The first cubane-type fluoro-bridged cluster

Abstract

CUBANE-TYPE clusters containing the unit M₄A₄ have been the subject of many investigations. Several of these systems have been characterised by X-ray structure determination. Such clusters are important from a bioinorganic point of view, for example, Fe₄S₄ clusters in iron–sulphur proteins¹. From a catalytic point of view, clustered compounds are becoming increasingly important as models for heterogeneous catalysts, and even as catalysts by themselves². Monoatomic ligands (for example, A = S, Cl, P, Br, Se) have been reported as bridging ligands in cubane-type clusters, but larger groups are known in which C, N, and O act in a similar way³. No cubane-type clusters have been previously reported with the highly electronegative F⁻ ion as a bridging ligand. We describe here the synthesis and structure of the first fluoro-bridged cubane-type cluster, having the formula [M₄F₄(L)₁₂.] (BF₄)₄, in which M = Mn, Cd, Co and L = N-ethylimidazole or N-propylimidazole. On reaction of M(H₂O)₆ (BF₄)₂ in ethanol–triethyl orthoformate with an ethanolic solution of the ligand, finely-divided white powders (Mn, Cd) or purple crystals (Co) separate, analysing as M(ligand)₃BF₅. In these circumstances the F⁻ ions are spontaneously generated by a slow decomposition of the BF₄⁻ ion. Such a decomposition of tetrafluoroborates, yielding fluoro complexes, has been reported in a few other cases, yielding polymeric fluoro-bridged systems⁴, dimers (M₂F₂)⁵ and also monomers MF₂ (ligand)₄ (ref. 6).