Sulfur Dioxide Oxidation by Zinc and Zinc Oxide Nitrate/Nitrite Complexes in the Gas Phase: An Interplay between Redox-Active Ligands and Metal.

Different oxidative pathways of sulfur dioxide promoted by ZnO(NO3 )2 - , Zn(NO3 )2 - and Zn(NO2 )(NO3 )- are revealed by a joint investigation by mass spectrometry and theoretical calculations. The reactions are triggered by [Zn2+ -O- ⋅]+ or by the low-valence Zn+ through oxygen ion transfer or electron transfer to SO2 , respectively. The NOx - ligands intervene in the oxidation only when sulfur dioxide is converted to SO3 - or SO2 - , leading to the formation of zinc sulfate and zinc sulfite coordinated to nitrate or nitrite anions. Kinetic analyses show that the reactions are fast and efficient, and theory discloses the elementary steps, namely oxygen ion transfer, oxygen atom transfer and electron transfer, occurring through similar energy landscapes for the three reactive anions.

Design and Modulation of Selectivity toward Vanadium(V) and Uranium(VI) Ions: Coordination Properties and Affinity of Hydroxylamino-Triazine Siderophores.

Based on the strong binding and high selectivity properties of 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine (H2bihyat) for [UVIO2]2+, novel binucleating ligands (BLs) N,N',N″,N‴-((1,4-phenylenebis(oxy))bis(1,3,5-triazine-6,2,4-triyl))tetrakis(N-methylhydroxylamine) (H4qtn), N1,N4-bis(4,6-bis(hydroxy(methyl)amino)-1,3,5-triazin-2-yl)benzene-1,4-diamine (H4pdl), and N1,N2-bis(4,6-bis(hydroxy(methyl)amino)-1,3,5-triazin-2-yl)ethane-1,2-diamine (H4enl) were synthesized. Binuclear complexes formed by coordination of hard metal ions with H4qtn are thermodynamically more stable than their mononuclear analogues with H2bihyat due to the increase in entropy accompanying the formation of more chelate rings. Reaction of either H4qtn or H4pdl or H4enl with [UVIO2]2+ and [VVO2]+ resulted in the isolation of the binuclear complexes [(UVIO2)2(µ-qtn)(H2O)4] (1), [(VVO2)2(µ-qtn)][PPh4]2[PPh4] (2), [(UVIO2)2(µ-pdl)(H2O)2(MeOH)2] (3), [(VVO2)2(µ-pdl)][PPh4]2 (4), [(UVIO2)2(µ-enl)(H2O)4] (5), and [(VVO2)2(µ-enl)][PPh4]2 (6). The binuclear complexes 1-6 were characterized by single-crystal X-ray diffraction analysis in solid state and by NMR and ESI-MS in solution. The comparison of the coordination ability of the BLs with either pyridine-2,6-dicarboxylic acid (H2dipic) or H2bihyat or CO32- toward [UVIO2]2+ and [VVO2]+ was investigated by NMR and UV-vis spectroscopies and DFT theoretical calculations, revealing a superior performance of BLs. The selectivity of the BLs for [UVIO2]2+ over [VVO2]+ is decreased compared to that of H2bihyat but increases considerably at pH > 9 values. Formation of the mixed-metal binuclear species [UVIO2(µ-O)VVO2] influences the selectivity and dynamics of the reaction of H4qtn for [UVIO2]2+ and [VVO2]+ in aqueous solution. The results of this study provide crucial information for the ligand design and the development of stronger and more selective systems.

The inert pair effect on heavy noble gases: insights from radon tetroxide.

A quantum chemical survey of radon and xenon tetroxides (NgO4, Ng = Xe, Rn) is reported herein. The intermediate species, which are formed in their explosive decomposition back to their elemental states (Ng and O2), were also studied and their energetics were compared. While Td symmetric RnO4 has a minimum energy structure, its standard enthalpy of formation is 88.6 kJ mol-1 higher than for XeO4. The reason for this higher instability lies in what is known as the inert pair effect. This work establishes that the high-valent chemical trends of the sixth period of groups 13-15 are indeed extended to group 18.

