Transfer Reagent for Bonding Isomers of Iron Complexes.

The cothermolysis of As4 and [Cp″2Zr(CO)2] (Cp″ = η5-C5H3tBu2) results in the formation of [Cp″2Zr(η1:1-As4)] (1) in high yields and the arsenic-rich complex [(Cp″2Zr)(Cp″Zr)(μ,η2:2:1-As5)] (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)]2 (Cp‴ = η5-C5H2tBu3) yields the unprecedented bond isomeric complexes [(Cp‴Fe)2(μ,η4:4-As4)] (3a) and [(Cp‴Fe)2(μ,η4:4-cyclo-As4)] (3b). In contrast, the analogous reaction with the CpBn derivative [CpBnFe(μ-Br)]2 (CpBn = η5-C5(CH2(C6H5)5) leads exclusively to the triple decker complex [(CpBnFe)2(μ,η4:4-As4)] (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 AgBF4 affords [(CpBnFe)2(μ,η5:5-As5)][BF4] (5), which is a product expanded by one arsenic atom, instead of the expected complex [(CpBnFe)2(μ,η4:4-cyclo-As4)]+.