Transmetalation Process Research Articles

Defect strategies in Mn-doped metal–organic frameworks to enhance adsorption-based NOx gas sensing performance

Post-synthetic metalation (PSM) represents a powerful toolkit for modifying the chemical composition of pre-formed metal-organic frameworks (MOFs). However, accessing defective MOFs with exchanged metal nodes via this approach remains challenging. Herein, we report a transmetalation strategy to induce desired metal exchange frustrated flexibility defects in a prototypical MOF platform. Specifically, the pristine Zn-based MOF undergoes selective Zn-to-Mn exchange through a carefully controlled solvothermal transmetalation process. This protocol generates a series of Mn-doped MOFs with increasing Mn integration and the concomitant formation of defect sites within the framework. Detailed structural characterization confirms the successful Mn doping and illuminates the generation of vacancies and distortions around the exchanged Mn nodes. The transmetallated Mn-doped MOF materials exhibit altered chemical environments, as probed by spectroscopic techniques, and display promising sensing activity for NOx through the MOF matrix membrane. The proposed application of the MOF samples as an adsorbent-based chemical sensor includes sensing mechanisms for NO2 gas.

Constructing Pd and Cu Crowding Single Atoms by Protein Confinement to Promote Sonogashira Reaction.

For multicenter-catalyzed reactions, it is important to accurately construct heterogeneous catalysts containing multiple active centers with high activity and low cost, whichis more challenging compared to homogeneous catalysts because of the low activity and spatial confinement of active centers in the loaded state. Herein, a convenient protein confinement strategy is reported to locate Pd and Cu single atoms in crowding state on carbon coated alumina for promoting Sonogashira reaction, the most powerful method for constructing the acetylenic moiety in molecules. The single-atomic Pd and Cu centers take advantage in not only the maximized atomic utilization for low cost, but also the much-enhanced performance by facilitating the activation of aryl halides and alkynes. Their locally crowded dispersion brings them closer to each other, which facilitates the transmetallation process of acetylide intermediates between them. Thus, the Sonogashira reaction is drove smoothly by the obtained catalyst with a turnover frequency value of 313 h-1, much more efficiently than that by commercial Pd/C and CuI catalyst, conventional Pd and Cu nanocatalysts, and mixed Pd and Cu single-atom catalyst. The obtained catalyst also exhibits the outstanding durability in the recycling test.

DFT mechanistic study on the synthesis of 2-fluorinated allylic scaffolds via PdII-catalyzed defluorinative arylation of gem-difluorocyclopropanes with phenylboronic acids

DFT calculations were conducted to understand the mechanism details and the Z/E-, regio-, and chemoselectivities observed in the synthesis of 2-fluorinated allylic scaffolds through PdII-catalyzed defluorinative arylation of gem-difluorocyclopropanes with arylboronic acids. The reaction is determined to proceed sequentially through oxidative addition, β-F elimination, two consecutive transmetalations, and reductive elimination. The β-F elimination dictates the Z/E-selectivity, primarily influenced by steric effects, while the reductive elimination step controls the regioselectivities, driven by electronic effects. The second transmetalation process serves as the bottleneck process with a free energy barrier of 26.5 kcal/mol. The chemoselective preference for gem-difluorocyclopropane over its corresponding chloride and its unsubstituted parent is attributed to the stronger electron-withdrawing capability of the F substituent compared to Cl and H, which polarizes the C1-C3 bond more than chlorine or hydrogen, making it easier to break.

Exploring the energetics of Suzuki cross-coupling reaction: A computational study of palladium and nickel catalysts

The present study employed density-functional theory (DFT) calculations to investigate the Suzuki-Miyaura Cross-Coupling reaction using single atom catalysis (SAC) via palladium or nickel. The reaction barriers for chloride bond cleavage in phenyl chloride were found to be 34.68 kJ/mol and 24.24 kJ/mol for palladium and nickel catalysts, respectively. During the transmetalation process, the energy barrier for boronic acid release from phenyl trihydroxyboronate using SAC via palladium and SAC via nickel was calculated to be 74.17 kJ/mol and 65.22 kJ/mol, respectively. The results indicated that in the oxidative addition and transmetalation reactions, the SAC via nickel outperforms the SAC via palladium, while the SAC via palladium excels in the reductive elimination reaction. These findings suggest the effectiveness of a SAC via nickel as a substitute for SAC via palladium in the synthesis of biphenyls in the Suzuki-Miyaura cross-coupling reaction.

General room-temperature Suzuki-Miyaura polymerization for organic electronics.

