Synthesis and Structure of Cubane-Type Tetrairon Clusters Possessing μ3-Isonitrile Ligands. Reductive Coupling of Two Isonitriles on Redox-Responsive Tetrairon Reaction Sites

A 3:1 molar ratio mixture of [(η5-C5H4Me)4Fe4(HCCBr)2]­(PF6) (1a) and [(η5-C5H4Me)4Fe4(HCCH)­(BrCCBr)]­(PF6) (1b) was converted to [(η5-C5H4Me)4Fe4(μ3-CH)2(μ3-CNPh)2]­(PF6)2 (2) upon treatment with aniline, followed by NiPr2Et and finally [Cp2Fe]­(PF6). The X-ray diffraction analysis revealed that 2 can be described as a cubane-type tetrairon cluster possessing two μ3-CH and two μ3-isonitrile ligands. Treatment of 2 with 2.5 equiv of [Cp2Co] gave the neutral form 3, formulated as [(η5-C5H4Me)4Fe4(HCCH)2(μ3-CNPh)2]. The redox reactions were chemically reversible; treatment of 3 with [Cp2Fe]­(PF6) reproduced 2 quantitatively. The structure of 3 was determined by X-ray diffraction analysis. The molecule exhibits a butterfly geometry resulting from the scission of one of the iron–iron bonds of the tetrahedron in 2. In accordance with the conversion of the core structure from tetrahedron to butterfly, the coupling of two μ3-CH ligands occurs to form an acetylene ligand. Further treatment of 3 with LiAlH4 followed by air-oxidation resulted in reductive coupling of two isonitrile ligands to give a bis­(acetylene) cluster, [(η5-C5H4Me)4Fe4(HCCH)2]­(PF6) (6).