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.