Change In Coordination Geometry Research Articles

Syntheses, Structures, Structural Transformation, and Properties of Neodymium Sulfate Based Inorganic Networks

AbstractTwo neodymium sulfate based purely inorganic networks, [NH4][Nd(SO4)2(H2O)3] ⋅ H2O (1) and [Nd2(SO4)3(H2O)4] ⋅ H2O (2) have been synthesized at different reaction temperatures and pH values. The structure of 1 features a two‐dimensional (2D) anionic corrugated sheet with an sql topological net built up by linking NdO8 units and μ2‐SO42− anions, in which the NH4+ charge‐compensating/templating cations occupy the intersheet layers. While 2 possesses a 3D neutral framework with an unprecedented (3,42,7‐c) net constructed by NdO9 units and μ4‐ and μ6‐SO42− linkers. They exhibit thermally induced irreversible structural transformation associated with the dehydration and the changes in coordination geometry and dimensionality. Both compound 1 and 2 display typical antiferromagnetic interactions between the Nd3+ centres. In addition, the CO2 adsorption isotherms of the desolvated samples 1 and 2 were also investigated.

X-ray and molecular dynamics study of the temperature-dependent structure of FLiNaK

The atomic structure of FLiNaK and its evolution with temperature are examined with x-ray scattering and molecular dynamics (MD) simulations in the temperature range 460–636 °C. In accord with previous studies, it’s observed that the average nearest-neighbor (NN) cation-anion coordination number increases with increasing cation size, going from ∼4 for Li-F to ∼6.4 for K-F. In addition, we find that there is a coupled change in local coordination geometry – going from tetrahedral for Li-F to octahedral for Na to very disordered quasi-cuboidal for K. The varying geometry and coordination distances for the cation-anion pairs cause a relatively constant F-F next-nearest neighbor (NNN) distance of approximately 3.1 Å. This relatively fixed distance allows the F anions to assume an overall correlated structure very similar to that of a hard-sphere liquid with an extended radius which is beyond the normal F ion size but reflects the cation-anion coordination requirements. Careful consideration of the evolution of the experimental atomic distribution functions with increasing temperature shows that the changes in correlation at each distance can be understood within the context of broadening asymmetric neighbor distributions. Within the temperature range studied, the evolution of F-F correlations with increasing temperature is consistent with changes expected in a hard-sphere liquid simply due to decreasing density.

Pressure-induced evolution of the lattice dynamics for selected UO3 polymorphs

Uranium trioxide (UO3) is a stable chemical form of uranium oxide with multiple polymorphs found throughout the nuclear fuel cycle. The pressure-induced changes in the structure and lattice dynamics of four of these polymorphs are simulated with density functional perturbation theory and analyzed. Two phases, α- and δ-UO3 are found to exhibit an isotropic response to pressure and do not undergo any changes in coordination geometry up to ∼40 GPa. In contrast, the other two phases investigated, β- and γ-UO3, exhibit an anisotropic response to pressure. Decomposition of the phonon eigenvectors allows us to assign specific pressure-induced structural changes to individual phonon modes. This analysis has been performed on a per atom basis for the relatively simple α- and δ-UO3 structures, which have one symmetrically unique uranium site, and on a per coordination environment basis for β- and γ-UO3, which have multiple U sites.

Toward Improved Charge Separation through Conformational Control in Copper Coordination Complexes.

