Sandwich-Type Germanotungstates:  Structure and Magnetic Properties of the Dimeric Polyoxoanions [M₄(H₂O)₂(GeW₉O₃₄)₂]^12- (M = Mn²⁺, Cu²⁺, Zn²⁺, Cd²⁺)

Abstract

The novel dimeric germanotungstates [M4(H2O)2(GeW9O34)2]12- (M = Mn2+, Cu2+, Zn2+, Cd2+) have been synthesized and characterized by IR spectroscopy, elemental analysis, magnetic measurements, and 183W-NMR spectroscopy. X-ray single-crystal analyses were carried out on Na12[Mn4(H2O)2(GeW9O34)2]·38H2O (Na12-1), which crystallizes in the monoclinic system, space group P21/n, with a = 13.0419(8) Å, b = 17.8422(10) Å, c = 21.1626(12) Å, β = 93.3120(10)°, and Z = 2; Na11Cs2[Cu4(H2O)2(GeW9O34)2]Cl·31H2O (Na11Cs-2) crystallizes in the triclinic system, space group P1̄, with a = 12.2338(17) Å, b = 12.3833(17) Å, c = 15.449(2) Å, α = 100.041(2)°, β = 97.034(2)°, γ = 101.153(2)°, and Z = 1; Na12[Zn4(H2O)2(GeW9O34)2]·32H2O (Na12-3) crystallizes in the triclinic system, space group P1̄, with a = 11.589(3) Å, b = 12.811(3) Å, c = 17.221(4) Å, α = 97.828(6)°, β = 106.169(6)°, γ = 112.113(5)°, and Z = 1; Na12[Cd4(H2O)2(GeW9O34)2]·32.2H2O (Na12-4) crystallizes also in the triclinic system, space group P1̄, with a = 11.6923(17) Å, b = 12.8464(18) Å, c = 17.616(2) Å, α = 98.149(3)°, β = 105.677(3)°, γ = 112.233(2)°, and Z = 1. The polyanions consist of two lacunary B-α-[GeW9O34]10- Keggin moieties linked via a rhomblike M4O16 (M = Mn, Cu, Zn, Cd) group leading to a sandwich-type structure. 183W-NMR studies of the diamagnetic Zn and Cd derivatives indicate that the solid-state polyoxoanion structures are preserved in solution. EPR measurements on Na12-1 at frequencies up to 188 GHz and temperatures down to 4 K yield a single, exchange-narrowed peak, at giso = 1.9949, typical of Mn systems, and an upper limit of |D| = 20.0 mT; its magnetization studies still await further theoretical treatment. Detailed EPR studies on Na11Cs-2 over temperatures down to 2 K and variable frequencies yield g∥ = 2.4303 and g⊥ = 2.0567 and A∥ = 4.4 mT (delocalized over the Cu4 framework), with |D| = 12.1 mT. Magnetization studies in addition yield the exchange parameters J1 = −11 and J2 = −82 cm-1, in agreement with the EPR studies