Arsenic-Rich Polyarsenides Stabilized by Cp*Fe Fragments

The redox chemistry of [Cp*Fe(η⁵ -As₅ )] (1, Cp*=η⁵ -C₅ Me₅ ) has been investigated by cyclic voltammetry, revealing a redox behavior similar to that of its lighter congener [Cp*Fe(η⁵ -P₅ )]. However, the subsequent chemical reduction of 1 by KH led to the formation of a mixture of novel As_n scaffolds with n up to 18 that are stabilized only by [Cp*Fe] fragments. These include the arsenic-poor triple-decker complex [K(dme)₂ ][{Cp*Fe(μ,η^2:2 -As₂ )}₂ ] (2) and the arsenic-rich complexes [K(dme)₃ ]₂ [(Cp*Fe)₂ (μ,η^4:4 -As₁₀ )] (3), [K(dme)₂ ]₂ [(Cp*Fe)₂ (μ,η^2:2:2:2 -As₁₄ )] (4), and [K(dme)₃ ]₂ [(Cp*Fe)₄ (μ₄ ,η^{4:3:3:2:2:1:1} -As₁₈ )] (5). Compound 4 and the polyarsenide complex 5 are the largest anionic As_n ligand complexes reported thus far. Complexes 2-5 were characterized by single-crystal X-ray diffraction, ¹ H NMR spectroscopy, EPR spectroscopy (2), and mass spectrometry. Furthermore, DFT calculations showed that the intermediate [Cp*Fe(η⁵ -As₅ )]^- , which is presumably formed first, undergoes fast dimerization to the dianion [(Cp*Fe)₂ (μ,η^4:4 -As₁₀ )]^2- .