Mechanistic Investigation of Dirhodium-Catalyzed Intramolecular Allylic C–H Amination versus Alkene Aziridination

The reaction mechanisms and chemoselectivity on the intramolecular allylic C–H amination versus alkene aziridination of 4-pentenylsulfamate promoted by four elaborately selected dirhodium paddlewheel complexes are investigated by a DFT approach. A predominant singlet concerted, highly asynchronous pathway and an alternative triplet stepwise pathway are obtained in either C–H amination or alkene aziridination reactions when mediated by weak electron-donating catalysts. A singlet stepwise C–H amination pathway is obtained under strongly donating catalysts. The rate-determining step in the C–H amination is the H-abstraction process. The subsequent diradical-rebound C–N formation in the triplet pathway or the combination of the allylic carbocation and the negative changed N center in the singlet pathway require an identical energy barrier. A mixed singlet–triplet pathway is preferred in either the C–H insertion or alkene aziridination in the Rh₂(NCH₃CHO)₄ entry that the triplet pathway is initially favorable in the rate-determining steps, and the resultant triplet intermediates would convert to a singlet reaction coordinate. The nature of C–H amination or alkene aziridination is estimated to be a stepwise process. The theoretical observations presented in the paper are consistent with the experimental results and, more importantly, provide a thorough understanding of the nature of the reaction mechanisms and the minimum-energy crossing points

Probing the Electric Field Effect on the Catalytic Performance of Mn-Doped Graphene to CO Oxidation

The electric field is an effective route to tune the performance of catalysts. Here, we have probed the reaction mechanism of CO oxidation on Mn-doped graphene (Mn-Gr), the effect as a result of the externally applied electric field, and provided a theoretical understanding of the rule by density functional theoy (DFT) calculation. On the basis of DFT calculations, we suggest that electric field has significant impact on the catalytic performance of CO oxidation on Mn-Gr. The reaction barriers for CO₂ formation decrease with increasing CO/O₂ adsorption on Mn-Gr as the electric field decreases from +0.5 to −0.75 V/Å, leading to a greater activation of the O–O bond and then accelerate the CO₂ formation. However, strong binding between CO₂ and Mn-Gr under a larger positive or negative electric field would result in CO₂ desorption difficult and hinder the catalyst regeneration. Therefore, it is proposed that −0.50 F/Å is more appropriate for CO oxidation on Mn-Gr with a lower determined reaction barrier of 0.55 eV when considering the CO₂ formation and desorption, in which the adsorption energy is neither too strong nor too weak. These findings highlight the possibility to manipulate the catalytic performance of the doped graphene to CO oxidation with the electric field controlled which would be helpful for future design and implementation of high performance catalysts

DFT Study of Acceptorless Alcohol Dehydrogenation Mediated by Ruthenium Pincer Complexes: Ligand Tautomerization Governing Metal Ligand Cooperation

Metal ligand cooperation (MLC) catalysis is a popular strategy to design highly efficient transition metal catalysts. In this presented theoretical study, we describe the key governing factor in the MLC mechanism, with the Szymczak’s NNN-Ru and the Milstein’s PNN-Ru complexes as two representative catalysts. Both the outer-sphere and inner-sphere mechanisms were investigated and compared. Our calculated result indicates that the PNN-Ru pincer catalyst will be restored to aromatic state during the catalytic cycle, which can be considered as the driving force to promote the MLC process. On the contrary, for the NNN-Ru catalyst, the MLC mechanism leads to an unfavored tautomerization in the pincer ligand, which explains the failure of the MLC mechanism in this system. Therefore, the strength of the driving force provided by the pincer ligand actually represents a prerequisite factor for MLC. Spectator ligands such as CO, PPh₃, and hydride are important to ensure the catalyst follow a certain mechanism as well. We also evaluate the driving force of various bifunctional ligands by computational methods. Some proposed pincer ligands may have the potential to be the new pincer catalysts candidates. The presented study is expected to offer new insights for MLC catalysis and provide useful guideline for future catalyst design

Mechanistic Insights Into the Factors That Influence the DNA Nuclease Activity of Mononuclear Facial Copper Complexes Containing Hetero-Substituted Cyclens

