Catalytic, Spectroscopic, and Theoretical Studies of Fe4S4-Based Coordination Polymers as Heterogenous Coupled Proton-Electron Transfer Mediators for Electrocatalysis

Iron-sulfur clusters play essential roles in biological systems, and thus synthetic [Fe4S4] clusters have been an area of active research. Recent studies have demonstrated that soluble [Fe4S4] clusters can serve as net H atom transfer mediators, improving the activity and selectivity of a homogeneous Mn CO2 reduction catalyst. Here, we demonstrate that incorporating these [Fe4S4] clusters into a coordination polymer enables heterogeneous H atom transfer from an electrode surface to a Mn complex dissolved in solution. A previously reported solution-processable Fe4S4-based coordination polymer was successfully deposited on the surfaces of different electrodes. The coated electrodes serve as H atom transfer mediators to a soluble Mn CO2 reduction catalyst displaying good product selectivity for formic acid. Furthermore, these electrodes are recyclable with a minimal decrease in activity after multiple catalytic cycles. The heterogenization of the mediator also enables the characterization of solution-phase and electrode surface species separately. Surface enhanced infrared absorption spectroscopy (SEIRAS) reveals spectroscopic signatures for an in situ generated active Mn-H species, providing a more complete mechanistic picture for this system. The active species, reaction mechanism, and the protonation sites on the [Fe4S4] clusters were further confirmed by density functional theory calculations. The observed H atom transfer reactivity of these coordination polymer-coated electrodes motivates additional applications of this composite material in reductive H atom transfer electrocatalysis.

Combining Donor Strength and Oxidative Stability in Scorpionates: A Strongly Donating Fluorinated Mesoionic Tris(imidazol-5-ylidene)borate Ligand

Strongly donating scorpionate ligands support the study of high-valent transition metal chemistry; however, their use is frequently limited by oxidative degradation. To address this concern, we report the synthesis of a tris(imidazol-5-ylidene)borate ligand featuring trifluoromethyl groups surrounding its coordination pocket. This ligand represents the first example of a chelating poly(imidazol-5-ylidene) mesoionic carbene ligand, a scaffold that is expected to be extremely donating. The {NiNO}10 complex of this ligand, as well as that of a previously reported strongly donating tris(imidazol-2-ylidene)borate, has been synthesized and characterized. This new ligand's strong donor properties, as measured by the υNO of its {NiNO}10 complex and natural bonding orbital second-order perturbative energy analysis, are at par with those of the well-studied alkyl-substituted tris(imidazol-2-ylidene)borates, which are known to effectively stabilize high-valent intermediates. The good donor properties of this ligand, despite the electron-withdrawing trifluoromethyl substituents, arise from the strongly donating imidazol-5-ylidene mesoionic carbene arms. These donor properties, when combined with the robustness of trifluoromethyl groups toward oxidative decomposition, suggest this ligand scaffold will be a useful platform in the study of oxidizing high-valent transition-metal species.

Hybrid organic-inorganic two-dimensional metal carbide MXenes with amido- and imido-terminated surfaces

Two-dimensional (2D) transition-metal carbides and nitrides (MXenes) combine the electronic and mechanical properties of 2D inorganic crystals with chemically modifiable surfaces, which provides an ideal platform for both fundamental and applied studies of interfaces. Good progress has been achieved in the functionalization of MXenes with small inorganic ligands, but relatively little work has been reported on the covalent bonding of various organic groups to MXene surfaces. Here we synthesize a family of hybrid MXenes (h-MXenes) that incorporate amido- and imido-bonding between organic and inorganic parts by reacting halogen-terminated MXenes with deprotonated organic amines. The resulting hybrid structures unite tailorability of organic molecules with electronic connectivity and other properties of inorganic 2D materials. Describing the structure of h-MXene necessitates the integration of concepts from coordination chemistry, self-assembled monolayers and surface science. The optical properties of h-MXenes reveal coherent coupling between the organic and inorganic constituents. h-MXenes also exhibit superior stability against hydrolysis.

Multi-functional Single-Source Molecular Precursors for Carbon-Coated Mixed-Metal Phosphates

We introduce a new synthetic concept that can be broadly adopted for the low-temperature preparation of mixed-metal energy storage materials, such as phosphates, silicates, fluorides, fluorophosphates, and fluorosulfates that exhibit intrinsic low electronic conductivity and thus require a carbon modulation. The development of novel low-temperature approaches for assembling energy-related materials with a complex core-shell microstructure is of great importance for expanding their application scope. The traditional definition of single-source precursors refers to their ability to yield a phase-pure material upon thermal decomposition. We have developed a new way for the utilization of heterometallic molecular precursors in synthesis that goes beyond its common delineation as a single-phase maker. The utility of this approach has been demonstrated upon the low-temperature synthesis of lithium-iron phosphate@C, which represents a celebrated cathode material for Li-ion batteries. The first atomically precise carbonaceous molecular precursors featuring a desired Li:Fe:P ratio of 1:1:1, divalent iron, and sufficient oxygen content for the target LiFeIIPO4 phosphate were shown to enable a spontaneous formation of both the olivine core and conductive carbon shell, yielding a carbon-coated mixed-metal phosphate.

Broad Electronic Modulation of Two-Dimensional Metal-Organic Frameworks over Four Distinct Redox States

Two-dimensional (2D) inorganic materials have emerged as exciting platforms for (opto)electronic, thermoelectric, magnetic, and energy storage applications. However, electronic redox tuning of these materials can be difficult. Instead, 2D metal-organic frameworks (MOFs) offer the possibility of electronic tuning through stoichiometric redox changes, with several examples featuring one to two redox events per formula unit. Here, we demonstrate that this principle can be extended over a far greater span with the isolation of four discrete redox states in the 2D MOFs Li_xFe₃(THT)₂ (x = 0-3, THT = triphenylenehexathiol). This redox modulation results in 10,000-fold greater conductivity, p- to n-type carrier switching, and modulation of antiferromagnetic coupling. Physical characterization suggests that changes in carrier density drive these trends with relatively constant charge transport activation energies and mobilities. This series illustrates that 2D MOFs are uniquely redox flexible, making them an ideal materials platform for tunable and switchable applications.

