Search

The capricious reactivity and speciation of earth-abundant metals (EAM) hinder the mechanistic understanding essential to boost their efficiency and versatility in catalysis. Moreover, metal’s solution chemistry and reactivity are conventionally controlled using organic ligands, while their fundamental chemistry in operando conditions is often overlooked. However, in this study, we showcase how a better understanding of in operando conditions may result in improved catalytic reactions. By assessing the composition and structure of active species for Zr-catalyzed direct amide bond formations under operating conditions, we discovered zirconium oxo clusters form quickly and are likely active species in the reactions. Formation of these clusters dismisses the use of additional organic ligands, inert atmosphere, anhydrous solvents, or even water scavenging to provide amides in good to excellent yields. More specifically, dodeca- and hexazirconium oxo clusters (Zr12and Zr6, respectively) rapidly form from commercial, readily available Zr salts under reaction conditions known to afford amides directly from nonactivated carboxylic acid and amine substrates. Extended X-ray absorption fine structure (EXAFS) experiments confirm the presence of oxo clusters in solution throughout the reaction, while their key role in the mechanism is supported by an in-depth computational study employing density functional theory (DFT) and molecular dynamics (MD) methods. These results underline the value of (earth-abundant) metals’ intrinsic solution chemistry to transformative mechanistic understanding and to enhance the sustainability of organic transformations.

The theoretical calculation of the temperature-dependent nonradiative decay rate constant is fundamental for predicting the usefulness of transition-metal complexes for technological applications. Such a computation implies the determination of the barriers separating the emitting triplet state from metal-centered states, which are key mediators of this type of radiationless relaxation. We here do so for the two green-emitting cyclometalated Ir(III) complexes, [Ir(ppy)2(pyim)]+and [Ir(diFppy)2(dtb-bpy)]+, of general formula [Ir(C∧N)2(N∧N)]+, performing DFT calculations with both B3LYP and PBE0 functionals. On the basis of the obtained results and the comparison with the experimental nonradiative decay rate constants, we conclude that B3LYP provides too low energy barriers to the metal-centered states, while the PBE0 provides reasonable values. We consequently recommend to avoid the use of the commonly employed B3LYP functional for the evaluation of such an energy barrier for cyclometalated Ir(III) complexes.

The metal-based deep eutectic solvent (MDES) formed by NiCl2·6H2O and urea in 1:3.5 molar ratio has been prepared for the first time and characterized from a structural point of view. Particular accent has been put on the role of water in the MDES formation, since the eutectic could not be obtained with the anhydrous form of the metal salt. To this end, mixtures at different water/MDES molar ratios (W) have been studied with a combined approach exploiting molecular dynamics and ab initiosimulations, UV–vis and near-infra-red spectroscopies, small- and wide-angle X-ray scattering, and X-ray absorption spectroscopy measurements. In the pure MDES, a close packing of Ni2+ion clusters forming oligomeric agglomerates is present thanks to the mediation of bridging chloride anions and water molecules. Conversely, urea poorly coordinates the metal ion and is mostly found in the interstitial regions among the Ni2+ion oligomers. This nanostructure is disrupted upon the introduction of additional water, which enlarges the Ni–Ni distances and dilutes the system up to an aqueous solution of the MDES constituents. In the NiCl2·6H2O 1:3.5 MDES, the Ni2+ion is coordinated on average by one chloride anion and five water molecules, while water easily saturates the metal solvation sphere to provide a hexa-aquo coordination for increasing Wvalues. This multidisciplinary study allowed us to reconstruct the structural arrangement of the MDES and its aqueous mixtures on both short- and intermediate-scale levels, clarifying the fundamental role of water in the eutectic formation and challenging the definition at the base of these complex systems.

The photophysical behavior of the cyclometalating Ir(III) complexes [Ir(ppy)2(bpy)]+, where Hppy is 2-phenylpyridine and bpy is 2,2′-bipyridine (complex 1), and [Ir(diFppy)2(dtb-bpy)]+, where diFppy is 2-(2,4-difluorophenyl)pyridine and dtb-bpy is 4,4′-di-tert-butyl-2,2′-bipyridine (complex 2), has been theoretically investigated by performing density functional theory calculations. The two complexes share the same molecular skeleton, complex 2being derived from complex 1through the addition of fluoro and tert-butyl substituents, but present notable differences in their photophysical properties. The remarkable difference in their emission quantum yields (0.196 for complex 1in dichloromethane and 0.71 for complex 2in acetonitrile) has been evaluated by characterizing both radiative and nonradiative decay paths. It has emerged that the probability of decaying through the nonradiative triplet metal-centered state, normally associated with the loss of the emission quantum yield, does not appear to be the reason behind the reported substantially different emission efficiency. A more critical factor appears to be the ability of complex 2to emit from both the usual metal-to-ligand charge-transfer state and from two additional ligand-centered states, as supported by the fact that the respective minima belong to the potential energy surface of the lowest triplet T1state and that their phosphorescence lifetimes are in the same order of magnitude. In contrast, the emission of complex 1can be originated only from the metal-to-ligand charge-transfer state, being the only emissive T1minimum. The results constitute a significant case in which the emission from ligand-centered states is the key for determining the high emission quantum yield of a complex.