Experimental and Computational Study of a Tetraazamacrocycle Bis(aryloxide) Uranyl Complex and of the Analogues {E═U═NR}2+ (E = O and NR).

The reaction of [U(κ6-{(t-Bu2ArO)2Me2-cyclam})I][I] (H2{(t-Bu2ArO)2Me2-cyclam} = 1,8-bis(2-hydroxy-3,5-di-tert-butyl)-4,11-dimethyl-1,4,8,11-tetraazacyclotetradecane) with 2 equiv of NaNO2 in acetonitrile results in the isolation of the uranyl complex [UO2{(t-Bu2ArO)2Me2-cyclam}] (3) in 31% yield, which was fully characterized, including by single-crystal X-ray diffraction. Density functional theory (DFT) computations were performed to evaluate and compare the level of covalency within the U═E bonds in 3 and in the analogous trans-bis(imido) [U(κ4-{(t-Bu2ArO)2Me2-cyclam})(NPh)2] (1) and trans-oxido-imido [U(κ4-{(t-Bu2ArO)2Me2-cyclam})(O)(NPh)] (2) complexes. Natural bond orbital (NBO) analysis allowed us to determine the mixing covalency parameter λ, showing that in 2, where both U-Ooxido and U-Nimido bonds are present, the U-Nimido bond registers more covalency with regard to 1, and the opposite is seen for U-Ooxido with respect to 3. However, the covalency driven by orbital overlap in the U-Nimido bond is slightly higher in 1 than in 2. The 15N-labeled complexes [U(κ4-{(t-Bu2ArO)2Me2-cyclam})(15NPh)2] (1-15N) and [U(κ4-{(t-Bu2ArO)2Me2-cyclam})(O)(15NPh)] (2-15N) were prepared and analyzed by solution 15N NMR spectroscopy. The calculated and experimental 15N chemical shifts are in good agreement, displaying the same trend of δN (1-15N) > δN (2-15N) and reveal that the 15N chemical shift may serve as a probe for the covalency of the U═NR bond.

Hafnium(IV) Chemistry with Imide-Dioxime and Catecholate-Oxime Ligands: Unique {Hf5} and Metalloaromatic {Hf6}-Oxo Clusters Exhibiting Fluorescence.

Hafnium(IV) molecular species have gained increasing attention due to their numerous applications ranging from high-resolution nanolithography, heterogeneous catalysis, and electronics to the design of molecule-based building blocks in metal-organic frameworks (MOFs), with applications in gas separation, sorption, luminescence sensing, and interim storage of radioactive waste. Despite great potential, their chemistry is relatively underdeveloped. Here, we use strong chelators (2Z-6Z)-piperidine-2,6-dione (H3pidiox) and 2,3-dihydroxybenzaldehyde oxime (H3dihybo) to synthesize the first ever reported pentanuclear {Hf5/H3pidiox} and hexanuclear {Hf6/H3dihybo} clusters (HfOCs). The {Hf6} clusters adopt unique core structures [Hf6IV(µ3-O)2(µ-O)3] with a trigonal-prismatic arrangement of the six hafnium atoms and have been characterized via single-crystal X-ray diffraction analysis, UV-vis spectroscopy in the solid state, NMR, fluorescence spectroscopy, and high-resolution mass spectrometry in solution. One-dimensional (1D) and two-dimensional (2D) 1H NMR and mass spectroscopies reveal the exceptional thermodynamic stability of the HfOCs in solution. Interestingly, the conjunction of the oxime group with the catechol resulted in the remarkable reduction of the clusters' band gap, below 2.51 eV. Another prominent feature is the occurrence of pronounced metalloaromaticity of the triangular {Hf3} metallic component revealed by its NICSzz scan curve calculated by means of density functional theory (DFT). The NICSzz(1) value of -44.6 ppm is considerably higher than the -29.7 ppm found at the same level of theory for the benzene ring. Finally, we investigated the luminescence properties of the clusters where 1 emits light in the violet region despite the lack of fluorescence of the free H3pidiox ligand, whereas the {Hf6} 3 shifts the violet-emitting light of the H3dihybo to lower energy. DFT calculations show that this fluorescence behavior stems from ligand-centered molecular orbital transitions and that HfIV coordination has a modulating effect on the photophysics of these HfOCs. This work not only represents a significant milestone in the construction of stable low-band-gap multinuclear HfIV clusters with unique structural features and metal-centered aromaticity but also reveals the potential of Hf(IV) molecule-based materials with applications in sensing, catalysis, and electronic devices.

