Chromium(II) phosphonates as hybrid organic-inorganic magnets

Metal phosphonates, M[(RPO3)(H2O)] (where M is a divalent metal ion and R is an alkyl or aryl group), are organic-inorganic hybrid materials in which the nature of the organic moiety can be designed to confer a specific property (such as intercalation, ionic exchange, proton conduction, catalysis etc.) to the whole lattice. An attractive feature of many metal(II) phosphonates is the two-dimensional character of the lattice, in which the metal containing layers are separated by the organic sheets. The organic part of the phosphonic acid, [RPO3]2- can be chemically modified by introducing another donor group, such as an amino, carboxylic or phosphonate group. The two centres of coordination then may induce self-assembling with the possible formation of a 3D hybrid inorganic-organic structure bound covalently and/or ionically. It has been shown that the structure of the divalent metal phosphonates tends to comprise zig-zag layers with the metal ion six-coordinated in a distorted octahedral symmetry. Moreover several paramagnetic metal(II) phosphonates [M(II)(RPO 3)(H2O)] (M(II) = Cr, Fe, Ni; Co, R = alkyl or phenyl) feature magnetic long-range ordering at low temperatures and have provided interesting examples of low-dimensional magnets. In this contribution we will report on the preparation, structure and magnetic investigation of several Cr(II) phosphonates prepared in our laboratory. In particular we will focus the attention on the correlation between their structure and magnetic properties. Finally a brief report on the use of metal(II) phosphonates as precursors for electrodic materials in lithium-ion batteries will be given.