Basab B. Dhar

Homogeneous Photochemical Water Oxidation by Biuret-Modified Fe-TAML: Evidence of FeV(O) Intermediate

Water splitting, leading to hydrogen and oxygen in a process that mimics natural photosynthesis, is extremely important for devising a sustainable solar energy conversion system. Development of earth-abundant, transition metal-based catalysts that mimic the oxygen-evolving complex of photosystem II, which is involved in oxidation of water to O2 during natural photosynthesis, represents a major challenge. Further, understanding the exact mechanism, including elucidation of the role of active metal-oxo intermediates during water oxidation (WO), is critical to the development of more efficient catalysts. Herein, we report Fe(III) complexes of biuret-modified tetra-amidomacrocyclic ligands (Fe-TAML; 1a and 1b) that catalyze fast, homogeneous, photochemical WO to give O2, with moderate efficiency (maximum TON = 220, TOF = 0.76 s(-1)). Previous studies on photochemical WO using iron complexes resulted in demetalation of the iron complexes with concomitant formation of iron oxide nanoparticles (NPs) that were responsible for WO. Herein, we show for the first time that a high valent Fe(V)(O) intermediate species is photochemically generated as the active intermediate for the oxidation of water to O2. To the best of our knowledge, this represents the first example of a molecular iron complex catalyzing photochemical WO through a Fe(V)(O) intermediate.

Formation of a Room Temperature Stable Fe V (O) Complex: Reactivity Toward Unactivated C−H Bonds

An Fe(V)(O) complex has been synthesized from equimolar solutions of (Et4N)2[Fe(III)(Cl)(biuret-amide)] and mCPBA in CH3CN at room temperature. The Fe(V)(O) complex has been characterized by UV-vis, EPR, Mössbauer, and HRMS and shown to be capable of oxidizing a series of alkanes having C-H bond dissociation energies ranging from 99.3 kcal mol(-1) (cyclohexane) to 84.5 kcal mol(-1) (cumene). Linearity in the Bell-Evans-Polayni graph and the finding of a large kinetic isotope effect suggest that hydrogen abstraction is engaged the rate-determining step.

Fe-TAML encapsulated inside mesoporous silica nanoparticles as peroxidase mimic: femtomolar protein detection.

Peroxidase, such as horseradish peroxidase (HRP), conjugated to antibodies are routinely used for the detection of proteins via an ELISA type assay in which a critical step is the catalytic signal amplification by the enzyme to generate a detectable signal. Synthesis of functional mimics of peroxidase enzyme that display catalytic activity which far exceeds the native enzyme is extremely important for the precise and accurate determination of very low quantities of proteins (fM and lower) that is necessary for early clinical diagnosis. Despite great advancements, analyzing proteins of very low abundance colorimetrically, a method that is most sought after since it requires no equipment for the analysis, still faces great challenges. Most reported HRP mimics that show catalytic activity greater than native enzyme (∼10-fold) are based on metal/metal-oxide nanoparticles such as Fe3O4. In this paper, we describe a second generation hybrid material developed by us in which approximately 25,000 alkyne tagged biuret modified Fe-tetraamido macrocyclic ligand (Fe-TAML), a very powerful small molecule synthetic HRP mimic, was covalently attached inside a 40 nm mesoporous silica nanoparticle (MSN). Biuret-modified Fe-TAMLs represent one of the best small molecule functional mimics of the enzyme HRP with reaction rates in water close to the native enzyme and operational stability (pH, ionic strength) far exceeding the natural enzyme. The catalytic activity of this hybrid material is around 1000-fold higher than that of natural HRP and 100-fold higher than that of most metal/metal oxide nanoparticle based HRP mimics reported to date. We also show that using antibody conjugates of this hybrid material it is possible to detect and, most importantly, quantify femtomolar quantities of proteins colorimetrically in an ELISA type assay. This represents at least 10-fold higher sensitivity than other colorimetric protein assays that have been reported using metal/metal oxide nanoparticles as HRP mimic. Using a human IgG expressing cell line, we were able to demonstrate that the protein of interest human IgG could be detected from a mixture of interfering proteins in our assay.

