Chemical and Electrochemical Investigation of the Oxidation of a Highly Reduced Fe₆C Iron Carbide Carbonyl Cluster: A Synthetic Route to Heteroleptic Fe₆C and Fe₅C Clusters

A chemical and electrochemical investigation of the redox chemistry of [Fe₆C(CO)₁₅]^4- is reported and supported by computational studies. Depending on the experimental conditions, the original Fe₆C cage is retained or partially degraded to Fe₅C. Chemical oxidation of [Fe₆C(CO)₁₅]^4- with [Cp₂Fe][PF₆], [C₇H₇][BF₄], or Me₃NO affords the previously reported [Fe₆C(CO)₁₆]^2-, whereas oxidation in the presence of a base (Na₂CO₃ or NaOH) results in the new carbonate-carbide cluster [Fe₆C(CO)₁₄(CO₃)]^4-. Oxidation of [Fe₆C(CO)₁₅]^4- in the presence of a phosphine ligand produces the heteroleptic species [Fe₆C(CO)₁₅(PTA)]^2- and [Fe₅C(CO)₁₃(PPh₃)]^2-. Reaction of [Fe₆C(CO)₁₅]^4- with alkylating or acylating agents (MeI, CF₃SO₃Me, and MeCOCl) affords the acetyl-carbide cluster [Fe₅C(CO)₁₃(COMe)]^3-, with partial oxidative degradation of the original Fe₆C cage. The new clusters have been spectroscopically and structurally characterized. The redox chemistry of [Fe₆C(CO)₁₅]^4- was further investigated by electrochemical and spectroelectrochemical methods. According to computational outcomes, the spectroelectrochemical oxidation of [Fe₆C(CO)₁₅]^4- follows an EEC mechanism, leading to the formation of [Fe₆C(CO)₁₆]^2-. The [Fe₆C(CO)₁₅]^3- intermediate can accumulate and be spectroscopically detected. These new chemical and electrochemical findings have been supported and corroborated by computational methods. DFT calculations suggest an EEC pathway also for the reverse electrochemical process, <i>i.e.</i>, reduction of [Fe₆C(CO)₁₆]^2- to [Fe₆C(CO)₁₅]^4-.