Mononuclear Rare-Earth Metalloligands Exploiting a Divergent Ligand.

Rare-earth tris-diketonato [RE(dike)3pyterpy] metalloligands can be prepared reacting at room temperature [RE(dike)3dme] (dme = 1,2-dimethoxyethane; dike = tta with Htta = 2-thenoyltrifluoroacetone and RE = La, 1; Y, 2; Eu, 3; Dy, 4; or dike = hfac with Hhfac hexafluoroacetylacetone, and RE = Eu, 5; Tb, 6; Yb 7) with 4'-(4‴-pyridil)-2,2':6',2″-terpyridine (pyterpy). The molecular structures of 1, 5, 6, and 7 have been studied through single-crystal X-ray diffraction showing mononuclear neutral complexes with the rare-earth ion in coordination number nine and with a muffin-like coordination geometry. [RE(tta)3pyterpy] promptly reacts with [M(tta)2dme] with formation of [Mpyterpy2][RE(tta)4]2 (M = Zn, RE = Y, 8; M = Co, RE = Dy, 9). Consistently, [Zn(hfac)2dme] reacts at room temperature with 2 equiv of pyterpy yielding [Znpyterpy2][hfac]2 10 that easily can be transformed by reaction with 2 equiv of [Eu(hfac)3] in [Znpyterpy2][Eu(hfac)4]2 11 that has been structurally characterized. Finally, 1, 2, 3, 5, and 7 metalloligands react at room temperature in few minutes with [PtCl(µ-Cl)PPh3]2 yielding the heterometallic molecular complexes [RE(dike)3pyterpyPtCl2PPh3] (dike = tta, RE = La, 12; Y, 13; Eu; 14; dike = hfac, RE = Eu, 15; Yb, 16).

Origin of the Unusual Ground-State Spin S = 9 in a Cr10 Single-Molecule Magnet.

The molecular wheel [Cr10(OMe)20(O2CCMe3)10], abbreviated {Cr10}, with an unusual intermediate total spin S = 9 and non-negligible cluster anisotropy, D/kB = -0.045(2) K, is a rare case among wheels based on an even number of 3d-metals, which usually present an antiferromagnetic (AF) ground state (S = 0). Herein, we unveil the origin of such a behavior. Angular magnetometry measurements performed on a single crystal confirmed the axial anisotropic behavior of {Cr10}. For powder samples, the temperature dependence of the susceptibility plotted as χT(T) showed an overall ferromagnetic (FM) behavior down to 1.8 K, whereas the magnetization curve M(H) did not saturate at the expected 30 µB/fu for 10 FM coupled 3/2 spin Cr3+ ions, but to a much lower value, corresponding to S = 9. In addition, the X-ray magnetic circular dichroism (XMCD) measured at high magnetic field (170 kOe) and 7.5 K showed the polarization of the cluster moment up to 23 µB/fu. The magnetic results can be rationalized within a model, including the cluster anisotropy, in which the {Cr10} wheel is formed by two semiwheels, each with four Cr3+ spins FM coupled (JFM/kB = 2.0 K), separated by two Cr3+ ions AF coupled asymmetrically (J23/kB = J78/kB = -2.0 K; J34/kB = J89/kB = -0.25 K). Inelastic neutron scattering and heat capacity allowed us to confirm this model leading to the S = 9 ground state and first excited S = 8. Single-molecule magnet behavior with an activation energy of U/kB = 4.0(5) K in the absence of applied field was observed through ac susceptibility measurements down to 0.1 K. The intriguing magnetic behavior of {Cr10} arises from the detailed asymmetry in the molecule interactions produced by small-angle distortions in the angles of the Cr-O-Cr alkoxy bridges coupling the Cr3+ ions, as demonstrated by ab initio and density functional theory calculations, while the cluster anisotropy can be correlated to the single-ion anisotropies calculated for each Cr3+ ion in the wheel.

Helicate versus Mesocate in Quadruple-Stranded Lanthanide Cages: A Computational Insight.

