Modern applications of low-valent early transition metals in synthesis and catalysis

Mechanistic study of pyrazole synthesis via oxidation-induced N-N coupling of diazatitanacycles

Metal-mediated inner-sphere N-N coupling is an uncommon route to N-N bond formation. Herein, we report a mechanistic study of pyrazole formation via oxidation-induced N-N coupling of diazatitanacycles. In TEMPO oxidation reactions, the first of two oxidations is rate limiting and TEMPO coordination to Ti is critical for reactivity. In oxidations with Fc+ salts, coordinating counteranions such (eg. Cl-) aid an "inner-sphere-like" oxidation.

Applications of low-valent compounds with heavy group-14 elements

Over the last two decades, the low-valent compounds of group-14 elements have received significant attention in several fields of chemistry owing to their unique electronic properties. The low-valent group-14 species include tetrylenes, tetryliumylidene, tetrylones, dimetallenes and dimetallynes. These low-valent group-14 species have shown applications in various areas such as organic transformations (hydroboration, cyanosilylation, N-functionalisation of amines, and hydroamination), small molecule activation (e.g. P4, As4, CO2, CO, H2, alkene, and alkyne) and materials. This review presents an in-depth discussion on low-valent group-14 species-catalyzed reactions, including polymerization of rac-lactide, L-lactide, DL-lactide, and caprolactone, followed by their photophysical properties (phosphorescence and fluorescence), thin film deposition (atomic layer deposition and vapor phase deposition), and medicinal applications. This review concisely summarizes current developments of low-valent heavier group-14 compounds, covering synthetic methodologies, structural aspects, and their applications in various fields of chemistry. Finally, their opportunities and challenges are examined and emphasized.

A Mixed-Valence Ti(II)/Ti(III) Inverted Sandwich Compound as a Regioselective Catalyst for the Uncommon 1,3,5-Alkyne Cyclotrimerization

The synthesis, structure, and catalytic activity of a Ti(II)/Ti(III) inverted sandwich compound are presented in this study. Synthesis of the arene-bridged dititanium compound begins with the preparation of the titanium(IV) precursor [TiCl2(MesPDA)(thf)2] (MesPDA = N,N'-bis(2,4,6-trimethylphenyl)-o-phenylenediamide) (2). The reduction of 2 with sodium metal results in species [{Ti(MesPDA)(thf)}2(μ-Cl)3{Na}] (3) in oxidation state III. To achieve the lower oxidation state II, 2 undergoes reduction through alkylation with lithium cyclopentyl. This alkylation approach triggers a cascade of reactions, including β-hydride abstraction/elimination, hydrogen evolution, and chemical reduction, to generate the Ti(II)/Ti(III) compound [Li(thf)4][(TiMesPDA)2(μ-η6: η6-C6H6)] (4). X-ray and EPR characterization confirms the mixed-valence states of the titanium species. Compound 4 catalyzes a mild, efficient, and regiospecific cyclotrimerization of alkynes to form 1,3,5-substituted arenes. Kinetic data support a mechanism involving a binuclear titanium arene compound, similar to compound 4, as the resting state. The active catalyst promotes the oxidative coupling of two alkynes in the rate-limiting step, followed by a rapid [4 + 2] cycloaddition to form the arene product. Computational analysis of the resting state for the cycloaddition of trimethylsilylacetylene indicates a thermodynamic preference for stabilizing the 1,3,5-arene within the space between the two [TiMesPDA] fragments, consistent with the observed regioselectivity.

Silica-based monoliths functionalized with DTPA for the removal of transition and lanthanide ions from aqueous solutions

Transition and rare earth metals serve as indispensable raw materials across a broad spectrum of technological applications. However, their utilization is frequently linked to substantial waste production. Consequently, the recycling and recovery of these metals from end-of-life products or metal-contaminated aqueous environments hold significant importance within the framework of a circular economy. In our investigation, we employed synthetic mesoporous silica monoliths, synthesized via the sol-gel method and functionalized with chelating groups, for the efficient recovery of metal ions from aqueous matrices. The monoliths were characterized using a multi-technique approach and were tested in the recovery of paramagnetic Gd3+, Cu2+ and Co2+ ions from aqueous solutions, using 1H-NMR relaxometry to evaluate their uptake performance in real time and under simple conditions. Detailed information on the kinetics of the capture process was also highlighted. Finally, the possibility to regenerate the solid sorbents was evaluated.

