Isoelectronic Compounds Research Articles

Nonalternating π-System Mediated Spin Coupling in Azulene Nitronyl Nitroxide Diradicals.

To explore the distinctions in spin coupling between the molecular bridges of alternating and nonalternating π-systems, we synthesized a pair of isoelectronic compounds, namely, 2,6-Na-NN and 2,6-Az-NN, by utilizing naphthalene and azulene (naphthalene = Na and azulene = Az) as the bridges, respectively. Moreover, we conducted assessments to predict the coupling paths for nonalternating azulene. Variable-temperature EPR (VT-EPR) and SQUID results consistently reveal that both 2,6-Na-NN and 2,6-Az-NN exhibit antiferromagnetic coupling interactions, with coupling constants of J(2,6-Na-NN) = -22.3 cm-1 and J(2,6-Az-NN) = -30.1 cm-1, respectively. Density functional theory computations support these discoveries by revealing negative coupling constants (J < 0) and the spin densities population of the diradicals are observed to delocalize into the molecular bridges. This work suggests the most suitable coupling path for 2,6-Az-NN. In addition, we have investigated the potential spatial resistance of the diradicals in conjunction with single-crystal data. Theoretical calculations underestimating the torsion angle of the diradicals and overestimating the value of the magnetic coupling provide an explanation for this phenomenon. The final experimental results and theoretical calculations show that the 2,6-Az-NN coupling path prefers short paths.

Low-energy high-spin binuclear hexamethylbenzene manganese carbonyl structures with partially bonded benzene rings

The geometries and energetics of binuclear hexamethylbenzene manganese carbonyls (Me6C6)2Mn2(CO)n (n = 4, 3, 2, 1), including the experimentally known tetracarbonyl, have been examined using density functional theory. Relatively low- energy structures include trans- and cis-(η6-Me6C6)2Mn2(CO)2(µ-CO)2 doubly bridged tetracarbonyl structures and a triplet (η6-Me6C6)2Mn2(µ-CO)3 triply bridged structure analogous to experimentally known isoelectronic binuclear cyclopentadienyliron carbonyls. However, in addition, a series of low-energy quintet spin state structures (η10–2n-Me6C6)Mn(CO)n–Mn(η6-C6Me6) (n = 4, 3, 2) is found. The tetracarbonyls and tricarbonyl of this series have partially bonded hexamethylbenzene rings suggesting a certain fragility of the hexamethylbenzene-manganese linkage not found in the isoelectronic cyclopentadienyliron compounds. For the dicarbonyl and monocarbonyl (Me6C6)2Mn2(CO)n (n = 2, 1) systems, singlet structures having the triple or quadruple manganese-manganese bonds necessary to give each manganese atom the favored 18-electron configuration are very high-energy structures relative to isomeric triplet and quintet spin state structures.

Investigation of structural, electronic, elastic and optical properties of Ge-halide perovskites NaGeX3 (X = Cl, Br and I): A first-principles DFT study

Lead-free perovskites are among materials that are currently most investigated for their potential application in photovoltaic and optoelectronic applications due to their non-toxic effect on the environment. In this study, a first-principle Density Functional Theory (DFT) calculation to study structural, elastic, electronic, and optical properties of Ge-halide perovskites namely cubic NaGeX3 (X = Cl, Br, and I) is presented and reported for the first time. The Quantum Espresso code with GGA-PBE functional was used for conducting the DFT calculation on these lead-free perovskite materials. The structural investigation revealed that the optimized lattice constants for NaGeCl3, NaGeBr3, and NaGeI3 are 5.24 Å, 5.50 Å, and 5.90 Å, respectively. Meanwhile, all compounds are shown to possess semiconducting behaviour and have direct R→R energy bandgap. Consistent with other isoelectronic compounds, it is observed that the energy gaps are tuned to slightly lower values as Cl is replaced by Br and I, with energy gaps 0.76 eV, 0.45 eV, and 0.44 eV for the respective compounds. From their Bulk modulus and other elastic parameters calculated, the materials are stable and ductile, making them easily incorporated into thin films in solar cells and other applications. The materials also exhibit excellent optical properties including high absorption coefficient and therefore they have the potential to be used in developing lead-free perovskite solar cells and other optoelectronic applications.

