Clusters stability

The definitions of the cluster ligands of the [3Fe-4S] cluster and of the related structural features are quite useful to predict cluster types in other Fds of known sequence, as well as to determine the nature of the cluster coordinating atoms (and variability) and their control on the type and performances of the metal sites, in particular in terms of cluster stability, cluster interconversion capability, and acceptance of other metals at the cluster. 

The former structure contains an intramolecular H-bond within the host, which stabilizes its planar conformation, and an N-H. ..N link to the guest species. The other structure with acetic acid contains hydrogen-bond stabilized clusters of two hosts and two guests around the crystallographic inversion centers. A distortion of... 

The ligand-stabilized cluster [Ru3(CO)7(y 3, rf x/ -4,6,8-lrimelhylazulene)] reacts with PhMe2Si-H to yield a new cluster containing a partially hydrogenated azu-lene ligand Ru3(CO)7(//3, if //5-4,5-dihydro-4,6,8-lrimethylazulene). Both of these complexes are efficient catalyst precursors for the hydrosilylation of aceto-... 

While the structure of clusters responds to directionally specific electrostatic interactions, their stabilization energy reflects also the intervention of less specific dispersion interactions. This is, e.g., the case for stacked DNA base pairs. Stability of these pairs stems from dispersion energy while their structure is determined by dipole-dipole electrostatic interactions. Dispersion energy plays an important role in stabilizing clusters of biomacromolecules, where it may be the dominant attractive term. 

One may obtain a stabilized cluster having different terminal groups. 

Stabilized cluster formation of supercritical Xe in carbon nanopores... 

Stabilized Cluster Formation of Supercritical Xe in Carbon Nanopores M. Aoshima, T. Suzuki and K. Kaneko... 

Based on synthetic methods developed for cysteine stabilized clusters, histidine encapsulated An nanoclusters were prepared by the formation of a histidine/Au (111) precursor complex with subsequent reduction by NaBELi. UV analysis confirmed the synthesis by the presence of a plasmon resonance peak for Au° at 518 mn. Further confirmation by TEM showed particles with an average diameter of 3.8 1.0 nm with a good deal of crystallinity as determined by XRD.i... 

Further indications that capping ligands stabilize clusters toward redox reactions is found with [Co3(/i,3-E)(/i.3-E )Cp3] (76), which adopt a structure similar to 66. Two reversible one-electron oxidations and one reduction are found for 76 (E = E = S), and the monocation and monoanion of 76 (E = S, E = CS) 149) were detected by cyclic voltammetry. 

Formic acid adds to the lightly stabilized cluster, Os3(CO)[g(h -CjH, )2 (C3H... 

Some reactions such as alkene isomerization, alkene hydrogenation, and H2 -I- D2 exchange can be used as sensitive chemical probes of the coordination environment of metal atoms associated with surface-bound metal clusters. Other catalytic reactions such as CO -I- H2 and alkane hydroge-nolysis, which are sensitive to metal ensemble sizes, are applied as a further structural probe. Several attempts have been made to stabilize cluster frameworks in such a way that catalytic activity is maintained. One of the more promising approaches involves the introduction of a capping group into the... 

Ligand-stabilized clusters could be suited to solve several problems in catalysis simultaneously by virtue of their uniform size, their solubility, their use on different supports, and the control of their selectivity by varying their ligand shells. 

Summarizing these experiments, the use of different ligand molecules, covering identical metal clusters, can lead to remarkable differences in activity and selectivity. The fact that ligand-stabilized clusters can be synthesized, modified, and investigated in advance of their immobilization on supports distinguishes these catalysts in a decisive manner from established metal catalysts. 

Support materials most adapted for stabilizing clusters on surfaces are oxide materials. They reveal a relatively large band gap and thus the characteristic, discrete electronic levels of the clusters are maintained to a certain extent. Furthermore, the interaction of the clusters with oxide surfaces, especially with their defects may be substantial and trapping of the clusters is feasible. Oxide supported metal particles are also relevant in industrial catalysis. 

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