Mechanistic Insights on the ortho-Hydroxylation of Aromatic Compounds by Non-heme Iron Complex: A Computational Case Study on the Comparative Oxidative Ability of Ferric-Hydroperoxo and High-Valent FeIV═O and FeV═O Intermediates

Origin of Unprecedented Formation and Reactivity of FeIV═O Species via Oxygen Activation: Role of Noncovalent Interactions and Magnetic Coupling

Emulating the capabilities of the soluble methane monooxygenase (sMMO) enzymes, which effortlessly activate oxygen at diiron(II) centers to form a reactive diiron(IV) intermediate Q, which then performs the challenging oxidation of methane to methanol, poses a significant challenge. Very recently, one of us reported the mononuclear complex [(cyclam)FeII(CH3CN)2]2+ (1), which performed a rare bimolecular activation of the molecule of O2 to generate two molecules of FeIV═O without the requirement of external proton or electron sources, similar to sMMO. In the present study, we employed the density functional theory (DFT) calculations to investigate this unique mechanism of O2 activation. We show that secondary hydrogen-bonding interactions between ligand N-H groups and O2 play a vital role in reducing the energy barrier associated with the initial O2 binding at 1 and O-O bond cleavage to form the FeIV═O complex. Further, the unique reactivity of FeIV═O species toward simultaneous C-H and O-H bond activation process has been demonstrated. Our study unveils that the nature of the magnetic coupling between the diiron centers is also crucial. Given that the influence of magnetic coupling and noncovalent interactions in catalysis remains largely unexplored, this unexplored realm presents numerous avenues for experimental chemists to develop novel structural and functional analogues of sMMO.

An Active Site Tyr Residue Guides the Regioselectivity of Lysine Hydroxylation by Nonheme Iron Lysine-4-hydroxylase Enzymes through Proton-Coupled Electron Transfer

Lysine dioxygenase (KDO) is an important enzyme in human physiology involved in bioprocesses that trigger collagen cross-linking and blood pressure control. There are several KDOs in nature; however, little is known about the factors that govern the regio- and stereoselectivity of these enzymes. To understand how KDOs can selectively hydroxylate their substrate, we did a comprehensive computational study into the mechanisms and features of 4-lysine dioxygenase. In particular, we selected a snapshot from the MD simulation on KDO5 and created large QM cluster models (A, B, and C) containing 297, 312, and 407 atoms, respectively. The largest model predicts regioselectivity that matches experimental observation with rate-determining hydrogen atom abstraction from the C4-H position, followed by fast OH rebound to form 4-hydroxylysine products. The calculations show that in model C, the dipole moment is positioned along the C4-H bond of the substrate and, therefore, the electrostatic and electric field perturbations of the protein assist the enzyme in creating C4-H hydroxylation selectivity. Furthermore, an active site Tyr233 residue is identified that reacts through proton-coupled electron transfer akin to the axial Trp residue in cytochrome c peroxidase. Thus, upon formation of the iron(IV)-oxo species in the catalytic cycle, the Tyr233 phenol loses a proton to the nearby Asp179 residue, while at the same time, an electron is transferred to the iron to create an iron(III)-oxo active species. This charged tyrosyl residue directs the dipole moment along the C4-H bond of the substrate and guides the selectivity to the C4-hydroxylation of the substrate.

A density functional theory analysis of the C-H activation reactivity of iron(IV)-oxo complexes with an 'O' substituted tetramethylcyclam macrocycle

In this article, we present a meticulous computational study to foresee the effect of an oxygen-rich macrocycle on the reactivity for C-H activation. For this study, a widely studied nonheme Fe(IV)O molecule with a TMC (1,4,8,11-tetramethyl 1,4,8,11-tetraazacyclotetradecane) macrocycle that is equatorially attached to four nitrogen atoms (designated as N4) and acetonitrile as an axial ligand has been taken into account. For the goal of hetero-substitution, step-by-step replacement of the N4 framework with O atoms, i.e., N4, N3O1, N2O2, N1O3, and O4 systems, has been considered, and dihydroanthracene (DHA) has been used as the substrate. In order to neutralise the system and prevent the self-interaction error in DFT, triflate counterions have also been included in the calculations. The study of the energetics of these C-H bond activation reactions and the potential energy surfaces mapped therefore reveal that the initial hydrogen abstraction, which is the rate-determining step, follows the two-state reactivity (TSR) pattern, which means that the originally excited quintet state falls lower in the transition state and the product. The reaction follows the hydrogen atom transfer (HAT) mechanism, as indicated by the spin density studies. The results revealed a fascinating reactivity order, in which the reactivity increases with the enrichment of the oxygen atom in the equatorial position, namely the order follows N4 < N3O1 < N2O2 < N1O3 < O4. The impacts of oxygen substitution on quantum mechanical tunneling and the H/D kinetic isotope effect have also been investigated. When analysing the causes of this reactivity pattern, a number of variables have been identified, including the reactant-like transition structure, spin density distribution, distortion energy, and energies of the electron acceptor orbital, i.e., the energy of the LUMO (σ*z2), which validate the obtained outcome. Our results also show very good agreement with earlier combined experimental and theoretical studies considering TMC and TMCO-type complexes. The DFT predictions reported here will undoubtedly encourage experimental research in this biomimetic field, as they provide an alternative with higher reactivity in which heteroatoms can be substituted for the traditional nitrogen atom.

Computational Study Into the Oxidative Ring-Closure Mechanism During the Biosynthesis of Deoxypodophyllotoxin

The nonheme iron dioxygenase deoxypodophyllotoxin synthase performs an oxidative ring-closure reaction as part of natural product synthesis in plants. How the enzyme enables the oxidative ring-closure reaction of (-)-yatein and avoids substrate hydroxylation remains unknown. To gain insight into the reaction mechanism and understand the details of the pathways leading to products and by-products we performed a comprehensive computational study. The work shows that substrate is bound tightly into the substrate binding pocket with the C7'-H bond closest to the iron(IV)-oxo species. The reaction proceeds through a radical mechanism starting with hydrogen atom abstraction from the C7'-H position followed by ring-closure and a final hydrogen transfer to form iron(II)-water and deoxypodophyllotoxin. Alternative mechanisms including substrate hydroxylation and an electron transfer pathway were explored but found to be higher in energy. The mechanism is guided by electrostatic perturbations of charged residues in the second-coordination sphere that prevent alternative pathways.

