Dr. Mursaleem Ansari | University of Girona (original) (raw)
Videos by Dr. Mursaleem Ansari
We are a group of people working in different areas of computational Inorganic chemistry, belongi... more We are a group of people working in different areas of computational Inorganic chemistry, belonging to the Molecular Modelling Group (MMG) at the Department of Chemistry, IITB. We are all fascinated by how computational tools to aid in understanding the structure and reactivity of open-shell systems possessing one or more unpaired electrons.
Our studies are aimed at understanding the structure, properties, and reactivity of metal complexes/clusters possessing unpaired electrons (open-shell systems). The activity diverges essentially into five components namely :
(I) studies on molecular nanomagnets.
(ii) reaction modeling for open-shell transition metal-mediated catalytic reactions.
(iii) modeling of metal-organic frameworks.
(iv) modeling molecular spintronic materials.
(v) modeling of metalloenzyme structures and reactivity.
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Papers by Dr. Mursaleem Ansari
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2022
Bridged dinuclear metal complexes have fascinated scientists worldwide, and remarkable success ha... more 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 (MIIMII); {[(MnII)2(O2CCH3)(N-Et-HPTB)]2+; Species I), [(FeII)2(O2CCH3)(N-Et-HPTB)]2+; Species II), [(CoII)2(O2CCH3)(N-Et-HPTB)]2+; Species III)} and heterovalent (MIIIMII): {[(MnIII)(MnII)(O2)(N-Et-HPTB)]2+; Species Ia) [(FeIII)(FeII)(O2)(N-Et-HPTB)]2+; Species IIa) and [(CoIII)(CoII)(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.
Physical Chemistry Chemical Physics, 2024
High-valent metal-oxo species serve as key intermediates in the activation of inert C-H bonds. He... more 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 Fe IV QO (species 1) and Co IV QO 2 Co III-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.
Journal of the American Chemical Society, 2024
Structural and spectroscopic investigations of compound II in ascorbate peroxidase (APX) have yie... more Structural and spectroscopic investigations of compound II in ascorbate peroxidase (APX) have yielded conflicting conclusions regarding the protonation state of the crucial Fe(IV) intermediate. Neutron diffraction and crystallographic data support an iron(IV)hydroxo formulation, whereas Mossbauer, X-ray absorption (XAS), and nuclear resonance vibrational spectroscopy (NRVS) studies appear consistent with an iron(IV)-oxo species. Here we examine APX with spectroscopy-oriented QM/MM calculations and extensive exploration of the conformational space for both possible formulations of compound II. We establish that irrespective of variations in the orientation of a vicinal arginine residue and potential reorganization of proximal water molecules and hydrogen bonding, the Fe−O distances for the oxo and hydroxo forms consistently fall within distinct, narrow, and nonoverlapping ranges. The accuracy of geometric parameters is validated by coupled-cluster calculations with the domain-based local pair natural orbital approach, DLPNO-CCSD(T). QM/MM calculations of spectroscopic properties are conducted for all structural variants, encompassing Mossbauer, optical, X-ray absorption, and X-ray emission spectroscopies and NRVS. All spectroscopic observations can be assigned uniquely to an Fe(IV)�O form. A terminal hydroxy group cannot be reconciled with the spectroscopic data. Under no conditions can the Fe(IV)�O distance be sufficiently elongated to approach the crystallographically reported Fe−O distance. The latter is consistent only with a hydroxo species, either Fe(IV) or Fe(III). Our findings strongly support the Fe(IV)�O formulation of APX-II and highlight unresolved discrepancies in the nature of samples used across different experimental studies.
![Research paper thumbnail of Diverse Reactivity of bis(Silylene) and bis(Germylene) [LE−EL (E=Si and Ge: L=PhC(NtBu)2)] in the Oxidative Activation of C−F Bond: Si−Si Bond is Retained While the Ge−Ge Bond Is Cleaved](https://attachments.academia-assets.com/113225446/thumbnails/1.jpg)
](European Journal of Inorganic Chemistry, 2023
Herein we report the reactions of 3,4,5,6‐tetrafluoroterephthalonitrile (1) with bis(silylene) an... more Herein we report the reactions of 3,4,5,6‐tetrafluoroterephthalonitrile (1) with bis(silylene) and bis(germylene) LE−EL [E=Si (2) and Ge(3): L=PhC(NtBu)2)]. The reaction of LSi−SiL (L=PhC(NtBu)2) (2) with two equivalents of 1 resulted in an unprecedented oxidative addition of a C−F bond of 1 leading to disilicon(III) fluoride {L(4‐C8F3N)FSi−SiF(4‐C8F3N)L}(4), wherein the Si−Si single bond was retained. In contrast, the reaction of LGe−GeL (L=PhC(NtBu)2) (3) with one equivalent of 1 resulted in the oxidative cleavage of Ge−Ge bond leading to L(4‐C8F3N2)Ge (5) and LGeF (6). All three compounds (4–6) were characterized by NMR spectroscopy, EI‐MS spectrometry, and elemental analysis. X‐ray single‐crystal structure determination of compound 4 unequivocally established that the SiIII−SiIII bond remains uncleaved.
