Transition Metal Chemistry Research Articles

Supervised learning of a chemistry functional with damped dispersion

Kohn–Sham density functional theory is widely used in chemistry, but no functional can accurately predict the whole range of chemical properties, although recent progress by some doubly hybrid functionals comes close. Here, we optimized a singly hybrid functional called CF22D with higher across-the-board accuracy for chemistry than most of the existing non-doubly hybrid functionals by using a flexible functional form that combines a global hybrid meta-nonseparable gradient approximation that depends on density and occupied orbitals with a damped dispersion term that depends on geometry. We optimized this energy functional by using a large database and performance-triggered iterative supervised training. We combined several databases to create a very large, combined database whose use demonstrated the good performance of CF22D on barrier heights, isomerization energies, thermochemistry, noncovalent interactions, radical and nonradical chemistry, small and large systems, simple and complex systems and transition-metal chemistry.

Anion-Coordination-Driven Assembly.

The assembly of discrete architectures has been an important subject in supramolecular chemistry because of their elegant structures and fascinating properties. During the last several decades, supramolecular chemists have developed manifold strategies for hierarchical assembly, which are normally classified by two main types of driving force: covalent and noncovalent interactions. Typical noncovalent interactions include metal coordination, hydrogen bonding, and other weak forces. These approaches have achieved great progress in the construction of various supramolecular structures, such as macrocycles, cages, polyhedra, and interlocked systems. Among these methods, metal-coordination-driven assembly is attractive due to the well-defined coordination properties of metal ions. Indeed, in terms of supramolecular chemistry, the concept of "coordination" has been expanded beyond transition metals. In particular, anion coordination chemistry, which was first proposed by Lehn in 1978 [ Acc. Chem. Res. 1978, 11, 49] and then elucidated in detail by Bowman-James two decades later [ Acc. Chem. Res. 2005, 38, 671], has grown up to a subfield of supramolecular chemistry. It is noticeable that anions also show "dual valencies" like transition metals, wherein the "primary valence" is the charge balance for anions by countercations while the "secondary valence", i.e., the coordination, refers to hydrogen bonding interactions where the electron flow is from the electron-rich anion (the coordination center) to hydrogen bonding donors (the ligands). Thus, anions also display certain coordination numbers and specific coordination geometries. Although such features are far less regular than those of transition metals, they are sufficient to allow anion coordination to serve as the driving force for assembling discrete supramolecular architectures. In this Account, the anion-coordination-driven assembly (ACDA), a new assembling strategy established by us during the past decade, will be presented. We summarize our work in the construction of a series of "aniono" supramolecular structures, especially triple helicates and tetrahedral cages, based on the coordination between oligourea ligands and anions (mostly phosphate). In particular, we will detail the considerations in the design of ligands, the assembling process including structural transformation, and functionalization of the systems toward guest inclusion, supramolecular catalysis, photoswitches, and molecular devices. These results demonstrate the great potential of ACDA in fabricating novel anion-based systems. Although the design concept was originally loaned from traditional coordination chemistry of transition metals, and structures of anion complexes bear some resemblance to metal complexes, there are significant differences of the aniono supramolecular assemblies from the metallo analogues. For example, these metal-free systems are held together by multiple hydrogen bonds (dozens to nearly 100), thus facilitating assembly/disassembly under mild conditions and relatively flexible structures for adaptive guest inclusion. To this end, intriguing applications (supramolecular chirality, catalysis, energy storage, etc.) may be expected for aniono systems. We hope the current Account will attract more attention from researchers in supramolecular assembly and inspire more efforts in this fascinating area.

Stimuli Responsive Silylene: Electromerism Induced Reversible Switching Between Mono- and Bis-Silylene.

