Ligand Effects on the Reactivity of [CoX]⁺ (X = CN, F, Cl, Br, O, OH) Towards CO₂: Gas-Phase Generation of the Elusive Cyanoformate by [Co(CN)]⁺ and [Fe(CN)]⁺

The thermal reactions of [CoX]⁺ (X = CN, F, Cl, Br, O, OH) with carbon dioxide have been investigated experimentally and theoretically by using electrospray ionization mass spectrometry (ESI-MS) and density functional theory. Surprisingly, in contrast to the complete inertness of [CoX]⁺ (X = F, Cl, Br, O, OH) toward carbon dioxide, [Co(CN)]⁺ activates carbon dioxide to form the elusive [NCCO₂Co]⁺ ion in the gas phase. Mechanistic investigation into this ligand-controlled CO₂ activation via C^_C bond formation, mediated by a first-row late transition-metal complex, reveals that the inertness of [CoX]⁺ (X = F, Cl, Br, O, OH) is due to kinetic barriers located above the entrance asymptote. The exception is the [Co(CN)]⁺/CO₂ couple, for which the thermal C–C bond formation is both thermochemically and kinetically accessible. Interestingly, a cyanoformate ligand is most likely also formed in the reaction of [Fe(CN)]⁺ with CO₂; cyanoformate formation had been suggested earlier as a protective mechanism to prevent cyanide complexation to the iron-containing active site of the enzyme ACC oxidase (Murphy et al., in Science 344:75–78, 2014).