To drive the synthesis of metallo-supramolecular assemblies (MSAs) and to fully exploit their functional properties, robust computational tools are crucial. The capability to model and to rationalize different parameters that can influence the outcome is mandatory. Here, we report a computational insight on the factors that can determine the relative stability of the supramolecular isomers helicate and mesocate in lanthanide-based quadruple-stranded assemblies. The considered MSAs have the general formula [Ln2L4]2- and possess a cavity suitable to allocate guests. The analysis was focused on three different factors: the ligand rigidity and the steric hindrance, the presence of a guest inside the cavity, and the guest dimension. Three different quantum mechanical calculation set-ups (in vacuum, with the solvent, and with the solvent and the dispersion correction) were considered. Comparison between theoretical and experimental outcomes suggests that all calculations correctly estimated the most stable isomer, while the inclusion of the dispersion correction is mandatory to reproduce the geometrical parameters. General guidelines can be drawn: less rigid and less bulky is the ligand and less stable is the helicate, and the presence of a guest can strongly affect the isomerism leading to an inversion of the stability by increasing the guest size when the ligand is flexible.

Carbodiimide-Mediated Beckmann Rearrangement of Oxyma-B as a Side Reaction in Peptide Synthesis.

The suppression of side reactions is one of the most important objectives in peptide synthesis, where highly reactive compounds are involved. Recently, the violuric acid derivative Oxyma-B was introduced into peptide synthesis protocols as a promising additive to efficiently control the optical purity of the amino acids prone to racemization. However, we discovered a side reaction involving the Beckmann rearrangement of Oxyma-B during the coupling reaction, which compromises the yield and purity of the target peptides. Here, we present the investigation of the mechanism of this rearrangement and the optimization of the coupling reaction conditions to control it. These results can be taken into account for the design of novel efficient oxime-based coupling reagents.

Multireference Ab Initio Investigation on Ground and Low-Lying Excited States: Systematic Evaluation of J-J Mixing in a Eu3+ Luminescent Complex.

A theoretical protocol combining density functional theory (DFT) and multireference (CAS) calculations is proposed for a Eu3+ complex. In the complex, electronic levels of the central Eu3+ ion are correctly calculated at the CASPT2 level of theory, and the effect of introducing different numbers of states in the configuration interaction matrices is highlighted as well as the shortcomings of DFT methods in the treatment of systems with high spin multiplicity and strong spin-orbit coupling effects. For the 5D0 state energy calculation, the inclusion of states with different multiplicity and the number of states considered for each multiplicity are crucial parameters, even if their relative weight is different. Indeed, the addition of triplet and singlets is important, while the number of states is relevant only for the quintets. The herein proposed protocol enables a rigorous, full ab initio treatment of Eu3+ complex, which can be easily extended to other Ln3+ ions.

Nature of the Ligand-Centered Triplet State in Gd3+ ß-Diketonate Complexes as Revealed by Time-Resolved EPR Spectroscopy and DFT Calculations.

A series of Gd3+ complexes (Gd1-Gd3) with the general formula GdL3(EtOH)2, where L is a ß-diketone ligand with polycyclic aromatic hydrocarbon substituents of increasing size (1-3), was studied by combining time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy and DFT calculations to rationalize the anomalous spectroscopic behavior of the bulkiest complex (Gd3) through the series. Its faint phosphorescence band is observed only at 80 K and it is strongly red-shifted (∼200 nm) from the intense fluorescence band. Moreover, the TR-EPR spectral analysis found that triplet levels of 3/Gd3 are effectively populated and have smaller |D| values than those of the other compounds. The combined use of zero-field splitting and spin density delocalization calculations, together with spin population analysis, allows us to explain both the large red shift and the low intensity of the phosphorescence band observed for Gd3. The large red shift is determined by the higher delocalization degree of the wavefunction, which implies a larger energy gap between the excited S1 and T1 states. The low intensity of the phosphorescence is due to the presence of C-H groups which favor non-radiative decay. These groups are present in all complexes; nevertheless, they have a relevant spin density only in Gd3. The spin population analysis on NaL models, in which Na+ is coordinated to a deprotonated ligand, mimicking the coordinative environment of the complex, confirms the outcomes on the free ligands.