π-Conjugated polymers (CPs) have broad applications in high-performance optoelectronics, energy storage, sensors and biomedicine. However, developing green and efficient methods to precisely synthesize alternating CP structures on a large scale remains challenging and critical for their industrialization. Here a room-temperature, scalable and homogeneous Suzuki-Miyaura-type polymerization reaction is developed with broad generality validated for 24 CPs including donor-donor, donor-acceptor and acceptor-acceptor connectivities, yielding device-quality polymers with high molecular masses. Furthermore, the polymerization protocol significantly reduces homocoupling structural defects, yielding more structurally regular and higher-performance electronic materials and optoelectronic devices than conventional thermally activated polymerizations. Experimental and theoretical studies reveal that a borate transmetalation process plays a key role in suppressing protodeboronation, which is critical for large-scale structural regularity. Thus, these results provide a general polymerization tool for the scalable production of device-quality CPs with alternating structural regularity.

Stabilization versus competing de-metalation, trans-metalation and (cyclo)-dehydrogenation of Pd porphyrins at a copper surface.

Metal-porphyrins are studied intensively due their potential applications, deriving from the variety of electronic and chemical properties, tunable by selecting metal centers and functional groups. Metalation, de- and trans-metalation processes are fundamental in this sense to investigate both the synthesis and the stability of these molecular building blocks. More specifically, Pd coordination in tetrapyrroles revealed to be potentially interesting in the fields of cancer therapy, drug delivery and light harvesting. Thus, we focused on the stability of palladium tetraphenyl porphyrins (PdTPPs) on a copper surface by means of combined spectroscopy and microscopy approaches. We find that PdTPPs undergo coverage-dependent trans-metalation accompanied by steric rearrangements already at room temperature, and fully trans-metalate to CuTPPs upon mild annealing. Side reactions such as (cyclo)-dehydrogenation and structural reorganization affect the molecular layer, with Pd-Cu alloying and segregation occurring at higher temperature. Instead, oxygen passivation of the Cu support prevents the metal-involving reactions, thus preserving the layer and increasing the chemical and temperature stability of the Pd porphyrins.

SYNTHESIS, CHARACTERIZATION OF NEW ASYMMETRICALLY IMIDAZOLIUM SALTS WITH THEIR Ag(I) AND Pd(II)-N-HETEROCYCLIC CARBENE COMPLEXES

New substituted asymmetrically imidazolium salts with Ag(I) and Pd(II) N-heterocyclic carbene complexes were synthesized and characterized. In order to produce asymmetrically substituted salts, imidazolium salts have been produced using a variety of substituents, including aliphatic chain and phenyl acetamide chloride (5,6,7). Using the carbene site deprotonation process, silver(I)-NHC complexes (8,9,10) were produced from the reaction of asymmetrically substituted imidazolium salts with Ag2O. The appropriate Pd(II)-NHC (11,12,13) were then synthesized using the transmetallation process using Ag(I)-complexes. NHC as transfer reagents. The complexes were identified using spectroscopy methods such as 1H-, 13C-NMR and FT-IR spectroscopy. The recorded spectra of the prepared complexes and their salts agree with theoretical ones.

Mechanistic study of the Ni-catalyzed hydroalkylation of 1,3-dienes: The origins of regio- and enantioselectivities and a further rational design.

The development of the catalytic regio- and enantioselective hydrofunctionalization of 1,3-dienes remains a challenge and requires deep insight into the reaction mechanisms. We herein thoroughly studied the reaction mechanism of the Ni-catalyzed hydroalkylation of 1,3-dienes with ketones by density functional theory (DFT) calculations. It reveals that the reaction is initiated by stepwise oxidative addition of EtO-H followed by 1,3-diene migratory insertion to generate the alkylnickel(II) intermediate, rather than the experimentally proposed ligand-to-ligand hydrogen transfer (LLHT) mechanism. In addition, we rationalized the role of t BuOK in the subsequent addition of enolate of ketone and transmetalation process. Based on the whole catalysis, the CC reductive elimination step, turns out to be the rate- and enantioselectivity-determining step. Furthermore, we disclosed the origins of the regio- and enantioselectivity of the product, and found that the 1,2-selectivity lies in the combination effects of the ligand-substrate electrostatic interactions, orbital interactions and Pauli repulsions, while the enantioselectivity mainly arises from substrate-ligand steric repulsions. Based on mechanistic study, new biaryl bisphosphine ligands affording higher enantioselectivity were designed, which will help to improve current catalytic systems and develop new transition-metal-catalyzed hydroalkylations.

