Unusual Oxidation Research Articles

Iridium/Acid-Dual Catalyzed Enantioselective Intramolecular Allylic Dearomatization Reaction of Allylic Alcohol Tethered α- and β-Naphthols.

The first catalytic enantioselective intramolecular allylic dearomatization of allylic alcohol tethered α- and β-naphthols has been developed with iridium/acid-dual catalysis. A wide range of polycyclic spiroketones containing vicinal tertiary and quaternary carbon stereocenters were readily prepared in good to high yields with high diastereo- and moderate to excellent enantioselectivities. An unusual anti-Markovnikov Wacker oxidation has also been shown in synthetic transformations.

Mechanochemical Synthesis and Magnetic Properties of the Mixed-Valent Binary Silver(I,II) Fluorides, AgI2AgIIF4 and AgIAgIIF3.

Fluoridoargentates(II) represent a fascinating class of silver(II) compounds with structural and magnetic similarities to cuprate superconductors. However, their synthesis is challenging, leaving their properties largely underexplored and hindering the discovery of new phases. This study introduces mechanochemistry as a novel approach for the synthesis of fluoridoargentates(II), avoiding the use of anhydrous HF or elemental fluorine and employing readily available equipment. Notably, ball milling of commercially available precursors successfully produced the long-sought-after first two examples of binary mixed-valent silver(I,II) phases, AgI2AgIIF4 (Ag3F4) and AgIAgIIF3 (Ag2F3). While the AgI2AgIIF4 phase was obtained at room temperature, the AgIAgIIF3 phase is metastable and required milling under cryogenic conditions. Characterization by synchrotron powder X-ray and neutron diffraction revealed that AgI2AgIIF4 crystallizes in the P21/c space group and is isostructural to β-K2AgF4. In this crystal structure, [AgIIF2F4/2]2- distorted octahedral units with 4 + 2 coordination, extend parallel to a-crystallographic axis giving a quasi-one-dimensional canted antiferromagnetic character, as shown by magnetic susceptibility. The triclinic perovskite AgIAgIIF3 phase adopts the P1̅ space group, is isostructural to AgCuF3 and also shows features of a one-dimensional antiferromagnet. This mechanochemical approach, also successfully applied to synthesize β-K2AgF4, is expected to expand the field of silver(II) chemistry, accelerating the search for silver analogs to cuprate superconductors and potentially extending to other cations in unusual oxidation states.

Prediction of stable structure and unique charge transfer in Li–Pt intermetallic compounds under pressure

The exploration of new intermetallic compounds is of great significance for basic research and practical application. There is a huge electronegativity difference between Li and Pt, and the metals exhibit interesting and diverse properties; however, the structural behavior of Li–Pt intermetallics with different ratios under high pressure has not been systematically studied. In this study, an intelligent structure search method based on a particle swarm optimization algorithm combined with first-principles calculations was used to extensively explore the stable structures and unique conditions governing charge transfer in Li–Pt intermetallic compounds under different pressures. In addition to reproducing the known LiPt (P6‾m2) and Li2Pt (P6/mmm), the simulation was consistent with the experimental results. New phases were also found by calculation: LiPt3(Cmmm), Li2Pt (P3‾m1), and Li4Pt (I4/m) at ambient pressure; Li3Pt (Fm3‾m), Li4Pt (R3‾m), and Li5Pt (P6/mmm) at 10 GPa; and Li5Pt (P3‾m1) at 20 GPa. Bader charge analysis and electron localization function (ELF) mapping showed that the transition metal (Pt) atoms exhibit unusual oxidation states both under ambient and high pressure, and more electrons were localized on Pt as the Li content increased. The highest negative valence state was approximately −4. Intermetallic compounds LiPt3 (Cmmm) and Li2Pt (P6/mmm) were prepared successfully by arc melting furnace. The reliability of structure prediction method and pseudo potential selection was verified. This work demonstrates that tuning the pressure and stoichiometry is an effective means of forming novel, stable intermetallic compounds.

