Hybrid organic-inorganic multilayer materials:: influence of π electrons as magnetic media in a series of bridged-layer compounds M2(OH)4-xAx/2 (M = Cu(II) or Co(II), A = dicarboxylate anion)

We report the preparation, structure and magnetic properties of a series of multilayer copper(II) hydroxy-dicarboxylates. These are prepared by anion-exchange, starting from copper(II) hydroxy-acetate Cu-2(OH)(3)(OAc). H2O. The acetate ion is substituted for n-methylene dicarboxylates O2CXCO22-, with X = (CH2), and n =1 to 8. Similar exchange reactions are carried out with unsaturated dicarboxylate anions (X = (CH)(n), with n = 2, 4 and X = C4H6). The compounds, of general formulation Cu-2(OH)(4-x)(XC2O4)(x/2). zH(2)O, exhibit a layered structure with a step-like variation of the basal spacing, according to n parity. IR spectroscopy indicates that the carboxylate functions are linked to the metal ions, each aliphatic chain bridging adjacent hydroxide layers. Only compounds with n = 2, 4 and 8 show a net magnetic moment within copper(II) layers. For the short chain anions, an antiferromagnetic order occurs at low temperature, with a metamagnetic transition in low field, except for the unsaturated dicarboxylate Cu-2(OH)(1.92)(C4H6C2O4)(1.04). 0.22H(2)O which exhibits a long-range ferromagnetic order at T-C = 13 K. For the long-chain anion, X = C8H16, a ferrimagnetic 3d order is stabilized below T-C= 17 K. On the basis of structural and magnetic findings, a comparison is made between the saturated and unsaturated aliphatic chain compounds. This study provides new insight on the influence of pi electrons on the ordering between magnetic layers. Finally, preliminary results for the cobalt(II) trans-hexenedioate analog indicate that a 3d ferromagnet is obtained below T-C = 56.5 K, thus emphasizing the key role of the in-plane spin-spin correlations on the ordering temperature. (C) 2000 Elsevier Science Ltd All rights reserved.