The synthetic fixation of the greenhouse gas carbon dioxide into fuel precursors and commodity chemicals is an important research challenge. When mediated by transition metal complexes, the so-called reductive disproportionation of CO₂ into carbon monoxide and carbonate occurs rapidly via catalytic intermediates that are thought to contain metal-carbon multiple bonds.

Using sterically encumbered ligands, Joshua S. Figueroa, University of California, San Diego, La Jolla, USA, and colleagues have trapped iron intermediates formed during CO₂ reductive disproportionation. The transformation of CO₂ took place when a dianionic iron complex bearing four bulky isocyanide ligands (namely tetraisocyanide dianion Na₂[Fe(CNArMeS₂)₄] ) was used. However, when the complex and CO₂ reacted in the presence of silyl triflates, the process was arrested by silylative esterification of a mono-CO₂ adduct.

An electronic rearrangement of this adduct produced an iron terminal carbyne complex, which was isolated (pictured).In this  unique trigonal-monopyramidal terminal carbynes, the Fe≡C bond is conformationally locked within the equatorial plane. Compounds featuring iron-carbon triple bonds are rare. They may yield interesting chemistry that can be further exploited for synthetic CO₂ fixation.

• Terminal Iron Carbyne Complexes Derived from Arrested CO₂ Reductive Disproportionation,
  Charles C. Mokhtarzadeh, Curtis E. Moore, Arnold L. Rheingold, Joshua S. Figueroa,
  Angew. Chem. Int. Ed. 2017.
  DOI: 10.1002/anie.201705877