Co-reporter:Fangfang Chen, Ni Wang, Haitao Lei, Dingyi Guo, Hongfei Liu, Zongyao Zhang, Wei Zhang, Wenzhen Lai, and Rui Cao
Inorganic Chemistry November 6, 2017 Volume 56(Issue 21) pp:13368-13368
Publication Date(Web):October 16, 2017
DOI:10.1021/acs.inorgchem.7b02125
Water-soluble copper(II) complexes of the dianionic tridentate pincer ligand N,N′-2,6-dimethylphenyl-2,6-pyridinedicarboxamidate (L) are catalysts for water oxidation. In [L-CuII-DMF] (1, DMF = dimethylformamide) and [L-CuII-OAc]− (2, OAc = acetate), ligand L binds CuII through three N atoms, which define an equatorial plane. The fourth coordination site of the equatorial plane is occupied by DMF in 1 and by OAc– in 2. These two complexes can electrocatalyze water oxidation to evolve O2 in 0.1 M pH 10 carbonate buffer. Spectroscopic, titration, and crystallographic studies show that both 1 and 2 undergo ligand exchange when they are dissolved in carbonate buffer to give [L-CuII-CO3H]− (3). Complex 3 has a similar structure as those of 1 and 2 except for having a carbonate group at the fourth equatorial position. A catalytic cycle for water oxidation by 3 is proposed based on experimental and theoretical results. The two-electron oxidized form of 3 is the catalytically active species for water oxidation. Importantly, for these two oxidation events, the calculated potential values of Ep,a = 1.01 and 1.59 V vs normal hydrogen electrode (NHE) agree well with the experimental values of Ep,a = 0.93 and 1.51 V vs NHE in pH 10 carbonate buffer. The potential difference between the two oxidation events is 0.58 V for both experimental and calculated results. With computational evidence, this Cu-bound carbonate group may act as a proton shuttle to remove protons for water activation, a key role resembling intramolecular bases as reported previously.
Co-reporter:Xudan Song;Jiarui Lu
Physical Chemistry Chemical Physics 2017 vol. 19(Issue 30) pp:20188-20197
Publication Date(Web):2017/08/02
DOI:10.1039/C7CP02687K
Herein, we use in-protein quantum mechanical/molecular mechanical (QM/MM) calculations to elucidate the mechanism of dioxygen activation, oxygen atom exchange and substrate epoxidation processes by AsqJ, an FeII/α-ketoglutarate-dependent dioxygenase (α-KGD) using a 2-His-1-Asp facial triad. Our results demonstrated that the whole reaction proceeds through a quintet surface. The dioxygen activation by AsqJ leads to a quintet penta-coordinated FeIV–oxo species, which has a square pyramidal geometry with the oxo group trans to His134. This penta-coordinated FeIV–oxo species is not the reactive one in the substrate epoxidation reaction since its oxo group is pointing away from the target CC bond. Instead, it can undergo the oxo group isomerization followed by water binding or the water binding followed by oxygen atom exchange to form the reactive hexa-coordinated FeIV–oxo species with the oxo group trans to His211. The calculated parameters of Mössbauer spectra for this hexa-coordinated FeIV–oxo intermediate are in excellent agreement with the experimental values, suggesting that it is most likely the experimentally trapped species. The calculated energetics indicated that the rate-limiting step is the substrate CC bond activation. This work improves our understanding of the dioxygen activation by α-KGD and provides important structural information about the reactive FeIV–oxo species.
