Metal Exchange Reactions Research Articles

Integrating Compositional and Structural Diversity in Heterometallic Titanium Frameworks by Metal Exchange Methods.

The increasing use of Metal-Organic Frameworks (MOFs) in separation, catalysis, or storage is linked to the targeted modification of their composition or porosity metrics. While modification of pore shape and size necessarily implies the assembly of alternative nets, compositional changes often rely on postsynthetic modification adapted to the functionalization or exchange of the organic linker or the modification of the inorganic cluster by metal exchange methods. We describe an alternative methodology that enables the integration of both types of modification, structural and compositional, in titanium MOFs by metal exchange reaction of the heterometallic cluster Ti2Ca2. A systematic analysis of this reactivity with MUV-10 is used to understand which experimental variables are crucial to enable replacement of calcium only or to integrate metal exchange with structural transformation. The isoreticular expanded framework, MUV-30, is next used to template the formation of MUV-301, a titanium framework not accessible by direct synthesis that displays the largest mesoporous cages reported to date. Given that the interest of Ti MOFs in photoredox applications often meets the limitations imposed by the challenges of titanium solution chemistry to design concrete candidates, this soft strategy based on preassembled frameworks will help integrate specific combinations of metals into high porosity architectures.

Synthesis, structure, and biological activity of sodium, zinc, and magnesium mono- and bis-1-alkoxy-4-(2′-naphthyl)-1,4-dioxo-2-buten-2-olates

The search for new biologically active compounds is a key area of chemical science. Therefore, the synthesis of compounds with a potentially bactericidal effect seems relevant. In order to extend the range of biologically active substances, the synthesis of sodium 4-(2’-naphthyl)-1,4-dioxo-1-ethoxy-2-buten-2-olate and sodium 1-butoxy-4-(2´-naphthyl)-1,4-dioxo-2-buten-2-olate was carried out by Claisen condensation of equimolar amounts of 2-acetonaphtone and dialkyloxalates in a non-polar medium in the presence of sodium as a condensing agent. Bidentate mononuclear metal chelates were obtained by the metal exchange reaction of sodium oxoenolates with Zn(II) and Mg(II) salts. The structure of the synthesized compounds was confirmed by infrared spectroscopy, (1Н, 13С) nuclear magnetic resonance spectroscopy, and mass spectrometry. The infrared spectra of solid samples of sodium oxoenolates and metal complexes contain bands of stretching vibrations of ester carbonyl groups, skeletal vibrations of aromatic rings, as well as “ether” bands due to vibrations of C–O–C bonds. The 1Н and 13С nuclear magnetic resonance spectra recorded in CDCl3 and Dimethyl sulfoxide-d6 are characterized by a complete set of all expected signals with chemical shift values that are well comparable with the reference data. In the mass spectra of the analyzed compounds, recorded in an acetonitrile solution in the electrospray mode, signals of [M+H]+ and [M+Na]+ protonated and cationized molecules are observed. The bactericidal action of the synthesized preparations was assessed using the agar well diffusion method in combination with the serial dilution method. The biological activity of sodium 4-(2’-naphthyl)-1,4-dioxo-1-ethoxy-2-buten-2-olate against Pseudomonas Aeruginosa and Staphylococcus Aureus, as well as sodium 1-butoxy-4-(2’-naphthyl)-1,4-dioxo-2-buten-2-olate against Pseudomonas Aeruginosa, was revealed. Pronounced resistance of Salmonella spp. to the studied compounds was established.

IPr*F - Highly Hindered, Fluorinated N-Heterocyclic Carbenes.

The introduction of fluorine atom has attracted considerable interest in molecular design owing to the high electronegativity and the resulting polarization of carbon-fluorine bonds. Simultaneously, sterically-hindered N-heterocyclic carbenes (NHCs) have received major interest due to high stabilization of the reactive metal centers, which has paved the way for the synthesis of stable and reactive organometallic compounds with broad applications in main group chemistry, inorganic synthesis and transition-metal-catalysis. Herein, we report the first class of sterically-hindered, fluorinated N-heterocyclic carbenes. These ligands feature variable fluorine substitution at the N-aromatic wingtip, permitting to rationally vary steric and electronic characteristics of the carbene center imparted by the fluorine atom. An efficient, one-pot synthesis of fluorinated IPr*F ligands is presented, enabling broad access of academic and industrial researchers to the fluorinated ligands. The evaluation of steric, electron-donating and π-accepting properties as well as coordination chemistry to Au(I), Rh(I) and Se is presented. Considering the unique properties of carbon-fluorine bonds, we anticipate that this novel class of fluorinated carbene ligands will find widespread application in stabilizing reactive metal centers.

