Divalent-metal Clusters Research Articles

Imine bond-directed assembly of polyoxometalate-based metal-organic frameworks.

Polyoxometalate-based metal-organic frameworks (POMOFs) are highly effective heterogeneous catalysts that combine the catalytic activity of polyoxometalates (POMs) with the high surface area, tunable porosity, and structural diversity of MOFs. Nevertheless, there is still a lack of a general method to integrate POMs with various transition metal-based building units into POMOFs under mild conditions. In this work, we employed imine bonds to link amino-functionalized Anderson-type POMs with aldehyde-terminated divalent metal clusters, resulting in a series of isostructural POMOFs, M(II)-POMOFs (M = Zn, Co, Mg, or Mn). Furthermore, we used post-synthetic metal exchange and oxidation to transform Zn-POMOF into Fe(III)-POMOF with strong Lewis acidic Fe3+ sites. Notably, both the synthesis and post-synthetic modifications were performed under mild conditions (room temperature, acid-free), preventing the decomposition of the POMs. Compared to M(II)-POMOFs or MOFs without POMs, the combination of Lewis acidic Fe3+ and POMs enhanced its catalytic activity for CO2 cycloaddition with epoxides, enabling efficient synthesis of cyclic carbonates. This versatile synthetic method could broaden the scope of POMOFs, extending their applications in catalysis and beyond.

Probing structural and electronic properties of divalent metal Mg n+1 and SrMg n (n = 2–12) clusters and their anions

Divalent metal clusters have received great attention due to the interesting size-induced nonmetal-to-metal transition and fascinating properties dependent on cluster size, shape, and doping. In this work, the combination of the CALYPSO code and density functional theory (DFT) optimization is employed to explore the structural properties of neutral and anionic Mg n + 1 and SrMg n (n = 2–12) clusters. The results exhibit that as the atomic number of Mg increases, Sr atoms are more likely to replace Mg atoms located in the skeleton convex cap. By analyzing the binding energy, second-order energy difference and the charge transfer, it can be found the SrMg9 cluster with tower framework presents outstanding stability in a studied size range. Further, bonding characteristic analysis reveals that the stability of SrMg9 can be improved due to the strong s–p interaction among the atomic orbitals of Sr and Mg atoms.

The Zn- and Cd-Clusters of Recombinant Mammalian MT1 and MT4 Metallothionein Domains Include Sulfide Ligands

Recombinant (E. coli ) synthesis of mammalian MT1 and MT4 domains as separate peptides in Zn(II) and Cd(II) enriched growth media has rendered metal complexes containing sulfide anions as additional ligands. The Cd preparations show higher sulfide content than the Zn preparations. Also, the betaMT1 and betaMT4 fragments exhibit higher sulfide/peptide ratios than the respective alpha fragments. Titration of Zn3-betaMT1 with Cd(II) followed by addition of several sodium sulfide equivalents shows that the Cd(II)-betaMT1 species can incorporate sulfide ligands in vitro, with a concomitant evolution of their UV-vis and CD fingerprints to those characteristic of the Cd-S2- chromophores. Current results have also provided full understanding of previous data collected by this group in the characterization of the Cd-betaMT1 preparations obtained from large-scale fermentor synthesis by allowing identification of at least 2S2- ligands per Cd-betaMT1 species. Furthermore, the results here presented have revealed that synthesis of betaMT4 in Cd-supplemented cultures yielded Cd,S(2-)-containing clusters instead of the proposed heterometallic Zn,Cd-betaMT4 complexes. Finally, a global evaluation of our results suggests that the higher the Cu-thionein character of a MT peptide, the higher is its tendency to harbor nonproteic ligands (i.e., sulfide anions) when building divalent metal clusters, especially Cd-MT complexes.

Cetirizine dihydrochloride interaction with some diclofenac complexes

IR, 1H NMR and mass spectrometric studies showed that cetirizine dihydrochloride interacted strongly with diclofenac sodium, even when the latter was metal bound, forming high molecular weight stable adducts. These new formations were unaffected by the possible steric constraints that may exist because of coordination yet did not have the power to break the formed coordinate bonds. The formed ionic bond took place between the carbonyl ion of diclofenac and the positively charged piperazine ring of cetirizine, forming a ternary compound in the case of the divalent metal clusters (Ca{(dic) 2·2H 2O}, Mg{(dic) 2·2H 2O}, Zn{(dic) 2·2H 2O}) and a quaternary one with the trivalent iron cluster (Fe{dic} 3·3H 2O). IR bands assigned to νNH, δNH and νC N were shifted to lower frequency values in the spectra of the complexes; thus showing that coordination took place at the NH of the diphenylamine. TG and elemental analysis confirmed these results.