On Haptotropic Rearrangements of Diphosphene and Diarsene Ligands in Titanium Complexes

Dipnictenes of the type RE=ER (E=P, As, Sb, Bi) are the isovalence electronic heavier analogs of alkenes. Although diphosphenes and dipnictenes in general show a variety of binding modes in metal complexes, little is known about haptotropic shift reactions involving these ligands. Herein, we report an unprecedented η2 to η1 rearrangement of the dipnictene ligands in titanocene complexes of the type Cp2Ti(Pn2Ar2) (Pn=P, As; Ar=2,4,6-Me3-C6H2, Mes; 2,6-iPr2-C6H3, Dip; 2,4,6-iPr3-C6H2, Tip), initiated by Lewis basic ligands (L=MeCN, PMe3, AdNC, CO). In the presence of L the dipnictene ligand changes its hapticity from η2 to η1 and complexes of the general form Cp2Ti(L)(Pn2Ar2) with a succinctly different electronic structure are obtained. Electronically, the new complexes are best described as biradicaloids with antiferromagnetically coupled (via a π-bond) [Cp2TiIII]⋅+ and [Pn2Ar2]⋅- fragments. However, the biradical character of these systems is affected by the electronic features of the co-ligand and significantly decreases moving from PMe3/MeCN (σ-donors) to CNAd/CO (σ-donors/π-acceptors).

Masked Reactivity of Hydrated Clusters of Monovalent Manganese Ions: Water Insertion versus Nitrous Oxide Activation-A Density Functional Theory Investigation

Previous mass spectrometric (MS) studies demonstrated that singly charged hydration clusters of manganese ions [Mn(H2O)n]+ were, on one hand, highly reactive toward intracluster water insertion but, on the other hand, inert toward nitrous oxide activation. This contrast in reactivity has been rationalized by our present theoretical investigation for the interconversion between the pristine Mn(I) monovalent form as a monatomic ion in [MnI(H2O)n]+ and the oxidized Mn(III) trivalent form as a hydride-hydroxide in [HMnIIIOH(H2O)n-1], as well as their reactivity toward nitrous oxide activation. Our theoretical interpretations are supported with quantum chemical calculations based on density functional theory (DFT), performed systematically for the cluster-size range of n = 1 - 12. Our DFT results show that water insertion is kinetically and thermodynamically favorable for n ≥ 8, suggesting [HMnIIIOH(H2O)n-1]+ is the predominant form, as observed in previous MS experiments. While [MnI(H2O)n]+ is capable of N2O reduction, the process of which is highly exothermic, similar reactions are unfavorable with [HMnIIIOH(H2O)n-1]+, which can only form weakly bound adducts with N2O. This work demonstrates the masking effect of water molecules over the high reactivity of the hydrated Mn(I) center and sheds light on the potential roles of water in transition metal systems.

Cinchona-Based Hydrogen-Bond Donor Organocatalyst Metal Complexes: Asymmetric Catalysis and Structure Determination

In this study, we describe the synthesis of cinchona (thio)squaramide and a novel cinchona thiourea organocatalyst. These catalysts were employed in pharmaceutically relevant catalytic asymmetric reactions, such as Michael, Friedel-Crafts, and A3 coupling reactions, in combination with Ag(I), Cu(II), and Ni(II) salts. We identified several organocatalyst-metal salt combinations that led to a significant increase in both yield and enantioselectivity. To gain insight into the active catalyst species, we prepared organocatalyst-metal complexes and characterized them using HRMS, NMR spectroscopy, and quantum chemical calculations (B3LYP-D4/def2-TZVP), which allowed us to establish a structure-activity relationship.

Low-Valent Transition Metalate Anions in Synthesis, Small Molecule Activation, and Catalysis

This review surveys the synthesis and reactivity of low-oxidation state metalate anions of the d-block elements, with an emphasis on contributions reported between 2006 and 2022. Although the field has a long and rich history, the chemistry of transition metalate anions has been greatly enhanced in the last 15 years by the application of advanced concepts in complex synthesis and ligand design. In recent years, the potential of highly reactive metalate complexes in the fields of small molecule activation and homogeneous catalysis has become increasingly evident. Consequently, exciting applications in small molecule activation have been developed, including in catalytic transformations. This article intends to guide the reader through the fascinating world of low-valent transition metalates. The first part of the review describes the synthesis and reactivity of d-block metalates stabilized by an assortment of ligand frameworks, including carbonyls, isocyanides, alkenes and polyarenes, phosphines and phosphorus heterocycles, amides, and redox-active nitrogen-based ligands. Thereby, the reader will be familiarized with the impact of different ligand types on the physical and chemical properties of metalates. In addition, ion-pairing interactions and metal-metal bonding may have a dramatic influence on metalate structures and reactivities. The complex ramifications of these effects are examined in a separate section. The second part of the review is devoted to the reactivity of the metalates toward small inorganic molecules such as H2, N2, CO, CO2, P4 and related species. It is shown that the use of highly electron-rich and reactive metalates in small molecule activation translates into impressive catalytic properties in the hydrogenation of organic molecules and the reduction of N2, CO, and CO2. The results discussed in this review illustrate that the potential of transition metalate anions is increasingly being tapped for challenging catalytic processes with relevance to organic synthesis and energy conversion. Therefore, it is hoped that this review will serve as a useful resource to inspire further developments in this dynamic research field.