Enhanced Reactivity through Equatorial Sulfur Coordination in Nonheme Iron(IV)-Oxo Complexes: Insights from Experiment and Theory

Sulfur ligation in metalloenzymes often gives the active site unique properties, whether it is the axial cysteinate ligand in the cytochrome P450s or the equatorial sulfur/thiol ligation in nonheme iron enzymes. To understand sulfur ligation to iron complexes and how it affects the structural, spectroscopic, and intrinsic properties of the active species and the catalysis of substrates, we pursued a systematic study and compared sulfur with amine-ligated iron(IV)-oxo complexes. We synthesized and characterized a biomimetic N4S-ligated iron(IV)-oxo complex and compared the obtained results with an analogous N5-ligated iron(IV)-oxo complex. Our work shows that the amine for sulfur replacement in the equatorial ligand framework leads to a rate enhancement for oxygen atom and hydrogen atom transfer reactions. Moreover, the sulfur-ligated iron(IV)-oxo complex reacts through a different reaction mechanism as compared to the N5-ligated iron(IV)-oxo complex, where the former reacts through hydride transfer with the latter reacting via radical pathways. We show that the reactivity differences are caused by a dramatic change in redox potential between the two complexes. Our studies highlight the importance of implementing a sulfur ligand into the equatorial ligand framework of nonheme iron(IV)-oxo complexes and how it affects the physicochemical properties of the oxidant and its reactivity.

C-H bond activation by high-valent iron/cobalt-oxo complexes: a quantum chemical modeling approach

High-valent metal-oxo species serve as key intermediates in the activation of inert C-H bonds. Here, we present a comprehensive DFT analysis of the parameters that have been proposed as influencing factors in modeled high-valent metal-oxo mediated C-H activation reactions. Our approach involves utilizing DFT calculations to explore the electronic structures of modeled FeIVO (species 1) and CoIVO ↔ CoIII-O˙ (species 2), scrutinizing their capacity to predict improved catalytic activity. DFT and DLPNO-CCSD(T) calculations predict that the iron-oxo species possesses a triplet as the ground state, while the cobalt-oxo has a doublet as the ground state. Furthermore, we have investigated the mechanistic pathways for the first C-H bond activation, as well as the desaturation of the alkanes. The mechanism was determined to be a two-step process, wherein the first hydrogen atom abstraction (HAA) represents the rate-limiting step, involving the proton-coupled electron transfer (PCET) process. However, we found that the second HAA step is highly exothermic for both species. Our calculations suggest that the iron-oxo species (Fe-O = 1.672 Å) exhibit relatively sluggish behavior compared to the cobalt-oxo species (Co-O = 1.854 Å) in C-H bond activation, attributed to a weak metal-oxygen bond. MO, NBO, and deformation energy analysis reveal the importance of weakening the M-O bond in the cobalt species, thereby reducing the overall barrier to the reaction. This catalyst was found to have a C-H activation barrier relatively smaller than that previously reported in the literature.

Unravelling the Effect of Acid-Driven Electron Transfer in High-Valent FeIV =O/MnIV =O Species and Its Implications for Reactivity

The electron transfer (ET) step is one of the crucial processes in biochemical redox reactions that occur in nature and has been established as a key step in dictating the reactivity of high-valent metal-oxo species. Although metalloenzymes possessing metal-oxo units at their active site are typically associated with outer-sphere electron transfer (OSET) processes, biomimetic models, in contrast, have been found to manifest either an inner-sphere electron transfer (ISET) or OSET mechanism. This distinction is clearly illustrated through the behaviour of [(N4Py)MnIV (O)]2+ (1) and [(N4Py)FeIV (O)]2+ (2) complexes, where complex 1 showcases an OSET mechanism, while complex 2 exhibits an ISET mechanism, especially evident in their reactions involving C-H bond activation and oxygen atom transfer reactions in the presence of a Lewis/Bronsted acid. However, the precise reason for this puzzling difference remains elusive. This work unveils the origin of the perplexing inner-sphere vs outer-sphere electron transfer process (ISET vs OSET) in [(N4Py)MnIV (O)]2+ (1) and [(N4Py)FeIV (O)]2+ (2) species in the presence of Bronsted acid. The calculations indicate that when the substrate (toluene) approaches both 1 and 2 that is hydrogen bonded with two HOTf molecules (denoted as 1-HOTf and 2-HOTf, respectively), proton transfer from one of the HOTf molecules to the metal-oxo unit is triggered and a simultaneous electron transfer occurs from toluene to the metal centre. Interestingly, the preference for OSET by 1-HOTf is found to originate from the choice of MnIV =O centre to abstract spin-down (β) electron from toluene to its δ(dxy ) orbital. On the other hand, in 2-HOTf, a spin state inversion from triplet to quintet state takes place during the proton (from HOTf) coupled electron transfer (from toluene) preferring a spin-up (α) electron abstraction to its σ* (dz 2 ) orbital mediated by HOTf giving rise to ISET. In addition, 2-HOTf was calculated to possess a larger reorganisation energy, which facilitates the ISET process via the acid. The absence of spin-inversion and smaller reorganisation energy switch the mechanism to OSET for 1-HOTf. Therefore, for the first time, the significance of spin-state and spin-inversion in the electron transfer process has been identified and demonstrated within the realm of high-valent metal-oxo chemistry. This discovery holds implications for the potential involvement of high-valent Mn-oxo species in performing similar transformative processes within Photosystem II.

Natural Abiotic Iron-Oxido-Mediated Formation of C1 and C2 Compounds from Environmentally Important Methyl-Substituted Substrates

Organic and inorganic volatile compounds containing one carbon atom (C1), such as carbon dioxide, methane, methanol, formaldehyde, carbon monoxide, and chloromethane, are ubiquitous in the environment, are key components in global carbon cycling, play an important role in atmospheric physics and chemistry, e.g., as greenhouse gases, destroy stratospheric and tropospheric ozone, and control the atmospheric oxidation capacity. Up to now, most C1 compounds in the environment were associated with complex metabolic and enzymatic pathways in organisms or to combustion processes of organic matter. We now present compelling evidence that many C1 and C2 compounds have a common origin in methyl groups of methyl-substituted substrates that are cleaved by the iron oxide-mediated formation of methyl radicals. This scenario is derived from experiments with a mechanistically well-studied bispidine-iron-oxido complex as oxidant and dimethyl sulfoxide as the environmentally relevant model substrate and is supported by computational modeling based on density functional theory and ab initio quantum-chemical studies. The exhaustive experimental model studies, also involving extensive isotope labeling, are complemented with the substitution of the bispidine model system by environmentally relevant iron oxides and, finally, a collection of soils with varying iron and organic matter contents. The combination of all data suggests that the iron oxide-mediated formation of methyl radicals from methyl-substituted substrates is a common abiotic source for widespread C1 and C2 compounds in the environment.