Electromerism is a very well-known phenomenon in transition metal chemistry. In main group chemistry, this concept has only started getting attention recently. We report stimuli responsive low-valent silicon compounds exhibiting electromerism. A mixed-valent silaiminyl-silylene 1, [LSi-Si(NDipp)L] (L=PhC(Nt Bu)2 ), was synthesized in a single step from amidinate-chlorosilylene. Compound 1 has two interconnected Si atoms in formally +I and +III oxidation states. Upon treatment with Lewis acidic CuI X (X=mesityl, Cl, Br, I), electron redistribution occurs resulting in the formation of [{LSi(NDipp)Si(L)}-CuX], in which both silicon atoms are in the +II formal oxidation state. Removal of the copper center from [{LSi(NDipp)Si(L)}-CuX] by using a Lewis basic carbene led to reformation of the precursor [LSi-Si(NDipp)L]. Thus, the process is fully reversible. This showcases the first example of Lewis acid/base-induced reversible electromerism in silicon chemistry.

Stimuli Responsive Silylene: Electromerism Induced Reversible Switching Between Mono‐ and Bis‐Silylene

AbstractElectromerism is a very well‐known phenomenon in transition metal chemistry. In main group chemistry, this concept has only started getting attention recently. We report stimuli responsive low‐valent silicon compounds exhibiting electromerism. A mixed‐valent silaiminyl‐silylene1, [LSi−Si(NDipp)L] (L=PhC(NtBu)2), was synthesized in a single step from amidinate‐chlorosilylene. Compound1has two interconnected Si atoms in formally +I and +III oxidation states. Upon treatment with Lewis acidic CuIX (X=mesityl, Cl, Br, I), electron redistribution occurs resulting in the formation of [{LSi(NDipp)Si(L)}−CuX], in which both silicon atoms are in the +II formal oxidation state. Removal of the copper center from [{LSi(NDipp)Si(L)}−CuX] by using a Lewis basic carbene led to reformation of the precursor [LSi−Si(NDipp)L]. Thus, the process is fully reversible. This showcases the first example of Lewis acid/base‐induced reversible electromerism in silicon chemistry.

Ion-Pairing Catalysis in Stereoselective, Light-Induced Transformations.

With the rapid development of photoredox catalysis, numerous concepts for asymmetric induction were successfully and broadly adapted from polar two-electron transformations to radical chemistry. While this applies to organocatalysis or transition metal chemistry, asymmetric ion-pairing catalysis remains a niche application within light-driven reactions today. This perspective gives an overview of recent examples, strategies, and their application in stereoselective transformations at the interface of ion-pairing and photo(redox) catalysis.

Molten Hydroxide Electrodeposition of Transition Metal Oxide Cathodes for Electrochemical Energy Storage

State-of-the-art synthesis for layered oxide cathodes for Li and Na-ion battery involves prolonged high temperature (>700°C) multistep processing for long reaction times (~24 hours) under high oxygen pressure, followed by slurry casting after mixing with binders and additives. Here, we demonstrate an intermediate temperature (250-350°C) molten hydroxide-based electrodeposition process to grow alkali ion (Li+, Na+) intercalated transition metal oxides across multiple transition metal chemistries of the Li system (from layered LiCoO2 to cobalt-less orthorhombic LiMnO2, layered Li2MnO3, spinel LiNixMn1-xO4, LiMn2O4,) and Na system (O3 and P2 NaxCoO2, O’3 NaxMnO2) in thick film form factors. The highly textured ([110]||ND), dense (>95%) electroplated LiCoO2 cathodes can perform at ultrahigh thickness of ~200 µm (areal capacity ~13.6 mAh/cm2) in comparison to 40-60 µm for state-of-the-art slurry cast cathodes (areal capacity ~3-4 mAh/cm2 with a porosity of ~20%), a fivefold increase in areal capacity and volumetric energy density. We discuss the interplay of bulk crystallographic orientation of the electro-active material (LiCoO2), surface terminating planes interacting with the electrolyte, and electrochemically grown conformal surface protective layers on the electrochemical-chemo-mechanical degradation mechanisms during intercalation and deintercalation for high voltage operation (>4.2 V vs. Li). We also demonstrate electrochemically controllable isovalent transition metal doping and substitution in ceramic oxides (LiCoxMn1-xO2) by electrodeposition. Our findings highlight the influence of the unique molten hydroxide-based solution chemistry platform and electrochemical processing parameters, on regulating the phase assemblage, designing the microstructure, and controlling crystallographic orientation during electrocrystallization.

