Synthesis and Reactivity of a Bipyridyl Tantalum Complex

The bipyridyl tantalum complex (2,6-ⁱPr₂C₆H₃O)₃Ta(bipy) (2) is synthesized by the reaction of (2,6-ⁱPr₂C₆H₃O)₃TaCl₂ (1) and 2,2′-bipyridine in the presence of excess potassium graphite (KC₈). Complex 2 coordinates readily with pyridine and 4-(dimethylamino)pyridine (dmap) to form Lewis base adducts (2,6-ⁱPr₂C₆H₃O)₃Ta(bipy)(L) (L = py (3), dmap (4)), and it exhibits rich redox reactivity toward small molecules: (a) single electron transfer (SET) occurs upon exposure of 2 to phenyl sulfide or selenide dimer, giving the open-shell, bipy-centered radical complexes (2,6-ⁱPr₂C₆H₃O)₃Ta(bipy^•)(PhE) (E = S (5), Se (6)). (b) Regioselective C–C σ-bond formation via radical coupling is observed in the SET reaction of 2 and aldehydes, ketones, or imines to furnish insertion products 7–13, namely, sterically more crowded benzophenone, acetophenone, 2,6-dichlorobenzaldehyde, and benzophenone imine couple with C6 or C6′ of bipy in 2, respectively, whereas sterically less hindered benzaldehyde, cyclohexanone, and benzylideneaniline couple with C2 or C2′ of bipy, respectively. (c) 2 serves as a synthon for (2,6-ⁱPr₂C₆H₃O)₃Ta(III) in contact with diazene, organic azides, and 1,4-diaza-1,3-diene and (conjugated)alkyne derivatives, in which tantalum imido/azido/hydrazido complexes (2,6-ⁱPr₂C₆H₃O)₃Ta(═NPh)(bipy⁰) (14), (2,6-ⁱPr₂C₆H₃O)₃Ta(═N₃(2,6-ⁱPr₂C₆H₃))(bipy⁰) (15), (2,6-ⁱPr₂C₆H₃O)₃Ta(═NSiMe₃)(bipy⁰) (16), and (2,6-ⁱPr₂C₆H₃O)₃Ta(═N₂CPh₂)(bipy⁰) (17); the tantalum σ²-enediamide complex (2,6-ⁱPr₂C₆H₃O)₃Ta(η²-N,N′-p-MeC₆H₄-DAD) (18); and tantalum metallacyclopropenes (2,6-ⁱPr₂C₆H₃O)₃Ta(η²-C₂Ph₂) (19) and (2,6-ⁱPr₂C₆H₃O)₃Ta[η²-C(Ph)═C(C≡CPh)](bipy⁰) (20) are formed via a two-electron redox process, respectively.