[Cu32(H)20{S2P(OiPr)2}12]: The Largest Number of Hydrides Recorded in a Molecular Nanocluster by Neutron Diffraction

An air‐ and moisture‐stable nanoscale polyhydrido copper cluster [Cu₃₂(H)₂₀{S₂P(OiPr)₂}₁₂] ( 1_H ) was synthesized and structurally characterized. The molecular structure of 1_H exhibits a hexacapped pseudo‐rhombohedral core of 14 Cu atoms sandwiched between two nestlike triangular cupola fragments of (2×9) Cu atoms in an elongated triangular gyrobicupola polyhedron. The discrete Cu₃₂ cluster is stabilized by 12 dithiophosphate ligands and a record number of 20 hydride ligands, which were found by high‐resolution neutron diffraction to exhibit tri‐, tetra‐, and pentacoordinated hydrides in capping and interstitial modes. This result was further supported by a density functional theory investigation on the simplified model [Cu₃₂(H)₂₀(S₂PH₂)₁₂].     

A Theoretical Study on the Mechanism of C2H4 Oxidation over a Neutral V3O8 Cluster

Density functional theory (DFT) calculations are used to investigate the reaction mechanism of V₃O₈+C₂H₄. The reaction of V₃O₈ with C₂H₄ produces V₃O₇CH₂+HCHO or V₃O₇+CH₂OCH₂ overall barrierlessly at room temperature, whereas formation of hydrogen‐transfer products V₃O₇+CH₃CHO is subject to a tiny overall free energy barrier (0.03 eV), although the formation of the last‐named pair of products is thermodynamically more favorable than that of the first two. These DFT results are in agreement with recent experimental observations. The (O_b)₂V(O_tO_t)^. (b=bridging, t=terminal) moiety containing the oxygen radical in V₃O₈ is the active site in the reaction with C₂H₄. Similarities and differences between the reactivities of (O_b)₂V(O_tO_t)^. in V₃O₈ and the small VO₃ cluster [(O_t)₂VO_t^.] are discussed. Moreover, the effect of the support on the reactivity of the (O_b)₂V(O_tO_t)^. active site is evaluated by investigating the reactivity of the cluster VX₂O₈, which is obtained by replacing the V atoms in the (O_b)₃VO_t support moieties of V₃O₈ with X atoms (X=P, As, Sb, Nb, Ta, Si, and Ti). Support X atoms with different electronegativities influence the oxidative reactivity of the (O_b)₂V(O_tO_t)^. active site through changing the net charge of the active site. These theoretical predictions of the mechanism of V₃O₈+C₂H₄ and the effect of the support on the active site may be helpful for understanding the reactivity and selectivity of reactive O^. species over condensed‐phase catalysts.     

Phosphinothiolates as Ligands for Polyhydrido Copper Nanoclusters

The reaction of [CuI(HSC₆H₄PPh₂)]₂ with NaBH₄ in CH₂Cl₂/EtOH led to air‐ and moisture‐stable copper hydride nanoparticles (CuNPs) containing phosphinothiolates as new ligands, one of which was isolated by crystallization. The X‐ray crystal structure of [Cu₁₈H₇L₁₀I] (L=^?S(C₆H₄)PPh₂) shows unprecedented features in its 28‐atom framework (18 Cu and 10 S atoms). Seven hydrogen atoms, in hydride form, are needed for charge balance and were located by density functional theory methods. H₂ was released from the copper hydride nanoparticles by thermolysis and visible light irradiation.     

Cationic Heterocycles as Ligands: Synthesis and Reactivity with Anionic Nucleophiles of Cationic Triruthenium Clusters Containing C‐Metalated N‐Methylquinoxalinium or N‐Methylpyrazinium Ligands