A new krypton complex - experimental and computational investigation of the krypton sulphur pentafluoride cation, [KrSF5]+, in the gas phase.

The gas-phase reactions of noble gas (Ng) cations, namely Kr+ and Xe+, with SF6 were investigated experimentally by Fourier transform ion cyclotron resonance mass spectrometry and computationally using RI-MP2 and BCCD(T) methods. The study revealed a new interaction between Kr+ and neutral SF6 that gave rise to a new cationic, weakly bound complex of Kr, [KrSF5]+, although the major reaction channel was dissociative electron transfer to yield SF5+ and {Kr, F}. Experimental studies examined the formation and stability of the new species and computational studies addressed the energetics of the reaction and indicated that [KrSF5]+ is stable by ca. 1 kcal mol-1. The same computational approach was used to examine the reaction of Xe+ with SF6 and showed it to be thermodynamically unfavourable by ca. 35 kcal mol-1, confirming the non-observation of reaction in the mass spectrometry experiments. An analysis of the bonding in [KrSF5]+ clearly showed that it is a non-covalently bound species, while in its presumed precursor [KrSF6]+ a partially covalent Kr-F bond is present.

Mechanistic Insights into Promoted Hydrolysis of Phosphoester Bonds by MoO2Cl2(DMF)2.

A phosphoester bond is a crucial structural block in biological systems, whose occurrence is regulated by phosphatases. Molybdenum compounds have been reported to be active in phosphate ester hydrolysis of model phosphates. Specifically, MoO2Cl2(DMF)2 is active in the hydrolysis of para-nitrophenyl phosphate (pNPP), leading to heteropolyoxometalate structures. We use density functional theory (DFT) to clarify the mechanism by which these species promote the hydrolysis of the phosphoester bond. The present calculations give insight into several key aspects of this reaction: (i) the speciation of this complex prior to interaction with the phosphate (DMF release, Mo-Cl hydrolysis, and pH influence on the speciation), (ii) the competition between phosphate addition and the molybdate nucleation process, (iii) and the mechanisms by which some plausible active species promote this hydrolysis in different conditions. We described thoroughly two different pathways depending on the nucleation possibilities of the molybdenum complex: one mononuclear mechanism, which is preferred in conditions in which very low complex concentrations are used, and another dinuclear mechanism, which is preferred at higher concentrations.

A lacunary tungstomolybdophosphate as an electronic pendulum: The "blue" electron under examination.

The photoreduction of a Keggin type lacunary tungstomolybdophosphate, α-(Bu4N)4[H3PW9Mo2O39], in acetonitrile, led to the formation of a monoreduced lacunary heteropoly anion, or a one electron reduced "heteropoly blue" species, whereby the added "blue" electron was captured by the molybdenum atoms. The magnetic properties and behavior of the "blue" electron were studied by a modified Evans nuclear magnetic resonance method (small downshift of the 31P signal) and variable-temperature electron paramagnetic resonance (g = 1.936 for MoV). The intermolecular exchange of the "blue" electron was limited by a geometrical factor, which requires the contact between Mo caps to exchange it between the heteropoly couple. The intramolecular exchange of the "blue" electron between Mo atoms was rather fast (5.3 × 109 s-1), with a rate of more than six orders of magnitude larger than the intermolecular exchange rate. Density functional theory was used to determine the most prevalent protonation sites in the mixed lacunary isomers with the aim of studying the intramolecular electron transfer pathway in the isolated [H4PW9Mo2O39]4- species. The singly occupied molecular orbital (SOMO) is essentially localized in one of the two nonequivalent molybdenum sites. The kinetics of the intramolecular electron exchange equilibrium MoV + MoVI → MoVI + MoV between the two molybdenum atoms bridged by an oxygen atom was found to be fast in agreement with the experimental result. The transition state is of mixed-valence type, with the SOMO delocalized over the Mo-O-Mo group. Spectroscopic parameters were found to be in fair agreement with experimental results.