Evolution, structure, and drug-metabolizing activity of mammalian prenylcysteine oxidases

Prenylcysteine oxidases (PCYOXs) metabolize prenylated cysteines produced by protein degradation. They utilize oxygen as a co-substrate to produce free cysteine, an aldehyde, and hydrogen peroxide through the unusual oxidation of a thioether bond. In this study, we explore the evolution, structure, and mechanism of the two mammalian PCYOXs. A gene duplication event in jawed vertebrates originated in these two paralogs. Both enzymes are active on farnesyl- and geranylgeranylcysteine, but inactive on molecules with shorter prenyl groups. Kinetics experiments outline a mechanism where flavin reduction and re-oxidation occur rapidly without any detectable intermediates, with the overall reaction rate limited by product release. The experimentally determined three-dimensional structure of PCYOX1 reveals long and wide tunnels leading from the surface to the flavin. They allow the isoprene substrate to curl up within the protein and position its reactive cysteine group close to the flavin. A hydrophobic patch on the surface mediates membrane association, enabling direct substrate and product exchange with the lipid bilayer. Leveraging established knowledge of flavoenzyme inhibition, we designed sub-micromolar PCYOX inhibitors. Additionally, we discovered that PCYOXs bind and slowly degrade salisirab, an anti-RAS compound. This activity suggests potential and previously unknown roles of PCYOXs in drug metabolism.

Crystal field anisotropy correlated charge disproportionation in Ruddlesden-Popper Sr3-xCaxFe2O7-δ perovskites

A series of Ca2+-doped Ruddlesden-Popper phase perovskites Sr3-xCaxFe2O7-δ (x = 0∼0.4) were synthesized. Strong asymmetric FeO6 octahedral distortion was confirmed at x = 0.2 owing to the anisotropic chemical pressure applied from preferable occupation of Ca2+ ion at Sr2 site. Unusual high-charged oxidation state of iron ion with enhanced p→d charge transfer was collectively evidenced by X-ray photoelectron spectroscopy and Mössbauer spectroscopy. Combined electron emission spectroscopy further revealed a narrowing of the surface band gap accompanied by an apparent lowering of the conduction band edge at x = 0.2. These observations can be explained by a nontrivial partial charge disproportionated (CD) regime: d5L + d5L → d5L1−n + d5L1+n, which reflects a significant correlation between anisotropic chemical pressure and CD ground state. The d5L1−n ground state, which dominates the conduction band, is largely stabilized by enhancing the d(Fe)-p(O) hybridization along the c-axis. In addition, abrupt changes in both magnetism and dc electrical properties were also observed at x = 0.2. It is believed that the anisotropic crystal field correlated electronic state fluctuation of iron may facilitate magnetic phase competition and carrier transport. This work provides a comprehensive understanding of the crystallographic anisotropy and correlated valence electron behavior in Fe-base Ruddlesden-Popper perovskites, as well as quasi two-dimensional perovskite materials.

Comparative genomics and evidence for an unusual polyamine oxidation pathway in aquatic duckweed (Spirodela polyrhiza L.)

Polyamines (PA) cellular levels are maintained through a balance between synthesis and catabolism, achieved by two classes of enzymes polyamine oxidases (PAOs) and copper amine oxidases (CuAO). Here we investigated the occurrence, molecular evolution and role(s) of PAOs and CuAO gene families in aquatic duckweed and their comparison with other aquatic plants -sea eelgrass, bladderwort, and Lotus. We identified eight bona fide PAO genes (SpPAO1–SpPAO8) and one SpCuAO1 in the greater duckweed genome from three genome assemblies. Interestingly, duckweed PAO genes increased their number through a tandem duplication event, while contrary to this CuAO genes were significantly lost to a single gene SpCuAO1. Phylogenetic analysis revealed that tandemly duplicated SpPAO2–7 share close similarity to well-known terminal catabolism (TC) pathway PAO genes while SpPAO1 and SpPAO8 seem to segregate along with back conversion (BC) participating known PAO genes, suggesting that all tandem duplicated PAOs are involved in TC pathway which is contrary to known trend in land plants where CuAOs are mainly involved in TC pathway. Comparative transcript abundance studies indicated that all eight PAOs and one CuAO gene respond to multiple stresses and principal component analysis identifies SpPAO4 as a highly active gene in response to multiple stresses. Results showed that oxidation of higher polyamines (SPD/SPM) through the TC pathway is diversified in duckweeds. Taken together this study reveals unique insights into the genomic losses and gains of polyamine metabolism possibly involved in achieving the structural and physiological adaptations required for aquatic lifestyle of duckweeds.