Exploring the Gas-Phase Formation and Chemical Reactivity of Highly Reduced M8 L6 Coordination Cages.

Coordination cages with well-defined cavities show great promise in the field of catalysis on account of their unique combination of molecular confinement effects and transition-metal redox chemistry. Here, three coordination cages are reduced from their native 16+ oxidation state to the 2+ state in the gas phase without observable structural degradation. Using this method, the reaction rate constants for each reduction step were determined, with no noticeable differences arising following either the incorporation of a C60 -fullerene guest or alteration of the cage chemical structure. The reactivity of highly reduced cage species toward molecular oxygen is "switched-on" after a threshold number of reduction steps, which is influenced by guest molecules and the structure of cage components. These new experimental approaches provide a unique window to explore the chemistry of highly-reduced cage species that can be modulated by altering their structures and encapsulated guest species.

Noble-Nanoparticle-Decorated Ti3C2T x MXenes for Highly Sensitive Volatile Organic Compound Detection.

Two-dimensional transition-metal carbides and nitrides (MXenes) have been regarded as promising sensing materials because of their high surface-to-volume ratios and outstanding electronic, optical, and mechanical properties with versatile transition-metal and surface chemistries. However, weak gas-molecule adsorption of MXenes poses a serious limitation to their sensitivity and selectivity, particularly for trace amounts of volatile organic compounds (VOCs) at room temperature. To deal with these issues, Au-decorated MXenes are synthesized by a facile solution mixing method for room-temperature sensing of a wide variety of oxygen-based and hydrocarbon-based VOCs. Dynamic sensing experiments reveal that optimal decoration of Au nanoparticles (NPs) on Ti3C2Tx MXene significantly elevates the response and selectivity of the flexible sensors, especially in detecting formaldehyde. Au–Ti3C2Tx gas sensors exhibited an extremely low limit of detection of 92 ppb for formaldehyde at room temperature. Au–Ti3C2Tx provides reliable gas response, low noise level, ultrahigh signal-to-noise ratio, high selectivity, as well as parts per billion level of formaldehyde detection. The prominent mechanism for Au–Ti3C2Tx in sensing formaldehyde is elucidated theoretically from density functional theory simulations. The results presented here strongly suggest that decorating noble-metal NPs on MXenes is a feasible strategy for the development of next-generation ultrasensitive sensors for Internet of Things.

Environmental, Health, and Legislation Considerations for Rational Design of Nonreactive Flame-Retardant Additives for Polymeric Materials: Future Perspectives.

Increasing polymer usage has demanded functional additives that decrease fire hazards for end users. While traditional flame-retardant (FR) additives, such as halogenated, phosphorus, and metal hydroxides, greatly reduce flammability and associated fire hazards, research has continually exposed a litany of health and environmental safety concerns. This perspective aims to identify the key components of a successful FR additive and address material, environmental, and health concerns of existing additives. Legislation surrounding FRs and persistent organic pollutants is also discussed to highlight political perception that has resulted in the increased chemical regulations and subsequent banning of FR additives. Finally, future directions of this field regarding nonreactive additives, focusing on the use of bioinspired materials and transition metal chemistries to produce alternatives for polymers with efficacies surpassing traditional additives are presented.

Enhanced Dihydrogen Activation by Mononuclear Iridium(II) Compounds: A Mechanistic Study

AbstractThe organometallic chemistry of 4d and 5d transition metals has been vastly dominated by closed‐shell states. The reactivity of their metalloradical species is though remarkable, albeit yet poorly understood and with limited mechanistic investigations available. In this work we report the synthesis and characterization of two mononuclear IrII species, including the first dinitrogen adduct. These compounds activate dihydrogen at a dissimilar rate, in the latter case several orders of magnitude faster than its IrI precursor. A combined experimental/computational investigation to ascertain the mechanism of this transformation in IrII compounds is reported.

