Tetrahedral Centers Research Articles

Magnesium 4, 5, and 6 coordinate complexes with ligands bound via sp or sp2 hybridized atoms

As a metallic element of high natural abundance, magnesium finds a wide range of uses in both stoichiometric and more recently catalytic applications. This often takes advantage of the basic or nucleophilic properties of its compounds or their ability to co-complex with other organometallic compounds. However, the homoleptic chemistry of Mg(II) is heavily skewed towards alkyl and aryl ligands bound via sp2 and sp3 hybridized atoms. Here, we report our combined NMR spectroscopic and X-ray crystallographic study into much rarer alternative THF solvates of homoleptic magnesium complexes using ligands which bind via sp (alkynyl) and sp2 (imido) atoms. Specifically, we exploit the high acidity of terminal alkynes and diphenylacetonitrile to prepare tetra-solvated distorted octahedral complexes Mg(CCC6H4R-p)2(THF)4 (R = Me, CF3) and Mg(NCCPh2)2(THF)4 starting from the commercial alkyl reagent MgBu2. Adopting a similar deprotonative strategy using benzophenoneimine Ph2CNH affords the heteroleptic trinuclear complex Mg3(NCPh2)4nBu2(THF)2 with distorted tetrahedral Mg centres. Related imidomagnesium halide complexes can be accessed either by deprotonation of the imine with a Grignard reagent, or nucleophilic addition of a Grignard reagent to a nitrile, but these unusual five-coordinate complexes are unresponsive to a 1,4-dioxane induced Schlenk equilibrium shift. Employing a higher reflux temperature switching from a THF to a toluene medium permits access to the THF solvate of a homoleptic imido complex which also possesses a trinuclear constitution in Mg3(NCPh2)6(THF)2.

Electrochemically Created Active Centers in a Bimetallic CoNi-Triazole Metal-Organic Framework for Enhanced Oxygen Evolution Reaction Activity.

Electrochemical water oxidation utilizing bimetallic CoNi-Tz (Tz = 1,2,4-triazole) framework is explored. Initially, CoNi-Tz possesses active tetrahedral Co center and electron-mediated octahedral Ni chain. After performing an electrochemical activation, the partial structural transformation on the Ni center occurs. This leads to the generation of excessive active centers which can promote catalytic activity of the framework. The activated CoNi-Tz catalyst displays a remarkably low OER overpotential of 293 mV at a current density of 10 mA cm-2 with a small Tafel slope of 49.98 mV dec-1, outperforming the single metal Co-Tz and benchmark IrO2 catalysts.

Selective oxidation of methane to formaldehyde and carbon monoxide over V/MSN catalysts with isolated VO4 sites

The selective oxidation of methane to value-added C1 chemicals (e.g., HCHO and CO) is a crucial process in the chemical industry. Recent studies indicate that the generation of highly dispersed isolated active sites is crucial for improving the performance in the selective oxidation of methane and studying the corresponding structure–activity relationship. Therefore, we used mesoporous silica nanoparticles (MSN) as supports and utilized grafting method to prepare catalysts with more selective oxidation active centers (isolated VO4). The study found that isolated VO4 centers are the active sites for the selective oxidation of methane to formaldehyde, and CO is the product of further oxidation of formaldehyde. The total selectivity of formaldehyde and CO on all catalysts is higher than 83.6 %. Among them, the highly dispersed and isolated VO4 tetrahedral active centers on the 2 V/MSN catalyst could stably convert methane to formaldehyde, achieving the highest formaldehyde yield of 3.7 % at 670 °C.

