Binding Preferences of the Tribenzylidenemethane Ligand in High-Oxidation-State Tantalum Complexes

Reaction of Li 2 (TBM)(TMEDA) 2 (TBM = tribenzylidenemethane) with TaMe 3 Cl 2 gives (TBM)TaMe 3 ( 1 ) in 44% yield. Structural characterization of the ( tert -Bu-TBM)TaMe 3 derivative 2 shows an eclipsed orientation of the TaMe 3 tripod relative to the inner core of the TBM ligand. Treatment of (TBM)TaMe 3 with ZnCl 2 cleanly replaces one methyl ligand for chloride to give (TBM)TaMe 2 Cl ( 3 ) which is a versatile precursor to (TBM)Ta-based complexes. Addition of LiNPh 2 to 3 gives (TBM)TaMe 2 (NPh 2 ) ( 4 ). Structural characterization reveals that both 3 and 4 have eclipsed frameworks. Metallocene-mimics are accessible by reacting LiCp (Cp = C 5 H 5 ), LiCp* (Cp* = C 5 Me 5 ), LiCp‘ (Cp‘ = C 5 H 4 Me), or LiFlu (Flu = fluorenyl) with 3 to give Cp(TBM)TaMe 2 ( 5 ), Cp*(TBM)TaMe 2 ( 6 ), Cp‘(TBM)TaMe 2 ( 7 ), and Flu(TBM)TaMe 2 ( 8 ), respectively. The solid-state structures of 5, 7, and 8 display gross molecular geometries similar to those of group 4 metallocenes. Complex 3 reacts with tris(pyrazolylborate) or bis(pyrazolylborate) salts. Thus, [HB(pz) 3 ](TBM)TaMe 2 ( 9 ), [HB(3,5-Me 2 -1-pz) 3 ](TBM)TaMe 2 ( 10 ), and [H 2 B(pz) 2 ](TBM)TaMe 2 ( 11 ) are obtained from Na[HB(pz) 3 ], K[HB(3,5-Me 2 -1-pz) 3 ], and K[H 2 B(pz) 2 ], respectively. Structural characterization of 9, 10, and 11 shows that TBM can adopt a continuum of bonding modes, from η 4 to η 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)TaMe 3 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.