[Ru₂Bi₁₄Br₄](AlCl₄)₄ by Mobilization and Reorganization of Complex Clusters in Ionic Liquids

Abstract

Two polymorphs of the new cluster compound [Ru₂Bi₁₄Br₄](AlCl₄)₄ have been synthesized from Bi₂₄Ru₃Br₂₀ in the Lewis acidic ionic liquid [BMIM]Cl/AlCl₃ ([BMIM]⁺: 1-n-butyl-3-methylimidazolium) at 140 °C. A large fragment of the precursor’s structure, namely the [(Bi₈)Ru(Bi₄Br₄)Ru(Bi₅)]⁵⁺ cluster, dissolved as a whole and transformed into a closely related symmetrical [(Bi₅)Ru(Bi₄Br₄)Ru(Bi₅)]⁴⁺ cluster through structural conversion of a coordinating Bi₈²⁺ to a Bi₅⁺ polycation, while the remainder was left intact. Both modifications have monoclinic unit cells that comprise two formula units (α form: P2₁/n, a=982.8(2), b=1793.2(4), c=1472.0(3) pm, β=109.05(3)°; β form: P2₁/n, a=1163.8(2), b=1442.7(3), c=1500.7(3), β=97.73(3)°). The [Ru₂Bi₁₄Br₄]⁴⁺ cluster can be regarded as a binuclear inorganic complex of two ruthenium(I) cations that are coordinated by terminal Bi₅⁺ square pyramids and a central Bi₄Br₄ ring. The presence of a covalent RuRu bond was established by molecular quantum chemical calculations utilizing real-space bonding indicator ELI-D. Structural similarity of the new and parent cluster suggests a structural reorganization or an exchange of the bismuth polycations as mechanisms of cluster formation. In this top-down approach a complex-structured unit formed at high temperature was made available for low-temperature use.