New Insights on the Interaction of Phenanthroline Based Ligands and Metal Complexes and Polyoxometalates with Duplex DNA and G-Quadruplexes.

This work provides new insights from our team regarding advances in targeting canonical and non-canonical nucleic acid structures. This modality of medical treatment is used as a form of molecular medicine specifically against the growth of cancer cells. Nevertheless, because of increasing concerns about bacterial antibiotic resistance, this medical strategy is also being explored in this field. Up to three strategies for the use of DNA as target have been studied in our research lines during the last few years: (1) the intercalation of phenanthroline derivatives with duplex DNA; (2) the interaction of metal complexes containing phenanthroline with G-quadruplexes; and (3) the activity of Mo polyoxometalates and other Mo-oxo species as artificial phosphoesterases to catalyze the hydrolysis of phosphoester bonds in DNA. We demonstrate some promising computational results concerning the favorable interaction of these small molecules with DNA that could correspond to cytotoxic effects against tumoral cells and microorganisms. Therefore, our results open the door for the pharmaceutical and medical applications of the compounds we propose.

Unveiling a Photoinduced Hydrogen Evolution Reaction Mechanism via the Concerted Formation of Uranyl Peroxide.

We present a density functional theory study for the photochemical water oxidation reaction promoted by uranyl nitrate upon sunlight radiation. First, we explored the most stable uranyl complex in the absence of light. The reaction in a dark environmen proceeds through the condensation of uranyl monomers to form dimeric hydroxo-bridged species, which is the first step toward a hydrogen evolution reaction (HER). We found a triplet-state-driven mechanism that leads to the formation of uranyl peroxide and hydrogen gas. To describe in detail this reaction path, we characterized the singlet and triplet low-lying states of the dimeric hydroxo-bridged species, including minima, transition states, minimal energy crossing points, and adiabatic energies. Our computational results provide mechanistic insights that are in good agreement with the experimental data available.

CoII Cryptates Convert CO2 into CO and CH4 under Visible Light.

Three CoII octaazacryptates, with different substituents on the aromatic rings (Br, NO2 , CCH), were synthesised and characterised. These and the already published non-substituted cryptate catalysed CO2 photoreduction to CO and CH4 under blue visible light at room temperature. Although CO was observed after short irradiation times and a large range of catalyst concentrations, CH4 was only observed after longer irradiation periods, such as 30 h, but with a small catalyst concentration (25 nm). Experiments with 13 C labelled CO2 showed that CO is formed and reacts further when the reaction time is long. The CCH catalyst is deactivated faster than the others and the more efficient catalyst for CH4 production is the one with Br. This reactivity trend was explained by an energy decomposition analysis based on DFT calculations.

Electronic Structure Studies on the Whole Keplerate Family: Predicting New Members.

A comprehensive study of the electronic structure of nanoscale molecular oxide capsules of the type [{MVI (MVI )5 O21 }12 {M'V2 O2 (µ-X)(µ-Y)(Ln- )}30 ](12+n)- is presented, where M,M'=Mo,W, and the bridging ligands X,Y=O,S, carried out by means of density functional theory. Discussion of the electronic structure of these derivatives is focused on the thermodynamic stability of each of the structures, the one having the highest HOMO-LUMO gap being M=W, M'=Mo, X=Y=S. For the most well-known structure M=M'=Mo, X=Y=O, [Mo132 O372 ]12- , the chemical bonding of several ligands to the {MoV2 O2 (µ-O)2 } linker moiety produces negligible effects on its stability, which is evidence of a strong ionic component in these bonds. The existence of a hitherto unknown species, namely W132 with both bridging alternatives, is discussed and put into context.

Thermodynamic Stability of Heterodimetallic [LnLn'] Complexes: Synthesis and DFT Studies.

