Transfer Reagent for Bonding Isomers of Iron Complexes.

The cothermolysis of As4 and [Cp″2 Zr(CO)2 ] (Cp″ = η5 -C5 H3 tBu2 ) results in the formation of [Cp″2 Zr(η1:1 -As4 )] (1) in high yields and the arsenic-rich complex [(Cp″2 Zr)(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 -C5 H2 tBu3 ) 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 [CpBn Fe(μ-Br)]2 (CpBn = η5 -C5 (CH2 (C6 H5 )5 ) leads exclusively to the triple decker complex [(CpBn Fe)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 [(CpBn Fe)2 (μ,η5:5 -As5 )][BF4 ] (5), which is a product expanded by one arsenic atom, instead of the expected complex [(CpBn Fe)2 (μ,η4:4 -cyclo-As4 )]+ .