METAL-ASSISTED LIGAND OXIDATION - SYNTHESIS AND CHARACTERIZATION OF NOVEL MANGANESE(III), IRON(III), AND NICKEL(II) COMPLEXES

The reaction of the ligand (2-pyridyl)bis(2-pyridylamino)methane (L1) in acetonitrile with nickel(II) perchlorate leads to the formation of the expected bis-chelated complex [Ni(L1)(2)](ClO4)2 2CH(3)CN, but in the presence of air L1 reacts with manganese(II) and iron(II) perchlorate salts to form the novel mononuclear complexes [M(L2)(2)]-ClO4 (M = Mn-III, Fe-III L2 = (2-pyridyl)(2-pyridylamino)methanolato anion). The crystal structures of the complexes have been determined by X-ray crystallography. The complex [Mn(L2)(2)]ClO4, C22H20N6MnClO6, crystallizes in the monoclinic space group C2/c with four molecules in a unit cell of dimensions a 13.538(3) Angstrom, b = 13.754(3) Angstrom, 13.222(3) Angstrom, and beta 106.47(3)degrees. The structure was solved by direct methods and refined by least-squares techniques to a final agreement factor of 0.0487 based on 1867 observed independent reflections. The structure of the [Mn(L2)(2)](+) cation reveals an axially compressed octahedral geometry at the manganese center. The complex [Fe(L2)(2)]ClO4, C22H20N6FeClO6, is isomorphous with the Mn complex, with a = 13.423(3) Angstrom, b = 13.572(3) Angstrom, c 13.371(3) Angstrom; and P 107.89(3)degrees. The structure was solved by direct methods and refined by least-squares techniques to a final agreement factor of 0.0521 based on 1723 observed independent reflections. The structure of the [Fe(L2)(2)](+) cation reveals a pseudooctahedral geometry for the iron center. The complex [Ni(L1)(2)](ClO4)2 2CH(3)CN, C32H36N12NiCl2O8, crystallizes in the orthorhombic space group Pbca with four molecules in a unit cell of dimensions a = 12.252(2) Angstrom, b = 15.146(3) Angstrom, and c 22.275(3) Angstrom. The structure was solved by direct methods and refined by least-squares techniques to a final agreement factor of 0.0653 based on 802 observed independent reflections. The structure of the [Ni(L1)(2)](2+) cation reveals a pseudooctahedral geometry for the nickel center. Metal-assisted deamination/oxidation of the L1 ligand units during complexation is evident from the structures of the Mn-III and Fe-III complexes, but such oxidation is not observed with the Ni-II ion. The room-temperature effective magnetic moments of 4.95 and 3.12 mu(B) observed for the Mn-III and Ni-II complexes, respectively, indicate that they are high-spin d(4) and d(8) systems, whereas the value of 2.56 mu(B) observed for the Fe-III complex suggests that this is a low-spin d(5) complex. The electronic spectrum of the Mn-III complex exhibits two weak spin-forbidden d-d transitions at 560 and 800 nm, whose presence is explained on the basis of axial compression of the Mn-III center due to the expected Jahn-Teller effect. The Mn-III and Fem complexes exhibit a quasi-reversible or reversible one-electron oxidation at +1.155 and +1.450 V (vs Ag/AgCl), respectively, corresponding to the M(III) <-> M(IV) redox process.