Radical Mechanisms in the Reaction of Organic Halides with Diiminepyridine Cobalt Complexes

The formally Co(0) complex LCo(N2) (L =2,6-bis(2,6-dimethylphenyliminoethyl)pyridine) can be prepared via either Na/Hg reduction of LCoCl2 or hydrogenolysis of LCoCH2SiMe3. In the latter reaction, LCoH could be trapped by reaction with NCC6H4-4-Cl to give LCoN=CHC6H4-4-Cl. LCo(N2) reacts with many alkyl and aryl halides RX, including aryl chlorides, to give a mixture of LCoR and LCoX in a halogen atom abstraction mechanism. Intermediacy of free alkyl and aryl radicals is confirmed by the ring-opening of cyclopropylmethyl to crotyl, and the rearrangement of 2,4,6-tBu3C6H2 to 3,5-tBu2C6H3CMe2CH2, before binding to Co. The organocobalt species generated in this way react further with activated halides R′X (alkyl iodides; allyl and benzyl halides) to give cross-coupling products RR′ in what is most likely again a halogen abstraction mechanism. DFT studies support the proposed radical pathways for both steps. MeI couples smoothly with LCoCH2SiMe3 to give LCoI and CH3CH2SiMe3, but the analogous reaction of tBuI leads in part to radical attack at the 3 and 4 positions of the pyridine ring to form (tBu2-L)CoI and (tBu2-L)CoI2.