Osmium complexes imido

There have also been significant advances in the imido chemistry of ruthenium and osmium. A variety of imido complexes in oxidation states +8 to +6 have been reported. Notably, osmium (VIII) imido complexes are active intermediates in osmium-catalyzed asymmetric aminohydroxyl-ations of alkenes. Ruthenium(VI) imido complexes with porphyrin ligands can effect stoichiometric and catalytic aziridination of alkenes. With chiral porphyrins, asymmetric aziridination of alkenes has also been achieved. Some of these imido species may also serve as models for biological processes. An imido species has been postulated as an intermediate in the nitrite reductase cycle. " ... 

No imido complexes of ruthenium(VII) have been reported and there are only two examples for osmium. 

The complex (PPh4)[Os NC(CCl3)NCCl(CCl3) Cl5] may be regarded as an imido complex of osmium(VI), made by the reaction of OS2CI10 with trichloroacetonitrile and has been characterized by X-ray structural studies. The Os—N distance of 1.97 A is slightly shorter than that expected for a single bond. 

The tetranuclear osmium(VI) oxo imido complex [(Bu N)20s(/u-NBu )20s(NBu )(/u-0)]2 (2) is obtained as a side product in the reaction of [OSO4] with neat NHBu (SiMe3) (Scheme 1). Its structure can be described as a dimer of dimers with Cjh symmetry. ... 

This is an interesting area of osmium chemistry which stands much in need of development. Apart from the two incompletely characterized NH complexes, most of the organoimido chemistry is that of osmium(VIII) species such as Os03(NR) will undergo oxyamination reactions with alkenic double bonds, and such reactions may be made catalytic. However, because of the high reactivity of osmium(VIII), only a small range of tertiary imido complexes have been prepared (R = Bu Am , 1-adamantyl, Oct1). 

The complexes will effect oxyamination reaction with alkenes in a stereospecific reaction (Scheme 8).290 After reductive cleavage of the intermediate alkanolaminato complex (I) (see below) vicinal amino alcohols (II) are formed. The reaction is unusual in that the new C—N bond is always formed at the least substituted terminal alkenic carbon atom, and there is a clear preference for the imido complex to use its NR group for coordination to the osmium despite the steric restraints of R.299,300 However, the least sterically hindered part of the alkene moiety is attached to. the nitrogen atom.290,291,300 The yields of amino alcohols in the reaction can be improved by addition of tertiary alkyl bulkhead amines (see below). 

The synthetic procedure is very critical. In our case, we believe that the imido-lithium compound (Li2NR) is present in the solution of butyl lithium and para-toluidine, in diethyl ether, as reported for the dilithiated a-naphtylamine. This is a noticeable difference in comparison to typical preparations of ruthenium, osmium, and iridium imido complexes, " in which a dichlorometal complex and the monolithium salt (LiNHR) in a molar ratio 1 2, appropriate for a ftA-amido precursor, are used. In these cases a subsequent removal of amine, or a dehydro-halogenation step with LiNHR, is required to afford the products and free amine. Equation (1) summarizes our synthetic procedure ... 

A similar scenario as for the dihydroxylation has recently been opened for the diamination Muniz et al. reported experimental studies on the asymmetric dia-mination of olefins with osmium(VIII) imido complexes using (-)-8-phenyl-menthyl esters as chiral auxiliaries [114-116]. The reaction of (Nt-BuljOsO with enantiopure acrylates and methacrylates yielded only two out of four possible stereoisomers, whose absolute configurations are inconsistent with a concerted [3-1-2] mechanism of olefin face differentiation. The authors considered a stepwise mechanism via an osma-2-azetidine intermediate as reasonable pathway (Fig. 16). 

Remarkably, both alkyl and aryl C-H activations have been described in the formation of the metallacyclic osmium(iv)-hydride complex 25, generated as the minor component (12%) of a mixture with the deprotonated methylene-amido complex [Os(=N=CH2)Gp(GH2SiMe3)2], from the reaction of the imido complex [Os(=NMe)Cp(CH2SiMe3)2][OTf] with PPhs. ... 

The vast majority of ruthenium(VI) and osmium(VI) complexes are those stabilized by metal-ligand multiple bonds. These are mainly imido, nitrido, and 0x0 species. 

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