C-H Activation by Isolable Cationic Bis(phosphine) Cobalt(III) Metallacycles.

Five- and six-coordinate cationic bis(phosphine) cobalt(III) metallacycle complexes were synthesized with the general structures, [(depe)Co(cycloneophyl)(L)(L')][BAr F 4 ] (depe = 1,2-bis(diethylphosphino)ethane; cycloneophyl = [κ- C : C -(CH 2 C(Me) 2 )C 6 H 4 ]; L/L' = pyridine, pivalonitrile, or the vacant site, BAr 4 F = B[(3,5-(CF 3 ) 2 )C 6 H 3 ] 4 ). Each of these compounds promoted facile directed C(sp 2 )-H activation with exclusive selectivity for ortho -alkylated products, consistent with the selectivity of reported cobalt-catalyzed arene-alkene-alkyne coupling reactions. The direct observation of C-H activation by cobalt(III) metallacycles provided experimental support for the intermediacy of these compounds in this class of catalytic C-H functionalization reaction. Deuterium labeling and kinetic studies provided insight into the nature of C-H bond cleavage and C-C bond reductive elimination from isolable cobalt(III) precursors.