Inorganic Chemistry, 2023
Here, we report a comparative study of nitric oxide oxidation (NOO) reactions of CoIII-peroxo (Co... more Here, we report a comparative study of nitric oxide oxidation (NOO) reactions of CoIII-peroxo (CoIII–O22–) and Co-nitrosyl ({CoNO}8) complexes bearing the same N4-donor ligand (HMTETA) framework. In this regard, we prepared and characterized two new [(HMTETA)CoIII(O22–)]+ (2, S = 2) and [(HMTETA)Co(NO)]2+ (3, S = 1) complexes from [(HMTETA)CoII(CH3CN)2]2+ (1). Both complexes (2 and 3) are characterized by different spectroscopic measurements, including their DFT-optimized structures. Complex 2 produces CoII-nitrato [(HMTETA)CoII(NO3–)]+ (CoII–NO3–, 4) complex in the presence of NO. In contrast, when 3 reacted with a superoxide (O2•–) anion, it generated CoII-nitrito [(HMTETA)CoII(NO2–)]+ (CoII–NO2–, 5) with O2 evolution. Experiments performed using 18/16O-labeled superoxide (18O2•–/16O2•–) showed that O2 originated from the O2•– anion. Both the NOO reactions are believed to proceed via a presumed peroxynitrite (PN) intermediate. Although we did not get direct spectral evidence for the proposed PN species, the mechanistic investigation using 2,4-di-tert-butylphenol indirectly suggests the formation of a PN intermediate. Furthermore, tracking the source of the N-atom in the above NOO reactions using 15N-labeled nitrogen (15NO) revealed N-atoms in 4 (CoII–15NO3–) and 5 (CoII–15NO2–) derived from the 15NO moiety.
New Journal of Chemistry, 2024
In enzymatic and synthetic catalytic oxidation, highly reactive iron–oxo intermediates play an im... more In enzymatic and synthetic catalytic oxidation, highly reactive iron–oxo intermediates play an important role as oxidants in the processes of hydroxylation, epoxidation, halogenation reactions, etc. Synthetic iron–oxo species also have the ability to catalyse chemo- or enantioselective reactions similar to enzymatic catalysis. In this context, a report on the [(PDP)FeII(CF3SO3)2)]/H2O2/AcOH system has gained attention as this activates the C[double bond, length as m-dash]C bond of the alkene and has the catalytic ability to perform chemo- or enantioselective conversion of the alkenes. In this study, we have employed density functional methods for the formation of FeIV[double bond, length as m-dash]O as well as FeV[double bond, length as m-dash]O from O⋯O bond cleavage and also shown that the formation of FeV[double bond, length as m-dash]O dominates over the corresponding FeIV[double bond, length as m-dash]O species. In addition to having favourable formation energy, a lower barrier height was computed for the C[double bond, length as m-dash]C bond activation of cis-2-butene in the S = 3/2 state in a concerted manner rather than forming a radical intermediate in a stepwise manner from the FeV[double bond, length as m-dash]O unit. The concerted mechanism was found to be responsible for the chemo- and enantioselective product, which was also observed in the experimental findings. Furthermore, NCI plots also support the less steric interaction during the formation of FeV[double bond, length as m-dash]O compared to FeIV[double bond, length as m-dash]O species. Additionally, during the epoxidation reaction, these steric effects significantly contributed towards the concerted route over the stepwise pathway.
Dalton Transactions, 2023
Over the years, mononuclear FeIV[double bond, length as m-dash]O species have been extensively st... more Over the years, mononuclear FeIV[double bond, length as m-dash]O species have been extensively studied, but the presence of dinuclear FeIV[double bond, length as m-dash]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) FeIV(μ-N)FeIV(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 FeIV[double bond, length as m-dash]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)FeIV(O)(Por)]− (1), [(HO)FeIV(O)(Por˙+)] (2), μ-oxo dinuclear iron species [(Por)(m-CBA)FeIV(μ-O)FeIV(O) (Por˙+)]− (3) and N-bridged dinuclear iron species [(Por)(m-CBA)FeIV(μ-N)FeIV(O)(Por˙+)]− (4) towards the activation of methane. Additionally, calculations were performed on the mononuclear models [(X)FeIV(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 FeIV 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 FeIV 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[double bond, length as m-dash]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.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2022
In the realm of dye sensitized solar cells (DSSCs), the 3d transition metals as photosensitizers ... more 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 NHCcarbon and metal atom and found that there is σ-bonding present between the metal and the NHCcarbon by the overlapping of sp-hybridized orbitals of carbenecarbon 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.
Inorganic Chemistry, 2023
Using a combination of density functional theory (DFT) and ab initio complete active space self-c... more Using a combination of density functional theory (DFT) and ab initio complete active space self-consistent field (CASSCF) calculations, various elementary steps in the mechanism of the reductive hydroboration of CO2 to two-electron-reduced boryl formate, four-electron-reduced bis(boryl)acetal, and six-electron-reduced methoxy borane by the [Fe(H)2(dmpe)2] catalyst were established. The replacement of hydride by oxygen ligation after the boryl formate insertion step is the rate-determining step. Our work unveils, for the first time, (i) how a substrate steers product selectivity in this reaction and (ii) the importance of configurational mixing in contracting the kinetic barrier heights. Based on the reaction mechanism established, we have further focused on the effect of other metals, such as Mn and Co, on rate-determining steps and on catalyst regeneration.
Inorganic Chemistry , 2021
To investigate the influence of the coordination geometry on the magnetization relaxation dynamic... more To investigate the influence of the coordination geometry on the magnetization relaxation dynamics, two geometric isomers of a fivecoordinate low-spin Co(II) complex with the general molecular formula [Co(DPPE) 2 Cl]SnCl 3 (DPPE = diphenylphosphinoethane) were synthesized and structurally characterized. While one isomer has a square pyramidal geometry (Co-SP (1)), the other isomer figures a trigonal bipyramidal geometry (Co-TBP (2)). Both complexes were already reported elsewhere. The spin state of these complexes is unambiguously determined by detailed direct current (dc) magnetic data, X-band, and high-frequency EPR measurements. Slow relaxation of magnetization is commonly observed for systems with S > 1/2. However, both 1 and 2 show field-induced slow relaxation of magnetization. Especially 1 shows relaxation times up to τ = 35 ms at T = 1.8 K, which is much longer than the reported values for undiluted Co(II) low-spin monomers. In 2, the maximal field-induced relaxation time is suppressed to τ = 5 ms. We attribute this to the change in g-anisotropy, which is, in turn, correlated to the spatial arrangement of ligands (i.e., coordination geometry) around the Co(II) ions. Besides the detailed electronic structure of these complexes, the experimental observations are further corroborated by theoretical calculations.
