Chemical reactivity clusters

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

Figure Cl. 1.3 shows a plot of tire chemical reactivity of small Fe, Co and Ni clusters witli FI2 as a function of size (full curves) [53]. The reactivity changes by several orders of magnitudes simply by changing tire cluster size by one atom. Botli geometrical and electronic arguments have been put fortli to explain such reactivity changes. It is found tliat tire reactivity correlates witli tire difference between tire ionization potential (IP) and tire electron affinity...

Further dramatic changes can happen in chemical reactivities of gold clusters with diameters smaller than 2nm. The contribution of support materials including metal oxides, carbons, and polymers to the genesis of unique catalytic properties may be much greater than in the case of NPs and therefore may provide an expanding new field of research. 

Figure 9.7 Temperature-programmed reaction (TPR) spectra for CO oxidation at a series of model catalysts prepared by the soft landing of mass-selected Aun and AunSr cluster ions on MgO(lOO) thin films which are vacancy free (typically 1 % of a monolayer), (a) MgO (b) Au3Sr (c) Au4 (d) Au8. Also shown is the chemical reactivity R of pure Aun and AunSr clusters with 1 < n < 9. (Reproduced from Ref. 21).

The chemical reactivity of cobalt cluster anions, Co (n = 2-8), toward 02, N2, and CO have been investigated using a flow tube reactor (226). The reactivity was found to be in the order 02 > CO > N2 the least reactive ligand N2 only reacted with C07 and Cog. The primary reaction of oxygen was the removal of one or two cobalt atoms from the cluster. Carbon monoxide reacts by multiple additions giving saturation limits shown in Table V. 

There is a marked difference in chemical reactivity between bridging and terminal hydrogens. Terminally bonded hydrogens readily react in a similar manner to that observed for mononuclear hydrides. Thus reactions with chlorinated hydrocarbons such as carbon tetrachloride yield the chloro cluster complexes and chloroform. In contrast, bridging hydrides are stable and may be studied in chlorinated sol-... 

Aromaticity remains a concept of central importance in chemistry. It is very useful to rationalize important aspects of many chemical compounds such as the structure, stability, spectroscopy, magnetic properties, and last but not the least, their chemical reactivity. In this chapter, we have discussed just a few examples in which the presence of chemical structures (reactants, intermediates, and products) and TSs with aromatic or antiaromatic properties along the reaction coordinate have a profound effect on the reaction. It is clear that many more exciting insights in this area, especially from the newly developed aromatic inorganic clusters, can be expected in the near future from both experimental and theoretical investigations. 

Despite its unsaturated nature, benzene with its sweet aroma, isolated by Michael Faraday in 1825 [1], demonstrates low chemical reactivity. This feature gave rise to the entire class of unsaturated organic substances called aromatic compounds. Thus, the aromaticity and low reactivity were connected from the very beginning. The aromaticity and reactivity in organic chemistry is thoroughly reviewed in the book by Matito et al. [2]. The concepts of aromaticity and antiaromaticity have been recendy extended into main group and transition metal clusters [3-10], The current chapter will discuss relationship among aromaticity, stability, and reactivity in clusters. 

Anderson and coworkers [59-66] produced boron cluster cations Bj-B in molecular beams using laser vaporization and studied their chemical reactivity and fragmentation properties. The structures of B3 —IBI3 cations have been established computationally (see review [7] for details) represented in Figure 29.1. In this chapter, we are discussing stability and reactivity of Bj — B 3 cations on the basis of their multifold aromaticity, multifold antiaromaticity, and conflicting aromaticity. 

The tetrahedral cluster compounds show an unprecedented chemical reactivity which led to the syntheses of a broad variety of fascinating products [59]. Some of these will be discussed in the Chapters 3.5. Only a short summary will be given here. 

Al, Ga, In and T1 differ sharply from boron. They have greater chemical reactivity at lower temperatures, well-defined cationic chemistry in aqueous solutions they do not form numerous volatile hydrides and cluster compounds as boron. Aluminium readily oxidizes in air, but bulk samples of the metal form a coherent protective oxide film preventing appreciable reaction aluminium dissolves in dilute mineral acids, but it is passivated by concentrated HN03. It reacts with aqueous NaOH, while gallium, indium and thallium dissolve in most acids. 

The application of computational algorithms for compound filtering and clustering is routinely used to eliminate undesired structures on the basis of chemically reactive fimctionalities, predicted liabilities (e.g., frequent hitters, hERG, cyp450 and so forth), or druglike properties. Computational methods are also used to group compoimds on... 

Polarographic and cycKc voltammetric studies showed the formation of dianions, with one of the dianionic complexes exhibiting further chemical reactivity. Bulk electrolysis and infrared spectroelectro-chemistry were used to confirm the decomposition of the dianionic tetranuclear clusters to stable tricobalt anions. 

It is very likely that the metal-insulator transition, the unusual catalytic properties, the unusual degree of chemical reactivity, and perhaps even some of the ultramagnetic properties of metal clusters are all linked intimately with the dynamic, vibronic processes inherent in these systems. Consequently, the combination of pump-probe spectroscopy on the femtosecond time scale with theoretical calculations of wavepacket propagation on just this scale offers a tantalizing way to address this class of problems [5]. Here we describe the application of these methods to several kinds of metal clusters with applications to some specific, typical systems first, to the simplest examples of unperturbed dimers then, to trimers, in which internal vibrational redistribution (IVR) starts to play a central role and finally, to larger clusters, where dissociative processes become dominant. 

In a detailed reference. Clrnni and Cooks (Purdue University) explain in considerable delail the combining of the latest advances in mass spectrometry with laser spectrometers. This technique is contributing in a major way to studies of chemical dynamics, cluster structures, and reactivity, and to the elucidation of the properties of highly excited molecules and ions. 

The subject of heteronuclear cluster compounds of the transition metals remains an active area of research interest, and was reviewed in the early 1980s by Geoffroy el al. (1,2). Clusters with novel architectures, exemplified by the star clusters of Stone and co-workers (5), continue to be synthesized. Whereas there is undoubtedly strong academic interest in the structure, bonding, and chemical reactivity of heteronuclear clusters in their own right, additional impetus to this field is given by the important relationship between heteronuclear clusters and bimetallic alloy catalysts. This relationship was the subject of a published symposium (4). 

It is convenient to discuss the structures and chemical reactivity of hetero-Pt clusters under the individual metals, because there is little homogeneity of structural type, particularly when the details of ligand coordination are taken into account. There are remarkably few examples common to more than one... 

Numerous triosmium-platinum clusters have been prepared, many of them derived from Os3Pt(/i-H)2(CO)10(PR3) (1) (40,40a). Although, as discussed in Section III, the participation in cluster bonding of the tangential orbitals of nonconical PtL units may be difficult to predict a priori, the observed tetrahedral skeleton of 1 (40,40a) and nonplanar coordination geometry at the Pt atom suggest that the Pt atom should behave as an 18-electron center. The 58-CVE count therefore implies that the cluster is unsaturated, and this is consistent with EHMO studies (88,98) and the high chemical reactivity of 1. The crystal structures of la (40,40a) and the 58-CVE derivatives 21 (108) and 22 (118) all show unusually short Os(/ -H)Os distances [2.789(1),... 

---