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

Abstract

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. Although several inorganic analogues of cyclopentadiene have been reported with group-15 elements, examples featuring the heavier elements Sb and Bi remain scarce. Now, a Bi5 ring has been stabilized between two Nb(cyclopentadienyl) units, forming a triple-decker sandwich structure; the bonding analysis indicates that a Nb–Nb bond goes through the centre of the Bi5 plane.