BeM(CO)3- (M = Co, Rh, Ir) and BeM(CO)3 (M = Ni, Pd, Pt): Triply bonded terminal beryllium in zero oxidation state.

We perform detailed potential energy surface explorations of BeM(CO)3- (M = Co, Rh, Ir) and BeM(CO)3 (M = Ni, Pd, Pt) using both single-reference and multireference-based methods. The present results at the CASPT2(12,12)/def2-QZVPD//M06-D3/def2-TZVPPD level reveal that the global minimum of BeM(CO)3- (M = Co, Rh, Ir) and BePt(CO)3 is a C3v symmetric structure with an 1A1 electronic state, where Be is located in a terminal position bonded to M along the center axis. For other cases, the C3v symmetric structure is a low-lying local minimum. Although the present complexes are isoelectronic with the recently reported BFe(CO)3- complex having a B-Fe quadruple bond, radial orbital-energy slope (ROS) analysis reveals that the highest occupied molecular orbital (HOMO) in the title complexes is slightly antibonding in nature, which bars a quadruple bonding assignment. Similar weak antibonding nature of HOMO in the previously reported BeM(CO)4 (M = Ru, Os) complexes is also noted in ROS analysis. The bonding analysis through energy decomposition analysis in combination with the natural orbital for chemical valence shows that the bonding between Be and M(CO)3q (q = -1 for M = Co, Rh, Ir and q = 0 for M = Ni, Pd, Pt) can be best described as Be in the ground state (1S) interacting with M(CO)30/- via dative bonds. The Be(spσ) → M(CO)3q σ-donation and the complementary Be(spσ) ← M(CO)3q σ-back donation make the overall σ bond, which is accompanied by two weak Be(pπ) ← M(CO)3q π-bonds. These complexes represent triply bonded terminal beryllium in an unusual zero oxidation state.

Developing strong-yet-ductile light-weight medium-entropy alloy via the unusual oxide doping effect

High or medium entropy alloys (M/HEAs) possess outstanding mechanical and thermal properties, but their high mass density severely restricts their practical applications. In this study, we designed a novel light-weight MEA based on the TiAlCrNb-x(ZrO2) system. These ZrO2 particles doped can completely dissolve in the β-phase structure of the TiAlCrNb MEA, which contrasts with conventional ceramic strengthening introduced through powder metallurgy. Consequently, it promotes the formation of Ti3Al nanoparticles and simultaneously substantially reduces grain size, thereby achieving a remarkable strength-ductility combination with a high specific yield strength of 225/250 MPa/(g·cm−3) while maintaining an excellent tensile ductility of 19.5/10.1% in the newly designed TiAlCrNb-(1.2/1.5)(ZrO2) MEAs. This performance outperforms most other light-weight M/HEAs. Strengthened mechanisms analysis indicates that the strength increment is attributed to grain-refinement-induced Hall-Petch strengthening and solid-solution strengthening from the dissolved oxygen. The results offer insight into the innovative design of ultrastrong and ductile light-weight MEAs for advanced structural applications.

Electrochemical Lactonization Enabled by Unusual Shono-Type Oxidation from Functionalized Benzoic Acids.

A novel method for electrochemical lactonization via C(sp3)-H functionalization was developed. This metal- and oxidant-free strategy enabled the efficient synthesis of various lactones. Gram-scale reaction and derivatization of the lactone product demonstrated the synthetic utility of this methodology. Mechanistic studies using control experiments and CV curves elucidated the proposed intramolecular HAT and the oxidative cyclization pathway. An unusual Shono-type oxidation was realized through this electrochemical approach, proceeding without a traditional nucleophilic addition process.

7.1',8.3'- and 7.3',8.5'-Connected Bicyclo[3.2.1]octanoids and Oxabicyclo[3.2.2]nonane-Type Neolignans from Ocotea aciphylla: Structures and Absolute Configurations.

