Synthesis and properties of oxygen-centered tetradecaimido hexatantalum clusters.

Hexanuclear transition-metal clusters are well-known, but examples displaying closely packed organic substituents are scarce.[1] The terminal oxo ligands in hexamolybdate and hexatungstate have been replaced to varying degrees by imido[2] or hydrazido[3] groups, and cyclopentadienyl-functionalized hetero-hexanuclear clusters have been prepared.[4] Still, incorporation of organic groups into the bridging sites has been more challenging.[5] Herein we report a high-yielding synthesis of neutral, oxygen-centered hexatantalum clusters with all other metal valencies occupied by terminal and µ3 arylimido ligands. Although chemically inert in their neutral form,[6] these “nearly homoleptic” tantalum imido species possess electronic structures which may facilitate interesting redox-promoted transformations. The discovery of these new compounds occurred serendipitously during our investigation of the hydroamination of alkynes by anilines with the imido catalyst [Bn3Ta=NtBu] (1).[7] Reaction mixtures often developed a red color during the course of the reaction, and catalyst deactivation was always observed within 24 hours regardless of the amount of 1 added (Scheme 1). A control experiment revealed that treatment of 1 with excess aniline at 135°C resulted in the precipitation of a sparingly soluble, highly air- and moisture-sensitive red crystalline material in low and erratic yield. Subsequent characterization showed that this material was the octahedral cluster [(PhN)14Ta6O] (2; Scheme 2). Scheme 1 Scheme 2 Single-crystal X-ray diffraction analysis[8] of 2 showed a centrosymmetric hexatantalum cluster[9] with six terminal and eight facially bridging phenylimido ligands (Figure 1). Whereas only two nonequivalent sets of phenyl resonances are observed in the 1H NMR spectrum of 2, in the solid state the cluster core is slightly distorted from Oh symmetry and the phenylimido ligands pack so that the potential D2d symmetry is not realized. A central atom was located that was initially assumed to be nitrogen[10] on the basis of precedent from known nitride-centered polyimido clusters.[11] However, the structure was refined equally well with oxygen in this position, and thus alternate methods of characterization were employed to resolve this issue (see below). Figure 1 ORTEP diagram of [(PhN)14Ta6O] (2) at 50% probability. Hydrogen atoms have been removed for clarity. Selected bond lengths (A): Ta-O 2.2035(5), 2.2079(4), 2.2119(4); Ta-Nterminal 1.790(3), 1.791(4), 1.793(4). C gray, N blue, O red, Ta orange. Irreproducible yields obtained in the synthesis of 2 led us to suspect the adventitious presence of a necessary reactant. Whereas oxygen sources such as peroxides, pyridine N-oxide, or phosphine oxides failed to yield 2, the addition of water prior to heating of the reaction mixture resulted in consistent yields of up to 57%. Surprisingly, tantalum oxide was also moderately effective as an oxygen source. This observation provides an explanation for the variable yield without added water: tantalum oxide may have remained on the surface of the glassware from one reaction to the next and may not have been inert as was assumed. The reaction is not limited to aniline itself. Substituted anilines p-toluidine and p-anisidine provided [(p-MeC6H4N)14Ta6O] (3) and [(p-MeOC6H4N)14Ta6O] (4) in 48% and 49% yield, respectively. Although p-chloro-, p-fluoro-, and p-trifluoromethylaniline did not furnish isolable products, m-chloroaniline gave [(m-ClC6H4N)14Ta6O] (5) in modest yield (15%). High-quality crystals of 5 were obtained from a reaction mixture, and structural elucidation[12] revealed a configuration analogous to that of 2. A screening of alternate metal sources showed that commercially available pentakis(dimethylamido)tantalum provided yields comparable to those obtained by employing 1 and allowed for the preparation of 2 and 3 on a much larger scale. Thus, it is likely that many reported reactions involving addition of an arylamine to tantalum coordination complexes have resulted in the formation, but not the detection, of these clusters. The presence of oxide in the cluster core was confirmed by electrospray mass spectrometry studies on 3 and 4 using THF as the matrix. Owing to extensive fragmentation and subsequent reaction with the matrix, none of the observed fragments corresponded to combinations of the cluster core and arylimido fragments. However, many of the high-molecular-weight fragments (up to ca. 2000 Da) possessed masses that were 16 Da greater for 4 (MeO) than for 3 (Me). Therefore, these fragments apparently each retained one imido ligand. Samples of 3 and 4 that had been prepared using 17O- and 18O- enriched water yielded spectra in which each high-mass fragment from the nonlabeled cluster was accompanied by fragments that were one and two Da heavier, including those that were inferred to still possess an imido ligand (see the Supporting Information). These data provide strong evidence for assignment of these compounds as oxide-centered clusters, with the oxide deriving from added water. Compounds 3 and 4 are soluble in benzene to about 10 mm and were recrystallized without decomposition. Both are significantly more air-sensitive than 2, decomposing in solution over about two days if stored in polypropylene-capped glass vials inside an inert-atmosphere glove box. Nonetheless, a sample of 3 in C6D6 showed no decomposition after it was heated at 135°C for two weeks under rigorously air-free conditions. Compound hues vary from red/orange (5) to red/black for the more electron-rich 4 (Figure 2, insert). Compounds 2, 3, and 4 exhibit intense UV absorptions quite similar in energy to those of the anilines from which they are derived (Figure 2). Fluorescence measurements, however, detected no emission between 300 and 1000 nm. The large number and density of vibrational modes accessible to these clusters probably makes thermal relaxation the dominant excited-state decay pathway. Figure 2 UV/Vis absorption spectra of 2 (solid trace), 3 (dotted), and 4 (dashed) in THF. Inset: visible-wavelength absorption onsets. Results of orbital calculations[13] (X3LYP/LACVP*)[14] on Ci-optimized 2 agree with the expected bonding scheme wherein all occupied frontier orbitals are ligand-centered. Both the geometry and orbital structure of optimized 2 are very similar to those calculated from the crystallographically determined coordinates. The calculated LUMO of 2 is delocalized over all six tantalum centers and is primarily nonbonding (Figure 3). This bonding scheme is consistent with the observed optical properties in that ligand-to-metal charge-transfer processes should be very facile. Furthermore, the computational results suggest that the title compounds may exhibit interesting redox behavior. Indeed, cursory reactivity studies have shown that although 2 and 3 are inert towards unsaturated small molecules, they are easily reduced to stable anions. Preliminary data suggest that further study of this novel family of compounds will yield new insights into the nature of polynuclear metal–imido bonding. Figure 3 LUMO of Ci-optimized 2 (X3LYP/LACVP*) viewed along the fourfold axis of the cluster core (nonbonding, localized on six tantalum centers). Ph groups have been removed for clarity.