Co-reporter:Binju Wang; Jiarui Lu; Kshatresh Dutta Dubey; Geng Dong; Wenzhen Lai;Sason Shaik
Journal of the American Chemical Society 2016 Volume 138(Issue 27) pp:8489-8496
Publication Date(Web):June 16, 2016
DOI:10.1021/jacs.6b03555
The iron(IV)–oxo (ferryl) intermediate has been amply established as the principal oxidant in nonheme enzymes and the key player in C–H bond activations and functionalizations. In contrast to this status, our present QM/MM calculations of the mechanism of fosfomycin biosynthesis (a broad range antibiotic) by the nonheme HppE enzyme rule out the iron(IV)–oxo as the reactive species in the hydrogen abstraction (H-abstraction) step of the pro-R hydrogen from the (S)-2-hydroxypropylphosphonic substrate. Moreover, the study reveals that the ferryl species is bypassed in HppE, while the actual oxidant is an HO• radical hydrogen-bonded to a ferric-hydroxo complex, resulting via the homolytic dissociation of the hydrogen peroxide complex, Fe(II)–H2O2. The computed energy barrier of this pathway is 12.0 kcal/mol, in fair agreement with the experimental datum of 9.8 kcal/mol. An alternative mechanism involves the iron-complexed hydroxyl radical (FeIII–(HO•)) that is obtained by protonation of the iron(IV)–oxo group via the O–H group of the substrate. The barrier for this pathway, 23.0 kcal/mol, is higher than the one in the first mechanism. In both mechanisms, the HO• radical is highly selective; its H-abstraction leading to the final cis-fosfomycin product. It appears that HppE is prone to usage of HO• radicals for C–H bond activation, because the ferryl oxidant requires a specific H-abstraction trajectory (∠FeOH ∼ 180°) that cannot be met for intramolecular H-abstraction. Thus, this work broadens the landscape of nonheme iron enzymes and makes a connection to Fenton chemistry, with implications on new potential biocatalysts that may harness hydroxyl radicals for C–H bond functionalizations.
Co-reporter:Geng Dong, Jiarui Lu, and Wenzhen Lai
ACS Catalysis 2016 Volume 6(Issue 6) pp:3796
Publication Date(Web):May 4, 2016
DOI:10.1021/acscatal.6b00372
2-Aminophenol 1,6-dioxygenase (APD) is an extradiol dioxygenase responsible for the ring cleavage of 2-aminophenol (2AP) at the position ortho to the hydroxyl substituent. To elucidate the reaction mechanism, we conducted quantum mechanical/molecular mechanical (QM/MM) calculations. The mode of binding of the substrate (monodentate or bidentate) to the iron center was found to have a crucial role in dioxygen activation. The Fe–O2 adducts with 2AP bound bidentately has a quintet ground state having a FeIII–superoxo character, while the Fe–O2 adducts with a monodentately bound substrate has been characterized as a substrate radical–FeII–superoxide. Unlike other extradiol dioxygenases that cleave catechol analogues using the superoxo moiety of the Fe–O2 adducts to attack the substrate, we found here an FeII–O(H)O intermediate formed through two sequential proton-coupled electron transfer steps from the initial FeIII–superoxo species is responsible for the attack. Importantly, the second-sphere His195 residue acts as an acid–base catalyst to mediate proton transfer (associated with electron transfer). The study presented here expands our understanding of the extradiol dioxygenases, especially those catalyzing the ring cleavage of noncatecholic substrates.Keywords: 2-His-1-carboxylate facial triad; extradiol dioxygenase; nonheme; oxygen activation; QM/MM
Co-reporter:Yongzhen Han;Dr. Huayi Fang;Huize Jing;Huiling Sun;Haitao Lei;Dr. Wenzhen Lai;Dr. Rui Cao
Angewandte Chemie International Edition 2016 Volume 55( Issue 18) pp:5457-5462
Publication Date(Web):
DOI:10.1002/anie.201510001
Abstract
A nickel(II) porphyrin Ni-P (P=porphyrin) bearing four meso-C6F5 groups to improve solubility and activity was used to explore different hydrogen-evolution-reaction (HER) mechanisms. Doubly reduced Ni-P ([Ni-P]2−) was involved in H2 production from acetic acid, whereas a singly reduced species ([Ni-P]−) initiated HER with stronger trifluoroacetic acid (TFA). High activity and stability of Ni-P were observed in catalysis, with a remarkable ic/ip value of 77 with TFA at a scan rate of 100 mV s−1 and 20 °C. Electrochemical, stopped-flow, and theoretical studies indicated that a hydride species [H-Ni-P] is formed by oxidative protonation of [Ni-P]−. Subsequent rapid bimetallic homolysis to give H2 and Ni-P is probably involved in the catalytic cycle. HER cycling through this one-electron-reduction and homolysis mechanism has been proposed previously but rarely validated. The present results could thus have broad implications for the design of new exquisite cycles for H2 generation.