Direct synthesis and chemical vapor deposition of 2D carbide and nitride MXenes

Two-dimensional transition-metal carbides and nitrides (MXenes) are a large family of materials actively studied for various applications, especially in the field of energy storage. MXenes are commonly synthesized by etching the layered ternary compounds, called MAX phases. We demonstrate a direct synthetic route for scalable and atom-economic synthesis of MXenes, including compounds that have not been synthesized from MAX phases, by the reactions of metals and metal halides with graphite, methane, or nitrogen. The direct synthesis enables chemical vapor deposition growth of MXene carpets and complex spherulite-like morphologies that form through buckling and release of MXene carpet to expose fresh surface for further reaction. The directly synthesized MXenes showed excellent energy storage capacity for lithium-ion intercalation.

Structure and Magnetic Properties of Pseudo-1D Chromium Thiolate Coordination Polymers

The synthesis, structure, and magnetic properties of two novel, pseudo-one-dimensional (1D) chromium thiolate coordination polymers (CPs), CrBTT and Cr₂BDT₃, are reported. The structures of these materials were determined using X-ray powder diffraction revealing highly symmetric 1D chains embedded within a CP framework. The magnetic coupling of this chain system was measured by SQUID magnetometry, revealing a switch from antiferromagnetic to ferromagnetic behavior dictated by the angular geometrical constraints within the CP scaffold consistent with the Goodenough-Kanamori-Anderson rules. Intrachain magnetic coupling constants J_NN of -32.0 and +5.7 K were found for CrBTT and Cr₂BDT₃, respectively, using the 1D Bonner-Fisher model of magnetism. The band structure of these materials has also been examined by optical spectroscopy and density functional theory calculations revealing semiconducting behavior. Our findings here demonstrate how CP scaffolds can support idealized low-dimensional structural motifs and dictate magnetic interactions through tuning of geometry and inter-spin couplings.

[Disruption of corpus callosum microstructural integrity by diffusion MRI as a predictor of progression of cerebral microangiopathy]

OBJECTIVE

To assess the microstructural integrity of the corpus callosum in patients with cerebral small vessel disease (cSVD) using signal and biophysical diffusion MRI models and to identify the most sensitive markers of disease progression.

MATERIAL AND METHODS

Diffusion MRI (3 Tesla) was performed in 166 patients (51.8% women; mean age 60.4±7.6) with cSVD and cognitive impairment of varying severity and in 44 healthy volunteers (65.9% women; mean age 59.6±6.8), followed by calculation of signal (diffusion tensor and diffusion kurtosis) and biophysical (WMTI, NODDI, MC-SMT) models, from which profiles of three corpus callosum segments were constructed.

RESULTS

The best results were obtained for metrics in the forceps minor and body of the corpus callosum. Among the metrics of the signal models in the forceps minor, fraction anisotropy (FA) and mean diffusion (MD), which characterize the overall loss of microstructural integrity and increase in extra-axonal water, as well as indirect markers of demyelination when considering transverse diffusion parameters (radial diffusion and radial kurtosis), had the larger area under the curve according to the ROC analysis. Among the metrics of the biophysical models in the forceps minor, a larger area under the curve was found in the MC-SMT model for extra-axonal transverse diffusion (ETR), mean diffusion (EMD), and intra-axonal water fraction (INTRA), and in the WMTI model for intra-axonal water fraction (AWF). ETR had high inverse correlations with INTRA and AWF, while INTRA and AWF had high direct intercorrelations.

CONCLUSION

Metrics of signaling (FA, MD, RD, RK) and biophysical patterns (ETR, EMD, INTRA, AWF) in the forceps minor and the corpus callosum body can be considered as indicators of cSVD progression. They indicate disease progression, mainly by an increase in extra-axonal water with the development of demyelination and tissue degeneration in the corpus callosum.

A Synthetic Cycle for Heteroarene Synthesis by Nitride Insertion

Recent interest in skeletal editing necessitates the continued development of reagent classes with the ability to transfer single atoms. Terminal transition metal nitrides hold immense promise for single-atom transfer, though their use in organic synthesis has so far been limited. Here we demonstrate a synthetic cycle with associated detailed mechanistic studies that primes the development of terminal transition metal nitrides as valuable single-atom transfer reagents. Specifically, we show [cis-terpyOsNCl₂ ]PF₆ inserts nitrogen into indenes to afford isoquinolines. Mechanistic studies for each step (insertion, aromatization, product release, and nitride regeneration) are reported, including crystallographic characterization of diverted intermediates, kinetics, and computational studies. The mechanistic foundation set by this synthetic cycle opens the door to the further development of nitrogen insertion heteroarene syntheses promoted by late transition metal nitrides.

Linker Redox Mediated Control of Morphology and Properties in Semiconducting Iron-Semiquinoid Coordination Polymers

The emergence of conductive 2D and less commonly 3D coordination polymers (CPs) and metal-organic frameworks (MOFs) promises novel applications in many fields. However, the synthetic parameters for these electronically complex materials are not thoroughly understood. Here we report a new 3D semiconducting CP Fe₅ (C₆ O₆ )₃ , which is a fusion of 2D Fe-semiquinoid materials and 3D cubic Fe_x (C₆ O₆ )_y materials, by using a different initial redox-state of the C₆ O₆ linker. The material displays high electrical conductivity (0.02 S cm^-1 ), broad electronic transitions, promising thermoelectric behavior (S² σ=7.0×10^-9  W m^-1  K^-2 ), and strong antiferromagnetic interactions at room temperature. This material illustrates how controlling the oxidation states of redox-active components in conducting CPs/MOFs can be a "pre-synthetic" strategy to carefully tune material topologies and properties in contrast to more commonly encountered post-synthetic modifications.

