Oxygen-Centered Hexatantalum Tetradecaimido Cluster Complexes

The syntheses and characterization of several octahedral hexatantalum cluster compounds of formula (ArN)₁₄Ta₆O are described (Ar = Ph, p-MeC₆H₄, p-MeOC₆H₄, p-t-BuC₆H₄, p-BrC₆H₄, m-ClC₆H₄). Treatment of Bn₃TaN-t-Bu (Bn = CH₂C₆H₅) or pentakis(dimethylamido)tantalum with an excess of the appropriate aniline and stoichiometric water or tantalum oxide afforded varying yields of arylimido clusters. The structures of two species were confirmed by X-ray diffraction (XRD), while the identity of the central oxygen atom was elucidated by electrospray mass spectrometry (MS) using ¹⁷O/¹⁸O-enriched material. The title species are very air- and moisture-sensitive but quite thermally stable in solution. Experimentally determined optical properties and oxidation/reduction potentials, as well as some computational results, indicate that they possess an electronic structure wherein the highest occupied molecular orbitals are ligand-centered, while the lowest unoccupied orbitals are metal-centered and delocalized throughout the tantalum cage. Whereas chemical oxidation resulted in cluster decomposition, reduction with decamethylcobaltocene yielded stable salts of formula [Cp*₂Co][(ArN)₁₄Ta₆O] (Ar = Ph, Ar = p-MeC₆H₄). Small-molecule reactivity studies on one of these clusters showed that its imido functionalities are moderately reactive toward oxide donors but inert with respect to metallaheterocycle-forming processes. Clean imido/oxo exchange was observed with aldehydes and ketones, leading cleanly to organic imines with no soluble byproducts being observed. This exchange was also observed with a rhenium oxo compound (generating an imidorhenium complex as the only soluble species). All 14 imido groups were transferred in these reactions, and no mixed-ligand cluster intermediates were ever observed.