Co-reporter:Yongzhen Han;Dr. Huayi Fang;Huize Jing;Huiling Sun;Haitao Lei;Dr. Wenzhen Lai;Dr. Rui Cao
Angewandte Chemie 2016 Volume 128( Issue 18) pp:5547-5552
Publication Date(Web):
DOI:10.1002/ange.201510001
Abstract
A nickel(II) porphyrin Ni-P (P=porphyrin) bearing four meso-C6F5 groups to improve solubility and activity was used to explore different hydrogen-evolution-reaction (HER) mechanisms. Doubly reduced Ni-P ([Ni-P]2−) was involved in H2 production from acetic acid, whereas a singly reduced species ([Ni-P]−) initiated HER with stronger trifluoroacetic acid (TFA). High activity and stability of Ni-P were observed in catalysis, with a remarkable ic/ip value of 77 with TFA at a scan rate of 100 mV s−1 and 20 °C. Electrochemical, stopped-flow, and theoretical studies indicated that a hydride species [H-Ni-P] is formed by oxidative protonation of [Ni-P]−. Subsequent rapid bimetallic homolysis to give H2 and Ni-P is probably involved in the catalytic cycle. HER cycling through this one-electron-reduction and homolysis mechanism has been proposed previously but rarely validated. The present results could thus have broad implications for the design of new exquisite cycles for H2 generation.
Co-reporter:Yue Qi, Jiarui Lu, and Wenzhen Lai
The Journal of Physical Chemistry B 2016 Volume 120(Issue 20) pp:4579-4590
Publication Date(Web):April 27, 2016
DOI:10.1021/acs.jpcb.6b03006
To elucidate the reaction mechanism of the ring cleavage of homogentisate by homogentisate dioxygenase, quantum mechanical/molecular mechanical (QM/MM) calculations were carried out by using two systems in different protonation states of the substrate C2 hydroxyl group. When the substrate C2 hydroxyl group is ionized (the ionized pathway), the superoxo attack on the substrate is the rate-limiting step in the catalytic cycle, with a barrier of 15.9 kcal/mol. Glu396 was found to play an important role in stabilizing the bridge species and its O–O cleavage product by donating a proton via a hydrogen-bonded water molecule. When the substrate C2 hydroxyl group is not ionized (the nonionized pathway), the O–O bond cleavage of the bridge species is the rate-limiting step, with a barrier of 15.3 kcal/mol. The QM/MM-optimized geometries for the dioxygen and alkylperoxo complexes using the nonionized model (for the C2 hydroxyl group) are in agreement with the experimental crystal structures, suggesting that the C2 hydroxyl group is more likely to be nonionized.
Co-reporter:Haitao Lei, Huayi Fang, Yongzhen Han, Wenzhen Lai, Xuefeng Fu, and Rui Cao
ACS Catalysis 2015 Volume 5(Issue 9) pp:5145
Publication Date(Web):July 27, 2015
DOI:10.1021/acscatal.5b00666
Several copper corrole complexes were synthesized, and their catalytic activities for hydrogen (H2) evolution were examined. Our results showed that substituents at the meso positions of corrole macrocycles played significant roles in regulating the redox and thus the catalytic properties of copper corrole complexes: strong electron-withdrawing substituents can improve the catalysis for hydrogen evolution, while electron-donating substituents are not favored in this system. The copper complex of 5,15-pentafluorophenyl-10-(4-nitrophenyl)corrole (1) was shown to have the best electrocatalytic performance among copper corroles examined. Complex 1 can electrocatalyze H2 evolution using trifluoroacetic acid (TFA) as the proton source in acetonitrile. In cyclic voltammetry, the value of icat/ip = 303 (icat is the catalytic current, ip is the one-electron peak current of 1 in the absence of acid) at a scan rate of 100 mV s–1 and 20 °C is remarkable. Electrochemical and spectroscopic measurements revealed that 1 has the desired stability in concentrated TFA acid solution and is unchanged by functioning as an electrocatalyst. Stopped-flow, spectroelectrochemistry, and theoretical studies provided valuable insights into the mechanism of hydrogen evolution mediated by 1. Doubly reduced 1 is the catalytic active species that reacts with a proton to give the hydride intermediate for subsequent generation of H2.Keywords: copper corroles; electrocatalysis; hydrogen evolution; reduction; stopped-flow
Co-reporter:Yongzhen Han, Yizhen Wu, Wenzhen Lai, and Rui Cao
