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. 2025 May 19;64(19):9744-9757.
doi: 10.1021/acs.inorgchem.5c01014. Epub 2025 May 6.

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

Tiziana Funaioli  1 , Cristiana Cesari  2 , Beatrice Berti  2 , Marco Bortoluzzi  3 , Cristina Femoni  2 , Francesca Forti  2 , Maria Carmela Iapalucci  2 , Giorgia Scorzoni  2 , Stefano Zacchini  2
Affiliations

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

Tiziana Funaioli et al. Inorg Chem. .

Abstract

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.e., reduction of [Fe6C(CO)16]2- to [Fe6C(CO)15]4-.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Oxidation Reactions of [Fe6C(CO)15]4– in MeCN, (a) without and (b) with a Base
All of the species have been structurally characterized by SC-XRD. CO ligands have been omitted for clarity. Both [NEt4]+ and [NMe3CH2Ph]+ can be used as counterions resulting in similar reactions and separation procedures. Oxidants employed: Me3NO, [Cp2Fe][PF6] and [C7H7][BF4].
Figure 1
Figure 1
Molecular structure of [Fe6C(CO)14(CO3)]4– as found in [NEt4]3[H3O][Fe6C(CO)14(CO3)]. The H-bond with the [H3O]+ cation is represented as a fragmented line (orange, Fe; red, O; gray, C; white H). Thermal ellipsoids are at the 30% probability level.
Scheme 2
Scheme 2. Oxidation Reactions of [Fe6C(CO)15]4– in MeCN in the Presence of a Phosphine Ligand
All of the species have been structurally characterized by SC-XRD. CO ligands have been omitted for clarity. [NEt4]+ has been used as counter ion.
Figure 2
Figure 2
Molecular structure of [Fe6C(CO)15(PTA)]2– as found in [NEt4]2[Fe6C(CO)15(PTA)] (orange, Fe; yellow, P; blue, N; red, O; gray, C; white, H). Thermal ellipsoids are at the 30% probability level.
Figure 3
Figure 3
Molecular structure of [Fe5C(CO)13(PPh3)]2– as found in [NEt4]2[Fe5C(CO)13(PPh3)] (orange, Fe; yellow, P; red, O; gray, C; white, H). Thermal ellipsoids are at the 30% probability level.
Scheme 3
Scheme 3. Reactions of [Fe6C(CO)15]4– in MeCN with Alkylating or Acylating Reactants, (a) without and (b) with a Base
All of the species have been structurally characterized by SC-XRD. CO ligands have been omitted for clarity. Both [NEt4]+ and [NMe3CH2Ph]+ can be used as counterions resulting in similar reactions and separation procedures.
Figure 4
Figure 4
Molecular structure of [Fe5C(CO)13(COMe)]3– as found in [NMe3CH2Ph]3[Fe5C(CO)13(COMe)] (orange, Fe; red, O; gray, C; white, H). Thermal ellipsoids are at the 30% probability level.
Figure 5
Figure 5
CV response at a Pt electrode in MeCN solution of (a) [Fe6C(CO)15]4– between −1.0 and +0.5 V; scan rate: 0.1 V s–1; (b) [Fe6C(CO)16]2– between −2.0 and 0.0 V, red line; between −1.65 and +0.5 V, black line; scan rate: 0.1 V s–1. Starred peaks are due to impurities. Inset: in blue, CV between −1.0 and 0.0 V (ordinate scale magnified). [NnBu4][PF6] (0.1 mol dm–3) supporting electrolyte.
Figure 6
Figure 6
IR spectra of a MeCN solution of [Fe6C(CO)15]4– recorded in an OTTLE cell during the progressive increase of the potential from −0.2 to +1.0 V (vs Ag pseudo reference electrode, scan rate 1 mV s–1). [NnBu4][PF6] (0.1 mol dm–3) was used as the supporting electrolyte. The absorptions of the solvent and supporting electrolyte have been subtracted. The two colors (red and black) of the arrows indicate which bands change simultaneously.
Figure 7
Figure 7
IR spectra of a MeCN solution of [Fe6C(CO)16]2– recorded in an OTTLE cell during (a) the progressive decrease of the potential from −0.6 to −1.2 V (vs Ag pseudo reference electrode, scan rate 1 mV s–1) and (b) the oxidation back-scan from −1.2 to 0.0 V (vs Ag pseudo reference electrode). [NnBu4][PF6] (0.1 mol dm–3) as the supporting electrolyte. The absorptions of the solvent and supporting electrolyte have been subtracted.
Scheme 4
Scheme 4. Proposed Mechanisms for the Electrochemical Oxidation of [Fe6C(CO)15]4–
See this image and copyright information in PMC

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