Transfer Reagent for Bonding Isomers of Iron Complexes.

The cothermolysis of As and [Cp″Zr(CO)] (Cp″ = η-CHtBu) results in the formation of [Cp″Zr(η-As)] (1) in high yields and the arsenic-rich complex [(Cp″Zr)(Cp″Zr)(μ,η-As)] (2) as a minor product. In contrast to yellow arsenic, 1 is a light-stable, weighable and storable arsenic source for subsequent reactions. The transfer reaction of 1 with [Cp‴Fe(μ-Br)] (Cp‴ = η-CHtBu) yields the unprecedented bond isomeric complexes [(Cp‴Fe)(μ,η-As)] (3a) and [(Cp‴Fe)(μ,η-cyclo-As)] (3b). In contrast, the analogous reaction with the Cp derivative [CpFe(μ-Br)] (Cp = η-C(CH(CH)) leads exclusively to the triple decker complex [(CpFe)(μ,η-As)] (4) possessing the tetraarsabutadiene-type ligand analogous to 3a. To elucidate the stability of the bonding isomers 3a and 3b, DFT calculations were performed. The oxidation of 4 with AgBF affords [(CpFe)(μ,η-As)][BF] (5), which is a product expanded by one arsenic atom, instead of the expected complex [(CpFe)(μ,η-cyclo-As)].