Application of EXAFS spectroscopy to probe structural properties of solid inclusion compounds containing halogenoalkane guest molecules within the catena-[(1,2-diaminopropane)cadmium(II) tetra-µ-cyanonickelate(II)] host structure

Abstract

Nickel K-edge EXAFS spectroscopy has been used to investigate, at 295 K and ca. 120 K, local structural properties of the inclusion compounds containing halogenoalkane guest molecules (2-chlorobutane, 2,3-dichlorobutane, 1,2-dichloroethane, 2-bromobutane, 2,3-dibromobutane and 1,2-dibromoethane) within the catena-[(1,2-diaminopropane)cadmium(II) tetra-µ-cyanonickelate(II)] host structure. For these inclusion compounds there are ambiguities in the interpretation of atomic positions ‘located’ for the guest molecules by diffraction-based approaches, and the work reported here has allowed an assessment of the scope of EXAFS spectroscopy for probing host–guest interaction in such situations. The optimum approach for handling the host structure within the EXAFS data analysis has also been assessed.

The only atoms of the host structure that make a significant contribution to the nickel K-edge EXAFS spectrum are the carbon and nitrogen atoms of the Ni(CN)₄ unit and the cadmium atoms, with the C—N—Cd angle refined in multiple scattering calculations. Carbon and nitrogen atoms in the host structure corresponding to Ni⋯C and Ni⋯N distances in the range 3.5–5.0 Å do not contribute significantly to the nickel K-edge EXAFS spectrum. For all the guest molecules studied (with the exception of 1,2-dibromoethane), the halogen atoms contribute significantly to the EXAFS spectrum, with physically sensible Ni⋯Cl and Ni⋯Br distances refined; this supports the suggestion that there is some degree of ordering of the guest molecules relative to the host structure in these inclusion compounds.