On the nature of K3Fe(CN)6 and K4Fe(CN)6 in aqueous environment: An ab-initio molecular dynamics study

The electron transfer reaction is pivotal in several branches of science, ranging from electrochemistry to catalysis. A suitable model system to inspect to the nature of reactants and products during a charge transfer reaction is made by a redox couple dissolved in a solvent. A typical example is made by [Fe(CN)<inf>6</inf>]^3-/[Fe(CN)<inf>6</inf>]^4- in water. Recently, it has been proposed that the charge transfer process is associated with ion-pairing, where counter ions like K⁺ take part in the coordination. In this work we performed density functional theory calculations in conjunction with ab-initio molecular dynamics to provide atomistic insights to the structure of [Fe(CN)<inf>6</inf>]^3- and [Fe(CN)<inf>6</inf>]^4- in water, and their interaction with K⁺. Our results show that [Fe(CN)<inf>6</inf>]^3- and [Fe(CN)<inf>6</inf>]^4- display a distinct interaction network with water molecules. The neutrality of K<inf>3</inf>Fe(CN)<inf>6</inf> and K<inf>4</inf>Fe(CN)<inf>6</inf> is retrieved by considering a sphere with a radius of about 10 Å, but not more than two K⁺ are located closer than 6 Å from the center of the complex. Our estimates indicate that when [Fe(CN)<inf>6</inf>]^4- is converted to [Fe(CN)<inf>6</inf>]^3-, not only the solvent reorganizes, but also K⁺ is reorganized and moves away from the complex by about 0.6 Å. The results may be of help for the fundamental understanding of the nature of this redox couple in water in the presence of supporting electrolyte.