Synthesis, characterization, and ligand exchange studies of W6S8L6 cluster compounds

Eleven organic Lewis bases were investigated as potential ligands (L) on W6S8L ' (6) clusters by exploring ligand exchange reactions to form W6S8L6 clusters. Six new homoleptic W6S8L6 cluster complexes were prepared and characterized with L = tri-n-butylphosphine ((PBu3)-Bu-n), triphenylphosphine (PPh3), tert-butylisocyanide ((BuNC)-Bu-t), morpholine, methylamine (MeNH2), and tert-butylamine ((BuNH2)-Bu-t). While partial replacement of ligands occurred with diethylamine (Et2NH) and dibutylamine (Bu2NH), homoleptic clusters could not be prepared by these exchange reactions. When aniline, tribenzylamine, and tri-tert-butylphosphine were the potential ligands, no exchange was observed. From ligand exchange studies of these Ligands and others previously studied, a thermodynamic series of binding free energies for ligands on W6S8L6 clusters was established as the following: non-Lewis base solvents, aniline, (PBu3)-Bu-t, etc. much less than Et2NH, Bu2NH < (BuNH2)-Bu-t < morpholine, piperidine less than or equal to (BuNH2)-Bu-n, MeNH2 less than or equal to 4-tert-butylpyridine, pyridine < (BuNC)-Bu-t < tricyclohexylphosphine (PCy3) < PPh3, (PBu3)-Bu-n less than or equal to triethylphosphine (PEt3). Structures of the new cluster complexes were determined by X-ray crystallography. The new compounds were also characterized by NMR spectroscopy and thermogravimetric analyses (TGA). The W-L bond orders and TGA data qualitatively agree with the thermodynamic series above.