11H-Benzo[4,5]imidazo[1,2-a]indol-11-one as a New Precursor of Azomethine Ylides: 1,3-Dipolar Cycloaddition Reactions with Cyclopropenes and Maleimides

The possibility of generating azomethine ylides from 11H-benzo[4,5]imidazo[1,2-a]indol-11-one and amino acids is shown for the first time. Based on the cycloaddition reactions of these azomethine ylides with cyclopropenes and maleimides, cyclopropa[a]pyrrolizines, 3-azabicyclo[3.1.0]hexanes, and pyrrolo[3,4-a]pyrrolizines spiro-fused with a benzo[4,5]imidazo[1,2-a]indole fragment were synthesized. Spirocyclic compounds were obtained in moderate to good yields, albeit with poor diastereoselectivity. Density functional theory calculations were performed to obtain an insight into the mechanism of the 1,3-dipolar cycloaddition of 11H-benzo[4,5]imidazo[1,2-a]indol-11-one-derived azomethine ylides to cyclopropenes. The cytotoxic activity of some of the obtained cycloadducts against the human erythroleukemia (K562) cell line was evaluated in vitro by MTS-assay.

Presynthetic Redox Gated Metal-to-Insulator Transition and Photothermoelectric Properties in Nickel Tetrathiafulvalene-Tetrathiolate Coordination Polymers

Photothermoelectric (PTE) materials are promising candidates for solar energy harvesting and photodetection applications, especially for near-infrared (NIR) wavelengths. Although the processability and tunability of organic materials are highly advantageous, examples of organic PTE materials are comparatively rare and their PTE performance is typically limited by poor photothermal (PT) conversion. Here, we report the use of redox-active Sn complexes of tetrathiafulvalene-tetrathiolate (TTFtt) as transmetalating agents for the synthesis of presynthetically redox tuned NiTTFtt materials. Unlike the neutral material NiTTFtt, which exhibits n-type glassy-metallic conductivity, the reduced materials Li_1.2Ni_0.4[NiTTFtt] and [Li(THF)_1.5]_1.2Ni_0.4[NiTTFtt] (THF = tetrahydrofuran) display physical characteristics more consistent with p-type semiconductors. The broad spectral absorption and electrically conducting nature of these TTFtt-based materials enable highly efficient NIR-thermal conversion and good PTE performance. Furthermore, in contrast to conventional PTE composites, these NiTTFtt coordination polymers are notable as single-component PTE materials. The presynthetically tuned metal-to-insulator transition in these NiTTFtt systems directly modulates their PT and PTE properties.

Generation and Aerobic Oxidative Catalysis of a Cu(II) Superoxo Complex Supported by a Redox-Active Ligand

Cu systems feature prominently in aerobic oxidative catalysis in both biology and synthetic chemistry. Metal ligand cooperativity is a common theme in both areas as exemplified by galactose oxidase and by aminoxyl radicals in alcohol oxidations. This has motivated investigations into the aerobic chemistry of Cu and specifically the isolation and study of Cu-superoxo species that are invoked as key catalytic intermediates. While several examples of complexes that model biologically relevant Cu(II) superoxo intermediates have been reported, they are not typically competent aerobic catalysts. Here, we report a new Cu complex of the redox-active ligand ^tBu,TolDHP (2,5-bis((2-t-butylhydrazono)(p-tolyl)methyl)-pyrrole) that activates O₂ to generate a catalytically active Cu(II)-superoxo complex via ligand-based electron transfer. Characterization using ultraviolet (UV)-visible spectroscopy, Raman isotope labeling studies, and Cu extended X-ray absorption fine structure (EXAFS) analysis confirms the assignment of an end-on κ¹ superoxo complex. This Cu-O₂ complex engages in a range of aerobic catalytic oxidations with substrates including alcohols and aldehydes. These results demonstrate that bioinspired Cu systems can not only model important bioinorganic intermediates but can also mediate and provide mechanistic insight into aerobic oxidative transformations.

Cobalt-Catalyzed Hydrogenation Reactions Enabled by Ligand-Based Storage of Dihydrogen

The use of supporting ligands that can store either protons or electrons has emerged as a powerful strategy in catalysis. While these strategies are potent individually, natural systems mediate remarkable transformations by combining the storage of both protons and electrons in the secondary coordination sphere. As such, there has been recent interest in using this strategy to enable fundamentally different transformations. Furthermore, outsourcing H-atom or hydrogen storage to ancillary ligands can also enable alternative mechanistic pathways and thereby selectivity. Here, we describe the application of this strategy to facilitate radical reactivity in Co-based hydrogenation catalysis. Metalation of previously reported dihydrazonopyrrole ligands with Co results in paramagnetic complexes, which are best described as having Co(II) oxidation states. These complexes catalytically hydrogenate olefins with low catalyst loadings under mild conditions (1 atm H₂, 23 °C). Mechanistic, spectroscopic, and computational investigations indicate that this system goes through a radical hydrogen-atom transfer (HAT) type pathway that is distinct from classic organometallic mechanisms and is supported by the ability of the ligand to store H₂. These results show how ancillary ligands can facilitate efficient catalysis, and furthermore how classic organometallic mechanisms for catalysis can be altered by the secondary coordination sphere.

