Crystal and molecular structures of alkali oxalates: first proof of a staggered oxalate anion in the solid state

In situ spectroelectrochemical probing of CO redox landscape on copper single-crystal surfaces

Electrochemical reduction of CO(2) to value-added chemicals and fuels is a promising strategy to sustain pressing renewable energy demands and to address climate change issues. Direct observation of reaction intermediates during the CO(2) reduction reaction will contribute to mechanistic understandings and thus promote the design of catalysts with the desired activity, selectivity, and stability. Herein, we combined in situ electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy and ab initio molecular dynamics calculations to investigate the CORR process on Cu single-crystal surfaces in various electrolytes. Competing redox pathways and coexistent intermediates of CO adsorption (*COatop and *CObridge), dimerization (protonated dimer *HOCCOH and its dehydrated *CCO), oxidation (*CO2- and *CO32-), and hydrogenation (*CHO), as well as Cu-Oad/Cu-OHad species at Cu-electrolyte interfaces, were simultaneously identified using in situ spectroscopy and further confirmed with isotope-labeling experiments. With AIMD simulations, we report accurate vibrational frequency assignments of these intermediates based on the calculated vibrational density of states and reveal the corresponding species in the electrochemical CO redox landscape on Cu surfaces. Our findings provide direct insights into key intermediates during the CO(2)RR and offer a full-spectroscopic tool (40-4,000 cm-1) for future mechanistic studies.

Adjusting Rotational Behavior of Molecular Rotors by a Rational Tuning of Molecular Structure

Many crystalline molecular rotors have been developed in the past decades. However, manipulating the rotational gesture that intrinsically controls the physical performance of materials remains a challenge. Herein, we report a series of crystalline rotors whose rotational gestures can be modulated by modifying the structures of molecular stators. In these dynamic crystals, the ox^2- (ox^2- = oxalate anion) behave as molecular rotators performing axial-free rotation in cavities composed of five complex cations, [M^II(en)₃]²⁺ (en = ethylenediamine). The structure of [M^II(en)₃]²⁺ that serves as a molecular stator can be tuned by varying the metal center with different ionic radii, consequently altering the chemical environment around the molecular rotator. Owing to the quasi-transverse isotropy of ox^2- and multiple hydrogen-bond interactions around it, the molecular rotator exhibits unusual motional malleability, i.e., it can rotate either longitudinally in the compound of Zn^II, or with a tilt angle of 42° in the compound of Fe^II, or even laterally in the compound of Cd^II. The atypical dynamic behavior demonstrated here provides a new chance for the development of exquisite crystalline molecular rotors with advanced tunable functionalities.

Tilting and twisting in a novel perovzalate, K3NaMn(C2O4)3

A unique variant on the perovskite structure, K₃NaMn(C₂O₄)₃, has been identified with unconventional octahedral tilting, interpenetration of two topologically identical perovskite-like frameworks and an unusual, twisted oxalate ligand.

Characterization of the alkali metal oxalates (MC2O4-) and their formation by CO2 reduction via the alkali metal carbonites (MCO2-)

The reduction of carbon dioxide to oxalate has been studied by experimental Collisionally Induced Dissociation (CID) and vibrational characterization of the alkali metal oxalates, supplemented by theoretical electronic structure calculations. The critical step in the reductive process is the coordination of CO2 to an alkali metal anion, forming a metal carbonite MCO2- able to subsequently receive a second CO2 molecule. While the energetic demand for these reactions is generally low, we find that the degree of activation of CO2 in terms of charge transfer and transition state energies is the highest for lithium and systematically decreases down the group (M = Li-Cs). This is correlated to the strength of the binding interaction between the alkali metal and CO2, which can be related to the structure of the oxalate moiety within the product metal complexes evolving from a planar to a staggered conformer with increasing atomic number of the interacting metal. Similar structural changes are observed for crystalline alkali metal oxalates, although the C2O42- moiety is in general more planar in these, a fact that is attributed to the increased number of interacting alkali metal cations compared to the gas-phase ions.