Enhancement of superconductivity at a quantum critical point in (Ca x Sr1−x )3Rh4Sn13

We report microscopic studies by muon spin spectroscopy of the superconducting properties as a function of chemical and hydrostatic pressure in the cubic ternary intermetallic (Ca x Sr1−x )3Rh4Sn13 compounds. We find evidence of a quantum critical point at a critical pressure p c in the superconducting phase, where the superfluid density increases by a factor of two and the superconducting pairing strength displays a pronounced maximum. The enhancement of superconductivity is related to the structural phase transition at p c , which is accompanied by profound changes of the Fermi surface associated to the suppression of a charge density wave (CDW). The quantum critical point separates a superconducting phase coexisting with CDW from a pure superconducting phase, while in both phases superconductivity has a strong-coupling phonon-mediated BCS-like s-wave character. Together with the related isoelectronic compound (Ca x Sr1−x )3Ir4Sn13, this system shows that conventional BCS superconductors in the presence of competing orders may display behaviour and characteristics of unconventional superconductors.

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII.

Many coordination complexes and organometallic compounds with the 4d6 and 5d6 valence electron configurations have outstanding photophysical and photochemical properties, which stem from metal-to-ligand charge transfer (MLCT) excited states. This substance class makes extensive use of the most precious and least abundant metal elements, and consequently there has been a long-standing interest in first-row transition metal compounds with photoactive MLCT states. Semiprecious copper(I) with its completely filled 3d subshell is a relatively straightforward and well explored case, but in 3d6 complexes the partially filled d-orbitals lead to energetically low-lying metal-centered (MC) states that can cause undesirably fast MLCT excited state deactivation. Herein, we discuss recent advances made with isoelectronic Cr0, MnI, FeII, and CoIII compounds, for which long-lived MLCT states have become accessible over the past five years. Furthermore, we discuss possible future developments in the search for new first-row transition metal complexes with partially filled 3d subshells and photoactive MLCT states for next-generation applications in photophysics and photochemistry.

Zinc(II) Complexes with Triplet Charge-Transfer Excited States Enabling Energy-Transfer Catalysis, Photoinduced Electron Transfer, and Upconversion.

Many CuI complexes have luminescent triplet charge-transfer excited states with diverse applications in photophysics and photochemistry, but for isoelectronic ZnII compounds, this behavior is much less common, and they typically only show ligand-based fluorescence from singlet π–π* states. We report two closely related tetrahedral ZnII compounds, in which intersystem crossing occurs with appreciable quantum yields and leads to the population of triplet excited states with intraligand charge-transfer (ILCT) character. In addition to showing fluorescence from their initially excited 1ILCT states, these new compounds therefore undergo triplet–triplet energy transfer (TTET) from their 3ILCT states and consequently can act as sensitizers for photo-isomerization reactions and triplet–triplet annihilation upconversion from the blue to the ultraviolet spectral range. The photoactive 3ILCT state furthermore facilitates photoinduced electron transfer. Collectively, our findings demonstrate that mononuclear ZnII compounds with photophysical and photochemical properties reminiscent of well-known CuI complexes are accessible with suitable ligands and that they are potentially amenable to many different applications. Our insights seem relevant in the greater context of obtaining photoactive compounds based on abundant transition metals, complementing well-known precious-metal-based luminophores and photosensitizers.