Synthesis of bis-spirocyclic derivatives of 3-azabicyclo[3.1.0]hexane via cyclopropene cycloadditions to the stable azomethine ylide derived from Ruhemann's purple

A reliable method for the synthesis of bis-spirocyclic derivatives of 3-azabicyclo[3.1.0]hexanes through the 1,3-dipolar cycloaddition (1,3-DC) reactions of cyclopropenes to the stable azomethine ylide – protonated form of Ruhemann's purple (PRP) has been developed. Both 3-substituted and 3,3-disubstituted cyclopropenes reacted with PRP, affording the corresponding bis-spirocyclic 3-azabicyclo[3.1.0]hexane cycloadducts in moderate to good yields with high diastereofacial selectivity. Moreover, several unstable 1,2-disubstituted cyclopropenes were successfully trapped by the stable 1,3-dipole under mild conditions. The mechanism of the cycloaddition reactions of cyclopropenes with PRP has been thoroughly studied using density functional theory (DFT) methods at the M11/cc-pVDZ level of theory. The cycloaddition reactions have been found to be HOMOcyclopropene–LUMOylide controlled while the transition-state energies for the reaction of 3-methyl-3-phenylcyclopropene with PRP are fully consistent with the experimentally observed stereoselectivity.

Electrostatic vs. inductive effects in phosphine ligand donor properties and reactivity

Enhanced rates and selectivity in enzymes are enabled in part by precisely tuned electric fields within active sites. Analogously, the use of charged groups to leverage electrostatics in molecular systems is a promising strategy to tune reactivity. However, separation of the through space and through bond effects of charged functional groups is a long standing challenge that limits the rational application of electric fields in molecular systems. To address this challenge we developed a method using the phosphorus selenium coupling value (J _P-Se) of anionic phosphine selenides to quantify the electrostatic contribution of the borate moiety to donor strength. In this analysis we report the synthesis of a novel anionic phosphine, PPh₂CH₂BF₃K, the corresponding tetraphenyl phosphonium and tetraethyl ammonium selenides [PPh₄][SePPh₂CH₂BF₃] and [TEA][SePPh₂CH₂BF₃], and the Rh carbonyl complex [PPh₄][Rh(acac)(CO)(PPh₂(CH₂BF₃))]. Solvent-dependent changes in J _P-Se were fit using Coulomb's law and support up to an 80% electrostatic contribution to the increase in donor strength of [PPh₄][SePPh₂CH₂BF₃] relative to SePPh₂Et, while controls with [TEA][SePPh₂CH₂BF₃] exclude convoluting ion pairing effects. Calculations using explicit solvation or point charges effectively replicate the experimental data. This J _P-Se method was extended to [PPh₄][SePPh₂(2-BF₃Ph)] and likewise estimates up to a 70% electrostatic contribution to the increase in donor strength relative to SePPh₃. The use of PPh₂CH₂BF₃K also accelerates C-F oxidative addition reactivity with Ni(COD)₂ by an order of magnitude in comparison to the comparatively donating neutral phosphines PEt₃ and PCy₃. This enhanced reactivity prompted the investigation of catalytic fluoroarene C-F borylation, with improved yields observed for less fluorinated arenes. These results demonstrate that covalently bound charged functionalities can exert a significant electrostatic influence under common solution phase reaction conditions and experimentally validate theoretical predictions regarding electrostatic effects in reactivity.

An Experimental and Theoretical Study of the 1,3-Dipolar Cyclo­addition of Alloxan-Derived Azomethine Ylides to Cyclopropenes

A diastereoselective synthesis of biologically interesting spirobarbiturates has been achieved via [3+2] cycloaddition of alloxan-derived azomethine ylides to 3-R-1,2-diphenylcyclopropenes. With this approach, a range of spirobarbiturate-3-azabicyclo[3.1.0]hexanes and spirobarbiturate-cyclopropa[a]pyrrolizines were obtained in moderate to good yields with excellent diastereoselectivities. DFT calculations (M11 density functional theory) were carried out to shed light on the molecular mechanism of 1,3-dipolar cycloaddition of alloxan-derived azomethine ylides to cyclopropenes. The cytotoxic activity of some obtained compounds against human erythroleukemia (K562) cell line was evaluated in vitro by MTS-assay.

Synthesis of Quinolino[1,2-c]quinazolin-6-one Derivatives via Formal (4+2)-Cycloaddition of Alkenes to Quinazolinе-Derived N-Acyliminium Cations: An Experimental and Theoretical Study

3-Aryl-4-hydroxy-1-methyl-3,4-dihydroquinazolin-2(1H)-ones were synthesized by reduction of 3-aryl-1-methylquinazoline-2,4(1H,3H)-diones with sodium triethylborohydride and studied as precursors of N-acyliminium cations that were expected to be trapped with various alkenes as (4+2)-cycloadducts. Unsubstituted 3-aryl-4-hydroxy-1-methyl-3,4-dihydroquinazolin-2(1H)-ones in the presence of BF3·Et2O failed to produce the desired cycloadducts probably due to a homooligomerization reaction involving N-acyliminium intermediates. To prevent this side reaction, we found it necessary to introduce substituents at both positions C6 and C8 of the quinazoline ring and C4′ of the 3-phenyl substituent. Utilizing bromine atoms as substituents at C6 and C8, N-acyliminium cations generated from 3-aryl-6,8-dibromo-4-hydroxy-1-methyl-3,4-dihydroquinazolin-2(1H)-ones in the presence of BF3·Et2O smoothly reacted with such alkenes as indene, acenaphthylene, styrene, α-methylstyrene to give new quinolino[1,2-c]quinazolin-6-one derivatives with high regio- and stereoselectivity. Density functional theory calculations were performed at the M06-2x/cc-pVDZ level to obtain an insight into the mechanism of the (4+2)-cycloaddition reaction of quinazoline-derived N-acyliminium cations to alkenes.