Inorganic Chemistry 2015 Volume 54(Issue 11) pp:5604-5613
Publication Date(Web):May 18, 2015
DOI:10.1021/acs.inorgchem.5b00924
The water-soluble cationic nickel(II) complex of meso-tetrakis(4-N-methylpyridyl)porphyrin (1) can electrocatalyze water oxidation to O2 in neutral aqueous solution (pH 7.0) with the onset of the catalytic wave appearing at ∼1.0 V (vs NHE). The homogeneous catalysis with 1 was verified. Catalyst 1 exhibited water oxidation activity in a pH range 2.0–8.0 and had a strict linear dependence of catalytic current on its concentration. After 10 h of constant potential electrolysis at 1.32 V (vs NHE), a negligible difference of the solution was observed by UV–vis. In addition, inspection of the working electrode by electrochemistry, scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy (EDX) showed no sign of deposition of NiOx films. These results strongly argued that 1 is a real molecular electrocatalyst for water oxidation. The turnover frequency (TOF) for this process was 0.67 s–1 at 20 °C. On the basis of results from the kinetic isotope effect (KIE) and inhibition experiments, electrochemical studies in various buffer solutions with different anions and pHs, and DFT calculations, a catalytic cycle of 1 for water oxidation via a formally Ni(IV) species was proposed.
Co-reporter:Lili Cao, Geng Dong, and Wenzhen Lai
The Journal of Physical Chemistry B 2015 Volume 119(Issue 13) pp:4608-4616
Publication Date(Web):March 9, 2015
DOI:10.1021/acs.jpcb.5b00613
The reaction mechanisms of cobalt-substituted homoprotocatechuate 2,3-dioxygenase (Co-HPCD) with electron-rich substrate homoprotocatechuate (HPCA) and electron-poor substrate 4-nitrocatechol (4NC) were investigated by quantum mechanical/molecular mechanical (QM/MM) calculations. Our results demonstrated that the Co-O2 adducts has doublet ground state with a CoIII-O2•– character when 4NC was used as the substrate, in good agreement with the EPR spectroscopic experiment. The reactive oxygen species is the doublet CoIII-O2•– for Co-HPCD/4NC and the quartet SQ•↑-CoII-O2•–↓ species for Co-HPCD/HPCA, indicating that the substrate plays important roles in the dioxygen activation by Co-HPCD. B3LYP was found to overestimate the rate-limiting barriers in Co-HPCD. TPSSh predicts barriers of 21.5 versus 12.0 kcal/mol for Co-HPCD/4NC versus Co-HPCD/HPCA, which is consistent with the fact that the rate of the reaction is decreased when the substrate was changed from HPCA to 4NC.
Co-reporter:Haitao Lei, Ali Han, Fengwang Li, Meining Zhang, Yongzhen Han, Pingwu Du, Wenzhen Lai and Rui Cao
Physical Chemistry Chemical Physics 2014 vol. 16(Issue 5) pp:1883-1893
Publication Date(Web):11 Nov 2013
DOI:10.1039/C3CP54361G
Six cobalt and manganese corrole complexes were synthesized and examined as single-site catalysts for water splitting. The simple cobalt corrole [Co(tpfc)(py)2] (1, tpfc = 5,10,15-tris(pentafluorophenyl)corrole, py = pyridine) catalyzed both water oxidation and proton reduction efficiently. By coating complex 1 onto indium tin oxide (ITO) electrodes, the turnover frequency for electrocatalytic water oxidation was 0.20 s−1 at 1.4 V (vs. Ag/AgCl, pH = 7), and it was 1010 s−1 for proton reduction at −1.0 V (vs. Ag/AgCl, pH = 0.5). The stability of 1 for catalytic oxygen evolution and hydrogen production was evaluated by electrochemical, UV-vis and mass measurements, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX), which confirmed that 1 was the real molecular catalyst. Titration and UV-vis experiments showed that the pyridine group on Co dissociated at the beginning of catalysis, which was critical to subsequent activation of water. A proton-coupled electron transfer process was involved based on the pH dependence of the water oxidation reaction catalyzed by 1. As for manganese corroles 2–6, although their oxidizing powers were comparable to that of 1, they were not as stable as 1 and underwent decomposition at the electrode. Density functional theory (DFT) calculations indicated that water oxidation by 1 was feasible through a proposed catalytic cycle. The formation of an O–O bond was suggested to be the rate-determining step, and the calculated activation barrier of 18.1 kcal mol−1 was in good agreement with that obtained from experiments.
