Osmium complexes dioxo

Many osmyl complexes with group VI ligands have been reported. The purple diamagnetic potassium osmate K2[0s(0H)4(0)2] is the best known and is a useful starting material for the preparation of other osmyl or osmium complexes. It is best prepared from the reaction of 0s04 with excess KOH. The X-ray crystal structure of K2[0s(0H)4(0)2] shows that the complex has the trans-dioxo unit, with a d(0s=0) of 1.77 A and a 0=0s=0 angle of 180° (238,239). The acid dissociation constants of H2[0s02(0H)4] have been determined. 

One example of a cis dioxo complex has been reported, cw-[0s02bipy,](C104)2, made by addition of CeiV to a solution containing m-[Osbipy2(H20)2]2+. The HNMR and IR spectra (vas(0s02) = 878, vs(0s02) = 858 cm-1) support this very unusual structure for an osmium VT) dioxo complex.176 An X-ray crystal structure would clearly be of great interest. 

Moreover, the authors reasoned that electron-withdrawing substituents R (e.g., R = CF3) on 0s02(NR)2 facilitate the cycloaddition step, whereas the reoxidation of Os(VI) to Os(Vlll) is hindered. For electron-donating substituents R (R = —Me, tert-Bu), the opposite behavior was predicted. The coordination of a chiral amino ligand (which might transfer stereochemical information) turned out to be hardly feasible due to the reduced Lewis acidity of the bisimido osmium complexes. All bisimido dioxo osmium compounds were calculated to be thermodynamically instable toward hydrolysis, with the degree of stability depending on the substituents R (Table 6). 

Monooxo and cis- and trans-6ioxo complexes of ruthenium(VI) and osmium(VI) are known, with the trans-6ioxo species being most common. In general, these complexes are all diamagnetic and are characterized by vibrational spectroscopy and/or X-ray crystallography. One intense metal-0x0 stretch is usually observed for monooxo and trans-dioxo complexes while two metal-oxo stretches, j/s(M(0)2) and J/as(M(0)2), are found for the cw-dioxo species in the IR spectra. The structural and vibrational data are listed in Table 4. 

The Criegee mechanism originally proposed involves the formation of an osmium(VI)-ester complex (106) from the [4+2] cycloaddition of the Osvul cis-dioxo moiety with an alkene, followed by the hydrolysis or reduction of (106) to cis-glycol and reduced osmium species. In support of this mechanism a variety of Osvl cyclic esters such as (107) or (108) (L = quinuclidine) have recently been synthesized from Os04 and the alkene, and characterized by an X-ray crystal structure.290,343 In solution the dimeric complex (108) dissociates to give the monomeric dioxo trigonal-bipyramidal complex (109), which is similar to (106).344... 

Figure 6D.1. Structureof the dioxo[(3.S,4 )-2,2,5,5-tetramethyl-3,4-hexanediolato]osmium(VI) complex with dihydroquinine p-chlorobenzoate (DHQ-CLB).

X-ray crystallographic structures of the cw-dioxo osmium(VI) esters derived from dihy-droquinine-4-chlorobcnzoate/Os04/( )-2,2,5,5-tetramethyl-3-hexene and dihydroquinidine 4-chlorobenzoate/Os04/(i )-2,5-dimethyl-3-hexene have also been reported 22. Inspection of these sixteen-electron complexes reveals that, despite the considerable distance between the ligand stereocenters and the reacting site, the 4-chlorobenzoyl moieties effectively shield one of the two possible modes of alkene approach. 

We have already mentioned that the easily attained configuration for osmium favours the formation of these species (just as d°, for other elements but apparently not for osmium, favours the formation of cis dioxo species ). The realization in 1960 that the long-known and supposedly tetrahedral K2[0s04]-2H2 0 was diamagnetic and its consequent reformulation as tra j-K2[0s02(0H)4] (subsequently confirmed by X-ray studies ) led to a rationalization of the osmyl complexes. 

Organoiron, 1181 Organoiron(I) complexes, 1203 Orthoperrhenates, 198 Osmates dioxo, 581 fluorohalo, 610 Osmium... 

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