Dependence of martensite transformation temperature on magnetic field in Ni2MnGa and Ni2MnGa0.95In0.05 single crystals

The shift of martensite transformation temperatures with magnetic field is an essential parameter for magnetic shape memory effect as well as for magnetocalorics. We compare the dependence of martensite transformation temperature on magnetic field in Ni2MnGa and Ni2MnGa0.95In0.05 single crystals. Choosing the stoichiometric and isoelectronic compound enables to observe the effect of In alloying free from the influence of off-stoichiometry. The martensite start temperature, Ms, decreases several kelvins in low magnetic fields (< 1 T). In high field (> 2 T), Ms increases with the magnetic field and the Ms(H) dependence is about linear with the slope of 0.26 K/T (Ni2MnGa) and 0.36 K/T (Ni2MnGa0.95In0.05). Modest 1.25 at% indium alloying decreases not only transformation temperature Ms by about 100 K but also considerably decreases the latent heat of martensite transformation from 1.99 kJ/kg (Ni2MnGa) to 0.29 kJ/kg (Ni2MnGa0.95In0.05) determined from Clausius-Clapeyron equation. The latent heat of premartensitic transformation also decreases, approximately five times.

Formation of the N≡N Triple Bond from Reductive Coupling of a Paramagnetic Diruthenium Nitrido Compound.

Construction of nitrogen-nitrogen triple bonds via homocoupling of metal nitrides is an important fundamental reaction relevant to a potential Nitrogen Economy. Here, we report that room temperature photolysis of Ru2(chp)4N3 (chp- = 2-chloro-6-hydroxypyridinate) in CH2Cl2 produces N2 via reductive coupling of Ru2(chp)4N nitrido species. Computational analysis reveals that the nitride coupling transition state (TS) features an out-of-plane "zigzag" geometry instead of the anticipated planar zigzag TS. However, with intentional exclusion of dispersion correction, the planar zigzag TS geometry can also be found. Both the out-of-plane and planar zigzag TS geometries feature two important types of orbital interactions: (1) donor-acceptor interactions involving intermolecular donation of a nitride lone pair into an empty Ru-N π* orbital and (2) Ru-N π to Ru-N π* interactions derived from coupling of nitridyl radicals. The relative importance of these two interactions is quantified both at and after the TS. Our analysis shows that both interactions are important for the formation of the N-N σ bond, while radical coupling interactions dominate the formation of N-N π bonds. Comparison is made to isoelectronic Ru2-oxo compounds. Formation of an O-O bond via bimolecular oxo coupling is not observed experimentally and is calculated to have a much higher TS energy. The major difference between the nitrido and oxo systems stems from an extremely large driving force, ∼-500 kJ/mol, for N-N coupling vs a more modest driving force for O-O coupling, -40 to -140 kJ/mol.

Half antiperovskites VII – DFT modelling of shandites that are isoelectronic to Co3Sn2S2

AbstractOur discovery of half metal ferromagnetic (HFM) properties on shandite type Co3Sn2S2=Sn2Co3S2 about 20 years ago by DFT calculations opened the gate for fascinating discoveries like giant anomalous Hall effect and Weyl semimetal characteristics. Thereby, interest arose on electronic and magnetic structure effects upon substitution of M=Co and A=Sn sites in and between Co Kagomé layers. Non isoelectronic substitution to A=In or M=Ni causes a decay of magnetic properties to semiconducting diamagnetic InSnCo3S2 or paramagnetic semi metal Ni3Sn2S2. The present study addresses simultanious substitutions on both A and M sites to novel isoelectronic compounds of Co3Sn2S2. Therefore, DFT calculations were performed on model structures MM'2AA'X2 (M=Fe, Co, Ni; A=In, Sn, Sb; X=S). By the given approach, target compositions are identified that are interesting for future investigations and discoveries.

Pyrochlore oxide Hg2Os2O7 on verge of metal–insulator boundary

Semimetallic osmium pyrochlore oxide Cd2Os2O7 undergoes a magnetic transition to an all-in–all-out (AIAO)-type order at 227 K, followed by a crossover to an AIAO insulator at around 210 K. Here, we studied the isostructural and isoelectronic compound Hg2Os2O7 through thermodynamic measurements, muon spin rotation (μSR) spectroscopy and neutron diffraction experiments. A similar magnetic transition, probably to an AIAO-type order, was observed at 88 K, while the resistivity showed a decrease at the transition and remained metallic down to 2 K. Thus, the ground state of Hg2Os2O7 is most likely an AIAO semimetal, which is analogous to the intermediate-temperature state of Cd2Os2O7. Hg2Os2O7 exists on the verge of the metal–insulator boundary on the metal side and provides an excellent platform for studying the electronic instability of 5d electrons with moderate electron correlations and strong spin–orbit interactions.