The cationic cluster complexes [Ru₃(CO)₁₀(μ‐H)(μ‐κ²N,C‐L¹Me)]⁺ ( 3 ⁺; HL¹=quinoxaline) and [Ru₃(CO)₁₀(μ‐H)(μ‐κ²N,C‐L²Me)]⁺ ( 5 ⁺; HL²=pyrazine) have been prepared as triflate salts by treatment of their neutral precursors [Ru₃(CO)₁₀(μ‐H)(μ‐κ²N,C‐Lⁿ)] with methyl triflate. The cationic character of their heterocyclic ligands is responsible for their enhanced tendency to react with anionic nucleophiles relative to that of hydrido triruthenium carbonyl clusters that have neutral N‐heterocyclic ligands. These clusters react instantaneously with methyl lithium and potassium tris‐sec‐butylborohydride (K‐selectride) to give neutral products that contain novel nonaromatic N‐heterocyclic ligands. The following are the products that have been isolated: [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L¹Me₂)] ( 6 ; from 3 ⁺ and methyl lithium), [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L¹HMe)] ( 7 ; from 3 ⁺ and K‐selectride), [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L²Me₂)] ( 8 ; from 5 ⁺ and methyl lithium), and [Ru₃(CO)₉(μ‐H)(μ₃‐κ²N,C‐L²HMe)] ( 11 ; from 5 ⁺ and K‐selectride). Whereas the reactions of 3 ⁺ lead to products that arise from the attack of the corresponding nucleophile at the C atom of the only CH group adjacent to the N‐methyl group, the reactions of 5 ⁺ give mixtures of two products that arise from the attack of the nucleophile at one of the C atoms located on either side of the N‐methyl group. The LUMOs and the atomic charges of 3 ⁺ and 5 ⁺ confirm that the reactions of these clusters with anionic nucleophiles are orbital‐controlled rather than charge‐controlled processes. The N‐heterocyclic ligands of all of these neutral products are attached to the metal atoms in nonconventional face‐capping modes. Those of compounds 6 – 8 have the atoms of a ligand C?N fragment σ‐bonded to two Ru atoms and π‐bonded to the other Ru atom, whereas the ligand of compound 11 has a C? N fragment attached to a Ru atom through the N atom and to the remaining two Ru atoms through the C atom. A variable‐temperature ¹H NMR spectroscopic study showed that the ligand of compound 7 is involved in a fluxional process at temperatures above ?93 °C, the mechanism of which has been satisfactorily modeled with the help of DFT calculations and involves the interconversion of the two enantiomers of this cluster through a conformational change of the ligand CH₂ group, which moves from one side of the plane of the heterocyclic ligand to the other, and a 180° rotation of the entire organic ligand over a face of the metal triangle.     

Stability and Nonlinear Optical Response of Alkalides that Contain a Completely Encapsulated Superalkali Cluster

Guided by density functional theory (DFT) computations, a new series of superalkali‐based alkalides, namely FLi₂⁺(aza222)K^?, OLi₃⁺(aza222)K^?, NLi₄⁺(aza222)K^?, and Li₃⁺(aza222)K^? were designed with various superalkali clusters embedded into an aza222 cage‐complexant. These species possess diverse isomeric structures in which the encapsulated superalkalis preserve their identities and behave as alkali metal atoms. The results show that these novel alkalides possess larger complexation energies and enhanced hyperpolarizabilities (β₀) compared with alkali‐metal‐based and previous superalkali‐based clusters. Especially, a prominent structural dependence of β₀ is observed for these studied compounds. Hence, the geometric factors that affect the nonlinear optical (NLO) response of such alkalides is elucidated in detail in this work. This study not only provides novel candidates for alkalides, it also offers an effective way to enhance the NLO response and stability of alkalides.     

Methane Activation Mediated by a Series of Cerium–Vanadium Bimetallic Oxide Cluster Cations: Tuning Reactivity by Doping

The reactions of cerium–vanadium cluster cations Ce_xV_yO_z⁺ with CH₄ are investigated by time‐of‐flight mass spectrometry and density functional theory calculations. (CeO₂)_m(V₂O₅)_n⁺ clusters (m=1,2, n=1–5; m=3, n=1–4) with dimensions up to nanosize can abstract one hydrogen atom from CH₄. The theoretical study indicates that there are two types of active species in (CeO₂)_m(V₂O₅)_n⁺, V[(O_t)₂]^. and [(O_b)₂CeO_t]^. (O_t and O_b represent terminal and bridging oxygen atoms, respectively); the former is less reactive than the latter. The experimentally observed size‐dependent reactivities can be rationalized by considering the different active species and mechanisms. Interestingly, the reactivity of the (CeO₂)_m(V₂O₅)_n⁺ clusters falls between those of (CeO₂)_2–4⁺ and (V₂O₅)_1–5⁺ in terms of C?H bond activation, thus the nature of the active species and the cluster reactivity can be effectively tuned by doping.     