Tuning the Reactivity of FeV(O) toward C–H Bonds at Room Temperature: Effect of Water

The presence of an FeV(O) species has been postulated as the active intermediate for the oxidation of both C–H and C=C bonds in the Rieske dioxygenase family of enzymes. Understanding the reactivity of these high valent iron–oxo intermediates, especially in an aqueous medium, would provide a better understanding of these enzymatic reaction mechanisms. The formation of an FeV(O) complex at room temperature in an aqueous CH3CN mixture that contains up to 90% water using NaOCl as the oxidant is reported here. The stability of FeV(O) decreases with increasing water concentration. We show that the reactivity of FeV(O) toward the oxidation of C–H bonds, such as those in toluene, can be tuned by varying the amount of water in the H2O/CH3CN mixture. Rate acceleration of up to 60 times is observed for the oxidation of toluene upon increasing the water concentration. The role of water in accelerating the rate of the reaction has been studied using kinetic measurements, isotope labeling experiments, and density functional theory (DFT) calculations. A kinetic isotope effect of ∼13 was observed for the oxidation of toluene and d8-toluene showing that C–H abstraction was involved in the rate-determining step. Activation parameters determined for toluene oxidation in H2O/CH3CN mixtures on the basis of Eyring plots for the rate constants show a gain in enthalpy with a concomitant loss in entropy. This points to the formation of a more-ordered transition state involving water molecules. To further understand the role of water, we performed a careful DFT study, concentrating mostly on the rate-determining hydrogen abstraction step. The DFT-optimized structure of the starting FeV(O) and the transition state indicates that the rate enhancement is due to the transition state’s favored stabilization over the reactant due to enhanced hydrogen bonding with water.

Mechanism of Oxygen Atom Transfer from FeV(O) to Olefins at Room Temperature

In biological oxidations, the intermediate Fe(V)(O)(OH) has been proposed to be the active species for catalyzing the epoxidation of alkenes by nonheme iron complexes. However, no study has been reported yet that elucidates the mechanism of direct O-atom transfer during the reaction of Fe(V)(O) with alkenes to form the corresponding epoxide. For the first time, we study the mechanism of O-atom transfer to alkenes using the Fe(V)(O) complex of biuret-modified Fe-TAML at room temperature. The second-order rate constant (k2) for the reaction of different alkenes with Fe(V)(O) was determined under single-turnover conditions. An 8000-fold rate difference was found between electron-rich (4-methoxystyrene; k2 = 216 M(-1) s(-1)) and electron-deficient (methyl trans-cinnamate; k2 = 0.03 M(-1) s(-1)) substrates. This rate difference indicates the electrophilic character of Fe(V)(O). The use of cis-stilbene as a mechanistic probe leads to the formation of both cis- and trans-stilbene epoxides (73:27). This suggests the formation of a radical intermediate, which would allow C-C bond rotation to yield both stereoisomers of stilbene-epoxide. Additionally, a Hammett ρ value of -0.56 was obtained for the para-substituted styrene derivatives. Detailed DFT calculations show that the reaction proceeds via a two-step process through a doublet spin surface. Finally, using biuret-modified Fe-TAML as the catalyst and NaOCl as the oxidant under catalytic conditions epoxide was formed with modest yields and turnover numbers.

Green synthesis of noble metal nanoparticles using cysteine-modified silk fibroin: catalysis and antibacterial activity

Noble metal nanoparticles (NPs) have shown remarkable potential for numerous applications. In this work, a simple, one-pot, green method for the synthesis of gold, silver, palladium, and platinum NPs by using thiol-modified silk fibroin (SF-SH) has been described. The incorporation of thiol groups into silk fibroin (SF) yields small, mono-dispersed metal nanoparticles with good colloidal stability. UV-Vis, transmission electron microscopy (TEM), and X-ray powder diffraction (XRD) analyses show the formation of NPs, and thermo-gravimetric analysis (TGA) data reveal interaction of the NPs with thiol-modified SF. We also show that all the NP–SF conjugates catalyse the reduction of p-nitrophenol to p-aminophenol in the presence of NaBH4 at room temperature. The NP–SF conjugate materials were processed into different material formats like porous scaffolds and films without compromising their individual properties. The Au–SF–SH composite scaffold was used successfully in the heterogeneous catalysis of p-nitrophenol reduction using NaBH4 while the Ag–SF–SH conjugated film showed good antibacterial activity.