Light-Triggered Catalytic Asymmetric Allylic Benzylation with Photogenerated C-Nucleophiles.

Herein is reported the asymmetric allylic benzylation of Morita-Baylis-Hillman (MBH) carbonates with 2-methylbenzophenone (MBP) derivatives as nonstabilized photogenerated C-nucleophiles. The dual activation of both reaction partners, chiral Lewis-base activation of the electrophile and light activation of the nucleophile, enables the stereoselective installation of benzyl groups at the allylic position to forge tertiary and quaternary carbon centers.

Possible Synthetic Approaches for Heterobimetallic Complexes by Using nNHC/tzNHC Heteroditopic Carbene Ligands.

The synthesis of heterobimetallic complexes remains a synthetic challenge in the field of organometallic chemistry. A possible approach in this regard might be the use of a bidentate heteroditopic bis(carbene) ligand that combines an imidazol-2-ylidene (nNHC) with a 1,2,3-triazol-5-ylidene (tzNHC) connected by an organic spacer. The optimized strategy to heterobimetallic complexes with this type of ligand involves a 3-step procedure: (i) Coordination of the nNHC, functionalized with a 1,2,3-triazole ring, to a metal center; (ii) formation of the triazolium ring by alkylation of the triazole N-3; (iii) deprotonation of the tzNHC precursor and coordination of the second metal center. Following this procedure, a novel Au(I)-Ag(I) dinuclear complex was isolated and its properties were compared to the analogous homobimetallic Ag(I)-Ag(I) and Au(I)-Au(I) complexes. The study was completed by the determination of the molecular structures of some synthetic intermediates.

Theoretical Investigation of the Electronic Properties of Three Vanadium Phthalocyaninato (Pc) Based Complexes: PcV, PcVO, and PcVI.

The electronic properties of three vanadium phthalocyaninato (Pc) based complexes (PcV, PcVO, and PcVI; I-III, respectively) were theoretically investigated and corresponding VL2,3-edge XAS spectra modeled. Ground state (GS) DFT outcomes indicated that II is more stable than III by 141 kcal/mol; moreover, the Ziegler transition state method allowed us to estimate the PcV-X bond dissociation energy and to quantify σ/π contributions to the V-X interaction. As such, the Nalewajski-Mrozek V-X and V-N bond multiplicity indexes (V-O/V-I = 2.48/1.22; V-N = 0.64, 0.51, and 0.58 in I-III, respectively) state that the V-X bond strength and nature affect the V-N interaction. The coordination of X to V in the I → II/I → III reactions implies the transfer of two/one electrons from I to X. In both cases, the oxidation involves only the V ion; moreover, V 3d based orbitals from which electrons are transferred were identified. Literature I/IIL2,3-edge XAS data were modeled by exploiting the DFT/ROCIS method. The same protocol was adopted to predict IIIL2,3-edge XAS spectra. Theoretical results indicated that, along the whole series, spectral features lying at the lowest excitation energies (EEs) are mostly generated by states having the same GS spin multiplicity and involve 2pV → SOMO (single occupied molecular orbital) single electronic excitations. XAS features at higher EEs include only states with the same GS spin multiplicity in I, while states with both ΔS = 0 and ΔS = +1 (S = total spin quantum number) are present in II and III with significant, in some cases prevailing, contributions from metal to ligand charge transfer (MLCT) excitations. Beyond the role played by MLCT transitions in determining XAS patterns, it is noteworthy that they involve only Pc-based empty orbitals with no participation of the X-based virtual levels.

A terminally protected dipeptide: from crystal structure and self-assembly, through co-assembly with carbon-based materials, to a ternary catalyst for reduction chemistry in water.