The Role of Solvent on the Mechanism of Proton Transfer to Hydride Complexes: The Case of the [W3PdS4H3(dmpe)3(CO)]+ Cubane Cluster

The kinetics of reaction of the [W₃PdS₄H₃(dmpe)₃(CO)]⁺ hydride cluster ( 1 ⁺) with HCl has been measured in dichloromethane, and a second‐order dependence with respect to the acid is found for the initial step. In the presence of added BF₄^? the second‐order dependence is maintained, but there is a deceleration that becomes more evident as the acid concentration increases. DFT calculations indicate that these results can be rationalized on the basis of the mechanism previously proposed for the same reaction of the closely related [W₃S₄H₃(dmpe)₃]⁺ cluster, which involves parallel first‐ and second‐order pathways in which the coordinated hydride interacts with one and two acid molecules, and ion pairing to BF₄^? hinders formation of dihydrogen bonded adducts able to evolve to the products of proton transfer. Additional DFT calculations are reported to understand the behavior of the cluster in neat acetonitrile and acetonitrile–water mixtures. The interaction of the HCl molecule with CH₃CN is stronger than the W? H???HCl dihydrogen bond and so the reaction pathways operating in dichloromethane become inefficient, in agreement with the lack of reaction between 1 ⁺ and HCl in neat acetonitrile. However, the attacking species in acetonitrile–water mixtures is the solvated proton, and DFT calculations indicate that the reaction can then go through pathways involving solvent attack to the W centers, while still maintaining the coordinated hydride, which is made possible by the capability of the cluster to undergo structural changes in its core.     

A New Quantum Chemical Approach to the Magnetic Properties of Oligonuclear Transition‐Metal Complexes: Application to a Model for the Tetranuclear Manganese Cluster of Photosystem II

Broken‐symmetry DFT calculations on transition‐metal clusters with more than two centers allow the hyperfine coupling constants to be extracted. Application of the proposed theoretical scheme to a tetranuclear manganese complex that models the S₂ state of the oxygen‐evolving complex of photosystem II yields hyperfine parameters that can be directly compared with experimental data. The picture shows the metal–oxo core of the model and the following parameters; exchange coupling constant J_ij, the expectation value of the site‐spin operator , and the isotropic hyperfine coupling parameters.

     

Inception of Acetic Acid/Water Cluster Growth in Molecular Beams

The influence of carboxylic acids on water nucleation in the gas phase has been explored in the supersonic expansion of water vapour mixed with acetic acid (AcA) at various concentrations. The sodium‐doping method has been used to detect clusters produced in supersonic expansions by using UV photoionisation. The mass spectra obtained at lower acid concentrations show well‐detected Na⁺?AcA(H₂O)_n and Na⁺?AcA₂(H₂O)_n clusters up to 200 Da and, in the best cooling expansions, emerging Na⁺?AcA_m(H₂O)_n signals at higher masses and unresolved signals that extend beyond m/e values >1000 Da. These signals, which increase with increasing acid content in water vapour, are an indication that the cluster growth taking place arises from mixed water–acid clusters. Theoretical calculations show that small acid–water clusters are stable and their formation is even thermodynamically favoured with respect to pure water clusters, especially at lower temperatures. These findings suggest that acetic acid may play a significant role as a pre‐nucleation embryo in the formation of aerosols in wet environments.     

Synthesis and characterization of new fluorescent styrene-containing carborane derivatives: the singular quenching role of a phenyl substituent