A thorough structural characterization of the La(NO3)3salt dissolved into several mixtures of ethyl ammonium nitrate (EAN) and methanol (MeOH) with EAN molar fraction χEANranging from 0 to 1 has been carried out by combining molecular dynamics (MD) and X-ray absorption spectroscopy (XAS). The XAS and MD results show that changes take place in the La3+first solvation shell when moving from pure MeOH to pure EAN. With increasing the ionic liquid content of the mixture, the La3+first-shell complex progressively loses MeOH molecules to accommodate more and more nitrate anions. Except in pure EAN, the La3+ion is always able to coordinate both MeOH and nitrate anions, with a ratio between the two ligands that changes continuously in the entire concentration range. When moving from pure MeOH to pure EAN, the La3+first solvation shell passes from a 10-fold bicapped square antiprism geometry where all the nitrate anions act only as monodentate ligands to a 12-coordinated icosahedral structure in pure EAN where the nitrate anions bind the La3+cation both in mono- and bidentate modes. The La3+solvation structure formed in the MeOH/EAN mixtures shows a great adaptability to changes in the composition, allowing the system to reach the ideal compromise among all of the different interactions that take place into it.

X-ray absorption spectroscopy (XAS) has been employed to study the coordination of the Ag+ion in aqueous solution. The conjunction of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) data analysis provided results suggesting the preference for a first shell linear coordination with a mean Ag–O bond distance of 2.34(2) Å, different from the first generally accepted tetrahedral model with a longer mean Ag–O bond distance. Ab initiomolecular dynamics simulations with the Car–Parrinello approach (CPMD) were also performed and were able to describe the coordination of the hydrated Ag+ion in aqueous solution in very good agreement with the experimental data. The high sensitivity for the closest environment of the photoabsorber of the EXAFS and XANES techniques, together with the long-range information provided by CPMD and large-angle X-ray scattering (LAXS), allowed us to reconstruct the three-dimensional model of the coordination geometry around the Ag+ion in aqueous solution. The obtained results from experiments and theoretical simulations provided a complex picture with a certain amount of water molecules with high configurational disorder at distances comprised between the first and second hydration spheres. This evidence may have caused the proliferation of the coordination numbers that have been proposed so far for Ag+in water. Altogether these data show how the description of the hydration of the Ag+ion in aqueous solution can be complex, differently from other metal species where hydration structures can be described by clusters with well-defined geometries. This diffuse hydration shell causes the Ag–O bond distance in the linear [Ag(H2O)2]+ion to be ca. 0.2 Å longer than in isolated ions in solid state.

Na-ion batteries are emerging alternatives to Li-ion chemistries for large-scale energy storage applications. Quaternary layered oxide Na0.76Mn0.5Ni0.3Fe0.1Mg0.1O2offers outstanding electrochemical performance in Na-ion batteries compared to pure-phase layered oxides because of the synergistic effect of the P/O-phase mixing. The material is indeed constituted by a mixture of P3, P2, and O3 phases, and a newly identified Na-free phase, i.e., nickel magnesium oxide phase, which improves heat removal and enhances the electrochemical performance. Herein, we structurally investigate, through synchrotron-radiation X-ray diffraction, the modifications occurring after full desodiation, detailing the material structural rearrangement upon Na removal and revealing the effect of two different charging protocols, i.e., constant current (CC) and constant current–constant voltage (CCCV). While the Na-free phase is electrochemically inactive, likely helping in homogenization of the thermal gradient in the electrode during cycling, O–P intergrown phases appear during the extraction of Na ions from interslab layers, and they are dependent on the desodiation level. The application of a constant voltage step at the end of the galvanostatic charge is responsible for a shortening of the interslab distance and a significant volume contraction (−11.9%).

Two isostructural coordination polymers with coinage metal(I) cations were compressed with the purpose of testing interactions between chains, which may trigger metallophilic interactions or otherwise expand the metal coordination. DFT calculations and X-ray diffraction studies reveal an extraordinary difference between Ag(I) and Cu(I) in homologous compounds. Argentophilic interactions are favored by a mild compression, and at P= 7.94 GPa, the Ag–Ag distance matches the value of metallic silver. On the other hand, no cuprophilic interaction is activated even by compression up to 8 GPa, and Cu–Cu distances remain outside the van der Waals spheres.