Dalton transactions, 2021
Two new copper clusters, {Cu4} and {Cu4Cd6}, with polydentate aminoalcohol ligands, diethanol pro... more 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 ...
Angewandte Chemie International Edition, 2021
μ‐1,2‐peroxo‐bridged diiron(III) intermediates P are proposed as reactive intermediates in variou... more μ‐1,2‐peroxo‐bridged diiron(III) intermediates P are proposed as reactive intermediates in various biological oxidation reactions. In sMMO, P acts as an electrophile, and performs hydrogen atom and oxygen atom transfers to electron‐rich substrates. In cyanobacterial ADO, however, P is postulated to react by nucleophilic attack on electrophilic carbon atoms. In biomimetic studies, the ability of μ‐1,2‐peroxo‐bridged dimetal complexes of Fe, Co, Ni and Cu to act as nucleophiles that effect deformylation of aldehydes is documented. By performing reactivity and theoretical studies on an end‐on μ‐1,2‐peroxodicobalt(III) complex 1 involving a non‐heme ligand system, L1, supported on a Sn6O6 stannoxane core, we now show that a peroxo‐bridged dimetal complex can also be a reactive electrophile. The observed electrophilic chemistry, which is induced by the constraints provided by the Sn6O6 core, represents a new domain for metal−peroxide reactivity.
Polyhedron, 2021
Changing the anion in a crystal lattice induces distinct structural distortions in the [CoS4]2+ c... more Changing the anion in a crystal lattice induces distinct structural distortions in the [CoS4]2+ core for a family of complexes with the general molecular formula [Co(L1)4]X2, where L1 = thiourea (NH2CSNH2) and X = I (1), Br (2), and [Co(L1)4](SiF6) (3). The magnetic anisotropy (D) for 1–3 was quantitatively determined with a magnitude of −153(2), −168(5) and < -400 GHz, respectively, by HF-EPR investigation. Also, intermolecular exchange interactions were determined experimentally, whereby an antiferromagnetic exchange for 1 (-5.5 GHz) and 2 (-4.1 GHz), and a ferromagnetic interaction for 3 (+3.5 GHz) are witnessed (based on H = -JS1S2). The exchange interactions, computed using DFT methods on a model complex of 1, disclose the presence of an antiferromagnetic exchange interaction, consistent with the experimental observation. Overall, the present study provides convincing experimental evidence for the sizable influence not only of the first but also of the second coordination sphere on the magnetic anisotropy and exchange interactions of Co(II) ions.
Structural Chemistry, 2021
Due to their rich structural chemistry and enormous catalytic applications, TPA (tris(2-pyridylme... more Due to their rich structural chemistry and enormous catalytic applications, TPA (tris(2-pyridylmethyl)amine) coordinated iron (Fe-TPA) complexes are widely studied as potent functional model in biomimetic area. We have done a thorough computational study on six diverse mononuclear and dinuclear FeII/FeIII-TPA complexes using density functional method. All the possible spin states for the six species were computed, and the ground state S value of these species was determined. The molecular structures, energetic profiles of spin states, electrostatic potentials, spin densities, and orbital energies of all the six complexes are reported. A comparative study among these six species was also done to gain insights about any sort of correlation in properties of these species. The observed variations as well as correlations in properties of the species are imputed to the diverse ligand substituents in Fe-TPA skeleton. Theoretical reactivity behavior was evaluated by doing frontier molecular orbital analysis for all the species by mapping HOMO-LUMO diagrams. Electrostatic potential surfaces were mapped to study charge energy distribution in the species. Our computed results are in well agreement with the experimental data.
Angewandte Chemie International Edition, 2021
The oxygen atom transfer (OAT) reactivity of the non-heme [FeIV(2PyN2Q)(O)]2+ (2) containing the ... more The oxygen atom transfer (OAT) reactivity of the non-heme [FeIV(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 [FeIV(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 urn:x-wiley:14337851:media:anie202102484:anie202102484-math-0001 and urn:x-wiley:14337851:media:anie202102484:anie202102484-math-0002 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.