Shell structure of magnesium and other divalent metal clusters

Clusters of the divalent metals magnesium, cadmium, and zinc have been grown in ultracold helium nanodroplets and studied by high-resolution mass spectrometry, with a special emphasis on magnesium. The mass spectra of all materials show similar characteristic features independent of the chosen ionization technique - i.e., electron impact ionization as well as nanosecond and femtosecond multiphoton excitation. In the lower-size range the abundance distributions can be explained by an electronic shell structure. The associated electron delocalization - i.e., metallic bonding - is found to set in at about N=20 atoms. For Mg{sub N} we have resolved crossings of electronic levels at the highest-occupied molecular orbital which result in additional magic numbers compared to the alkali metals: e.g., Mg{sub 40} with 80 electrons. This specific electronic shell structure is also present in the intensity pattern of doubly charged Mg{sub N}. For larger clusters (N{>=}92) a coexistence of electronic shell effects and geometrical packing is observed and a clear signature of icosahedral structure is present beyond N{>=}147.

When are thin films of metals metallic? Part III

A large amount of experimental information has indicated that very thin films of metallic elements can exhibit nonmetallic behavior, even on metal substrates. These films undergo a gradual nonmetal to metal transition with increasing film density or thickness. The nonmetallic behavior can be related to electron localization due to strong electron-electron correlation in low dimensional systems, as indicated by the strong enhancement of electron effective mass. The evolution in the electronic structure associated with the nonmetal to metal transition bears a striking resemblance to the behavior observed for free metal clusters. Part I [1], outlined the general concepts of a nonmetal to metal transition in a thin film, while part II [2] critically examined three examples of nonmetal to metal transitions for thin film overlayers. This paper will discuss some surprising correlations between the electronic structure of divalent thin films on metal substrates and divalent metal clusters that have been reported in the literature.

Slave-boson approach to the size-dependent transition from van der Waals to covalent bonding in Hg n clusters

We use a microscopic theory to describe the dynamics of the valence electrons in divalent-metal clusters. The theory is based on a many-body model Hamiltonian H which takes into account, on the same electronic level, the van der Waals and the covalent bonding. In order to study the ground-state properties of H we have developed an extended slave-boson method. We have studied the bonding character and the degree of electronic delocalization in Hg n clusters as a function of cluster size. Results show that, for increasing cluster size, an abrupt change occurs in the bond character from van der Waals to covalent bonding at a critical cluster size n c ∼ 10–20. This change also involves a transition from localized to delocalized valence electrons, as a consequence of the competition between both bonding mechanisms.

Relativistic effects in mercury: Atom, clusters, and bulk.

We study the effects of relativity on the electronic structure of Hg as it evolves from an atom to a cluster, and then to the bulk. A comparison of nonrelativistic and scalar-relativistic results for bulk Hg, obtained self-consistently, clearly shows the importance of including the relativistic terms as well as treating the 5[ital d] electrons as valencelike. Our cluster calculations show that the relativistic effects are equally important for clusters, and that the differences in many electronic properties such as the size dependence of the gap energy and the [ital l] character of Hg clusters with respect to other divalent-metal clusters, for example, Be and Mg, arise due to the relativistic terms, thus providing further insight into the electronic origins of these properties.

Detection of electronic-shell structure in divalent-metal clusters (Hg)n.

