴]⁺.