![Research paper thumbnail of Deciphering the Origin of Million-fold Reactivity Observed for the Open Core Diiron [HO-FeIII-O-FeIV=O]2+ Species Towards C-H Bond Activation: Role of Spin-states, Spin-coupling, and Spin-cooperation](https://attachments.academia-assets.com/64453582/thumbnails/1.jpg)
](Chemical Science , 2020
High-valent metal-oxo species have been characterised as key intermediates in both heme and non-h... more High-valent metal-oxo species have been characterised as key intermediates in both heme and non-heme enzymes that are
found to perform efficient aliphatic hydroxylation, epoxidation, halogenation, and dehydrogenation reactions. Several
biomimetic model complexes have been synthesised over the years to mimic both the structure and function of the
metalloenzymes. The diamond-core [Fe2(-O)2] is one of the celebrated models in this context as this has been proposed as
the catalytically active species in soluble methane monooxygenase enzyme (sMMO) which perform the challenging chemical
conversion of methane to methanol at ease. In this context, report of an open core [(HO(L)FeIII–O–FeIV(O)(L)]2+ (1) gains
attention as this activates C-H bonds a million-fold faster compared to the diamond-core structure and has the dual catalytic
ability to perform hydroxylation as well as desaturation with organic substrates. In this study, we have employed density
functional methods to probe the origin of the very high reactivity observed for this complex and also to shed light on how
this complex performs efficient hydroxylation and desaturation of alkanes. By modelling fifteen possible spin-state for 1 that
could potentially participate in the reaction mechanism, our calculations reveal a doublet ground state for 1 arising from
antiferromagnetic coupling between quartet FeIV centre and sextet FeIII centre regulate the reactivity of this species. The
unusual stabilisation of high-spin ground state for FeIV=O is due strong overlap of FeIVσ
*
z
2 with FeIIIπ
*
xz orbital, reducing the
antibonding interactions via spin-cooperation. The electronic structure features computed for 1 is consistent with
experiments offering confidence on the methodology chosen. Further, we have probed various mechanistic pathways for
the C-H bond activation as well as –OH rebound/desaturation of alkane. An extremely small barrier height computed for the
first hydrogen atom abstraction by the terminal FeIV=O unit found to be responsible for the million-fold activation observed
in the experiments. The barrier height computed for –OH rebound by the FeIII
-OH unit is also smaller suggesting a facile
hydroxylation of organic substrates by 1. A strong spin-cooperation between the two iron centres also reduces the barrier
for second hydrogen atom abstraction, thus making the desaturation pathway competitive. Both the spin-state as well as
spin-coupling between the two metal centres are playing a crucial role in dictating the reactivity for species 1. By exploring
various mechanistic pathways, our study unveils the fact that the bridged -oxo group is poor electrophile for both C-H
activation as well for –OH rebound. As more and more evidence is gathered in recent years for the open core geometry of
sMMO enzymes, the idea of enhancing the reactivity via an open-core motif has far-reaching consequences.
Coordination Chemistry Reviews, 2020
High-valent iron-oxo species are ubiquitous in nature and are present at the active site of sever... more High-valent iron-oxo species are ubiquitous in nature and are present at the active site of several metal-loenzymes which perform challenging organic transformations. Mimicking these metalloenzyme reactivities is one of the growing areas of research, and over the last two decades, tremendous progress has been made to mimic both the structure and function of various heme and non-heme metalloenzymes. Understanding the mechanism of catalytic reactions of these enzymes and their biomimetic models are extremely important to improvise the models further. However, due to the open-shell nature of the catalyst with often close-lying spin-states, the mechanistic aspects associated are highly complex. In this regard, computational tools have played a pivotal role in underpinning the mechanism and several important concepts such as two-state/multi-state reactivity, exchange-enhanced reactivity has emerged. While there are several reviews written already on the reactivity of the popular high-valent Fe IV =O species , the comparative oxidative ability of this species to other oxidants has not been reviewed. Our group has been working actively in this area, and here we have compared the oxidative ability of the Fe IV =O to other species with variation arising due to (i) oxidation state (ii) ligand architecture (iii) substitution of oxo by the isoelectronic nitrene species. In this review, theoretical studies undertaken in this spirit are summarised to provide birds-eye-view on the reactivity of the popular Fe IV =O species. The facts/concepts discussed here will undoubtedly be helpful to design efficient bioinspired catalysts in the years to come.
ACS catalysis, 2019
Copper amine oxidase (CAO), consisting of the topoquinone (TPQ) cofactor, catalyzes the oxidation... more Copper amine oxidase (CAO), consisting of the topoquinone (TPQ) cofactor, catalyzes the oxidation of primary amines to aldehyde. We have successfully addressed this issue through isolation of a copper complex which mimics the active-site structure as well as the function of CAO. This inimitable complex, consisting of two TPQ-like side-arms, formed by ambient aerial oxidation of a precursor Schiff base complex, is the most efficient homogeneous catalyst for quantitative oxidation of primary benzylic amines to corresponding secondary imines under ambient conditions within 30 min. The longstanding contention of actual involvement of Cu(II) in the catalysis is resolved through quenching experiments of Cu(II) superoxo species and detailed density functional theory studies.
![Research paper thumbnail of Expanding the limits of catalysts with low-valent main-group elements for the hydroboration of aldehydes and ketones using [L † Sn(II)][OTf ] (L † = aminotroponate; OTf = triflate](https://attachments.academia-assets.com/58991156/thumbnails/1.jpg)
](Dalton Transaction , 2019
A triflatostannylene [L † Sn(II)][OTf ] (2) is reported here as an efficient catalyst with low-va... more A triflatostannylene [L † Sn(II)][OTf ] (2) is reported here as an efficient catalyst with low-valent main-group element for the hydroboration of aldehydes and ketones (L † = aminotroponate). Using 0.025-0.25 mol% of compound 2, hydroboration of various aldehydes and ketones is accomplished in 0.13-1.25 h at room temperature; the aliphatic aldehydes show an impressive TOF of around 30 000 h −1. DFT calculations are performed to explore the mechanistic aspects of this reaction suggesting that the reaction proceeds via a stepwise pathway with hydridostannylene [L † Sn(II)H] (2a) as the active catalyst and the H atom transfer from the Sn-H bond to the carbonyl carbon being the rate determining step.
We are a group of people working in different areas of computational Inorganic chemistry, belongi... more We are a group of people working in different areas of computational Inorganic chemistry, belonging to the Molecular Modelling Group (MMG) at the Department of Chemistry, IITB. We are all fascinated by how computational tools to aid in understanding the structure and reactivity of open-shell systems possessing one or more unpaired electrons.
Our studies are aimed at understanding the structure, properties, and reactivity of metal complexes/clusters possessing unpaired electrons (open-shell systems). The activity diverges essentially into five components namely :
(I) studies on molecular nanomagnets.