DFT and TDDFT exploration on the role of pyridyl ligands with copper toward bonding aspects and light harvesting

CONTEXT

Schiff base-containing metal complexes have been the subject of extensive research. In this work, a coordination polymer-derived complex called [Cu(L)] that is solution-stable (L = 2-(2-hydroxybenzylidene-amino)phenol) has been explored theoretically with five different pyridyl-based ligands using DFT/TDDFT in order to understand the structural-functional and electronic transitions of these five complexes. Frontier molecular orbital (FMO) analysis was carried out to assess the reactivity behavior of all five complexes. For the purpose of studying the charge energy distribution over complexes, electrostatic potential maps were also drawn. Furthermore, in order to identify any stabilizing interactions that may be present in the given complexes, an NBO analysis was studied. To learn more about any potential correlations between the properties of these five complexes, a comparative analysis was explored. Our calculations demonstrate that complex 3 having pyridine-4-carboxamide as a ligand has a lower energy gap and a higher negative electrostatic potential which may indicate its higher reactivity and this may be due to the electron withdrawing group (carboxamide). TDDFT results show that the highest light harvesting efficiency (LHE) of all the studied complexes is found in the range of 440-448 nm. Complexes 1, 2, and 4 show the higher light harvesting efficiency as compared to complexes 3 and 5. Our findings are in good accordance with the available experimental data.

METHODS

All DFT computations were performed using the Gaussian16 with unrestricted B3LYP-D2 functional with the basis sets 6-31G(d,p) for O, N, C, and H while LanL2DZ for Cu. The polarized continuum model (PCM) was used for the solvation. The software GaussView6.1 was utilized for the modeling of initial geometries and the plotting of MEP maps. The NBO6.0 program which is incorporated in Gaussian16 was utilized to investigate the bonding nature and stabilization energies of the complexes. The ORCA program was used to simulate the absorption spectra.

Theoretical study of the formation of metal-oxo species of the first transition series with the ligand 14-TMC: driving factors of the "Oxo Wall"

Terminal metal-oxo species of the early transition metal series are well known, whereas those for the late transition series are rare, and this is related to the "Oxo Wall". Here, we have undertaken a theoretical study on the formation of metal-oxo species from the metal hydroperoxo species of the 3d series (Cr, Mn, Fe, Co, Ni, and Cu) with the ligand 14-TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) via O⋯O bond cleavage. DFT calculations reveal that the barrier for O⋯O bond cleavage is higher with the late transition metals (Co, Ni, and Cu) than the early transition metals (Cr, Mn, and Fe), and the formed late metal-oxo species are also thermodynamically less stable. The higher barrier may be due to electronic repulsion because of the pairing of d electrons. In the late transition metal series, the electron goes into an antibonding orbital, which decreases the bond order and hence decreases the possibility of metal-oxo formation. Computed structural parameters and spin densities suggest that valence tautomerism occurs in the late transition metal-oxo species which remain as a metal-oxyl. Our findings support the concept of the "Oxo Wall".

Probing the Origins of Puzzling Reactivity in Fe/Mn-Oxo/Hydroxo Species toward C-H Bonds: A DFT and Ab Initio Perspective

Activation of C-H bonds using an earth-abundant metal catalyst is one of the top challenges of chemistry, where high-valent Mn/Fe-oxo(hydroxo) biomimic species play an important role. There are several open questions related to the comparative oxidative abilities of these species, and a unifying concept that could accommodate various factors influencing reactivity is lacking. To shed light on these open questions, here, we have used a combination of density functional theory (DFT) (B3LYP-D3/def2-TZVP) and ab initio (CASSCF/NEVPT2) calculations to study a series of high-valent metal-oxo species [Mn+H3buea(O/OH)] (M = Mn and Fe, n = II to V; H3buea = tris[(N'-tert-butylureaylato)-N-ethylene)]aminato towards the activation of dihydroanthracene (DHA). The H-bonding network in the ligand architecture influences the ground state-excited state gap and brings several excited states of the same spin multiplicity closer in energy, which triggers reactivity via one of those excited states, reducing the kinetic barriers for the C-H bond activation and rationalizing several puzzling reactivity trends observed in various high-valent Mn/Fe-oxo(hydroxo) species.

Elusive Active Intermediates and Reaction Mechanisms of ortho-/ipso-Hydroxylation of Benzoic Acid by Hydrogen Peroxide Mediated by Bioinspired Iron(II) Catalysts

Aromatic hydroxylation of benzoic acids (BzOH) to salicylates and phenolates is fundamentally interesting in industrial chemistry. However, key mechanistic uncertainties and dichotomies remain after decades of effort. Herein, the elusive mechanism of the competitive ortho-/ipso-hydroxylation of BzOH by H2O2 mediated by a nonheme iron(II) catalyst was comprehensively investigated using density functional theory calculations. Results revealed that the long-postulated FeV(O)(anti-BzO) oxidant is an FeIV(O)(anti-BzO•) species 2 (anti- and syn- are defined by the orientation of the carboxyl oxygen of BzO to the oxo), which rules out the noted two-oxidant mechanism proposed previously. We propose a new mechanism in which, following the formation of an FeV(O)(syn-BzO) species (3) and its electromer FeIV(O)(syn-BzO•) (3'), 3/3' either converts to salicylate and phenolate via intramolecular self-hydroxylation (route A) or acts as an oxidant to oxygenate another BzOH to generate the same products (route B). In route A, the rotation of the BzO group along the C-O bond forms 2, in which the BzO group is orientated by π-π stacking interactions. An electrophilic ipso-addition forms a phenolate by concomitant decarboxylation or an ortho-attack forms a cationic complex, which readily undergoes an NIH shift and a BzOH-assisted proton shift to form a salicylate. In route B, 3 oxidizes an additional BzOH molecule directed by hydrogen bonding and π-π stacking interactions. In both routes, selectivity is determined by the chemical property of the BzO ring. These mechanistic findings provide a clear mechanistic scenario and enrich the knowledge of hydroxylation of aromatic acids.

