Modification of Alkylpyridine Ligands in the Reactions of Osmium(III) Complexes with Anions

Osmium(III) complexes supported by N-methyl-di(pyridin-2-ylmethyl)amine (DPA), 4,4'-dimethyl-2,2'-bipyridyl (Me₂-bpy), and N¹,N¹,N³,N³-tetrakis(pyridin-2-ylmethyl)propane-1,3-diamine (TPPN) ligands were synthesized, and their reactivities toward a fluoride anion were examined. Crystal structures of the osmium(III) complexes showed that the metal centers adopted octahedral iso-structures with the osmium(II) centers of the one-electron reduced complexes. In the reaction with a fluoride anion, C(sp³)-H bonds of the pyridin-2-ylmethyl groups of TPPN and DPA ligands were fluorinated to give F-TPPN and F-DPA ligands containing a C(sp³)-F bond. In the case of the osmium(III) complex with the Me₂-bpy ligand, one of the methyl groups was fluorinated. The product complexes were characterized by ¹H NMR, ¹⁹F NMR, and mass spectra. From the osmium(III) complexes of TPPN and DPA ligands, two geometric isomers were formed by the fluorination reaction. One of the isomers has the fluorinated methylene group at the axial position whereas another isomer has the fluorinated methylene group at the equatorial position. There is no need to worry about the formation of such stereoisomers in the reaction with the osmium(III) complex of the Me₂-bpy ligand. The C(sp³)-H bond modification in the osmium(III) complex of the Me₂-bpy ligand also proceeded in the reactions with chloride, acetate, and benzoate anions. The reaction mechanism that involves a metallo-quinonoid canonical form comprising the osmium center and the deprotonated pyridin-2-ylmethyl or methylpyridine (picoline) moiety is proposed for the present C(sp³)-H bond modification reactions based on mechanistic studies including density functional theory calculations.