Hetero and Homo Metal Exchange of Au25(SR)18 - and Ag25(SR)18 - Clusters with Metal-Thiolate Complexes: Ab Initio Molecular Dynamics Simulation Studies.

The hetero and homo metal exchange of Au25(SR)18 - and Ag25(SR)18 - nanoclusters with metal-thiolate (M-SR) complexes (AuI(SR), AgI(SR), CuI(SR), and CuII(SR)2) are studied using ab initio molecular dynamics (AIMD) simulations. The AIMD simulation results unveil that the M-SR complexes directly displace Au(SR) or Ag(SR) units on the gold or silver core surface through an "anchoring effect". The whole process of metal-exchange reactions can be divided into three steps, including the adsorption of M-SR complexes on clusters, the formation of new staple motif, and the displacement of Au(SR) or Ag(SR) units by M-SR complexes. The key role of sulfur atoms in metal exchange reactions in M-SR complexes is revealed, which facilitates formation of new staple motifs and doping of M-SR complexes into gold and silver cores. This work provides a theoretical basis for further exploring the metal exchange reaction between noble metal nanoclusters and metal-thiolate complexes, as well as the isotope exchange reactions.

Chemical implication of the partition coefficient of 137Cs between the suspended and dissolved phases in natural water

After the Fukushima Daiichi nuclear power plant accident, the terrestrial environment became severely contaminated with radiocesium. Consequently, the river and lake water in the Fukushima area exhibited high radiocesium levels, which declined subsequently. The partition coefficient of 137Cs between the suspended sediment (SS) and dissolved phases, Kd, was introduced to better understand the dynamic behavior of 137Cs in different systems. However, the Kd values in river water, ranging from 2 × 104 to 7 × 106 L kg−1, showed large spatiotemporal variability. Therefore, the factors controlling the 137Cs partition coefficient in natural water systems should be identified. Herein, we introduce a chemical model to explain the variability in 137Cs Kd in natural water systems. The chemical model includes the complexation of Cs+ with mineral and organic binding sites in SS, metal exchange reactions, and the presence of colloidal species. The application of the chemical model to natural water systems revealed that Cs+ is strongly associated with binding sites in SS, and a major chemical interaction between 137Cs and the binding sites in SS is the isotope exchange reaction between stable Cs and 137Cs, rather than metal exchange reactions with other metal ions such as potassium ions. To explain the effect of the SS concentration on Kd, the presence of colloidal 137Cs passing through a filter is significant as the dominant dissolved species of 137Cs in river water. These results suggest that a better understanding of stable Cs dissolved in natural water is important for discerning the geochemical and ecological behaviors of 137Cs in natural water.

Mechanism Insight into Metal Exchange between Au25(SR)18-/Ag25(SR)18- Clusters and Metal Ions from Ab Initio Molecular Dynamics Simulations.

The metal exchange reaction has emerged as an efficient method to synthesize ligand-protected alloy nanoclusters with precise compositions and structure. However, the understanding of the mechanism of these metal exchange processes is quite limited. Herein, the dynamic process of metal exchange of Au25(SR)18- and Ag25(SR)18- (R = CH3) nanoclusters with metal ions (Au+, Ag+, Cu2+, Cu+, Cd2+, and Hg2+) is investigated using ab initio molecular dynamics simulations. Computational results unveiled a multifaceted nature of the metal exchange process, dictated by several variables, including thermodynamic stability, electrochemical activity, metal affinity to ligand, and the coordination mode of metal ions. As a result of these factors, metal ions may either directly exchange with Au or Ag atoms on the icosahedral core surface by a "knock-off" mechanism or be stably adsorbed at the core-motif interface of Au25(SR)18- and Ag25(SR)18- nanoclusters. Meanwhile, we also discovered that counterions can promote adsorbed Ag and Cu atoms to diffuse into the gold core. Finally, the driving force of the galvanic reduction and antigalvanic reduction reactions is discussed. The formation of a more stable core-doping product nanocluster is the major driving force of metal exchange reactions.

