Metal Coordination Environment Research Articles

Carbon‐Supported Single‐Atom Catalysts: Carbon‐Supported Single Metal Site Catalysts for Electrochemical CO2 Reduction to CO and Beyond (Small 16/2021)

The carbon-supported single metal site catalysts have great potential for electrochemical CO2 reduction due to their flexibility to regulate metal centers, coordination environments, and local structures for enhancing catalytic activity and selectivity. Yi Mei, Gang Wu, and co-workers report that besides the high efficiency of CO2 to CO conversion, single metal sites could provide a synergy to design composite catalysts for producing other C2 products (article number 2005148).

New heptacoordinate tungsten(II) complexes with α-diimine ligands in the catalytic oxidation of multifunctional olefins

New tungsten(II) and molybdenum(II) heptacoordinate complexes [MX2(CO)3(LY)] (MXLy: M = W, Mo; X = Br, I; LY = C5H4NCY = N(CH2)2CH3 with Y = H (L1), Me (L2), Ph (L3)) were synthesized and characterized by spectroscopic techniques and elemental analysis. The two tungsten complexes WXL1 (X = Br, I) were also structurally characterized by single crystal X-ray diffraction. The metal coordination environment is in both a distorted capped octahedron. The complexes with L1 and L2 ligands were grafted in MCM-41, after functionalization of the ligands with a Si(OEt)3 group. The new materials were characterized by elemental analysis, N2 adsorption isotherms, 29Si MAS and 13C MAS NMR. The tungsten(II) complexes and materials were the first examples of this type reported. All complexes and materials were tested as homogeneous and heterogeneous catalysts in the oxidation of multifunctional olefins (cis-hex-3-en-1-ol, trans-hex-3-en-1-ol, geraniol, S-limonene, and 1-octene), with tert-butyl hydroperoxide (TBHP) as oxidant. The molybdenum(II) catalyst precursors are in general very active, reaching 99% conversion and 100% selectivity in the epoxidation of trans-hex-3-en-1-ol. Their performance is comparable with that of the [Mo(η3-C3H5)X(CO)2(LY)] complexes, but it increases with immobilization. On the other hand, most of the W(II) complexes display an activity similar or inferior to that of the Mo(II) analogues and it decreases after they are supported in MCM-41. DFT calculations show that tungsten complexes and iodide ligands are more easily oxidized from M(II) to M(VI) than molybdenum ones, while the energies of relevant species in the catalytic cycle are very similar for all complexes, making the theoretical rationalization of experimental catalytic data difficult.

Influence of the primary and secondary coordination spheres on nitric oxide adsorption and reactivity in cobalt(ii)-triazolate frameworks.

Nitric oxide (NO) is an important signaling molecule in biological systems, and as such, the ability of porous materials to reversibly adsorb NO is of interest for potential medical applications. Although certain metal–organic frameworks are known to bind NO reversibly at coordinatively unsaturated metal sites, the influence of the metal coordination environment on NO adsorption has not been studied in detail. Here, we examine NO adsorption in the frameworks Co2Cl2(bbta) (H2bbta = 1H,5H-benzo(1,2-d:4,5-d′)bistriazole) and Co2(OH)2(bbta) using gas adsorption, infrared spectroscopy, powder X-ray diffraction, and magnetometry. At room temperature, NO adsorbs reversibly in Co2Cl2(bbta) without electron transfer, with low temperature data supporting spin-crossover of the NO-bound cobalt(ii) centers of the material. In contrast, adsorption of low pressures of NO in Co2(OH)2(bbta) is accompanied by charge transfer from the cobalt(ii) centers to form a cobalt(iii)–NO− adduct, as supported by diffraction and infrared spectroscopy data. At higher pressures of NO, characterization data indicate additional uptake of the gas and disproportionation of the bound NO to form a cobalt(iii)–nitro (NO2−) species and N2O gas, a transformation that appears to be facilitated by secondary sphere hydrogen bonding interactions between the bound NO2− and framework hydroxo groups. These results provide a rare example of reductive NO binding in a cobalt-based metal–organic framework, and they demonstrate that NO uptake can be tuned by changing the primary and secondary coordination environment of the framework metal centers.