(ii) reaction modeling for open-shell transition metal-mediated catalytic reactions.
(iii) modeling of metal-organic frameworks.
(iv) modeling molecular spintronic materials.
(v) modeling of metalloenzyme structures and reactivity.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2022
Bridged dinuclear metal complexes have fascinated scientists worldwide, and remarkable success ha... more 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 (MIIMII); {[(MnII)2(O2CCH3)(N-Et-HPTB)]2+; Species I), [(FeII)2(O2CCH3)(N-Et-HPTB)]2+; Species II), [(CoII)2(O2CCH3)(N-Et-HPTB)]2+; Species III)} and heterovalent (MIIIMII): {[(MnIII)(MnII)(O2)(N-Et-HPTB)]2+; Species Ia) [(FeIII)(FeII)(O2)(N-Et-HPTB)]2+; Species IIa) and [(CoIII)(CoII)(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.
Physical Chemistry Chemical Physics, 2024
High-valent metal-oxo species serve as key intermediates in the activation of inert C-H bonds. He... more 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 Fe IV QO (species 1) and Co IV QO 2 Co III-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.
Journal of the American Chemical Society, 2024
Structural and spectroscopic investigations of compound II in ascorbate peroxidase (APX) have yie... more Structural and spectroscopic investigations of compound II in ascorbate peroxidase (APX) have yielded conflicting conclusions regarding the protonation state of the crucial Fe(IV) intermediate. Neutron diffraction and crystallographic data support an iron(IV)hydroxo formulation, whereas Mossbauer, X-ray absorption (XAS), and nuclear resonance vibrational spectroscopy (NRVS) studies appear consistent with an iron(IV)-oxo species. Here we examine APX with spectroscopy-oriented QM/MM calculations and extensive exploration of the conformational space for both possible formulations of compound II. We establish that irrespective of variations in the orientation of a vicinal arginine residue and potential reorganization of proximal water molecules and hydrogen bonding, the Fe−O distances for the oxo and hydroxo forms consistently fall within distinct, narrow, and nonoverlapping ranges. The accuracy of geometric parameters is validated by coupled-cluster calculations with the domain-based local pair natural orbital approach, DLPNO-CCSD(T). QM/MM calculations of spectroscopic properties are conducted for all structural variants, encompassing Mossbauer, optical, X-ray absorption, and X-ray emission spectroscopies and NRVS. All spectroscopic observations can be assigned uniquely to an Fe(IV)�O form. A terminal hydroxy group cannot be reconciled with the spectroscopic data. Under no conditions can the Fe(IV)�O distance be sufficiently elongated to approach the crystallographically reported Fe−O distance. The latter is consistent only with a hydroxo species, either Fe(IV) or Fe(III). Our findings strongly support the Fe(IV)�O formulation of APX-II and highlight unresolved discrepancies in the nature of samples used across different experimental studies.
European Journal of Inorganic Chemistry, 2023
Herein we report the reactions of 3,4,5,6‐tetrafluoroterephthalonitrile (1) with bis(silylene) an... more Herein we report the reactions of 3,4,5,6‐tetrafluoroterephthalonitrile (1) with bis(silylene) and bis(germylene) LE−EL [E=Si (2) and Ge(3): L=PhC(NtBu)2)]. The reaction of LSi−SiL (L=PhC(NtBu)2) (2) with two equivalents of 1 resulted in an unprecedented oxidative addition of a C−F bond of 1 leading to disilicon(III) fluoride {L(4‐C8F3N)FSi−SiF(4‐C8F3N)L}(4), wherein the Si−Si single bond was retained. In contrast, the reaction of LGe−GeL (L=PhC(NtBu)2) (3) with one equivalent of 1 resulted in the oxidative cleavage of Ge−Ge bond leading to L(4‐C8F3N2)Ge (5) and LGeF (6). All three compounds (4–6) were characterized by NMR spectroscopy, EI‐MS spectrometry, and elemental analysis. X‐ray single‐crystal structure determination of compound 4 unequivocally established that the SiIII−SiIII bond remains uncleaved.
Inorganic Chemistry, 2023
Here, we report a comparative study of nitric oxide oxidation (NOO) reactions of CoIII-peroxo (Co... more Here, we report a comparative study of nitric oxide oxidation (NOO) reactions of CoIII-peroxo (CoIII–O22–) and Co-nitrosyl ({CoNO}8) complexes bearing the same N4-donor ligand (HMTETA) framework. In this regard, we prepared and characterized two new [(HMTETA)CoIII(O22–)]+ (2, S = 2) and [(HMTETA)Co(NO)]2+ (3, S = 1) complexes from [(HMTETA)CoII(CH3CN)2]2+ (1). Both complexes (2 and 3) are characterized by different spectroscopic measurements, including their DFT-optimized structures. Complex 2 produces CoII-nitrato [(HMTETA)CoII(NO3–)]+ (CoII–NO3–, 4) complex in the presence of NO. In contrast, when 3 reacted with a superoxide (O2•–) anion, it generated CoII-nitrito [(HMTETA)CoII(NO2–)]+ (CoII–NO2–, 5) with O2 evolution. Experiments performed using 18/16O-labeled superoxide (18O2•–/16O2•–) showed that O2 originated from the O2•– anion. Both the NOO reactions are believed to proceed via a presumed peroxynitrite (PN) intermediate. Although we did not get direct spectral evidence for the proposed PN species, the mechanistic investigation using 2,4-di-tert-butylphenol indirectly suggests the formation of a PN intermediate. Furthermore, tracking the source of the N-atom in the above NOO reactions using 15N-labeled nitrogen (15NO) revealed N-atoms in 4 (CoII–15NO3–) and 5 (CoII–15NO2–) derived from the 15NO moiety.
