Characterization of a Ferryl Flip in Electronically Tuned Nonheme Complexes. Consequences in Hydrogen Atom Transfer Reactivity

Two oxoiron(IV) isomers (^R 2a and ^R 2b) of general formula [Fe^IV (O)(^R PyNMe₃ )(CH₃ CN)]²⁺ are obtained by reaction of their iron(II) precursor with NBu₄ IO₄ . The two isomers differ in the position of the oxo ligand, cis and trans to the pyridine donor. The mechanism of isomerization between ^R 2a and ^R 2b has been determined by kinetic and computational analyses uncovering an unprecedented path for interconversion of geometrical oxoiron(IV) isomers. The activity of the two oxoiron(IV) isomers in hydrogen atom transfer (HAT) reactions shows that ^R 2a reacts one order of magnitude faster than ^R 2b, which is explained by a repulsive noncovalent interaction between the ligand and the substrate in ^R 2b. Interestingly, the electronic properties of the R substituent in the ligand pyridine ring do not have a significant effect on reaction rates. Overall, the intrinsic structural aspects of each isomer define their relative HAT reactivity, overcoming changes in electronic properties of the ligand.

C-H Bond Cleavage by Bioinspired Nonheme Metal Complexes

The functionalization of C-H bonds is one of the most challenging transformations in synthetic chemistry. In biology, these processes are well-known and are achieved with a variety of metalloenzymes, many of which contain a single metal center within their active sites. The most well studied are those with Fe centers, and the emerging experimental data show that high-valent iron oxido species are the intermediates responsible for cleaving the C-H bond. This Forum Article describes the state of this field with an emphasis on nonheme Fe enzymes and current experimental results that provide insights into the properties that make these species capable of C-H bond cleavage. These parameters are also briefly considered in regard to manganese oxido complexes and Cu-containing metalloenzymes. Synthetic iron oxido complexes are discussed to highlight their utility as spectroscopic and mechanistic probes and reagents for C-H bond functionalization. Avenues for future research are also examined.

Semiempirical method for examining asynchronicity in metal-oxido-mediated C-H bond activation

The oxidation of substrates via the cleavage of thermodynamically strong C-H bonds is an essential part of mammalian metabolism. These reactions are predominantly carried out by enzymes that produce high-valent metal-oxido species, which are directly responsible for cleaving the C-H bonds. While much is known about the identity of these transient intermediates, the mechanistic factors that enable metal-oxido species to accomplish such difficult reactions are still incomplete. For synthetic metal-oxido species, C-H bond cleavage is often mechanistically described as synchronous, proton-coupled electron transfer (PCET). However, data have emerged that suggest that the basicity of the M-oxido unit is the key determinant in achieving enzymatic function, thus requiring alternative mechanisms whereby proton transfer (PT) has a more dominant role than electron transfer (ET). To bridge this knowledge gap, the reactivity of a monomeric MnIV-oxido complex with a series of external substrates was studied, resulting in a spread of over 104 in their second-order rate constants that tracked with the acidity of the C-H bonds. Mechanisms that included either synchronous PCET or rate-limiting PT, followed by ET, did not explain our results, which led to a proposed PCET mechanism with asynchronous transition states that are dominated by PT. To support this premise, we report a semiempirical free energy analysis that can predict the relative contributions of PT and ET for a given set of substrates. These findings underscore why the basicity of M-oxido units needs to be considered in C-H functionalization.

A Pseudotetrahedral Terminal Oxoiron(IV) Complex: Mechanistic Promiscuity in C-H Bond Oxidation Reactions

S=2 oxoiron(IV) species act as reactive intermediates in the catalytic cycle of nonheme iron oxygenases. The few available synthetic S=2 FeIV =O complexes known to date are often limited to trigonal bipyramidal and very rarely to octahedral geometries. Herein we describe the generation and characterization of an S=2 pseudotetrahedral FeIV =O complex 2 supported by the sterically demanding 1,4,7-tri-tert-butyl-1,4,7-triazacyclononane ligand. Complex 2 is a very potent oxidant in hydrogen atom abstraction (HAA) reactions with large non-classical deuterium kinetic isotope effects, suggesting hydrogen tunneling contributions. For sterically encumbered substrates, direct HAA is impeded and an alternative oxidative asynchronous proton-coupled electron transfer mechanism prevails, which is unique within the nonheme oxoiron community. The high reactivity and the similar spectroscopic parameters make 2 one of the best electronic and functional models for a biological oxoiron(IV) intermediate of taurine dioxygenase (TauD-J).