Intrinsic Activity of Metal Centers in Metal-Nitrogen-Carbon Single-Atom Catalysts for Hydrogen Peroxide Synthesis.

Metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) show high catalytic activity for many important chemical reactions. However, an understanding of their intrinsic catalytic activity remains ambiguous because of the lack of well-defined atomic structure control in current M-N-C SACs. Here, we use covalent organic framework SACs with an identical metal coordination environment as model catalysts to elucidate the intrinsic catalytic activity of various metal centers in M-N-C SACs. A pH-universal activity trend is discovered among six 3d transition metals for hydrogen peroxide (H2O2) synthesis, with Co having the highest catalytic activity. Using density functional calculations to access a total of 18 metal species, we demonstrate that the difference in the binding energy of O2* and HOOH* intermediates (EO2* - EHOOH*) on single metal centers is a reliable thermodynamic descriptor to predict the catalytic activity of the metal centers. The predicted high activity of Ir centers from the descriptor is further validated experimentally. This work suggests a class of structurally defined model catalysts and clear mechanistic principles for metal centers of M-N-C SACs in H2O2 synthesis, which may be further extendable to other reactions.

Rational Preparation of Atomically Precise Non-Alkyl Tin-Oxo Clusters with Theoretical to Experimental Insights into Electrocatalytic CO 2 Reduction Applications

Tin oxides (SnO2) have been widely utilized in electronics, nanolithography, and catalysis. As the atomically precise models of SnO2, tin-oxo clusters (TOCs) not only provide opportunities for mech...

Recent Advances in the Development of Single‐Atom Catalysts for Oxygen Electrocatalysis and Zinc–Air Batteries

AbstractRechargeable zinc–air batteries (ZABs) are presently attracting a lot of attention for electrical energy storage, owing to their low manufacturing cost and very high theoretical specific energy density. Currently, the large‐scale application of ZABs is hampered by the sluggish kinetics of the oxygen‐reduction reaction (ORR) and oxygen evolution reaction (OER), which underpin battery discharging and charging processes, respectively. In recent years, metal single‐atom catalysts (SACs) have emerged as promising candidates for driving oxygen electrocatalysis in ZABs, offering both high electrocatalytic activity and high metal atom utilization through unique metal coordination environments (typically porphyrin‐like MNx species on N‐doped carbon supports). Herein, recent breakthroughs in the design of SACs for ORR and OER electrocatalysis are summarized, with a general view towards improving ZAB performance. This Review begins by introducing the operating principles of ZABs and the reaction mechanisms of the ORR and the OER on the air electrode, after which the various types of SAC‐based materials developed to date for oxygen electrocatalysis and ZABs are discussed. Special emphasis is placed on the relationships between the structure of the SAC active site and electrocatalytic performance. Finally, challenges and opportunities for SACs in practical ZABs are explored.

Contributions of primary coordination ligands and importance of outer sphere interactions in UFsc, a de novo designed protein with high affinity for metal ions

Metalloproteins constitute nearly half of all proteins and catalyze some of the most complex chemical reactions. Recently, we reported a design of 4G-UFsc (Uno Ferro single chain), a single chain four-helical bundle with extraordinarily high (30 pM) affinity for zinc. We evaluated the contribution of different side chains to binding of Co(II), Ni(II), Zn(II) and Mn(II) using systematic mutagenesis of the amino acids that constitute the primary metal coordination and outer spheres. The binding affinity of proteins for metals was then measured using isothermal titration calorimetry. Our results show that both primary metal coordination environment and side chains in the outer sphere of UFsc are highly sensitive to even slight changes and can be adapted to binding different 3d metals, including hard-to-tightly bind metal ions such as Mn(II). The studies on the origins of tight metal binding will guide future metalloprotein design efforts.