A set of neutral and anionic carborane derivatives in which the styrenyl fragment is introduced as a fluorophore group has been successfully synthesized and characterized. The reaction of the monolithium salts of 1‐Ph‐1,2‐C₂B₁₀H₁₁, 1‐Me‐1,2‐C₂B₁₀H₁₁ and 1,2‐C₂B₁₀H₁₂ with one equivalent of 4‐vinylbenzyl chloride leads to the formation of compounds 1 – 3 , whereas the reaction of the dilithium salt of 1,2‐C₂B₁₀H₁₂ with two equivalents of 4‐vinylbenzyl chloride gives disubstituted compound 4 . The closo clusters were degraded using the classical method, KOH in EtOH, to afford the corresponding nido species, which were isolated as tetramethylammonium salts. The crystal structure of the four closo compounds 1 – 4 were analyzed by X‐ray diffraction. All compounds, except 1 , display emission properties, with quantum yields dependent on the nature of the cluster (closo or nido) and the substituent on the second C_cluster atom. In general, closo compounds 2 – 4 exhibit high fluorescence emission, whereas the presence of a nido cluster produces a decrease of the emission intensity. The presence of a phenyl group bonded to the C_cluster results in an excellent electron‐acceptor unit that produces a quenching of the fluorescence. DFT calculations have confirmed the charge‐separation state in 1 to explain the quenching of the fluorescence and the key role of the carboranyl fragment in this luminescent process.     

Unprecedented Solvent‐Assisted Reactivity of Hydrido W3CuS4 Cubane Clusters: The Non‐Innocent Behaviour of the Cluster‐Core Unit

Opening the cluster core : Substitution of the chloride ligand in the novel cationic cluster [W₃CuS₄H₃Cl(dmpe)₃]⁺ (see figure; dmpe=1,2‐bis(dimethylphosphino)ethane) by acetonitrile is promoted by water addition. Kinetic and density functional theory studies lead to a mechanistic proposal in which acetonitrile or water attack causes the opening of the cluster core with dissociation of one of the Cu? S bonds to accommodate the entering ligand.

     

Amphiphilic and magnetic properties of a new class of cluster-bearing [L2Cu4(mu4-O)(mu2-carboxylato)4] soft materials

A general approach toward amphiphilic systems bearing multimetallic clusters and their ability to form Langmuir-Blodgett films is presented. The synthetic strategy to stabilize these clusters involves the use of a ligand (HL) containing an N(2)O-donor set and long octadecanoic chains to obtain the carboxylate-supported [L(2)Cu(4)(mu(4)-O)(mu(2)-OAc)(4)]EtOH (1) and [L(2)Cu(4)(mu(4)-O)(mu(2)-OBz)(4)] (2) in which OAc(-) and OBz(-) represent acetate (1) and benzoate (2) co-ligands. These species were thoroughly characterized and had their structures solved by X-ray crystallography. We observed that the mu-oxo Cu(4) cluster is antiferromagnetically coupled and used broken-symmetry density functional theory (DFT) calculations to describe the main superexchange pathways of the tetracopper core. We also describe the amphiphilic properties of the ligand and the cluster-containing systems by means of area versus pressure isotherms and show that these cluster-bearing species can be transferred onto solid substrates yielding homogeneous Langmuir-Blodgett films, as characterized by atomic force microscopy and contact angle measurements.     

Expansion of Magnetic Aromaticity Criteria to Multilayer Structures: Magnetic Response and Spherical Aromaticity of Matryoshka-Like Cluster [Sn@Cu12@Sn20]12−

Structural characterization of the discrete [Sn@Cu₁₂@Sn₂₀]¹²⁻ cluster exposed a fascinating architecture composed of three concentric structural layers in which an endohedral Sn atom is enclosed in a Cu₁₂ icosahedron, which in turn is embedded in an Sn₂₀ dodecahedron. Herein, the possibility of sustaining aromatic behavior for this prototypical multilayered species was evaluated, in order to extend this concept to more complex clusters on the basis of magnetic response and bonding analysis by the AdNDP approach. This revealed characteristic features of spherical aromatics, given by the ability to sustain the shielding cone property, similar to archetypal aromatics. The favorable bonding pattern in the [Sn@Cu₁₂@Sn₂₀]¹²⁻ cluster fulfills the 2(N+1)² Hirsch rule for aromaticity; thus, the cluster could be regarded as a first member of aromatic multilayered structures. The set of four 13c–2e aromatic bonds that was identified in the internal SnCu₁₂ structure results in spherical aromatic character of this multilayered cluster. This insight builds a bridge between the traditional concept of Hückel's aromaticity and the aromaticity of complex and stable 3D systems that may be explored on the basis of magnetic response and bonding analysis. It also may open a way to novel findings in bottled clusters displaying aromatic behavior in multilayer structures, which are of great interest for inorganic nano- and material sciences due to their unprecedented stability.     

