Search

The one-step oxidation of benzene to phenol represents a significant and promising advancement in modern industries focused on the production of high-value-added chemical products. Nevertheless, challenges persist in achieving sufficient catalytic selectivity and preventing over-oxidation. Inspired by copper enzymes, we present a nonsymmetric dicopper complex ([CuII2(TPMAN)(μ-OH)(H 2 O)] 3+ , 1) for the selective oxidation of benzene to phenol. Utilizing H 2 O 2 as the oxidant, complex 1 demonstrates remarkable catalytic activity (a TON of 14 000 within 29 hours) and selectivity exceeding 97%, comparable to the finest homogeneous catalyst derived from first-row transition metals. It is noteworthy that the significant substituent effect, alongside a negligible kinetic isotope effect (KIE = 1.05), radical trapping experiments, and an inconsistent standard selectivity test of the ˙OH radicals, all contradict the conventional Fenton mechanism and rebound pathway. Theoretical investigations indicate that the active Cu II (μ-O˙)Cu II -OH species generated through the cleavage of the O-O bond in the Cu II (μ-1,1-OOH)Cu I intermediate facilitates the hydroxylation of benzene via an electrophilic attack mechanism. The nonsymmetric coordination geometry is crucial in activating H 2 O 2 and in the process of O-O bond cleavage.

The tetradentate PNNP-type Ir III complex Mes-IrPCY2 ([Cl-Ir III -H] + ) is reported to be an efficient catalyst for the reduction of CO 2 to formate with excellent selectivity under visible light irradiation. Density functional calculations have been carried out to elucidate the mechanism and the origin of selectivity in the present work. Calculations suggest that the double-reduced complex 1-H ( 1 [Ir I -H] 0 ) demonstrates higher activity than the single-reduced complex 2-H ( 2 [Ir III (L˙ - )-H] + ), possibly owing to the higher hydride donor ability of the former compared to the latter; thus 1-H functions as the active species in the overall CO 2 reduction reaction. In the HCOO - formation pathway, the hydride of 1-H performs a nucleophilic attack on CO 2 via an outer-sphere fashion to generate species 1-OCHO ( 1 [Ir I -OCHO] 0 ), which then releases HCOO - to produce an Ir I intermediate. A subsequent protonation and chloride coordination of the Ir center leads to the regeneration of catalyst 1 [Cl-Ir III -H] + . For the CO production, a nucleophilic attack on CO 2 takes place by the Ir atom of 1-H via an inner-sphere manner to afford complex O2C-3-H ( 1 [O 2 C-Ir III -H] 0 ), followed by a two-proton-one-electron reduction to furnish the OC-2-H complex ( 2 [OC-Ir III (L˙ - )-H] + ) after liberating a H 2 O. Ultimately, CO is released to form 2-H. The stronger nucleophilicity as well as smaller steric hindrance of the hydride than the Ir atom of the active species 1-H ( 1 [Ir I -H] 0 ) is found to account for the favoring of formate formation over CO formation. Meanwhile, the CO 2 reduction reaction is calculated to be preferred over the hydrogen evolution reaction, and this is consistent with the experimental product distributions.

Mmp10 is a B 12 -dependent SAM radical enzyme that catalyzes C δ -methylation of arginine. The quantum chemical cluster calculations of Mmp10 revealed a "pull-push" radical transfer mechanism in which a 5'-deoxyadenosine radical first abstracts a hydrogen atom from the arginine residue and then methylcobalamin donates a methyl group to the arginine residue. The stereoselectivity and regioselectivity of Mmp10 originated in a specific substrate binding mode enabled by the active site.