(Invited) Probing Spin Selective Surface Chemistry with Circularly Polarized XUV Light

Controlling spin states of surface reaction intermediates is an important but often overlooked parameter to designing semiconductor photocatalysts to mimic nature’s best photosynthetic complexes. Extreme Ultraviolet Reflection-Absorption (XUV-RA) spectroscopy enables direct observation of surface electron dynamics by combining element, oxidation, and spin state resolution, with surface sensitivity and ultrafast time resolution. While absorption of linearly polarized XUV light can probe the local oxidation and spin states of individual elements, it is insensitive to absolute spin alignment. To enable the study of spin-selective electron dynamics at interfaces, we have recently implemented ultrafast XUV Magnetic Circular Dichroism (XUV-MCD). Employing XUV-MCD in a reflection geometry, we are now studying the ultrafast surface electron dynamics that give rise to spin polarized photocurrents in magnetic and chiral semiconductors. As an example, yttrium iron garnet (Y3Fe5O12, YIG) is a ferrimagnetic oxide with a visible band gap, consisting of two sub-lattices based on octahedrally and tetrahedrally coordinated Fe(III) centers. Ultrafast XUV measurements show that electron-phonon coupling and polaron formation rates differ significantly between octahedral and tetrahedral centers leading to very different charge carrier recombination kinetics. Circular measurements show that this difference in recombination rate gives rise to long-lived, spin-polarized charge accumulation at the YIG surface that drives spin selective water oxidation. This new ability to observe real-time spin polarization at surfaces promises to provide key design parameters for semiconductor photocatalysts capable of spin selective surface chemistry.

Indigo Carmine Binding to Cu(II) in Aqueous Solution and Solid State: Full Structural Characterization Using NMR, FTIR and UV/Vis Spectroscopies and DFT Calculations.

The food industry uses indigo carmine (IC) extensively as a blue colorant to make processed food for young children and the general population more attractive. Given that IC can act as a ligand, this raises concerns about its interactions with essential metal ions in the human body. In view of this interest, in the present investigation, the copper(II)/indigo carmine system was thoroughly investigated in aqueous solution and in the solid state, and the detailed structural characterization of the complexes formed between copper(II) and the ligand was performed using spectroscopic methods, complemented with DFT and TD-DFT calculations. NMR and UV/Vis absorption spectroscopy studies of the ligand in the presence of copper(II) show changes that clearly reveal strong complexation. The results point to the formation of complexes of 1:1, 1:2 and 2:1 Cu(II)/IC stoichiometry in aqueous solution, favored in the pH range 6-10 and stable over time. DFT calculations indicate that the coordination of the ligand to the metal occurs through the adjacent carbonyl and amine groups and that the 1:1 and the 2:1 complexes have distorted tetrahedral metal centers, while the 1:2 structure is five-coordinate with a square pyramidal geometry. FTIR results, together with EDS data and DFT calculations, established that the complex obtained in the solid state likely consists of a polymeric arrangement involving repetition of the 1:2 complex unit. These results are relevant in the context of the study of the toxicity of IC and provide crucial data for future studies of its physiological effects. Although the general population does not normally exceed the maximum recommended daily intake, young children are highly exposed to products containing IC and can easily exceed the recommended dose. It is, therefore, extremely important to understand the interactions between the dye and the various metal ions present in the human body, copper(II) being one of the most relevant due to its essential nature and, as shown in this article, the high stability of the complexes it forms with IC at physiological pH.

Enantioselective domino alkyl arylation of vinyl phosphonates by combining photoredox and nickel catalysis

A nickel/photoredox mediated asymmetric domino alkyl arylation of vinyl phosphonates to generate a diverse array of enantioenriched α-aryl phosphonates is disclosed. This asymmetric three-component difunctionalization couples aryl halides and alkyl bromides with vinyl phosphonates, exhibiting excellent chemo- and regioselectivity under mild reaction conditions. The method avoids the need for pre-formed organometallics and phosphorus halides. Mechanistic and DFT studies suggest that photoexcited [4CzIPN]* oxidizes diethyl 1,4-dihydro-2,6-dimethyl-3,5-pyridinedicarboxylate (HEH) to generate the [4CzIPN]•–, which then reduces the alkyl bromide to form alkyl radicals that undergo Giese addition to the vinyl phosphonate. At the same time, Ni0 oxidatively adds the aryl bromide followed by enantiodetermining oxidative radical trapping of the phosphonate-based radical by the tetrahedral NiII center followed by reductive elimination. Independent gradient model based on Hirshfeld partition (IGMH) analysis suggests that the orientation of the phosphonate group (P=O…π interaction) is expected to play an essential role in controlling the enantioselectivity.

