MAGNETO-STRUCTURAL CORRELATIONS IN LOW-DIMENSIONAL MAGNETS FORMED BY NON-COVALENT INTERACTIONS
A. Orendáčová, E. Čižmár, M. Orendáč, M. Kajňaková, J. Černák, A. Feher.
Over the past few years, non-covalent interactions, namely hydrogen bonds (HBs) have attracted a considerable amount of attention due to their important role in biological systems as well as in structural chemistry. Their flexibility and variability enables to organize the assembly of building blocks into stabile three-dimensional crystal structures. Recent quantum mechanical calculations of exchange coupling in Cu(II) dimers formed by HBs of O-H···O sort revealed that the HBs play predominantly structural role, e.i. they are responsible for the shortening of the distance between atoms in the absence of covalent bonding. The shortening enables the formation of unconventional exchange pathways . This property together with the flexibility and variability predetermines HBs as a promising new way for creation of novel quantum magnetic systems.
Cu(II) cyanocomplexes characterized by a chain-like crystal structure proved to represent spatially anisotropic two-dimensional (2d) magnets. The local surroundings of Cu(II) ion formed by a ligand octahedron elongated due to Jahn-Teller distortion, stabilizes dz2 electronic ground state of the Cu(II) ion. It implicates a preferred formation of exchange pathways within the equatorial plane of the local octahedron along dx2-y2 lobes (Fig.1), perpendicular to the covalent chains. Since the chain-like structure does not offer any covalent bonds between the chains, the main role in the formation of exchange pathways is played be the present HBs. In other words, the chain-like crystal structure does not coincide with the magnetic structure which is built in the layers perpendicular to the covalent chains. The symmetry of the 2d lattice is determined by the position of Cu(II) ions and HBs geometry and can vary from square to triangular pattern. Our systematic investigations of magnetic properties of cyanocomplexes are summarized in the Ref. 2.
A cyanocomplex compound [CuII(en)2(H2O)] [CuII(en)2Ni2Cu2I(CN)10].2H2O deserves a spatial attention as an example of a molecular magnet in which copper paramagnetic centers are connected into centro symmetric tetrameric units via the HBs . Corresponding magnetic subsystem can be approximated by a model of a planar tetramer with three different couplings J1, J2 and J3 » 1K (Fig.2). However, specific heat data clearly indicate the need to incorporate further interactions introduced by HBs.
 C. Desplanches, E. Ruiz, A. Rodríguez-Fortea, S. Alvarez, J. Am. Chem. Soc. 124 (2002)