The phosphine-substituted Ru(II) polypyridyl complex, [Ru II -(tpy)(pqn)(MeCN)] 2+ (RuP), was disclosed to be an efficient photocatalyst for the reduction of CO 2 to CO with excellent selectivity. In this work, density functional calculations were performed to elucidate the reaction mechanism and understand the origin of selectivity. The calculations showed that RuP was first excited to the singlet excited state, followed by intersystem crossing to produce a triplet species (3RuIII(L˙-)-S), which was then reduced by the sacrificial electron donor BIH to generate a Ru II (L˙ - ) intermediate. The ligand of Ru II (L˙ - ) was further reduced to produce a Ru II (L 2- ) intermediate. The redox non-innocent nature of the tpy and pqn ligands endows the Ru center with an oxidation state of +2 after two one-electron reductions. Ru II (L 2- ) nucleophilically attacks CO 2 , in which two electrons are delivered from the ligands to CO 2 , affording a Ru II -COOH species after protonation. This is followed by the protonation of the hydroxyl moiety of Ru II -COOH, coupled with the C-O bond cleavage, resulting in the formation of Ru II -CO. Ultimately, CO is dissociated after two one-electron reductions. Protonation of Ru II (L 2- ) to generate a Ru II -hydride, a critical intermediate for the production of formate and H 2 , turns out to be kinetically less favorable, even though it is thermodynamically more favorable. This fact is due to the presence of a Ru 2+ ion in the reduced catalyst, which disfavors its protonation.

Chiral photosensitizer-catalyzed stereoselective olefin cyclization has shown its significance in organic synthesis. In this work, we investigated the reaction mechanism, regioselectivity and stereoselectivity of photochemical intramolecular [2 + 2] cycloaddition reaction catalyzed by a chiral thioxanthone molecule using quantum chemical calculations. The reaction proceeded via an energy transfer from the triplet thioxanthone to the substrate, involving stepwise and sequential C-C bond formation. The first C-C bond formation was calculated to be the rate-limiting and selectivity-controlling step. The origin of stereoselectivity was found to be interaction-controlled by distortion/interaction analysis. In addition, the catalyst substituent effects (O vs. S vs. Se) on the stereoselectivity of the photocycloadditions were explored, which provides helpful mechanistic information for the design of related photoinduced reactions.

The conversion of carbon dioxide (CO 2 ) to fuels or value-added chemicals by a photocatalytic system has recently been of growing research interest. One of the challenges is the development of new catalysts with high activity and low cost. Cobalt complexes have long been used as catalysts for the reduction of CO 2 in either electrochemical or photochemical systems. Recently, a series of cis-Co II complexes of tetradentate pyridine-amine ligands (N 4 -ligands) exhibited high activity in the reduction of CO 2 in homogeneous photocatalytic systems. However, only CO was obtained as the reduction product. In this regard, herein, we report a novel cis-Co II complex C1 supported by an N 4 ligand derivatized with TPA (TPA = tris(2-pyridylmethyl)amine). In contrast to the aforementioned Co II catalysts, which contain two halogen atoms at cis-positions, C1 contains one oxygen atom at one cis-coordination site. The structure of C1 was fully characterized by MS, elemental analysis, and single-crystal X-ray diffraction. Experiments on the photocatalytic reduction of CO 2 revealed that C1 is able to convert CO 2 to not only CO but also formate in a homogeneous system containing C1 as a catalyst, Ir(ppy) 3 as a photosensitizer, and triethylamine as an electron donor under visible-light irradiation. The catalytic activity and distribution of reduction products of this system are highly affected by the solvent environment. The presence of water in this system enhances the efficiency of 2H + -to-H 2 and CO 2 -to-formate conversions. Electrochemical and steady-state emission quenching experiments indicate that photoinduced electron transfer from excited Ir(ppy) 3 to C1 is thermodynamically feasible. A photogenerated Co I species is suggested to be the active species involved in the reduction of CO 2 and protons. DFT calculations were performed to elucidate the catalytic pathways of the formation of CO, formate, and H 2 in this system; four pathways, namely, one for the formation of CO, one for the formation of hydrogen, and two for the formation of formate, were suggested. The results revealed that the oxygen atom at the cis-coordination site in C1 plays an important role in stabilizing the transition state during the transformation of CO 2 at the cobalt center.