Accessing Bioactive Hydrazones by the Hydrohydrazination of Terminal Alkynes Catalyzed by Gold(I) Acyclic Aminooxy Carbene Complexes and Their Gold(I) Arylthiolato and Gold(III) Tribromo Derivatives: A Combined Experimental and Computational Study

Hydrohydrazination of terminal alkynes with hydrazides yielding hydrazones 5-14 were successfully catalyzed by a series of gold(I) acyclic aminooxy carbene complexes of the type [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuCl, where R² = H, R¹ = Me (1b); R² = H, R¹ = Cy (2b); R² = t-Bu, R¹ = Me (3b); R² = t-Bu, R¹ = Cy (4b). The mass spectrometric evidence corroborated the existence of the catalytically active solvent-coordinated [(AAOC)Au(CH₃CN)]SbF₆ (1-4)A species and the acetylene-bound [(AAOC)Au(HC≡CPhMe)]SbF₆ (3B) species of the proposed catalysis cycle. The hydrohydrazination reaction was successfully employed in synthesizing several bioactive hydrazone compounds (15-18) with anticonvulsant properties using a representative precatalyst (2b). The DFT studies favored the 4-ethynyltoluene (HC≡CPhMe) coordination pathway over the p-toluenesulfonyl hydrazide (NH₂NHSO₂C₆H₄CH₃) coordination pathway, and that proceeded by a crucial intermolecular hydrazide-assisted proton transfer step. The gold(I) complexes (1-4)b were synthesized from the {[(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)]CH}⁺OTf^- (1-4)a by treatment with (Me₂S)AuCl in the presence of NaH as a base. The reactivity studies of (1-4)b yielded the gold(III) [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuBr₃ (1-4)c complexes upon reaction with molecular bromine and the gold(I) perfluorophenylthiolato derivatives, [{(4-R²-2,6-t-Bu₂-C₆H₂O)(N(R¹)₂)}methylidene]AuSC₆F₅ (1-4)d, upon treatment with C₆F₅SH.

Aromatic and aliphatic hydrocarbon hydroxylation via a formally NiIVO oxidant

The reaction of (NMe₄)₂[Ni^II(L^Ph)(OAc)] (1[OAc], L^Ph = 2,2',2''-nitrilo-tris-(N-phenylacetamide); OAc = acetate) with 3-chloroperoxybenzoic acid (m-CPBA) resulted in the formation of a self-hydroxylated Ni^III-phenolate complex, 2, where one of the phenyl groups of L^Ph underwent hydroxylation. 2 was characterised by UV-Vis, EPR, and XAS spectroscopies and ESI-MS. 2 decayed to yield a previously characterised Ni^II-phenolate complex, 3. We postulate that self-hydroxylation was mediated by a formally Ni^IVO oxidant, formed from the reaction of 1[OAc] with m-CPBA, which undergoes electrophilic aromatic substitution to yield 2. This is supported by an analysis of the kinetic and thermodynamic properties of the reaction of 1[OAc] with m-CPBA. Addition of exogenous hydrocarbon substrates intercepted the self-hydroxylation process, producing hydroxylated products, providing further support for the formally Ni^IVO entity. This study demonstrates that the reaction between Ni^II salts and m-CPBA can lead to potent metal-based oxidants, in contrast to recent studies demonstrating carboxyl radical is a radical free-chain reaction initiator in Ni^II/m-CPBA hydrocarbon oxidation catalysis.

Electronic, geometrical and photophysical facets of five coordinated porphyrin N-heterocyclic carbene transition metals complexes: A theoretical study

In the realm of dye sensitized solar cells (DSSCs), the 3d transition metals as photosensitizers are scarcely studied. In the present work, electronic structures, FMO, MEP surfaces, NBO analysis, energetics and photophysical properties of earth abundant metals (Mn, Fe and Co) based metalloporphyrins coordinated with NHC-carbene have been explored by using DFT and TDDFT calculations. According to formation energies and energy decomposition analysis (EDA), the cobalt based metalloporphyrins species are found to be more stable while in contrast manganese based species are predicted as more reactive among all. Also, from the ligation point of view, the TPP (meso-tetraphenylporphyrin) ligand forms more steady and rigid coordination as compare to the TTP (meso-tetratolylporphyrin) ligand. FMO analysis also support these observations. NBO and SNO results support the electronic configurations as well as unveil the controversial bonding pattern of NHC_carbon and metal atom and found that there is σ-bonding present between the metal and the NHC_carbon by the overlapping of sp-hybridized orbitals of carbene_carbon and sp/d hybrid orbital of the metal atom. TDDFT results show that the highest light harvesting efficiency (LHE) of all the studied species is found under the range of 360 nm - 380 nm (λ) and this may due to the presence of longer π-conjugations. In-depth investigation of this work may help to design new robust energy harvesting systems for high energy conversion efficiency based on earth abundance metals. Our results are in well agreement with the available experimental findings.

Comparative oxidative ability of mononuclear and dinuclear high-valent iron-oxo species towards the activation of methane: does the axial/bridge atom modulate the reactivity?

Over the years, mononuclear Fe^IVO species have been extensively studied, but the presence of dinuclear Fe^IVO species in soluble methane monooxygenase (sMMO) has inspired the development of biomimic models that could activate inert substrates such as methane. There are some successful attempts; particularly the [(Por)(m-CBA) Fe^IV(μ-N)Fe^IV(O)(Por˙⁺)]^- species has been reported to activate methane and yield decent catalytic turnover numbers and therefore regarded as the closest to the sMMO enzyme functional model, as no mononuclear Fe^IVO analogues could achieve this feat. In this work, we have studied a series of mono and dinuclear models using DFT and ab initio DLPNO-CCSD(T) calculations to probe the importance of nuclearity in enhancing the reactivity. We have probed the catalytic activities of four complexes: [(HO)Fe^IV(O)(Por)]^- (1), [(HO)Fe^IV(O)(Por˙⁺)] (2), μ-oxo dinuclear iron species [(Por)(m-CBA)Fe^IV(μ-O)Fe^IV(O) (Por˙⁺)]^- (3) and N-bridged dinuclear iron species [(Por)(m-CBA)Fe^IV(μ-N)Fe^IV(O)(Por˙⁺)]^- (4) towards the activation of methane. Additionally, calculations were performed on the mononuclear models [(X)Fe^IV(O)(Por˙⁺)]ⁿ {X = N 4a (n = -2), NH 4b (n = -1) and NH₂4c (n = 0)} to understand the role of nuclearity in the reactivity. DFT calculations performed on species 1-4 suggest an interesting variation among them, with species 1-3 possessing an intermediate spin (S = 1) as a ground state and species 4 possessing a high-spin (S = 2) as a ground state. Furthermore, the two Fe^IV centres in species 3 and 4 are antiferromagnetically coupled, yielding a singlet state with a distinct difference in their electronic structure. On the other hand, species 2 exhibits a ferromagnetic coupling between the Fe^IV and the Por˙⁺ moiety. Our calculations suggest that the higher barriers for the C-H bond activation of methane and the rebound step for species 1 and 3 are very high in energy, rendering them unreactive towards methane, while species 2 and 4 have lower barriers, suggesting their reactivity towards methane. Studies on the system reveal that model 4a has multiple FeN bonds facilitating greater reactivity, whereas the other two models have longer Fe-N bonds and less radical character with steeper barriers. Strong electronic cooperativity is found to be facilitated by the bridging nitride atom, and this cooperativity is suppressed by substituents such as oxygen, rendering them inactive. Thus, our study unravels that apart from enhancing the nuclearity, bridging atoms that facilitate strong cooperation between the metals are required to activate very inert substrates such as methane, and our results are broadly in agreement with earlier experimental findings.