A Stable Homoleptic Organometallic Iron(IV) Complex

A homoleptic organometallic Fe^IV complex that is stable in both solution and in the solid state at ambient conditions has been synthesized and isolated as [Fe(phtmeimb)₂ ](PF₆ )₂ (phtmeimb=[phenyl(tris(3-methylimidazolin-2-ylidene))borate]^- ). This Fe^IV N-heterocyclic carbene (NHC) complex was characterized by ¹ H NMR, HR-MS, elemental analysis, scXRD analysis, electrochemistry, Mößbauer spectroscopy, and magnetic susceptibility. The two latter techniques unequivocally demonstrate that [Fe(phtmeimb)₂ ](PF₆ )₂ is a triplet Fe^IV low-spin S=1 complex in the ground state, in agreement with quantum chemical calculations. The electronic absorption spectrum of [Fe(phtmeimb)₂ ](PF₆ )₂ in acetonitrile shows an intense absorption band in the red and near IR, due to LMCT (ligand-to-metal charge transfer) excitation. For the first time the excited state dynamics of a Fe^IV complex was studied and revealed a ≈0.8 ps lifetime of the ³ LMCT excited state of [Fe(phtmeimb)₂ ](PF₆ )₂ in acetonitrile.

Ligand Taxonomy for Bioinorganic Modeling of Dioxygen-Activating Non-Heme Iron Enzymes

Novel functions emerge from novel structures. To develop efficient catalytic systems for challenging chemical transformations, chemists often seek inspirations from enzymatic catalysis. A large number of iron complexes supported by nitrogen-rich multidentate ligands have thus been developed to mimic oxo-transfer reactivity of dioxygen-activating metalloenzymes. Such efforts have significantly advanced our understanding of the reaction mechanisms by trapping key intermediates and elucidating their geometric and electronic properties. Critical to the success of this biomimetic approach is the design and synthesis of elaborate ligand systems to balance the thermodynamic stability, structural adaptability, and chemical reactivity. In this Concept article, representative design strategies for biomimetic atom-transfer chemistry are discussed from the perspectives of "ligand builders". Emphasis is placed on how the primary coordination sphere is constructed, and how it can be elaborated further by rational design for desired functions.

A Switch from Mechanistic Competition Mediated by a Combination of Temperature and Concentration Effects in the Oxidation Reaction of [FeII (N4Py/TPA)](OTf)2

The formation of [(N4Py)Fe^IV =O]²⁺ species was accomplished by the reaction of [Fe^II (N4Py)]²⁺ with 20 equivalents of tBuO₂ H (TBHP, 70 % in H₂ O). The temperature, [Fe^II (N4Py)]²⁺ -concentration and H₂ O-concentration in anhydrous TBHP (5.5 m in decane) dependences of its yields and rates were analyzed to indicate that the proton migration from [(N4Py)Fe^II -HOOtBu]²⁺ to [(N4Py)Fe^II -OO^⊕ HtBu]²⁺ is the rate-determining step followed by rapid heterolytic O-O bond cleavage of Fe^II -OO^⊕ HtBu to Fe^IV =O complex. The formation of [(TPA)Fe^IV =O]²⁺ is thus revealed to be greatly enhanced by the similar oxidation of [Fe^II (TPA)]²⁺ (40 mm) with 10 equivalents of tBuO₂ H at -45 °C. These results demonstrate the heterolytic O-O bond cleavage of Fe^II -alkylperoxo species to form Fe^IV =O originating from the direct reaction of iron(II) complexes/TBHP. The observation of concentration and temperature effects leads to the hypothesis that O-O bond homolysis is a kinetic control pathway and O-O bond heterolysis is a thermodynamic control pathway.

Synthesis of an Iron(IV) Aqua-Oxido Complex Using Ozone as an Oxidant

The iron(IV) oxido complex [(tmc)Fe=O(OTf)]OTf with the macrocyclic ligand 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclo-tetradecane (tmc) has been synthesized using ozone as an oxidant. By adding water to this compound the complex [(H₂ O)(tmc)Fe=O)](OTf)₂ could be prepared. This complex is important in regard to a better understanding of the reactivity of Fe^IV oxido complexes. Mössbauer measurements using the solid compound showed an isomer shift of δ=0.19 mm s^-1 and a quadrupole splitting ΔE_Q =1.38 mm s^-1 , confirming the high-valent Fe^IV state. DFT calculations were performed and led to an assignment of triplet spin multiplicity. Crystallographic characterization of [(H₂ O)(tmc)Fe=O)](OTf)₂ as well as of starting materials [(tmc)Fe(CH₃ CN)](OTf)₂ and [(tmc)Fe(OTf)]OTf together with previous results strongly suggest that [(H₂ O)(tmc)Fe=O)](OTf)₂ was formed similar to the oxido-hydroxido tautomerism analogous to heme systems.