01413nas a2200229 4500008004100000022001400041245010000055210006900155260007100224300001400295490000800309520056500317100001800882700002200900700002100922700001800943700002600961700002600987700002001013700002201033856012801055 2014 eng d a0002-786300aFormation of a room temperature stable Fe-V(O) complex: reactivity toward unactivated C-H bonds0 aFormation of a room temperature stable FeVO complex reactivity t a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcJUL a9524-95270 v1363 a

An Fe-V(O) complex has been synthesized from equimolar solutions of (Et4N)(2)[Fe-III(Cl)(biuretamide)] and mCPBA in CH3CN at room temperature. The Fe-V(O) complex has been characterized by UV-vis, EPR, Mossbauer, 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 mori (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.

1 aGhosh, Munmun1 aSingh, Kundan, K.1 aPanda, Chakadola1 aWeitz, Andrew1 aHendrich, Michael, P.1 aCollins, Terrence, J.1 aDhar, Basab, B.1 aGupta, Sayam, Sen uhttp://library.ncl.res.in/content/formation-room-temperature-stable-fe-vo-complex-reactivity-toward-unactivated-c-h-bonds-001617nas a2200205 4500008004100000022001400041245010700055210006900162260007100231300001600302490000800318520083300326100002101159700002401180700002201204700002201226700002201248700002001270856012101290 2014 eng d a0002-786300aHomogeneous photochemical water oxidation by biuret-modified Fe-TAML: evidence of Fe-V(O) intermediate0 aHomogeneous photochemical water oxidation by biuretmodified FeTA a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcSEP a12273-122820 v1363 a

{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 O-2 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 OD with moderate efficiency (maximum TON = 220

1 aPanda, Chakadola1 aDebgupta, Joyashish1 aDiaz, David, Diaz1 aSingh, Kundan, K.1 aGupta, Sayam, Sen1 aDhar, Basab, B. uhttp://library.ncl.res.in/content/homogeneous-photochemical-water-oxidation-biuret-modified-fe-taml-evidence-fe-vo-002237nas a2200193 4500008004100000022001400041245008200055210006900137260007100206300001400277490000700291520153500298100002201833700002701855700002001882700001701902700002201919856010201941 2015 eng d a0020-166900aMechanism of oxygen atom transfer from Fe-V(O) to olefins at room temperature0 aMechanism of oxygen atom transfer from FeVO to olefins at room t a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcJUL a6112-61210 v543 a

In biological oxidations, the intermediate Fe-V(0)(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(0) 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(0) complex of biuret-modified Fe-TAML at room temperature. The second-order rate constant (k(2)) for the reaction of different alkenes with Fe-V(0) was determined under single-turnover conditions. An 8000-fold rate difference was found between electron-rich (4-methoxystyrene; k(2) = 216 M-1 s(-1)) and electron-deficient (methyl transcinnamate; k(2) = 0.03 M-1 s(-1)) substrates. This rate difference indicates the electrophilic character of Fe-V(0). 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 rho 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.

