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 the ketones. Complex 1 is also capable of activating ortho-CF bonds of fluorinated aromatic ketones. Thus, the reactions of this complex with pentafluoroacetophenone, decafluorobenzophenone, and 2,6-difluoroacetophenone give OsH3{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 diffraction. The geometry around the osmium atom can be described as a distorted pentagonal bipyramid with the phosphine ligands occupying axial positions. Complexes 4 and 6 can be also obtained by reaction of 1 with 2,3,4,5-tetrafluoroacetophenone and 2-fluoroacetophenone, respectively. This selective C-H activation of the ortho-CH bond of the above-mentioned ketones is in contrast 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)-Pr-i)(2) (7). The structure of 7 has also been determined by X-ray diffraction. The geometry around the osmium is the same as that of 4. DFT calculations suggest that in fluorinated aromatic ketones the ortho-CF activation is thermodynamically favored over the ortho-CH activation and that the kinetically 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 of the OsH3 unit of 2-7 undergo two different thermally activated exchange processes, which involve the central hydride with each hydride ligand situated close to the donor atoms of the chelate group. The exchange involving the hydride ligand disposed cis to the carbonyl group is faster than the other 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 activated exchange process.