Solid-state phase transformations toward a metal-organic framework of 7-connected Zn4O secondary building units

In the development of metal-organic frameworks (MOFs), secondary building units (SBUs) have been utilized as molecular modules for the construction of nanoporous materials with robust structures. Under solvothermal synthetic conditions, dynamic changes in the metal coordination environments and ligand coordination modes of SBUs determine the resultant product structures. Alternatively, MOF phases with new topologies can also be achieved by post-synthetic treatment of as-synthesized MOFs via the introduction of acidic or basic moieties that cause the simultaneous cleavage/reformation of coordination bonds in the solid state. In this sense, we studied the solid-state transformation of two ndc-based Zn-MOFs (ndc = 1,4-naphthalene dicarboxylate) with different SBUs but the same pcu topology to another MOF with sev topology. One of the chosen MOFs with pcu nets is [Zn2(ndc)2(bpy)]n (bpy = 4,4′-bipyridine), (6Cbpy-MOF) consisting of a 6-connected pillared-paddlewheel SBU, and the other is IRMOF-7 composed of 6-connected Zn4O(COO)6 SBUs and ndc. Upon post-structural modification, these pcu MOFs were converted into the same MOF with sev topology constructed from the uncommon 7-connected Zn4O(COO)7 SBU (7C-MOF). The appropriate post-synthetic conditions for the transformation of each SBUs were systematically examined. In addition, the effect of the pillar molecules in the pillared-paddlewheel MOFs on the topology conversion was studied in terms of the linker basicity, which determines the inertness during the solid-state phase transformation. This post-synthetic modification approach is expected to expand the available methods for designing and synthesizing MOFs with controlled topologies.

Isoreticular Linker Substitution in Conductive Metal–Organic Frameworks with Through‐Space Transport Pathways

AbstractThe extension of reticular chemistry concepts to electrically conductive three‐dimensional metal–organic frameworks (MOFs) has been challenging, particularly for cases in which strong interactions between electroactive linkers create the charge transport pathways. Here, we report the successful replacement of tetrathiafulvalene (TTF) with a nickel glyoximate core in a family of isostructural conductive MOFs with Mn2+, Zn2+, and Cd2+. Different coordination environments of the framework metals lead to variations in the linker stacking geometries and optical properties. Single‐crystal conductivity data are consistent with charge transport along the linker stacking direction, with conductivity values only slightly lower than those reported for the analogous TTF materials. These results serve as a case study demonstrating how reticular chemistry design principles can be extended to conductive frameworks with significant intermolecular contacts.

Selective electroreduction of CO2 to acetone by single copper atoms anchored on N-doped porous carbon

Efficient electroreduction of CO2 to multi-carbon products is a challenging reaction because of the high energy barriers for CO2 activation and C–C coupling, which can be tuned by designing the metal centers and coordination environments of catalysts. Here, we design single atom copper encapsulated on N-doped porous carbon (Cu-SA/NPC) catalysts for reducing CO2 to multi-carbon products. Acetone is identified as the major product with a Faradaic efficiency of 36.7% and a production rate of 336.1 μg h−1. Density functional theory (DFT) calculations reveal that the coordination of Cu with four pyrrole-N atoms is the main active site and reduces the reaction free energies required for CO2 activation and C–C coupling. The energetically favorable pathways for CH3COCH3 production from CO2 reduction are proposed and the origin of selective acetone formation on Cu-SA/NPC is clarified. This work provides insight into the rational design of efficient electrocatalysts for reducing CO2 to multi-carbon products.

Which Stereoinductor Is Better for Asymmetric Functionalization of α-Amino Acids in a Nickel(II) Coordination Environment? Experimental and DFT Considerations.