The electronic-shell structure in divalent-metal clusters (Hg${)}_{\mathit{n}}$ was observed from the size distribution of mercury-cesium clusters, (Hg${)}_{\mathit{n}}$${\mathrm{Cs}}^{+}$. The cluster ions (Hg${)}_{\mathit{n}}$${\mathrm{Cs}}^{+}$ were produced by ${\mathrm{Cs}}^{+}$ ion bombardment of an amalgam of mercury and silver, and the mass analysis was performed using a grand-scale sector mass spectrometer. For the cluster size n>40, the spectral behavior exhibited consecutive hills and valleys, which was interpreted in terms of electronic-shell structure of pure mercury clusters (Hg${)}_{\mathit{n}}$. The evidence for electronic-shell structure of clusters containing up to 1500 valence electrons was clearly shown. The presumptive evidence for the phase shift of magic numbers theoretically expected from a quantum beating effect were found at the region of the cluster size around n=500.

Metallic character of small metal aggregates as a function of their atomic structure

The electronic properties of small clusters are investigated by various techniques as a function of their atomic structure. In alkali clusters, most electronic properties may be understood in terms of a very simple electronic shell model. In divalent metal clusters, a transition occurs as a function of the size from the Van der Waals structure to the metallic one. This transition has been characterized in mercury. In trivalent metal clusters, a different behaviour has been found for aluminium and indium clusters. This difference could be due to a solid-like structure for aluminium and a liquid-like structure for indium in our experimental conditions.

Calculation of the electronic properties of neutral and ionized divalent-metal clusters

The electronic properties of neutral and ionized divalent-metal clusters have been studied using a microscopic theory, which takes into account the interplay between van der Waals (vdW) and covalent bonding in the neutral clus­ ters, and the competition between hole delocalization and p0­ larization energy in the ionized clusters. By calculating the ground-state energies of neutral and ionized. Hgn clusters, we determine the size dependence of the bond character and the ionization potential Ip(n). For neutral Hgn clusters we ob­ tain a transition from van del Waals to covalent behaviour at the critical size n c ~ 10-20 atoms. Results for Ip(Hgn ) with n ~ 20 are in good agreement with experiments, and suggest that small Hg~ clusters can be viewed as consisting of a positive trimer core Hgt surrounded by n - 3 polarized neutral atoms.

Theory for the change of the bond character in divalent-metal clusters

To determine the size dependence of the bonding in divalent-metal clusters we use a many-electron Hamiltonian describing the interplay between van der Waals (vdW) and covalent interactions. Using a saddle-point slave-boson method and taking into account the size-dependent screening of charge fluctuations, we obtain for ${\mathrm{Hg}}_{\mathit{n}}$ a sharp transition with vdW to covalent bonding for increasing n. We show also, by solving the model Hamiltonian exactly, that for divalent metals vdW and covalent bonding coexist already in the dimers.

On the transition from localized to delocalized electronic states in divalent-metal clusters

The transition from van der Waals to covalent bonding, which is expected to occur in divalent-metal clusters with increasing cluster size, is discussed. We propose a model which takes into account, within the same electronic theory, the three main competing contributions, namely the kinetic energy of the electrons, the Coulomb interactions between electrons, and thes ↔p intraatomic transitions responsible for van der Waals like bonding. The model is solved by taking into account electron correlations using a generalized Gutzwiller approximation (slave boson method). The occurrence of electron localization is studied as a function of the interaction parameters and cluster size.

Electronic properties of divalent-metal clusters

The transition from van der Waals to metallic bonding expected to occur in divalent-metal clusters (e.g., Be n , Mg n , Hg n ) as a function of cluster size is discussed. Theoretical results for several electronic properties reflecting this transition in Hg n -clusters are briefly reviewed and compared with available experiments. The limitations of the present theory particularly concerning the role of correlations and van der Waals interactions are discussed and possible improvements are suggested.

Spectroscopy and photoionization spectra of small metal clusters

The properties of small clusters can be investigated by various methods. For very small clusters having 3 or 4 atoms, the spectroscopic techniques are suitable and results are presented on Na3 for which the dynamical Jahn-Teller effect, the ground state structure and predissociated states have been observed. For larger clusters, various other methods can be used and some of them are detailed in the paper. General trends in the cluster properties are given for selected examples of atomic clusters studied in the literature. Rare gas clusters are dominated by the van der Waals bonding. Alkaline clusters are well interpreted by a very simple shell model. In divalent metal clusters, a transition from a van der Waals to metallic structure is expected and has been observed in mercury clusters. Finally, the more complicated case of semi-metal clusters is also discussed. Results are presented on antimony and bismuth clusters.