The phytochemical investigation of the leaves and trunk bark of a specimen of Ocotea aciphylla collected in the southern portion of the Amazon forest led to the isolation of an oxabicyclo[3.2.2]nonane-type neolignan and 15 bicyclo[3.2.1]octanoid neolignans, 14 of which are unreported compounds (2-15), including one with an unusual oxidation pattern of the side chain at C-1' and two rare 7.1',8.3'-connected bicyclo[3.2.1]octanoid derivatives. Their structures and relative configurations were determined by extensive spectrometric analysis based on 1D- and 2D-NMR spectroscopy and HRESIMS data, while their absolute configurations were unambiguously assigned using electronic and vibrational circular dichroism data assisted by density functional theory calculations. Additionally, known sesquiterpenes, phenylpropanoids, and phytosterols were also isolated.

Synthesis and α-Functionalisation of Cyclic Imines

Abstractα-Functionalisation of cyclic imines is explored. The cyclic imine substrates are synthesised from their respective halonitrile precursors using a nucleophilic addition/cyclisation sequence. Selective monohalogenation of the cyclic imines yields α-haloimines, which serve as a platform to obtain various α-hydroxyimine derivatives. In addition, an unusual tautomerisation and oxidation sequence is observed in the attempted preparation of α-hydroxyimines.

Cyclometalated Ruthenium Catalyst Enables Selective Oxidation of N-Substituted Tetrahydroquinolines to Lactams.

Herein, we report an unusual α-methylene oxidation of N-substituted tetrahydroquinoline to lactams using the cyclometalated Ru(II)-complex as a catalyst. Cyclic-α-methylene C-H bonds are selectively oxidized under the reaction conditions even in the presence of α-methyl or reactive α-methylene C-H bonds. This methodology is also useful in the late-stage functionalization of pharmaceuticals. Mechanistic study demonstrates that the high-valent Ru(VI)-cis-dioxo species plays an important role in controlling selectivity.

Pyridine Diphosphonate Ligand for Stabilization of Tetravalent Uranium and Neptunium in Aqueous Medium under Aerobic Conditions.

Though uranium is usually present in its +6 oxidation state (as uranyl ion) in aqueous solutions, its conversion to oxidation states such as +4 or +5 is a challenging task. Electrochemical reduction and axial oxo activation are the preferred methods to get stable unusual oxidation states of uranium in an aqueous medium. In previous studies, dicarboxylic acid has been used to stabilize UO2+ in aqueous alkaline solutions. In the present work, a diphosphonate ligand was chosen due to its higher complexing ability compared to that of the carboxylate ligands. Neptunium complexation studies with 2,6-pyridinediphosphonic acid (PyPOH) indicated the formation of different species at different pH values and the complexation facilitates disproportionation of NpO2+ to Np4+ and NpO22+ at pH 2. Hexavalent actinides form insoluble complexes in aqueous media at pH = 2, as confirmed by UO22+ complexation studies. The in situ complexation-driven precipitation resulted in conversion to pure Np4+ in aqueous media as the Np4+-PyPOH complex. A strong complexing ability of the PyPOH ligand toward the Np4+ ion is also seen for the stabilization of the electrochemically generated U4+ in aqueous medium under aerobic conditions. The U4+-PyPOH complex was found to be stable for 3 months. Raman, UV-vis, fluorescence, and cyclic voltametric studies along with density functional theory (DFT) calculations were done to get structural insights into the PyPOH complexes of actinides in different oxidation states.

S-Block metal complexes of superbulky (tBu3Si)2N-: a new weakly coordinating anion?

Sterically hindered amide anions have found widespread application as deprotonation agents or as ligands to stabilize metals in unusual coordination geometries or oxidation states. The use of bulky amides has also been advantageous in catalyst design. Herein we present s-block metal chemistry with one of the bulkiest known amide ligands: (tBu3Si)2N- (abbreviated: tBuN-). The parent amine (tBuNH), introduced earlier by Wiberg, is extremely resistant to deprotonation (even with nBuLi/KOtBu superbases) but can be deprotonated slowly with a blue Cs+/e- electride formed by addition of Cs0 to THF. (tBuN)Cs crystallized as a separated ion-pair, even without cocrystallized solvent. As salt-metathesis reactions with (tBuN)Cs are sluggish and incomplete, it has only limited use as an amide transfer reagent. However, ball-milling with LiI led to quantitative formation of (tBuN)Li and CsI. Structural characterization shows that (tBuN)Li is a monomeric contact ion-pair with a relatively short N-Li bond, an unusual T-shaped coordination geometry around N and extremely short Li⋯Me anagostic interactions. Crystal structures are compared with Li and Cs complexes of less bulky amide ligands (iPr3Si)2N- (iPrN-) and (Me3Si)2N- (MeN-). DFT calculations show trends in the geometries and electron distributions of amide ligands of increasing steric bulk (MeN- < iPrN- < tBuN-) and confirm that tBuN- is a rare example of a halogen-free weakly coordinating anion.

