Regioselective Nitration and/or Halogenation of Iridabenzofurans through Electrophilic Substitution

Regioselective electrophilic substitution reactions of the iridabenzofurans [Ir­(C7H5O­{OMe-7})­(CO)­(PPh3)2]­[OTf] (1) and IrCl­(C7H5O­{OMe-7})­(PPh3)2 (2) provide a convenient route to mononitro-, dinitro-, and mixed nitro-/halo-substituted derivatives. Treatment of cationic 1 with copper­(II) nitrate in acetic anhydride (“Menke” nitration conditions) gives the mononitrated iridabenzofuran [Ir­(C7H4O­{NO2-2}­{OMe-7})­(CO)­(PPh3)2]­[O3SCF3] (3). Under the same conditions neutral 2 undergoes dinitration to form IrCl­(C7H3O­{NO2-2}­{NO2-6}­{OMe-7})­(PPh3)2 (5). Simple substitution of the carbonyl ligand in 3 with chloride gives the neutral mononitro derivative IrCl­(C7H4O­{NO2-2}­{OMe-7})­(PPh3)2 (4). Depending on the conditions employed, treatment of the iridabenzofurans 1 and 2 with Cu­(NO3)2 and either lithium chloride or lithium bromide in acetic anhydride gives either the mixed nitro-/halo-substituted iridabenzofurans IrCl­(C7H3O­{NO2-2}­{Cl-6}­{OMe-7})­(PPh3)2 (6) and IrCl­(C7H2O­{NO2-2}­{NO2-4}­{Cl-6}­{OMe-7})­(PPh3)2 (7) or the simple halo-substituted iridabenzofurans [Ir­(C7H4O­{Cl-6}­{OMe-7})­(CO)­(PPh3)2]­[OTf] (8), [Ir­(C7H4O­{Br-6}­{OMe-7})­(CO)­(PPh3)2]­[OTf] (9), and IrBr­(C7H3O­{Br-2}­{Br-6}­{OMe-7})­(PPh3)2 (10). Bromination of 4 with pyridinium tribromide gives IrCl­(C7H3O­{NO2-2}­{Br-6}­{OMe-7})­(PPh3)2 (11). The molecular structures of 3–7 and 11 have been obtained by X-ray crystallography.