The results of extended comparative investigation of nickel(II) Schiff base complexes (containing various auxiliary chiral moieties) commonly used as a methodological platform for the asymmetric synthesis of tailor-made α-amino acids are provided. The following issues are addressed: 1) redox activity (determining the possibility for electrochemically induced reactions); 2) quantitative estimation of the reactivity of deprotonated complexes towards electrophiles; and 3) quantum-chemical estimation of noncovalent interactions in the metal coordination environment (which shed light on the origin of the stereochemical outcome observed for different stereoinductors). Possible mechanisms that determine the relationship between the stereochemical configuration of a molecule and its electronic structure are discussed. The DFT-calculated HOMO-LUMO energies and localization, as well as relative energies for the (S)- and (R)-alanine derivatives, that determine the stereoinduction efficiency in thermodynamically controlled reactions in nickel(II) coordination are provided. The computational data are supported by experimental results on the monobenzylation of glycine derivatives.

Metal-Containing Polydopamine Nanomaterials: Catalysis, Energy, and Theranostics.

Polydopamine (PDA) is a major type of artificial melanin material with many interesting properties such as antioxidant activity, free-radical scavenging, high photothermal conversion efficiency, and strong metal-ion chelation. The high affinity of PDA to a wide range of metals/metal ions has offered a new class of functional metal-containing polydopamine (MPDA) nanomaterials with promising functions and extensive applications. Understanding and controlling the metal coordination environment is vital to achieve desirable functions for which such materials can be exploited. MPDA nanomaterials with metal/metal ions as the active functions are reviewed, including their synthesis and metal coordination environment and their applications in catalysis, batteries, solar cells, capacitors, medical imaging, cancer therapy, antifouling, and antibacterial coating. The current trends, limitations, and future directions of this area are also explored.

The effect of coordinated solvent molecules on metal coordination environments in single-crystal-to-single-crystal transformations

We summarized the effect of coordinated solvent molecules on the metal coordination environments in single-crystal to single-crystal transformations.

Vapor ammonization strategy towards surface-reconstructed and N-modified three-dimensional Cu foam electrocatalyst for efficient alkaline hydrogen production

Development of highly active and stable noble-metal-free electrocatalysts is of great significance for large-scale electrocatalytic hydrogen evolution reaction (HER). Herein, we develop an effective vapor ammonization-electroreduction strategy for fabricating a surface-reconstructed and N-modified three dimensional (3D) Cu foam (SN-CF) cathode for highly efficient alkaline HER. The vapor ammonization is effective to enhance the surface roughness of 3D Cu foam (CF), which is beneficial to enhance the catalytically active surface areas and active site density. Meanwhile, the in situ formation of the surface Cu–N species can effectively regulate the electronic structures of surface active sites. As a result, the thus-obtained SN-CF electrocatalyst exhibits a superior electrocatalytic activity towards HER in 1.0 M KOH solution, and it requires low overpotentials of 188 and 377 mV at a current density of 10 and 100 mA cm−2, respectively. Moreover, SN-CF also exhibits a good cycling stability (95% after 2000 cycles) and long-term durability for 12 h at a current density of 10 mA cm−2. This work provides a simple and effective strategy for rationally fabricating highly active HER electrocatalysts by synergistically tuning the surface nanostructures and coordination environment of transition metals.

In Situ Coordination Environment Tuning of Cobalt Sites for Efficient Water Oxidation

Most transition-metal-based oxygen-evolving catalyst surfaces typically experience irreversible compositional and structural variations during oxygen evolution reaction (OER) in hydrolytic and corrosive alkaline media, degrading the coordination environment of active metal sites into unified (oxy)hydroxides. Here, we present an in situ electrochemical coordination tuning of cobalt sites for OER in a strong base, where an electrolyzing soluble cobalt-2,2′-bipyridine (Co-bpy) complex partially splits the bpy ligand, leading to the deposition of active Co sites with fine coordination at room temperature. We deposited the Co sites while catalyzing water oxidation at the same condition so that this catalyst can adapt the hostile alkaline condition. This robust coordination environment involving the remaining bpy and generated (oxy)hydroxide ligands (Co–BH catalyst) sustains the highly improved OER activity over 500 h at 200 mA cm–2, outperforming other fragile Co sites with only (oxy)hydroxides. In addition, this work presents an efficient tuning of metal coordination environments to in situ generate highly active and stable metal sites in alkaline electrolytes for water splitting.