Ligand- and metal-exchange reactions of robust nickel complexes with pentagonal bipyramidal structures.

The coordination chemistries of transition metal complexes with pentagonal bipyramidal geometries were investigated, and the highly stable nature of a cyclic pentapyridyl ligand was disclosed. A NiCl2 complex with the pentapyridyl ligand was found to be stable toward water and acidic conditions. The stable complex underwent ligand-exchange reactions with nucleophilic reagents, and a series of pentagonal bipyramidal complexes with different apical ligands was prepared. Crystallographic analyses with the aid of theoretical calculations revealed that the complexes were constructed by electrostatic threading of a divalent linear nickel complex into the ring of the neutral, cyclic pentapyridyl, which resulted in robust water- and acid-resistant complexes with unique pentagonal bipyramidal structures. A reductive metal exchange reaction was then discovered, which enabled "linear divalent metal + electrostatic ring" formulations with different metal atoms.

Wingtip-Flexible N-Heterocyclic Carbenes: Unsymmetrical Connection between IMes and IPr.

IMes (IMes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and IPr (IPr=1,3- bis(2,6-diisopropylphenyl)imidazol-2-ylidene) represent by far the most frequently used N-heterocyclic carbene ligands in homogeneous catalysis, however, despite numerous advantages, these ligands are limited by the lack of steric flexibility of catalytic pockets. We report a new class of unique unsymmetrical N-heterocyclic carbene ligands that are characterized by freely-rotatable N-aromatic wingtips in the imidazol-2-ylidene architecture. The combination of rotatable N-CH2 Ar bond with conformationally-fixed N-Ar linkage results in a highly modular ligand topology, entering the range of geometries inaccessible to IMes and IPr. These ligands are highly reactive in Cu(I)-catalyzed β-hydroboration, an archetypal borylcupration process that has had a transformative impact on the synthesis of boron-containing compounds. The most reactive Cu(I)-NHC in this class has been commercialized in collaboration with MilliporeSigma to enable broad access of the synthetic chemistry community. The ligands gradually cover %Vbur geometries ranging from 37.3 % to 52.7 %, with the latter representing the largest %Vbur described for an IPr analogue, while retaining full flexibility of N-wingtip. Considering the modular access to novel geometrical space in N-heterocyclic carbene catalysis, we anticipate that this concept will enable new opportunities in organic synthesis, drug discovery and stabilization of reactive metal centers.

Wingtip‐Flexible N‐Heterocyclic Carbenes: Unsymmetrical Connection between IMes and IPr

AbstractIMes (IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene) and IPr (IPr=1,3‐ bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) represent by far the most frequently used N‐heterocyclic carbene ligands in homogeneous catalysis, however, despite numerous advantages, these ligands are limited by the lack of steric flexibility of catalytic pockets. We report a new class of unique unsymmetrical N‐heterocyclic carbene ligands that are characterized by freely‐rotatable N‐aromatic wingtips in the imidazol‐2‐ylidene architecture. The combination of rotatable N−CH2Ar bond with conformationally‐fixed N−Ar linkage results in a highly modular ligand topology, entering the range of geometries inaccessible to IMes and IPr. These ligands are highly reactive in Cu(I)‐catalyzed β‐hydroboration, an archetypal borylcupration process that has had a transformative impact on the synthesis of boron‐containing compounds. The most reactive Cu(I)‐NHC in this class has been commercialized in collaboration with MilliporeSigma to enable broad access of the synthetic chemistry community. The ligands gradually cover %Vbur geometries ranging from 37.3 % to 52.7 %, with the latter representing the largest %Vbur described for an IPr analogue, while retaining full flexibility of N‐wingtip. Considering the modular access to novel geometrical space in N‐heterocyclic carbene catalysis, we anticipate that this concept will enable new opportunities in organic synthesis, drug discovery and stabilization of reactive metal centers.

