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.