Electronic structures, bonding aspects and spectroscopic parameters of homo/hetero valent bridged dinuclear transition metal complexes

Bridged dinuclear metal complexes have fascinated scientists worldwide, and remarkable success has been achieved to unravel the electronic structures, structure-function relationship, coordination environments, and fine mechanistic details of the enzymes owing to the repercussion of biomimetic studies carried out on dinuclear model systems. Molecular level study of these systems integrated with spectroscopic study helps in gaining deep insights about structural and electronic aspects of natural enzymatic systems. Considering the same, here first time we report DFT study on bridged non-heme metal complexes based on N-Et-HPTB ligand system containing homovalent (M^IIM^II); {[(Mn^II)₂(O₂CCH₃)(N-Et-HPTB)]²⁺; Species I), [(Fe^II)₂(O₂CCH₃)(N-Et-HPTB)]²⁺; Species II), [(Co^II)₂(O₂CCH₃)(N-Et-HPTB)]²⁺; Species III)} and heterovalent (M^IIIM^II): {[(Mn^III)(Mn^II)(O₂)(N-Et-HPTB)]²⁺; Species Ia) [(Fe^III)(Fe^II)(O₂)(N-Et-HPTB)]²⁺; Species IIa) and [(Co^III)(Co^II)(O₂)(N-Et-HPTB)]²⁺; Species IIIa)} dinuclear metal centres. Bridging oxygen bears a significant spin density which may prompt important chemical reactions involving activation of bonds like C-H/O-H/N-H etc. TD-DFT calculations for UV-Visible absorption have been carried out to further shed light on structural-functional and electronic structures of these dinuclear species. Studying these dinuclear species may be a good starting point for the study of active sites of the bimetallic centre of dinuclear enzymes and thus may serve as fascinating spectroscopic models. Further, FMO analysis, MEP mapping, and NBO calculations were employed to analyze bonding aspects predict theoretical reactivity behaviour and any kind of stabilizing interactions present in the reported species.

Theoretical exploration on structures, bonding aspects and molecular docking of α-aminophosphonate ligated copper complexes against SARS-CoV-2 proteases

Recent years have witnessed a growing interest in the biological activity of metal complexes of α-aminophosphonates. Here for the first time, a detailed DFT study on five α-aminophosphonate ligated mononuclear/dinuclear CuII complexes is reported using the dispersion corrected density functional (B3LYP-D2) method. The electronic structures spin densities, FMO analysis, energetic description of spin states, and theoretical reactivity behaviour using molecular electrostatic potential (MEP) maps of all five species are reported. All possible spin states of the dinuclear species were computed and their ground state S values were determined along with the computation of their magnetic coupling constants. NBO analysis was also performed to provide details on stabilization energies. A molecular docking study was performed for the five complexes against two SARS-CoV-2 coronavirus protein targets (PDB ID: 6LU7 and 7T9K). The docking results indicated that the mononuclear species had a higher binding affinity for the targets compared to the dinuclear species. Among the species investigated, species I showed the highest binding affinity with the SARS-CoV-2 Omicron protease. NPA charge analysis showed that the heteroatoms of model species III had a more nucleophilic nature. A comparative study was performed to observe any variations and/or correlations in properties among all species.

What Drives Radical Halogenation versus Hydroxylation in Mononuclear Nonheme Iron Complexes? A Combined Experimental and Computational Study

Nonheme iron halogenases are unique enzymes in nature that selectively activate an aliphatic C–H bond of a substrate to convert it into C–X (X = Cl/Br, but not F/I). It is proposed that they generate an Fe^III(OH)(X) intermediate in their catalytic cycle. The analogous Fe^III(OH) intermediate in nonheme iron hydroxylases transfers OH^• to give alcohol product, whereas the halogenases transfer X^• to the carbon radical substrate. There remains significant debate regarding what factors control their remarkable selectivity of the halogenases. The reactivity of the complexes Fe^III(BNPA^Ph2O)(OH)(X) (X = Cl, Br) with a secondary carbon radical (R^•) is described. It is found that X^• transfer occurs with a secondary carbon radical, as opposed to OH^• transfer with tertiary radicals. Comprehensive computational studies involving density functional theory were carried out to examine the possible origins of this selectivity. The calculations reproduce the experimental findings, which indicate that halogen transfer is not observed for the tertiary radicals because of a nonproductive equilibrium that results from the endergonic nature of these reactions, despite a potentially lower reaction barrier for the halogenation pathway. In contrast, halogen transfer is favored for secondary carbon radicals, for which the halogenated product complex is thermodynamically more stable than the reactant complex. These results are rationalized by considering the relative strengths of the C–X bonds that are formed for tertiary versus secondary carbon centers. The computational analysis also shows that the reaction barrier for halogen transfer is significantly dependent on secondary coordination sphere effects, including steric and H-bonding interactions.

Effect of the ring size of TMC ligands in controlling C-H bond activation by metal-superoxo species

Metal-superoxo species play a very important role in many metal-mediated catalytic transformation reactions. Their catalytic reactivity is affected by many factors such as the nature of metal ions and ring size of ligands. Herein, for the first time, we report DFT calculations on the electronic structures of a series of metal-superoxo species (M = V, Cr, Mn, Fe, and Co) with two ring size ligands, i.e., 13-TMC/14-TMC, and a detailed mechanistic study on the C-H bond activation of cyclohexa-1,4-diene followed by the effect of the ring size of ligands. Our DFT results showed that the electron density at the distal oxygen plays an important role in C-H bond activation. By computing the energetics of C-H bond activation and mapping the potential energy surface, it was found that the initial hydrogen abstraction is the rate-determining step with both TMC rings and all the studied metal-superoxo species. The significant electron density at the cyclohex-1,4-diene carbon indicates that the reaction proceeds via the proton-coupled electron transfer mechanism. By mapping the potential energy surfaces, we found that the 13-TMC ligated superoxo with the anti-isomer are more reactive than the 14-TMC superoxo species except for the iron-superoxo species where the 14-TMC ligated superoxo species is more reactive i.e. smaller ring size TMC is more reactive towards C-H bond activation. This is also supported by the structural correlation, i.e., the greater contraction in the smaller ring results in the metal being pushed out of plane along the z-axis, which reduces the steric hindrance. Thus, the ring size can help in designing catalysts with better efficiency for catalytic reactions.

