Direct Comparison of C-H Bond Amination Efficacy through Manipulation of Nitrogen-Valence Centered Redox: Imido versus Iminyl

Reduction of previously reported iminyl radical (^ArL)FeCl(^•N(C₆H₄-p-^tBu)) (2) with potassium graphite furnished the corresponding high-spin (S = ⁵/₂) imido (^ArL)Fe(N(C₆H₄-p-^tBu)) (3) (^ArL = 5-mesityl-1,9-(2,4,6-Ph₃C₆H₂)dipyrrin). Oxidation of the three-coordinate imido (^ArL)Fe(NAd) (5) with chlorotriphenylmethane afforded (^ArL)FeCl(^•NAd) (6) with concomitant expulsion of Ph₃C(C₆H₅)CPh₂. The respective aryl/alkyl imido/iminyl pairs (3, 2; 5, 6) have been characterized by EPR, zero-field ⁵⁷Fe Mössbauer, magnetometry, single crystal X-ray diffraction, XAS, and EXAFS for 6. The high-spin (S = ⁵/₂) imidos exhibit characteristically short Fe-N bonds (3: 1.708(4) Å; 5: 1.674(11) Å), whereas the corresponding iminyls exhibit elongated Fe-N bonds (2: 1.768(2) Å; 6: 1.761(6) Å). Comparison of the pre-edge absorption feature (1s → 3d) in the X-ray absorption spectra reveals that the four imido/iminyl complexes share a common iron oxidation level consistent with a ferric formulation (3: 7111.5 eV, 2: 7111.5 eV; 5: 7112.2 eV, 6: 7112.4 eV) as compared with a ferrous amine adduct (^ArL)FeCl(NH₂Ad) (7: 7110.3 eV). N K-edge X-ray absorption spectra reveal a common low-energy absorption present only for the iminyl species 2 (394.5 eV) and 6 (394.8 eV) that was assigned as a N 1s promotion into a N-localized, singly occupied iminyl orbital. Kinetic analysis of the reaction between the respective iron imido and iminyl complexes with toluene yielded the following activation parameters: E_a (kcal/mol) 3: 12.1, 2: 9.2; 5: 11.5, 6: 7.1. The attenuation of the Fe-N bond interaction on oxidation from an imido to an iminyl complex leads to a reduced enthalpic barrier [Δ(ΔH^‡) ≈ 5 kcal/mol]; the alkyl iminyl 6 has a reduced enthalpic barrier (1.84 kcal/mol) as compared with the aryl iminyl 2 (3.84 kcal/mol), consistent with iminyl radical delocalization into the aryl substituent in 2 as compared with 6.