A Chiral Titanocene Complex as Regiodivergent Photoredox Catalyst: Synthetic Scope and Mechanism of Catalyst Generation

We describe a combined synthetic, spectroscopic, and computational study of a chiral titanocene complex as a regiodivergent photoredox catalyst (PRC). With Kagan's complex catCl2 either monoprotected 1,3-diols or 1,4-diols can be obtained in high selectivity from a common epoxide substrate in a regiodivergent epoxide opening depending on which enantiomer of the catalyst is employed. Due to the catalyst-controlled regioselectivity of ring opening and the broader substrate scope, the PRC with catCl2 is also a highly attractive branching point for diversity-oriented synthesis. The photochemical processes of cat(NCS)2, a suitable model for catCl2, were probed by time-correlated single-photon counting. The photoexcited complex displays a thermally activated delayed fluorescence as a result of a singlet-triplet equilibration, S1 ⇄ T1, via intersystem crossing and recrossing. Its triplet state is quenched by electron transfer to the T1 state. Computational and cyclic voltammetry studies highlight the importance of our sulfonamide additive. By bonding to sulfonamide additives, chloride abstraction from [catCl2]- is facilitated, and catalyst deactivation by coordination of the sulfonamide group is circumvented.

Phosphinoquinoline supported CoII, NiII, and FeII complexes: divergent behaviour upon reduction

The reduction of [CoLBr2], a CoII complex supported by a diisopropylphosphinoquinoline (L) ligand, induced a ligand coupling giving access to a (PNNP) supported CoII complex which was isolated in 70% yield. This complex was formed using a minimum of 2 equivalents of a reductant (either Mn or KC8). The fate of [CoLBr2] in the presence of 1 equivalent of a reductant was more difficult to study; nevertheless, a CoI complex was characterised in the solid state. In order to determine whether this ligand coupling could occur with other 3d metals, L supported FeII and NiII complexes were synthesised. While no compound could be identified upon reduction of [FeLBr2], both [NiLBr2] and [NiL2Br](Br) led to the reduction at the metal center allowing the isolation of an original Ni0 trimer in a satisfactory yield. This study shows the different behaviours of these 3d metal complexes in the presence of a reductant.

Cooperation towards nobility: equipping first-row transition metals with an aluminium sword

The exploration for noble metals substitutes in catalysis has become a highly active area of research, driven by the pursuit of sustainable chemical processes. Although the utilization of base metals holds great potential as an alternative, their successful implementation in predictable catalytic processes necessitates the development of appropriate ligands. Such ligands must be capable of controlling their intricate redox chemistry and promote two-electron events, thus mimicking well-established organometallic processes in noble metal catalysis. While numerous approaches for infusing nobility to base metals have been explored, metal-ligand cooperation has garnered significant attention in recent years. Within this context, aluminium-based ligands offer interesting features to fine-tune the activity of metal centres, but their application in base metal catalysis remains largely unexplored. This perspective seeks to highlight the most recent breakthroughs in the reactivity of heterobimetallic aluminium-base-metal complexes, while also showcasing their potential to develop novel and predictable catalytic transformations. By turning the spotlight on such heterobimetallic species, we aim to inspire chemists to explore aluminium-base-metal species and expand the range of their applications as catalysts.