Design principles for zero-strain Li-ion cathodes

<h2>Summary</h2> The cycling of cathode materials for Li-ion batteries is often accompanied by a change in volume, posing a challenge to the integrity of cathode particles and electrolyte/cathode interface in solid-state batteries. To enhance capacity retention, it is thus crucial to design materials that remain structurally invariant during electrochemical cycling. Here, we use well-calibrated first-principles calculations to systematically investigate the effect of transition-metal chemistry, cation ordering, Li site occupancy, redox-inactive species, anion substitution, and cation migration on the volume change associated with the delithiation of cathode materials with an FCC anion framework. Suggested by an in-depth first-principles Monte Carlo simulation of the Li⁺–V³⁺–Nb⁵⁺–O²⁻–F⁻ system, we experimentally confirm Li_1.3V_0.4Nb_0.3O₂ and Li_1.25V_0.55Nb_0.2O_1.9F_0.1 as nearly zero-strain cathodes. Our study establishes a fundamental understanding of the important physical descriptors that determine the dimensional change of materials during cycling and provides general guidelines for designing low- or zero-strain cathodes.

Synthesis of a Triphenylphosphinimide-Substituted Silirane as a "Masked" Acyclic Silylene.

Phosphinimides are long known as useful ligands in transition metal chemistry, but examples of these in low-valent silicon chemistry are rather rare. Hence, in this work, we report on the implementation of a triphenylphosphinimide moiety as a ligand of a novel silylene that is trapped as a silirane with cyclohexene. By performing activation reactions with B(p-Tol)3, HSiEt3, N2O, and NH3, we demonstrate that the silirane exhibits a silylene-like behavior, making it a "masked" silylene. Furthermore, we treated the silirane with ethylene, propylene, and trans-butene, which led to an olefin exchange. In the case of ethylene and propylene, an additional insertion of the olefin into the silicon-silicon bonds of the respective siliranes could be achieved. As the insertion of trans-butene was not feasible, we surmise that the scope of this reactivity is restricted by the steric demand of the olefin.

Synthesis, Characterization and Photophysical Properties of Mesoionic N‐Heterocyclic Imines

N-heterocyclic imines are widely used in transition-metal chemistry, main-group chemistry as well as catalysis, due to their enhanced basicity and nucleophilicity which benefit from their ylidic form. As their analogs, mesoionic N-heterocyclic imines, which feature more highly ylidic form, is still in its infancy though excellent works also achieved. Here we reported the synthesis, characterization and photophysical properties of mesoionic N-heterocyclic imines. TD-DFT are employed to get deeper insight into the mechanism of the photophysical behaviors. The unsubstituted mesoionic N-heterocyclic imines (4-6) displayed considerable quantum yields (QY: up to 43.8%) and could be potentially applied as luminescent materials.

Molecular orbital projectors in non-empirical jmDFT recover exact conditions in transition-metal chemistry.

Low-cost, non-empirical corrections to semi-local density functional theory are essential for accurately modeling transition-metal chemistry. Here, we demonstrate the judiciously modified density functional theory (jmDFT) approach with non-empirical U and J parameters obtained directly from frontier orbital energetics on a series of transition-metal complexes. We curate a set of nine representative Ti(III) and V(IV) d1 transition-metal complexes and evaluate their flat-plane errors along the fractional spin and charge lines. We demonstrate that while jmDFT improves upon both DFT+U and semi-local DFT with the standard atomic orbital projectors (AOPs), it does so inefficiently. We rationalize these inefficiencies by quantifying hybridization in the relevant frontier orbitals. To overcome these limitations, we introduce a procedure for computing a molecular orbital projector (MOP) basis for use with jmDFT. We demonstrate this single set of d1 MOPs to be suitable for nearly eliminating all energetic delocalization and static correlation errors. In all cases, MOP jmDFT outperforms AOP jmDFT, and it eliminates most flat-plane errors at non-empirical values. Unlike DFT+U or hybrid functionals, jmDFT nearly eliminates energetic delocalization and static correlation errors within a non-empirical framework.