Photoredox Nickel‐Catalysed Stille Cross‐Coupling Reactions

AbstractThe Stille cross‐coupling reaction is one of the most common strategies for the construction of C−C bonds. Despite notable strides in the advancement of the Stille reaction, persistent challenges persist in hindering its greener evolution. These challenges encompass multiple facets, such as the high cost of precious metals and ligands, the demand for various additives, and the slow reaction rate. In comparison to the dominant palladium‐catalysed Stille reactions, cost‐effective nickel‐catalysed systems lag behind, and enantioconvergent Stille reactions of racemic stannanes remain undeveloped. Herein, we present a pioneering instance of nickel‐catalysed enantioconvergent Stille cross‐coupling reactions of racemic stannane reagents, resulting in the formation of C−C bonds in good to high yields with excellent stereoselectivity. This strategy provides a practical, scalable, and operationally straightforward method for the synthesis of C(sp3)−C(sp3), C(sp3)−C(sp2), and C(sp3)−C(sp) bonds under exceptionally mild conditions (without additives and bases, ambient temperature). The innovative use of synergistic photoredox/nickel catalysis enables a novel single‐electron transmetalation process of stannane reagents, providing a new research paradigm of Stille reactions.

Photoredox Nickel-Catalysed Stille Cross-Coupling Reactions.

The Stille cross-coupling reaction is one of the most common strategies for the construction of C-C bonds. Despite notable strides in the advancement of the Stille reaction, persistent challenges persist in hindering its greener evolution. These challenges encompass multiple facets, such as the high cost of precious metals and ligands, the demand for various additives, and the slow reaction rate. In comparison to the dominant palladium-catalysed Stille reactions, cost-effective nickel-catalysed systems lag behind, and enantioconvergent Stille reactions of racemic stannanes remain undeveloped. Herein, we present a pioneering instance of nickel-catalysed enantioconvergent Stille cross-coupling reactions of racemic stannane reagents, resulting in the formation of C-C bonds in good to high yields with excellent stereoselectivity. This strategy provides a practical, scalable, and operationally straightforward method for the synthesis of C(sp3 )-C(sp3 ), C(sp3 )-C(sp2 ), and C(sp3 )-C(sp) bonds under exceptionally mild conditions (without additives and bases, ambient temperature). The innovative use of synergistic photoredox/nickel catalysis enables a novel single-electron transmetalation process of stannane reagents, providing a new research paradigm of Stille reactions.

The synthesis and characterization of a bidentate complex of 1,1′-(1,3-phenylene)bis(3-butylimidazolium) pincer proligand with molybdenum – A non-pincer binding mode

The synthesis of the bidentate Mo complex tetrachlorido[6-(3′-butylimidazolium-1′-yl)-2-(3″-butylimidazol-1″-yl-2″-idene-κC2)phenyl-κC1]molybdenum(IV) 3 was carried out using the metalation followed by transmetalation methodology. The transmetalation process led to a bidentate complex after the reaction of DCM with HNMe2 formed an acidic ammonium ion that protonated the bidentate complex. Optimization of the synthetic methodology provided the tetrachlorido[6-(3′-butylimidazolium-1′-yl)-2-(3″-butylimidazol-1″-yl-2″-idene-κC2)phenyl-κC1]molybdenum(IV) complex in high yield. The crystal structure of the bidentate complex, 3, is reported herein. Attempts to avoid the acidic reaction conditions with different solvents or starting materials produced the bis-ligated Mo complex bis[2,6-bis(3′-butylimidazol-1′-yl-2′-idene-κC2)phenyl-κC1]molybdenum(IV) dichloride based on MS analysis. Electronic and coordinative unsaturation in the resulting bidentate complex, 3, open new possibilities for coordination of incoming substrates while also allowing access to the pincer-like motif via oxidative addition. Access to the interconversion of a NHC to/from imidazolium opens new avenues of non-innocent ligand pathways for proton shuttling in reactions such as nitrogen reduction to ammonia.

Spontaneous transmetalation at the ZnPc/Al(100) interface

Metal-phthalocyanines (MPcs) are a class of organic macrocycles that have received much attention for their potential use in various applications, including molecular electronics. In recent years, the on-surface synthesis of different MPcs has been demonstrated via deposition of atomic species, sample heating and scanning tunneling microscopy (STM) manipulation. However, the spontaneous substitution of the central metal atom with a substrate one, known as transmetalation, remains a less explored process. Here, we study the transmetalation of ZnPc on Al(100) using a combination of STM and density functional theory (DFT) computations. Previously published X-ray photoelectron spectroscopy (XPS) data indicate the absence of Zn atoms from the interface, while our STM images show that a central atom is still present. This suggests that the transmetalation process occurs spontaneously for all ZnPc molecules. Our DFT calculations reveal that the transmetalation of Zn with Al releases 2 eV per molecule, providing thermodynamic evidence for the process. The on-surface synthesized aluminium phthalocyanine (AlPc) molecules are stabilised by the formation of a bond between the central metal atom and the substrate, similar to that found in chloroaluminium phthalocyanine (ClAlPc), with the consequent suppresion of the paramagnetism of free AlPc.