Reactive X (where X = O, N, S, C, Cl, Br, and I) species nanomedicine

Reactive oxygen, nitrogen, sulfur, carbonyl, chlorine, bromine, and iodine species (RXS, where X = O, N, S, C, Cl, Br, and I) have important roles in various normal physiological processes and act as essential regulators of cell metabolism; their inherent biological activities govern cell signaling, immune balance, and tissue homeostasis. However, an imbalance between RXS production and consumption will induce the occurrence and development of various diseases. Due to the considerable progress of nanomedicine, a variety of nanosystems that can regulate RXS has been rationally designed and engineered for restoring RXS balance to halt the pathological processes of different diseases. The invention of radical-regulating nanomaterials creates the possibility of intriguing projects for disease treatment and promotes advances in nanomedicine. In this comprehensive review, we summarize, discuss, and highlight very-recent advances in RXS-based nanomedicine for versatile disease treatments. This review particularly focuses on the types and pathological effects of these reactive species and explores the biological effects of RXS-based nanomaterials, accompanied by a discussion and the outlook of the challenges faced and future clinical translations of RXS nanomedicines.

Ti(III) Catalysts for CO2/Epoxide Copolymerization at Unusual Ambient Pressure Conditions

Titanium compounds in low oxidation states are highly reducing species and hence powerful tools for the functionalization of small molecules. However, their potential has not yet been fully realized because harnessing these highly reactive complexes for productive reactivity is generally challenging. Advancing this field, herein we provide a detailed route for the formation of titanium(III) orthophenylendiamido (PDA) species using [LiBHEt3] as a reducing agent. Initially, the corresponding lithium PDA compounds [Li2(ArPDA)(thf)3] (Ar = 2,4,6-trimethylphenyl (MesPDA), 2,6-diisopropylphenyl (iPrPDA)) are combined with [TiCl4(thf)2] to form the heterobimetallic complexes [{TiCl(ArPDA)}(μ-ArPDA){Li(thf)n}] (n = 1, Ar = iPr 3 and n = 2, Ar = Mes 4). Compound 4 evolves to species [Ti(MesPDA)2] (6) via thermal treatment. In contrast, the transformation of 3 into [Ti(iPrPDA)2] (5) only occurs in the presence of [LiNMe2], through a lithium-assisted process, as revealed by density functional theory (DFT). Finally, the Ti(IV) compounds 3-6 react with [LiBHEt3] to give rise to the Ti(III) species [Li(thf)4][Ti(ArPDA)2] (Ar = iPr 8, Mes 9). These low-valent compounds in combination with [PPN]Cl (PPN = bis(triphenylphosphine)iminium) are proved to be highly selective catalysts for the copolymerization of CO2 and cyclohexene epoxide. Reactions occur at 1 bar pressure with activity/selectivity levels similar to Salen-Cr(III) compounds.

Coupling Titanium and Chromium Catalysis in a Reaction Network for the Reprogramming of [BH4 ]- as Electron Transfer and Hydrogen Atom Transfer Reagent for Radical Chemistry

We describe a unique catalytic system with an efficient coupling of Ti- and Cr-catalysis in a reaction network that allows the use of [BH4 ]- as stoichiometric hydrogen atom and electron donor in catalytic radical chemistry. The key feature is a relay hydrogen atom transfer from [BH4 ]- to Cr generating the active catalysts under mild conditions. This enables epoxide reductions, regiodivergent epoxide opening and radical cyclizations that are not possible with cooperative catalysis with radicals or by epoxide reductions via Meinwald rearrangement and ensuing carbonyl reduction. No typical SN 2-type reactivity of [BH4 ]- salts is observed.

Observing the Entry Events of a Titanium-Based Photoredox Catalytic Cycle in Real Time

Titanium-based catalysis in single electron transfer (SET) steps has evolved into a versatile approach for the synthesis of fine chemicals and first attempts have recently been made to enhance its sustainability by merging it with photo-redox (PR) catalysis. Here, we explore the photochemical principles of all-Ti-based SET-PR-catalysis, i.e. in the absence of a precious metal PR-co-catalyst. By combining time-resolved emission with ultraviolet-pump/mid-infrared-probe (UV/MIR) spectroscopy on femtosecond-to-microsecond time scales we quantify the dynamics of the critical events of entry into the catalytic cycle; namely, the singlet-triplet interconversion of the do-it-all titanocene(IV) PR-catalyst and its one-electron reduction by a sacrificial amine electron donor. The results highlight the importance of the PR-catalyst's singlet-triplet gap as a design guide for future improvements.