The first successful synthesis and characterization of group 10 complexes featuring flexible scorpionate ligands based on 7-azaindole heterocycles are reported herein. Addition of 2 equiv of either K[HB(azaindolyl)3] or Li[HB(Me)(azaindolyl)2] to [M(μ-Cl)(η,1η2-COEOMe)]2leads to the formation of 2 equiv of the complexes [M{κ3-N,N,H-HB(azaindolyl)3}(η,1η2-COEOMe)] and [M{κ3-N,N,H-HB(Me)(azaindolyl)2}(η,1η2-COEOMe)] (where M = Pt, Pd; COEOMe= 8-methoxycyclooct-4-en-1-ide), respectively. In these reactions, the borohydride group is directed toward the metal center forming square based pyramidal complexes. In contrast to analogous complexes featuring other flexible scorpionate ligands, no hydrogen migration from boron is observed in the complexes studied. The fortuitous line widths observed in some of the 11B NMR spectra allow for a closer inspection of the B–H···metal unit in scorpionate complexes than has previously been possible.

The structural and dynamic properties of the Ba2+cation in water have been studied by combining quantum mechanical (QM) calculations, molecular dynamics (MD) simulations, and extended X-ray absorption fine structure (EXAFS) spectroscopy. An effective Ba2+-water interaction potential, to be used in the MD simulation of a Ba2+aqueous solution, has been developed by means of QM methods, and the validity of the whole procedure has been assessed by comparing the theoretical structural results with the EXAFS experimental data. By combining distance and angular distribution functions it was possible to unambiguously identify the geometry adopted by the water molecules surrounding the ion in the solution. The Ba2+ion was found to preferentially form an 8-fold first shell complex with a bicapped trigonal prism (BTP) geometry. The 8-fold complex is in equilibrium with a 9-fold structure having a tricapped trigonal prism (TTP) geometry, and the hydration shell is very diffuse and flexible, being characterized by a very fast solvent exchange process on the picosecond time scale.

By utilizing a flexible tetrapyridinate ligand, tetrakis(4-pyridyloxymethylene) methane(L), a novel multifunctional soft porous framework, [Co(NCS)2(L)]·2H2O·CH3OH (1), was constructed. This framework exhibits quick and selective solvatochromic and vapochromic behavior during a reversible crystal-to-amorphous-to-crystal (CAC) transformation. Importantly, the rapid CAC transition can selectively be triggered by methanol molecules, even at low concentrations of liquid or gaseous methanol. This reproducible transition can be monitored by single-crystal and power X-ray analysis, IR, and UV–vis, which all powerfully illustrate the selectivity and sensitivity of this system. In addition, the typical magnetic behavior of single ion magnets (SIMs) has been successfully introduced into this 3D framework, and the modified dynamic relaxations have been investigated via experimental and theoretical analysis. The consistent observations of both experimental and theoretical results support that the distortions of the metal coordination environments should be responsible for the finely tuned SIM behavior.

The structural modifications induced on a 0.5 M Zn2+aqueous solution by increasing the pressure to 6.4 GPa were investigated using a combination of X-ray absorption near edge structure (XANES) spectroscopy and molecular dynamics (MD) simulations. The Zn K-edge XANES experimental spectra show two different trends depending on the pressure and temperature conditions of the system. On the one hand, when the pressure is increased to 1.0 GPa while keeping the temperature at 300 K, the highly structured nature of Zn2+second hydration shell is preserved. On the other hand, when the Zn2+aqueous solution is simultaneously pressurized and heated to follow the melting curve above 1.0 GPa, the Zn2+second shell loses its high degree of structuring and becomes much more disordered and unstructured. These results are confirmed by the analysis of MD simulations of Zn2+aqueous solutions under high pressure. By combining distance and angular distribution functions it is possible to highlight the loss of water structuring in the Zn2+second coordination shell that takes place upon pressurization and heating. A progressive crowding of the Zn2+second shell is observed with increasing pressure; the water structure becomes remarkably different from that found at ambient conditions, and for pressure values higher than 1.0 GPa the tetrahedral arrangements of water molecules is highly distorted. Moreover, MD simulations of Zn2+aqueous solutions performed at 1.0 GPa and at increasing temperature values have shown that the loss of water structuring in the Zn2+second coordination shell observed by simultaneously pressurizing and heating is due to a combined effect of pressure and temperature, both producing an increase of the Zn2+second-shell disorder.