Sila-polyethers as innocent crystallization reagents for heavy alkali metal compounds

Sila-polyethers are used as innocent crystallization reagents for the formation of single crystals of different elusive rubidium and cesium salts.

Density functional theory modelling of protective agents for carbonate stones: a case study of oxalate and oxamate inorganic salts

DFT calculations allowed investigating the ability of oxalate monoesters and monoamides salts to act as protective agents for carbonate stones, such as marble or limestones, of historical interest in the field of cultural heritage.

Evaluation of terrestrial plants extracts for uranium sorption and characterization of potent phytoconstituents

Sorption capacity of four plants (Funaria hygrometrica, Musa acuminata, Brassica juncea and Helianthus annuus) extracts/fractions for uranium, a radionuclide was investigated by EDXRF and tracer studies. The maximum sorption capacity, i.e., 100% (complete sorption) was observed in case of Musa acuminata extract and fractions. Carbohydrate, proteins, phenolics and flavonoids contents in the active fraction (having maximum sorption capacity) were also determined. Further purification of the most active fraction provided three pure molecules, mannitol, sorbitol and oxo-linked potassium oxalate. The characterization of isolated molecules was achieved by using FTIR, NMR, GC-MS, MS-MS, and by single crystal-XRD analysis. Of three molecules, oxo-linked potassium oxalate was observed to have 100% sorption activity. Possible binding mechanism of active molecule with the uranyl cation has been purposed.

Crystal structure of bis[3-methoxy-17β-estra-1,3,5(10)-trien-17-yl] oxalate

The title symmetrical steroid oxalate diester is substantially twisted about the central O₂C—CO₂ bond, leading to an overall shallow V-shape for the molecule, which may correlate with its reactivity under flash vacuum pyrolysis. C—H⋯O hydrogen bonds help to establish the packing.

In the title compound, C₄₀H₅₀O₆, a symmetrical steroid oxalate diester, the dihedral angle between the CO₂ planes of the oxalate linker is 61.5 (5)° and the C—C bond length is 1.513 (6) Å. The steroid B, C and D rings adopt half-chair, chair and envelope conformations, respectively, in both halves of the mol­ecule, which adopts an overall shallow V-shaped conformation. In the crystal, mol­ecules are linked by weak C—H⋯O inter­actions, forming a three-dimensional network.

Poly[diaquadi-μ6-oxalato-μ5-oxalato-chromium(III)rubidium(I)]: a new supramolecular isomer

The title compound, [CrRb(C(2)O(4))(2)(H(2)O)(2)](n), obtained under hydrothermal conditions and investigated structurally at 100 K, is a three-dimensional supramolecular isomer of the layered structure compound studied at room temperature. This novel polymer is built up from crosslinked heterobimetallic units. The linkage of alternating edge- and vertex-shared RbO(7)(H(2)O)(2) and CrO(4)(H(2)O)(2) polyhedra running along three different directions gives a dense packing. The two independent ligands display two η(4)-chelation modes and two conventional carboxylate bridges. However, the pentadentate ligand connects the Cr(III) and Rb(I) ions through one O-atom bridge, while the hexadentate ligand exhibits an additional η(3)-chelation and two O-atom bridges. Each coordinated water molecule forms an O-atom bridge between the two metals. Moreover, in the absence of protonated ligands, these water molecules act as donors through their four H atoms in strong-to-weak hydrogen bonds. This results in zigzag chains of alternating oxalate and aqua ligands parallel to the twofold screw axis. The six double oxalates known to date containing an alkali and Cr(III) all present layered two-dimensional structures. In the series, this supramolecular isomer is the first three-dimensional framework.