Ti-Catalyzed Modular Ketone Synthesis from Carboxylic Derivatives and gem-Dihaloalkanes

Ketones are ubiquitous in organic synthesis. However, the general method to convert widely available carboxylic acids, unactivated esters, and amides into ketones remains elusive. Herein, we describe the Ti-catalyzed modular ketone synthesis from carboxylic derivatives and easily accessed gem-dihaloalkanes. Notably, this protocol could achieve the direct catalytic olefination of carboxylic acids. This method features a sequence of olefination and electrophilic transformation and good functional group compatibility and allows rapid access to various functionalized ketones. Preliminary mechanistic studies provide insights into the reaction pathway and support the intermediacy of plausible alkylidene titanocene and gem-bimetallic complexes.

Reductive Hydrogenation of Sulfido-Bridged Tantalum Alkyl Complexes: A Mechanistic Insight

Hydrogenolysis of a series of alkyl sulfido-bridged tantalum(IV) dinuclear complexes [Ta(η⁵-C₅Me₅)R(μ-S)]₂ [R = Me, nBu (1), Et, CH₂SiMe₃, C₃H₅, Ph, CH₂Ph (2), p-MeC₆H₄CH₂ (3)] has led quantitatively to the Ta(III) tetrametallic sulfide cluster [Ta(η⁵-C₅Me₅)(μ₃-S)]₄ (4) along with the corresponding alkane. Mechanistic information for the formation of the unique low-valent tetrametallic compound 4 was gathered by hydrogenation of the phenyl-substituted precursor [Ta(η⁵-C₅Me₅)Ph(μ-S)]₂, which proceeds through a stepwise hydrogenation process, disclosing the formation of the intermediate tetranuclear hydride sulfide [Ta₂(η⁵-C₅Me₅)₂(H)Ph(μ-S)(μ₃-S)]₂ (5). Extending our studies toward tantalum alkyl precursors containing functional groups susceptible to hydrogenation, such as the allyl-and benzyl-substituted compounds [Ta(η⁵-C₅Me₅)(η³-C₃H₅)(μ-S)]₂ and [Ta(η⁵-C₅Me₅)(CH₂Ph)(μ-S)]₂ (2), enables alternative reaction pathways en route to the formation of 4. In the former case, the dimetallic system undergoes selective hydrogenation of the unsaturated allyl moiety, forming the asymmetric complex [{Ta(η⁵-C₅Me₅)(η³-C₃H₅)}(μ-S)₂{Ta(η⁵-C₅Me₅)(C₃H₇)}] (6) with only one propyl fragment. Species 2, in addition to the hydrogenation of one benzyl fragment and concomitant toluene release, also undergoes partial hydrogenation and dearomatization of the phenyl ring on the vicinal benzyl unity to give a η⁵-cyclohexadienyl complex [Ta₂(η⁵-C₅Me₅)₂(μ-CH₂C₆H₆)(μ-S)₂] (7). The mechanistic implications of the latter hydrogenation process are discussed by means of DFT calculations.

Direct Amination of α-Triaryl Alcohols via Vanadium Catalysis

α-Triaryl amines have been used as pharmaceuticals and pharmaceutical intermediates for antifungal and anticancer applications. Current methods to synthesize such compounds require at least two steps, and no direct amination of tertiary alcohols has been reported. Herein, we disclose efficient catalytic conditions for the direct amination of α-triaryl alcohols to access α-triaryl amines. VO(OⁱPr)₃, a commercially available reagent, has been identified as an effective catalyst for the direct amination of several α-triaryl alcohols. This process is scalable, as demonstrated by a gram-scale synthesis, and the reaction still works at as low as a 0.01 mol % catalyst loading with the turnover number reaching 3900. Moreover, commercial pharmaceuticals including clotrimazole and flutrimazole have been successfully prepared rapidly and efficiently using this newly developed method.

Synthesizing Cr-Based Two-Dimensional Conjugated Metal-Organic Framework Through On-Surface Substitution Reaction

A single-layer Cr₃ (HITP)₂ (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) conjugated metal-organic framework (c-MOF) is synthesized under ultrahigh vacuum conditions by substituting Cr for Ni in Ni₃ (HITP)₂ template. As revealed by low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy, while codeposition of Cr atoms and 2,3,6,7,10,11-hexaaminotriphenylene precursors produces irregular branches, crystalline Cr₃ (HITP)₂ frameworks are obtained by depositing Cr atoms to the Ni₃ (HITP)₂ templates. The density functional theory calculations reveal that the binding energy between Cr and HITP ligands is much higher than that for Ni, which hampers the growth of crystalline Cr₃ (HITP)₂ frameworks through direct coordination assembly but makes the substitution reaction energetically favorable. This work demonstrates a new strategy to prepare high-quality early-transition-metal-based c-MOFs under ultrahigh vacuum conditions.

