Binding preferences of the tribenzylidenemethane ligand in high-oxidation-state tantalum complexes

Reaction of Li-2(TBM)(TMEDA)(2) (TBM = tribenzylidenemethane) with TaMe3Cl2 gives (TBM)-TaMe3 (1) in 44% yield. Structural characterization of the (tert-Bu-TBM)TaMe3 derivative 2 shows an eclipsed orientation of the TaMe3 tripod relative to the inner core of the TBM ligand. Treatment of(TBM)TaMe3 with ZnCl2 cleanly replaces one methyl ligand for chloride to give (TBM)TaMe2Cl (3) which is a versatile precursor to (TBM)Ta-based complexes. Addition of LiNPh2 to 3 gives (TBM)TaMe2(NPh2) (4). Structural characterization reveals that both 3 and 4 have eclipsed frameworks. Metallocene-mimics are accessible by reacting LiCp (Cp = C5H5), LiCp* (Cp* = C5Me5), LiCp' (Cp' = C5H4Me), or LiFlu (Flu = fluorenyl) with 3 to give Cp(TBM)TaMe2 (5), Cp*(TBM)TaMe2 (6), Cp'(TBM)TaMe2 (7), and Flu(TBM)TaMe2 (8), respectively. The solid-state structures of 5, 7, and 8 display gross molecular geometries similar to those of group 3 metallocenes. Complex 3 reacts with tris(pyrazolylborate) or bis(pyrazolylborate) salts. Thus, [HB (pz)(3)](TBM)TaMe2 (9), [HB(3,5-Me-2-1-pz)(3)](TBM)TaMe2 (10), and [H2B(pz)(2)] (TBM)TaMe2 (11) are obtained from Na[HB(pz)(3)], K[HB(3,5-Me-2-1-pz)(3)], and K[H2B(pz)(2)], respectively. Structural characterization of 9, 10, and 11 shows that TBM can adopt a continuum of bonding modes, from eta(4) to eta(2), depending on the steric hindrance around the metal center. The TMM ligand participates in hydrogenation and insertion reactions, indicating that TMM is a weak ancillary ligand. Combining 6, 7, or 8 with MAO results in short-lived ethylene polymerization catalysts. Finally, an electronic description of the model complex (TMM)TaMe3 is developed to account for the eclipsed molecular structures of 1-4. A comparison against the orbital description of staggered (TMM)Fe(CO)(3) is also made.