Synthesis and properties of asymmetricly substituted Mn(III)-nitro-phenylporphirins

Mn(III)-5-(4-nitrophenyl)-10,15,20-triphenylporphyrin and Mn(III)-5,10,15-tri-(4-nitrophenyl)-20-phenylporphyrin were synthesized using the reactions of complex formation of corresponding porphyrins and metal exchange of their Cd complexes with MnCl2 in dimethylformamide. Partial reduction of Mn(III) to Mn(II) is observed upon dissolution of manganese(III) complexes in dimethylformamide. When NaOH solid is added to a solution of dimethylformamide and in ethanolamine unstable Mn(II) porphyrins are formed. The photochemical stability and oxidative degradation of Mn(III)-porphyrins have been studied. The metal exchange reaction of asymmetricaly substituted Cd(II)-porphyrins with manganese chloride in dimethylformamide has been studied. The kinetic parameters of the reaction were calculated. The influence of substituends and the nature of the salt on the kinetic parameters of the metal exchange reaction was revealed. The synthesized compounds were identified by methods UV-Vis, IR, 1H NMR spectroscopy and mass spectrometry.

Metal exchange kinetics of asymmetrically substituted cd(II)- tetraarylporphyrins with cobalt and zinc chlorides in dmf

The metal exchange reactions of Cd(II)-5-(4-nitrophenyl)-10,15,20-triphenylporphyrin and Cd(II)-5,10,15-tri-(4-nitrophenyl)-20-phenylporphyrin with cobalt and zinc chlorides in dimethylformamide were studied by the spectrophotometric method. The kinetic parameters of metal exchange were calculated and a possible stoichiometric mechanism of the process is established. The influence of the nature of the solvate salt and the chemical modification of the tetrapyrrole macrocycle on the kinetic parameters of the metal exchange reaction was revealed.

Synthesis and properties of metal complexes with halogen-substituted octaphenyltetraazaporphyrins

Mg(II)-octa-(4-chlorophenyl)tetraazaporphyrin, Mg(II)-octa-(4-fluorophenyl)tetraazaporphyrin, Zn(II)octa-(4-chlorophenyl)tetraazaporphyrin and Zn(II)-octa-(4-fluorophenyl)tetraazaporphyrin were synthesized by cyclization of di-(4-halophenyl)maleinonitriles with magnesium and zinc(II) acetates in refluxing ethylene glycol. Zn(II)-Octaphenyltetraazaporphyrin was obtained by cyclization of diphenylmaleininitrile with zinc acetate in ethylene glycol. Using the metal exchange reaction of halogen-substituted magnesium complexes with copper(II) and manganese(II) chlorides in dimethylformamide, the corresponding Cu(II) and Mn(III)octaphenyltetraazaporphyrins were synthesized. Structure of the obtained compounds were identified by UV-Vis, IR, 1H NMR spectroscopy and mass spectrometry methods. The photochemical stability of the synthesized complexes was studied.

Complex formation of halogen substituted tetraphenylporphyrins and Cd(II)-tetraphenylporphyrins with MnCl2 in DMF

Reactions of complex formation of 5,10,15,20-tetra-(2,6-dichlorophenyl)porphyrin, 5,10,15,20-tetra-(2,6-difluorophenyl)porphyrin and metal exchange of their cadmium complexes with manganese chloride(II) in dimethylformamide were investigated. ortho -Substituted manganese complexes were synthesized by prolonged refluxing of the corresponding porphyrins with an excess of MnCl2 in dimethylformamide. Using the metal exchange reaction of Cd(II)-5,10,15,20-tetra-(2,6-dichlorophenyl)porphyrin and Cd(II)-5,10,15,20-tetra-(2,6-difluorophenyl) porphyrin, the corresponding Mn(III)-tetraphenylporphyrins were obtained. On the contrary, the coordination reactions of porphyrins substituted at the pyrrole and phenyl rings with manganese chloride in dimethylformamide proceed under mild conditions with the formation of complexes Mn(II)-2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetra-(2,6-dichlorophenyl)porphyrin [the compound oxidizes in air to a mixture of Mn(II) and Mn(III)-porphyrins] and Mn(II)-2,3,7,8,12,13,17,18-octachloro-5,10,15,20-tetra-(2,6-difluorophenyl)porphyrin. The synthesized compounds were identified using UV-Vis, 1H NMR spectroscopy and mass-spectrometry. Metal exchange reaction of ortho -substituted Cd(II)-porphyrins and complex formation of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetra-(2,6-dichlorophenyl)porphyrin with manganese chloride in dimethylformamide were studied by the spectrophotometric method. The kinetic parameters of the reactions were calculated. A strong effect of β- and ortho -substitution on the reactions of complex formation and metal exchange of the studied compounds was found.

