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)]+.