Visual analysis of nigrosome-1 in the differential diagnosis of Parkinson's disease and essential tremor

Differentiation between Parkinson’s disease, especially in its early stages, and essential tremor, which is a phenotypically similar movement disorder, still remains an unsolved challenge for neurology. The aim of this study was to assess the diagnostic significance of nigrosome imaging (nigrosomes are dopaminergic neuron clusters in the substantia nigra of the midbrain) using 3T high-resolution SW-MRI. The study was conducted in 20 patients with Parkinson’s disease and 10 patients with essential tremor. Visual analysis of the acquired nigrosome-1 images was performed using a 4-point ordinal rating scale. Differences in sex, age and duration of the disease were calculated using the Fisher exact test and the Mann–Whitney U test. The diagnostic value of the method was assessed using Pearson’s chisquared test. Nigrosome-1 was bilaterally or unilaterally absent in 70% of parkinsonian patients. Less specific changes to the substantia nigra (SN) were observed in two more parkinsonian patients (10%), whose nigrosome-1 appeared reduced in size. By contrast, nigrosome-1 was bilaterally intact in all patients (100%) with essential tremor (p < 0.001). Our preliminary findings demonstrate the high potential of noninvasive nigrosome-1 imaging in the differential diagnosis of Parkinson’s disease and essential tremor.

Enzyme-Like Hydroxylation of Aliphatic C-H Bonds From an Isolable Co-Oxo Complex

The selective hydroxylation of aliphatic C-H bonds remains a challenging but broadly useful transformation. Nature has evolved systems that excel at this reaction, exemplified by cytochrome P450 enzymes, which use an iron-oxo intermediate to activate aliphatic C-H bonds with k₁ > 1400 s^-1 at 4 °C. Many synthetic catalysts have been inspired by these enzymes and are similarly proposed to use transition metal-oxo intermediates. However, most examples of well-characterized transition metal-oxo species are not capable of reacting with strong, aliphatic C-H bonds, resulting in a lack of understanding of what factors facilitate this reactivity. Here, we report the isolation and characterization of a new terminal Co^III-oxo complex, PhB(^AdIm)₃Co^IIIO. Upon oxidation, a transient Co^IV-oxo intermediate is generated that is capable of hydroxylating aliphatic C-H bonds with an extrapolated k₁ for C-H activation >130 s^-1 at 4 °C, comparable to values observed in cytochrome P450 enzymes. Experimental thermodynamic values and DFT analysis demonstrate that, although the initial C-H activation step in this reaction is endergonic, the overall reaction is driven by an extremely exergonic radical rebound step, similar to what has been proposed in cytochrome P450 enzymes. The rapid C-H hydroxylation reactivity displayed in this well-defined system provides insight into how hydroxylation is accomplished by biological systems and similarly potent synthetic oxidants.

Identification of Spiro-Fused Pyrrolo[3,4-a]pyrrolizines and Tryptanthrines as Potential Antitumor Agents: Synthesis and In Vitro Evaluation

A series of heterocyclic compounds containing a spiro-fused pyrrolo[3,4-a]pyrrolizine and tryptanthrin framework have been synthesized and studied as potential antitumor agents. Cytotoxicity of products was screened against human erythroleukemia (K562) and human cervical carcinoma (HeLa) cell lines. Among the screened compounds. 4a, 4b and 5a were active against human erythroleukemia (K562) cell line, while 4a and 5a were active against cervical carcinoma (HeLa) cell line. In agreement with the DNA cytometry studies, the tested compounds have achieved significant cell-cycle perturbation with higher accumulation of cells in G2/M phase and induced apoptosis. Using confocal microscopy, we found that with 4a and 5a treatment of HeLa cells, actin filaments disappeared, and granular actin was distributed diffusely in the cytoplasm in 76–91% of cells. We discovered that HeLa cells after treatment with compounds 4a and 5a significantly reduced the number of cells with filopodium-like membrane protrusions (from 63 % in control cells to 29% after treatment) and a decrease in cell motility.

Direct Aerobic Generation of a Ferric Hydroperoxo Intermediate Via a Preorganized Secondary Coordination Sphere

Enzymes exert control over the reactivity of metal centers with precise tuning of the secondary coordination sphere of active sites. One particularly elegant illustration of this principle is in the controlled delivery of proton and electron equivalents in order to activate abundant but kinetically inert oxidants such as O2 for oxidative chemistry. Chemists have drawn inspiration from biology in designing molecular systems where the secondary coordination sphere can shuttle protons or electrons to substrates. However, a biomimetic activation of O2 requires the transfer of both protons and electrons, and molecular systems where ancillary ligands are designed to provide both of these equivalents are comparatively rare. Here, we report the use of a dihydrazonopyrrole (DHP) ligand complexed to Fe to perform exactly such a biomimetic activation of O2. In the presence of O2, this complex directly generates a high spin Fe(III)-hydroperoxo intermediate which features a DHP• ligand radical via ligand-based transfer of an H atom. This system displays oxidative reactivity and ultimately releases hydrogen peroxide, providing insight on how secondary coordination sphere interactions influence the evolution of oxidizing intermediates in Fe-mediated aerobic oxidations.