The solid-state and solution configurations of the heterodimetallic complexes (Hpy)[LaEr(HL)3 (NO3 )(py)(H2 O)] (1), (Hpy)[CeEr(HL)3 (NO3 )(py)(H2 O)] (2), (Hpy)[CeGd(HL)3 (NO3 )(py)(H2 O)] (3), (Hpy)[PrSm(HL)3 (NO3 )(py)(H2 O)] (4), and (Hpy)2 [LaYb(HL)3 (NO3 )(H2 O)](NO3 ) (5), in which H3 L is 6-(3-oxo-3-(2-hydroxyphenyl)propionyl)pyridine-2-carboxylic acid and py is pyridine, were analyzed experimentally and by using DFT calculations. Complexes 3, 4, and 5 are described here for the first time, and were analyzed by using single-crystal X-ray diffraction and mass spectrometry. The theoretical study was also extended to the [LaCe] and [LaLu] analogues. The results are consistent with a remarkable selectivity of the metal distribution within the molecule in the solid state, enhanced by the size difference between the different ions. This selectivity was reduced in solution, particularly for ions with the most similar radii. This unique entry into 4f-4f'' heterometallic chemistry establishes for the first time the difference between the selectivity in solution and that in the solid state, as a result of changes to the coordination that follow the dissociation of terminal ligands upon dissolution of the complexes.

Anions coordinating anions: analysis of the interaction between anionic Keplerate nanocapsules and their anionic ligands.

Keplerates are a family of anionic metal oxide spherical capsules containing up to 132 metal atoms and some hundreds of oxygen atoms. These capsules holding a high negative charge of -12 coordinate both mono-anionic and di-anionic ligands thus increasing their charge up to -42, even up to -72, which is compensated by the corresponding counter-cations in the X-ray structures. We present an analysis of the relative importance of several energy terms of the coordinate bond between the capsule and ligands like carbonate, sulphate, sulphite, phosphinate, selenate, and a variety of carboxylates, of which the overriding component is contributed by solvation/de-solvation effects.

Synthesis, structure and bonding of actinide disulphide dications in the gas phase.

Actinide disulphide dications, AnS22+, were produced in the gas phase for An = Th and Np by reaction of An2+ cations with the sulfur-atom donor COS, in a sequential abstraction process of two sulfur atoms, as examined by FTICR mass spectrometry. For An = Pu and Am, An2+ ions were unreactive with COS and did not yield any sulphide species. High level multiconfigurational (CASPT2) calculations were performed to assess the structures and bonding of the new AnS22+ species obtained for An = Th, Np, as well as for An = Pu to examine trends along the An series, and for An = U to compare with a previous experimental study and DFT computational scrutiny of US22+. The CASPT2 results showed that, like in the case of uranium, the new AnS22+ ions have ground states with triangular geometries, corresponding to the presence of a persulphide in the case of thorium that formally leads to a stable ThIVS22+ species, while a supersulphide appears to be present in the case of U, Np and Pu, formally leading to a AnIIIS22+ species. The computations also revealed that linear thioactinyl structures are higher in energy, with a difference that increases fourfold upon moving from U to Pu, apparently indicating that it will be even more pronounced for Am.

Magneto-Structural Analysis of Iron(III) Keggin Polyoxometalates.

A computational study and magnetic susceptibility measurements of three homonuclear Fe(III) Keggin structures are herein presented: the [FeO4@Fe12F24(µ-OCH3)12]5- anion (1), the [Bi6{FeO4@Fe12O12(OH)12}(µ-O2CCCl3)12]+ cation (2) and its polymorph [Bi6{FeO4@Fe12O12(OH)10(H2O)2}(µ-O2CCF3)10]3+ (3). These results are contrasted with the exchange interactions present in the previously characterized [Fe6(OH)3Ge2W18O68(OH)6]11- and [H12As4Fe8W30O120(H2O)2]4- anions. The computational analysis shows that the most significant antiferromagnetic spin coupling takes place at the junction between each of the {Fe3O6(OH)3}/{Fe3F6(OCH3)3} framework motifs, a possibility that had been previously discarded in the literature on the basis of the Fe-Fe distances. For all the examined iron(III) Keggin structures, it is found that the magnitude of the magnetic couplings within each structural subunit follows the same trend.