The continued development of solar energy as a renewable resource necessitates new approaches to sustaining photodriven charge separation (CS). We present a bioinspired approach in which photoinduced conformational rearrangements at a ligand are translated into changes in coordination geometry and environment about a bound metal ion. Taking advantage of the differential coordination properties of CuI and CuII, these dynamics aim to facilitate intramolecular electron transfer (ET) from CuI to the ligand to create a CS state. The synthesis and photophysical characterization of CuCl(dpaaR) (dpaa = dipicolylaminoacetophenone, with R = H and OMe) are presented. These ligands incorporate a fluorophore that gives rise to a twisted intramolecular charge transfer (TICT) excited state. Excited-state ligand twisting provides a tetragonal coordination geometry capable of capturing CuII when an internal ortho-OMe binding site is present. NMR, IR, electron paramagnetic resonance (EPR), and optical spectroscopies, X-ray diffraction, and electrochemical methods establish the ground-state properties of these CuI and CuII complexes. The photophysical dynamics of the CuI complexes are explored by time-resolved photoluminescence and optical transient absorption spectroscopies. Relative to control complexes lacking a TICT-active ligand, the lifetimes of CS states are enhanced ∼1000-fold. Further, the presence of the ortho-OMe substituent greatly enhances the lifetime of the TICT* state and biases the coordination environment toward CuII. The presence of CuI decreases photoinduced degradation from 14 to <2% but does not result in significant quenching via ET. Factors affecting CS in these systems are discussed, laying the groundwork for our strategy toward solar energy conversion.

A Rare Flexible Metal-Organic Framework Based on a Tailorable Mn8 -Cluster Showing Smart Responsiveness to Aromatic Guests and Capacity for Gas Separation.

The design and creation of soft porous crystals combining regularity and flexibility may promote potential applications for gas storage and separation due to their deformable framework's responsiveness to external stimuli. The flexibility of metal-organic frameworks (MOFs) relies on alterable degrees of freedom that are mainly provided by organic linkers or the junctions linking organic and inorganic building units. Herein, we report a new dynamic MOF whose flexibility originates from an unprecedented tailorable Mn8 O38 -cluster and shows simultaneous coordination geometry changes and ligand migration that are reversibly driven by guest exchange. This provides an extra degree of freedom to the framework's deformation, resulting in three-dimensional variations in the framework that subtly respond to varied aromatic molecules. The gas adsorption behavior of this flexible MOF was evaluated, and the selective separation of light hydrocarbons and Freon gases is achieved.

A Rare Flexible Metal–Organic Framework Based on a Tailorable Mn8‐Cluster Showing Smart Responsiveness to Aromatic Guests and Capacity for Gas Separation

AbstractThe design and creation of soft porous crystals combining regularity and flexibility may promote potential applications for gas storage and separation due to their deformable framework's responsiveness to external stimuli. The flexibility of metal–organic frameworks (MOFs) relies on alterable degrees of freedom that are mainly provided by organic linkers or the junctions linking organic and inorganic building units. Herein, we report a new dynamic MOF whose flexibility originates from an unprecedented tailorable Mn8O38‐cluster and shows simultaneous coordination geometry changes and ligand migration that are reversibly driven by guest exchange. This provides an extra degree of freedom to the framework's deformation, resulting in three‐dimensional variations in the framework that subtly respond to varied aromatic molecules. The gas adsorption behavior of this flexible MOF was evaluated, and the selective separation of light hydrocarbons and Freon gases is achieved.

Dimerization of Paramagnetic Trinuclear Complexes by Coordination Geometry Changes Showing Mixed Valency and Significant Antiferromagnetic Coupling through -Pt···Pt- Bonds.

Paramagnetic trinuclear complexes, trans-[Pt2M(piam)4(NH3)4](ClO4)x (t-M; piam = pivalamidate, M = Mn, Fe, Co, Ni, and Cu, x = 2 or 3), aligned as Pt-M-Pt were successfully synthesized and characterized. The dihedral angles between the Pt and M coordination planes in t-M are approximately parallel, showing straight metal-metal bonds with distances of approximately 2.6 Å. Except for t-Fe, the trinuclear complexes are dimerized with close contact (approximately 3.9 Å) between the end Pt atoms to form Pt-M-Pt···Pt-M-Pt alignments with high-spin M(+2) containing five (t-Mn), three (t-Co), two (t-Ni), and one (t-Cu) unpaired electrons localized on M atoms. Several physical measurements and calculations revealed that the dimerized structures were maintained in MeCN, where cyclic voltammograms for t-M exhibited two-step oxidation and reduction attributed to Pt-M(+2)-Pt···Pt-M(+2)-Pt ↔ Pt-M(+3)-Pt···Pt-M(+2)-Pt ↔ Pt-M(+3)-Pt···Pt-M(+3)-Pt via mixed-valent states. Magnetic susceptibility measurements for t-M showed antiferromagnetic interaction, t-Mn: J = -0.9 cm-1, t-Co: J = -3.5 cm-1, t-Ni: J = -7.3 cm-1, and t-Cu: J = 0.0 cm-1, between the two M centers with distances of 9.0 Å through Pt···Pt bonds.

