a, To probe the oxidation state of the active iron catalyst, comparative studies on catalytic cyclopropanation of alkenes 2 with in situ-generated α-trifluoromethyldiazomethane (1′) by [Fe(P3)Cl] were performed under a nitrogen atmosphere and in air, as well as in the absence and presence of DMAP. As a parallel comparison, the catalytic cyclopropanation reaction was also carried out with the [Co(P3)] as the catalyst in the absence and presence of DMAP. b, HRMS-ESI detection of α-Fe(IV)-alkyl radical I[Fe(p3)Cl]/4b from a reaction mixture of ethyl diazoacetate (4b) and [Fe(P3)Cl]. c, HRMS-ESI detection of α-Fe(IV)-alkyl radical I[Fe(P3)Cl]/4l from a reaction mixture of α-phenyldiazomethane (4l′) and [Fe(P3)Cl].