Spin-degree manipulation for one-dimensional room-temperature ferromagnetism in a haldane system.

The intricate correlation between lattice geometry, topological behavior and charge degrees of freedom plays a key role in determining the physical and chemical properties of a quantum-magnetic system. Herein, we investigate the introduction of the unusual oxidation state as an alternative pathway to modulate the magnetic ground state in the well-known S = 1 Haldane system nickelate Y2BaNiO5 (YBNO). YBNO is topologically reduced to incorporate d9-Ni+ (S = 1/2) in the one-dimensional Haldane chain system. The random distribution of Ni+ for the first time results in the emergence of a one-dimensional ferromagnetic phase with a transition temperature far above room temperature. Theoretical calculations reveal that the antiferromagnetic interplay can evolve into ferromagnetic interactions with the presence of oxygen vacancies, which promotes the formation of ferromagnetic order within one-dimensional nickel chains. The unusual electronic instabilities in the nickel-based Haldane system may offer new possibilities towards unconventional physical and chemical properties from quantum interactions.

Unusual redox stability of pentavalent uranium with hetero-bifunctional phosphonocarboxylate: insight into aqueous speciation.

The +5 state is an unusual oxidation state of uranium due to its instability in the aqueous phase. As a result, gaining information about its aqueous speciation is extremely difficult. The present work is an attempt in that direction and it provides insight into the existence of a new pentavalent species in the presence of hetero-bifunctional phosphonocarboxylate (PC) chelators, other than the carbonate ion, in the aqueous medium. The aqueous chemistry of pentavalent uranium species with three environmentally relevant PCs was probed using electrochemical and DFT methods to understand the redox energy and kinetics of conversion of the U(VI)/U(V) couple, stability, structure, stoichiometry, binding modes, etc. Interestingly, pentavalent uranium complexes with PCs are quite persistent over a wide range of pH starting from acidic to alkaline conditions. The PC chelators block the cation-cation interaction (CCI) of U(V) through strong hetero-bidentate chelation and intermolecular hydrogen bonding (IMHB) interactions which stabilize the pentavalent metal ion against disproportionation. For uranyl species in the presence of PCs, acting as chelators, CV plots were obtained at varying pH values from 2 to 8. The obtained results indicate an irreversible single redox peak involving U(VI) to U(V) conversion and association of a coupled chemical reaction with the electron transfer step. ESI-MS studies were performed to understand the speciation effect on the U(VI)/U(V) redox couple with varying pH. Speciation modelling of U(V) with the PC ligands was carried out, which indicated that the U(V) is redox stable in nearly 47% of the pH region in the presence of the PCs as compared to the carboxylate-based chelators. The free energy and reduction potential of the U(V) complexes and the reduction free energy and disproportionation free energy for the U(VI)/U(V) couple were determined by DFT computations in the presence of the PCs. In situ spectroelectrochemical spectra were recorded to provide evidence for the existence of U(V) species with PCs in the aqueous medium and to acquire its absorption spectra. The present study is highly significant for understanding the coordination chemistry of pentavalent uranium species, accurate modelling of uranium, and isolation of U(V).

Crystal structure of a water oxidation catalyst solvate with composition (NH4)2[FeIV(L-6H)]·3CH3COOH (L = clathrochelate ligand).

