Oxide Ligands Research Articles

Phosphineoxide-Chelated Europium(III) Nanoparticles for Ceftriaxone Detection.

The present work demonstrates the optimization of the ligand structure in the series of bis(phosphine oxide) and β-ketophosphine oxide representatives for efficient coordination of Tb3+ and Eu3+ ions with the formation of the complexes exhibiting high Tb3+- and Eu3+-centered luminescence. The analysis of the stoichiometry and structure of the lanthanide complexes obtained using the XRD method reveals the great impact of the bridging group nature between two phosphine oxide moieties on the coordination mode of the ligands with Tb3+ and Eu3+ ions. The bridging imido-group facilitates the deprotonation of the imido- bis(phosphine oxide) ligand followed by the formation of tris-complexes. The spectral and PXRD analysis of the separated colloids indicates that the high stability of the tris-complexes provides their safe conversion into polystyrenesulfonate-stabilized colloids using the solvent exchange method. The red Eu3+-centered luminescence of the tris-complex exhibits the same specificity in the solutions and the colloids. The pronounced luminescent response on the antibiotic ceftriaxone allows for sensing the latter in aqueous solutions with an LOD value equal to 0.974 μM.

Two Novel Schiff Base Manganese Complexes as Bifunctional Electrocatalysts for CO2 Reduction and Water Oxidation.

One mononuclear Mn(III) complex [MnIIIL(H2O)(MeCN)](ClO4) (1) and one hetero-binuclear complex [(CuIILMnII(H2O)3)(CuIIL)2](ClO4)2·CH3OH (2) have been synthesized with the Schiff base ligand (H2L = N,N'-bis(3-methoxysalicylidene)-1,2-phenylenediamine). Single crystal X-ray structural analysis manifests that the Mn(III) ion in 1 has an octahedral coordination structure, whereas the Mn(II) ion in 2 possesses a trigonal bipyramidal configuration and the Cu(II) ion in 2 is four-coordinated with a square-planar geometry. Electrochimerical catalytic investigation demonstrates that the two complexes can electrochemically catalyze water oxidation and CO2 reduction simultaneously. The coordination environments of the Mn(III), Mn(II), and Cu(II) ions in 1 and 2 were provided by the Schiff base ligand (L) and labile solvent molecules. The coordinately unsaturated environment of the Cu(II) center in 2 can perfectly facilitate the catalytic performance of 2. Complexes 1 and 2 display that the over potentials for water oxidation are 728 mV and 216 mV, faradaic efficiencies (FEs) are 88% and 92%, respectively, as well as the turnover frequency (TOF) values for the catalytic reduction of CO2 to CO are 0.38 s-1 at -1.65 V and 15.97 s-1 at -1.60 V, respectively. Complex 2 shows much better catalytic performance for both water oxidation and CO2 reduction than that of complex 1, which could be owing to a structural reason which is attributed to the synergistic catalytic action of the neighboring Mn(III) and Cu(II) active sites in 2. Complexes 1 and 2 are the first two compounds coordinated with Schiff base ligand for both water oxidation and CO2 reduction. The finding in this work can offer significant inspiration for the future development of electrocatalysis in this area.

Impact of added ligand oxides in the sulfoxidation of phenylmercaptoacetic acids with oxovanadium(IV)‐salen complexes

AbstractThe mechanistic aspects covering the effect of added ligand oxides, picoline N‐oxide (PicNO), pyridine N‐oxide (PyNO), and triphenyl phosphine oxide (TPPO) on the selective sulfoxidation of phenylmercaptoacetic acids to PSAA by H2O2 catalyzed by oxovanadium(IV)‐salen complexes have been studied spectrophotometrically in 100% acetonitrile medium. Hydroperoxovanadium(V)‐salen is the reactive species in this sulfoxidation. The rate of sulfoxidation is found to be enhanced significantly by increasing the concentrations of PMAA, salen, and H2O2. The ascertained order of reactivity among the oxovanadium(IV)‐salen complexes is 5,5′‐dimethyl > unsubstituted salen > 5,5′‐dichloro. The reaction is found to be impeded by the addition of ligand oxide and introduction of substituents in PMAA. The observed order of rate retardation among ligand oxides is TPPO < PyNO < PicNO. Downward Hammett plot obtained has been rationalized by change in rate‐determining step from coordination of PMAA to active species for electron‐withdrawing substituents to transfer of oxygen to PMAA for electron‐donating substituents within the same mechanism.