Synthesis and Structures of Bis- and Tris-(triphenylarsine)gold(I) Iodides

The title compounds [(Ph3As)2AuI] and [(Ph3As)3AuI] have been crystallized from equimolar solutions of Bu4NAuI2 and AsPh3 in dimethylformamide and structurally characterized by single crystal X-ray diffraction studies. [(Ph3As)2AuI] crystallizes in space group C2/c, Z 4, and is isomorphous with other [(Ph3E)2MX] (MX=coinage metal(i) salt) arrays, with the Au–I bond being disposed on a crystallographic 2-axis: Au–I, As 2.7008(2), 2.4337(2) Å, As–Au–As, I 125.736(8)°, 117.132(4)° (153K). [(Ph3As)3AuI] crystallizes as a triclinic phase in space group , Z 4, and is isomorphous with [(Ph3Sb)3CuI] and [(Ph3P)3AgI]: Au–As 2.4847–2.5049(10), Au–I 2.8518(8), 2.8597(7) Å with As–Au–As, I 109.67(3)–115.97(3)°, 101.33(2)–106.85(3)°. A second ‘[(Ph3As)3AuI]’ product was obtained as a co-crystalline phase in space group P21/n containing [(Ph3As)3AuI], and [(Ph3As)2AuI] accompanied by an additional unbound Ph3As molecule, i.e. [(Ph3As)3AuI]·[(Ph3As)2AuI·Ph3As], with structural parameters closely similar to those for the corresponding separate [(Ph3As)3AuI] and [(Ph3As)2AuI] complexes described above. Comparison of the bond lengths for these and related complexes show that they are generally consistent with the ‘gold is smaller than silver’ phenomenon caused by relativistic orbital contraction effects in gold, but the results also show that the magnitude of this effect is dependent on the nature of the metal–ligand bonds involved, and on changes in the metal coordination environment, which can in some circumstances yield trends in which the effect on particular bonds is partially masked or even reversed.

Triplet-State Position and Crystal-Field Tuning in Opto-Magnetic Lanthanide Complexes: Two Sides of the Same Coin.

Lanthanide-complex-based luminescence thermometry and single-molecule magnetism are two effervescent fields of research, owing to the great promise they hold from an application standpoint. The high thermal sensitivity achievable, their contactless nature, along with sub-micrometric spatial resolution make these luminescent thermometers appealing for accurate temperature probing in miniaturised electronics. To that end, single-molecule magnets (SMMs) are expected to revolutionise the field of spintronics, thanks to the improvements made in terms of their working temperature-now surpassing that of liquid nitrogen-and manipulation of their spin state. Hence, the combination of such opto-magnetic properties in a single molecule is desirable in the aim of overcoming, among others, addressability issues. Yet, improvements must be made through design strategies for the realisation of the aforementioned goal. Moving forward from these considerations, we present a thorough investigation of the effect that changes in the ligand scaffold of a family of terbium complexes have on their performance as luminescent thermometers and SMMs. In particular, an increased number of electron-withdrawing groups yields modifications of the metal coordination environment and a lowering of the triplet state of the ligands. These effects are tightly intertwined, thus, resulting in concomitant variations of the SMM and the luminescence thermometry behaviour of the complexes. Supported by ab initio calculations, we can rationally interpret the observed trends and provide solid foundations for the development of opto-magnetic lanthanide complexes.

