Carbonyl-isocyanide mono-substitution in [Fe2Cp2(CO)4]: A re-visitation

The reactions of [Fe2Cp2(CO)4] with a series of isocyanides, CNR, were conducted in acetonitrile and afforded, after a thermal treatment, the mono-isocyanide derivatives [Fe2Cp2(CO)3(CNR)] [R = 1H-indol-5-yl, 1; CH2P(O)(OEt)2, 2; Cy = C6H11, 3; 4-C6H4OMe, 4; Xyl = 2,6-C6H3Me2, 5; Me, 6; 2-naphthyl, 7; Bn = CH2Ph, 8]. In order to avoid multiple substitution, the diiron reactant was used in a molar excess with respect to the isocyanide (1.6 equivalents; 1.1 for the synthesis of 8). The products were separated from unreacted [Fe2Cp2(CO)4] by chromatography or via reversible protonation, and finally isolated in 50–83% yields. IR and NMR spectroscopy indicate that the isocyanide ligand is bridging coordinated in 2 and 7, terminal in 3 and 5, while in the remaining cases a mixture of terminal- and bridging-CNR isomers is obtained. The molecular structure of 5 was ascertained by X-ray diffraction. In general, the coordination mode of the isocyanide is scarcely influenced by the environment (solvents with different polarities, solid state). Sluggish partial isocyanide migration from terminal to bridging position was recognized for 3 and 5 upon heating in refluxing toluene, a process which was reproduced by DFT calculations.