A Cationic Catechol Derivative Binds Anions in Competitive Aqueous Media.

A simple dihydroxy isoquinolinium molecule (3+) was prepared by a modification of a literature procedure. Interestingly, during optimisation of the synthesis a small amount of the natural product pseudopalmatine was isolated, and characterised for the first time by X-ray crystallography. Compound 3+ contains a catechol motif and positive charge on the same scaffold and was found to be a potent anion receptor, binding sulfate strongly in 8 : 2 d6-acetone:D2O and 7 : 3 d6-acetone:D2O (Ka>104 and 2,100 M-1, respectively). Unsurprisingly, chloride binding was much weaker, even in the less polar solvent mixture 9 : 1 d6-acetone:D2O. The sulfate binding is remarkably strong for such a simple molecule, however anion binding studies were complicated by the tendency of the molecule to react with BPh4 - or BF4 - species during anion metathesis reactions. This gave two unusual zwitterions containing tetrahedral boronate centres, which were both characterised by X-ray crystallography.

Unveiling the crystallization mechanism of cadmium selenide via molecular dynamics simulation with machine-learning-based deep potential

Cadmium selenide (CdSe) is an inorganic semiconductor with unique optical and electronic properties that make it useful in various applications, including solar cells, light-emitting diodes, and biofluorescent tagging. In order to synthesize high-quality crystals and subsequently integrate them into devices, it is crucial to understand the atomic scale crystallization mechanism of CdSe. Unfortunately, such studies are still absent in the literature. To overcome this limitation, we employed an enhanced sampling-accelerated active learning approach to construct a deep neural potential with ab initio accuracy for studying the crystallization of CdSe. Our brute-force molecular dynamics simulations revealed that a spherical-like nucleus formed spontaneously and stochastically, resulting in a stacking disordered structure where the competition between hexagonal wurtzite and cubic zinc blende polymorphs is temperature-dependent. We found that pure hexagonal crystal can only be obtained approximately above 1430 K, which is 35 K below its melting temperature. Furthermore, we observed that the solidification dynamics of Cd and Se atoms were distinct due to their different diffusion coefficients. The solidification process was initiated by lower mobile Se atoms forming tetrahedral frameworks, followed by Cd atoms occupying these tetrahedral centers and settling down until the third-shell neighbor of Se atoms sited on their lattice positions. Therefore, the medium-range ordering of Se atoms governs the crystallization process of CdSe. Our findings indicate that understanding the complex dynamical process is the key to comprehending the crystallization mechanism of compounds like CdSe, and can shed lights in the synthesis of high-quality crystals.

Spin state-tailored tetrahedral and octahedral cobalt centers on millimetric Co-Al oxide catalysts as dual sites for synergistic peroxymonosulfate activation

Rationally developing millimetric metal catalysts with tailored electron function is crucial for water decontamination, but challenges remain due to insufficient understanding on their electronic structure engineering and the underlying mechanism. Here, through constructing bonded Co-O-Al unit sites on the millimetric γ-Al2O3 spheres, we report a feasible and scalable strategy of modulating the 3d electron configurations and spin states of supported tetrahedral and octahedral cobalt cations just by adjusting the radius of support curvature. The tetrahedral and octahedral cobalt centers with tailored electron spin states showed a synergism in peroxymonosulfate activation to generate SO4• and reactive oxygen (O*), in which the latter was converted into 1O2 and O2•. The electronic structure-oriented spin catalysis affected the formation of reactive species and the pathway of pollutant degradation. This work will stimulate the design and mass production of macroscopic spin catalysts with superior activity, reusability and stability for catalytic water purification.

Cationic Cobalt(II) Bisphosphine Hydroformylation Catalysis: In Situ Spectroscopic and Reaction Studies.

