title: Ferromagnetic exchange interactions for Cu6(12+) and Mn6(12+) hexagons sandwiched by two B-alpha-[XW9O33]9- (X = As(III) and Sb(III)) ligands in D3d-symmetric polyoxotungstates.
abstract: Two novel Cu6 and Mn6 hexagon sandwiched polyoxometalates, [(CuCl)6(AsW9O33)2]12- (1a) and [(MnCl)6(SbW9O33)2]12- (2a), have been synthesized and characterized by X-ray single-crystal analysis and magnetic measurements. These complexes are D3d symmetric and were isolated as [n-BuNH3]+ salts from aqueous solutions: (n-BuNH3)12[(CuCl)6(AsW9O33)2].6H2O (1), rhombohedral, R, a = 20.33(1) A, c = 26.35(2) A, Z = 3 and (n-BuNH3)12[(MnCl)6(SbW9O33)2].6H2O (2). Six Cu (or Mn) atoms, each of which shows 5-fold coordination, make an approximately equatorial hexagon with a first-neighboring Cu...Cu (Mn...Mn) distance of 2.913(2) A (3.248(1) A) and a Cu-O-Cu (Mn-O-Mn) bond angle of 94.5(2) degrees (100.4(2) degrees ). The magnetic behavior investigated by magnetic susceptibility measurements shows the ferromagnetic exchange interactions with J/k = +12.7 K (J/hc = +8.82 cm(-1)) and a S = 3 ground state for 1 and J/k = +0.20 K (J/hc = +0.14 cm(-1)) and a S = 15 ground state for 2, when only J refers to the isotropic magnetic-exchange interactions for first-neighbor atoms of the approximately equilateral Cu6(12+) and Mn6(12+) hexagons. The single-crystal ESR spectroscopy of 1 under the orientation of the magnetic field along a Cu6 hexagon's nearly 6-fold axis equal to the c axis on the variation of temperature supports the S = 3 ground state and allows an estimate of the zero-field (fine-structure) energy separation between Sz = 0 and Sz = +/-1 of D = -0.182 K to be obtained.
authors.affiliation: Chemical Resources Laboratory, Tokyo Institute of Technology, R1-21 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. tyamase@res.titech.ac.jp
authors.forename: Toshihiro
authors.initials: T
authors.lastname: Yamase
authors.forename: Keisuke
authors.initials: K
authors.lastname: Fukaya
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Nojiri
authors.forename: Yuhgo
authors.initials: Y
authors.lastname: Ohshima