Species fine structure of transition metal Cu(II) in aqueous chloride-bearing solutions: Insights from X-ray absorption spectroscopy and ab initio XANES calculations

Zn, Cd and Cu Coordination Polymers for Metronidazole Sensing and for Ullmann and Chan-Lam Coupling Reactions

Five compounds, [Zn2(bpe)(BPTA)2(H2O)2] ⋅ 2H2O (1); [Zn(bpe)(BPTA)] (2); [Cd(bpe)(BPTA)H2O] (3); [Cd(BPTA) (bpmh)] ⋅ 2H2O (4); and Cu2(BPTA)2(bpmh)3(H2O)2] ⋅ 2H2O (5) were prepared employing 2,5-bis(prop-2-yn-1-yloxy)terephthalic acid (2, 5 BPTA) as the primary ligand and 1,2-di(pyridin-4-yl)ethane (4, 4' bpe) (1-3) and 1,2-bis(pyridin-3-ylmethylene)hydrazine (bpmh) (4-5) as the secondary ligands. Single crystal studies indicated that the compounds 1, 3 and 5 have two-dimensional layer structures and compounds 2 and 4 three-dimensional structures. The luminescence behaviour of the compounds 2 and 3 were explored for the sensing of metronidazole in aqueous medium. The studies indicated that the compounds can detect metronidazole in ppm level both in solution as well as simple paper strips. The Cu compound 5 was found to lose the coordinated water molecule at 100 °C without any structural change. The coordinatively unsaturated Cu-centre were examined towards the Lewis acidic character by carrying out the Ullmann type C-C homocoupling reaction of the aromatic halide compounds. The compounds, 4 and 5, also have the Lewis basic functionality arising out the =N-N=, aza groups. The bifunctional nature of the coordination polymers (CP) was explored towards the Chan-Lam coupling reaction between phenyl boronic acid and aniline derivatives in the ethanol medium. In both the catalytic reactions, good yields and recyclability were observed. The present studies illustrated the rich diversity that the transition metal containing compounds exhibit in extended framework structures.

Fingerprint of Dipole Moment Orientation of Water Molecules in Cu2+ Aqueous Solution Probed by X-ray Photoelectron Spectroscopy

The electronic structure and geometrical organization of aqueous Cu2+ have been investigated by using X-ray photoelectron spectroscopy (XPS) at the Cu L-edge combined with state-of-the-art ab initio molecular dynamics and a quantum molecular approach designed to simulate the Cu 2p X-ray photoelectron spectrum. The calculations offer a comprehensive insight into the origin of the main peak and satellite features. It is illustrated how the energy drop of the Cu 3d levels (≈7 eV) following the creation of the Cu 2p core hole switches the nature of the highest singly occupied molecular orbitals (MOs) from the dominant metal to the dominant MO nature of water. It is particularly revealed how the repositioning of the Cu 3d levels induces the formation of new bonding (B) and antibonding (AB) orbitals, from which shakeup mechanisms toward the relaxed H-SOMO operate. As highlighted in this study, the appearance of the shoulder near the main peak corresponds to the characteristic signature of shakeup intraligand (1a1 → H-SOMO(1b1)) excitations in water, providing insights into the average dipole moment distribution (≈36°) of the first-shell water molecules surrounding the metal ion and its direct impact on the broadening of the satellite. It is also revealed that the main satellite at 8 eV from the main peak corresponds to (metal/1b2 → H-SOMO(1b1) of water) excitations due to a bonding/antibonding (B/AB) interaction of Cu 3d levels with the deepest valence O2p/H1s 1b2 orbitals of water. This finding underscores the sensitivity of XPS to the electronic structure and orientation of the nearest water molecules around the central ion.

Fixing CO2 under Atmospheric Conditions and Dual Functional Heterogeneous Catalysis Employing Cu MOFs: Polymorphism, Single-Crystal-to-Single-Crystal (SCSC) Transformation and Magnetic Studies

New copper compounds, [Cu(C14H8O6)(C10H8N2)(H2O)] (1), [Cu(C14H8O6)(C10H8N2)(H2O)]·(C3H7ON)2 (2), [Cu(C14H8O6)(C10H8N2)(H2O)2]·(C3H7ON) (3), [Cu(C14H8O6)(C10H8N4)] (4), and [Cu(C14H8O6)(C10H8N4)]·(H2O) (5), were prepared employing 2,5-bis(prop-2-yn-1-yloxy)terephthalic acid (2,5-BPTA) as the primary ligand and 4,4'-bipyridine (1-3) and 4,4'-azopyridine (4-5) as the secondary ligands. Single-crystal studies indicated that compounds 1-4 have two-dimensional layer structures and compound 5 has a three-dimensional structure. Compounds 1-3 were isolated from the same reaction mixture but by varying the time of reaction. The framework structures of compounds 1-3 are similar and may be considered as polymorphic structures. Compounds 4 and 5 can also be considered polymorphic with a change in dimensionality of the structure. Compounds 1-3 can be formed through a single-crystal-to-single-crystal transformation under a suitable solvent mixture. The Cu center was explored for the Lewis acid-catalyzed cycloaddition reaction of epoxide and CO2 under ambient conditions in a solventless condition and also for the synthesis of propargylamine derivatives by three-component coupling reactions (A3 coupling) in a DCM medium. The Lewis basic functionality of the MOF (-N═N- group) has been explored for the Henry reaction (aldol condensation) in a solventless condition. In all of the catalytic reactions, good yields and recyclability were observed. The magnetic studies indicated that compounds 1 and 4 have antiferromagnetic interactions and compound 5 has ferromagnetic interactions. The present studies illustrated the rich diversity that the copper-containing compounds exhibit in extended framework structures.