Substrate Plasticity Enables Group-Selective Transmetalation: Catalytic Stereospecific Cross-Couplings of Tertiary Boronic Esters.

Activation of enantiomerically enriched tertiary alkylboronic esters with adamantyllithium generated in situ enables stereoretentive boron-to-copper transmetalation. The resulting alkylcopper species can undergo cross-coupling reactions with an array of electrophiles to furnish synthetically useful compounds bearing quaternary stereocenters. DFT calculations of the transmetalation process provide insights for reactivity and selectivity.

Mechanistic Insights into Oxazolone Synthesis by Bimetallic Au-Pd-Catalyzed Catalysis and Catalyst Design: DFT Investigations.

Bimetallic synergistic catalysis is one of the most effective and powerful strategies for the synthesis of oxazolones, an important species in organic synthesis. In this work, the mechanism of AuCl(PMe3)/AgOTf-Pd(0) ([Au-Pd]) bimetallic catalyst-catalyzed oxazolone synthesis using N-alkynyl carbamates as precursors was studied in detail by DFT calculations and the catalytic performances of a series of bimetallic catalysts were evaluated. The results show that the reaction begins from the [Au]-catalyzed cycloisomerization of N-alkynyl carbamates. After the five-membered intermediate is formed, the [Pd(0)]-catalyzed cycle starts, which contains three steps: oxidation addition, transmetalation, and reductive elimination. The whole reaction belongs to a catalyzed catalysis, and the reductive elimination is the rate-determining step. In the transmetalation process, both the [Pd(0)] catalyst and the ionic bridge are necessary. For the [Au-Pd]-catalyzed process, it is Cl- as the bridge, not OTf-. The cheaper metal compound, AgCl(PMe3), can serve as the alternative of AuCl(PMe3) to co-catalyze with the [Pd(0)] catalyst for the title reaction.

Enhanced Gas Adsorption on Cu3(BTC)2 Metal-Organic Framework by Post-Synthetic Cation Exchange and Computational Analysis.

Increased gas adsorption in a series of post-synthetically modified metal-organic frameworks (MOFs) of the type HKUST-1 was achieved by the partial cation exchange process. Manipulation of post-synthetic conditions demonstrates high tunability in the site substitution and gas adsorption properties during the dynamic equilibrium process. In this work, post-synthetic modification of Cu3(BTC)2 is carried on by exposure to TM2+ solutions (TM = Mn, Fe, Co, Ni) at different time intervals. The crystal structure, composition, and morphology were studied by powder X-ray diffraction, Fourier-transform infrared spectroscopy, inductively coupled plasma optical emission spectroscopy, and scanning electron microscopy. Structural analysis supports the retention of the crystal structure and partial substitution of the Cu metal nodes within the framework. A linear increase in the transmetalation process is observed with Fe and Co with a maximum percentage of 39 and 18%, respectively. Conversely, relatively low cation exchange is observed with Mn having a maximum percentage of 2.40% and Ni with only 2.02%. Gas adsorption measurements and surface area analysis were determined for each species. Interestingly, (Cu/Mn)3(BTC)2 revealed the highest CO2 adsorption capacity of 5.47 mmol/g, compared to 3.08 mmol/g for Cu3(BTC)2. The overall increased gas adsorption can be attributed to the formation of defects in the crystal structure during the cation exchange process. These results demonstrate the outstanding potential of post-synthetic ion exchange as a general approach to fine-tuning the physical properties of existing MOF architectures.

A Zinc(II) Schiff Base Complex as Fluorescent Chemosensor for the Selective and Sensitive Detection of Copper(II) in Aqueous Solution.

The development of chemosensors able to detect analytes in a variety of sample matrices through a low-cost, fast, and direct approach is of current interest in food, health, industrial, and environmental fields. This contribution presents a simple approach for the selective and sensitive detection of Cu2+ ions in aqueous solution based on a transmetalation process of a fluorescent substituted Zn(salmal) complex. Transmetalation is accompanied by relevant optical absorption changes and quenching of the fluorescence emission, leading to high selectivity and sensitivity of the chemosensor, with the advantage of not requiring any sample pretreatment or pH adjustment. Competitive experiments demonstrate a high selectivity of the chemosensor towards Cu2+ with respect to the most common metal cations as potential interferents. A limit of detection down to 0.20 μM and a dynamic linear range up to 40 μM are achieved from fluorometric data. By exploiting the fluorescence quenching upon formation of the copper(II) complex, simple paper-based sensor strips, visible to naked eyes under UV light, are used for the rapid, qualitative, and quantitative in situ detection of Cu2+ ions in aqueous solution over a wide concentration range, up to 10.0 mM, in specific environments, such as in industrial wastewater, where higher concentrations of Cu2+ ions can occur.