Synthesis, Structure and Thermal Behavior of Oxalato-Bridged Rb+ and H3O+ Extended Frameworks with Different Dimensionalities

Correlative studies of three oxalato-bridged polymers, obtained under hydrothermal conditions for the two isostructural compounds {Rb(HC₂O₄)(H₂C₂O₄)(H₂O)₂}_∞¹, 1, {H₃O(HC₂O₄)(H₂C₂O₄).2H₂O}_∞¹, 2, and by conventional synthetic method for {Rb(HC₂O₄)}_∞³, 3, allowed the identification of H-bond patterns and structural dimensionality. Ferroïc domain structures are confirmed by electric measurements performed on 3. Although 2 resembles one oxalic acid sesquihydrate, its structure determination doesn’t display any kind of disorder and leads to recognition of a supramolecular network identical to hybrid s-block series, where moreover, unusual H₃O⁺ and NH₄⁺ similarity is brought out. Thermal behaviors show that 1D frameworks with extended H-bonds, whether with or without a metal center, have the same stability. Inversely, despite the dimensionalities, the same metallic intermediate and final compounds are obtained for the two Rb⁺ ferroïc materials.

The common role of water molecule and lone electron pair as a bond-valence mediator in oxalate complexes: the crystal structures of Rb2(C2O4) · H2O and Tl2(C2O4)

The crystal structure of rubidium oxalate monohydrate, Rb2(C2O4) · H2O, has been refined using an imaging-plate diffractometer system and graphite-monochromatized MoK α radiation. The complex crystallizes in the monoclinic system, space group C2/c, with unit cell dimensions of a = 9.617(6), b = 6.353(5), c = 11.010(8) Å, β = 109.46(3)°, V = 634.2(8) Å3, and Z = 4. The structure was solved and refined to R = 0.026 for 2646 independent reflections [I o > 2σ(I o)]. In Rb2(C2O4) · H2O, oxalate anions [(C2O4)2–] and cations (Rb+) constitute a two-dimensional layered structure parallel to (001); interlayered water molecules (H2O)0 occupy a well-defined position and form hydrogen bonds to the layers to prop up the structure, where (H2O)0 is structurally an essential component. The relationship among the crystal structures of M2(C2O4) · nH2O (M = Li+, Na+, K+, Rb+, Cs+, Tl+, NH4 +, Ag+; n = 0, 1, 2) showed that ionic radii of the cations decide whether the corresponding oxalate complexes have water molecules or not. On the other hand, anhydrous thallium oxalate, Tl2(C2O4), has a similar structure to Rb2(C2O4) · H2O except for the water molecules. Tl+ cation exhibits stereochemical activity of its 6s2 lone electron pairs (LEP), which is absent from Rb+ in the latter oxalate. Comparison between interlayer spaces occupied by LEP and (H2O)0 in these two structures reveals the existence of the first uncharged substitution: 2 × LEP → (H2O)0, where the directions of the former occupants are opposite to each other, and nearly perpendicular to a dipole direction of the latter. In the layer sites of these two structures, two LEP of Tl+ seem to work like a dipole moment of the water molecule in Rb-oxalate monohydrate by shifting away from the nucleus to stabilize the structure of Tl2C2O4. The crucial role common to water molecules and LEP which acts as a bond-valence mediator in oxalate complexes is discussed quantitatively in terms of a valence-matching principle.

Effect of hydrostatic pressure on the crystal structure of sodium oxalate: X-ray diffraction study and ab initio simulations

Effect of hydrostatic pressures up to 8 GPa on the crystals of Na2C2O4 (sp. gr. P21/c) was studied in situ in the diamond anvil cells a) in neon, b) in methanol-ethanol mixture by high-resolution X-ray powder diffraction (synchrotron radiation, λ = 0.7 Å, MAR345-detector). Below 3.3–3.8 GPa, anisotropic structural distortion was observed, which was similar to, but not identical with that on cooling. At 3.8 GPa, a reversible isosymmetric first-order phase transition without hysteresis occurred. The orientation of the oxalate anions changed at the transition point by a jump, and so did the coordination of the sodium cations by oxygen atoms. Ab initio simulations based on the generalized gradient approximation of density functional theory have reproduced the main features of the structural changes in the crystals of sodium oxalate with increasing pressure. The theoretical pressure for the isosymmetric phase transition is 3.65 GPa, close to the experimental value; in agreement with experiment the transition was predicted to be reversible. Ab initio calculations gave a pronounced hysteresis for this transition, and have also predicted a further isosymmetric phase transition at 10.9 GPa, also with a hysteresis. The role of temperature in the pressure-induced phase transitions in sodium oxalate is discussed.