The factors that influence the DNA nuclease activity of mononuclear facial copper complexes containing heterosubstituted cyclens were systematically investigated in this work using density functional theory (DFT) calculations. The heterosubstitution of cyclens were found to significantly affect the dimerization tendency of the mononuclear Cu­(II) complexes examined and their respective p<i>K</i>_a values of the metal-bonded water molecules. The Cu­(II)–oxacyclen complex was found to be more favorable for the hydrolytic cleavage of the DNA dinucleotide analogue BNPP^–(bis (<i>p</i>-nitrophenyl) phosphate). This was due to this species having a higher dimerization resistance to give rise to a higher concentration of the active catalyst and a lower p<i>K</i>_a value of the Cu­(II)-coordinated water molecule to facilitate an easier generation of the better nucleophile hydroxyl ion, which gave a lower reaction barrier. The dimerization of the Cu­(I) complexes studied and their corresponding redox potentials were determined, and a remarkable reaction barrier was observed for the generation of a superoxide ROS (reactive oxygen species) mediated by the Cu­(I)–oxacyclen complex. This behavior was attributed to the higher electronegativity of the O heteroatom, which facilitates the nucleophilic attack of the oxygen molecule and the Cu–O­(OH₂) bond fission via an enhancement of the Lewis acidity of the metal center and the formation of a significant hydrogen bond between the heterocyclic oxygen and the metal-bonded water molecule. The theoretical results reported here are in good agreement with the literature experimental observations and more importantly help to systematically elucidate the factors that influence the DNA nuclease activity of mononuclear facial copper complexes containing heterosubstituted cyclens in detail

When Bifunctional Catalyst Encounters Dual MLC Modes: DFT Study on the Mechanistic Preference in Ru-PNNH Pincer Complex Catalyzed Dehydrogenative Coupling Reaction

Metal ligand cooperation (MLC) plays an important role in the development of homogeneous catalysts. Two major MLC modes have generally been proposed, known as the M-L bond mode and the (de)­aromatization mode. To reveal the role of the dual potential functional sites on the MLC process, we present a detailed mechanistic study on a novel-designed Ru-PNNH complex possessing dual potential MLC functional sites for the M-L bond mode and the (de)­aromatization mode, respectively. Our results indicate that the Ru-PNNH complex prefers the M-L mode exclusively during different stages of the catalytic cycle. The unusual double deprotonation process and the mechanistic preference are rationalized. The N-arm deprotonation is attributed to the small steric hindrance of the amido N-arm and the conjugation stabilization effect of the amido group. The origin of the unexpected exclusive mechanistic preference on the M-L bond mechanism is due to the conjugation effect of the amido group, which stabilizes the dearomatized complex and diminishes the driving force of the (de)­aromatization mode. This study highlights the pivotal role of the ligand’s electronic effect on the MLC mechanism and should provide valuable information for the development of highly efficient bifunctional catalysts

The Effect of HSAB on Stereoselectivity: Copper- and Gold-Catalyzed 1,3-Phosphatyloxy and 1,3-Halogen Migration Relay to 1,3-Dienes

The origin of stereodivergence between copper- and gold-catalyzed cascade 1,3-phosphatyloxy and 1,3-halogen migration from α-halo-propargylic phosphates to 1,3-dienes is rationalized with density functional theory (DFT) studies. Our studies reveal the significant role of the relative hardness/softness of the metal centers in determining the reaction mechanism and the stereoselectivity. The relative harder Cu­(I/III) center prefers an associative pathway with the aid of a phosphate group, leading to the (<i>Z</i>)-1,3-dienes. In contrast, the relative softer Au­(I/III) center tends to undergo a dissociative pathway without coordination to a phosphate group, resulting in the (<i>E</i>)-1,3-dienes, where the <i>E</i> type of transition state is favored due to the steric effect. Our findings indicate the intriguing role of hard–soft/acid–base (HSAB) theory in tuning the stereoselectivity of metal-catalyzed transformations with functionalized substrates

Key Mechanistic Features of Ni-Catalyzed C–H/C–O Biaryl Coupling of Azoles and Naphthalen-2-yl Pivalates

The mechanism of the Ni-dcype-catalyzed C–H/C–O coupling of benzoxazole and naphthalen-2-yl pivalate was studied. Special attention was devoted to the base effect in the C–O oxidative addition and C–H activation steps as well as the C–H substrate effect in the C–H activation step. No base effect in the C­(aryl)–O oxidative addition to Ni-dcype was found, but the nature of the base and C–H substrate plays a crucial role in the following C–H activation. In the absence of base, the azole C–H activation initiated by the C–O oxidative addition product Ni­(dcype)­(Naph)­(PivO), <b>1B</b>, proceeds via Δ<i>G</i> = 34.7 kcal/mol barrier. Addition of Cs₂CO₃ base to the reaction mixture forms the Ni­(dcype)­(Naph)­[PivOCs·CsCO₃], <b>3_Cs_clus</b>, cluster complex rather than undergoing PivO^– → CsCO₃^– ligand exchange. Coordination of azole to the resulting <b>3_Cs_clus</b> complex forms intermediate with a weak Cs–heteroatom­(azole) bond, the existence of which increases acidity of the activated C–H bond and reduces C–H activation barrier. This conclusion from computation is consistent with experiments showing that the addition of Cs₂CO₃ to the reaction mixture of <b>1B</b> and benzoxazole increases yield of C–H/C–O coupling from 32% to 67% and makes the reaction faster by 3-fold. This emerging mechanistic knowledge was validated by further exploring base and C–H substrate effects via replacing Cs₂CO₃ with K₂CO₃ and benzoxazole (<b>1a</b>) with 1<i>H</i>-benzo­[<i>d</i>]­imidazole (<b>1b</b>) or quinazoline (<b>1c</b>). We proposed the modified catalytic cycle for the Ni­(cod)­(dcype)-catalyzed C–H/C–O coupling of benzoxazole and naphthalen-2-yl pivalate

