An EPR and 1H NMR Active Mixed-Valence Manganese (III/II/III) Trinuclear Compound
journal contributionposted on 20.05.2000, 00:00 by Vasilis Tangoulis, Dora A. Malamatari, Georgios A. Spyroulias, Catherine P. Raptopoulou, Aris Terzis, Dimitris P. Kessissoglou
A mixed-valence Mn(III)−Mn(II)−Mn(III) trinuclear complex of stoichiometry MnIIIMnIIMnIII(Hsaladhp)2(Sal)4·2CH3CN (1), where H3saladhp is a tridentate Schiff-base ligand, has been structurally characterized with X-ray crystallography. The Mn(III)Mn(II)Mn(III) angles are strictly 180° as required by crystallographic inversion symmetry. The complex is valence-trapped with two terminal Mn(III) ions in a distorted square pyramidal geometry. The Mn(III)···Mn(II) separation is 3.495 Å. The trinuclear complex shows small antiferromagnetic exchange J coupling. The magnetic parameters obtained from the fitting procedure in the temperature range 10−300 K are J1 = −5.7 cm-1, g = 2.02, zJ = −0.19 cm-1, and R = 0.004. The EPR spectrum was obtained at 4 K in CHCl3 and in tetrahydrofuran glasses. The low-field EPR signal is a superposition of two signals, one centered around g = 3.6 and the other, for which hyperfine structure is observed, centered around g = 4.1 indicating an S = 3/2 state. In addition, there is a 19-line signal at g = 2.0. The multiline signal compares well with that observed for the S2 or S0* states of the oxygen-evolving complex. 1H NMR data reveal that the trinuclear compound keeps its integrity into the CHCl3 solution. Crystal data for complex 1: [C54H52N4O18Mn3], M = 1209.82, triclinic, space group P1̄, a = 10.367(6) Å, b =11.369(6) Å, c = 13.967(8) Å; α = 112.56(1)°, β = 93.42(2)°, γ = 115.43(1)°, Z = 1.