Multireversible redox processes in pentanuclear bis(triple-helical) manganese complexes featuring an oxo-centered triangular {Mn II 2Mn III(μ 3-O)} 5+ or {Mn IIMn III 2(μ 3-O)} 6+ core wrapped by two {Mn II 2(bpp) 3} -

Abstract

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} 2Mn II 2Mn III(μ-O)] 3+ (1 3+) and [{Mn II(μ-bpp) 3} 2Mn IIMn III 2(μ-O)] 4+ (1 4+). The structure consists of a central {Mn 3(μ 3-O)} core of Mn II 2Mn III (1 3+) or Mn IIMn 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 3CN. 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 4Mn III/Mn II 5, Mn II 3Mn III 2/Mn II 4Mn III, Mn II 2Mn III 3/Mn II 3Mn III 2, Mn IIMn III 4/Mn II 2Mn III 3, and Mn III 5/Mn IIMn III 4 redox couples. The two first oxidation processes leading to Mn II 3Mn III 2 (1 4+) and Mn II 2Mn 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 2Mn III(μ 3-O)} 5+ and {Mn IIMn 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