New Journal of Chemistry, 2024
In enzymatic and synthetic catalytic oxidation, highly reactive iron–oxo intermediates play an im... more In enzymatic and synthetic catalytic oxidation, highly reactive iron–oxo intermediates play an important role as oxidants in the processes of hydroxylation, epoxidation, halogenation reactions, etc. Synthetic iron–oxo species also have the ability to catalyse chemo- or enantioselective reactions similar to enzymatic catalysis. In this context, a report on the [(PDP)FeII(CF3SO3)2)]/H2O2/AcOH system has gained attention as this activates the C[double bond, length as m-dash]C bond of the alkene and has the catalytic ability to perform chemo- or enantioselective conversion of the alkenes. In this study, we have employed density functional methods for the formation of FeIV[double bond, length as m-dash]O as well as FeV[double bond, length as m-dash]O from O⋯O bond cleavage and also shown that the formation of FeV[double bond, length as m-dash]O dominates over the corresponding FeIV[double bond, length as m-dash]O species. In addition to having favourable formation energy, a lower barrier height was computed for the C[double bond, length as m-dash]C bond activation of cis-2-butene in the S = 3/2 state in a concerted manner rather than forming a radical intermediate in a stepwise manner from the FeV[double bond, length as m-dash]O unit. The concerted mechanism was found to be responsible for the chemo- and enantioselective product, which was also observed in the experimental findings. Furthermore, NCI plots also support the less steric interaction during the formation of FeV[double bond, length as m-dash]O compared to FeIV[double bond, length as m-dash]O species. Additionally, during the epoxidation reaction, these steric effects significantly contributed towards the concerted route over the stepwise pathway.
Dalton Transactions, 2023
Over the years, mononuclear FeIV[double bond, length as m-dash]O species have been extensively st... more Over the years, mononuclear FeIV[double bond, length as m-dash]O species have been extensively studied, but the presence of dinuclear FeIV[double bond, length as m-dash]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) FeIV(μ-N)FeIV(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 FeIV[double bond, length as m-dash]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)FeIV(O)(Por)]− (1), [(HO)FeIV(O)(Por˙+)] (2), μ-oxo dinuclear iron species [(Por)(m-CBA)FeIV(μ-O)FeIV(O) (Por˙+)]− (3) and N-bridged dinuclear iron species [(Por)(m-CBA)FeIV(μ-N)FeIV(O)(Por˙+)]− (4) towards the activation of methane. Additionally, calculations were performed on the mononuclear models [(X)FeIV(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 FeIV 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 FeIV 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[double bond, length as m-dash]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.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 2022
In the realm of dye sensitized solar cells (DSSCs), the 3d transition metals as photosensitizers ... more 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 NHCcarbon and metal atom and found that there is σ-bonding present between the metal and the NHCcarbon by the overlapping of sp-hybridized orbitals of carbenecarbon 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.
Inorganic Chemistry, 2023
Using a combination of density functional theory (DFT) and ab initio complete active space self-c... more Using a combination of density functional theory (DFT) and ab initio complete active space self-consistent field (CASSCF) calculations, various elementary steps in the mechanism of the reductive hydroboration of CO2 to two-electron-reduced boryl formate, four-electron-reduced bis(boryl)acetal, and six-electron-reduced methoxy borane by the [Fe(H)2(dmpe)2] catalyst were established. The replacement of hydride by oxygen ligation after the boryl formate insertion step is the rate-determining step. Our work unveils, for the first time, (i) how a substrate steers product selectivity in this reaction and (ii) the importance of configurational mixing in contracting the kinetic barrier heights. Based on the reaction mechanism established, we have further focused on the effect of other metals, such as Mn and Co, on rate-determining steps and on catalyst regeneration.
Inorganic Chemistry , 2021
To investigate the influence of the coordination geometry on the magnetization relaxation dynamic... more To investigate the influence of the coordination geometry on the magnetization relaxation dynamics, two geometric isomers of a fivecoordinate low-spin Co(II) complex with the general molecular formula [Co(DPPE) 2 Cl]SnCl 3 (DPPE = diphenylphosphinoethane) were synthesized and structurally characterized. While one isomer has a square pyramidal geometry (Co-SP (1)), the other isomer figures a trigonal bipyramidal geometry (Co-TBP (2)). Both complexes were already reported elsewhere. The spin state of these complexes is unambiguously determined by detailed direct current (dc) magnetic data, X-band, and high-frequency EPR measurements. Slow relaxation of magnetization is commonly observed for systems with S > 1/2. However, both 1 and 2 show field-induced slow relaxation of magnetization. Especially 1 shows relaxation times up to τ = 35 ms at T = 1.8 K, which is much longer than the reported values for undiluted Co(II) low-spin monomers. In 2, the maximal field-induced relaxation time is suppressed to τ = 5 ms. We attribute this to the change in g-anisotropy, which is, in turn, correlated to the spatial arrangement of ligands (i.e., coordination geometry) around the Co(II) ions. Besides the detailed electronic structure of these complexes, the experimental observations are further corroborated by theoretical calculations.
Dalton transactions, 2021
Two new copper clusters, {Cu4} and {Cu4Cd6}, with polydentate aminoalcohol ligands, diethanol pro... more 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 ...