Air-Stable Dy(III)-Macrocycle Enantiomers: From Chiral to Polar Space Group

Air-Stable Dy(III)-Macrocycle Enantiomers: From Chiral to Polar Space Group

Sign control of circularly polarized luminescence of chiral Schiff-base Zn(II) complexes through coordination geometry changes.

The handedness of circularly polarized luminescence (CPL) of Zn(II) complexes with a chiral Schiff-base ligand can be controlled by coordination geometry changes in solution and in the solid state. The relationships between molecular structure and chiroptical properties were discussed with the results of X-ray diffraction analysis and DFT calculations.

Understanding the Mechanism of Amorphization for Co‐URJC‐5

AbstractIrreversible transition from monocrystalline to amorphous phase accompanied by a change in coordination geometry (octahedral to tetrahedral) was first observed in a Co(II)‐based MOF material, named Co‐URJC‐5, via SO2 uptake. This structural change is associated with the total loss of the pyridine ligand from the axial positions. These results are corroborated using PXRD and FT‐IR spectroscopy. A plausible amorphization mechanism is proposed using crystal field theory.

Homobinuclear compounds based on a chiral oxazolidine ligand: From solid state study to aqueous solution dynamics

Two binuclear complexes of molecular formula [CoIICl(H3L)(H2O)]2Cl2(1) and [CuIICl2(H3L)]2(2), where H3L stands for the chiral ligand (2-pyridin-2-yl-1,3-oxazolidine-4,4-diyl)dimethanol) were obtained. The complexes and H3L ligand were formed using an one-pot strategy and characterized by spectroscopy techniques (FT-IR and UV–Vis), X-ray diffraction and theoretical calculations. The crystal structure revealed that both compounds feature a [M2Cl2] core and two ligand enantiomers, namely 8S,6R,1R-H3L and 8R,6S,1S-H3L in each molecular unit. Absorption spectroscopy supported by DFT studies indicated that the H3L pyridine moiety plays a key role to the charge transfer observed in 1 and 2. Finally, a time-dependent change in the absorption spectra profiles of 1 and 2 was seen in stability studies in aqueous solutions, which was associated to the ligand replacement reactions and/or changes in coordination geometry. Molecular dynamics confirmed that aqueous solutions of both compounds are quickly undergoing a symmetrical breakdown of the former binuclear core, leading to mononuclear units. For 1, this process goes towards the H3L release, while 2 remains as a mononuclear H3L-containing species in penta- and hexacoordinate forms.

Investigation of Substitution Reactions Between Zinc(II) Complexes with Different Geometries and N-Bonding Nucleophiles