Co-reporter:Rui Zhang, Zhenning Liang, Ali Han, Haotian Wu, Pingwu Du, Wenzhen Lai and Rui Cao
CrystEngComm 2014 vol. 16(Issue 25) pp:5531-5542
Publication Date(Web):25 Mar 2014
DOI:10.1039/C4CE00514G
The vapochromism of the platinum(II) terpyridyl complex [Pt(tpy)Cl](PF6) (1, tpy = 2,2′:6′,2′′-terpyridine) was investigated. Complex 1 was found to exist in two forms. The yellow form of 1 turned to red by exposing the solid to either vapor or solution of acetonitrile, accompanied by changes in luminescence spectroscopy. This process could be reversed upon the loss of acetonitrile. Complex 1 was also demonstrated to show aggregation in a diethyl ether–acetonitrile system through Pt⋯Pt and terpyridyl π–π interactions to afford the red form of 1. Crystals of the red and yellow forms of 1 were analyzed. The red form of 1 crystallizes in the orthorhombic space group Pnma with a co-crystallized acetonitrile solvent molecule and thus is defined as 1-MeCN, while the yellow form is found to have the previously reported non-solvated crystal structure of 1. In 1-MeCN, the square planar [Pt(tpy)Cl]+ monocations stack to give an extended chain-like array of Pt atoms with a short Pt⋯Pt distance of 3.362 Å, while in 1, there are two alternating Pt⋯Pt distances (4.032 and 3.340 Å). Time-dependent density functional theory (TDDFT) calculations demonstrated that Pt⋯Pt and terpyridyl π–π interactions play important roles in electronic absorption features of the [Pt(tpy)Cl]+ system. Compared to dimers, the stack of three molecules of 1 with a short Pt⋯Pt distance considerably lowers the transition energy of metal–metal-to-ligand charge transfer (MMLCT), which causes a dramatic red shift in UV-vis spectroscopy.
Co-reporter:Geng Dong and Wenzhen Lai
The Journal of Physical Chemistry B 2014 Volume 118(Issue 7) pp:1791-1798
Publication Date(Web):January 27, 2014
DOI:10.1021/jp411812m
The reaction mechanism of the dioxygen activation by homoprotocatechuate 2,3-dioxygenase (HPCD) with the substrate 4-nitrocatechol was investigated by quantum mechanical/molecular mechanical calculations. Our results demonstrated that the experimentally determined side-on iron–oxygen complex in crystallo is a semiquinone substrate radical (SQ•)–FeIII–hydroperoxo species, which could not act as the reactive species. In fact, the FeIII–superoxo species with a hydrogen bond between His200 and the proximal oxygen is the reactive oxygen species. The second-sphere His200 residue was found to play an important role in manipulating the orientation of the superoxide in the Fe–O2 adduct for the further reaction. The rate-limiting step is the attack of the superoxo group on the substrate with a barrier of 17.2 kcal/mol, in good agreement with the experimental value of 16.8 kcal/mol. The reaction mechanism was then compared with the one for HPCD with its native substrate homoprotocatechuate studied recently by the same methods, in which a hybrid SQ•–FeII–O2•–/FeIII–O2•– was suggested to be the reactive species. Therefore, our studies suggested that the substrate plays important roles in the dioxygen activation by HPCD.