The performance of water oxidation catalysis by a Cu-based polypyridyl complex, [CuII(TPA)(OH2)]2+ (1H; TPA = tris-(pyridylmethyl)amine), has been investigated in neutral aqueous solution by electrochemical methods. Compared with our previously reported binuclear catalyst, [(BPMAN)(CuII)2(μ-OH)]3+ (2; BPMAN = 2,7-[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine), mononuclear catalyst 1 has a higher overpotential and lower catalytic activity toward water oxidation under the same conditions. Experimental results revealed that the O-O bond formation occurred via a water nucleophilic attack mechanism in which formal CuIV(O) is proposed as a key intermediate for the mononuclear catalyst 1H. In contrast, for the binuclear catalyst, O-O bond formation was facilitated by bimetallic cooperation between the two CuIII centers.

The reaction mechanism and chemoselectivity of nickel-dependent quercetin 2,4-dioxygenase (2,4-QueD) have been investigated using the QM/MM approach. The protonation state of the Glu74 residue, a first-shell ligand of Ni, has been considered to be either neutral or deprotonated. QM/MM calculations predict that Glu74 must be deprotonated to rationalize the chemoselectivity and steer the 2,4-dioxygenolytic cleavage of quercetin, which harvests the experimentally-observed product, 2-protocatechuoylphloroglucinol carboxylic acid, coupled with the release of carbon monoxide. If the enzyme has a neutral Glu74 residue, the undesired 2,3-dioxygenolytic cleavage of quercetin becomes the dominant pathway, leading to the formation of α-keto acid. The calculations suggest that the reaction takes place via three major steps: (1) attack of the superoxide on the C2 of the substrate pyrone ring to generate a NiII-peroxide intermediate; (2) formation of the second C-O bond between C4 and the peroxide to produce a peroxide bridge; (3) simultaneous cleavage of the C2-C3, C3-C4, and O1-O2 bonds with the formation of 2-protocatechuoylphloroglucinol carboxylic acid and carbon monoxide. The third step was found to be rate-limiting, with a barrier of 17.4 kcal mol-1, which is in very good agreement with the experimental kinetic data. For the second C-O bond formation, an alternative pathway is that the peroxide attacks the C3 of the substrate pyrone ring, leading to the formation of a four-membered ring intermediate, which then undergoes concerted C2-C3 and O1-O2 bond cleavages to produce an α-keto acid. This pathway is associated with a barrier of 30.6 kcal mol-1, which is much higher than the major pathway. When Glu74 is protonated, the 2,3-dioxygenolytic pathway, however, has a lower barrier (21.8 kcal mol-1) than the 2,4-dioxygenolytic pathway.

The reaction mechanism of the electro-catalytic proton reduction in neutral phosphate buffer enabled by mononuclear cobalt and iron complexes containing a tetra-dentate phosphine ligand (MP 4 N 2 , M = Fe, Co) has been elucidated by density functional calculations. The phosphate from the buffer was found to play a crucial role by coordinating to the metal and delivering a proton to the metal hydride in the H-H bond formation. For the more efficient cobalt catalyst, the starting species is a Co II complex with a hydrogen phosphate and a water molecule ligated at the two vacant coordination sites. Two sequential proton-coupled electron transfer reductions lead to the formation of a Co II -H intermediate with a dihydrogen phosphate ligand, and the reduction potentials for these two steps were calculated to be -0.58 V and -0.72 V, respectively. Subsequently, the H-H bond formation takes place via coupling of the Co II -H and the proton from the dihydrogen phosphate ligand. The total barrier was calculated to be 18.2 kcal mol -1 with an applied potential of -0.5 V, which can further decrease to only 11.2 kcal mol -1 with an applied potential of -0.8 V. When the phosphate is displaced by a water molecule, the total barrier for the dihydrogen formation increases by 7.3 kcal mol -1 . For the iron catalyst, the overall mechanism is essentially the same; however, the first reduction (Fe II /Fe I , potential of -1.13 V) is likely the rate-limiting step. The calculated results are in good agreement with the experimental data, which showed an onset potential of -0.50 V for the cobalt complex and -1.03 V for the iron complex.

