Determination of the In-Solution Molecular Structure of Reactive Osmium Compounds Involved in the Synthesis of Vicinal Diols

Extended X-ray absorption fine structure spectroscopy, EXAFS, has been used to determine the in-solution structure of the two principal reactive components involved in the production of vicinal diols by olefin dihydroxylation using osmium tetroxide. These components are osmium tetroxide (OsO 4 ) and the corresponding diester osmium glycolate complex. The results show that initially the osmium tetroxide molecules are found free and unbound in the investigated solvent (6:1 tertiary butanol in water), with a molecular structure close to that found in the solid state. On addition of an excess of 2,3-dimethyl-2-butene, a compound of formula OsO(O 2 C 6 H 1 2 ) 2 , a monomeric diester of osmium tetroxide, is formed. The structural refinement of this compound indicates that the osmium center is surrounded by five oxygen atoms, one located at a shorter distance than the other four (1.69 and 1.90 A, respectively). The same structure has been found for the osmium complex formed as the result of the slow reaction of osmium tetroxide with an excess of ethylene glycol in solution, OSO(O 2 C 2 H 4 ) 2 . This last complex is compared with the known solid state structure.