Layered hybrid organic–inorganic Co(II) alkylphosphonates. Synthesis, crystal structure and magnetism of the first two members of the series: Co[(CH 3PO 3)(H 2O)] and Co[(C 2H 5PO 3)(H 2O)

Abstract

Co[(CH 3PO 3)(H 2O)] ( 1) and Co[(C 2H 5PO 3)(H 2O)] ( 2) were prepared by the hydrothermal method and isolated as blue-violet platelet crystals. They were characterized by X-ray diffraction, FT-IR, TGA-DSC techniques and their magnetic properties studied by a dc-SQUID magnetometer. Compound ( 1) shows an hybrid layered structure, made of alternating inorganic and organic layers along the a-direction of the unit cell. The inorganic layers contain Co(II) ions six-coordinated by five phosphonate oxygen atoms and one from the water molecule. These layers are separated by bi-layers of methyl groups and van der Waals contacts are established between them. In compound ( 2), the layered hybrid structure is rather similar to that described for compound ( 1), but the alternation of the inorganic and organic layers is along the b-direction of the unit cell. The magnetic behavior of ( 1) and ( 2) as function of temperature and magnetic field was studied. The compounds obey the Curie–Weiss law at temperatures above 100 K, the Curie C, and Weiss θ constants for the methyl derivative being C = 3.3 6 cm 3 K mol - 1 and θ = - 5 3 K and for the ethyl derivative C = 3.6 2 cm 3 K mol - 1 and θ = - 7 5 K , respectively. The observed magnetic moments for Co atom at room temperature (i.e. μ eff = 5.1 8 and 5.38 BM, respectively) are higher than those expected for a spin-only value for high spin Co(II) ( S = 3 / 2 ), revealing a substantial orbital contribution to the magnetic moment. The negative values of θ are an indication of the presence of antiferromagnetic exchange couplings between the near-neighbors Co(II) ions, within the layers. [Co(C n H 2 n +1PO 3)(H 2O)] ( n = 1 , 2 ) are 2D Ising antiferromagnets at low temperatures.