Nucleophilic aromatic substitution on aryl-amido ligands promoted by oxidizing osmium(IV) centers.
Addition of amine nucleophiles to acetonitrile solutions of the OsIV 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)5)]Cl2 (2) from piperidine and TpOs[NH-p-C6H4N(CH2)4]Cl2 (3) from pyrrolidine. These materials are formed in approximately 30% yield under anaerobic conditions, together with approximately 60% yields of the OsIII aniline complex TpOs(NH2Ph)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-C6H4Cl)Cl2 (7). Binding the anilide ligands to an oxidizing OsIV 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 TpOsIV center with bond formation at the coordinated ligand.