Di- versus Trinuclear Copper(II) Cryptate for the Uptake of Dicarboxylate Anions.

Searching for receptors selective for the binding of dicarboxylate anions, the copper(II) complexes of the known ditopic octaazacryptand (t2pN8), derived from bistren [tren = tris(2-aminoethyl)amine] linked by p-xylyl spacers, were re-examined, with the expectation of observing a selective binding of oxalate or malonate by bridging the two copper centers of the [Cu2(t2pN8)(H2O)2](4+) receptor. Solution studies involving the supramolecular species formed by the receptor and oxalate (oxa(2-)), malonate (mal(2-)), and succinate (suc(2-)) anions are reported. The determined association constants revealed the unexpected formation of a 3:1:1 Cu/t2pN8/anion stoichiometry for the cascade species with oxa(2-) and mal(2-), and the single crystal X-ray structural characterization confirmed the presence of tricopper(II) complexes, with an unusual binding mode for the dicarboxylate anions. Each of the two copper atoms binds four nitrogen donor atoms of the t2pN8 cryptand and one additional hydroxide group, which bridges to the third copper. The square planar environment of this one is complete with two oxygen atoms from the oxalate (or the malonate). The two copper centers bound to the tren heads are ∼6.5 Å apart, each one at about 3.5 Å from the third Cu center. These studies were complemented by SQUID magnetization measurements and DFT calculations. The magnetic susceptibility measurements of the oxalate cascade complex showed a strong magnetic coupling (J = - 210 cm(-1)) between the Cu centers at a short distance (3.5 Å), while the coupling between the two equivalent Cu atoms (∼6.5 Å) was only -70 cm(-1). This result was well reproduced by DFT calculations.

Polynuclear Gold [Au(I) ]4 , [Au(I) ]8 , and Bimetallic [Au(I) 4 Ag(I) ] Complexes: C-H Functionalization of Carbonyl Compounds and Homogeneous Carbonylation of Amines.

The synthesis of tetranuclear gold complexes, a structurally unprecedented octanuclear complex with a planar [Au(I) 8 ] core, and pentanuclear [Au(I) 4 M(I) ] (M=Cu, Ag) complexes is presented. The linear [Au(I) 4 ] complex undergoes C-H functionalization of carbonyl compounds under mild reaction conditions. In addition, [Au(I) 4 Ag(I) ] catalyzes the carbonylation of primary amines to form ureas under homogeneous conditions with efficiencies higher than those achieved by gold nanoparticles.

Highly Efficient Chirality Transfer from Diamines Encapsulated within a Self-Assembled Calixarene-Salen Host.

A calix[4]arene host equipped with two bis-[Zn(salphen)] complexes self-assembles into a capsular complex in the presence of a chiral diamine guest with an unexpected 2:1 ratio between the host and the guest. Effective chirality transfer from the diamine to the calix-salen hybrid host is observed by circular dichroism (CD) spectroscopy, and a high stability constant K2,1 of 1.59×10(11) M(-2) for the assembled host-guest ensemble has been determined with a substantial cooperativity factor α of 6.4. Density functional calculations are used to investigate the origin of the stability of the host-guest system and the experimental CD spectrum compared with those calculated for both possible diastereoisomers showing that the M,M isomer is the one that is preferentially formed. The current system holds promise for the chirality determination of diamines, as evidenced by the investigated substrate scope and the linear relationship between the ee of the diamine and the amplitude of the observed Cotton effects.

A Metal-Free Synthesis of N-Aryl Carbamates under Ambient Conditions.

The first chemo- and site-selective process for the formation of N-aryl-carbamates from cyclic organic carbonates and aromatic amines is reported. The reactions proceed smoothly under extremely mild reaction conditions using TBD (triazabicyclodecene) as an effective and cheap organocatalyst, thus providing a sustainable and new methodology for the formation of a wide variety of useful N-aryl carbamate synthons in good to excellent yields. Computational investigations have been performed and show the underlying reason for the observed unique reactivity as related to an effective proton-relay mechanism mediated by the bicyclic guanidine base.