EFFECT OF pH CHANGE ON THE STABILITY OF HETEROMETALIC COMPLEX COMPOUNDS OF NICKEL (II)–COBALT (III) WITH MONOETHANOLAMINE IN ALCOHOL SOLUTIONS

Background. The application of the cobalt (III) nickel (II) heterometalic complex with monoethanolamine, which is used as a precursor for the preparation of electrocatalytic materials on inert carriers, remain insufficiently studied. Of particular importance is the control of the pH range of the reaction medium, which can significantly affect the composition and structure of the active centers after deposition and pyrolytic decomposition.Objective. The purpose of the paper is the experimental determination of the pH range in which the composition of 2Co–Ni complex compounds with monoethanolamine will remain unchanged and study of the transformations that occur with complex compounds depending on the change in pH value.Methods. Synthesis of {Ni[CoEtm3]2}(NO3)2 and studies of its durability by methods of potentiometric titration with chloride acid (C = 0.1 M) and sodium hydroxide (C = 0.1 M). Studying the structural transformations of the obtained complex compound and its decomposition products in alcohol solutions using electronic absorption spectra.Results. The study of the stability of alcoholic solutions of the intracomplex cobalt (III) compound with monoethanolamine and the heterometallic compound 2Co–Ni with monoethanolamine in alkaline and acidic media made it possible to establish the limits of the pH range in which the synthesis of the synthesized complex compounds does not occur. For the results of the analysis of the obtained absorption spectra and redox titration, it can be argued that there is no change in the coordination environment of metals (Co (III), Ni (II)) in the range of 7 to 8 pH. With an increase in the pH of the medium above 8, the complex compounds are destroyed with the formation of Co2+ and Ni2+ salts. In the case of lowering the pH below 7, the above complex compounds are converted to chloroacidic amines. The corresponding transformations that occur with CoEtm3 and {Ni[CoEtm3]2}(NO3)2 under the influence of acid and alkali in alcohol solutions are described by the proposed decomposition schemes.Conclusions. The obtained results indicate the need to control the pH of the reaction medium with the promising development of technology for producing catalytic materials using the {Ni[CoEtm3]2}(NO3)2 heteronuclear complex as precursors and clearly set the pH limits in the range from 7 to 8 units.

An overly anionic metal coordination environment eliminates the T1-weighted response of quinol-containing MRI contrast agent sensors to H2O2

A previously reported quinol- and Mn(II)-containing MRI contrast agent sensor for H2O2 has the drawback of releasing the metal ion upon oxidation of the organic ligand. The release of potentially neurotoxic Mn(II) limits the sensor’s in vivo applicability. We prepared N,N′-bis(2,5-dihydroxybenzyl)ethanediamine-N,N′-diacetic acid (H6qc1) as a substitute ligand that could potentially remain bound to the metal ion after oxidation of the quinol portions to more weakly metal-binding para-quinones. The carboxylic acid groups deprotonate at ambient pH, providing a more anionic coordination environment that stabilizes its Mn(II) complex in water. Although the more anionic coordination sphere doesn’t introduce air sensitivity, it does render the metal center more susceptible to oxidation by hydrogen peroxide, as evidenced by electron paramagnetic resonance. The oxidation of the metal to less paramagnetic Mn(III) is proposed to lower the r1 enough to completely counter any increase in T1-weighted relaxivity that would result from improved aquation.

(Invited) Atomically Dispersed Eletrocatalysts for High-Efficiency Ambient N2 Fixation

NH3 synthesis by the electrocatalytic N2 reduction reaction (NRR) under ambient conditions is regarded as an appealing alternative to the industrial method that requires high temperature and pressure. Finding a material that can efficiently catalyze electrochemical N2 fixation at ambient condition is a subject of considerable current interest. Atomically dispersed catalysts with mononuclear metal complexes or single metal atoms anchored on supports would be a promising candidate, because of their maximum atom efficiency, unique catalytic performances, and the similarity of the metal coordination environment to the ligand fields in molecular catalysts. A series of cost-effective and optimized atomically-dispersed eletrocatalysts for NRR will be reported. Our results show that the catalysts are featured with high density of single metal atoms supported on hierarchically porous carbon frameworks. They exhibited remarkable selectivity of NH3 formation and high NH3 yield rate at low applied potentials at room temperature. Moreover, the catalysts show negligible activity decay in an NRR electrolysis as long as 50,000 s. On the basis of our results and the previously reported work, a possible mechanism for NRR on the catalyst will be proposed to provide a fundamental insight into the high-efficiency NRR.