Electrostatic Perturbations in the Substrate-Binding Pocket of Taurine/α-Ketoglutarate Dioxygenase Determine its Selectivity

Taurine/α-ketoglutarate dioxygenase is an important enzyme that takes part in the cysteine catabolism process in the human body and selectively hydroxylates taurine at the C1 -position. Recent computational studies showed that in the gas-phase the C2 -H bond of taurine is substantially weaker than the C1 -H bond, yet no evidence exists of 2-hydroxytaurine products. To this end, a detailed computational study on the selectivity patterns in TauD was performed. The calculations show that the second-coordination sphere and the protonation states of residues play a major role in guiding the enzyme to the right selectivity. Specifically, a single proton on an active site histidine residue can change the regioselectivity of the reaction through its electrostatic perturbations in the active site and effectively changes the C1 -H and C2 -H bond strengths of taurine. This is further emphasized by many polar and hydrogen bonding interactions of the protein cage in TauD with the substrate and the oxidant that weaken the pro-R C1 -H bond and triggers a chemoselective reaction process. The large cluster models reproduce the experimental free energy of activation excellently.

Organometallic 3d transition metal NHC complexes in oxidation catalysis

The development of processes for the selective oxidation of hydrocarbons is a major focus in catalysis research. Making this process simultaneously environmentally friendly is still challenging. 3d transition metals are...

Theoretical insights for generation of terminal metal-oxo species and involvement of the “oxo wall”

This work is based on a deep insight on the formation of high-valent metal-oxo by the O⋯O bond cleavage of metal hydroperoxo species and our theoretical findings also illustrate the concept “oxo wall”.

Biodegradation of herbicides by a plant nonheme iron dioxygenase: mechanism and selectivity of substrate analogues

Aryloxyalkanoate dioxygenases are unique herbicide biodegrading nonheme iron enzymes found in plants and hence, from environmental and agricultural point of view they are important and valuable. However, they often are substrate specific and little is known on the details of the mechanism and the substrate scope. To this end, we created enzyme models and calculate the mechanism for 2,4-dichlorophenoxyacetic acid biodegradation and 2-methyl substituted analogs by density functional theory. The work shows that the substrate binding is tight and positions the aliphatic group close to the metal center to enable a chemoselective reaction mechanism to form the C 2 -hydroxy products, whereas the aromatic hydroxylation barriers are well higher in energy. Subsequently, we investigated the metabolism of R - and S -methyl substituted inhibitors and show that these do not react as efficiently as 2,4-dichlorophenoxyacetic acid substrate due to stereochemical clashes in the active site and particularly for the R -isomer give high rebound barriers.

Mechanistic Insights into the Oxygen Atom Transfer Reactions by Nonheme Manganese Complex: A Computational Case Study on the Comparative Oxidative Ability of Manganese-Hydroperoxo vs High-Valent MnIV═O and MnIV-OH Intermediates

Understanding the comparative oxidative abilities of high-valent metal-oxo/hydroxo/hydroperoxo species holds the key to robust biomimic catalysts that perform desired organic transformations with very high selectivity and efficiency. The comparative oxidative abilities of popular high-valent iron-oxo and manganese-oxo species are often counterintuitive, for example, oxygen atom transfer (OAT) reaction by [(Me₂EBC)Mn^IV-OOH]³⁺, [(Me₂EBC)Mn^IV-OH]³⁺, and [(Me₂EBC)Mn^IV═O]²⁺ (Me₂EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane) shows extremely high reactivity for Mn^IV-OOH species and no reactivity for Mn^IV-OH and Mn^IV═O species toward alkyl/aromatic sulfides. Using a combination of density functional theory (DFT) and ab initio domain-based local pair natural orbital coupled-cluster with single, double, and perturbative triples excitation (DLPNO-CCSD(T)) and complete-active space self-consistent field/N-electron valence perturbation theory second order (CASSCF/NEVPT2) calculations, here, we have explored the electronic structures and sulfoxidation mechanism of these species. Our calculations unveil that Mn^IV-OOH reacts through distal oxygen atom with the substrate via electron transfer (ET) mechanism with a very small kinetic barrier (16.5 kJ/mol), placing this species at the top among the best-known catalysts for such transformations. The Mn^IV-OH and Mn^IV═O species have a much larger barrier. The mechanism has also been found to switch from ET in the former to concerted in the latter, rendering both unreactive under the tested experimental conditions. Intrinsic differences in the electronic structures, such as the presence and absence of the multiconfigurational character coupled with the steric effects, are responsible for such variations observed. This comparative oxidative ability that runs contrary to the popular iron-oxo/hydroperoxo reactivity will have larger mechanistic implications in understanding the reactivity of biomimic catalysts and the underlying mechanisms in PSII.

Novel {Cu4} and {Cu4Cd6} clusters derived from flexible aminoalcohols: synthesis, characterization, crystal structures, and evaluation of anticancer properties

Two new copper clusters, {Cu₄} and {Cu₄Cd₆}, with polydentate aminoalcohol ligands, diethanol propanolamine (H₃L¹) and bis-tris{2-[bis(2-hydroxyethyl)amino]-2-(hydroxymethyl)propane-1,3-diol} (H₆L²), have been synthesized under mild conditions and characterized thoroughly by single-crystal X-ray diffraction (XRD), infrared spectroscopy, elemental analysis, powder XRD, magnetic and DFT studies, and absorption and fluorescence spectroscopy. The cluster {Cu₄} exhibits a rare tetranuclear copper cubane core whereas {Cu₄Cd₆} forms an unusual heterometallic cage owing to the introduction of the second metal Cd into the ligand. A hexapodal ligand (H₆L²) with N and O donor atoms was chosen deliberately for the construction of a high-nuclearity cluster, i.e., {Cu₄Cd₆}. Interestingly, both the clusters displayed significant cytotoxicity towards human cervical (HeLa) and lung (A549) cancer cells as evident from the shallow IC₅₀ values [15.6 ± 0.8 μM (HeLa), 18.5 ± 1.9 μM (A549) for {Cu₄}, and 11.1 ± 1.5 μM (HeLa), 10.2 ± 1.3 μM (A549) for {Cu₄Cd₆}] obtained after a 24 h incubation. However, moderate toxicity was observed toward immortalized lung epithelial normal cells (HPL1D) with IC₅₀ values of 32.4 ± 1.2 μM for {Cu₄} and 27.6 ± 1.7 μM for {Cu₄Cd₆}. A cellular apoptotic study using HeLa cells revealed that the {Cu₄} cluster triggered apoptosis at both the early and late phases while the {Cu₄Cd₆} cluster facilitate apoptosis mainly at the late apoptotic stage. A standard 2',7'-dichlorodihydrofluorescein-diacetate (DCFH-DA) test affirms that both the clusters enhanced ROS production inside the cancer cells, responsible for promoting cell apoptosis. The decanuclear {Cu₄Cd₆} clusters demonstrated better anticancer activity compared to the tetranuclear {Cu₄} clusters, indicating the role of high nuclearity and additional Cd metal in the enhanced intracellular production of ROS.

