Magnetic polyoxometalates: Anisotropic antiferro- and ferromagnetic exchange interactions in the pentameric cobalt(II) cluster [Co3W(D2O)(2)(CoW9O34)(2)](12-). A magnetic and inelastic neutron scattering study

The ground-state properties of the pentameric Co(II) cluster [Co3W(D2O)(2)( CoW9O34)(2)] (12-) were investigated by combining magnetic susceptibility a nd low-temperature magnetization measurements with a detailed inelastic neu tron scattering (INS) study on a fully deuterated polycrystalline sample of Na-12[Co3W(D2O)(2)(CoW9O34)(2)]. 46D(2)O. The encapsulated magnetic Cos un it consists of three octahedral and two tetrahedral ore-coordinated Co(II) ions. Thus, two different types of exchange interactions are present within this cluster: a ferromagnetic interaction between the octahedral Co(II) io ns and an antiferromagnetic interaction between the octahedral and the tetr ahedral Co(II) ions. As a result of the single-ion anisotropy of the octahe dral Co(II) ions, the appropriate exchange Hamiltonian to describe the grou nd-state properties of the Co-5 spin cluster is anisotropic and is expresse d as (H) over cap = -2 Sigma (i=x,y,z)J(1i)[(S) over cap (1i)(S) over cap ( 2i) + (S) over cap (2i)(S) over cap (3i)] + J(2i)[(S) over cap (1i)(S) over cap (5i) + (S) over cap (2i)(S) over cap (5i) + (S) over cap (2i)(S) over cap (6i) + (S) over cap (3i)(S) over cap (6i)], where J(1i) are the compone nts of the exchange interaction between the octahedral Co(II) ions and J(2i ) are the components of the exchange interaction between the octahedral and tetrahedral Co(II) ions (see Figure Id). The study of the exchange interac tions in the two structurally related polyoxoanions [Co-4(H2O)(2)(PW9O34)(2 )](10-) and [Co3W(H2O)(2)(ZnW9O34)(2)](12-) allowed an independent determin ation of the ferromagnetic exchange parameters J(1x) = 0.70 meV, J(1y) = 0. 43 meV, and J(1Z) = 1.51 meV (set a) and J(1x) = 1.16 meV, J(1y) = 1.16 meV and J(1z) = 1.73 meV (set b), respectively. Our analysis proved to be much more sensitive to the size and anisotropy of the antiferromagnetic exchang e interaction J(2). We demonstrate that this exchange interaction exhibits a rhombic anisotropy with exchange parameters J(2x) = -1.24 meV, J(2y) = -0 .53 meV, and J(2z) = - 1.44 meV (set a) or J(1x) = -1.19 meV, J(1y) = -0.53 meV, and J(1z) = -1.44 meV (set b). The two parameter sets reproduce in a satisfactory manner the susceptibility, magnetization, and INS properties o f the title compound.