A study of chemically prepared small metal clusters in the nonmetallic regime

Small gold clusters [mean diameter (d)[less, similar] 1.4 nm], unlike larger clusters, show a higher Au(4f) binding energy relative to the bulk value and the presence of a conductance gap in tunnelling measurements, just as the molecular cluster compound, Au55(PPh3)12Cl6; small platinum clusters show similar nonmetallic features.

Fascinating chemistry of metal clusters and carbon clusters (fullerenes)

Investigations of a variety of transition metal clusters by means of high-energy spectroscopies including BIS show the occurrence of a metal-insulator transition with decrease in the cluster size. The chemical reactivity of the clusters also varies significantly with the size. Among the many fascinating properties of the fullerenes C60 and C70, a noteworthy one is the interaction between metal clusters and fullerenes. Phase transitions of fullerenes involving orientational disorder and pressure-induced decrease in the band gap of C60 are other novel features of interest.

Dependence of the strength of interaction of carbon monoxide with transition metal clusters on the cluster size

The interaction of CO with Cu, Pd, and Ni at different coverages of the metals on solid substrates has been investigated by He II and core-level spectroscopies, after the nature of variation of the metal core-level binding energies with the coverage or the cluster size is established. The separation between the (1 pi + 5 sigma) and 4 sigma levels of CO increases with a decrease in the size of the metal clusters, accompanied by an increase in the desorption temperature. In the case of Cu, the intramolecular shakeup satellite of CO disappears on small clusters. More importantly, CO dissociates on small Ni clusters, clearly confirming that metal-CO interaction strength increases with a decrease in the cluster size.

Metal-insulator transitions in metal clusters: a high-energy spectroscopy study of palladium and silver clusters

Bremsstrahlung isochromat spectroscopy (BIS) along with ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) has been employed to investigate the electron states of Pd and Ag deposited on amorphous graphite at different coverages. The metal core level binding energies increase with decreasing cluster size while the UPS valence bands show a decrease in the 4d states at E(F) accompanied by a shift in the intensity maximum to higher binding energies. BIS measurements show the emergence of new states closer to E(F) with increase in the cluster size. It is pointed out that the observed spectral shifts cannot be accounted for by final-state effects alone and that initial-state effects have a significant role. It therefore appears that a decrease in cluster size is accompanied by a metal-insulator transition.

Transition metal clusters supported by multinucleating ligand frameworks as models of biological active sites

This dissertation describes efforts to model biological active sites with small molecule clusters. The approach used took advantage of a multinucleating ligand to control the structure and nuclearity of the product complexes, allowing the study of many different homo- and heterometallic clusters. Chapter 2 describes the synthesis of the multinucleating hexapyridyl trialkoxy ligand used throughout this thesis and the synthesis of trinuclear first row transition metal complexes supported by this framework, with an emphasis on tricopper systems as models of biological multicopper oxidases. The magnetic susceptibility of these complexes were studied, and a linear relation was found between the Cu-O(alkoxide)-Cu angles and the antiferromagnetic coupling between copper centers. The triiron(II) and trizinc(II) complexes of the ligand were also isolated and structurally characterized.

Chapter 3 describes the synthesis of a series of heterometallic tetranuclear manganese dioxido complexes with various incorporated apical redox-inactive metal cations (M = Na⁺, Ca²⁺, Sr²⁺, Zn²⁺, Y³⁺). Chapter 4 presents the synthesis of heterometallic trimanganese(IV) tetraoxido complexes structurally related to the CaMn₃ subsite of the oxygen-evolving complex (OEC) of Photosystem II. The reduction potentials of these complexes were studied, and it was found that each isostructural series displays a linear correlation between the reduction potentials and the Lewis acidities of the incorporated redox-inactive metals. The slopes of the plotted lines for both the dioxido and tetraoxido clusters are the same, suggesting a more general relationship between the electrochemical potentials of heterometallic manganese oxido clusters and their “spectator” cations. Additionally, these studies suggest that Ca²⁺ plays a role in modulating the redox potential of the OEC for water oxidation.

