Synthesis and Reactivity of a Bipyridyl Tantalum Complex

The bipyridyl tantalum complex (2,6-<i>ⁱ</i>Pr₂C₆H₃O)₃Ta(bipy) (<b>2</b>) is synthesized by the reaction of (2,6-<i>ⁱ</i>Pr₂C₆H₃O)₃TaCl₂ (<b>1</b>) and 2,2'-bipyridine in the presence of excess potassium graphite (KC₈). Complex <b>2</b> coordinates readily with pyridine and 4-(dimethylamino)pyridine (dmap) to form Lewis base adducts (2,6-<i>ⁱ</i>Pr₂C₆H₃O)₃Ta(bipy)(L) (L = py (<b>3</b>), dmap (<b>4</b>)), and it exhibits rich redox reactivity toward small molecules: (a) single electron transfer (SET) occurs upon exposure of <b>2</b> to phenyl sulfide or selenide dimer, giving the open-shell, bipy-centered radical complexes (2,6-<i>ⁱ</i>Pr₂C₆H₃O)₃Ta(bipy^•)(PhE) (E = S (<b>5</b>), Se (<b>6</b>)). (b) Regioselective C-C σ-bond formation via radical coupling is observed in the SET reaction of <b>2</b> and aldehydes, ketones, or imines to furnish insertion products <b>7</b>-<b>13</b>, namely, sterically more crowded benzophenone, acetophenone, 2,6-dichlorobenzaldehyde, and benzophenone imine couple with C6 or C6' of bipy in <b>2</b>, respectively, whereas sterically less hindered benzaldehyde, cyclohexanone, and benzylideneaniline couple with C2 or C2' of bipy, respectively.