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 (te rt-Bu-TBM)TaMe3 derivative 2 shows an eclipsed orientation of the TaMe3 tri pod relative to the inner core of the TBM ligand. Treatment of(TBM)TaMe3 wi th ZnCl2 cleanly replaces one methyl ligand for chloride to give (TBM)TaMe2 Cl (3) which is a versatile precursor to (TBM)Ta-based complexes. Addition of LiNPh2 to 3 gives (TBM)TaMe2(NPh2) (4). Structural characterization reve als that both 3 and 4 have eclipsed frameworks. Metallocene-mimics are acce ssible by reacting LiCp (Cp = C5H5), LiCp* (Cp* = C5Me5), LiCp' (Cp' = C5H4 Me), or LiFlu (Flu = fluorenyl) with 3 to give Cp(TBM)TaMe2 (5), Cp*(TBM)Ta Me2 (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(pyrazolylborat e) 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 obtain ed from Na[HB(pz)(3)], K[HB(3,5-Me-2-1-pz)(3)], and K[H2B(pz)(2)], respecti vely. 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 ster ic hindrance around the metal center. The TMM ligand participates in hydrog enation and insertion reactions, indicating that TMM is a weak ancillary li gand. Combining 6, 7, or 8 with MAO results in short-lived ethylene polymer ization 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.