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.

Molecular Capacitors: Accessible 6- and 8-Electron Redox Chemistry from Dimeric "Ti(I)" and "Ti(0)" Synthons Supported by Imidazolin-2-Iminato Ligands

Reduction of the diamagnetic Ti(III)/Ti(III) dimer [Cl₂Ti(μ-NIm^Dipp)]₂ (1) (NIm^Dipp = [1,3-bis(Dipp)imidazolin-2-iminato]^-, Dipp = C₆H₃-2,6-ⁱPr₂) with 4 and 6 equiv of KC₈ generates the intramolecularly arene-masked, dinuclear titanium compounds [(μ-N-η⁶-Im^Dipp)Ti]₂ (2) and {[(Et₂O)₂K](μ-N-μ-η⁶:η⁶-Im^Dipp)Ti}₂ (3), respectively, in modest yields. The compounds have been structurally characterized by X-ray crystallographic analysis, and inspection of the bond metrics within the η⁶-coordinated aryl substituent of the bridging imidazolin-2-iminato ligand shows perturbation of the aromatic system most consistent with two-electron reduction of the ring. As such, 2 and 3 can be assigned respectively as possessing metal centers in formal Ti(III)/Ti(III) and Ti(II)/Ti(II) oxidation states. Exploration of their redox chemistry reveal the ability to reduce several substrate equivalents. For instance, treatment of 2 with excess C₈H₈ (COT) forms the novel COT-bridged complex [(Im^DippN)(η⁸-COT)Ti](μ-η²:η³-COT)[Ti(η⁴-COT)(NIm^Dipp)] (4) that dissociates in THF solutions to give mononuclear (Im^DippN)Ti(η⁸-COT)(THF) (5). Addition of COT to 3 yields heterometallic [(Im^DippN)(η⁴-COT)Ti(μ-η⁴:η⁵-COT)K(THF)(μ-η⁶:η⁴-COT)Ti(NIm^Dipp)(μ-η⁴:η⁴-COT)K(THF)₂]_n (6). Compounds 4 and 5 are the products of the 4-electron oxidation of 2, while 6 stands as the 8-electron oxidation product of 3. Reduction of organozides was also explored. Low temperature reaction of 2 with 4 equiv of AdN₃ gives the terminal and bridged imido complex [(Im^DippN)Ti(═NAd)](μ-NAd)₂[Ti(NIm^Dipp)(N₃Ad)] (7) that undergoes intermolecular C-H activation of toluene at room temperature to afford the amido compound [(Im^DippN)Ti(NHAd)](μ-NAd)₂[Ti(C₆H₄Me)(NIm^Dipp)] (8-tol). These complexes are the 6-electron oxidation products of the reaction of 2 with AdN₃. Furthermore, treatment of 3 with 4 equiv of AdN₃ produces the thermally stable Ti(III)/Ti(III) terminal and bridged imido [K(18-crown-6)(THF)₂]{[(Im^DippN)Ti(NAd)](μ-NAd)₂K[Ti(NIm^Dipp)]} (10). Altogether, these reactions firmly establish 2 and 3 as unprecedented Ti(I)/Ti(I) and Ti(0)/Ti(0) synthons with the clear capacity to effect multielectron reductions ranging from 4 to 8 electrons.

Nitrile regio-synthesis by Ni centers on a siliceous surface: implications in prebiotic chemistry

By means of quantum chemistry (PBE0/def2-TZVPP; DLPNO-CCSD(T)/cc-pVTZ) and small, but reliable models of Polyhedral Oligomeric Silsesquioxanes (POSS), an array of astrochemically-relevant catalysis products, related to prebiotic and origin of life chemistry, has been theoretically explored. In this work, the heterogeneous phase hydrocyanation reaction of an unsaturated CC bond (propene) catalyzed by a Ni center complexed to a silica surface is analyzed. Of the two possible regioisomers, the branched iso-propyl-cyanide is thermodynamically and kinetically preferred over the linear n-propyl-cyanide (T = 200 K). The formation of nitriles based on a regioselective process has profound implications on prebiotic and origin of life chemistry, as well as deep connections to terrestrial surface chemistry and geochemistry.