Co-reporter:Rui-Min Han, Hong Cheng, Ruopei Feng, Dan-Dan Li, Wenzhen Lai, Jian-Ping Zhang, and Leif H. Skibsted
The Journal of Physical Chemistry B 2014 Volume 118(Issue 40) pp:11659-11666
Publication Date(Web):September 16, 2014
DOI:10.1021/jp5075626
The efficient bleaching following continuous bubbling of gaseous nitric oxide (NO•) to β-carotene (β-Car) dissolved in n-hexane under anaerobic conditions results from an initial addition of two NO• followed by fragmentation coupled with further NO• addition as shown by mass spectrometry (MS). Density functional theory (DFT) calculations demonstrated that hydrogen atom transfer (HAT) and electron transfer (ET) from β-Car to NO• are strongly energetically unfavorable in contrast to radical adduct formation (RAF) followed by degradation. The results indicated the lowest energy for addition of the first NO• at C7 with an activation free energy of ΔG≠ = 74.40 kJ mol–1 and a rate constant of 0.56 s–1, followed by trans-addition of a second NO• at C8 with ΔG≠ = 55.51 kJ mol–1. MS confirmed the formation of a dinitrosyl-β-Car (596.6 m/z), and of a β-Car fragment (400.4 m/z) formed by C7/C8 bond cleavage and suggested to be of importance for progression of bleaching. Up to eight reaction products with increasing mass of 28 m/z are assigned to continuous addition of NO• to the initially formed fragment forming nitroxides. Continuous wave photolysis of sodium nitroprusside (SNP) as a NO• source dissolved together with β-Car in 4:1 (v/v) methanol:tetrahydrofuran gradually bleached β-Car. Nanosecond laser flash photolysis at 355 nm followed by transient absorption spectroscopy showed a β-Car derived intermediate with an absorption maximum around 420 nm in agreement with a prediction (425 nm) from time-dependent DFT (TDDFT) for the trans-C7,8 dinitrosyl adduct of β-Car. The NO• adduct of β-Car decays with a rate constant of ∼107 s–1 at 25 °C.
Co-reporter:Geng Dong, Sason Shaik and Wenzhen Lai
Chemical Science 2013 vol. 4(Issue 9) pp:3624-3635
Publication Date(Web):13 Jun 2013
DOI:10.1039/C3SC51147B
Oxygen activation by homoprotocatechuate 2,3-dioxygenase (HPCD) was investigated by quantum mechanical/molecular mechanical (QM/MM) calculations. Our results demonstrated that the semiquinone substrate radical-FeII-superoxo (SQ•–FeII–O2•−) and the corresponding FeIII-superoxo species are both present within the protein environment. Moreover, we also located a species, which possesses a hybrid SQ•–FeII–O2•−/FeIII–O2•− character (so-called hybrid state) with a hydrogen bond between His200 and the proximal oxygen. His200 was found to play an important role in controlling the electronic configuration of the superoxide species. A mere reorientation of the hydrogen bonding donated by His200, from its interaction with the substrate's oxygen to interaction with the proximal oxygen of the dioxygen moiety, causes a fast rearrangement from FeIII-superoxo to the hybrid state with partial electron transfer from the substrate to the Fe center. Since the hybrid state reacts further with a low barrier, then during the oxidation process all the FeIII-superoxo species are converted to the hybrid state, which is consumed rapidly by the substrate oxidation process. This theoretical result agrees quite well with the mechanism proposed in previous experimental investigation by Lipscomb et al., Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 16788–16793, where the FeIII–O2•− was suggested to be able to convert to the true reactive species, the SQ•–FeII–O2•− species, rapidly with one-electron transfer from the substrate to iron.
Co-reporter:Wenzhen Lai, Jiannian Yao, Sason Shaik, and Hui Chen
Journal of Chemical Theory and Computation 2012 Volume 8(Issue 9) pp:2991-2996
Publication Date(Web):August 22, 2012
DOI:10.1021/ct3005936
Using the recently proposed corrective LCCSD(T) method as a reference, we systematically assess the widely used approximate density functionals to reproduce C–H bond activation barriers by pincer complexes of the late platinum group transition metals (TMs) (TM = Rh, Pd, Ir, Pt). The pincer ligands explored here cover a wide range of PNP, PCP, POCOP, NCN, and SCS types. Interestingly, B3LYP is found to be the most accurate functional, followed by several others previously identified as well-performing functionals, like B2GP-PLYP, B2-PLYP, and PBE0. However, all tested functionals were found to exhibit the following uniform trends: (1) the DFT barriers for reactions of group 9 TM (Rh and Ir) pincer complexes show higher accuracy compared with those for group 10 TM (Pd and Pt) reactions; (2) within the same group, 5d TM pincer complexes have higher accuracy than 4d TM ones. Consequently, the barriers for C–H activation by Pd(II) pincer complexes were found to be the least accurate among the four TMs in almost all functionals tested here. The DFT empirical dispersion correction (DFT-D3) is shown to have a very small effect on barrier height. This study has some implications for other σ-bond activations like H–H, C–C, and C–halogen bonds by late platinum group pincer complexes.
