Radical Hydrodehalogenation of Aryl Halides with H2 Catalyzed by a Phenanthroline-Based PNNP Cobalt(I) Complex

Radical hydrodehalogenation of aryl halides (Ar–X; X = Cl, Br, I) is achieved in the presence of atmospheric pressure H2 as a H-atom donor using a Co(I) catalyst bearing a phenanthroline-based PNNP ligand (2,9-bis((diphenylphosphanyl)methyl)-1,10-phenanthroline). The reaction proceeds under mild conditions (1 atm H2) and is applicable to aryl bromides and aryl chlorides with various functional groups. A mechanistic study revealed that the PNNP–Co complex underwent facile H–H cleavage and facilitated a H-atom transfer. This process is mediated by a long-range metal–ligand cooperation of the PNNP–Co system, which includes the dearomatization/aromatization sequence of the phenanthroline ligand backbone. A radical clock experiment demonstrated the Ar–X bond cleavage via a radical mechanism. Further kinetic study supported that the rate-determining step includes electron transfer from the Co center to the substrate, affording a radical pair ArX•– and an odd-electron metal-halide complex [Co(II) + ArX•–]⧧ as a transition state.