A terminally protected, hydrophobic dipeptide Boc-L-Cys(Me)-L-Leu-OMe (1) was synthesized and its 3D-structure was determined by single crystal X-ray diffraction analysis. This peptide is able to hierarchically self-assemble in a variety of superstructures, including hollow rods, ranging from the nano- to the macroscale, and organogels. In addition, 1 is able to drive fullerene (C60) or multiwalled carbon nanotubes (MWCNTs) in an organogel by co-assembling with them. A hybrid 1-C60­MWCNT organogel was prepared and converted (through a high vacuum-drying process) into a robust, high-volume, water insoluble, solid material where C60 is well dispersed over the entire superstructure. This ternary material was successfully tested as a catalyst for: (i) the reduction reaction of water-soluble azo compounds mediated by NaBH4 and UV-light with an overall performance remarkably better than that provided by C60 alone, and (ii) the NaBH4-mediated reduction of benzoic acid to benzyl alcohol. Our results suggest that the self-assembly properties of 1 might be related to the occurrence in its single crystal structure of a sixfold screw axis, a feature shared by most of the linear peptides known so far to give rise to nanotubes.

Double level selection in a constitutional dynamic library of coordination driven supramolecular polygons.

A constitutional dynamic library (CDL) of Cu(II) metallo-supramolecular polygons has been studied as a bench test to examine an interesting selection case based on molecular recognition. Sorting of the CDL polygons is achieved through a proper guest that is hosted into the triangular metallo-macrocycle constituent. Two selection mechanisms are observed, a guest induced path and a guest templated self-assembly (virtual library approach). Remarkably, the triangular host can accommodate several guests with a degree of selectivity ranging from ∼1 to ∼10(4) for all possible guest pairs. A double level selection operates: guests drive the CDL toward the triangular polygon, and, at the same time, this is able to pick a specific guest from a set of competitive molecules, according to a selectivity-affinity correlation. Association constants of the host-guest systems have been determined. Guest competition and exchange studies have been analyzed through variable temperature UV-Vis absorption spectroscopy and single crystal X-ray diffraction studies. Molecular structures and electronic properties of the triangular polygon and of the host-guest systems also have been studied by means of all electrons density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations including dispersive contributions. DFT outcomes ultimately indicate the dispersive nature of the host-guest interactions, while TDDFT results allow a thorough assignment of the host and host-guests spectral features.

Graphene Oxide-Arginine Composites: Efficient Dual Function Materials for Integrated CO2 Capture and Conversion.

The "on-demand" capture and utilization of CO2 is effectively realized with a readily accessible dual function organic composite. The covalent and controlled derivatization of graphene oxide (GO) surface with naturally occurring arginine led to a "smart" material capable of capturing (chemisorption) CO2 from high-purity flue-gas as well as low-concentration streams (i. e. direct air capture) and concomitant chemical activation toward the incorporation into cyclic carbonates. The overall integrated CO2 capture and conversion (ICCC) strategy has been fully elucidated mechanistically via dedicated computational, spectroscopic and thermal analyses.

Supported MOCVD TiO2 Thin Films Grown on Modified Stainless Steel Mesh for Sensing Applications.

Among semiconductor metal oxides, that are an important class of sensing materials, titanium dioxide (TiO2) thin films are widely employed as sensors because of their high chemical and mechanical stability in harsh environments, non-toxicity, eco-compatibility, and photocatalytic properties. TiO2-based chemical oxygen demand (COD) sensors exploit the photocatalytic properties of TiO2 in inducing the oxidation of organic compounds to CO2. In this work, we discuss nanostructured TiO2 thin films grown via low-pressure metal organic chemical vapor deposition (MOCVD) on metallic AISI 316 mesh. To increase the surface sensing area, different inorganic acid-based chemical etching protocols have been developed, determining the optimal experimental conditions for adequate substrate roughness. Both chemically etched pristine meshes and the MOCVD-coated ones have been studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX) microanalysis, and X-ray photoelectron spectroscopy (XPS). We demonstrate that etching by HCl/H2SO4 at 55 °C provides the most suitable surface morphology. To investigate the behavior of the developed high surface area TiO2 thin films as COD sensors, photocatalytic degradation of functional model pollutants based on ISO 10678:2010 has been tested, showing for the best performing acid-etched mesh coated with polycrystalline TiO2 an increase of 60% in activity, and degrading 66 µmol of MB per square meter per hour.

