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: Ea (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.