Multireversible redox processes in pentanuclear bis(triple-helical) manganese complexes featuring an oxo-centered...

A new pentanuclear bis(triple-helical) manganese complex has been isolated and characterized by X-ray diffraction in two oxidation states: [{Mn(II)(μ-bpp)(3)}(2)Mn(II)(2)Mn(III)(μ-O)](3+) (1(3+)) and [{Mn(II)(μ-bpp)(3)}(2)Mn(II)Mn(III)(2)(μ-O)](4+) (1(4+)). The structure consists of a central {Mn(3)(μ(3)-O)} core of Mn(II)(2)Mn(III) (1(3+)) or Mn(II)Mn(III)(2) ions (1(4+)) which is connected to two apical Mn(II) ions through six bpp(-) ligands. Both cations have a triple-stranded helicate configuration, and a pair of enantiomers is present in each crystal. The redox properties of 1(3+) have been investigated in CH(3)CN. A series of five distinct and reversible one-electron waves is observed in the -1.0 and +1.50 V potential range, assigned to the Mn(II)(4)Mn(III)/Mn(II)(5), Mn(II)(3)Mn(III)(2)/Mn(II)(4)Mn(III), Mn(II)(2)Mn(III)(3)/Mn(II)(3)Mn(III)(2), Mn(II)Mn(III)(4)/Mn(II)(2)Mn(III)(3), and Mn(III)(5)/Mn(II)Mn(III)(4) redox couples. The two first oxidation processes leading to Mn(II)(3)Mn(III)(2) (1(4+)) and Mn(II)(2)Mn(III)(3) (1(5+)) are related to the oxidation of the Mn(II) ions of the central core and the two higher oxidation waves, close in potential, are thus assigned to the oxidation of the two apical Mn(II) ions. The 1(4+) and 1(5+) oxidized species and the reduced Mn(4)(II) (1(2+)) species are quantitatively generated by bulk electrolyses demonstrating the high stability of the pentanuclear structure in four oxidation states (1(2+) to 1(5+)). The spectroscopic characteristics (X-band electron paramagnetic resonance, EPR, and UV-visible) of these species are also described as well as the magnetic properties of 1(3+) and 1(4+) in solid state. The powder X- and Q-band EPR signature of 1(3+) corresponds to an S = 5/2 spin state characterized by a small zero-field splitting parameter (|D| = 0.071 cm(-1)) attributed to the two apical Mn(II) ions. At 40 K, the magnetic behavior is consistent for 1(3+) with two apical S = 5/2 {Mn(II)(bpp)(3)}(-) and one S = 2 noninteracting spins (11.75 cm(3) K mol(-1)), and for 1(4+) with three S = 5/2 noninteracting spins (13.125 cm(3) K mol(-1)) suggesting that the {Mn(II)(2)Mn(III)(μ(3)-O)}(5+) and {Mn(II)Mn(III)(2)(μ(3)-O)}(6+) cores behave at low temperature like S = 2 and S = 5/2 spin centers, respectively. The thermal behavior below 40 K highlights the presence of intracomplex magnetic interactions between the two apical spins and the central core, which is antiferromagnetic for 1(3+) leading to an S(T) = 3 and ferromagnetic for 1(4+) giving thus an S(T) = 15/2 ground state.