Catalytic activity of a new dioxidomolybdenum(vi) complex with a bipodal N2O-donor ligand in sulfur compound oxidation

A new dinuclear dioxidomolybdenum(vi) complex [(MoO2)2(BPA)2(μ-O)] was synthesized and evaluated as a catalyst in the reaction involving the oxidation of sulfur compounds.

Pressure-induced postsynthetic cluster anion substitution in a MIL-53 topology scandium metal-organic framework.

Postsynthetic modification of metal-organic frameworks (MOFs) has proven to be a hugely powerful tool to tune physical properties and introduce functionality, by exploiting reactive sites on both the MOF linkers and their inorganic secondary building units (SBUs), and so has facilitated a wide range of applications. Studies into the reactivity of MOF SBUs have focussed solely on removal of neutral coordinating solvents, or direct exchange of linkers such as carboxylates, despite the prevalence of ancillary charge-balancing oxide and hydroxide ligands found in many SBUs. Herein, we show that the μ2-OH ligands in the MIL-53 topology Sc MOF, GUF-1, are labile, and can be substituted for μ2-OCH3 units through reaction with pore-bound methanol molecules in a very rare example of pressure-induced postsynthetic modification. Using comprehensive solid-state NMR spectroscopic analysis, we show an order of magnitude increase in this cluster anion substitution process after exposing bulk samples suspended in methanol to a pressure of 0.8 GPa in a large volume press. Additionally, single crystals compressed in diamond anvil cells with methanol as the pressure-transmitting medium have enabled full structural characterisation of the process across a range of pressures, leading to a quantitative single-crystal to single-crystal conversion at 4.98 GPa. This unexpected SBU reactivity - in this case chemisorption of methanol - has implications across a range of MOF chemistry, from activation of small molecules for heterogeneous catalysis to chemical stability, and we expect cluster anion substitution to be developed into a highly convenient novel method for modifying the internal pore surface and chemistry of a range of porous materials.

Intensive emission of Eu(iii) β-diketonate complexes with arsine oxide ligands

Europium(iii) hexafluoroacetylacetonates incorporating arsine oxides were synthesized. Tris(1-naphthyl)arsine oxide yielded a notably more intense emission and higher thermal stability than triphenylphosphine oxide, which is conventionally used.

Multifunctional LnIII complexes with SMM, luminescence and MCE properties constructed using selected phosphine oxide and diketonate

A series of lanthanide-based multifunctional materials with SMM, luminescence and MCE properties were constructed by deliberately selecting β-diketonate and phosphine oxide ligands combined with lanthanide ions.

Amine-assisted catechol-based nanocoating on ultrasmall iron oxide nanoparticles for high-resolution T1 angiography.

Surface engineered iron oxide nanoparticles (IONPs) with catecholic ligands have been investigated as alternative T1 contrast agents. However, complex oxidative chemistry of catechol during IONP ligand exchange causes surface etching, heterogeneous hydrodynamic size distribution, and low colloidal stability because of Fe3+ mediated ligand oxidation. Herein, we report highly stable and compact (∼10 nm) Fe3+ rich ultrasmall IONPs functionalized with a multidentate catechol-based polyethylene glycol polymer ligand through amine-assisted catecholic nanocoating. The IONPs exhibit excellent stability over a broad range of pHs and low nonspecific binding in vitro. We also demonstrate that the resultant NPs have a long circulation time (∼80 min), enabling high resolution T1 magnetic resonance angiography in vivo. These results suggest that the amine assisted catechol-based nanocoating opens a new potential of metal oxide NPs to take a step forward in exquisite bio-application fields.