On the non-existence of a square-planar pentaiodide coordination complex I(I)4.

The properties of two conformers of the pentaiodide ion, a V-shaped and regularly observed I5- ion, and a so far undetected square-planar coordination complex of II4- composition, have been investigated by computational methods. The latter compound is indicated by the analogy to the coordination chemistry of gold with halide ligands, as well as isoelectronic main-group compounds. Static and dynamic simulations at density-functional and semi-empirical level including effects of solvent and counter ions indicate that the square-planar II4- indeed represents a well-defined local minimum on the pentaiodide potential energy surface, albeit less stable than the typically observed V-shaped I5-. No simple pathway of transformation between the two forms of the pentaiodide ion can be identified. Molecular dynamics simulations indicate that the presence of cations, unavoidable during the synthesis of polyiodide compounds, may trigger decomposition of the II4- coordination complex into smaller polyiodide building blocks and thus constitute the main reason why this conformer so far has not been identified in solid polyiodide compounds. However, its intrinsic stability indicates that the square-planar form should be possible to isolate in solid compounds given the right conditions of synthesis.

Integrating Computation and Experiment to Investigate Photoelectrodes for Solar Water Splitting at the Microscopic Scale.

ConspectusPhotoelectrochemical water-splitting is a promising and sustainable way to store the energy of the sun in chemical bonds and use it to produce hydrogen gas, a clean fuel. The key components in photoelectrochemical cells (PECs) are photoelectrodes, including a photocathode that reduces water to hydrogen gas and a photoanode that oxidizes water to oxygen gas. Materials used in photoelectrodes for PECs must effectively absorb sunlight, yield photogenerated carriers, and exhibit electronic properties that enable the efficient shuttling of carriers to the surface to participate in relevant water-splitting reactions. Discovering and understanding the key characteristics of optimal photoelectrode materials is paramount to the realization of PEC technologies.Oxide-based photoelectrodes can satisfy many of these materials requirements, including stability in aqueous environments, band edges with reasonable alignment with the redox potentials for water splitting, and ease of synthesis. However, oxide photoelectrodes generally suffer from poor charge transport properties and considerable bulk electron–hole separation, and they have relatively large band gaps. Numerous strategies have been proposed to improve these aspects and understand how these improvements are reflected in the photoelectrochemical performance. Unfortunately, the structural and compositional complexity of multinary oxides accompanied by the inherent complexity of photoelectrochemical processes makes it challenging to understand the individual effects of composition, structure, and defects in the bulk and on the surface on a material’s photoelectrochemical properties. The integration of experiment and theory has great potential to increase our atomic-level understanding of structure–composition–property relationships in oxide photoelectrodes.In this Account, we describe how integrating experiment and theory is beneficial for achieving scientific insights at the microscopic scale. We highlight studies focused on understanding the role of (i) bulk composition via solid-state solutions, intercalation, and comparison with isoelectronic compounds, (ii) dopants for both the anion and cation and their interactions with oxygen vacancies, and (iii) surface/interface structure in the photocurrent generation and photoelectrochemical performance in oxide photoelectrodes. In each instance, we outline strategies and considerations for integrating experiment and theory and describe how this integration led to valuable insights and new directions in uncovering structure–composition–property relationships. Our aim is to demonstrate the unique value of combining experiment and theory in studying photoelectrodes and to encourage the continued effort to bring experiment and theory in closer step with each other.

