A Mononuclear Nonheme Iron(IV)-Oxo Complex of a Substituted N4Py Ligand: Effect of Ligand Field on Oxygen Atom Transfer and C–H Bond Cleavage Reactivity

Abstract

A m o n o n u c l e a r i r o n ( I I ) c o m p l e x [Fe II (N4Py Me2 )(OTf)](OTf)(1), supported by a new pentadentate ligand, bis(6-methylpyridin-2-yl)-N,N-bis((pyridin-2yl)methyl)methanamine (N4Py Me2 ), has been isolated and characterized. Introduction of methyl groups in the 6-position of two pyridine rings makes the N4Py Me2 a weaker field ligand compared to the parent N4Py ligand. Complex 1 is high-spin in the solid state and converts to [Fe II (N4Py Me2 )(CH 3 CN)]-(OTf) 2 (1a) in acetonitrile solution. The iron(II) complex in acetonitrile displays temperature-dependent spin-crossover behavior over a wide range of temperature. In its reaction with m-CPBA or oxone in acetonitrile at −10 °C, the iron(II) complex converts to an iron(IV)-oxo species, [Fe IV (O)(N4Py Me2 )] 2+ (2). Complex 2 exhibits the Mossbauer parameters δ = 0.05 mm/s and ΔE Q = 0.62 mm/s, typical of N-ligated S = 1 iron(IV)-oxo species. The iron(IV)-oxo complex has a half-life of only 14 min at 25 °C and is reactive toward oxygen-atom-transfer and hydrogen-atom-transfer (HAT) reactions. Compared to the parent complex [Fe IV (O)(N4Py)] 2+ , 2 is more reactive in oxidizing thioanisole and oxygenates the C−H bonds of aliphatic substrates including that of cyclohexane. The enhanced reactivity of 2 toward cyclohexane results from the involvement of the S = 2 transition state in the HAT pathway and a lower triplet-quintet splitting compared to [Fe IV (O)(N4Py)] 2+ , as supported by DFT calculations.The second-order rate constants for HAT by 2 is well correlated with the C−H bond dissociation energies of aliphatic substrates. Surprisingly, the slope of this correlation is different from that of [Fe IV (O)(N4Py)] 2+ , and 2 is more reactive only in the case of strong C−H bonds (>86 kcal/mol), but less reactive in the case of weaker C−H bonds. Using oxone as the oxidant, the iron(II) complex displays catalytic oxidations of substrates with low activity but with good selectivity.

Structurally Modelling the 2‐His‐1‐Carboxylate Facial Triad with a Bulky N,N,O Phenolate Ligand

“…The 2H1C has been an inspiration for the design of many biomimetic ligands, [13] some of the most well‐known being the Tp , [14] TPA , [15] 14‐TMC , [16] PyTACN , [17] PyNMe 3 , [18] BPBP , [19] and N4Py [20] ligands (Figure 2, top). These polydentate N‐donor ligands support the formation of high‐valent iron‐oxo species and have greatly helped the scientific community understand the role of such entities in enzymatic reactions and oxidation catalysis [21, 22] .…”

Effect of Ligand Fields on the Reactivity of O2‐Activating Iron(II)‐Benzilate Complexes of Neutral N5 Donor Ligands

“…However, complex 3 is found to oxidize adamantane with a C3/C2 normalized selectivity of 26 (Figure S17, Table ). Recently, similar C3/C2 selectivity has been reported in oxidation of adamantane with m ‐CPBA or oxone by the iron(II) complex [Fe II (N4Py Me2 )(CH 3 CN)](OTf) 2 complex . To rule out the involvement of a free radical mechanism, substrate oxidations were carried out in the presence of radical scavengers such as tert ‐butanol and 1,4‐benzoquinone.…”

“…These results indicate the generation of an electrophilic oxidant which could carry out epoxidation of alkenes and the C−H bond oxidation of cyclohexane (Table ). The fact that the N4Py and N4Py Me2 ligands can support the iron(IV)‐oxo unit and the results from mechanistic and interception studies together indicate that iron(IV)‐oxo species is generated in situ in the dioxygen‐dependent decarboxylation of the iron‐benzilate complexes. As reported for the Tp Ph2 system, an iron(IV)‐oxo species is proposed as the active oxidant from the respective iron(II)‐benzilate complexes that performs the epoxidation reactions in the presence of a protic acid.…”

“…The steric hindrance of the 6‐Me substituents is manifested in the longer metal‐ligand bonds and weaker ligand field. The effect of 6‐Me substituents on the pyridine rings of the pentadentate N4Py scaffold in weakening the ligand field strength of the bis(6‐methylpyridin‐2‐yl)‐ N , N ‐bis((pyridin‐2‐yl)methyl)methanamine (N4Py Me2 ) ligand in the equatorial plane has been reported recently …”

Facile Synthesis and Characterization of a Bromine-Substituted (Chloromethyl)Pyridine Precursor towards the Immobilization of Biomimetic Metal Ion Chelates on Functionalized Carbons

“…Many non-haem iron containing metalloenzymes bind iron through moieties known as the 2-His-1-carboxylate, 3-His facial triads or other combinations of histidine and carboxylate endogenous ligating residues [14][15][16][17]. These structural motifs have been extensively modeled using pyridyl based ligands (Figure 1) that have been shown to support high valent metal-oxo species capable of activating C-H bonds of substrates similarly to their enzymatic counterparts [18][19][20][21][22][23]. The appeal of these heterocyclic N-donor groups is that they function as strong σ donors to support high oxidation states and provide flexibility in their functional derivatization for the tuning of primary coordination spheres [24].…”