A Cobalt(III)−Hydroxo Complex Bearing a Pentadentate Amidate Ligand as an Electrocatalyst for Water Oxidation

AbstractA Co(III)−hydroxo complex, [CoIII(dpaq)OH]ClO4 (1‐OH) bearing a pentadentate ligand, H‐dpaq, (H‐dpaq=(2‐[bis(pyridin‐2‐ylmethyl)]amino‐N‐quinolin‐8‐yl‐acetamidate]) catalyses water oxidation in mildly alkaline medium (pH 8.0) at a potential of 1.4 VNHE with an average Turn‐Over‐Frequency (TOFmax) of 2.8×104 s−1 and faradaic efficiency of 88 %. Post‐electrolysis characterization of the electrode rules out the formation of any heterogeneous electroactive species. Electrochemical results and theoretical calculations confirm the occurrence of both metal and ligand centered PCET processes during anodic scanning. The resulting formally Co(V)−oxo/oxyl intermediate undergoes water nucleophilic attack to install the O−O bond. The role of axial ligand in water oxidation by Co(III)−dpaq system has been examined by comparing the reactivity of the Co‐hydroxide complex (1‐OH) with that of its chloride‐ligated counterpart, [CoIII(dpaq)Cl]Cl (1‐Cl). The results confirm the ability of the Co‐dpaq complexes to bind water/or water derived ligands over chloride or non‐aqueous solvents. The interplay of ligand redox non‐innocence and σ‐donating ability of the N5‐carboxamido ligand helps to store oxidizing equivalents and triggers O−O bond formation.

Strong hydrogen bond in the crystal structure design of CuNbOF5·4H2O

Fluoride metal-organic frameworks (MOFs) based on pillared [NbOF5]2– anion have been recently received research attention as ultramicroporous materials for gas storage and separation. The trans-directing property of the NbOF5 octahedron plays a significant role in designing mixed metal oxyfluoride compounds composed of the alternating transition metal polyhedra, which are linked via oxide and fluoride ligands. Mixed metal CuNbOF5·4H2O is the only unique example when the trans-directing properties of [NbOF5]2– are realized in the formation of a strong O–H⋯F hydrogen bond (HB). Well-shaped single crystals of this compound were grown and their structure was refined by X-ray diffraction. Under quasi-isobaric conditions, a phase transition at 88 ​°C takes place, which is associated with the weakening of HBs and formation of ordinary chains through conventional trans-(O,F) bridges. The character of hydrogen bonding in CuNbOF5·4H2O affects its thermal behavior described by the dehydration and pyrohydrolysis processes.

Ruthenium Complexes of Rigid, Dianionic, Tetradentate N‐Donor Ligands and their Potential as Catalysts for Water Oxidation

AbstractTwo mononuclear ruthenium(II) complexes based on dianionic {N4} ligands and with axial pyridines have been prepared and characterized crystallographically (1) or by 2D NMR spectroscopy using residual dipolar couplings (2). The {N4} ligands provide a constrained equatorial coordination with one large N−Ru−N angle, and additional non‐coordinating N atoms in case of 2. Their redox properties have been investigated (spectro)electrochemically, and their potential to serve as water oxidation catalysts has been probed using cerium ammonium nitrate (CAN) at pH 1.0. Complex 1 undergoes rapid degradation, likely via ligand oxidation, whereas 2 is more rugged and exhibits 80 % efficiency in the CeIV‐driven water oxidation, with a high initial turnover frequency (TOFi) of 3.07×10−2 s−1 (at 100 equiv. CAN). The initial rate of O2 evolution exhibits 1st order dependence on catalyst concentration, suggesting a water nucleophilic attack mechanism. Repeated addition of CAN and control experiments show that high ionic strength conditions (both NO3− and CeIII) significantly decrease the TOF.