Understanding the Coordination Chemistry of AmIII/CmIII in the DOTA Cavity: Insights from Energetics and Electronic Structure Theory.

The minor actinides Am/Cm show multiple possibilities for coordination, providing great opportunities for their extraction and adsorption separation. Herein, we report complexation in an aqueous medium of AmIII/CmIII in the DOTA (H4DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) cavity with axial ligands (OH-, F-, and H2O), based on the energetics and electronic structure properties using density functional theory (DFT). The formation and substitution reactions of OH--capped complexes are more likely to occur due to their enhanced hydration Gibbs free energies, followed by F-, and then H2O. Both the longer An-ODOTA bond lengths and the larger bite angle (∠O-An-O) in the OH--capped complexes reflect the enhanced coordination provided by the axial ligand, slightly less so for F-. Energy decomposition analysis based on the electronic structure supports the preference for OH--capped complexes with a near-perfect balance between attractive and repulsive contributions toward the interaction. Furthermore, molecular orbital analysis revealed that the frontier molecular orbitals of Am and Cm complexes are substantially different; that is, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) compositions of the Am complexes are all contributed by 5f, while the HOMO and LUMO compositions of the Cm complexes are derived from 5f and 6d, respectively. Finally, the metal-exchange reactions demonstrate competitive complexation of DOTA toward AmIII over CmIII for the OH--capped system. These results imply the importance of coordination chemistry in actinide chemistry in general and specifically in AmIII/CmIII solution chemistry.

Impact of human serum albumin on CuII and ZnII complexation by ATSM (diacetyl-bis(N4-methylthiosemicarbazone)) and a water soluble analogue.

The chelator diacetyl-bis(N4-methylthiosemicarbazone) (ATSM) and its complexes with CuII and ZnII are becoming increasingly investigated for medical applications such as PET imaging for anti-tumour therapy and the treatment of amyotrophic lateral sclerosis. However, the solubility in water of both the ligand and the complexes presents certain limitations for in vitro studies. Moreover, the stability of the CuII and ZnII complexes and their metal exchange reaction against the potential biological competitor human serum albumin (HSA) has not been studied in depth. In this work it was observed that the ATSM with an added carboxylic group into the structure increases its solubility in aqueous solutions without altering the coordination mode and the conjugated system of the ligand. The poorly water-soluble CuII- and ZnII-ATSM complexes were prevented from precipitating due to the binding to HSA. Both HSA and ATSM show a similar thermodynamic affinity for ZnII. Finally, the CuII-competition experiments with EDTA and the water-soluble ATSM ligands yielded an apparent log Kd at pH 7.4 of about -19. When ATSM was added to CuII- and ZnII-loaded HSA, withdrawing of ZnII was kinetically favoured, but this metal is slowly substituted by the CuII afterwards taken from HSA so that this protein could be considered as a source of CuII for ATSM.

Intracluster ligand rearrangement: an NMR-based thermodynamic study.

Ligand and metal exchange reactions are powerful methods to tailor the properties of atomically precise metal nanoclusters. Hence, a deep understanding of the mechanisms behind the dynamics that rule the ligand monolayer is crucial for its specific functionalization. Combining variable-temperature NMR experiments and dynamic-NMR simulations, we extract the thermodynamic activation parameters of a new exchange reaction: the intracluster ligand rearrangement between the two symmetry-unique positions in [Ag25(DMBT)18]- and [Ag24Au(DMBT)18]- clusters. We report for the first time that this peculiar intracluster modification does not seem to proceed via metal-sulphur bond breaking and follows a first-order rate law, being therefore a process independent from the well-described collisional ligand exchange.

