Sandwich-type germanotungstates:: Structure and magnetic properties of the dimeric polyoxoanions [M4(H2O)2(GeW9O34)2]12- (M = Mn2+, Cu2+, Zn2+, Cd2+)

The novel dimeric germanotungstates [M-4(H2O)(2)(GeW9O34)(2)](12-) (M = Mn2+, Cu2+, Zn2+, Cd2+) have been synthesized and characterized by IR spectroscopy, elemental analysis, magnetic measurements, and W-183-NMR spectroscopy. X-ray single-crystal analyses were carried out on Na-12[Mn-4(H2O)(2)(GeW9O34)(2)].38H(2)O (Na-12-1), which crystallizes in the monoclinic system, space group P2(1)/n, with a = 13.0419(8) Angstrom, b = 17.8422(10) Angstrom, c = 21.1626(12) Angstrom, beta = 93.3120(10)degrees, and Z = 2; Na11Cs2[Cu-4(H2O)(2)(GeW9O34)(2)]Cl.31H(2)O (Na11Cs-2) crystallizes in the triclinic system, space group P(1) over bar1, with a = 12.2338(17) Angstrom, b = 12.3833(17) Angstrom, c = 15.449(2) Angstrom, alpha = 100.041(2)degrees, beta = 97.034(2)degrees, gamma = 101.153(2)degrees, and Z = 1; Na-12[Zn-4(H2O)(2)(GeW9O34)(2)].32H(2)O (Na-12-3) crystallizes in the triclinic system, space group P1, with a 11.589(3) Angstrom, b = 12.811(3) Angstrom, c = 17.221(4) Angstrom, alpha = 97.828(6)degrees, beta = 106.169(6)degrees, gamma = 112.113(5)degrees, and Z = 1; Na-12[Cd-4(H2O)(2)(GeW9O34)(2)].32.2H(2)O (Na-12-4) crystallizes also in the triclinic system, space group P(1) over bar, with a 11.6923(17) Angstrom, b = 12.8464(18) Angstrom, c = 17.616(2) Angstrom, (x = 98.149(3)0, alpha = 105.677(3)degrees, gamma = 112.233(2)degrees, and Z = 1. The polyanions consist of two lacunary B-alpha-[GeW9O34](10-) Keggin moieties linked via a rhomblike M4O16 (M = Mn, Cu, Zn, Cd) group leading to a sandwich-type structure. W-183-NMR studies of the diamagnetic Zn and Cd derivatives indicate that the solid-state polyoxoanion structures are preserved in solution. EPR measurements on Na-12-1 at frequencies up to 188 GHz and temperatures down to 4 K yield a single, exchange-narrowed peak, at g(iso) = 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(II) = 2.4303 and g(perpendicular to) = 2.0567 and A(II) = 4.4 mT (delocalized over the Cu-4 framework), with \D\ = 12.1 mT. Magnetization studies in addition yield the exchange parameters J(1) = -11 and J(2) = -82 cm(-1), in agreement with the EPR studies.