Synthesis of triple-decker sandwich compounds featuring a M–M bond through cyclo-Bi5 and cyclo-Sb5 rings

The cyclopentadienyl anion is a π-aromatic five-membered ring ligand that is widely used in organometallic chemistry. By replacing the CH groups in cyclopentadiene with isoelectronic group-15 elements, an inorganic analogue can be obtained. In this line, Pn₅ (Pn = P, Sb) rings have been stabilized in a triple-decker sandwich structure, prepared via high-temperature reactions, and an example of a Bi₅^- ring stabilized in a cobalt-based inverse-sandwich-type complex has been reported. Here we report the synthesis and structural characterization of two complexes, [Cp-V(cyclo-Sb₅)V-Cp]^2- and [Cp-Nb(cyclo-Bi₅)Nb-Cp]^2-, which are stabilized by [K(18-crown-6)]⁺ or [K(2.2.2-crypt)]⁺ cations at room temperature under mild conditions. Our bonding analysis through various quantum-chemical methods reveals that V‒V and Nb‒Nb bonds pass through the centre of the E₅ rings (E = Sb, Nb). In contrast to free cyclo-E₅ (E = Sb, Bi) the cyclo-E₅ moieties between Cp-E units do not possess any aromatic character because the M‒M (M = V, Nb) bond passes through the centre of the ring.