An EPR and 1H NMR active mixed-valence manganese (III/II/III) trinuclear compound

A mixed-valence Mn(III)-Mn(II)-Mn(III) trinuclear complex of stoichiometry (MnMnMnIII)-Mn-III-Mn-II(Hsaladhp)(2)(Sal)(4).2CH(3)CN (1), where H(3)saladhp 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 degrees 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 Angstrom. 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 J(1) = -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 S-2 or S-0* states of the oxygen-evolving complex. H-1 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 P (1) over bar, a = 10.367(6) Angstrom, b = 11.369(6) Angstrom, c = 13.967(8) Angstrom; alpha = 112.56(1)degrees, beta = 93.42(2)degrees, gamma = 115.43(1)degrees, Z = 1.