Nitrogen oxide clusters

Keywords Valence electron rule, Metal ring, Metal cluster, AN +2 valence electron rule, 8/V +6 valence electron rule, 6N +14 valence electron rule, Pentagon stability, Cyclopentaphosphane, Hydronitrogen, Polynitrogen, Triazene, 2-Tetrazene, Tetrazadiene, Pentazole, Hexazine, Nitrogen Oxide, Disiloxane, Disilaoxirane, 1,3-Cyclodisiloxane, Metallacycle, Inorganic heterocycle... 

The native iron-sulfur clusters of many enzymes can be directly observed by EPR spectroscopy (Beinert, 1978). In several cases, appearance of the DNIC signal from nitrogen oxide-treated enzyme (Salerno et al., 1976 Dervartanian et al., 1973 Hyman et al., 1992) or cells (Reddy et al., 1983 Payne et al., 1990) results in the disappearance of these native signals, indicating conversion of the iron from the native cluster coordination to DNIC complexes. However, loss of EPR signal from native clusters does not necessarily correlate with appearance of the DNIC signal (Hyman et al., 1992). 

Few studies have also been reported with metal oxide clusters in mesoporous materials. MCM-41 modified by deposition of vanadium and titanium oxides exhibited catalytic activity for reduction of nitrogen oxides by NH3 [186]. 

No detailed mechanistic study of these reactions has been made, and there is little evidence to support the intermediacy of nitrosyl or dinitrogen oxide complexes. Since this report, much interest has been shown in the efficient synthesis of oxide-bridged clusters and dimers of IIIA-VIA metals using nitrogen oxides as the oxidizing agents. The following reactions provide t5q)ical examples (205 207) ... 

Catalytic removal of nitrogen oxides [1] and sulfur oxides [2] is one of the most important issues in environmental problems. We have investigated supported ruthenium carbido-cluster catalysts derived from [RUjC(CO),J crystal. The supported cluster was... 

The exceptional activity exhibited by ion-exchanged copper ZSM-5 zeolite catalysts for nitric oxide (NO) decomposition, and for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) in the presence of excess oxygen is well documented [1-10]. The nature of the active copper species in the SCR reaction however still remains uncertain. We and others have recognised that there are two different types of copper species within the ZSM-5 zeolite channels [11]. Isolated copper ions exist in low symmetry environments, and small clusters, where the copper atoms are linked by extra-lattice oxygen species such as [Cu(II)-0-Cu(n)] dimers, are also present. Recent studies have also suggested that the isolated copper ions in ZSM-5 occupy two types of sites [11], which may have different SCR reactivity. It is likely... 

Finally, as shown in Table V, activity of ferrochelatase is also decreased by -NO. This mitochondrial enzyme catalyzes the final step in heme synthesis, the insertion of ferrous iron into porphyrin, and has been shown to contain an Fc2S2 nonheme iron-sulfur cluster which is required for activity (Dailey et aL, 1994). This result suggests that loss of activity may occur by nitrogen oxide-mediated [i.e., -NO in the presence of dioxygen (Wink et aL, 1993a)] destruction of its iron-sulfur cluster. Consistent with this result, while inhibition of ALAS and increase in HO induced by SNAP requires a period of 4-8 hr (consistent with the established effects of heme on these enzymes, as described above), the inhibition of ferrochelatase is virtually complete within 1 hr (Kim et aL, 1995). 

The spaces between the natural layers can be enlarged to form pillared interlayered clays. This is carried out by ion exchanging the charge compensation cations with polynuclear metal ion hydro-complexes which are formed in hydrolysed solutions of polyvalent metal ions such as Al(III) or Zr(IV). The polynuclear cations dehydrate on calcination to create metal oxide clusters which act as pillars between the clay layers and create spaces of molecular dimensions. Example separations with pillared clays include the separation of oxygen and nitrogen, and the separation of isomers. 

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