[HCo(CO)x(bisphosphine)](BF4), x = 1-3, is a highly active hydroformylation catalyst system, especially for internal branched alkenes. In situ infrared spectroscopy (IR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance studies support the proposed catalyst formulation. IR studies reveal the formation of a dicationic Co(I) paramagnetic CO-bridged dimer, [Co2(μ-CO)2(CO)(bisphosphine)2]2+, at lower temperatures formed from the reaction of two catalyst complexes via the elimination of H2. DFT studies indicate a dimer structure with square-pyramidal and tetrahedral cobalt centers. This monomer-dimer equilibrium is analogous to that seen for HCo(CO)4, reacting to eliminate H2 and form Co2(CO)8. EPR studies on the catalyst show a high-spin (S = 3/2) Co(II) complex. Reaction studies are presented that support the cationic Co(II) bisphosphine catalyst as the catalyst species present in this system and minimize the possible role of neutral Co(I) species, HCo(CO)4 or HCo(CO)3(phosphine), as catalysts.

Rapid microwave synthesis of tetrahedral pyrazole/Co(II) complex: [N[sbnd]H···Cl] synthon, XRD/HSA-interactions, DFT/TD-DFT, physiochemical, antifungal, antibacterial, and POM bio-calculations

A new coordination complex, CoCl2(NNH)2, was synthesized by complexation of the pyrazole (NNH) ligand with CoCl2 in less than 5 min with a high yield (>85%). The synthesis was done using microwave radiation in water solvent where no side products were detected. The complexation was confirmed using UV–visible, CHN-analysis, and FT-IR analysis and compared to theoretical IR, DFT and TD-DFT calculations. CoCl2(NNH)2 crystallizes in a C2/c space group with distorted tetrahedral center as shown by single-crystal X-ray diffraction, two types of heteromeric [NH···Cl] synthons have been recorded, such interactions type and ratios were confirmed also via Hirsfield surface analysis (HSA). The desired complex was evaluated for its antibacterial activity against four classes of gram-positive and gram-negative bacteria, and its antifungal activity against four types of fungi. The complex's physio-chemical properties, toxicity risk, lipophilicity, and pharmacophore sites were studied using the POM (Petra-Osiris-Molinspiration) bio-computational theory.

Multistep Cascade Catalytic Reactions Employing Bifunctional Framework Compounds.

Multistep cascade reactions are important to achieve atom as well as step economy over conventional synthesis. This approach, however, is limited due to the incompatibility of the available reactive centers in a catalyst. In the present study, new MOF compounds, [Zn2(SDBA)(3-ATZ)2]·solvent, I and II, with tetrahedral Zn centers as good Lewis acidic sites and the free amino group of the 3-amino triazole ligand as a strong Lewis base center were shown to perform 4-step cascade/tandem reaction in a facile manner. Effective conversion of benzaldehyde dimethyl acetal in the presence of excess nitromethane at 100 °C in water to 1-(1,3-dinitropropan-2-yl) benzene was achieved in 10 h with yields of ∼95% (I) and ∼94% (II). This 4-step cascade reaction proceeds via deacetalization (Lewis acid), Henry (Lewis base), and Michael (Lewis base) reactions. The present work highlights the importance of spatially separated functional groups in multistep tandem catalysis─the examples of which are not common.

Rational Design of Cu-Doped Tetrahedron of Spinel Oxide for High-Performance Nitric Oxide Electrochemical Sensor