Dinuclear Cu(II) complexes of isomeric bis-(3-acetylacetonate)benzene ligands: synthesis, structure, and magnetic properties.

Highly versatile coordinating ligands are designed and synthesized with two ß-diketonate groups linked at the carbon 3 through a phenyl ring. The rigid aromatic spacer is introduced in the molecules to orient the two acetylacetone units along different angles and coordination vectors. The resulting para, meta, and ortho bis-(3-acetylacetonate)benzene ligands show efficient chelating properties toward Cu(II) ions. In the presence of 2,2'-bipyridine, they promptly react and yield three dimers, 1, 2, and 3, with the bis-acetylacetonate unit in bridging position between two metal centers. X-ray single crystal diffraction shows that the compounds form supramolecular chains in the solid state because of intermolecular interactions. Each of the dinuclear complexes shows a magnetic behavior which is determined by the combination of structural parameters and spin polarization effects. Notably, the para derivative (1) displays a moderate antiferromagnetic coupling (J = -3.3 cm(-1)) along a remarkably long Cu···Cu distance (12.30 Å).

Investigations on Caesium Dispersion and Molybdenum Coating on SPIDER Components.

SPIDER is the 100 keV full-size Negative Ion Source prototype of the ITER Neutral Beam Injector, operating at Consorzio RFX in Padova, Italy. The largest Negative Ion Source in the world, SPIDER generates an RF driven plasma from which Deuterium or Hydrogen negative ions are produced and extracted. At the end of 2021, a scheduled long-term shutdown started to introduce major modifications and improvements aiming to solve issues and drawbacks identified during the first three years of SPIDER operations. The first action of the shutdown period was the disassembly and characterization of the SPIDER beam source after removal from the vacuum vessel and its placement inside the clean room. Each component was carefully assessed and catalogued, following a documented procedure. Some source components, i.e., the Plasma Grid, Extraction Grid and Bias Plate, revealed the presence of different and non-uniform red, white and green coatings that might be correlated to back-streaming positive ions impinging on grid surfaces, electrical discharges and caesium evaporation. Thus, several analyses have been carried out to understand the nature of such coatings, with the study still ongoing. The evidence of caesium evaporation and deposition on molybdenum-coated SPIDER components, such as the formation of oxides and hydroxides, is demonstrated through surface characterization analyses with the use of the Scanning Electron Microscope (SEM), X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS).

Design of a Zn Single-Site Curing Activator for a More Sustainable Sulfur Cross-Link Formation in Rubber.

ZnO is a worldwide used activator for a rubber vulcanization process, which promotes fast curing kinetics and high cross-linking densities of rubber nanocomposites (NCs). However, its extended use together with leaching phenomena occurring during the production and life cycle of rubber products, especially tires, entails potential environmental risks, as ecotoxicity toward aquatic organisms. Pushed by this issue, a novel activator was developed, which introduces highly dispersed and active zinc species in the vulcanization process, reducing the amount of employed ZnO and keeping high the curing efficiency. The activator is constituted by Zn(II) single sites, anchored on the surface of SiO2 nanoparticles (NPs) through the coordination with functionalizing amino silane groups. It behaves as a double-function material, acting at the same time as a rubber reinforcing filler and a curing activator. The higher availability and reactivity of the single-site Zn(II) centers toward curative agents impart faster kinetics and higher efficiency to the vulcanization process of silica/isoprene NCs, compared to conventionally used ZnO activators. Moreover, the NCs show a high cross-linking degree and improved dynamic mechanical properties, despite the remarkably lower amount of zinc employed than that normally used for rubber composites in tires. Finally, the structural stability of Zn(II) single sites during the curing reactions and in the final materials may represent a turning point toward the elimination of zinc leaching phenomena.

