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) serves as a synthon for (2,6-PrCHO)Ta(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-PrCHO)Ta(═NPh)(bipy) (), (2,6-PrCHO)Ta(═N(2,6-PrCH))(bipy) (), (2,6-PrCHO)Ta(═NSiMe)(bipy) (), and (2,6-PrCHO)Ta(═NCPh)(bipy) (); the tantalum σ-enediamide complex (2,6-PrCHO)Ta(η-,--MeCH-DAD) (); and tantalum metallacyclopropenes (2,6-PrCHO)Ta(η-CPh) () and (2,6-PrCHO)Ta[η-C(Ph)═C(C≡CPh)](bipy) () are formed via a two-electron redox process, respectively.