The real-time detection of nitric oxide (NO) in living cells is essential to reveal its physiological processes. However, the popular electrochemical detection strategy is limited to the utilization of noble metals. The development of new detection candidates without noble metal species still maintaining excellent catalytic performance has become a big challenge. Herein, we propose a spinel oxide doped with heteroatom-Cu-doped Co3O4 (Cu-Co3O4) for the sensitive and selective detection of NO release from the living cells. The material is strategically designed with Cu occupying the tetrahedral (Td) center of Co3O4 through the formation of a Cu-O bond. The introduced Cu regulates the local coordination environment and optimizes the electronic structure of Co3O4, hybridizing with the N 2p orbital to enhance charge transfer. The CuTd site can well inhibit the current response to nitrite (NO2-), resulting in a high improvement in the electrochemical oxidation of NO. The selectivity of Cu-Co3O4 can be markedly improved by the pore size of the molecular sieve and the negative charge on the surface. The rapid transmission of electrons is due to the fact that Cu-Co3O4 can be uniformly and densely in situ grown on Ti foil. The rationally designed Cu-Co3O4 sensor displays excellent catalytic activity toward NO oxidation with a low limit of detection of 2.0 nM (S/N = 3) and high sensitivity of 1.9 μA nM-1 cm-2 in cell culture medium. The Cu-Co3O4 sensor also shows good biocompatibility to monitor the real-time NO release from living cells (human umbilical vein endothelial cells: HUVECs; macrophage: RAW 264.7 cells). It was found that a remarkable response to NO was obtained in different living cells when stimulated by l-arginine (l-Arg). Moreover, the developed biosensor could be used for real-time monitoring of NO released from macrophages polarized to a M1/M2 phenotype. This cheap and convenient doping strategy shows universality and can be used for sensor design of other Cu-doped transition metal materials. The Cu-Co3O4 sensor presents an excellent example through the design of proper materials to implement unique sensing requirements and sheds light on the promising strategy for electrochemical sensor fabrication.

Elucidation of Metal-Sugar Complexes: When Tungstate Combines with d-Mannose.

The control of metal-sugar complexes speciation in solution is crucial in an energy transition context. Herein, the formation of tungstate-mannose complexes is unraveled in aqueous solution using a multitechnique experimental and theoretical approach. 13C nuclear magnetic resonance (NMR), as well as 13C-1H and 1H-1H correlation spectra, analyzed in the light of coordination-induced shift method and conformation analysis, were employed to characterize the structure of the sugar involved in the complexes. X-ray absorption near edge structure spectroscopy was performed to provide relevant information about the metal electronic and coordination environment. The calculation of 13C NMR chemical shifts for a series of tungstate-mannose complexes using density functional theory (DFT) is a key to identify the appropriate structure among several candidates. Furthermore, a parametric study based on several relevant parameters, namely, pH and tungstate concentration, was carried out to look over the change of the nature and concentrations of the complexes. Two series of complexes were detected, in which the metallic core is either in a ditungstate or a monotungstate form. With respect to previous proposals, we identify two new species. Dinuclear complexes involve both α- and β-furanose forms chelating the metallic center in a tetradentate fashion. A hydrate form chelating a ditungstate core is also revealed. One monotungstate complex appears at high pH, in which a tetrahedral tungstate center is bound to α-mannofuranose through a monodentate site at the second deprotonated hydroxyl group. This unequalled level of knowledge opens the door to structure-reactivity relationships.

Construction of a Chiral β-Fluoroamine by Asymmetric Mannich Reaction of 2-Fluoroindanone with Pyrazolinone Ketimines.

The asymmetric Mannich reaction of 2-fluoroindanone with ketimine was developed under the catalysis of a kind of chiral copper complex, affording a chiral tetrahedral center containing fluorine. A series of β-fluoroamine derivatives can be obtained in excellent yields (73-94%) with high diastereoselectivities (>99:1 dr) and enantioselectivities (89-99%). The possible transition state was supported by density functional theory calculation.

Lithium, Tin(II), and Zinc Amino-Boryloxy Complexes: Synthesis and Characterization.

Analogous to the ubiquitous alkoxide ligand, metal boroxide and boryloxy complexes are an underexplored class of hard anionic O- ligand. A new series of amine-stabilized Li, Sn(II), and Zn boryloxy complexes, comprising electron-rich tetrahedral boron centers have been synthesized and characterized. All complexes have been characterized by one-dimensional (1D), two-dimensional (2D), and DOSY NMR, which are consistent with the solid-state structures unambiguously determined via single-crystal X-ray diffraction. Electron-rich μ2- (Sn and Zn) and μ3- (Li) boryloxy binding modes are observed. Compounds 6-9 are the first complexes of this class, with the chelating bis- and tris-phenol ligands providing a scaffold that can be easily functionalized and provides access to the boronic acid pro-ligand, hence allowing facile direct synthesis of the resulting compounds. Computational quantum chemical studies suggest a significant enhancement of the π-donor ability of the amine-stabilized boryloxy ligand because of electron donation from the amine functionality into the p-orbital of the boron atom.