Aims: The study aimed at investigating interactions between zinc(II) complexes with different geometrical structures and relevant nitrogen donor nucleophiles at physiological pH. Background: Lack of clear distinction between the therapeutic and toxic doses of platinum drugs is a major challenge for the design of novel non-platinum DNA and protein targeting metal-based anticancer agents. The non-platinum antitumor complexes could be alternatives to platinum-based drugs due to their better characteristics and different mechanisms of action. Objective: This study could provide more information regarding the design of future zinc-based anticancer drugs, providing a better understanding of the mechanism of interactions between Zn(II) complexes and nitrogen-donor nucleophiles (important from the medical point of view), and clarifying the changes in geometrical structures of zinc(II) that are referred to structure-reactivity correlations. Methods: Mole-ratio method and UV-V is spectroscopic kinetic method have been used. Results: The results indicated additional coordination of chlorides in the first coordination sphere with changes in coordination geometry and formation of the octahedral complex anion [ZnCl4(en)]2- while the excess of chloride did not affect the square-pyramidal structure of [ZnCl2(terpy)]. The substitutions of studied complexes and relevant nucleophiles proceeded in two consecutive reaction steps that depended on the nucleophile concentration. Octahedral complex anion [ZnCl4(en)]2- forms rapidly and all substitution processes of this complex species should be considered. We assume that the first reaction step is accompanied by the dissociation of chloride ligands. Nucleophile 1,2,4- triazoles have shown the highest affinity toward [ZnCl2(en)], and rates of both steps were almost of the same value, indicating parallel reactions. Conclusion: The different order of reactivity of relevant N-donor ligands toward [ZnCl2(en)] and [ZnCl2(terpy)] complexes for the first reaction step occurred because of the influence of different geometrical structures of complexes, while low reaction rates for the second reactions of [ZnCl2(en)] complex with imidazole and pyrazine were a consequence of interconversion between octahedral and tetrahedral structure during substitution processes.

Exploiting coordination geometry to tune the dimensions and processability of metallosupramolecular polymers.

Achieving precise control over the morphology, dimensions and processability of functional materials is a key but challenging requirement for the fabrication of smart devices. To address this issue, we herein compare the self-assembly behavior of two new Pt(ii) complexes that differ in the molecular and coordination geometry through implementation of either a monodentate (pyridine) or bidentate (bipyridine) ligand. The molecular preorganization of the bipyridine-based complex enables effective self-assembly in solution involving Pt⋯Pt interactions, while preserving aggregate solubility. On the other hand, increased steric effects of the linear bispyridine-based complex hinder an effective preorganization leading to poorly solvated aggregates when a critical concentration is exceeded.

Coordination modulated on-off switching of flexibility in a metal-organic framework.

Stimuli-responsive metal–organic frameworks (MOFs) exhibit dynamic, and typically reversible, structural changes upon exposure to external stimuli. This process often induces drastic changes in their adsorption properties. Herein, we present a stimuli-responsive MOF, 1·[CuCl], that shows temperature dependent switching from a rigid to flexible phase. This conversion is associated with a dramatic reversible change in the gas adsorption properties, from Type-I to S-shaped isotherms. The structural transition is facilitated by a novel mechanism that involves both a change in coordination number (3 to 2) and geometry (trigonal planar to linear) of the post-synthetically added Cu(i) ion. This process serves to ‘unlock’ the framework rigidity imposed by metal chelation of the bis-pyrazolyl groups and realises the intrinsic flexibility of the organic link.

Molecular Sensors Operating by a Spin‐State Change in Solution: Application to Magnetic Resonance Imaging

AbstractRecent proposals for molecular switches are reviewed that adopt a different spin state before and after the encounter with an analyte (metabolite) or physical stimulus (light). Spin‐state switching has significant potential to provide molecular MRI with a first line of probes operating by an off/on mode. Past efforts to design MRI sensors relied on strategies other than spin‐switching and thus mostly comprised probes only modulating the signal before and after encounter with the analyte (X%‐on/Y%‐on). The article thus essentially treats small‐molecule metal ion chelates, i.e. coordination compounds. Initially, a comprehensive treatment of design considerations for optimal spin‐state switches is furnished. The article then enters into a treatment of reported examples of spin‐state switches according to three response mechanisms: 1) changes in the redox state of the metal center; 2) changes in coordination geometry; and 3) modification of the surrounding ligand field, either by peripheral interaction with the analyte, or by replacement of a coordinating ligand by another.