Co-reporter:Wenzhen Lai, Rui Cao, Geng Dong, Sason Shaik, Jiannian Yao, and Hui Chen
The Journal of Physical Chemistry Letters 2012 Volume 3(Issue 17) pp:2315-2319
Publication Date(Web):August 6, 2012
DOI:10.1021/jz3008535
O–O bond formation catalyzed by a variety of β-octafluoro hangman corrole metal complexes was investigated using density functional theory methods. Five transition metal elements, Co, Fe, Mn, Ru, and Ir, that are known to lead to water oxidation were examined. Our calculations clearly show that the formal CoV catalyst has a CoIV–corrole•+ character and is the most efficient water oxidant among all eight transition-metal complexes. The O–O bond formation barriers were found to change in the following order: Co(V) ≪ Fe(V) < Mn(V) < Ir(V) < Co(IV) < Ru(V) < Ir(IV) < Mn(IV). The efficiency of water oxidation is discussed by analysis of the O–O bond formation step. Thus, the global trend is determined by the ability of the ligand d-block to accept two electrons from the nascent OH–, as well as by the OH• affinity of the TM(IV)═O species of the corresponding TM(V)═O·H2O complex. Exchange-enhanced reactivity (EER) is responsible for the high catalytic activity of the Co(V) species in its S = 1 state.Keywords: density functional theory; hangman corrole; nucleophilic attack; O−O bond formation; transition metal; water oxidation;
Co-reporter:Haitao Lei, Ali Han, Fengwang Li, Meining Zhang, Yongzhen Han, Pingwu Du, Wenzhen Lai and Rui Cao
Physical Chemistry Chemical Physics 2014 - vol. 16(Issue 5) pp:NaN1893-1893
Publication Date(Web):2013/11/11
DOI:10.1039/C3CP54361G
Six cobalt and manganese corrole complexes were synthesized and examined as single-site catalysts for water splitting. The simple cobalt corrole [Co(tpfc)(py)2] (1, tpfc = 5,10,15-tris(pentafluorophenyl)corrole, py = pyridine) catalyzed both water oxidation and proton reduction efficiently. By coating complex 1 onto indium tin oxide (ITO) electrodes, the turnover frequency for electrocatalytic water oxidation was 0.20 s−1 at 1.4 V (vs. Ag/AgCl, pH = 7), and it was 1010 s−1 for proton reduction at −1.0 V (vs. Ag/AgCl, pH = 0.5). The stability of 1 for catalytic oxygen evolution and hydrogen production was evaluated by electrochemical, UV-vis and mass measurements, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX), which confirmed that 1 was the real molecular catalyst. Titration and UV-vis experiments showed that the pyridine group on Co dissociated at the beginning of catalysis, which was critical to subsequent activation of water. A proton-coupled electron transfer process was involved based on the pH dependence of the water oxidation reaction catalyzed by 1. As for manganese corroles 2–6, although their oxidizing powers were comparable to that of 1, they were not as stable as 1 and underwent decomposition at the electrode. Density functional theory (DFT) calculations indicated that water oxidation by 1 was feasible through a proposed catalytic cycle. The formation of an O–O bond was suggested to be the rate-determining step, and the calculated activation barrier of 18.1 kcal mol−1 was in good agreement with that obtained from experiments.