Spectroscopic Properties of a Biologically Relevant [Fe2(μ-O)2] Diamond Core Motif with a Short Iron-Iron distance.

Diiron cofactors in enzymes perform diverse challenging transformations. The structures of high valent intermediates (Q in methane monooxygenase and X in ribonucleotide reductase) are debated since Fe-Fe distances of 2.5-3.4 Å were attributed to "open" or "closed" cores with bridging or terminal oxo groups. We report the crystallographic and spectroscopic characterization of a FeIII2(µ-O)2 complex (2) with tetrahedral (4C) centres and short Fe-Fe distance (2.52 Å), persisting in organic solutions. 2 shows a large Fe K-pre-edge intensity, which is caused by the pronounced asymmetry at the TD Fe(III) centres due to the short Fe-µ-O bonds. A ~2.5 Å Fe-Fe distance is unlikely for six-coordinate sites in Q or X, but for a Fe2(μ-O)2 core containing four-coordinate (or by possible extension five-coordinate) iron centres there may be enough flexibility to accommodate a particularly short Fe-Fe separation with intense pre-edge transition. This finding may broaden the scope of models considered for the structure of high-valent diiron intermediates formed upon O2 activation in biology.

B-site mixed cationic tetrahedral layer confined the concentration and mobility of interstitial oxygen in mellite family

The subtle difference in the cationic size and covalent bonding between Ga and Ge enables B-site mixed Ga/Ge tetrahedral centers to confine the concentration and mobility of interstitial oxygen in mellite family.

Formic Acid Generation from CO2 Reduction by MOF-253 Coordinated Transition Metal Complexes: A Computational Chemistry Perspective

The inclusion of transition metal elements within metal–organic frameworks (MOFs) is considered one of the most promising approaches for enhancing the catalytic capability of MOFs. In this study, MOF-253 containing bipyridine coordination sites is investigated for possible transition metal chelation, and a consequent possible CO2 reduction mechanism in the formation of formic acid. All transition metal elements of the third, fourth and fifth periods except hafnium and the lanthanide series are considered using density functional theory calculations. Two distinct types of CO2 reduction mechanisms are identified: (1) the five-coordination Pd center, which promotes formic acid generation via an intramolecular proton transfer pathway; (2) several four-coordination metal centers, including Mn, Pd, and Pt, which generate formic acid by means of heterolytic hydrogen activation. The MOF-253 environment is found to promote beneficial steric hindrance, and to constrain metal–ligand orientation, which consequently facilitates the formation of formic acid, particularly with the tetrahedral Mn center at high-spin electronic state.

Zeolite NPO-Type Azolate Frameworks.

Three-membered rings (3-rings) are an important structural motif in zeolite chemistry, but their formation remains serendipitous in reticular chemistry when designing zeolitic imidazolate frameworks (ZIFs). Herein, we report a design principle for constructing four new ZIFs, termed ZIF-1001 to -1004, from tetrahedral ZnII centers (T), benzotriazolate (bTZ), and different functionalized benzimidazolates (RbIM) that adopt a new zeolite NPO-type topology built from 3-rings. Two factors were critical for this discovery: i) incorporating the bTZ linker within the structures formed 3-rings due to a ∠(T-bTZ-T) angle of 120-130° reminiscent of the ∠(Ge-O-Ge) angle (130°) observed in germanate zeolite-type structures having 3-rings; and ii) RbIM guided the coordination chemistry of bTZ to bind preferentially in an imidazolate-type mode. This series' ability to selectively capture CO2 from high-humidity flue gas and trap ethane from tail gas during shale gas extraction was demonstrated.