Bis(diphenylphosphino)methane Dioxide Complexes of Lanthanide Trichlorides: Synthesis, Structures and Spectroscopy

Bis(diphenylphosphino)methane dioxide (dppmO2) forms eight-coordinate cations [M(dppmO2)4]Cl3 (M = La, Ce, Pr, Nd, Sm, Eu, Gd) on reaction in a 4:1 molar ratio with the appropriate LnCl3 in ethanol. Similar reaction in a 3:1 ratio produced seven-coordinate [M(dppmO2)3Cl]Cl2 (M = Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb), whilst LuCl3 alone produced six-coordinate [Lu(dppmO2)2Cl2]Cl. The complexes have been characterised by IR, 1H and 31P{1H}-NMR spectroscopy. X-ray structures show that [M(dppmO2)4]Cl3 (M = Ce, Sm, Gd) contain square antiprismatic cations, whilst [M(dppmO2)3Cl]Cl2 (M = Yb, Dy, Lu) have distorted pentagonal bipyramidal structures with apical Cl. The [Lu(dppmO2)2Cl2]Cl has a cis octahedral cation. The structure of [Yb(dppmO2)3(H2O)]Cl3·dppmO2 is also reported. The change in coordination numbers and geometry along the series is driven by the decreasing lanthanide cation radii, but the chloride counter anions also play a role.

Uranium Isotope Fractionation (238U/235U) during U(VI) Uptake by Freshwater Plankton.

Uranium contamination in the environment is a serious public health concern. Biotic U(VI) reduction and nonreductive U(VI) uptake by microorganisms (e.g., U(VI) biosorption by cyanobacteria) are effective U remediation techniques. Variations of 238U/235U have been extensively explored to track biotic U(VI) reduction in laboratory experiments and field applications. However, U isotope fractionation during nonreductive U(VI) uptake by microorganisms is poorly constrained. To investigate U isotope fractionation in this process, we cultured freshwater plankton in the presence of U(VI) and measured 238U/235U in the culture media and biomass. We found that nonreductive U(VI) uptake by freshwater plankton fractionated U isotopes in the opposite direction compared to biotic U(VI) reduction. δ238U values in freshwater plankton were consistently ∼0.23 ± 0.06‰ lighter than those in dissolved U in the culture medium at various fractions of U removal (12-30%), consistent with equilibrium isotope fractionation in a closed system. The equilibrium isotope fractionation observed in our experiments possibly results from changes in coordination geometry between dissolved U(VI) in the culture media and adsorbed U(VI) on cell surfaces. Our experimental results highlight the need to consider U isotope fractionation during nonredox U(VI) uptake by microorganisms and organic matter when applying variations of 238U/235U to track biogeochemical processes and evaluate U remediation.

Dehydration–Hydration Switching of Single-Molecule Magnet Behavior and Visible Photoluminescence in a Cyanido-Bridged DyIIICoIII Framework

Microporous magnets compose a class of multifunctional molecule-based materials where desolvation-driven structural transformation leads to the switching of magnetic properties. Herein, we present a special type of microporous magnet where a dehydration-hydration process within a bimetal coordination framework results in the switching of emissive DyIII single-molecule magnets (SMMs). We report a three-dimensional (3-D) cyanido-bridged coordination polymer, {[DyIII(H2O)2][CoIII(CN)6]}·2.2H2O (1), and its dehydrated form of {DyIII[CoIII(CN)6]} (2), which was obtained through a reversible single-crystal-to-single-crystal transformation. Both phases are composed of paramagnetic DyIII centers alternately arranged with diamagnetic hexacyanidocobaltates(III). The hydrated phase contains eight-coordinated [DyIII(μ-NC)6(H2O)2]3- complexes of a square antiprism geometry, while the dehydrated form contains six-coordinated [DyIII(μ-NC)6]3- moieties of a trigonal prism geometry. This change in coordination geometry results in the generation of DyIII single-molecule magnets in 2, whereas slow magnetic relaxation effect is not observed for DyIII sites in 1. The D4d-to-D3h symmetry change of DyIII complexes produces also the shift of photoluminescent color from nearly white to deep yellow thanks to the modulation of emission bands of f-f electronic transitions. A combined approach utilizing dc magnetic data and low-temperature emission spectra confirmed an axial crystal field of trigonal prismatic DyIII complexes in 2, which produces an Orbach type of slow magnetic relaxation. Therefore, we present a unique route to the efficient switching of SMM behavior and photoluminescence of DyIII complexes embedded in a 3-D cyanido-bridged framework.