Co-reporter:Geng Dong, Sason Shaik and Wenzhen Lai
Chemical Science (2010-Present) 2013 - vol. 4(Issue 9) pp:NaN3635-3635
Publication Date(Web):2013/06/13
DOI:10.1039/C3SC51147B
Oxygen activation by homoprotocatechuate 2,3-dioxygenase (HPCD) was investigated by quantum mechanical/molecular mechanical (QM/MM) calculations. Our results demonstrated that the semiquinone substrate radical-FeII-superoxo (SQ•–FeII–O2•−) and the corresponding FeIII-superoxo species are both present within the protein environment. Moreover, we also located a species, which possesses a hybrid SQ•–FeII–O2•−/FeIII–O2•− character (so-called hybrid state) with a hydrogen bond between His200 and the proximal oxygen. His200 was found to play an important role in controlling the electronic configuration of the superoxide species. A mere reorientation of the hydrogen bonding donated by His200, from its interaction with the substrate's oxygen to interaction with the proximal oxygen of the dioxygen moiety, causes a fast rearrangement from FeIII-superoxo to the hybrid state with partial electron transfer from the substrate to the Fe center. Since the hybrid state reacts further with a low barrier, then during the oxidation process all the FeIII-superoxo species are converted to the hybrid state, which is consumed rapidly by the substrate oxidation process. This theoretical result agrees quite well with the mechanism proposed in previous experimental investigation by Lipscomb et al., Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 16788–16793, where the FeIII–O2•− was suggested to be able to convert to the true reactive species, the SQ•–FeII–O2•− species, rapidly with one-electron transfer from the substrate to iron.
Co-reporter:Xudan Song, Jiarui Lu and Wenzhen Lai
Physical Chemistry Chemical Physics 2017 - vol. 19(Issue 30) pp:NaN20197-20197
Publication Date(Web):2017/06/28
DOI:10.1039/C7CP02687K
Herein, we use in-protein quantum mechanical/molecular mechanical (QM/MM) calculations to elucidate the mechanism of dioxygen activation, oxygen atom exchange and substrate epoxidation processes by AsqJ, an FeII/α-ketoglutarate-dependent dioxygenase (α-KGD) using a 2-His-1-Asp facial triad. Our results demonstrated that the whole reaction proceeds through a quintet surface. The dioxygen activation by AsqJ leads to a quintet penta-coordinated FeIV–oxo species, which has a square pyramidal geometry with the oxo group trans to His134. This penta-coordinated FeIV–oxo species is not the reactive one in the substrate epoxidation reaction since its oxo group is pointing away from the target CC bond. Instead, it can undergo the oxo group isomerization followed by water binding or the water binding followed by oxygen atom exchange to form the reactive hexa-coordinated FeIV–oxo species with the oxo group trans to His211. The calculated parameters of Mössbauer spectra for this hexa-coordinated FeIV–oxo intermediate are in excellent agreement with the experimental values, suggesting that it is most likely the experimentally trapped species. The calculated energetics indicated that the rate-limiting step is the substrate CC bond activation. This work improves our understanding of the dioxygen activation by α-KGD and provides important structural information about the reactive FeIV–oxo species.
![Nickel(4 ), [[4,4',4'',4'''-(21H,23H-porphine-5,10,15,20-tetrayl-κN21,κN22,κN23,κN24)tetrakis[1-methylpyridiniumato]](2-)]-, (SP-4-1)-](/data/chemimg/68600/48242-71-3.png)
![Nickel(4 ), [[4,4',4'',4'''-(21H,23H-porphine-5,10,15,20-tetrayl-κN21,κN22,κN23,κN24)tetrakis[1-methylpyridiniumato]](2-)]-, (SP-4-1)-](/data/chemimg/68600/48242-71-3_b.png)
![Nickel, [5,10,15,20-tetraphenyl-21H,23H-porphinato(2-)-κN21,κN22,κN23,κN24]-, (SP-4-1)- (9CI)](/data/chemimg/478900/14172-92-0.png)
![Nickel, [5,10,15,20-tetraphenyl-21H,23H-porphinato(2-)-κN21,κN22,κN23,κN24]-, (SP-4-1)- (9CI)](/data/chemimg/478900/14172-92-0_b.png)
![2,2'-[(2,3,4,5,6-pentafluorophenyl)methylene]bis-1H-Pyrrole](http://img.cochemist.com/ccimg/167500/167482-91-9.png)
![2,2'-[(2,3,4,5,6-pentafluorophenyl)methylene]bis-1H-Pyrrole](http://img.cochemist.com/ccimg/167500/167482-91-9_b.png)