Aberrant Cerebral Iron Trafficking Co-morbid With Chronic Inflammation: Molecular Mechanisms and Pharmacologic Intervention.

The redox properties that make iron an essential nutrient also make iron an efficient pro-oxidant. Given this nascent cytotoxicity, iron homeostasis relies on a combination of iron transporters, chaperones, and redox buffers to manage the non-physiologic aqueous chemistry of this first-row transition metal. Although a mechanistic understanding of the link between brain iron accumulation (BIA) and neurodegenerative diseases is lacking, BIA is co-morbid with the majority of cognitive and motor function disorders. The most prevalent neurodegenerative disorders, including Alzheimer's Disease (AD), Parkinson's Disease (PD), Multiple System Atrophy (MSA), and Multiple Sclerosis (MS), often present with increased deposition of iron into the brain. In addition, ataxias that are linked to mutations in mitochondrial-localized proteins (Friedreich's Ataxia, Spinocerebellar Ataxias) result in mitochondrial iron accumulation and degradation of proton-coupled ATP production leading to neuronal degeneration. A comorbidity common in the elderly is a chronic systemic inflammation mediated by primary cytokines released by macrophages, and acute phase proteins (APPs) released subsequently from the liver. Abluminal inflammation in the brain is found downstream as a result of activation of astrocytes and microglia. Reasonably, the iron that accumulates in the brain comes from the cerebral vasculature via the microvascular capillary endothelial cells whose tight junctions represent the blood-brain barrier. A premise amenable to experimental interrogation is that inflammatory stress alters both the trans- and para-cellular flux of iron at this barrier resulting in a net accumulation of abluminal iron over time. This review will summarize the evidence that lends support to this premise; indicate the mechanisms that merit delineation; and highlight possible therapeutic interventions based on this model.

Applications of Low-Valent Transition Metalates: Development of a Reactive Noncarbonyl Rhenium(I) Anion.

Low-valent transition metalates─anionic, electronic-rich organometallic complexes─comprise a class of highly reactive chemical reagents that find integral applications in organic synthesis, small-molecule activation, transient species stabilization, and M-E bond formation, among others. The inherent reactivity of such electron-rich metal centers has necessitated the widespread use of strong backbonding ligands, particularly carbonyls, to aid in the isolation and handling of metalate reagents, albeit sometimes at the expense of partially masking their full reactivity. However, recent synthetic explorations into transition-metalate complexes devoid of archetypic back-bonding ligands have led to the discovery of highly reactive metalates capable of performing a variety of novel chemical transformations.Building on our group's long-standing interest in reactive organometallic species, a series of rational progressions in early-to-middle transition-metal chemistry ultimately led to our isolation of a rhenium(I) β-diketiminate cyclopentadienide metalate that displays exceptional reactivity. We have found this Re(I) metalate to be capable of small-molecule activation; notably, the complex reversibly binds dinitrogen in solution and can be utilized to trap N2 for the synthesis of functionalized diazenido species. By employing isolobal analogues to N2 (CO and RNC), we were able to thoroughly monitor the mechanism of activation and conclude that the metalate's sodium counterion plays an integral role in promoting dinitrogen activation through a novel side-on interaction. The Re(I) metalate is also used in forming a variety of M-E bonds, including a series of uncommon rhenium-tetrylene (Si, Ge, and Sn) complexes that display varying degrees of multiple bonding. These metal tetrylenes act to highlight deviations in chemical properties within the group 14 elements. Our metalate's utility also applies to metal-metal bond formation, as demonstrated through the synthesis of a heterotetrametallic rhenium-zinc dimer. In this reaction, the Re(I) metalate performs a dual role as a reductant and metalloligand to stabilize a transient Zn22+ core fragment. Finally, the metalate displays unique reactivity with uranium(III) to yield the first transition metal-actinide inverse-sandwich bonds, in this case with three rhenium fragments bound through their Cp moieties surrounding the uranium center. Notably, throughout these endeavors we demonstrate that the metalate displays reactivity at multiple locations, including directly at the rhenium metal center, at a Cp carbon, through a Cp-sandwich mode, or through reversibly bound dinitrogen.Overall, the rhenium(I) metalate described herein demonstrates utility in diverse applications: small-molecule activation, the stabilization of reduced and/or unstable species, and the formation of unconventional M-E/M-M bonds or heterometallic complexes. Moving forward, we suggest that the continued discovery of noncarbonyl, electron-rich transition-metal anions featuring new or unconventional ligands should produce additional reactive organometallic species capable of stabilizing unique structural motifs and performing novel and unusual chemical transformations.

