Nitrogen oxide, bridged

This chapter focuses on the chemistry ofbiomimetic copper nitrosyl complexes relevant to the NO-copper interactions in proteins that are central players in dissimilatory nitrogen oxide reduction (denitrification). The current state of knowledge of NO-copper interactions in nitrite reductase, a key denitrifying enzyme, is briefly surveyed the syntheses, structures, and reactivity of copper nitrosyl model complexes prepared to date are presented and the insight these model studies provide into the mechanisms of denitrification and the structures of other copper protein nitrosyl intermediates are discussed. Emphasis is placed on analysis of the geometric features, electronic structures, and biomimetic reactivity with NO or NOf of the only structurally characterized copper nitrosyls, a dicopper(II) complex bridged by NO and a mononuclear tris(pyrazolyl)hydroborate complex having a Cu(I)-NO formulation. 

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) ... 

Now, when the (Lewis acid) osmium(VIII) tetroxide (OSO4) is added to a solution containing either (DHQ)2-PHAL or (DHQD)2-PHAL, the metal oxide bridges the two basic nitrogens in the azabicy-clo[2.2.2]octyl systems so that only a narrow channel is available into which the unsymmetrically substituted alkene will fit. Thus, one face of the alkene is more readily oxidized than the other. 

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

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