pH Changes That Induce an Axial Ligand Effect on Nonheme Iron(IV) Oxo Complexes with an Appended Aminopropyl Functionality

Nonheme iron enzymes often utilize a high-valent iron(IV) oxo species for the biosynthesis of natural products, but their high reactivity often precludes structural and functional studies of these complexes. In this work, a combined experimental and computational study is presented on a biomimetic nonheme iron(IV) oxo complex bearing an aminopyridine macrocyclic ligand and its reactivity toward olefin epoxidation upon changes in the identity and coordination ability of the axial ligand. Herein, we show a dramatic effect of the pH on the oxygen-atom-transfer (OAT) reaction with substrates. In particular, these changes have occurred because of protonation of the axial-bound pendant amine group, where its coordination to iron is replaced by a solvent molecule or anionic ligand. This axial ligand effect influences the catalysis, and we observe enhanced cyclooctene epoxidation yields and turnover numbers in the presence of the unbound protonated pendant amine group. Density functional theory studies were performed to support the experiments and highlight that replacement of the pendant amine with a neutral or anionic ligand dramatically lowers the rate-determining barriers of cyclooctene epoxidation. The computational work further establishes that the change in OAT is due to electrostatic interactions of the pendant amine cation that favorably affect the barrier heights.

Constructing an Acidic Microenvironment by MoS2 in Heterogeneous Fenton Reaction for Pollutant Control

Although Fenton or Fenton-like reactions have been widely used in the environment, biology, life science, and other fields, the sharp decrease in their activity under macroneutral conditions is still a large problem. This study reports a MoS₂ cocatalytic heterogeneous Fenton (CoFe₂ O₄ /MoS₂ ) system capable of sustainably degrading organic pollutants, such as phenol, in a macroneutral buffer solution. An acidic microenvironment in the slipping plane of CoFe₂ O₄ is successfully constructed by chemically bonding with MoS₂ . This microenvironment is not affected by the surrounding pH, which ensures the stable circulation of Fe³⁺ /Fe²⁺ on the surface of CoFe₂ O₄ /MoS₂ under neutral or even alkaline conditions. Additionally, CoFe₂ O₄ /MoS₂ always exposes "fresh" active sites for the decomposition of H₂ O₂ and the generation of ¹ O₂ , effectively inhibiting the production of iron sludge and enhancing the remediation of organic pollutants, even in actual wastewater. This work not only experimentally verifies the existence of an acidic microenvironment on the surface of heterogeneous catalysts for the first time, but also eliminates the pH limitation of the Fenton reaction for pollutant remediation, thereby expanding the applicability of Fenton technology.

μ-Oxo-bridged diiron(iii) complexes of tripodal 4N ligands as catalysts for alkane hydroxylation reaction using m-CPBA as an oxidant: substrate vs. self hydroxylation

A series of non-heme μ-oxo-bridged dinuclear iron(iii) complexes of the type [Fe₂(μ-O)(L1–L6)₂Cl₂]Cl₂1–6 have been isolated and their catalytic activity towards oxidative transformation of alkanes into alcohols has been studied using m-choloroperbenzoic acid (m-CPBA) as an oxidant. All the complexes were characterized by CHN, electrochemical, and UV-visible spectroscopic techniques. The molecular structures of 2 and 5 have been determined successfully by single crystal X-ray diffraction analysis and both possesses octahedral coordination geometry and each iron atom is coordinated by four nitrogen atoms of the 4N ligand and a bridging oxygen. The sixth position of each octahedron is coordinated by a chloride ion. The (μ-oxo)diiron(iii) core is linear in 2 (Fe–O–Fe, 180.0°), whereas it is non-linear (Fe–O–Fe, 161°) in 5. All the diiron(iii) complexes show quasi-reversible one electron transfer in the cyclic voltammagram and catalyze the hydroxylation of alkanes like cyclohexane, adamantane with m-CPBA as an oxidant. In acetonitrile solution, adding excess m-CPBA to the diiron(iii) complex 2 without chloride ions leads to intramolecular hydroxylation reaction of the oxidant. Interestingly, 2 catalyzes alkane hydroxylation in the presence of chloride ions, but intramolecular hydroxylation in the absence of chloride ions. The observed selectivity for cyclohexane (A/K, 5–7) and adamantane (3°/2°, 9–18) suggests the involvement of high-valent iron–oxo species rather than freely diffusing radicals in the catalytic reaction. Moreover, 4 oxidizes (A/K, 7) cyclohexane very efficiently up to 513 TON while 5 oxidizes adamantane with good selectivity (3°/2°, 18) using m-CPBA as an oxidant. The electronic effects of ligand donors dictate the efficiency and selectivity of catalytic hydroxylation of alkanes.

The ligand stereoelectronic effect of diiron(iii) complexes determines the efficiency and selectivity of catalytic alkane hydroxylation with m-CPBA as an oxidant.

Effect of the Ligand Backbone on the Reactivity and Mechanistic Paradigm of Non-Heme Iron(IV)-Oxo during Olefin Epoxidation

The oxygen atom transfer (OAT) reactivity of the non-heme [Fe^IV (2PyN2Q)(O)]²⁺ (2) containing the sterically bulky quinoline-pyridine pentadentate ligand (2PyN2Q) has been thoroughly studied with different olefins. The ferryl-oxo complex 2 shows excellent OAT reactivity during epoxidations. The steric encumbrance and electronic effect of the ligand influence the mechanistic shuttle between OAT pathway I and isomerization pathway II (during the reaction stereo pure olefins), resulting in a mixture of cis-trans epoxide products. In contrast, the sterically less hindered and electronically different [Fe^IV (N4Py)(O)]²⁺ (1) provides only cis-stilbene epoxide. A Hammett study suggests the role of dominant inductive electronic along with minor resonance effect during electron transfer from olefin to 2 in the rate-limiting step. Additionally, a computational study supports the involvement of stepwise pathways during olefin epoxidation. The ferryl bend due to the bulkier ligand incorporation leads to destabilization of both d z 2 and d x 2 - y 2 orbitals, leading to a very small quintet-triplet gap and enhanced reactivity for 2 compared to 1. Thus, the present study unveils the role of steric and electronic effects of the ligand towards mechanistic modification during olefin epoxidation.

