Mono complexes, osmium

Mono(p-oxo) complexes, osmium, 37 311 Monooxygenases, Fe—S clusters as electron carriers, 38 305... 

A more detailed study of the decomposition of osmium(ii) dinitrogen species in aqueous solutions confirms that both [Os(NH3)s(N2)] and [Os(NH3)4-(N2)2] are very stable in neutral water. However, in both acidic and basic solution the decomposition of the former complex is subject to autocatalysis. Surprisingly, cis-[Os(NH3)4(N2)2] is stable between pH 1 and 14, although it was expected to be less stable than the mono-complex on the basis of i.r. frequency positions. These observations can be rationalized if one assumes Sf 2 attack on osmium by OH , and that the positive charge on the metal is smaller for [Os(NH3)4(N2)2] than for the mono-complex. 

The bulky ruthenium TMP complex Ru(TMP) is very electron deficient in the absence of any coordinating ligand, and a tt-complex with benzene has been proposed. In fact, it readily coordinates dinitrogen, forming the mono- and bis-N adducts Ru(TMP)(N2)(THF) and Ru(TMP)(N2)2, - As a result, the use of the TMP ligand for careful stereochemical control of the chemistry at the metal center, which has been very successful for the isolation of elusive rhodium porphyrin complexes, is less useful for ruthenium (and osmium) because of the requirement to exclude all potential ligands, including even N2,... 

Both rhodium and osmium porphyrins are active for the cyclopropanation of alkenes. The higher activity of the rhodium porphyrin catalysts can possibly be attributed to a more reactive, cationic carbene intermediate, which so far has defied isolation. The neutral osmium carbene complexes are less active as catalysts but the mono- and bis-carbene complexes can be isolated as a result. 

Of the Ru(IV) complexes recorded here most are mono-oxo species which, despite the strong axial distortion brought about by the terminal oxo ligand, are probably all paramagnetic. Semi-empirical molecular orbital calculations (INDO/1) for epoxidations effected by oxo-Ru(IV) complexes have been reported (a non-concerted [1 h- 2] pathway was preferred) [642], [643] and for alcohol oxidations by octahedral species containing an Ru" (0) unit [644]. The reactivity of high oxidation-state polypyridyl complexes of osmium and Ru, with particular emphasis on Ru(IV) and Os(IV) oxo species, has been reviewed [43]. 

Instead, evidence now favors the intermediacy of the highly organized mono-osmium complex 15. Various factors could contribute to its excellent enantioselectivity, the most important one being the formation of a binding site consisting of the two methoxy quinolines and the pyridazine linker. As confirmed by X-ray analysis and NMR studies [35,36], this pocket adopts a U-shaped conformation and is capable of perfectly binding aromatic substrates such as styrene through attractive interactions with the methoxyquinolines. In contrast, bulky... 

Osmium(vi).— [0s04] has been shown to react with monoalkenes (R) to give five-co-ordinate mono- and di-esters of the saturated diolato ligand (02R)2-, namely [0s204(02R)2] and [OsO(02R)2].104 These complexes were assigned structures (21) and (22). respectively, on the basis of i.r. and molecular weight measurements. 

Many methods of preparing mono or dihydrido mononuclear osmium complexes have been reported. Monohydrido osmium complexes 167 can be obtained by treatment of 146 with zinc in methanol at room temperature... 

Obviously the chemistry here is dominated by that of the tetroxide. We briefly consider the hexa-and penta-oxo species, then Os04, followed by species of the type Os04 L and OsO, . The few tri-, di- and mono-oxo complexes of osmium(VIII) conclude the section, (see Table 23). 

In this section we consider the complexes of mono-, di- and poly-carboxylato complexes. There appear to be marked differences from ruthenium carboxylato chemistry particularly in respect of the II/III and III states. A number of osmyl complexes are known with monodentate or bidentate carboxylates (p. 582) but apparently none of ruthenium(VT). On the other hand, the binuclear ruthenium species [Ru30(OCOR)6L3]"+, in which OCOR functions as a bidentate ligand, does not appear to have an osmium counterpart both elements form lantem -like species containing [M2(OCOR)4]"+ cores, but again with marked differences between ruthenium and osmium. This is clearly an interesting and rewarding field for further study. 

Che and coworkers [152] were able to isolate and characterize a pure bis-carbene (TPFPP)Os(CPh2)2 (Fig. 3). The bis-carbene species represents the first structurally characterized fra s-bis-carbene metal complex whose carbene groups are not stabilized by heteroatoms. The related pentacoor-dinated mono-carbene complex was also prepared and characterized by an X-ray structure. A comparison of the reactivity of these complexes with olefins suggests that the bis-carbene species acts as an intermediate in cy-clopropanation. Thus, the inertness of the mono-carbene complex towards stoichiometric styrene cyclopropanation and the observation of an efficient cyclopropanation of styrene in the presence of the bis-carbene complex as a catalyst support this suggestion [152]. A recent X-ray structure determination for (TPFPP)Os(CPh2)(MeIm) revealed an Os = C distance of 1.902(3) A (Table 3) [141]. Recently, Che and coworkers [153] and Miyamoto and coworkers [154] reported oxo-bridged carbene complexes of osmium porphyrins (see Table 3). These compounds are rare examples of oxo-binuclear carbene complexes. 

Further insight into the mechanism of osmium(ll) porphyrin-catalyzed cyclopropanation of alkenes by diazoalkanes was reported by Woo and coworkers [169]. A mono-carbene complex, (TTP)0s(CHC02Et), has been isolated but is not the catalytically active species. An electron-withdrawing Hg-and trans to the carbene activates the carbon fragment towards transfer to an olefin. Substrate reactivity profiles and labeling studies are consistent with a trans-osmiimi(II) bis-carbene species as the active catalyst [169]. 

Factors affecting simple reductive elimination versus other forms of elimination (e.g. a), especially with respect to dihydrides, dialkyls, and mixed alkyl-hydrides (which eliminate fastest) of mono- and di-nuclear osmium complexes, have been discussed. In compounds such as [OsHMe(CO)4] or [OsMe2(CO)4] simple reductive elimination does not occur for example, a mixture of [OsH(CD3)(CO>4] and [OsD(CH3)(CO)4] gave CH4, CH3D, CD3H, and CD4, and a dinuclear mechanism is proposed. ... 

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