Nucleophilic aromatic substitution on aryl-amido ligands promoted by oxidizing osmium(IV) centers

Addition of amine nucleophiles to acetonitrile solutions of the Os ^IV anilido complex TpOs(NHPh)Cl₂ (1) [Tp = hydrotris(1-pyrazolyl)borate] gives products with derivatized anilido ligands, i.e., TpOs[NH-p-C₆H₄(N(CH₂)S)]Cl₂ (2) from piperidine and TpOs[NH-p-C₆H₄N(CH ₂)₄]Cl₂ (3) from pyrrolidine. These materials are formed in ≃30% yield under anaerobic conditions, together with ≃60% yields of the Os^III aniline complex TpOs(NH ₂Ph)Cl₂ (5). Formation of the para-substituted materials 2 or 3 from 1 involves oxidative removal of two hydrogen atoms (two H⁺ and two e^-). The oxidation can be accomplished by 1, forming 5, or by O₂. Related reactions have been observed with other amines and with the 2-naphthylamido derivative, which gives an ortho-substituted product. Kinetic studies indicate an addition-elimination mechanism involving initial attack of the amine nucleophile on the anilido ligand. These are unusual examples of nucleophilic aromatic substitution of hydrogen. Ab initio calculations on 1 show that the LUMO has significant density at the ortho and para positions of the anilido ligand, resembling the LUMO of nitrobenzene. By analogy with nucleophilic aromatic substitution, 2 is quantitatively formed from piperidine and the p-chloroanilide TpOs(NH-P-C₈H₄Cl) Cl₂ (7). Binding the anilide ligands to an oxidizing Os^IV center thus causes a remarkable umpolung or inversion of chemical character from a typically electron-rich anilido to an electron-deficient aromatic functionality. This occurs because of the coupling of redox changes at the TpOs^IV center with bond formation at the coordinated ligand.