How to identify a smoker: a salient crystallographic approach to detect thiocyanate content

There is an increasing demand for monitoring environmental pollutants and the control requires new sensing materials with better sensitivity, selectivity and reliability. In this study, a series of Co₇ clusters incorporating various flexible polyhydroxyamine ligands are explored, with the first report of thiocyanate recognition triggered by crystal formation using a Co₇ crystal (1). For this, we have fortunately synthesized three new mixed metal Co₇ clusters with fascinating structural features. The clusters were characterized by spectroscopic and single crystal X-ray diffraction methods and later by DFT calculations. Due to its better emission spectrum, 1 was further utilized for evaluating its sensing ability towards various anions in water. Surprisingly, 1 shows better quenching ability towards the recognition of SCN⁻ with a better binding constant. The luminescence quenching towards SCN⁻ detection was further verified by the single crystal method, HSAB principle (symbiosis) and theoretical calculations such as DFT studies. The SCXRD data clearly suggest that the Co₇ (1) can be converted into Co₁₄ (1a) by direct reaction with NaSCN under ambient conditions. Besides the soft/hard acid–base concept (symbiosis), the energies of formation, and Co–NCS and Co–OH₂ bond energies (as unravelled by DFT) are responsible for this transformation. Therefore, 1 can be used as a selective and sensitive sensor for the detection of thiocyanate anions based on the fluorescence amplification and quenching method. Further, the designed cluster has also been utilized to detect anions in human blood samples to differentiate a smoker and a non-smoker. It has been concluded that the samples of smokers have a high degree of thiocyanate (∼12 or 9.5 mg L⁻¹) in comparison to those of non-smokers (2–3 mg L⁻¹). Thus, this kind of cluster material has high potentiality in the field of bio-medical science in future endeavours for identification of the extent of thiocyanate content in smokers.

A new Co based sensor for thiocyanate recognition by formation of the dimeric crystals is designed to distinguish a smoker from a non-smoker.

Enhancing Chemo- and Stereoselectivity in C-H Bond Oxygenation with H2O2 by Nonheme High-Spin Iron Catalysts: The Role of Lewis Acid and Multimetal Centers

Spin states of iron often direct the selectivity in oxidation catalysis by iron complexes using hydrogen peroxide (H₂O₂) on an oxidant. While low-spin iron(III) hydroperoxides display stereoselective C-H bond hydroxylation, the reactions are nonstereoselective with high-spin iron(II) catalysts. The catalytic studies with a series of high-spin iron(II) complexes of N4 ligands with H₂O₂ and Sc³⁺ reported here reveal that the Lewis acid promotes catalytic C-H bond hydroxylation with high chemo- and stereoselectivity. This reactivity pattern is observed with iron(II) complexes containing two cis-labile sites. The enhanced selectivity for C-H bond hydroxylation catalyzed by the high-spin iron(II) complexes in the presence of Sc³⁺ parallels that of the low-spin iron catalysts. Furthermore, the introduction of multimetal centers enhances the activity and selectivity of the iron catalyst. The study provides insights into the development of peroxide-dependent bioinspired catalysts for the selective oxygenation of C-H bonds without the restriction of using iron complexes of strong-field ligands.

A side-on Mn(III)-peroxo supported by a non-heme pentadentate N3Py2 ligand: synthesis, characterization and reactivity studies

A mononuclear manganese(iii)-peroxo complex [MnIII(N3Py2)(O2)]+ (1a) bearing a non-heme N,N'-dimethyl-N-(2-(methyl(pyridin-2-ylmethyl)amino)ethyl)-N'-(pyridin-2-ylmethyl)ethane-1,2-diamine (N3Py2) ligand was synthesized by the reaction of [Mn(N3Py2)(H2O)](ClO4)2 (1) with hydrogen peroxide and triethylamine in CH3CN at 25 °C. The reactivity of 1a in aldehyde deformylation using 2-phenyl propionaldehyde (2-PPA) was studied and the reaction kinetics was monitored by UV-visible spectroscopy. A kinetic isotope effect (KIE) = 1.7 was obtained in the reaction of 1a with 2-PPA and α-[D1]-PPA, suggesting nucleophilic character of 1a. The activation parameters ΔH‡ and ΔS‡ were determined using the Eyring plot while Ea was obtained from the Arrhenius equation by performing the reaction between 288 and 303 K. Hammett constants (σp) of para-substituted benzaldehydes p-X-Ph-CHO (X = Cl, F, H, and Me) were linear with a slope (ρ) = 3.0. Computational study suggested that the side-on structure of 1a is more favored over the end-on structure and facilitates the reactivity of 1a.

Effect of Metal Ions on Oxidation of Micropollutants by Ferrate(VI): Enhancing Role of FeIV Species

This paper investigated the oxidation of recalcitrant micropollutants [i.e., atenolol (ATL), flumequine, aspartame, and diatrizoic acid] by combining ferrate(VI) (Fe^VIO₄^2-, Fe^VI) with a series of metal ions [i.e., Fe(III), Ca(II), Al(III), Sc(III), Co(II), and Ni(II)]. An addition of Fe(III) to Fe^VI enhanced the oxidation of micropollutants compared solely to Fe^VI. The enhanced oxidation of studied micropollutants increased with increasing [Fe(III)]/[Fe^VI] to 2.0. The complete conversion of phenyl methyl sulfoxide (PMSO), as a probe agent, to phenyl methyl sulfone (PMSO₂) by the Fe^VI-Fe(III) system suggested that the highly reactive intermediate Fe^IV/Fe^V species causes the increased oxidation of all four micropollutants. A kinetic modeling of the oxidation of ATL demonstrated that the major species causing the increase in ATL removal was Fe^IV, which had an estimated rate constant as (6.3 ± 0.2) × 10⁴ M^-1 s^-1, much higher than that of Fe^VI [(5.0 ± 0.4) × 10^-1 M^-1 s^-1]. Mechanisms of the formed oxidation products of ATL by Fe^IV, which included aromatic and/or benzylic oxidation, are delineated. The presence of natural organic matter significantly inhibited the removal of four pollutants by the Fe^VI-Fe(III) system. The enhanced effect of the Fe^VI-Fe(III) system was also seen in the oxidation of the micropollutants in river water and lake water.