Vibrational Spectroscopy and ab Initio Studies of Lithium Bis(oxalato)borate (LiBOB) in Different Solvents

The effect of lithium ion coordination with the bis(oxalato)borate (BOB-) [B(C2O4)2]- anion in DMSO, PEG, PPG, and d-PPG has been studied in detail by IR and Raman spectroscopy. Ab initio calculations were performed to allow a consistent analysis of the experimental data. The main features observed in the IR and Raman spectra correspond to the presence of "free", un-coordinated, BOB- anions. Only with use of d-PPG as solvent a small amount of Li+...BOB- ion pairs were detected. The Raman spectra and the calculations together indicate that Li+ coordinates bidentately with two end-oxygen atoms of the BOB- anion. The identification of ion pairs can be used to reveal limitations of LiBOB based electrolytes. The results for LiBOB are compared with literature on other Li salts.

Crystal Structures and Topological Aspects of the High-Temperature Phases and Decomposition Products of the Alkali-Metal Oxalates M2[C2O4] (M=K, Rb, Cs)

The high-temperature phases of the alkali-metal oxalates M2[C2O4] (M = K, Rb, Cs), and their decomposition products M2[CO3] (M = K, Rb, Cs), were investigated by fast, angle-dispersive X-ray powder diffraction with an image-plate detector, and also by simultaneous differential thermal analysis (DTA)/thermogravimetric analysis (TGA)/mass spectrometry (MS) and differential scanning calorimetry (DSC) techniques. The following phases, in order of decreasing temperature, were observed and crystallographically characterized (an asterisk denotes a previously unknown modification): *alpha-K2[C2O4], *alpha-Rb2[C2O4], *alpha-Cs2[C2O4], alpha-K2[CO3], *alpha-Rb2[CO3], and *alpha-Cs2[CO3] in space group P6(3)/mmc; *beta-Rb2[C2O4], *beta-Cs2[C2O4], *beta-Rb2[CO3], and *beta-Cs2[CO3] in Pnma; gamma-Rb2[C2O4], gamma-Cs[C2O4], gamma-Rb2[CO3], and gamma-Cs2[CO3] in P2(1)/c; and delta-K2[C2O4] and delta-Rb2[C2O4] in Pbam. With respect to the centers of gravity of the oxalate and carbonate anions, respectively, the crystal structures of all known alkali-metal oxalates and carbonates belong to the AlB2 family, and adopt either the AlB2 or the Ni2In arrangement depending on the size of the cation and the temperature. Despite the different sizes and constitutions of the carbonate and oxalate anions, the high-temperature phases of the alkali-metal carbonates M2[CO3] (M = K, Rb, Cs), exhibit the same sequence of basic structures as the corresponding alkali-metal oxalates. The topological aspects and order-disorder phenomena at elevated temperature are discussed.

Modifying Electronic Communication in Dimolybdenum Units by Linkage Isomers of Bridged Oxamidate Dianions

Reactions of Mo(2)(O(2)CCH(3))(DAniF)(3), DAniF = N,N'-di-p-anisylformamidinate, with oxamidate dianions [ArNC(O)C(O)NAr](2-), Ar = C(6)H(5) and p-anisyl, give pairs of isomeric compounds where the [Mo(2)] units are bridged by the oxamidate anions. For the alpha isomers, the C-C unit of the dianion is nearly perpendicular to the Mo-Mo bonds, and these are essentially perpendicular to each other. For the beta isomers, the corresponding C-C unit and the Mo-Mo bonds are essentially parallel to each other. Each type of isomer is stable in solution. The electronic communication as measured by the DeltaE(1/2) for the oxidation of each of the Mo(2) units is significantly better for the beta isomers. This is supported also by the appearance of what is conventionally called an intervalence charge-transfer band in the near infrared region upon oxidation of the beta isomers but not the alpha isomers. Molecular mechanics and DFT calculations help explain the relative conformations in the alpha isomers and the relative energy differences between the alpha and beta isomers.