Solvation Structure, Dynamics, and Charge Transfer Kinetics of Cu2+ and Cu+ in Choline Chloride Ethylene Glycol Electrolytes

Experimental measurements and classical molecular dynamics (MD) simulations were carried out to study electrolytes containing CuCl₂ and CuCl salts in mixtures of choline chloride (ChCl) and ethylene glycol (EG). The study focused on the concentration of 100 mM of both CuCl₂ and CuCl with the ratio of ChCl/EG varied from 1:2, 1:3, 1:4, to 1:5. It was found that the Cu²⁺ and Cu⁺ have different solvation environments in their first solvation shell. Cu²⁺ is coordinated by both Cl^- anions and EG molecules, whereas Cu⁺ is only solvated by EG. However, both Cu²⁺ and Cu⁺ show strong interactions with their second solvation shells, which include both Cl^- anions and EG molecules. Considering both the first and second solvation shells, the concentrations of Cu²⁺ and Cu⁺ that have various coordination numbers in each solution were calculated and were found to correlate qualitatively with the exchange current density trends reported in previous experiments of Cu²⁺ reduction to Cu⁺. This finding makes a connection between atomic solvation structure observed in MD simulations and redox reaction kinetics measured in electrochemical experiments, thus revealing the significance of the solvation environment of reduced and oxidized species for electrokinetics in deep eutectic solvents.

Spectroscopic Study of the Behavior of Mo(VI) and W(VI) Polyanions in Sulfuric-Phosphoric Acid Mixtures

The solution chemistry of Mo(VI) and W(VI) in mixtures of sulfuric and phosphoric acids is relevant to the development of practicable hydrometallurgical processes for the recovery and separation of these two elements from low-grade scheelite ores. The behavior of Mo(VI) and W(VI) in such mixtures has been studied using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), nuclear magnetic resonance (NMR), and small-angle X-ray scattering (SAXS) spectroscopies, along with electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS). Where applicable, these techniques have produced a self-consistent picture of the similarities and differences between the chemical speciation of Mo(VI) and W(VI) as functions of solution composition, mostly at a constant phosphorous/metal (P/M; M = Mo(VI) or W(VI)) ratio of ∼1. In dilute acidic media (0.02 mol·kg^-1 H⁺, without H₂SO₄), Mo(VI) exists mostly (∼60%) as P₂Mo₅O₂₃^6- with the remaining ∼40% as β-Mo₈O₂₆^4-. Under the same conditions, W(VI) is largely present as NaPW₁₁O₃₉^6- (∼80%) and P₂W₅O₂₃^6- (∼10%), with the remainder probably occurring as isopolytungstates such as W₁₂O₄₂^12- and some tungstophosphate dimers such as P₂W₁₈O₆₂^6-. At higher acid concentrations (≲5 mol·kg^-1 H₂SO₄), polymeric Mo(VI) anions are broken down to form the oxocations MoO₂²⁺ and Mo₂O₅²⁺ and their protonated forms, with the dimers becoming increasingly dominant at higher acidities (∼80% in 5 mol·kg^-1 H₂SO₄). In stark contrast, W(VI) polyanions do not decompose at higher acidities but instead form (∼70% in 0.6 mol·kg^-1 H₂SO₄) a Keggin ion, PW₁₂O₄₀^3-. Further acidification with H₂SO₄ results in the agglomeration of this Keggin ion, forming clusters of about 50 and 100 Å in diameter that ultimately produce crystalline precipitates, which could be identified in part by their X-ray diffraction patterns. Possible application of these findings to the hydrometallurgical separation of Mo and W using acidic solutions is briefly discussed, based on a limited number of batch solvent extractions.

A spectroscopic study of solvent effects on the formation of Cu(ii)-chloride complexes in aqueous solution

A combination of electronic (UV-vis) and X-ray absorption (EXAFS, XANES) spectroscopies has been used to investigate the formation of copper(ii)/chloride complexes in concentrated aqueous solutions. It is established that lowering the water activity by the addition of Mg(ClO₄)₂ at a constant Cl^-/Cu(ii) ratio results in the replacement of water molecules by Cl^- ions in the primary coordination shell of Cu(ii). This behavior closely parallels the effect of increasing the Cl^-/Cu(ii) ratio and demonstrates that full understanding of the stoichiometry and structures of the complexes formed in concentrated metal-ion chloride solutions requires explicit consideration of the role of the solvent.

Nature of Monomeric Molybdenum(VI) Cations in Acid Solutions Using Theoretical Calculations and Raman Spectroscopy

The composition and structures of the two protonated species formed from uncharged molybdic acid, MoO₂(OH)₂(OH₂)₂⁰, in strongly acidic solutions have been investigated using a combination of density functional theory calculations, first-principles molecular dynamics simulations, and Raman spectroscopy. The calculations show that both protonated species maintain the original octahedral structure of molybdic acid. Computed p K_a values indicated that the ═O moieties are the proton acceptor sites and, therefore, that MoO(OH)₃(OH₂)₂⁺ and Mo(OH)₄(OH₂)₂²⁺ are the probable protonated forms of Mo(VI) in strong acid solutions, rather than the previously accepted MoO₂(OH)_{2- x}(OH₂)_{2+ x} ^x+ ( x = 1, 2) species. This finding is shown to be broadly consistent with the observed Raman spectra. Structural details of MoO(OH)₃(OH₂)₂⁺ and Mo(OH)₄(OH₂)₂²⁺ are reported.