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Oxalate-Bridged Bimetallic Complexes {NH(prol)3}[MCr(ox)3] (M = MnII, FeII, CoII; NH(prol)3+ = Tri(3-hydroxypropyl)ammonium) Exhibiting Coexistent Ferromagnetism and Proton Conduction

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posted on 23.09.2009, 00:00 by Hisashi O̅kawa, Akihito Shigematsu, Masaaki Sadakiyo, Takuya Miyagawa, Ko Yoneda, Masaaki Ohba, Hiroshi Kitagawa
The oxalate-bridged bimetallic complexes {NH(prol)3}[MIICrIII(ox)3] (MII = MnII, FeII, CoII) with hydrophilic tri(3-hydroxypropyl)ammonium (NH(prol)3+) were prepared by a new synthetic procedure, and the effects of the NH(prol)3+ ion upon the structure, magnetism, and electrical conduction were studied. An X-ray crystallographic study of the MnCr dihydrate, {NH(prol)3}[MnCr(ox)3]·2H2O, was performed. Crystal data: hexagonal, P63, a = b = 9.3808(14) Å, c = 15.8006(14) Å, Z = 2. The structure comprises oxalate-bridged bimetallic layers interleaved by NH(prol)3+ ions. The ions assume a tripodal configuration and are hydrogen bonded to the bimetallic layers together with water molecules, giving rise to a short interlayer separation (7.90 Å) and unsymmetrical faces to the bimetallic layer. Cryomagnetic studies demonstrate ferromagnetic ordering with transition temperature of 5.5 K for the MnCr complex, 9.0 K for the FeCr complex, and 10.0 K for the CoCr complex. The interlayer magnetic interaction is negligibly weak in all of the complexes despite the short interlayer separation. A slow magnetization is observed in all the complexes. This is explained by spin canting associated with the unsymmetrical feature of the bimetallic layer. The complexes show proton conduction of 1.2 × 10−10 to 4.4 × 10−10 S cm−1 under 40% relative humidity (RH) and ∼1 × 10−4 S cm−1 under 75% RH. On the basis of water adsorption/desorption profiles, the conduction under 40% RH is mediated through the hydrogen-bonded network formed by the bimetallic layer, NH(prol)3+ ions, and water molecules (two per MCr). Under 75% RH, additional water molecules (three per MCr) are concerned with the high proton conduction. This is the first example of a metal complex system exhibiting coexistent ferromagnetism and proton conduction.