Axial ligand tuning of a nonheme iron(IV)-oxo unit for hydrogen atom abstraction

Abstract

The reactivities of mononuclear nonheme iron(IV)-oxo complexes bearing different axial ligands, [Fe(IV)(O)(TMC)(X)](n+) [where TMC is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and X is NCCH(3) (1-NCCH(3)), CF(3)COO(-) (1-OOCCF(3)), or N(3)(-) (1-N(3))], and [Fe(IV)(O)(TMCS)](+) (1'-SR) (where TMCS is 1-mercaptoethyl-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane), have been investigated with respect to oxo-transfer to PPh(3) and hydrogen atom abstraction from phenol O H and alkylaromatic C H bonds. These reactivities were significantly affected by the identity of the axial ligands, but the reactivity trends differed markedly. In the oxidation of PPh(3), the reactivity order of 1-NCCH(3) > 1-OOCCF(3) > 1-N(3) > 1'-SR was observed, reflecting a decrease in the electrophilicity of iron(IV)-oxo unit upon replacement of CH(3)CN with an anionic axial ligand. Surprisingly, the reactivity order was inverted in the oxidation of alkylaromatic C H and phenol O H bonds, i.e., 1'-SR > 1-N(3) > 1-OOCCF(3) > 1-NCCH(3). Furthermore, a good correlation was observed between the reactivities of iron(IV)-oxo species in H atom abstraction reactions and their reduction potentials, E(p,c), with the most reactive 1'-SR complex exhibiting the lowest potential. In other words, the more electron-donating the axial ligand is, the more reactive the iron(IV)-oxo species becomes in H atom abstraction. Quantum mechanical calculations show that a two-state reactivity model applies to this series of complexes, in which a triplet ground state and a nearby quintet excited-state both contribute to the reactivity of the complexes. The inverted reactivity order in H atom abstraction can be rationalized by a decreased triplet-quintet gap with the more electron-donating axial ligand, which increases the contribution of the much more reactive quintet state and enhances the overall reactivity.

Fig. 1.

Schematic structures of [Fe^IV(O)(TMC)(X)]ⁿ⁺ (1-X) and [Fe^IV(O)(TMCS)]⁺ (1′-SR).

Fig. 2.

Second-order rate constants determined in the reactions of [Fe^IV(O)(TMC)(X)]ⁿ⁺ (0.5 mM) at 0°C with PPh₃ (a), 2,4-t-Bu₂C₆H₃OH (b), and DHA (c). Blue diamonds indicate 1′-SR; black squares indicate 1-N₃; green circles indicate 1-OOCCF₃; and red inverted triangles indicate 1-NCCH₃.

Scheme 1.

Scheme 2.

Fig. 3.

Correlations of reaction rates with BDE. (a) Plot of log k_rel of [Fe^IV(O)(TMC)(N₃)]⁺ (1-N₃) against OH BDE of p-Y-2,6-t- Bu₂C₆H₃OH in CH₃CN at 25°C. (b) Plot of log k′₂ of [Fe^IV(O)(TMC)(N₃)]⁺ (1-N₃) against CH BDE of substrates. Second-order rate constants, k₂, were determined at 25°C and then adjusted for reaction stoichiometry to yield k′₂ based on the number of equivalent target CH bonds of substrates (e.g., four for DHA and CHD and two for xanthene and fluorene).

Fig. 4.

Electrochemical comparisons. (a) Cyclic and differential pulse voltamograms of [Fe^IV(O)(TMC)(NCCH₃)]²⁺ (1-NCCH₃) in CH₃CN at 25°C (Left) and [Fe^IV(O)(TMCS)]⁺ (1′-SR) in 1:1 CH₃OH/CH₃CN at −30°C (Right). (b) Plot of log k₂ determined in the oxidation of DHA at 0°C against E_p,c values of [Fe^IV(O)(TMC)(X)]ⁿ⁺ complexes and [Fe^IV(O)(TMCS)]⁺ in 1:1 CH₃CN/CH₃OH measured at −30°C.

Fig. 5.

Key geometric features of 1-NCCH₃, 1-OOCCF₃, 1-N₃, and 1′-SR optimized at the B3LYP/LACVP level (bond lengths in angstroms) in the triplet (quintet) states, along with the amounts of charge shifted from ligand to the (TMC)FeO moiety (Δq_CT) and the quintet–triplet energy gap (ΔE_Q-T, in kcal/mol).

Fig. 6.

Plots of log k₂ values for the reactions of 1-X and 1′-SR with DHA (red open circles), 2,4-t-Bu₂C₆H₃OH (green filled triangles), and PPh₃ (black filled squares) against the computed quintet-triplet energy gaps (ΔE_Q-T) for 1-X and 1′-SR.