The first cubane-type fluoro-bridged cluster

CUBANE-TYPE clusters containing the unit M4A4have 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, Fe4S4clusters 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)2in 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)5and also monomers MF2(ligand)4(ref. 6).