A Borane Lewis Acid in the Secondary Coordination Sphere of a Ni(II) Imido Imparts Distinct C-H Activation Selectivity

Two borane-functionalized bidentate phosphine ligands that vary in tether length have been prepared to examine cooperative metal-substrate interactions. Ni(0) complexes react with aryl azides at low temperatures to form structurally unusual κ2-(N,N)-N3Ar adducts. Warming these adducts affords products of N2 extrusion and in one case, a Ni-imido compound that is capped by the appended borane. Reactions with 1-azidoadamantane (AdN3) provide a distinct outcome, where a proposed nickel imido intermediate activates the sp2 C-H bonds of arenes, even in the presence of benzylic C-H sites. Combined experimental and computational mechanistic studies demonstrate that the unique reactivity is a consequence of Lewis-acid-induced polarization of the Ni-NR bond, potentially providing a synthetic strategy for chemoselective reaction engineering.

Synthesis and reactivity of dinuclear copper(I) pyridine diimine complexes

The reaction of a tethered pyridine diimine (PDI) ligand with copper(I) chloride yielded a μ-chlorido bridged cationic dicopper(I) PDI complex, which is a rare structural motif. The geometric constraint of the ligand is fostering attractive van der Waals interactions between the coplanar pyridine units. This is supported by an Atoms in Molecules (AIM) and NCI (non-covalent interaction) analysis. Reaction with carbon monoxide yields the corresponding mono- and dicarbonyl complexes, which display reversible binding of carbon monoxide. This equilibrium was studied by ¹³C-NMR exchange spectroscopy and complemented by DFT and LNO-CCSD(T) calculations.

Direct and Selective Electrocarboxylation of Styrene Oxides with CO2 for Accessing β-Hydroxy Acids

Highly selective and direct electroreductive ring-opening carboxylation of epoxides with CO2 in an undivided cell is reported. This reaction shows broad substrate scopes within styrene oxides under mild conditions, providing practical and scalable access to important synthetic intermediate β-hydroxy acids. Mechanistic studies show that CO2 functions not only as a carboxylative reagent in this reaction but also as a promoter to enable efficient and chemoselective transformation of epoxides under additive-free electrochemical conditions. Cathodically generated α-radical and α-carbanion intermediates lead to the regioselective formation of α-carboxylation products.

Engendering reactivity at group 5-heteroatom multiple bonds via π-loading

In this Perspective, we discuss the strategy of π-loading, i.e., coordination of two or more strongly π-donating ligands to a single metal center, as it applies to promoting reactivity at group 5 transition metal-imido groups. When multiple π-donor ligands compete to interact with the same symmetrically-available metal dπ orbitals, the energy of the imido-based frontier molecular orbitals increases, leading to amplified imido-based reactivity. This strategy is of particular relevance to group 5 metals, as mono(imido) complexes of these metals tend to be inert at the imido group. Electronic structure studies of group 5 bis(imido) complexes are presented, and examples of catalytically and stoichiometrically active group 5 bis(imido) and chalcogenido-imido complexes are reviewed. These examples are intended to encourage future work exploring π-loaded bis(imido) systems of the group 5 triad.

Cu-Catalyzed Chan-Evans-Lam Coupling reactions of 2-Nitroimidazole with Aryl boronic acids: An effort toward new bioactive agents against S. pneumoniae

The coupling of phenylboronic acids with poorly-activated imidazoles is studied as a model system to explore the use of copper-catalyzed Chan-Evans-Lam (CEL) coupling for targeted C-N bond forming reactions. Optimized CEL reaction conditions are reported for four phenanthroline-based ligand systems, where the ligand 4,5-diazafluoren-9-one (dafo, L2 ) with 1 molar equivalent of potassium carbonate yielded the highest reactivity. The substrate 2-Nitroimidazole (also known as azomycin) has documented antimicrobial activity against a range of microbes. Here N-arylation of 2-nitroimidazole with a range of aryl boronic acids has been successfully developed by copper(II)-catalyzed CEL reactions. Azomycin and a range of newly arylated azomycin derivatives were screened against S. pneumoniae , where 1-(4-(benzyloxy)phenyl)-2-nitro-1H-imidazole ( 3d ) was demonstrated to have a minimal inhibition concentration value of 3.3 μg/mL.