Angewandte Chemie International Edition, 2021
μ‐1,2‐peroxo‐bridged diiron(III) intermediates P are proposed as reactive intermediates in variou... more μ‐1,2‐peroxo‐bridged diiron(III) intermediates P are proposed as reactive intermediates in various biological oxidation reactions. In sMMO, P acts as an electrophile, and performs hydrogen atom and oxygen atom transfers to electron‐rich substrates. In cyanobacterial ADO, however, P is postulated to react by nucleophilic attack on electrophilic carbon atoms. In biomimetic studies, the ability of μ‐1,2‐peroxo‐bridged dimetal complexes of Fe, Co, Ni and Cu to act as nucleophiles that effect deformylation of aldehydes is documented. By performing reactivity and theoretical studies on an end‐on μ‐1,2‐peroxodicobalt(III) complex 1 involving a non‐heme ligand system, L1, supported on a Sn6O6 stannoxane core, we now show that a peroxo‐bridged dimetal complex can also be a reactive electrophile. The observed electrophilic chemistry, which is induced by the constraints provided by the Sn6O6 core, represents a new domain for metal−peroxide reactivity.
Polyhedron, 2021
Changing the anion in a crystal lattice induces distinct structural distortions in the [CoS4]2+ c... more Changing the anion in a crystal lattice induces distinct structural distortions in the [CoS4]2+ core for a family of complexes with the general molecular formula [Co(L1)4]X2, where L1 = thiourea (NH2CSNH2) and X = I (1), Br (2), and [Co(L1)4](SiF6) (3). The magnetic anisotropy (D) for 1–3 was quantitatively determined with a magnitude of −153(2), −168(5) and < -400 GHz, respectively, by HF-EPR investigation. Also, intermolecular exchange interactions were determined experimentally, whereby an antiferromagnetic exchange for 1 (-5.5 GHz) and 2 (-4.1 GHz), and a ferromagnetic interaction for 3 (+3.5 GHz) are witnessed (based on H = -JS1S2). The exchange interactions, computed using DFT methods on a model complex of 1, disclose the presence of an antiferromagnetic exchange interaction, consistent with the experimental observation. Overall, the present study provides convincing experimental evidence for the sizable influence not only of the first but also of the second coordination sphere on the magnetic anisotropy and exchange interactions of Co(II) ions.
Structural Chemistry, 2021
Due to their rich structural chemistry and enormous catalytic applications, TPA (tris(2-pyridylme... more Due to their rich structural chemistry and enormous catalytic applications, TPA (tris(2-pyridylmethyl)amine) coordinated iron (Fe-TPA) complexes are widely studied as potent functional model in biomimetic area. We have done a thorough computational study on six diverse mononuclear and dinuclear FeII/FeIII-TPA complexes using density functional method. All the possible spin states for the six species were computed, and the ground state S value of these species was determined. The molecular structures, energetic profiles of spin states, electrostatic potentials, spin densities, and orbital energies of all the six complexes are reported. A comparative study among these six species was also done to gain insights about any sort of correlation in properties of these species. The observed variations as well as correlations in properties of the species are imputed to the diverse ligand substituents in Fe-TPA skeleton. Theoretical reactivity behavior was evaluated by doing frontier molecular orbital analysis for all the species by mapping HOMO-LUMO diagrams. Electrostatic potential surfaces were mapped to study charge energy distribution in the species. Our computed results are in well agreement with the experimental data.
Angewandte Chemie International Edition, 2021
The oxygen atom transfer (OAT) reactivity of the non-heme [FeIV(2PyN2Q)(O)]2+ (2) containing the ... more The oxygen atom transfer (OAT) reactivity of the non-heme [FeIV(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 [FeIV(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 urn:x-wiley:14337851:media:anie202102484:anie202102484-math-0001 and urn:x-wiley:14337851:media:anie202102484:anie202102484-math-0002 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.
Chemical Science , 2020
High-valent metal-oxo species have been characterised as key intermediates in both heme and non-h... more High-valent metal-oxo species have been characterised as key intermediates in both heme and non-heme enzymes that are
found to perform efficient aliphatic hydroxylation, epoxidation, halogenation, and dehydrogenation reactions. Several
biomimetic model complexes have been synthesised over the years to mimic both the structure and function of the
metalloenzymes. The diamond-core [Fe2(-O)2] is one of the celebrated models in this context as this has been proposed as
the catalytically active species in soluble methane monooxygenase enzyme (sMMO) which perform the challenging chemical
conversion of methane to methanol at ease. In this context, report of an open core [(HO(L)FeIII–O–FeIV(O)(L)]2+ (1) gains
attention as this activates C-H bonds a million-fold faster compared to the diamond-core structure and has the dual catalytic
ability to perform hydroxylation as well as desaturation with organic substrates. In this study, we have employed density
functional methods to probe the origin of the very high reactivity observed for this complex and also to shed light on how
this complex performs efficient hydroxylation and desaturation of alkanes. By modelling fifteen possible spin-state for 1 that
could potentially participate in the reaction mechanism, our calculations reveal a doublet ground state for 1 arising from
antiferromagnetic coupling between quartet FeIV centre and sextet FeIII centre regulate the reactivity of this species. The
unusual stabilisation of high-spin ground state for FeIV=O is due strong overlap of FeIVσ
*
z
2 with FeIIIπ
*
xz orbital, reducing the
antibonding interactions via spin-cooperation. The electronic structure features computed for 1 is consistent with
experiments offering confidence on the methodology chosen. Further, we have probed various mechanistic pathways for
the C-H bond activation as well as –OH rebound/desaturation of alkane. An extremely small barrier height computed for the
first hydrogen atom abstraction by the terminal FeIV=O unit found to be responsible for the million-fold activation observed
in the experiments. The barrier height computed for –OH rebound by the FeIII
-OH unit is also smaller suggesting a facile
hydroxylation of organic substrates by 1. A strong spin-cooperation between the two iron centres also reduces the barrier
for second hydrogen atom abstraction, thus making the desaturation pathway competitive. Both the spin-state as well as
spin-coupling between the two metal centres are playing a crucial role in dictating the reactivity for species 1. By exploring
various mechanistic pathways, our study unveils the fact that the bridged -oxo group is poor electrophile for both C-H
activation as well for –OH rebound. As more and more evidence is gathered in recent years for the open core geometry of
sMMO enzymes, the idea of enhancing the reactivity via an open-core motif has far-reaching consequences.