Investigation of Ionic Liquid interaction with ZnBDC-Metal Organic Framework through Scanning EXAFS and Inelastic Neutron Scattering

Zn based Metal-Organic Framework (MOF), Zinc 1,4-benzenedicarboxylate (ZnBDC) has been synthesized and incorporated with ionic liquid (IL) 1-butyl-3 methylimidazolium bromide (BMIMBr) at optimum 50 wt% of IL. Interaction of BMIMBr in the micropores of ZnBDC-MOF is investigated by XPS, scanning EXAFS and inelastic neutron spectroscopy. Significant increase in binding energies of Zn spin-orbit peaks upon IL incorporation is observed from XPS spectra indicating motion of Br− anions of the IL BMIMBr towards the unsaturated Zn cluster of ZnBDC-MOF. The k-space periodicity as well as the local coordination geometry of Zn K-edge is investigated from the scanning EXAFS and XANES spectra. Asymmetric oscillation periodicity has been observed from the k-space scanning XANES spectra upon IL incorporation. Difference in peak positions of oxygen and zinc are observed from the R-space scanning EXAFS spectra suggesting change in coordination geometry due to dehydration of Zn2+ ion in the ZnBDC-MOF upon IL incorporation. Incorporation of IL in the pores of ZnBDC-MOF gives rise to increased scattering intensity in the Inelastic Neutron Scattering (INS) spectra, which is attributed to the displacement of IL ions in the MOF pores.

Exposing the inadequacy of redox formalisms by resolving redox inequivalence within isovalent clusters

In this report we examine a family of trinuclear iron complexes by multiple-wavelength, anomalous diffraction (MAD) to explore the redox load distribution within cluster materials by the free refinement of atomic scattering factors. Several effects were explored that can impact atomic scattering factors within clusters, including 1) metal atom primary coordination sphere, 2) M-M bonding, and 3) redox delocalization in formally mixed-valent species. Complexes were investigated which vary from highly symmetric to fully asymmetric by 57Fe Mössbauer and X-ray diffraction to explore the relationship between MAD-derived data and the data available from these widely used characterization techniques. The compounds examined include the all-ferrous clusters [ n Bu4N][(tbsL)Fe3(μ3-Cl)] (1) ([tbsL]6- = [1,3,5-C6H9(NC6H4-o-NSi t BuMe2)3]6-]), (tbsL)Fe3(py) (2), [K(C222)]2[(tbsL)Fe3(μ3-NPh)] (4) (C222 = 2,2,2-cryptand), and the mixed-valent (tbsL)Fe3(μ3-NPh) (3). Redox delocalization in mixed-valent 3 was explored with cyclic voltammetry (CV), zero-field 57Fe Mössbauer, near-infrared (NIR) spectroscopy, and X-ray crystallography techniques. We find that the MAD results show an excellent correspondence to 57Fe Mössbauer data; yet also can distinguish between subtle changes in local coordination geometries where Mössbauer cannot. Differences within aggregate oxidation levels are evident by systematic shifts of scattering factor envelopes to increasingly higher energies. However, distinguishing local oxidation levels in iso- or mixed-valent materials can be dramatically obscured by the degree of covalent intracore bonding. MAD-derived atomic scattering factor data emphasize in-edge features that are often difficult to analyze by X-ray absorption near edge spectroscopy (XANES). Thus, relative oxidation levels within the cluster were most reliably ascertained from comparing the entire envelope of the atomic scattering factor data.