IPr*Oxa - a new class of sterically-hindered, wingtip-flexible N,C-chelating oxazole-donor N-heterocyclic carbene ligands.

N-heterocyclic carbenes (NHCs) have emerged as a major direction in ancillary ligand development for stabilization of reactive metal centers in inorganic and organometallic chemistry. In particular, wingtip-flexible NHCs have attracted significant attention due to their unique ability to provide a sterically-demanding environment for transition metals in various oxidation states. Herein, we report a new class of sterically-hindered, wingtip-flexible NHC ligands that feature N,C-chelating oxazole donors. These ligands are readily accessible through a modular arylation of oxazole derivatives. We report their synthesis and complete structural and electronic characterization. The evaluation of steric, electron-donating and π-accepting properties and coordination chemistry to Ag(I), Pd(II) and Rh(I) is described. Preliminary studies of catalytic activity in Ag, Pd and Rh-catalyzed coupling and hydrosilylation reactions are presented. This study establishes the fluxional behavior of a freely-rotatable oxazole unit, wherein the oxazolyl ring adjusts to the steric and electronic environment of the metal center. Considering the tremendous impact of sterically-hindered NHCs and their potential to stabilize reactive metals by N-chelation, we expect that this class of NHC ligands will be of broad interest in inorganic and organometallic chemistry.

Formation of Metallosupramolecular Helicates and Mesocates from Poly-N-Heterocyclic Carbene Ligands.

In this work, a series of poly-NHC-based tetranuclear silver helicates and mesocates were synthesized from the silver-mediated self-assembly of the ligands involving multiple tridentate CNC-type pincer units and NHC coordination sites. The silver helicate was found to be transferred to a gold mesocate upon metal exchange reaction. The metallosupramolecular helicates and mesocates have been fully characterized by single-crystal X-ray crystallography, mass spectrometry, and multinuclear nuclear magnetic resonance spectroscopies. This study provides an example of the selective preparation of poly-NHC-based helicates or mesocates depending on the size of metal ions and the steric effect of ligands.

Alkali Metal–Metal Exchange Reaction for Effective Synthesis of Functional Phosphate Salts

AMPO4 and MPO4 systems (A = alkali metal, M = metal) exhibit various optical and electrochemical functions. This study shows that such systems can be synthesized via an alkali metal–metal exchange reaction between A3PO4 and the metal salt of an acid with lower pKa than phosphoric acid. This method significantly improves the material designs and syntheses of various phosphate salts.

Shining Visible Light on Reductive Elimination: Acridine-Pd-Catalyzed Cross-Coupling of Aryl Halides with Carboxylic Acids.

Despite the recent tremendous progress on transition-metal/photoredox dual catalysis in organic synthesis, single transition-metal catalysis under visible-light irradiation, which can utilize light energy more efficiently, is still underdeveloped. Herein, we report the design of photosensitizing phosphinoacridine bidentate ligands for visible-light-induced transition-metal catalysis, expecting that the electron-accepting acridine moiety would create a highly reactive electron-deficient metal center toward reductive elimination via metal-to-ligand charge transfer (MLCT). Using these ligands, we have achieved a palladium-catalyzed cross-coupling reaction of aryl halides with carboxylic acids under visible-light irradiation. Electronic tuning of the phosphinoacridine ligands not only enabled the use of a variety of aryl halides as the coupling partner, including less reactive aryl chlorides, under blue light irradiation, but also realized the employment of lower-energy green and red light for the cross-coupling. Experimental mechanistic studies have proved that the reductive elimination of aryl esters is induced by photoirradiation of phosphinoacridine-ligated arylpalladium(II) carboxylate complexes. The theoretical calculation suggests that the reductive elimination in the excited state is promoted by decreasing the electron density of the Pd center through photoinduced intramolecular electron transfer, i.e., MLCT, in the transition state owing to the electron-deficient acridine scaffold. This is a very rare example of photoinduced reductive elimination on palladium(II) complexes.