The synthesis of two molecular iron complexes, a dinuclear iron(iii,iii) complex and a nonanuclear iron complex, based on the dinucleating ligand 2,2'-(2-hydroxy-5-methyl-1,3-phenylene)bis(1H-benzo[d]imidazole-4-carboxylic acid) is described. The two iron complexes were found to drive the oxidation of water by the one-electron oxidant [Ru(bpy)3](3+).

Developing new catalytic technologies through C-H bond activation to synthesize versatile pharmaceuticals has attracted much attention in recent decades. This work introduces a new strategy in catalyst design for Pd(ii)-catalyzed C-H bond activation in which non-redox metal ions serving as Lewis acids play significant roles. In the oxidative coupling of indoles with olefins using dioxygen, it was found that Pd(OAc)2 alone as the catalyst is very sluggish at ambient temperature which provided a low yield of the olefination product, whereas adding non-redox metal ions to Pd(OAc)2 substantially improves its catalytic efficiency. In particular, it provided bis(indolyl)methane derivatives as the dominant product, a category of pharmacological molecules which could not be synthesized by Pd(ii)-catalyzed oxidative coupling previously. Detailed investigations revealed that the reaction proceeds by heterobimetallic Pd(ii)/Sc(iii)-catalyzed oxidative coupling of an indole with an olefin followed by Sc(iii)-catalyzed addition with a second indole molecule. DFT calculations disclosed that the formation of heterobimetallic Pd(ii)/Sc(iii) species substantially decreases the C-H bond activation energy barrier, and shifts the rate determining step from C-H bond activation of indole to the olefination step. This non-redox metal ion promoted Pd(ii)-catalyzed C-H bond activation may offer a new opportunity for catalyst design in organic synthesis, which has not been fully recognized yet.

The mechanism of water reduction catalysed by a mononuclear copper complex Cu(bztpen) (bztpen = N-benzyl-N,N',N'-tris(pyridine-2-ylmethyl)ethylenediamine) has been elucidated by DFT calculations, revealing that hydrogen evolution proceeds via coupling of a Cu(ii)-hydride and a pendant pyridinium, and providing important implications for the future design of new catalytic systems for water reduction.

Benzoyl-CoA epoxidase is a dinuclear iron enzyme that catalyzes the epoxidation reaction of the aromatic ring of benzoyl-CoA with chemo-, regio- and stereo-selectivity. It has been suggested that this enzyme may also catalyze the deoxygenation reaction of epoxide, suggesting a unique bifunctionality among the diiron enzymes. We report a density functional theory study of this enzyme aimed at elucidating its mechanism and the various selectivities. The epoxidation is suggested to start with the binding of the O 2 molecule to the diferrous center to generate a diferric peroxide complex, followed by concerted O-O bond cleavage and epoxide formation. Two different pathways have been located, leading to (2 S ,3 R )-epoxy and (2 R ,3 S )-epoxy products, with barriers of 17.6 and 20.4 kcal mol -1 , respectively. The barrier difference is 2.8 kcal mol -1 , corresponding to a diastereomeric excess of about 99 : 1. Further isomerization from epoxide to phenol is found to have quite a high barrier, which cannot compete with the product release step. After product release into solution, fast epoxide-oxepin isomerization and racemization can take place easily, leading to a racemic mixture of (2 S ,3 R ) and (2 R ,3 S ) products. The deoxygenation of epoxide to regenerate benzoyl-CoA by a diferrous form of the enzyme proceeds via a stepwise mechanism. The C2-O bond cleavage happens first, coupled with one electron transfer from one iron center to the substrate, to form a radical intermediate, which is followed by the second C3-O bond cleavage. The first step is rate-limiting with a barrier of only 10.8 kcal mol -1 . Further experimental studies are encouraged to verify our results.

