Osmium dioxo

There is a vigorously expanding chemistry of compounds of ruthenium and osmium in high oxidation states [3, 4, 11, 12], particularly of dioxo and nitrido compounds, though recently some striking developments have taken place in imide chemistry. 

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).

Fig. 12. Structure of trimeric dimethyl(dioxo)(pyridme)osmium(VI), 0s306(py)3(Me)6. From Ref. U81).

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. 

The coordination about the osmium is very similar to that found for the monoesters, the /r,//-dioxo bridge being replaced by a second oxo ester ring. The 0s=0 distance is 1.670(12) A and the mean 0(terminal)0s0(ester) angle is 110.1 the mean Os—O (ester) distance is 1.885 A and mean C—O distance 1.452 A.4" 532 Very similar parameters have been found for 0s0(02C2Me4)2 (Os=0 = 1.62, Os—O (ester) = 1.84.A).500... 

Diamagnetic, brown, monomeric diolatodioxobis(amine)osmium(VI) esters of adenosine 411, or five-coordinated monodiolato-amine osmium(VI) esters 5 with m-dioxo coordination12... 

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. 

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