Two-Dimensional Charge-Separated Metal–Organic Framework for Hysteretic and Modulated Sorption

A charge-separated metal–organic framework (MOF) has been successfully synthesized from an imidazolium tricarboxylate ligand, <i>N</i>-(3,5-dicarboxylphenyl)-<i>N</i>′-(4-carboxylbenzyl)­imidazolium chloride (DCPCBImH₃Cl), and a zinc­(II) dimeric secondary building unit, namely, <b>DCPCBim-MOF-Zn</b>, which shows an unprecedented 3,6-connected two-dimensional net topology with the point (Schläfli) symbol (4².6)₂(4⁴.6⁹.8²). The framework contains one-dimensional highly polar channels, and density functional theory calculations show that positive charges are located on the imidazolium/phenyl rings and negative charges on the carboxylate moieties. The charge-separated nature of the pore surface has a profound effect in their adsorption behavior, resulting in remarkable hysteretic sorption of various gases and vapors. For CO₂, the hysteretic sorption was observed to occur even up to 298 K. Additionally, trace chloride anions present in the pore channels are able to modulate the gas-sorption behavior

Mechanistic Implications in the Phosphatase Activity of Mannich-Based Dinuclear Zinc Complexes with Theoretical Modeling

An “end-off” compartmental ligand has been synthesized by an abnormal Mannich reaction, namely, 2-[bis­(2-methoxyethyl)­aminomethyl]-4-isopropylphenol yielding three centrosymmetric binuclear μ-phenoxozinc­(II) complexes having the molecular formula [Zn₂(L)₂X₂] (<b>Zn-1</b>, <b>Zn-2</b>, and <b>Zn-3</b>), where X = Cl^–, Br ^–, and I ^–, respectively. X-ray crystallographic analysis shows that the ZnO₃NX chromophores in each molecule form a slightly distorted trigonal-bipyramidal geometry (τ = 0.55–0.68) with an intermetallic distance of 3.068, 3.101, and 3.083 Å (<b>1</b>–<b>3</b>, respectively). The spectrophotometrical investigation on their phosphatase activity established that all three of them possess significant hydrolytic efficiency. Michaelis–Menten-derived kinetic parameters indicate that the competitiveness of the rate of P–O bond fission employing the phosphomonoester (4-nitrophenyl)­phosphate in 97.5% <i>N</i>,<i>N</i>-dimethylformamide is <b>3</b> > <b>1</b> > <b>2</b> and the <i>k</i>_cat value lies in the range 9.47–11.62 s^–1 at 298 K. Theoretical calculations involving three major active catalyst forms, such as the dimer-cis form (D-Cis), the dimer-trans form (D-Trans), and the monoform (M-1 and M-2), systematically interpret the reaction mechanism wherein the dimer-cis form with the binuclear-bridged hydroxide ion acting as the nucleophile and one water molecule playing a role in stabilizing the leaving group competes as the most favored pathway

Hydrogenation of Carbon Dioxide Using Half-Sandwich Cobalt, Rhodium, and Iridium Complexes: DFT Study on the Mechanism and Metal Effect

The hydrogenation of carbon dioxide catalyzed by half-sandwich transition metal complexes (M = Co, Rh, and Ir) was studied systematically through density functional theory calculations. All metal complexes are found to process a similar mechanism, which involves two main steps, the heterolytic cleavage of H₂ and the hydride transfer. The heterolytic cleavage of H₂ is the rate-determining step. The comparison of three catalytic systems suggests that the Ir catalyst has the lowest activation free energy (13.4 kcal/mol). In contrast, Rh (14.2 kcal/mol) and Co (18.3 kcal/mol) catalysts have to overcome relatively higher free energy barriers. The different catalytic efficiency of Co, Rh, and Ir is attributed to the back-donation ability of different metal centers, which significantly affects the H₂ heterolytic cleavage. The highest activity of an iridium catalyst is attributed to its strong back-donation ability, which is described quantitatively by the second order perturbation theory analysis. Our study indicates that the functional group of the catalyst plays versatile roles on the catalytic cycle to facilitate the reaction. It acts as a base (deprotonated) to assist the heterolytic cleavage of H₂. On the other hand, during the hydride transfer, it can also serve as Brønsted acid (protonated) to lower the LUMO of CO₂. This ligand assisted pathway is more favorable than the direct attack of hydride to CO₂. These finds highlight that the unique features of the metal center and the functional ligands are crucial for the catalyst design in the hydrogenation of carbon dioxide