Identification of Spiro-Fused [3-azabicyclo[3.1.0]hexane]oxindoles as Potential Antitumor Agents: Initial In Vitro Evaluation of Anti-Proliferative Effect and Actin Cytoskeleton Transformation in 3T3 and 3T3-SV40 Fibroblast

Novel heterocyclic compounds containing 3-spiro[3-azabicyclo[3.1.0]hexane]oxindole framework (4a, 4b and 4c) have been studied as potential antitumor agents. The in silico ADMET (adsorption, distribution, metabolism, excretion and toxicity) analysis was performed on 4a-c compounds with promising antiproliferative activity, previously synthetized and screened against human erythroleukemic cell line K562 tumor cell line. Cytotoxicity of 4a-c against murine fibroblast 3T3 and SV-40 transformed murine fibroblast 3T3-SV40 cell lines were evaluated. The 4a and 4c compounds were cytotoxic against 3T3-SV40 cells in comparison with those of 3T3. In agreement with the DNA cytometry studies, the tested compounds have achieved significant cell-cycle perturbation with higher accumulation of cells in G0/G1 phase. Using confocal microscopy, we found that with 4a and 4c treatment of 3T3 cells, actin filaments disappeared, and granular actin was distributed diffusely in the cytoplasm in 82-97% of cells. The number of 3T3-SV40 cells with stress fibers increased to 7-30% against 2% in control. We discovered that transformed 3T3-SV40 cells after treatment with compounds 4a and 4c significantly reduced the number of cells with filopodium-like membrane protrusions (from 86 % in control cells to 6-18% after treatment), which indirectly suggests a decrease in cell motility. We can conclude that the studied compounds 4a and 4c have a cytostatic effect, which can lead to a decrease in the number of filopodium-like membrane protrusions.

Steric and electronic effects of ligand substitution on redox-active Fe4S4-based coordination polymers

One of the notable advantages of molecular materials is the ability to precisely tune structure, properties, and function via molecular substitutions. While many studies have demonstrated this principle with classic carboxylate-based coordination polymers, there are comparatively fewer examples where systematic changes to sulfur-based coordination polymers have been investigated. Here we present such a study on 1D coordination chains of redox-active Fe₄S₄ clusters linked by methylated 1,4-benzene-dithiolates. A series of new Fe₄S₄-based coordination polymers were synthesized with either 2,5-dimethyl-1,4-benzenedithiol (DMBDT) or 2,3,5,6-tetramethyl-1,4-benzenedithiol (TMBDT). The structures of these compounds have been characterized based on synchrotron X-ray powder diffraction while their chemical and physical properties have been characterized by techniques including X-ray photoelectron spectroscopy, cyclic voltammetry and UV-visible spectroscopy. Methylation results in the general trend of increasing electron-richness in the series, but the tetramethyl version exhibits unexpected properties arising from steric constraints. All these results highlight how substitutions on organic linkers can modulate electronic factors to fine-tune the electronic structures of metal-organic materials.

Construction of Spiro[3-azabicyclo[3.1.0]hexanes] via 1,3-Dipolar Cycloaddition of 1,2-Diphenylcyclopropenes to Ninhydrin-Derived Azomethine Ylides

The multi-component 1,3-dipolar cycloaddition of ninhydrin, α-amino acids (or peptides), and cyclopropenes for the synthesis of spirocyclic heterocycles containing both 3-azabicyclo[3.1.0]hexane and 2H-indene-1,3-dione motifs has been developed. This method provides easy access to 3-azabicyclo[3.1.0]hexane-2,2′-indenes with complete stereoselectivity and a high degree of atom economy under mild reaction conditions. A broad range of cyclopropenes and α-amino acids have been found to be compatible with the present protocol, which offers an opportunity to create a new library of biologically significant scaffold (3-azabicyclo[3.1.0]hexane). In addition, the сomprehensive study of mechanism of azomethine ylide formation from ninhydrin and sarcosine was performed by means of M11 density functional theory (DFT) calculations. It has been revealed that experimentally observed 1-methylspiro[aziridine-2,2′-indene]-1′,3′-dione is a kinetically controlled product of this reaction and appears to act as a 1,3-dipole precursor. This theoretical study also shed light on the main transformations of the azomethine ylide derived from ninhydrin and sarcosine such as a 1,3-dipolar cycloaddition to cyclopropene dipolarophiles, a dimerization reaction and a (1+5) electrocyclization reaction. The antitumor activity of some synthesized compounds against cervical carcinoma (HeLa­) cell line was evaluated in vitro by MTS-assay.

Catalytic hydrogenation enabled by ligand-based storage of hydrogen

Biology employs exquisite control over proton, electron, H-atom, or H₂ transfer. Similar control in synthetic systems has the potential to facilitate efficient and selective catalysis. Here we report a dihydrazonopyrrole Ni complex where an H₂ equivalent can be stored on the ligand periphery without metal-based redox changes and can be leveraged for catalytic hydrogenations. Kinetic and computational analysis suggests ligand hydrogenation proceeds by H₂ association followed by H-H scission. This complex is an unusual example where a synthetic system can mimic biology's ability to mediate H₂ transfer via secondary coordination sphere-based processes.

[Clustering of diagnostic MRI signs of cerebral microangiopathy and its relationship with markers of inflammation and angiogenesis]

OBJECTIVE

To perform cluster analysis of MRI signs of cerebral microangiopathy (small vessel disease, SVD) and to clarify the relationship between the isolated groups and circulating markers of inflammation and angiogenesis.

MATERIAL AND METHODS

The identification of groups of MRI signs (MRI types) using cluster hierarchical agglomerative analysis and iterative algorithm of k-means and assessment of their relationship with serum concentrations of tumor necrosis factor-α (TNF-α), transforming growth factor-β1 (TGF-β1), vascular endothelial growth factor-A (VEGF-A), hypoxia-inducible factor 1-α (HIF1-α) determined by ELISA were performed in 96 patients with SVD (STRIVE, 2013) (65 women, average age 60.91±6.57 years).

