Reactions of a hexahydride-osmium complex with aromatic ketones: C-H activation versus C-F activation

Treatment of OsH6((PPr3)-Pr-i)(2) (1) with benzophenone and acetophenone in toluene under reflux affords OsH3{C6H4C(O)R}((PPr3)-Pr-i)(2) (R = Ph (2), CH3 (3)), as a result of the ortho-CH activation of the aromatic group of t he ketones. Complex 1 is also capable of activating ortho-CF bonds of fluor inated aromatic ketones. Thus, the reactions of this complex with pentafluo roacetophenone, decafluorobenzophenone, and 2,6-difluoroacetophenone give O sH3{C6F4C(O)R}(PiPr(3))(2) (R = CH3 (4), C6F5 (5)) and OsH3{C6H3FC(O)CH3}(( PPr3)-Pr-i)(2) (6). The structure of 4 has been determined by X-ray diffrac tion. The geometry around the osmium atom can be described as a distorted p entagonal bipyramid with the phosphine ligands occupying axial positions. C omplexes 4 and 6 can be also obtained by reaction of 1 with 2,3,4,5-tetrafl uoroacetophenone and 2-fluoroacetophenone, respectively. This selective C-H activation of the ortho-CH bond of the above-mentioned ketones is in contr ast with the selective C-F activation observed for the reaction of 1 with 2 ,3,4,5,6-pentafluorobenzophenone, which affords OsH3{C6F4C(O)C6H5}((PPr3)-P r-i)(2) (7). The structure of 7 has also been determined by X-ray diffracti on. The geometry around the osmium is the same as that of 4. DFT calculatio ns suggest that in fluorinated aromatic ketones the ortho-CF activation is thermodynamically favored over the ortho-CH activation and that the kinetic ally preferred ortho-CH activation of 2,3,4,5-tetrafluoroacetophenone and 2 -fluoroacetophenone is in part due to the preferred anti arrangement of the F-C-C-C=O unit of the starting ketones. In solution, the hydride ligands o f the OsH3 unit of 2-7 undergo two different thermally activated exchange p rocesses, which involve the central hydride with each hydride ligand situat ed close to the donor atoms of the chelate group. The exchange involving th e hydride ligand disposed cis to the carbonyl group is faster than the othe r one in all the cases. For 2, 3, and 6, quantum exchange coupling is also observed between the hydride ligands involved in the faster thermally activ ated exchange process.