1 aSingh, Kundan, K.1 aTiwari, Mrityunjay, K.1 aDhar, Basab, B.1 aVanka, Kumar1 aGupta, Sayam, Sen uhttp://library.ncl.res.in/content/mechanism-oxygen-atom-transfer-fe-vo-olefins-room-temperature-001055nas a2200181 4500008004100000245011300041210006900154260000800223300001600231490000700247520038800254100001800642700002400660700002200684700001800706700002200724856012700746 2016 eng d00aElectrocatalytic water oxidation by a molecular cobalt complex through a high valent cobalt oxo intermediate0 aElectrocatalytic water oxidation by a molecular cobalt complex t cAUG a11787-117900 v523 aBiuret-modified tetraamidomacrocyclic cobalt complex [Co-III-bTAML]- is shown to catalyze electrochemical water oxidation at basic pH leading to the formation of O-2. Electrochemical and spectroscopic studies indicate a high valent cobalt oxo intermediate isoelectronic to Co-V(O) as the active oxidant. The kinetic isotope effect of 8.63 indicates an atom proton transfer mechanism.1 aDas, Debasree1 aPattanayak, Santanu1 aSingh, Kundan, K.1 aGarai, Bikash1 aGupta, Sayam, Sen uhttp://library.ncl.res.in/content/electrocatalytic-water-oxidation-molecular-cobalt-complex-through-high-valent-cobalt-oxo02274nas a2200205 4500008004100000245010900041210006900150260000800219300001400227490000800241520153900249100002401788700002901812700001801841700002201859700001501881700002001896700002201916856013001938 2017 eng d00aElectrochemical formation of Fe-V (O) and mechanism of its reaction with water during O-O bond formation0 aElectrochemical formation of FeV O and mechanism of its reaction cMAR a3414-34240 v23 3 aA detailed electrochemical investigation of a series of iron complexes (biuret-modified tetraamido iron macrocycles FeIII-bTAML), including the first electrochemical generation of Fe-V(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 Fe-IV(O)/Fe-III(OH2)and Fe-V(O)/Fe-IV(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 H2O on FeV(O). A kinetic isotope effect of 3.2 indicates an atom-proton transfer (APT) mechanism. The reaction of chemically synthesised Fe-V(O) in CH3CN and water was directly probed by electrochemistry and was found to be first-order in water. The pK(a) 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.1 aPattanayak, Santanu1 aChowdhury, Debarati, Roy1 aGarai, Bikash1 aSingh, Kundan, K.1 aPaul, Amit1 aDhar, Basab, B.1 aGupta, Sayam, Sen uhttp://library.ncl.res.in/content/electrochemical-formation-fe-v-o-and-mechanism-its-reaction-water-during-o-o-bond-formation00934nas a2200145 4500008004100000245009600041210006900137260000800206300001400214490000700228520039100235100002200626700002200648856011800670 2017 eng d00aReductive activation of O-2 by a bioinspired Fe complex for catalytic epoxidation reactions0 aReductive activation of O2 by a bioinspired Fe complex for catal cMAY a5914-59170 v533 aAerobic epoxidation of olefins catalyzed by iron complexes without the use of a sacrificial coreductant is unknown. We report the reductive activation of O-2 by a bioinspired [(bTAML)Fe-III(H2O)](-) (1) complex to catalyze the epoxidation of alkenes with TONs of up to 80. Spectroscopic and kinetic evidence indicates the involvement of FeV(O) as the active oxidant during the reaction.1 aSingh, Kundan, K.1 aGupta, Sayam, Sen uhttp://library.ncl.res.in/content/reductive-activation-o-2-bioinspired-fe-complex-catalytic-epoxidation-reactions02250nas a2200241 4500008004100000245010400041210006900145260000800214300001400222490000700236520143100243100002401674700002601698700001601724700002501740700002201765700001801787700002201805700001801827700001801845700002201863856012301885 2017 eng d00aSpectroscopic and reactivity comparisons of a pair of bTAML complexes with Fe-V=O and Fe-IV=O units0 aSpectroscopic and reactivity comparisons of a pair of bTAML comp cJUN a6352-63610 v563 aIn this report we compare the geometric and electronic structures and reactivities of [Fe-V(O)](-) and [Fe-IV(O)](2-) species supported by the same ancillary nonheme biuret tetraamido macrocyclic ligand (bTAML). Resonance Raman studies show that the Fe-O vibration of the [Fe-IV(O)](2-) complex 2 is at 798 cm(-1), compared to 862 cm(-1) for the corresponding [Fe-V(O)](-) species 3, a 64 cm(-1) frequency difference reasonably reproduced by density functional theory calculations. These values are, respectively, the lowest and the highest frequencies observed thus far for nonheme high-valent Fe-O complexes. Extended X-ray absorption fine structure analysis of 3 reveals an Fe-O bond length of 1.59 angstrom, which is 0.05 angstrom shorter than that found in complex 2. The redox potentials of 2 and 3 are 0.44 V (measured at pH 12) and 1.19 V (measured at pH 7) versus normal hydrogen electrode, respectively, corresponding to the [Fe-IV(O)](2-)/[Fe-III(OH)](2-) and [Fe-V(O)](-)/[Fe-IV(O)](2-) couples. Consistent with its higher potential (even after correcting for the pH difference), 3 oxidizes benzyl alcohol at pH 7 with a second-order rate constant that is 2500-fold bigger than that for 2 at pH 12. Furthermore, 2 exhibits a classical kinteic isotope effect (KIE) of 3 in the oxidation of benzyl alcohol to benzaldehyde versus a nonclassical KIE of 12 for 3, emphasizing the reactivity differences between 2 and 3.1 aPattanayak, Santanu1 aJasniewsk, Andrew, J.1 aRana, Atanu1 aDraksharapu, Apparao1 aSingh, Kundan, K.1 aWeitz, Andrew1 aHendrich, Michael1 aQue, Lawrence1 aDey, Abhishek1 aGupta, Sayam, Sen uhttp://library.ncl.res.in/content/spectroscopic-and-reactivity-comparisons-pair-btaml-complexes-fe-vo-and-fe-ivo-units