Experimental and First‐Principles Characterization of Functionalized Magnetic Nanoparticles

Magnetic iron oxide nanoparticles synthesized by coprecipitation and thermal decomposition yield largely monodisperse size distributions. The diameters of the coprecipitated particles measured by X‐ray diffraction and transmission electron microscopy are between approximately 9 and 15 nm, whereas the diameters of thermally decomposed particles are in the range of 8 to 10 nm. Coprecipitated particles are indexed as magnetite‐rich and thermally decomposed particles as maghemite‐rich; however, both methods produce a mixture of magnetite and maghemite. Fourier transform IR spectra reveal that the nanoparticles are coated with at least two layers of oleic acid (OA) surfactant. The inner layer is postulated to be chemically adsorbed on the nanoparticle surface whereas the rest of the OA is physically adsorbed, as indicated by carboxyl O? H stretching modes above 3400 cm^?1. Differential thermal analysis (DTA) results indicate a double‐stepped weight loss process, the lower‐temperature step of which is assigned to condensation due to physically adsorbed or low‐energy bonded OA moieties. Density functional calculations of Fe–O clusters, the inverse spinel cell, and isolated OA, as well as OA in bidentate linkage with ferrous and ferric atoms, suggest that the higher‐temperature DTA stage could be further broken down into two regions: one in which condensation is due ferrous/ferrous– and/or ferrous/ferric–OA and the other due to condensation from ferrous/ferric– and ferric/ferric–OA complexes. The latter appear to form bonds with the OA carbonyl group of energy up to fivefold that of the bond formed by the ferrous/ferrous pairs. Molecular orbital populations indicate that such increased stability of the ferric/ferric pair is due to the contribution of the low‐lying Fe³⁺ t_2g states into four bonding orbitals between ?0.623 and ?0.410 a.u.     

Density Functional Investigations of Electronic Structure and Dehydrogenation Reactions of Al‐ and Si‐Substituted Magnesium Hydride

The effect on the hydrogen storage attributes of magnesium hydride (MgH₂) of the substitution of Mg by varying fractions of Al and Si is investigated by an ab initio plane‐wave pseuodopotential method based on density functional theory. Three supercells, namely, 2×2×2, 3×1×1 and 5×1×1 are used for generating configurations with varying amounts (fractions x=0.0625, 0.1, and 0.167) of impurities. The analyses of band structure and density of states (DOS) show that, when a Mg atom is replaced by Al, the band gap vanishes as the extra electron occupies the conduction band minimum. In the case of Si‐substitution, additional states are generated within the band gap of pure MgH₂—significantly reducing the gap in the process. The reduced band gaps cause the Mg? H bond to become more susceptible to dissociation. For all the fractions, the calculated reaction energies for the stepwise removal of H₂ molecules from Al‐ and Si‐substituted MgH₂ are much lower than for H₂ removal from pure MgH₂. The reduced stability is also reflected in the comparatively smaller heats of formation (ΔH_f) of the substituted MgH₂ systems. Si causes greater destabilization of MgH₂ than Al for each x. For fractions x=0.167 of Al, x=0.1, 0.167 of Si (FCC) and x=0.0625, 0.1 of Si (diamond), ΔH_f is much less than that of MgH₂ substituted by a fraction x=0.2 of Ti (Y. Song, Z. X. Guo, R. Yang, Mat. Sc. & Eng. A 2004 , 365, 73). Hence, we suggest the use of Al or Si instead of Ti as an agent for decreasing the dehydrogenation reaction and energy, consequently, the dehydrogenation temperature of MgH₂, thereby improving its potential as a hydrogen storage material.     

Thermal Conversion of Methane to Formaldehyde Promoted by Gold in AuNbO3+ Cluster Cations

In addition to generation of a methyl radical, formation of a formaldehyde molecule was observed in the thermal reaction of methane with AuNbO₃⁺ heteronuclear oxide cluster cations. The clusters were prepared by laser ablation and mass‐selected to react with CH₄ in an ion‐trap reactor under thermal collision conditions. The reaction was studied by mass spectrometry and DFT calculations. The latter indicated that the gold atom promotes formaldehyde formation through transformation of an Au?O bond into an Au?Nb bond during the reaction.