Ti-Catalyzed Diastereoselective Cyclopropanation of Carboxylic Derivatives with Terminal Olefins

Cyclopropanols and cyclopropylamines not only serve as important structural motifs in medicinal chemistry but also show diverse reactivities in organic synthesis. Owing to the high ring strain energy, the development of a general protocol from stable and readily available starting materials to afford these cyclopropyl derivatives remains a compelling challenge. Herein, we describe that a Ti-based catalyst can effectively promote the diastereoselective syntheses of cyclopropanols and cyclopropylamines from widely accessible carboxylic derivatives (acids, esters, amides) with terminal olefins. To the best of our knowledge, this method represents the first example of direct converting alkyl carboxylic acids into cyclopropanols. Distinct from conventional studies in Ti-mediated cyclopropanations with reactive alkyl Grignard reagents as nucleophiles or reductants, this protocol utilizes Mg and Me₂SiCl₂ to turn over the Ti catalyst. Our method exhibits broad substrate scope with good functional group compatibility and is amenable to late-stage synthetic manipulations of natural products and biologically active molecules.

Ultrasensitive electrochemical detection of furazolidone in biological samples using 1D-2D BiVO4@MoS2 hierarchical nano-heterojunction composites armed electrodes

In this work, the hydrothermally synthesized of BiVO₄@MoS₂ hierarchical nano-heterojunction composite is employed as a novel electrocatalyst for electrochemical sensing of Furazolidone (FZE) drug by modifying the glassy carbon electrodes (GCE). The Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy are used to thoroughly investigate the functional groups, vibrational modes, crystal structure, elemental composition and surface topography of the heterojunction composite. The physical characterization results revealed the successful construction of 1D-2D BiVO₄@MoS₂ hierarchical nano-heterojunction composite. When these unique architectures are reinforced on GCE surface, we achieved an enhanced electroactive surface area of 0.154 cm². The electrochemical performance of 1D-2D BiVO₄@MoS₂ is examined though cyclic voltammetry and differential pulse voltammetry (DPV) analysis. The BiVO₄@MoS₂ composites exhibited an excellent electrocatalytic activity in sensing of FZE with superior linear detection ranges of 0.01-14 and 14-614 μM. The limit of detection (LOD) of the BiVO₄@MoS₂ based sensor is determined to be 2.9 nM which is far superior than other reported FZE sensors. Consequently, it is evident from the investigation that the BiVO₄@MoS₂ based FZE sensor can be recommended for analyzing real time samples like human urine and blood serum with appreciable recovery.

Ruthenium(II) Phosphine/Picolylamine Dichloride Complexes Hydrogenation and DFT Calculations

Treating [Ru(PPh3)3Cl2] with the amine 2-picolylamine (Picam) ligand in a 1:1 molar ratio, the Ru(II) complex trans-Ru(PPh3)2(Picam)Cl2 (1) is obtained in methylene chloride and can be isolated as a pure solid compound. The single-crystal structure of 1 was determined by X-ray crystallography. The geometry at the Ru metal center is a distorted octahedral environment with a trans arrangement of the two chlorides. A trans effect of the bond lengths was observed within the structure. Similarly, treating [Ru(PPh3)3Cl2] with 1:1:1 molar ratios of 2-picolylamine (Picam) and 1,1′-bis(diphenylphosphine)ferrocene (DPPF) ligands yielded the Ru(II) complex trans-Ru(DPPF)(Picam)Cl2 (2). In identical conditions, the homogeneous hydrogen transfer catalytic reactivity of complexes 1 and 2 has been tested in a basic 2-propanol solution and they indicate different catalytic activity. It was discovered that monodentate and bidentate phosphine ligands of Ru(II) complexes, as well as cis- and trans-chloro configuration display different catalytic properties from our experimental data, in agreement with literature data. Based on DFT calculations, the relative molecular catalytic reactivity of all available experimental data is understood from the relative calculated molecular energy.

Spin-flip luminescence

In molecular photochemistry, charge-transfer emission is well understood and widely exploited. In contrast, luminescent metal-centered transitions only came into focus in recent years. This gave rise to strongly phosphorescent Cr^III complexes with a d³ electronic configuration featuring luminescent metal-centered excited states which are characterized by the flip of a single spin. These so-called spin-flip emitters possess unique properties and require different design strategies than traditional charge-transfer phosphors. In this review, we give a brief introduction to ligand field theory as a framework to understand this phenomenon and outline prerequisites for efficient spin-flip emission including ligand field strength, symmetry, intersystem crossing and common deactivation pathways using Cr^III complexes as instructive examples. The recent progress and associated challenges of tuning the energies of emissive excited states and of emerging applications of the unique photophysical properties of spin-flip emitters are discussed. Finally, we summarize the current state-of-the-art and challenges of spin-flip emitters beyond Cr^III with d², d³, d⁴ and d⁸ electronic configuration, where we mainly cover pseudooctahedral molecular complexes of V, Mo, W, Mn, Re and Ni, and highlight possible future research opportunities.