Dioxygen Splitting by a Tantalum(V) Complex Ligated by a Rigid, Redox Non-Innocent Pincer Ligand.

The reaction of TaMe3 Cl2 with the rigid acridane-derived trisamine H3 NNN yields the tantalum(V) complex [TaCl2 (NNNcat )]. Subsequent reaction with dioxygen results in the full four-electron reduction of O2 yielding the oxido-bridged bimetallic complex [{TaCl2 (NNNsq )}2 O]. This dinuclear complex features an open-shell ground state due to partial ligand oxidation and was comprehensively characterized by single crystal X-ray diffraction, LIFDI mass spectrometry, NMR, EPR, IR and UV/VIS/NIR spectroscopy. The mechanism of O2 activation was investigated by DFT calculations revealing initial binding of O2 to the tantalum(V) center followed by complete O2 scission to produce a terminal oxido-complex.

Stabilizing Doubly Deprotonated Diazomethane: Isolable Complexes with CN22- and CN2- Radical Ligands.

Transition metal complexes with a doubly deprotonated diazomethane (CNN2-) ligand have been proposed as fleeting intermediates in nitrogen transfer reactions. However, in contrast to isoelectronic azide (N3-), well-defined examples are unknown. We here report the synthesis and characterization of isolable complexes with terminal and bridging CNN2- ligands, stabilized by platinum(II) pincer fragments. Bonding within the allenic dimetallanitrilimine core (Pt-N═N═C-Pt) was probed by oxidation of the bridging ligand. Enhanced reactivity toward [3 + 2]-cycloaddition with CO2 was obtained. Photofragmentation favors N-NC over NN-C bond cleavage as a route to cyanide and a transient metallonitrene complex.

Mechanistic Insights into the Oxidative and Reductive Quenching Cycles of Transition Metal Photoredox Catalysts through Effective Oxidation State Analysis.

The electronic structures of the ground and excited electronic states involved in the oxidative and reductive quenching cycles of 12 relevant ruthenium, iridium, and copper photoredox catalysts (S0, T1, Dox, and Dred) are characterized using the recently developed effective oxidation state (EOS) analysis, allowing the monitoring of metal and ligand oxidation states (OSs) along the catalytic cycles. The formal oxidation state assignments derived from the EOS analysis are in agreement with those commonly assumed for these complexes in both ground and excited states. Rather clean and separate ligand- and metal-centered redox events along the different quenching cycles are observed in most of the studied molecular systems. The reliability index obtained for the OS assignations can be readily interpreted in terms of the ionic/covalent character of metal-ligand interactions and ligand non-innocent character. In addition, EOS analysis reveals the high-degree localization of the ligand-centered redox event to one or two redox-active ligand(s) in heteroleptic complexes. Ligand- and metal-condensed spin populations were also computed and analyzed for all the open-shell species involved in this study, showing promises for rapid oxidation state assignments in certain systems, especially Ru complexes, however, suffering from severe defects in other cases.

Synthesis and Characterisation of Novel Bis(diphenylphosphane oxide)methanidoytterbium(III) Complexes.

Reaction of [YbCp2(dme)] (Cp = cyclopentadienyl, dme = 1,2 dimethoxyethane) with bis(diphenylphosphano)methane dioxide (H2dppmO2) leads to deprotonation of the ligand H2dppmO2 and oxidation of ytterbium, forming an extremely air-sensitive product, [YbIII(HdppmO2)3] (1), a six-coordinate complex with three chelating (OPCHPO) HdppmO2 ligands. Complex 1 was also obtained by a redox transmetallation/protolysis synthesis from metallic ytterbium, Hg(C6F5)2, and H2dppmO2. In a further preparation, the reaction of [Yb(C6F5)2] with H2dppmO2, not only yielded compound 1, but also gave a remarkable tetranuclear cage, [Yb4(µ-HdppmO2)6(µ-F)6] (2) containing two [Yb(µ-F)]2 rhombic units linked by two fluoride ligands and the tetranuclear unit is encapsulated by six bridging HdppmO2 donors. The fluoride ligands of the cage result from C-F activation of pentafluorobenzene and concomitant formation of p-H2C6F4 and m-H2C6F4, the last being an unexpected product.