Development of a multifunctional catalyst for a “relay” reaction

In the area of catalysis, nanoparticles and enzymes are two of the most important systems. By amalgamating the two, we present here proof of the concept that it is possible to prepare a multifunctional catalyst that can carry out a “relay” reaction. The catalyst consists of a surface bound enzyme on a metalcore–silicashell nanoparticle architecture. Here the enzyme catalyzes the 1st reaction and the metal nanoparticles act as a catalyst for the 2nd reaction of the product from the 1st reaction. In particular, we have studied the catalytic activity of glucosidase grafted Au@mSiO2 on 4-nitrophenyl-β-glucopyranoside, where glucosidase will catalyse the 1st step to generate 4-nitrophenol, which acts as a substrate for the next reduction step which is catalysed by the Au nanoparticles present inside the mesoporous silica shell.

Electrochemical formation of FeV(O) and mechanism of its reaction with water during O-O bond formation.

A detailed electrochemical investigation of a series of iron complexes (biuret-modified tetraamido iron macrocycles FeIII -bTAML), including the first electrochemical generation of FeV (O), and demonstration of their efficacy as homogeneous catalysts for electrochemical water oxidation (WO) in aqueous medium are reported. Spectroelectrochemical and mass spectral studies indicated FeV (O) as the active oxidant, formed due to two redox transitions, which were assigned as FeIV (O)/FeIII (OH2 ) and FeV (O)/FeIV (O). The spectral properties of both of these high-valent iron oxo species perfectly match those of their chemically synthesised versions, which were thoroughly characterised by several spectroscopic techniques. The O-O bond-formation step occurs by nucleophilic attack of H2 O on FeV (O). A kinetic isotope effect of 3.2 indicates an atom-proton transfer (APT) mechanism. The reaction of chemically synthesised FeV (O) in CH3 CN and water was directly probed by electrochemistry and was found to be first-order in water. The pKa value of the buffer base plays a critical role in the rate-determining step by increasing the reaction rate several-fold. The electronic effect on redox potential, WO rates, and onset overpotential was studied by employing a series of iron complexes. The catalytic activity was enhanced by the presence of electron-withdrawing groups on the bTAML framework. Changing the substituents from OMe to NO2 resulted in an eightfold increase in reaction rate, while the overpotential increased threefold.

Novel porous silica encapsulated Au nanoreactors as peroxidase mimic for one-pot glucose detection

The peroxidase family of natural enzymes facilitate H2O2 reduction by one electron transfer through aromatic substrates, if chromogenic, they can be used in the colorimetric detection of H2O2 and subsequently glucose in tandem with glucose oxidase. Au nanoparticles encapsulated within porous silica exhibited peroxidase mimetic activity and kinetic parameter evaluation indicates an excellent affinity for H2O2. It is also shown to have a detection capability for glucose in very low concentrations and could be used for glucose detection in a one-pot assay. The material is synthesised by using cation-ended thiol-stabilised ultra small gold clusters as precursors. Removal of the thiols creates micropores within the silica, making this material unique in that the active centres are protected inside the silica, yet are accessible to reactant molecules. This characteristic makes the material ideal as a detection tool where attrition resistance will be advantageous.

Selective C–H Bond Oxidation Catalyzed by the Fe-bTAML Complex: Mechanistic Implications

Nonheme iron complexes bearing tetradentate N-atom-donor ligands with cis labile sites show great promise for chemoselective aliphatic C-H hydroxylation. However, several challenges still limit their widespread application. We report a mechanism-guided development of a peroxidase mimicking iron complex based on the bTAML macrocyclic ligand framework (Fe-bTAML: biuret-modified tetraamido macrocyclic ligand) as a catalyst to perform selective oxidation of unactivated 3° bonds with unprecedented regioselectivity (3°:2° of 110:1 for adamantane oxidation), high stereoretention (99%), and turnover numbers (TONs) up to 300 using mCPBA as the oxidant. Ligand decomposition pathways involving acid-induced demetalation were identified, and this led to the development of more robust and efficient Fe-bTAML complexes that catalyzed chemoselective C-H oxidation. Mechanistic studies, which include correlation of the product formed with the FeV(O) reactive intermediates generated during the reaction, indicate that the major pathway involves the cleavage of C-H bonds by FeV(O). When these oxidations were performed in the presence of air, the yield of the oxidized product doubled, but the stereoretention remained unchanged. On the basis of 18O labeling and other mechanistic studies, we propose a mechanism that involves the dual activation of mCPBA and O2 by Fe-bTAML, leading to formation of the FeV(O) intermediate. This high-valent iron oxo remains the active intermediate for most of the reaction, resulting in high regio- and stereoselectivity during product formation.