Multireversible Redox Processes in Pentanuclear Bis(Triple-Helical) Manganese Complexes Featuring an Oxo-Centered triangular {MnII 2MnIII(μ3-O)}5+ or {MnIIMnIII 2(μ3-O)}6+ Core Wrapped by Two {MnII2(bpp)3}-
Résumé
A new pentanuclear bis(triple-helical) manganese complex has been isolated and characterized by X-ray diffraction in two oxidation states: [{MnII(μ-bpp)3}2MnII 2- MnIII(μ-O)]3+ (13+) and [{MnII(μ-bpp)3}2MnIIMnIII 2(μ-O)]4+ (14+). The structure consists of a central {Mn3(μ3-O)} core of MnII2MnIII (13+) or MnIIMnIII 2 ions (14+) which is connected to two apical MnII 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 13+ have been investigated in CH3CN. 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 MnII 4MnIII/MnII5, MnII3MnIII2/MnII4MnIII, MnII2MnIII3/MnII3MnIII2, MnIIMnIII4/MnII2MnIII3, and MnIII5/MnIIMnIII4 redox couples. The two first oxidation processes leading to MnII3MnIII 2 (14+)and MnII2MnIII3 (15+) are related to the oxidation of the MnII ions of the central core and the two higher oxidation waves, close in potential, are thus assigned to the oxidation of the two apical MnII ions. The 14+ and 15+ oxidized species and the reduced Mn4II(12+) species are quantitatively generated by bulk electrolyses demonstrating the high stability of the pentanuclear structure in four oxidation states (12+ to 15+). 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 13+ and 14+ in solid state. The powder X- and Q-band EPR signature of 13+ 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 MnII ions. At 40 K, the magnetic behavior is consistent for 13+ with two apical S = 5/2 {MnII(bpp)3}- and one S = 2 noninteracting spins (11.75 cm3Kmol-1), and for 14+ with three S = 5/2 noninteracting spins (13.125 cm3 K mol-1) suggesting that the {MnII 2MnIII(μ3-O)}5+ and {MnIIMnIII2(μ3-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 13+ leading to an ST = 3 and ferromagnetic for 14+ giving thus an ST = 15/2 ground state.