An Air‐ and Water‐Stable B4N4‐Heteropentalene Serving as a Host Material for a Phosphorescent OLED

AbstractReplacement of the carbon–carbon bonds of antiaromatic compounds with polar boron–nitrogen bonds often provides isoelectronic BN compounds with excellent thermodynamic stability and interesting photophysical properties. By this element‐substitution strategy, we synthesized a new B4N4‐heteropentalene derivative, 1, which is fully substituted with mesityl groups. Owing to kinetic protection by the sterically bulky substituents, 1 is remarkably stable toward air and even water. Single‐crystal X‐ray analysis of 1 revealed the bonding characteristics of the B4N4‐heteropentalene structure. In a glassy matrix, 1 emitted short‐wavelength phosphorescence with an onset at 350 nm, indicating that the triplet energy is substantially high. DFT calculations reasonably explained the ground‐ and excited‐state electronic structures of 1 as well as its emission properties. Motivated by the high‐energy triplet state of 1, we used it as a host material to fabricate a phosphorescent organic light‐emitting diode with an external quantum efficiency of 15 %.

An Air- and Water-Stable B4 N4 -Heteropentalene Serving as a Host Material for a Phosphorescent OLED.

Replacement of the carbon-carbon bonds of antiaromatic compounds with polar boron-nitrogen bonds often provides isoelectronic BN compounds with excellent thermodynamic stability and interesting photophysical properties. By this element-substitution strategy, we synthesized a new B4 N4 -heteropentalene derivative, 1, which is fully substituted with mesityl groups. Owing to kinetic protection by the sterically bulky substituents, 1 is remarkably stable toward air and even water. Single-crystal X-ray analysis of 1 revealed the bonding characteristics of the B4 N4 -heteropentalene structure. In a glassy matrix, 1 emitted short-wavelength phosphorescence with an onset at 350 nm, indicating that the triplet energy is substantially high. DFT calculations reasonably explained the ground- and excited-state electronic structures of 1 as well as its emission properties. Motivated by the high-energy triplet state of 1, we used it as a host material to fabricate a phosphorescent organic light-emitting diode with an external quantum efficiency of 15 %.

Rhodocenium Functionalization Enabled by Half-Sandwich Capping, Zincke Reaction, Diazoniation and Sandmeyer Chemistry.

In continuation of our exploration of metallocenium chemistry we report here on innovative ways toward monofunctionalized rhodocenium salts applying half‐sandwich capping reactions of cyclopentadienyl rhodium(III) halide synthons with cyclopentadienyl ylides containing pyridine, phosphine or dinitrogen leaving groups, followed by Zincke and Sandmeyer reactions. Thereby amino, diazonio, bromo, azido and iodo rhodocenium salts containing valuable functional groups are accessible for the first time. Target compounds were characterized by spectroscopic (1H/13C/103Rh‐NMR, IR, HR‐MS), structural (single crystal XRD) and electrochemical (CV) methods and their properties were compared to those of isoelectronic cobaltocenium compounds. These new functionalized rhodocenium complexes significantly expand the so far extremely limited chemical space of rhodocenium salts with promising options for the future development in the area of rhodocenium chemistry.

First principles investigation of intrinsic and Na defects in XTe (X=Ca, Sr, Ba) nanostructured PbTe

Embedding isoelectronic and isostructural XTe (X = Ca, Sr, Ba) compounds in Na-doped PbTe can significantly boost thermoelectric performance through interface engineering and phonon scattering. Thus, defects in PbTe-based compounds play an essential role in improving the thermoelectric properties. In this study, we investigate the formation energies of charged intrinsic and extrinsic (Na) defects in a PbTe/PbXTe/XTe pseudo-interface. We find that different synthesis conditions of PbTe uniquely determine the lowest-energy defects. The low formation energies of NaSr1− (and NaPb1−) play an important role in increasing the Na concentration in the solid solution PbTe/SrTe interface, in good agreement with the experimental observations. A low energy Na charged defect (n-type NaTe3+) has been distinctly identified in the PbXTe solid solutions as well. Thus, the defect should be eliminated in the SrTe precipitated PbTe system for the p-type purpose. However, if experiments could synthesize the Pb0.5Sr0.5Te solid solution ingot, NaTe3+ will play an important role to achieve the n-type behavior. While low energy defects have little effect on the electronic structures in PbTe and XTe, they enhance the density of states around the Fermi level in PbXTe solid solutions. Our work therefore not only elucidates the lowest energy defects in PbTe-based materials, but it also paves the way to understanding and designing promising thermoelectrics with interface phases.

