Releasing Cyanide from Ferrocyanide through Carbon Monoxide Ligand Exchange in Alkaline Aqueous Environments

Abstract

Cyanide is a critical reagent in prebiotic chemistry to promote the synthesis of precursors to biomolecules. Due to its strong nucleophilic properties, cyanide is integral to fundamental prebiotic pathways such as Strecker synthesis of amino acids, Oro’s synthesis of adenine, and the synthesis of pentose-like sugars and plays a key role in many prebiotic chemical networks. In aqueous systems with free ferrous iron, cyanide is strongly complexed by ferrous iron, forming stable ferrocyanide ([Fe(CN)₆]^4–) complexes that limit aqueous HCN pools. Here, we show that dissolved carbon monoxide, another prebiotically relevant molecule often present in environments in contact with Earth’s early atmosphere, can drive ligand-exchange reactions with ferrocyanide. Utilizing alkaline and hyperalkaline fluid compositions to simulate waters that have reacted with mafic and ultramafic rocks on early Earth, and moderate temperatures, we demonstrate that carbon monoxide is able to replace one or more cyanide ligands in ferrocyanide, consequently producing free cyanide and ferrocyanocarbonyl complexes. We also demonstrate that this [CN^–] can further react with prebiotic reagents, such as polysulfide, producing thiocyanate. Thus, this CO–CN ligand-exchange mechanism with ferrocyanide provides a plausible source of free cyanide in early Earth conditions, without a temperature extreme, to drive prebiotic reaction networks.