Département de chimie moléculaire

A new pentanuclear bis(triple-helical) manganese complex has been isolated and characterized by x-ray diffraction in two oxidn. states: [{\{}MnII($μ$-bpp)3{\}}2MnII2MnIII($μ$-O)]3+ (13+) and [{\{}MnII($μ$-bpp)3{\}}2MnIIMnIII2($μ$-O)]4+ (14+). The structure consists of a central {\{}Mn3($μ$3-O){\}} core of MnII2MnIII (13+) or MnIIMnIII2 ions (14+) which is connected to two apical MnII ions through six bpp- ligands. Both cations have a triple-stranded helicate configuration, and a pair of enantiomers is present in each crystal. The redox properties of 13+ have been studied in CH3CN. A series of five distinct and reversible one-electron waves is obsd. in the -1.0 and +1.50 V potential range, assigned to the MnII4MnIII/MnII5, MnII3MnIII2/MnII4MnIII, MnII2MnIII3/MnII3MnIII2, MnIIMnIII4/MnII2MnIII3, and MnIII5/MnIIMnIII4 redox couples. The two first oxidn. processes leading to MnII3MnIII2 (14+) and MnII2MnIII3 (15+) are related to the oxidn. of the MnII ions of the central core and the two higher oxidn. waves, close in potential, are thus assigned to the oxidn. of the two apical MnII ions. The 14+ and 15+ oxidized species and the reduced MnII4 (12+) species are quant. generated by bulk electrolyzes demonstrating the high stability of the pentanuclear structure in four oxidn. states (12+ to 15+). The spectroscopic characteristics (X-band ESR, EPR, and UV-visible) of these species are also described as well as the magnetic properties of 13+ and 14+ in solid state. The powder X- and Q-band EPR signature of 13+ corresponds to an S = 5/2 spin state characterized by a small zero-field splitting parameter (|D| = 0.071 cm-1) attributed to the two apical MnII ions. At 40 K, the magnetic behavior is consistent for 13+ with two apical S = 5/2 {\{}MnII(bpp)3{\}}- and one S = 2 noninteracting spins (11.75 cm3 K mol-1), and for 14+ with three S = 5/2 noninteracting spins (13.125 cm3 K mol-1) suggesting that the {\{}MnII2MnIII($μ$3-O){\}}5+ and {\{}MnIIMnIII2($μ$3-O){\}}6+ cores behave at low temp. like S = 2 and S = 5/2 spin centers, resp. The thermal behavior {\textless}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. [on SciFinder(R)]