Three-centre electron sharing indices (3c-ESIs) as a tool to differentiate among (an)agostic interactions and hydrogen bonds in transition metal complexes.

The agostic bond plays an important role in chemistry, not only in transition metal chemistry but also in main group chemistry. In some complexes with M⋯H-X (X = C, N) interactions, differentiation among agostic, anagostic, and hydrogen bonds is challenging. Here we propose the use of three-centre electron sharing indices to classify M⋯H-X (X = C, N) interactions.

Electronic structure of strongly correlated systems: recent developments in multiconfiguration pair-density functional theory and multiconfiguration nonclassical-energy functional theory.

Strong electron correlation plays an important role in transition-metal and heavy-metal chemistry, magnetic molecules, bond breaking, biradicals, excited states, and many functional materials, but it provides a significant challenge for modern electronic structure theory. The treatment of strongly correlated systems usually requires a multireference method to adequately describe spin densities and near-degeneracy correlation. However, quantitative computation of dynamic correlation with multireference wave functions is often difficult or impractical. Multiconfiguration pair-density functional theory (MC-PDFT) provides a way to blend multiconfiguration wave function theory and density functional theory to quantitatively treat both near-degeneracy correlation and dynamic correlation in strongly correlated systems; it is more affordable than multireference perturbation theory, multireference configuration interaction, or multireference coupled cluster theory and more accurate for many properties than Kohn–Sham density functional theory. This perspective article provides a brief introduction to strongly correlated systems and previously reviewed progress on MC-PDFT followed by a discussion of several recent developments and applications of MC-PDFT and related methods, including localized-active-space MC-PDFT, generalized active-space MC-PDFT, density-matrix-renormalization-group MC-PDFT, hybrid MC-PDFT, multistate MC-PDFT, spin–orbit coupling, analytic gradients, and dipole moments. We also review the more recently introduced multiconfiguration nonclassical-energy functional theory (MC-NEFT), which is like MC-PDFT but allows for other ingredients in the nonclassical-energy functional. We discuss two new kinds of MC-NEFT methods, namely multiconfiguration density coherence functional theory and machine-learned functionals.

The synthetic versatility of fluoroiodomethane: recent applications as monofluoromethylation platform.

In recent years, fluoroiodomethane (CH2FI) has emerged as an easy-to-handle, non-ozone depleting agent and readily available platform for monofluoromethylation strategies. Recent applications in nucleophilic substitutions, lithiation reactions, transition-metal catalyzed transformations, radical processes, and 18F-radiolabelling chemistry showcase the potential of this reagent for the preparation of organofluorine compounds. In this minireview, we provide an update to the field covering the recent relevant literature on the use of CH2FI.

Study of Hydrazine Carboxamide, Characterization Method and Biological Activities with Transition Metal Using Cu (II) and Mn (II) as Central Metal

The co-ordination chemistry of transition metals and their derivatives has received much attention in the recent years, primarily because of their biological importance1-3, Nitrogen, Oxygen and Sulphur donor ligands possess a range of biological applications like antitomour4, antiviral5, antimaterial7 and antifungal activities. The legends of Hydrazine with their metal complexes with Cu (II) and Mn (II). Fungicidal activity of the ligand and their metal complexes were also done to establish any relation with the metal and ligands.