Dativeversuselectron-sharing bonding in the isoelectronic argon compounds ArR+(R = CH3, NH2, OH, and F)

For the series of isoelectronic ArR+(R = CH3, NH2, OH, and F) complexes, the nature of the bonding between Ar and R shifts from an Ar → R+dative σ bond in ArCH3+and ArNH2+to an Ar+–R electron-sharing σ bond in ArOH+and ArF+.

Hydrogen bonding to metals as a probe for an inverted ligand field.

Electron-rich, late transition metals are known to act as hydrogen-bonding (HBd) acceptors. In this regard, Pt(ii) centres in square-planar environments are particularly efficient. It is however puzzling that no convincing experimental evidence is currently available for the isoelectronic neighbour Au(iii) being involved in HBd interactions. We report now on the synthesis and characterisation of two series of isoleptic and isoelectronic (d8) compounds [(CF3)3Pt(L)]- and (CF3)3Au(L), where the L ligands are based on the quinoline frame and have been selected to favour HBd with the metal centre. Strong HBd interactions were actually found in the Pt(ii) compounds, based on structural and spectroscopic evidence, and they were further confirmed by theoretical calculations. In contrast, no evidence was obtained in the Au(iii) case. In order to find the reason underlying this general disparity, we undertook a detailed theoretical analysis of the model systems [(CF3)3Pt(py)]- and (CF3)3Au(py). This study revealed that the filled dz2 orbital is the HOMO in the case of Pt(ii), but is buried in the lower energy levels in the case of Au(iii). The sharply different electronic configurations involve ligand-field inversion on going from Pt to the next element Au. This is not a gradual but an abrupt change, which invalidates Au(iii) as a HBd-acceptor wherever ligand-field inversion occurs.

Structural stability and anomalous compressibility of isoelectronic compounds- URh3 and UIr3

The compound was synthesized using arc-melting technique and high-pressure X-ray diffraction studies were carried out up to 25 GPa. It is found to remain in its parent AuCu3 type structure up to the highest pressure studied. Birch-Murnaghan equation of state fit of pressure-volume data gives the bulk modulus to be 262 GPa while the bulk modulus of the similar isoelectronic compound – URh3 is 133 GPa. Rh and Ir being isoelectronic transition metals, their compounds with uranium metal show a dramatic change in the compressibility of the material. First principle computations indicate a significantly large contribution of 5-f electrons of uranium in UIr3. A strong f-d hybridization in UIr3, as compared to URh3, results in high bulk modulus values of UIr3. The high structural stability of UIr3 is also predicted from the density of state plots as compared to URh3. Computed elastic constants of UIr3 indicate the mechanical stability of the lattice.

Rhodocenium Monocarboxylic Acid Hexafluoridophosphate and Its Derivatives: Synthesis, Spectroscopy, Structure, and Electrochemistry

As an extension of our continuing work in metallocenium chemistry, we report here on new functionalized rhodocenium salts. In contrast to isoelectronic cobaltocenium compounds, rhodium as a 4d metal allows synthetic routes via prefunctionalized cyclopentadienyl half‐sandwich precursors, thereby facilitating access to monofunctionalized rhodocenium salts containing substituents comprising methyl, trimethylsilyl, carboxylate and carboxylate ester as well as amide derivatives. Synthetic aspects, scope and limitations, as well as spectroscopic (1H/13C‐NMR, IR, HR‐MS), and structural (XRD) properties are reported. Like the parent rhodocenium ion, all new derivatives undergo two chemically consecutive reductions at potentials that depend on the respective ring substituent. The second reduction competes with a rapid reaction of the corresponding rhodocenes to their 18 VE dimers. Rate constants for this process range from 2 × 103 s–1 to 2.5 × 105 s–1 as estimated from digital simulations of experimental voltammograms. Rhodocenium carboxylic acid (8) constitutes a special case in that proton instead of metal reduction is observed at Pt or Au electrodes.