Coordination Chemistry Reviews, 2020
High-valent iron-oxo species are ubiquitous in nature and are present at the active site of sever... more High-valent iron-oxo species are ubiquitous in nature and are present at the active site of several metal-loenzymes which perform challenging organic transformations. Mimicking these metalloenzyme reactivities is one of the growing areas of research, and over the last two decades, tremendous progress has been made to mimic both the structure and function of various heme and non-heme metalloenzymes. Understanding the mechanism of catalytic reactions of these enzymes and their biomimetic models are extremely important to improvise the models further. However, due to the open-shell nature of the catalyst with often close-lying spin-states, the mechanistic aspects associated are highly complex. In this regard, computational tools have played a pivotal role in underpinning the mechanism and several important concepts such as two-state/multi-state reactivity, exchange-enhanced reactivity has emerged. While there are several reviews written already on the reactivity of the popular high-valent Fe IV =O species , the comparative oxidative ability of this species to other oxidants has not been reviewed. Our group has been working actively in this area, and here we have compared the oxidative ability of the Fe IV =O to other species with variation arising due to (i) oxidation state (ii) ligand architecture (iii) substitution of oxo by the isoelectronic nitrene species. In this review, theoretical studies undertaken in this spirit are summarised to provide birds-eye-view on the reactivity of the popular Fe IV =O species. The facts/concepts discussed here will undoubtedly be helpful to design efficient bioinspired catalysts in the years to come.
ACS catalysis, 2019
Copper amine oxidase (CAO), consisting of the topoquinone (TPQ) cofactor, catalyzes the oxidation... more Copper amine oxidase (CAO), consisting of the topoquinone (TPQ) cofactor, catalyzes the oxidation of primary amines to aldehyde. We have successfully addressed this issue through isolation of a copper complex which mimics the active-site structure as well as the function of CAO. This inimitable complex, consisting of two TPQ-like side-arms, formed by ambient aerial oxidation of a precursor Schiff base complex, is the most efficient homogeneous catalyst for quantitative oxidation of primary benzylic amines to corresponding secondary imines under ambient conditions within 30 min. The longstanding contention of actual involvement of Cu(II) in the catalysis is resolved through quenching experiments of Cu(II) superoxo species and detailed density functional theory studies.
Dalton Transaction , 2019
A triflatostannylene [L † Sn(II)][OTf ] (2) is reported here as an efficient catalyst with low-va... more A triflatostannylene [L † Sn(II)][OTf ] (2) is reported here as an efficient catalyst with low-valent main-group element for the hydroboration of aldehydes and ketones (L † = aminotroponate). Using 0.025-0.25 mol% of compound 2, hydroboration of various aldehydes and ketones is accomplished in 0.13-1.25 h at room temperature; the aliphatic aldehydes show an impressive TOF of around 30 000 h −1. DFT calculations are performed to explore the mechanistic aspects of this reaction suggesting that the reaction proceeds via a stepwise pathway with hydridostannylene [L † Sn(II)H] (2a) as the active catalyst and the H atom transfer from the Sn-H bond to the carbonyl carbon being the rate determining step.
Chemistry: A European Journal, 2017
Activationo fi nert CÀHb onds such as those of methane are extremely challenging for chemists but... more Activationo fi nert CÀHb onds such as those of methane are extremely challenging for chemists but in nature, the soluble methane monooxygenase (sMMO) enzyme readily oxidizes methane to methanol by using adiir-on(IV) species. This has prompted chemists to look for similar model systems. Recently,a(m-oxo)bis(m-carboxamido)diir-on(IV) ([Fe IV 2 O(L) 2 ] 2 + L = N,N-bis-(3',5'-dimethyl-4'-methoxy-pyridyl-2'-methyl)-N'-acetyl-1,2-diaminoethane) complex has been generated by bulk electrolysis and this species activates inert CÀHb ondsa lmost1 000 times faster than mono-nuclear Fe IV =Os peciesa nd at the same time selectively activates OÀHb onds of alcohols. The very high reactivity and selectivity of this speciesi sp uzzlinga nd herein we use extensive DFT calculations to shed light on this aspect. We have studied the electronic and spectralf eatures of diiron {Fe III-m(O)-Fe III } + 2 (complex I), {Fe III-m(O)-Fe IV } + 3 (II), and {Fe IV-m(O)-Fe IV } + 4 (III)c omplexes. Strong antiferromagnetic coupling between the Fe centers leads to spin-coupled S = 0, S = 3/2, and S = 0g round state for species I-III respectively. The mechanistic study of the CÀHa nd OÀHb ond activation reveals am ultistater eactivity scenario where CÀHb ond activation is found to occur through the S = 4s pin-coupled state corresponding to the high-spin state of individual Fe IV centers.T he OÀHb ond activation on the other hand, occurs through the S = 2s pin-coupled state corresponding to an intermediate state of individual Fe IV centers. Molecular orbital analysisr eveals s-p/p-p channels for the reactivity.T he nature of the magnetic exchange interaction is found to be switched during the course of the reactiona nd this offers lower energy pathways. Significant electronic cooperativity between two metal centers during the course of the reaction has been witnessed and this uncovers the reason behind the efficiency and selectivity observed. The catalyst is found to prudently choose the desired spin states based on the natureo ft he substrate to effect the catalytic transformations. These findings suggestt hat the presence of such factors play ar ole in the reactivity of dinuclear metalloenzymes such as sMMO.