Modulation of Charge Distribution in Cobalt-α-Diimine Complexes toward Valence Tautomerism.

Valence tautomerism (VT) and spin crossover (SCO) are promising avenues for developing a range of molecular materials for sensing, memory, and optoelectronic applications. However, these phenomena arise only when specific metal-ligand combinations are employed. The underexplored combination of cobalt(II/III) paired with bis((aryl)imino)acenapthene (Ar-BIAN) ligands, which can exist as neutral Ar-BIAN0 (L0), monoanionic radical Ar-BIAN•- (L•-), and dianionic Ar-BIAN2- (L2-) forms, has potential to afford both VT and SCO. Aiming to develop a new family of switchable molecules, we systematically explored a dual-tuning approach by varying the redox state and aryl substituents in a series of homoleptic [Co(Ar-BIAN)3]n+ complexes (Ar = Ph, n = 2 (12+), 1 (1+), 0 (1); Ar = 3,5-CF3-Ph, n = 0 (2); Ar = 4-MeO-Ph, n = 2 (32+), 0 (3)). As a prelude to synthetic and experimental studies, density functional theory (DFT) calculations were used to explore the structure and relative energies of the different electronic forms of each complex, comprising different cobalt oxidation and spin states and different ligand oxidation states. Except for compound 3, DFT identified a HS-CoII-L0 containing ground state for all complexes, precluding thermally induced SCO or VT. For 3, calculations suggested a possible thermally accessible LS-CoIII-(L•-)3 ⇌ HS-CoII-(L•-)2(L0) VT interconversion. Experimentally, structural and magnetic data reveal a HS-CoII-L0 containing ground state for all six compounds in the solid state, including 3, discounting thermally induced VT or SCO. In solution, electrochemical and spectroscopic analysis also indicate that all compounds exist as the HS-CoII-L0-containing electromer at 298 K. Intervalence charge transfer (IVCT) bands observed for neutral 1, 2, and 3 at room temperature suggest the mixed-valence HS-CoII-(L•-)2(L0) charge distribution. However, cooling 3 to 243 K in acetonitrile uniquely affords a substantial reduction in the intensity of this IVCT band, consistent with thermally induced VT interconversion to the LS-CoIII-(L•-)3 ground state as predicted by DFT calculations. This study emphasizes the utility of computationally guided molecular design for complicated systems with redox activity at the metal and multiple ligands, thus opening new avenues for tuning electronic structure and developing new families of switchable molecules.

Surface Exchange Reaction of Mixed Conductive La0.65Ca0.35FeO3−δ during Oxygen Evolution and Incorporation as Traced by Operando X-ray Photoelectron Spectroscopy

High oxygen permeability of mixed conductive La0.65Ca0.35FeO3-δ (LCF) is applicable to pure oxygen gas generators and cathodes for solid oxide fuel cells, etc.; however, lower surface exchange reactions at temperatures below 800 °C reduce permeability. To understand the microscopic surface reaction mechanism, operando soft X-ray photoelectron spectroscopy of an LCF film surface was conducted during the evolution and incorporation of oxygen. LCF film was prepared on yttria-stabilized zirconia and a current was applied throughout the film at ∼600 °C. From operando X-ray photoelectron spectra, surface oxide species involved in the surface exchange reaction obviously appeared on the film during the evolution of oxygen from the surface. The number of surface oxide species abruptly decreased during incorporation of oxygen. By applying the current from a negative to positive value, the numbers of surface oxide species and ligand holes near Fe3+ ions on the surface both significantly increased. The results infer that ligand holes in the Fe 3d-O 2p hybrid orbitals correspond to active reaction sites at which surface oxide species change to oxygen molecules. Increasing the number of active reaction sites is key to improving oxygen evolution of mixed conductive oxides.