Chapter 5 presents studies of the effects of the redox-inactive metals on the reactivities of the heterometallic manganese complexes discussed in Chapters 3 and 4. Oxygen atom transfer from the clusters to phosphines is studied; although the reactivity is kinetically controlled in the tetraoxido clusters, the dioxido clusters with more Lewis acidic metal ions (Y³⁺ vs. Ca²⁺) appear to be more reactive. Investigations of hydrogen atom transfer and electron transfer rates are also discussed.

Appendix A describes the synthesis, and metallation reactions of a new dinucleating bis(N-heterocyclic carbene)ligand framework. Dicopper(I) and dicobalt(II) complexes of this ligand were prepared and structurally characterized. A dinickel(I) dichloride complex was synthesized, reduced, and found to activate carbon dioxide. Appendix B describes preliminary efforts to desymmetrize the manganese oxido clusters via functionalization of the basal multinucleating ligand used in the preceding sections of this dissertation. Finally, Appendix C presents some partially characterized side products and unexpected structures that were isolated throughout the course of these studies.

Construction of coordination networks with high connectivity: a new 8-connected self-penetrating network based on tetranuclear metal clusters

Two highly connected cobalt(II) and zinc(II) coordination polymers with tetranuclear metal clusters as the nodes of network have been prepared, being the first example of an 8-connected self-penetrating net based on a cross-linked alpha-Po subnet.

Effects of tip structure on the generation of metal clusters by an STM tip: a way to control the orientation of nanocrystallites?

The role of the crystalline orientation of the STM tip in the generation of metal clusters is studied by atom dynamics simulations. When a (111) facet is facing the surface, the process is accompanied by a perturbation of the surface stronger than that observed for more open tip structures. This implies a technological application: the possibility of orienting a nanocrystallite deposited on a tip according to the changes observed in the force on the tip.

On the generation of metal clusters with the electrochemical scanning tunneling microscope

In the present work we consider two aspects of the deposition of metal clusters on an electrode surface. The formation of such clusters with the tip of a scanning tunneling microscope is simulated by atom dynamics. Subsequently the stability of these clusters is investigated by Monte Carlo simulations in a grand-canonical ensemble. In particular, the following systems were considered explicitly: Pd clusters on Au(111), Cu on Au(111), Ag on Au(111), Pb on Au(111) and Cu on Ag(111). The analysis of the results obtained for the different systems leads to the conclusion that optimal systems for nanostructuring are those where the metals participating have similar cohesive energies and negative heats of alloy formation. In this respect, the system Cu-Pd(111) is predicted as a good candidate for the formation of stable clusters. (c) 2005 Elsevier B.V. All rights reserved.

The basis for the formation of stable metal clusters on an electrode surface

Metal nanoclusters can be produced cheaply and precisely in an electrochemical environment. Experimentally this method works in some systems, but not in others, and the unusual stability of the clusters has remained a mystery. We have simulated the deposition of the clusters using classical molecular dynamics and studied their stability by grand-canonical Monte Carlo simulations. We find that electrochemically stable clusters occur only in those cases where the two metals involved form stable alloys.

Deformed-jellium model for the fission of multiply charged metal clusters

A deformed-jellium model is used to calculate the fission barrier height of positive doubly charged sodium clusters within an extended Thomas-Fermi approximation. The fissioning cluster is continuously deformed from the parent configuration until it splits into two fragments. Although the shape of the fission barrier obviously depends on the parametrization of the fission path, we have found that remarkably, the maximum of the barrier corresponds to a configuration in which the emerging fragments are already formed and rather well apart. The implication of this finding in the calculation of critical numbers for fission is illustrated in the case of multiply charged Na clusters.

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 clusters, and the competition between hole delocalization and polarization energy in the ionized clusters. By calculating the ground-state energies of neutral and ionized. Hg_n clusters, we determine the size dependence of the bond character and the ionization potential I_p(n). For neutral Hg_n clusters we obtain a transition from van del Waals to covalent behaviour at the critical size n_c ~ 10-20 atoms. Results for I_p(Hg_n) with n \le 20 are in good agreement with experiments, and suggest that small Hg_n^+ clusters can be viewed as consisting of a positive trimer core Hg_3^+ surrounded by n - 3 polarized neutral atoms.

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 the s \gdw 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.

Theory for the change of 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 Hg_n a sharp transition from 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.