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 (PⁿBu₃), triphenylphosphine (PPh₃), tert-butylisocyanide (^tBuNC), morpholine, methylamine (MeNH₂), and tert-butylamine (^tBuNH₂). 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, P^tBu₃, etc. ≪ Et₂NH, Bu₂NH < ^tBuNH₂ < morpholine, piperidine ≤ ⁿBuNH₂, MeNH₂ ≤ 4-tert-butylpyridine, pyridine < ^tBuNC < tricyclohexylphosphine (PCy₃) < PPh₃, PⁿBu₃ ≤ 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.