Antenna triplet DFT calculations to drive the design of luminescent Ln3+ complexes.

Density functional theory-based methods have been exploited to look into the structural, vibrational and electronic properties of antenna ligands, all of them being crucial factors for the reliable design of customized luminescent lanthanide (Ln3+) complexes. The X-ray structures, UV-Vis absorption spectra and triplet (T1) energies of three novel ß-diketone ligands with a thienyl group and naphthyl (L1), phenanthryl (L2), and pyrenyl (L3) polycyclic aromatic hydrocarbons as substituents have been modelled. Vibronic progressions provide a strong contribution to the L1 and L2 absorption spectra, while the L3 absorption spectrum needs the assumption of different conformational isomers in solution. T1 energies have been estimated either through the vertical- or the adiabatic-transition approach. The comparison with the phosphorescence spectra of Gd3+ complexes allowed us to infer that the latter approach is the most suitable one, in particular when sizable ligands are involved. Results obtained for the isolated antennas can be directly compared with those of the corresponding Ln3+ complexes, due to the unanimously accepted assumption that the excitation is ligand-centred.

Nanostructured copper oxide on silica-zirconia mixed oxides by chemical implantation.

N,N-Dialkylcarbamato complexes of copper(II), [Cu(O(2)CNR(2))(2)] (R = All = allyl, C(3)H(5); iPr, CH(CH(3))(2)) were prepared with the aim of functionalizing silica and nanostructured silica-zirconia matrices. The mixed matrices for the grafting reactions were prepared by copolymerizing MAPTMS (methacryloxypropyltrimethoxysilane), the precursor for the silica matrix, with the zirconium tetranuclear derivative [Zr(4)O(2)(OMc)(12)] (OMc = methacrylate), the precursor for the zirconia nanoparticles. Suspension of the silica and silica-zirconia matrices in a solution of the copper dialkylcarbamate led to the functionalization of the respective substrates. The composition, microstructure, morphology, and physicochemical nature of the copper species grafted on the matrices were investigated by FTIR, X-ray photoelectron spectroscopy (XPS), EPR, X-ray absorption spectroscopy (XAS), XRD, TEM, and dinitrogen adsorption. The effect of selected experimental parameters (the nature of the copper precursor and of the matrix, grafting time, thermal treatment) on the grafting reaction was investigated. The Cu/Si ratio is increased by increasing the grafting time and the ZrO(2)-SiO(2) matrix is more reactive to attack by the carbamato complexes than either prepared or commercial SiO(2). After functionalization of the matrix, thermal treatment yielded nanostructured copper(II) oxide clusters, average diameter 12-15 nm, uniformly supported on the silica and on the silica-zirconia matrices.

Chemistry and supramolecular chemistry of chromium horseshoes.

Synthetic and structural studies of Cr horseshoes are reported which show that these compounds demonstrate a rich supramolecular chemistry through H-bonding interactions, and can act as ligands for metal clusters.

A Cu(ii) metallocycle for the reversible self-assembly of coordination-driven polyrotaxane-like architectures.

We report the design and synthesis of a Cu(ii) metallocycle (1) and use the possibility to expand the Cu(ii) coordination sphere to self-assemble mechanically interlocked species via interpenetration. Metallocycle 1 can be used as a platform to reversibly assemble a 1D + 1D → 1D coordination-driven polyrotaxane (3), where 1 acts as the hosting ring as well as the stopper, and 4,4'-bipyridine is the guest-axle. A coordinating solvent can substitute the 4,4'-bipyridine axle to disassemble the polyrotaxane (3 → 2) that is easily restored by further adding 4,4'-bipyridine (2 → 3). Other polyrotaxanes can be isolated by reacting 1 with pyridine (4) and phenylpyridine (5). Interconversion among the presented species is demonstrated and ensured by the open position of each copper center in platform 1.