Framework Rearrangement

The 18-electron rule generally governs the bonding in clusters with up to five metal atoms. The cluster core shape, i.e., the number of metal-metal bonds, can be deduced from the number of electrons provided by the metal 

NMR all three osmium atoms are chemically equivalent (200). This may involve changing the bridging sulfur atom from a four-electron donor to a two-electron donor in order to gain a third Os-Os bond and an equilateral metal triangle. These ligand induced cluster rearrangements are a further demonstration of metal-metal bond weakness as the reason for cluster mobility. For application-oriented purposes they may be the most important ones. 

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However, only one hydrido ligand resonance is observed in the 1H NMR spectrum of each cluster at temperatures low enough to prevent the metal framework rearrangement process from operating, although for the... 

The cyclic processes coupled with the framework rearrangement lead to total exchange of all the carbonyl ligands of the cluster in this final fluxional process. 

The open frameworks of zeolites are slightly less stable than the corresponding condensed structures [15,16] into which they will transform during severe thermal treatment. Nevertheless, the difference in energy between a-quartz, the most stable polymorph of silica, and siliceous faujasite, one of the most open and least stable, is only about 15 kj mol k The extensive occurrence of aluminosilicate zeolites and their widespread utility in industry therefore depend heavily upon both the strengths of their T-O bonds (e.g. Si-O 466 kJ mol ), which render them stable with respect to framework rearrangement. The challenge with many of the newer materials is that their stability with respect to transformation into alternative condensed structures is considerably lower and they frequently collapse on dehydration or other means of activation. It is for this reason that only a small subset of the many open-framework families of materials can be rendered truly nanoporous,... 

A novel metal framework rearrangement occurs for the Hg-bridged Fe—Rh carbonyl cluster In solution the Hg atom migrates around the Fe2Rh... 

Although very many carboxylate and amine-based hybrid frameworks have been prepared, solid state NMR has not been used as a characterisation tool in the same way as it has for inorganic zeolitic frameworks. This is probably because they are commonly prepared as crystals suitable for single crystal diffraction, and often with paramagnetic metals. It is likely, however, that NMR will reveal important structural details as these materials are studied more carefully, particularly in monitoring interactions and framework rearrangements upon adsorption of molecules.Studies of framework and sorbate motion by deuterium NMR are also highly relevant here (Chapter 7). 

A lot of flexible MOFs have been synthesized and characterized by gas adsorption, and some of them showed unique selective adsorptions of CO2 over other gases. The mechanisms of selective adsorption in flexible MOFs are more complicated than that in rigid MOFs. In some cases, the gas sorption isotherms show hyster-etic behaviors due to framework rearrangements during adsorption/desorption processes. Besides size/shape exclusion and adsorbate-surface interactions, structural rearrangement must thus also be taken into account in these cases. 

Reaction of Osg(CO)ig with (SPPh2)2NH at low temperature results in a metal framework rearrangement to give the spiked-bridged-tetrahedral hydrido complex 39 ... 

Lipscomb, W. N. (1966). Framework rearrangement in boranes and carboranes. Science, 253(3734),... 

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