RESULTS

Cluster analysis of MRI signs identified two MRI types of SVD with Fazekas grade 3 of white matter hyperintensity (WMH). MRI type 1 (n=18; 6 women, mean age 59.1±6.8 years) and MRI type 2 (n=22, 15 f., mean age 63.5±6.2 years) did not differ by age, sex, severity of hypertension, presence of other risk factors. MRI type 1 had a statistically significantly more pronounced WMH in the periventricular regions, multiple lacunes and microbleeds, atrophy, severe cognitive impairment and gait disorders compared with MRI type 2. Its formation was associated with a decrease in VEGF-A level. MRI type 2 had the significantly more pronounced juxtacortical WMH, white matter lacunes, in the absence of microbleeds and atrophy, and less severe clinical manifestations compared with MRI type 1. Its formation was associated with an increase in TNF-α level.

CONCLUSION

Clustering of diagnostic MRI signs into MRI types of SVD with significant differences in the severity of clinical manifestations suggests the pathogenetic heterogeneity of age-related SVD. The relationship of MRI types with circulating markers of different mechanisms of vascular wall and brain damage indicates the dominant role of depletion of angiogenesis in the formation of MRI type 1 and increased inflammation in the formation of MRI type 2. Further studies are needed to clarify the criteria and diagnostic value of differentiation of MRI types of SVD, and also their mechanisms with the definition of pathogenetically justified prevention and treatment of various forms of SVD.

Generation and Reactivity of a NiIII2(μ-1,2-peroxo) Complex

High-valent transition metal-oxo, -peroxo, and -superoxo complexes are crucial intermediates in both biological and synthetic oxidation of organic substrates, water oxidation, and oxygen reduction. While high-valent oxygenated complexes of Mn, Fe, Co, and Cu are increasingly well-known, high-valent oxygenated Ni complexes are comparatively rarer. Herein we report the isolation of such an unusual high-valent species in a thermally unstable Ni^III₂(μ-1,2-peroxo) complex, which has been characterized using single-crystal X-ray diffraction and X-ray absorption, NMR, and UV-vis spectroscopies. Reactivity studies show that this complex is stable toward dissociation of oxygen but reacts with simple nucleophiles and electrophiles.

Heterotrimetallic Precursor with 2:2:1 Metal Ratio Requiring at Least a Pentanuclear Molecular Assembly

This work represents an important step in the quest to make heteromultimetallic molecules featuring specific metal types and complicated metal ratios. The rational design, synthesis, and characterization of a complex heterotrimetallic single-source molecular precursor for the next generation sodium-ion battery cathode material, Na₂Mn₂FeO₆, is described. A unique pentametallic platform [Mn^II(ptac)₃-Na-Mn^III(acac)₃-Na-Mn^II(ptac)₃] (1) was derived from the known polymeric structure of [NaMn^II(acac)₃]_∞, through a series of elaborate design procedures, such as mixed-ligand, unsymmetric ligand, and mixed-valent approaches. Importantly, the application of those techniques results in a molecule with distinctively different transition metal positions in terms of ligand environment and oxidation states. An isovalent substitution of Fe^III for the central Mn^III ion forms the target heterotrimetallic precursor [Mn^II(ptac)₃-Na-Fe^III(acac)₃-Na-Mn^II(ptac)₃] (3) with an appropriate metal ratio of Na:Mn:Fe = 2:2:1. The arrangement of metal ions and ligands in this pentametallic assembly was confirmed by single crystal X-ray investigation. The unambiguous assignment of the positions and oxidation states of the Periodic Table neighbors Fe and Mn in 3 has been achieved by a combination of investigative techniques that include synchrotron resonant diffraction, X-ray multiwavelength anomalous diffraction, X-ray fluorescence spectroscopy, Mössbauer spectroscopy, and gas-phase DART mass spectrometry. The heterotrimetallic single-source precursor 3 was shown to exhibit a clean decomposition pattern yielding the phase-pure P2-Na₂Mn₂FeO₆ quaternary oxide with high uniformity of metal ion distribution as confirmed by electron microscopy.

Covalent surface modifications and superconductivity of two-dimensional metal carbide MXenes

Versatile chemical transformations of surface functional groups in two-dimensional transition-metal carbides (MXenes) open up a previously unexplored design space for this broad class of functional materials. We introduce a general strategy to install and remove surface groups by performing substitution and elimination reactions in molten inorganic salts. Successful synthesis of MXenes with oxygen, imido, sulfur, chlorine, selenium, bromine, and tellurium surface terminations, as well as bare MXenes (no surface termination), was demonstrated. These MXenes show distinctive structural and electronic properties. For example, the surface groups control interatomic distances in the MXene lattice, and Tin+1Cn (n = 1, 2) MXenes terminated with telluride (Te2−) ligands show a giant (>18%) in-plane lattice expansion compared with the unstrained titanium carbide lattice. The surface groups also control superconductivity of niobium carbide MXenes.

Generation and Oxidative Reactivity of a Ni(II) Superoxo Complex via Ligand-Based Redox Non-Innocence

Metal ligand cooperativity is a powerful strategy in transition metal chemistry. This type of mechanism for the activation of O₂ is best exemplified by heme centers in biological systems. While aerobic oxidations with Fe and Cu are well precedented, Ni-based oxidations are frequently less common due to less-accessible metal-based redox couples. Some Ni enzymes utilize special ligand environments for tuning the Ni(II)/(III) redox couple such as strongly donating thiolates in Ni superoxide dismutase. A recently characterized example of a Ni-containing protein, however, suggests an alternative strategy for mediating redox chemistry with Ni by utilizing ligand-based reducing equivalents to enable oxygen binding. While this mechanism has little synthetic precedent, we show here that Ni complexes of the redox-active ligand ^tBu,TolDHP (^tBu,TolDHP = 2,5-bis((2-t-butylhydrazono)(p-tolyl)methyl)-pyrrole) activate O₂ to generate a Ni(II) superoxo complex via ligand-based electron transfer. This superoxo complex is competent for stoichiometric oxidation chemistry with alcohols and hydrocarbons. This work demonstrates that coupling ligand-based redox chemistry with functionally redox-inactive Ni centers enables oxidative transformations more commonly mediated by metals such as Fe and Cu.

