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Reversible Bromide−Chloride Exchange in Zirconium Cluster Compounds:  Synthesis by Means of Ionic Liquids, Structures, and Some Spectral Data of (EMIm)4[(Zr6Z)Br18] Cluster Phases (Z = Be, Fe)

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posted on 23.07.2007, 00:00 by Johannes B. Willems, Henning W. Rohm, Christine Geers, Martin Köckerling
The solid-state precursor cluster chlorides Na4[(Zr6Be)Cl16] and K[(Zr6Fe)Cl15] readily dissolve in Lewis-basic ionic liquids consisting of mixtures of EMIm-Br and AlBr3 (EMIm:  1-ethyl-3-methyl-imidazolium) to give dark colored solutions. From these solutions, the cluster phases (EMIm)4[(Zr6Fe)Br18] (1) and (EMIm)4[(Zr6Be)Br18] (2) were obtained in acceptable yields. Crystallographic data of the isostructural phases are the following:  monoclinic, P21/c, Z = 2. The data for 1 follow:  a = 10.5746(4) Å, b = 22.6567(9) Å, and c = 13.0260(5) Å, β = 111.279(2)°. The data for 2 follow:  a = 10.574(2) Å, b = 22.681(4) Å, and c = 13.041(2) Å, β = 111.31(2)°. Compound 1 is the first detailed structurally characterized molecular Fe-centered zirconium bromide cluster phase. In the bromide based ionic liquid, a complete exchange of all the outer and inner chlorides by bromide takes place. Since the inverse reaction, the exchange of all bromides by chlorides, was reported before, this complete ligand exchange can be considered as reversible, with the equilibrium being largely determined by the free ligand concentration. The electronic spectra of a chloride supported cluster precursor in different ionic liquids were measured and analyzed.