Synthetic, structural, and redox studies of arene alkyl complexes of tantalum(III) supported by aryloxide and arenethiolate ligands

A series of eta(6)-hexamethylbenzene alkyl and aryl complexes of tantalum(III) supported by aryloxide and arenethiolate ligands have been prepared, characterized, and compared to their halide analogues. Thus, (eta(6)-C6Me6)Ta(OAr)(2)Cl (1, Ar = 2,6-(C6H3Pr2)-Pr-i) reacts with MeMgBr at low temperature to afford (eta(6)-C6Me6)Ta(OAr)(2)Me (3). Low-temperature alkylation of (eta(6)-C6Me6)Ta(OAr)Cl-2 (2) with 2 equiv of RMgBr forms (eta(6)-C6Me6)Ta(OAr)R-2 (4, R = Me; 5, R = Et) and with 2 equiv of RLi affords (eta(6)-C6Me6)Ta(OAr)R-2 (6, R = CH2SiMe3; 7, R = Ph). Complexes 3-7 are more stable than their halide precursors; no products arising from alpha- or beta-H elimination processes mere identified upon thermolysis. In addition to NMR studies of these compounds, cyclic voltammetry experiments show two oxidation processes; the Ta(III) reversible arrow Ta(IV) couple is quasi-reversible, and the Ta(IV) --> Ta(V) process is irreversible. Molecules of 5 exhibit a folded arene Ligand with pi-electron localization (diene-diyl structure) and normal ethyl ligands (no evidence for agostic interactions). Under the appropriate conditions, (eta(6)-C6Me6)Ta(OAr)Cl-2 (2) can be monoalkylated using 1 equiv of LiCH2SiMe3 or LiPh to afford (eta(6)-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) and (eta(6)-C6Me6)Ta(OAr)(Ph)Cl (9). However, attempts to monoalkylate (eta(6)-C6Me6)Ta(OAr)Cl-2 with 1 equiv of either MeMgBr or EtMgBr provide the ''double-exchange'' products (eta(6)-C6Me6)Ta(OAr)(Me)Br (10) and eta(6)-C6Me6)Ta(OAr)(Et)Br (11), respectively. The metathesis product (eta(6)-C6Me6)Ta(OAr)(Et)Cl (12) is isolated in good yield upon attempts to alkylate (eta(6)-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) with ZnEt2. However, (eta(6)-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) reacts with PhLi to afford (eta(6)-C6Me6)Ta(OAr)(CH2SiMe3)Ph (13). The halide alkyl complexes (eta(6)-C6Me6)Ta(OAr)(Et)Br (11) and (eta(6)-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) react with LiBEt3H to provide the hydride complexes (eta(6)-C6Me6)Ta(OAr)(Et)H (14) and (eta(6)-C6Me6)Ta(OAr)(CH2SiMe3)H (15), respectively. The arenethiolate complexes (eta(6)-C6Me6)Ta(OAr)(SAr')Cl (16) (Ar' = 2,4,6-(C6H2Pr3)-Pr-i) and (eta(6)-C6Me6)Ta(OAr)(S(mes>)Cl (17) (mes = 2,4,6-C6H2Me3) are formed upon reacting (eta(6)-C6Me6)Ta(OAr)Cl-2 (2) with the appropriate lithium arenethiolate reagent, and the characterization of these species is discussed.