Nickel(ii)-methyl complexes adopting unusual seesaw geometries

Tris-carbene borate supported nickel methyl complexes adopt unusual seesaw geometries due to competing electronic and chelate effects.

Redox, transmetalation, and stacking properties of tetrathiafulvalene-2,3,6,7-tetrathiolate bridged tin, nickel, and palladium compounds

Here we report that capping the molecule TTFtt (TTFtt = tetrathiafulvalene-2,3,6,7-tetrathiolate) with dialkyl tin groups enables the isolation of a stable series of redox congeners and facile transmetalation to Ni and Pd. TTFtt has been proposed as an attractive building block for molecular materials for two decades as it combines the redox chemistry of TTF and dithiolene units. TTFttH₄, however, is inherently unstable and the incorporation of TTFtt units into complexes or materials typically proceeds through the in situ generation of the tetraanion TTFtt^4-. Capping of TTFtt^4- with Bu₂Sn²⁺ units dramatically improves the stability of the TTFtt moiety and furthermore enables the isolation of a redox series where the TTF core carries the formal charges of 0, +1, and +2. All of these redox congeners show efficient and clean transmetalation to Ni and Pd resulting in an analogous series of bimetallic complexes capped by 1,2-bis(diphenylphosphino)ethane (dppe) ligands. Furthermore, by using the same transmetalation method, we synthesized analogous palladium complexes capped by 1,1'-bis(diphenylphosphino)ferrocene (dppf) which had been previously reported. All of these species have been thoroughly characterized through a systematic survey of chemical and electronic properties by techniques including cyclic voltammetry (CV), ultraviolet-visible-near infrared spectroscopy (UV-vis-NIR), electron paramagnetic resonance spectroscopy (EPR), nuclear magnetic resonance spectroscopy (NMR) and X-ray diffraction (XRD). These detailed synthetic and spectroscopic studies highlight important differences between the transmetalation strategy presented here and previously reported synthetic methods for the installation of TTFtt. In addition, the utility of this stabilization strategy can be illustrated by the observation of unusual TTF radical-radical packing in the solid state and dimerization in the solution state. Theoretical calculations based on variational 2-electron reduced density matrix methods have been used to investigate these unusual interactions and illustrate fundamentally different levels of covalency and overlap depending on the orientations of the TTF cores. Taken together, this work demonstrates that tin-capped TTFtt units are ideal reagents for the installation of redox-tunable TTFtt ligands enabling the generation of entirely new geometric and electronic structures.

Colloidal Gelation in Liquid Metals Enables Functional Nanocomposites of 2D Metal Carbides (MXenes) and Lightweight Metals

Nanomaterials dispersed in different media, such as liquids or polymers, generate a variety of functional composites with synergistic properties. In this work, we discuss liquid metals as the nanomaterials' dispersion media. For example, 2D transition-metal carbides and nitrides (MXenes) can be efficiently dispersed in liquid Ga and lightweight alloys of Al, Mg, and Li. We show that the Lifshitz theory predicts strong van der Waals attraction between nanoscale objects interacting through liquid metals. However, a uniform distribution of MXenes in liquid metals can be achieved through colloidal gelation, where particles form self-supporting networks stable against macroscopic phase segregation. This network acts as a reinforcement boosting mechanical properties of the resulting metal-matrix composite. By choosing Mg-Li alloy as an example of ultralightweight metal matrix and Ti₃C₂T_x MXene as a nanoscale reinforcement, we apply a liquid metal gelation technique to fabricate functional nanocomposites with an up to 57% increase in the specific yield strength without compromising the matrix alloy's plasticity. MXenes largely retain their phase and 2D morphology after processing in liquid Mg-Li alloy at 700 °C. The 2D morphology enables formation of a strong semicoherent interface between MXene and metal matrix, manifested by biaxial strain of the MXene lattice inside the metal matrix. This work expands applications for MXenes and shows the potential for developing MXene-reinforced metal matrix composites for structural alloys and other emerging applications with metal-MXene interfaces, such as batteries and supercapacitors.

Mono-reduced Corannulene: To Couple and Not to Couple in One Crystal

One-electron reduction of corannulene, C₂₀ H₁₀ , with Li metal in diglyme resulted in crystallization of [{Li⁺ (diglyme)₂ }₄ (C₂₀ H₁₀ ^.- )₂ (C₂₀ H₁₀ -C₂₀ H₁₀ )^2- ] (1), as revealed by single-crystal X-ray diffraction. This hybrid product contains two corannulene monoanion-radicals along with a dianionic dimer, crystallized with four Li⁺ ions wrapped by diglyme molecules. The dimeric (C₂₀ H₁₀ -C₂₀ H₁₀ )^2- anion provides the first crystallographically confirmed example of spontaneous radical dimerization for C₂₀ H₁₀ ^.- . The C-C bond length between the two C₂₀ H₁₀ ^.- bowls of 1.588(5) Å is consistent with the single σ-bond character of the linker. The trans-disposition of two bowls in the centrosymmetric (C₂₀ H₁₀ -C₂₀ H₁₀ )^2- dimer is observed with the torsion angle around the central C-C bond of 180°. Comprehensive theoretical analysis of formation/decomposition processes of the dimeric dianion has been carried out in order to evaluate the nature of bonding and energetics of the C₂₀ H₁₀ ^.- coupling. It is found that such σ-bonded dimers are thermodynamically unstable due to large preparation energy and repulsive Pauli component of the bonding, but kinetically persistent due to a high energy barrier provided by the existing spin-crossing point.