The oxygen uptake ability of Pr-CeO2-based oxygen carriers, catalysts, and solid oxide fuel cells can be attributed to 3+ cation generation and the presence of vacant oxygen sites. Oxygen occupancies of CeO2, Pr-CeO2, and 5% Cu-doped Pr-CeO2were investigated using neutron diffraction and related to the oxygen uptake as determined using thermogravimetric analysis (TGA). The presence of vacant tetrahedral oxygen sites at room temperature did not correspond to low-temperature oxygen uptake. The materials did not uptake oxygen at 420 °C, but oxygen uptake was observed at 600 °C, which indicated that a minimum temperature needs to be met to generate sufficient vacancies/3+ cations. Variations in the lattice parameter as a function of temperature were revealed using in situ X-ray diffraction (XRD). With increasing temperature the lattice parameter increased linearly due to thermal expansion and was followed by an exponential increase at ∼300–400 °C as cations were reduced. Despite segregation of Cu into CuO at high dopant concentration, at 600 °C a higher O2uptake was obtained for Ce0.65Pr0.20Cu0.15O2−δ(120 μmol g–1), in comparison to Ce0.75Pr0.2Cu0.05O2−δ(92 μmol g–1), and was higher than that for Ce0.8Pr0.2O2−δ(55 μmol g–1). Both Pr and Cu introduce vacancies and promote the O2uptake of CeO2.

The self-exchange kinetics of CO ligands in the solvated forms of the commonly used complex [MBr3(CO)3]2–(M = Re, 99Tc) were investigated in-depth by 13C NMR spectroscopy in organic solvents such as dimethylformamide and methanol. The two homologues exhibit surprisingly different chemical behavior. In the case of rhenium, the stable intermediate [NEt4][ReBr2(CO)4] was isolated and characterized by 13C NMR and IR spectroscopy as well as by single-crystal X-ray diffraction. For technetium, no such intermediate could be identified. The activation parameters (ΔH⧧= 110 ± 7 kJ mol–1and ΔS⧧= 127 ± 22 J mol–1K–1) and the observed influences of different ligands and solvents suggest a dissociative-interchange-type mechanism with a second-order rate constant for the formation of [NEt4][ReBr2(CO)4], k1= 0.039 ± 0.001 M–1s–1at 274 K. On the basis of variable-temperature NMR experiments, kinetic simulations, and density functional theory calculations, a complete model for the CO self-exchange, including all respective rate constants, is reported.

The hydration structure and dynamics of Sc3+in aqueous solution have been investigated using a combined approach based on quantum mechanical (QM) calculations, molecular dynamics (MD) simulations, and extended X-ray absorption fine structure (EXAFS) spectroscopy. An effective Sc–water two-body potential has been generated from QM calculations and then used in the MD simulation of Sc3+in water, and the reliability of the entire procedure has been assessed by comparing the theoretical structural results with the EXAFS experimental data. The outstanding outcome of this work is that the Sc3+ion forms a well-defined capped square antiprism (SAP) complex in aqueous solution, where the eight water molecules closest to the ion are located at the vertexes of a SAP polyhedron, while the ninth water molecule occupying the capping position is unusually found at a very long distance from the ion. This far-coordinated water molecule possesses a degree of structure comparable with the other first shell molecules surrounding the ion at much shorter distances, and its presence gave us the unique opportunity to easily identify the geometry of the Sc3+coordination polyhedron. Despite very strong ion–water interactions, the Sc3+hydration shell is very labile, as the far-coordinated ligand allows first shell water molecules to easily exchange their positions both inside the solvation shell and with the rest of the solvent molecules.

Luminescent, mixed metal d–f complexes have the potential to be used for dual (magnetic resonance imaging (MRI) and luminescence) in vivoimaging. Here, we present dinuclear and trinuclear d–f complexes, comprising a rigid framework linking a luminescent Ir center to one (Ir·Ln) or two (Ir·Ln2) lanthanide metal centers (where Ln = Eu(III) and Gd(III), respectively). A range of physical, spectroscopic, and imaging-based properties including relaxivity arising from the Gd(III) units and the occurrence of Ir(III) → Eu(III) photoinduced energy-transfer are presented. The rigidity imposed by the ligand facilitates high relaxivities for the Gd(III) complexes, while the luminescence from the Ir(III) and Eu(III) centers provide luminescence imaging capabilities. Dinuclear (Ir·Ln) complexes performed best in cellular studies, exhibiting good solubility in aqueous solutions, low toxicity after 4 and 18 h, respectively, and punctate lysosomal staining. We also demonstrate the first example of oxygen sensing in fixed cells using the dyad Ir·Gd, via two-photon phosphorescence lifetime imaging (PLIM).