The synthetic availability of mol-ecular water oxidation catalysts containing high-valent ions of 3d metals in the active site is a prerequisite to enabling photo- and electrochemical water splitting on a large scale. Herein, the synthesis and crystal structure of di-ammonium {μ-1,3,4,7,8,10,12,13,16,17,19,22-dodeca-aza-tetra-cyclo-[8.8.4.13,17.18,12]tetra-cosane-5,6,14,15,20,21-hexa-onato}ferrate(IV) acetic acid tris-olvate, (NH4)2[FeIV(C12H12N12O6)]·3CH3COOH or (NH4)2[FeIV(L-6H)]·3CH3COOH is reported. The FeIV ion is encapsulated by the macropolycyclic ligand, which can be described as a dodeca-aza-quadricyclic cage with two capping tri-aza-cyclo-hexane fragments making three five- and six six-membered alternating chelate rings with the central FeIV ion. The local coord-ination environment of FeIV is formed by six deprotonated hydrazide nitro-gen atoms, which stabilize the unusual oxidation state. The FeIV ion lies on a twofold rotation axis (multiplicity 4, Wyckoff letter e) of the space group C2/c. Its coordination geometry is inter-mediate between a trigonal prism (distortion angle φ = 0°) and an anti-prism (φ = 60°) with φ = 31.1°. The Fe-N bond lengths lie in the range 1.9376 (13)-1.9617 (13) Å, as expected for tetra-valent iron. Structure analysis revealed that three acetic acid mol-ecules additionally co-crystallize per one iron(IV) complex, and one of them is positionally disordered over four positions. In the crystal structure, the ammonium cations, complex dianions and acetic acid mol-ecules are inter-connected by an intricate system of hydrogen bonds, mainly via the oxamide oxygen atoms acting as acceptors.

Preparation of a high-coordinated-silicon-centered spiro-cyclic compound.

Silicon compounds containing silicon-silicon bond with a variety of unusual oxidation states are quite important, because their high reactivity leads to the formation of a variety of silicon compounds. The isolation of such compounds with unusual oxidation states requires a resilient synthetic strategy. Herein, we report the synthesis of a silicon based spirocyclic compound containing a hyper-valent silicon atom and a silicon-silicon bond. The computational calculations employing natural bond orbital (NBO) analysis and energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) reveal that the nature of bonding between the silicon atoms is of an electron sharing nature.

Unveiling the Anomaly of Reduction of Carborane-bis-silylene-Stabilised Silylone/Germylone Leading to Unusual Oxidation of Si0 /Ge0 to SiI /GeI with EDA-NOCV Analyses.

Researchers have successfully isolated Si0 /Ge0 species, termed silylone and germylone, with two lone pairs of electrons on them. These elusive compounds have been stabilised in singlet ground states by using different donor base ligands. Driess et al. in particular have made strides in this area, isolating carborane-bis-silylene-stabilised silylone/germylone and their N+ /Pb analogues. Carborane (C2 B10 H10 ) plays a pivotal role as a redox-active ligand, converting from closo-carborane to nido-carborane with the addition of two electrons. Notably, anomalous oxidation of Si0 /Ge0 centres in carborane-bis-silylene-stabilised species to SiI /GeI has been reported, resulting in the formation of dimeric SiI -SiI /GeI -GeI di-cationic units. The energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) study focuses on the carborane-bis-silylene ligand in the free state, and its three other species, including silylone/germylone species. Interestingly, it reveals that the carborane unit in an anionic doublet state tends to form one electron-sharing bond and one dative bond with the counter fragment in its cationic doublet state. This helps us to rationalise why the carborane unit undergoes intramolecular electronic rearrangements leading to the formation of a di-anionic carborane unit with a significantly elongated C-C bond (2.38-2.68 Å) and undergoes unusual oxidation of Si0 /Ge0 to SiI /GeI .

Role of Bis(phosphinimino)methanides as Universal Ligands in the Coordination Sphere of Metals across the Periodic Table.

The coordination chemistry of bis(phosphinimino)methanide ligands is widespread and accompanies a large number of metal ions in the periodic table ranging from lithium to neptunium. This unique class of ligand systems show copious coordination chemistry with the main-group, transition, rare-earth, and actinide metals and are considered to be among the most attractive ligand systems to researchers. The bis(phosphinimino)methanide metal complexes offer an extensive range of applications in various fields and have been demonstrated as one of the universal ligand systems to stabilize the metal ions in not only their usual but also their unusual oxidation states. The main-group and transition metal chemistry using bis(phosphinimino)methanides as ligands was last updated almost a decade ago. In this review, we provide a comprehensive overview of various state-of-the-art bis(phosphinimino)methanide-supported metal complexes by dealing with their synthesis, characterization, reactivity, and catalytic studies which were not included in the last critical reviews.