Pushing the Emission Envelope for Full-Color Realization of Colloidal Semiconductor Core/Shell Nanoplatelets

Colloidal semiconductor nanoplatelets (NPLs) have been known to exhibit favorable optical characteristics, such as a narrow emission bandwidth, giant oscillator strength, suppressed Auger recombination, and polarized emission, which can merit their use in light-emitting diodes and lasing devices. However, to date, NPLs have shown a relatively limited range of emission energy because of their thickness-dependent discrete band gap and restricted choice of chemical composition (mostly CdSe-based), impeding the realization of R, G, and B full-color display based on NPLs. CdSe-based core/shell NPLs can only cover the emission range from yellowish green (∼550 nm) to red (∼620 nm). In this study, we synthesize CdZnSe/ZnS core/shell NPLs with significantly enhanced spectral tunability over a broad spectral range (425–570 nm). Particularly, to overcome the difficulty in the synthesis of composition-controlled CdZnSe NPLs, we carried out a Cd-to-Zn direct cation exchange reaction on CdSe/ZnS core/shell NPLs using ZnI2 and an oleylamine (or trioctylphosphine oxide) ligand, which allows for the composition change in NPL emitting core while maintaining the original shape of NPLs. With the control of the reaction temperature and time as well as the ligands, the exchange reaction enables us to demonstrate an unprecedentedly wide emission range and spectral tunability of NPLs with the emission wavelength reaching 425 nm. Furthermore, as the core alloying proceeds, NPLs maintain excellent color purity and exhibit linearly polarized emission. Our light-emitting devices using the cation-exchanged CdZnSe/ZnS NPLs hinted at the possibility of using the NPLs in full-color display applications. Based on the elemental mapping analysis and experimental results, we proposed the temperature-dependent cation exchange reaction process in core/shell NPLs.

(Digital Presentation) Elucidation of Active Oxygen Sites upon Delithiation of Li3IrO4

Transformational increases in the storage capacity of battery cathodes could be achieved by tapping into the redox activity at oxide ligands in addition to conventional transition metal couples. Yet the key signatures that govern such lattice oxygen redox (LOR) have not been ascertained. Li3IrO4 has the largest reversible LOR, rendering it a unique model system. Here, X-ray spectroscopy and computational simulations reveal that LOR in Li3IrO4 is selectively compensated via O sites with 3 lone pairs, which are activated by Li/Ir disorder. The 2-electron LOR can be reversed to regenerate the initial state without unlocking competing bulk reactions observed in many other compounds. We uncover an intricate interplay between stoichiometry, O coordination and non-bonding states in LOR and pinpoint spectroscopic signatures. This interplay is indispensable to design materials with 3d metals that fulfill the promise of LOR to overcome the bottlenecks of current cathodes for future implementation in practical batteries.

Solution Phase Deposition of Metal Oxides Directly Onto Perovskite Absorbers

Perovskite solar cells have been discussed as a low-cost alternative to traditional technologies based on the simple deposition mechanisms and inexpensive materials. These materials include metal oxides that are relatively straightforward to manufacture and can be suspended in inexpensive solvent systems including water, alcohols and aromatic hydrocarbons. However, these solvents are not appropriate for the perovskite materials and hence most efforts do not include the deposition of a metal oxide onto a perovskite. In this talk we will discuss the avenues to establish direct deposition of metal oxides onto a perovskite. This includes solvent choice, metal oxide choice, ligands and interfacial layers. The talk will then demonstrate the successful deposition of metal oxides by scalable solution phase processes with efficiencies greater than 18%. Results will include means to assess the potential damage to the films with an emphasis towards scalable manufacturing.