The iridium-catalyzed C(sp 3 )-H borylation of methylchlorosilanes is investigated by means of density functional theory, using the B3LYP and M06 functionals. The calculations establish that the resting state of the catalyst is a seven-coordinate Ir(v) species that has to be converted into an Ir(iii)tris(boryl) complex in order to effect the oxidative addition of the C-H bond. This is then followed by a C-B reductive elimination to yield the borylated product, and the catalytic cycle is finally completed by the regeneration of the active catalyst over two facile steps. The two employed functionals give somewhat different conclusions concerning the nature of the rate-determining step, and whether reductive elimination occurs directly or after a prior isomerization of the Ir(v) hydride intermediate complex. The calculations reproduce quite well the experimentally-observed trends in the reactivities of substrates with different substituents. It is demonstrated that the reactivity can be correlated to the Ir-C bond dissociation energies of the corresponding Ir(v) hydride intermediates. The effect of the chlorosilyl group is identified to originate from the α-carbanion-stabilizing effect of the silicon, which is further reinforced by the presence of an electron-withdrawing chlorine substituent. Furthermore, the source of selectivity for the borylation of primary over secondary C(sp 3 )-H can be explained on a steric basis, by repulsion between the alkyl group and the Ir/ligand moiety. Finally, the difference in the reactivity between C(sp 3 )-H and C(sp 2 )-H borylation is investigated and rationalized in terms of distortion/interaction analysis.

Herein is described the preparation of a dinuclear molecular Ru catalyst for H2O oxidation. The prepared catalyst mediates the photochemical oxidation of H2O with an efficiency comparable to state-of-the-art catalysts.

During recent years significant progress has been made towards the realization of a sustainable and carbon-neutral energy economy. One promising approach is photochemical splitting of H2O into O2 and solar fuels, such as H2. However, the bottleneck in such artificial photosynthetic schemes is the H2O oxidation half reaction where more efficient catalysts are required that lower the kinetic barrier for this process. In particular catalysts based on earth-abundant metals are highly attractive compared to catalysts comprised of noble metals. We have now synthesized a library of dinuclear Mn2(II,III) catalysts for H2O oxidation and studied how the incorporation of different substituents affected the electronics and catalytic efficiency. It was found that the incorporation of a distal carboxyl group into the ligand scaffold resulted in a catalyst with increased catalytic activity, most likely because of the fact that the distal group is able to promote proton-coupled electron transfer (PCET) from the high-valent Mn species, thus facilitating O-O bond formation.

We present a systematic theoretical investigation on the overall ground state and excited-state isomerization reaction mechanism of ortho-green fluorescent protein chromophore (o-HBDI) using the density functional theory and the multireference methods. The calculated results and subsequent analysis suggest the possible isomerization mechanism for o-HBDI. By comparison with experimental observation and detailed analysis, it is concluded that as initiated by the excited-state intramolecular proton transfer reaction, the conical intersection between the ground state and the excited state along the C4-C5 single-bond rotational coordinate is responsible for the rapid deactivation of o-HBDI.

We describe theoretical insights into the mechanism of Hg-C bond protonolysis in methyl mercury coordinated by the tris(2-mercapto-1-tert-butylimidazolyl)hydroborato ligand, the structural and functional analogue of the organomercurial lyase MerB. Different cleavage pathways including both frontside and backside attack transition states were systematically studied by the hybrid density functional method B3LYP. Dependence of Hg-C bond activation on the primary sulfur coordination number of mercury was elaborated, and conceptual DFT indexes were suggested to be more appropriate than gross charge of atom sites in interpreting the dependence. Furthermore, absence of configurational inversion in MerB-catalyzed reactions was accounted for by examinations of the backside protonolysis pathways in the present system. Lastly, a rationalization was provided about the choice between different characteristics of transition states including both four-center and six-center ones.