Synthetic, Structural, and Redox Studies of Arene Alkyl Complexes of Tantalum(III) Supported by Aryloxide and Arenethiolate Ligands

Abstract

A series of η6-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, (η6-C6Me6)Ta(OAr)2Cl (1, Ar = 2,6-C6H3iPr2) reacts with MeMgBr at low temperature to afford (η6-C6Me6)Ta(OAr)2Me (3). Low-temperature alkylation of (η6-C6Me6)Ta(OAr)Cl2 (2) with 2 equiv of RMgBr forms (η6-C6Me6)Ta(OAr)R2 (4, R = Me; 5, R = Et) and with 2 equiv of RLi affords (η6-C6Me6)Ta(OAr)R2 (6, R = CH2SiMe3; 7, R = Ph). Complexes 3−7 are more stable than their halide precursors; no products arising from α- or β-H elimination processes were identified upon thermolysis. In addition to NMR studies of these compounds, cyclic voltammetry experiments show two oxidation processes; the Ta(III) ⇄ Ta(IV) couple is quasi-reversible, and the Ta(IV) → Ta(V) process is irreversible. Molecules of 5 exhibit a folded arene ligand with π-electron localization (diene−diyl structure) and normal ethyl ligands (no evidence for agostic interactions). Under the appropriate conditions, (η6-C6Me6)Ta(OAr)Cl2 (2) can be monoalkylated using 1 equiv of LiCH2SiMe3 or LiPh to afford (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) and (η6-C6Me6)Ta(OAr)(Ph)Cl (9). However, attempts to monoalkylate (η6-C6Me6)Ta(OAr)Cl2 with 1 equiv of either MeMgBr or EtMgBr provide the “double-exchange” products (η6-C6Me6)Ta(OAr)(Me)Br (10) and (η6-C6Me6)Ta(OAr)(Et)Br (11), respectively. The metathesis product (η6-C6Me6)Ta(OAr)(Et)Cl (12) is isolated in good yield upon attempts to alkylate (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) with ZnEt2. However, (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) reacts with PhLi to afford (η6-C6Me6)Ta(OAr)(CH2SiMe3)Ph (13). The halide alkyl complexes (η6-C6Me6)Ta(OAr)(Et)Br (11) and (η6-C6Me6)Ta(OAr)(CH2SiMe3)Cl (8) react with LiBEt3H to provide the hydrido complexes (η6-C6Me6)Ta(OAr)(Et)H (14) and (η6-C6Me6)Ta(OAr)(CH2SiMe3)H (15), respectively. The arenethiolate complexes (η6-C6Me6)Ta(OAr)(SAr‘)Cl (16) (Ar‘ = 2,4,6-C6H2iPr3) and (η6-C6Me6)Ta(OAr)(S(mes))Cl (17) (mes = 2,4,6-C6H2Me3) are formed upon reacting (η6-C6Me6)Ta(OAr)Cl2 (2) with the appropriate lithium arenethiolate reagent, and the characterization of these species is discussed.