Building Square Planar Cobalt (II) Complexes via Sodium Mediated Cobaltation of Fluoroarenes

AbstractWhile cobalt complexes have already shown their potential for C−H and C−F bond activation of fluoroarenes, their reactivity as metalating agents via Co−H exchange towards these substrates has not been explored. Herein, we report a Co(HMDS)2 [HMDS=N(SiMe3)2] system which, when synergistically enhanced via sodium amide Na(HMDS) mediation, can render chemo‐ and regioselective cobaltation of a series of fluoroarenes to produce a new class of homoleptic square planar [Na2CoAr4] complexes. Density functional theory calculations elucidate the key roles of the Na/Co counterparts in a stepwise sodiation/cobalt transmetalation process, leading to this novel C−H metalation. Depending on the reaction stoichiometry, this process can occur inter‐ or intramolecularly, furnishing transient [NaCo(HMDS)2Ar] intermediates which can undergo ligand rearrangement to afford [Na2CoAr4] with concomitant formation of Co(HMDS)2 and [NaCo(HMDS)3].

Building Square Planar Cobalt (II) Complexes via Sodium Mediated Cobaltation of Fluoroarenes.

While cobalt complexes have already shown their potential for C-H and C-F bond activation of fluoroarenes, their reactivity as metalating agents via Co-H exchange towards these substrates has not been explored. Herein, we report a Co(HMDS)2 [HMDS=N(SiMe3 )2 ] system which, when synergistically enhanced via sodium amide Na(HMDS) mediation, can render chemo- and regioselective cobaltation of a series of fluoroarenes to produce a new class of homoleptic square planar [Na2 CoAr4 ] complexes. Density functional theory calculations elucidate the key roles of the Na/Co counterparts in a stepwise sodiation/cobalt transmetalation process, leading to this novel C-H metalation. Depending on the reaction stoichiometry, this process can occur inter- or intramolecularly, furnishing transient [NaCo(HMDS)2 Ar] intermediates which can undergo ligand rearrangement to afford [Na2 CoAr4 ] with concomitant formation of Co(HMDS)2 and [NaCo(HMDS)3 ].

Derivatives of GdAAZTA Conjugated to Amino Acids: A Multinuclear and Multifrequency NMR Study.

The GdAAZTA (AAZTA = 6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid) complex represents a platform of great interest for the design of innovative MRI probes due to its remarkable magnetic properties, thermodynamic stability, kinetic inertness, and high chemical versatility. Here, we detail the synthesis and characterization of new derivatives functionalized with four amino acids with different molecular weights and charges: l-serine, l-cysteine, l-lysine, and l-glutamic acid. The main reason for conjugating these moieties to the ligand AAZTA is the in-depth study of the chemical properties in aqueous solution of model compounds that mimic complex structures based on polypeptide fragments used in molecular imaging applications. The analysis of the 1H NMR spectra of the corresponding Eu(III)-complexes indicates the presence of a single isomeric species in solution, and measurements of the luminescence lifetimes show that functionalization with amino acid residues maintains the hydration state of the parent complex unaltered (q = 2). The relaxometric properties of the Gd(III) chelates were analyzed by multinuclear and multifrequency NMR techniques to evaluate the molecular parameters that determine their performance as MRI probes. The relaxivity values of all of the novel chelates are higher than that of GdAAZTA over the entire range of applied magnetic fields because of the slower rotational dynamics. Data obtained in reconstituted human serum indicate the occurrence of weak interactions with the proteins, which result in larger relaxivity values at the typical imaging fields. Finally, all of the new complexes are characterized by excellent chemical stability in biological matrices over time, by the absence of transmetallation processes, or the formation of ternary complexes with oxyanions of biological relevance. In particular, the kinetic stability of the new complexes, measured by monitoring the release of Gd3+ in the presence of a large excess of Zn2+, is ca. two orders of magnitude higher than that of the clinical MRI contrast agent GdDTPA.

In Situ Monitoring of Transmetallation in Electric Potential-Promoted Oxidative Coupling in a Single-Molecule Junction

In Situ Monitoring of Transmetallation in Electric Potential-Promoted Oxidative Coupling in a Single-Molecule Junction