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

Abstract

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.

Synthesis, characterization and C–H amination reactivity of nickel iminyl complexes

“…Close inspection of the C-C bond lengths within the mesityl group of 3 ( Fig. 3a and b) reveals the elongation of the C ipso -C ortho bonds (1.423(4), 1.429(4)Å), analogous to our previously reported Fe III iminyl species [37][38][39][40] as well as all the previously reported Ni aryl imido complexes ( Fig. S7 †).…”

“…[1][2][3][4][5][6]8,9,[14][15][16][17][18][35][36][37][38][39][40][41][42][43] Our group has focused on the generation of highly electrophilic MLMBs by employing weak-eld dipyrrin ancillary ligands. 37,39,40,42,44 Ferric imido complexes bearing the dipyrrin ligand feature high-spin (S ¼ 5/2) electronic structures, [37][38][39][40] while the Co III analogues thermally access open-shell excited states. 41,42 The open-shell electronic structures for the Fe [37][38][39][40] and Co 41,42 complexes provide access to reactivity along the MLMB vector, where both inter-and intramolecular amination transfer to C-H bond substrates were observed.…”

“…37,39,40,42,44 Ferric imido complexes bearing the dipyrrin ligand feature high-spin (S ¼ 5/2) electronic structures, [37][38][39][40] while the Co III analogues thermally access open-shell excited states. 41,42 The open-shell electronic structures for the Fe [37][38][39][40] and Co 41,42 complexes provide access to reactivity along the MLMB vector, where both inter-and intramolecular amination transfer to C-H bond substrates were observed. In both cases, however, the MLMBs were best formulated as imido functionalities.…”

“…Oxidation of the Fe III imidos elicits the formation of an open-shell iminyl, where oxidation is borne out at the NR functionality, not the metal center, leading to faster oxidative group transfer than their high spin imido analogues. 39,40 Formation of later transition metal MLMBs can also diminish MLMB bond order in two ways: (1) higher d-electron counts necessarily populate more M-L antibonding orbitals;…”

Unprecedented Five‐Coordinate Iron(IV) Imides Generate Divergent Spin States Based on the Imide R‐Groups

“…Since the synthesis of the first monomeric iron imide, [PhBP 3 ]Fe(N‐ p ‐tolyl), by Peters and co‐workers in 2003, iron imides have been studied extensively because of their purported roles in oxidative group transfer catalysis, such as aziridination and amination, as well as for structural models of metalloenzymes . As such, a relatively varied set of iron imides have been reported to date, with the oxidation states on the iron ranging from iron(II) to iron(V) (Figure ) . Despite the diversity of oxidation states isolated thus far, it is striking that these species are almost exclusively low‐coordinate (four or less) about the iron center.…”