Hanspeter Andres, Juan M. Clemente-Juan, Reto Basler, Michael Aebersold, Hans-Ulrich Güdel, Juan J. Borrás-Almenar, A. Gaita, Eugenio Coronado, Herma Büttner and Stefan Janssen
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
Inorg. Chem. 40, 1943-1950 (2001)                 

Abstract: The ground-state properties of the pentameric Co(II) cluster [Co₃W(D₂O)₂(CoW₉O₃₄)₂]^12- were investigated by combining magnetic susceptibility and low-temperature magnetization measurements with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na₁₂[Co₃W(D₂O)₂(CoW₉O₃₄)₂]·46D₂O. The encapsulated magnetic Co₅ unit consists of three octahedral and two tetrahedral oxo-cooordinated Co(II) ions. Thus, two different types of exchange interactions are present within this cluster: a ferromagnetic interaction between the octahedral Co(II) ions and an antiferromagnetic interaction between the octahedral and the tetrahedral Co(II) ions. As a result of the single-ion anisotropy of the octrahedral Co(II) ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co₅ spin cluster is anisotropic and is expressed as H = -2S_i=x,y,zJ_1i[S_1iS_2i + S_2iS_3i] + J_2i[S_1iS_5i + S_2iS_5i + S_2iS_6i + S_3iS_6i], where J_1i are the components 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. The study of the exchange interactions in the two structurally related polyoxoanions [Co₄(H₂O)₂(PW₉O₃₄)₂]^10- and [Co₃W(H₂O)₂(ZnW₉O₃₄)₂]^12- allowed an independent determination 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 exchange interaction J₂. 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) of 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, magneization, and INS properties of the title compound.
The ground-state properties of the pentameric Co(II) cluster [Co₃W(D₂O)₂(CoW₉O₃₄)₂]^12- were investigated by combining magnetic susceptibility and low-temperature magnetization measurements with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na₁₂[Co₃W(D₂O)₂(CoW₉O₃₄)₂]·46D₂O. The encapsulated magnetic Co₅ unit consists of three octahedral and two tetrahedral oxo-cooordinated Co(II) ions. Thus, two different types of exchange interactions are present within this cluster: a ferromagnetic interaction between the octahedral Co(II) ions and an antiferromagnetic interaction between the octahedral and the tetrahedral Co(II) ions. As a result of the single-ion anisotropy of the octrahedral Co(II) ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co₅ spin cluster is anisotropic and is expressed as H = -2S_i=x,y,zJ_1i[S_1iS_2i + S_2iS_3i] + J_2i[S_1iS_5i + S_2iS_5i + S_2iS_6i + S_3iS_6i], where J_1i are the components 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. The study of the exchange interactions in the two structurally related polyoxoanions [Co₄(H₂O)₂(PW₉O₃₄)₂]^10- and [Co₃W(H₂O)₂(ZnW₉O₃₄)₂]^12- allowed an independent determination 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 exchange interaction J₂. 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) of 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, magneization, and INS properties of the title compound.