Structure and stability of a clathrate of bis(dibenzoylmethanato)-dipyridine-nickel(II) with acetone (1:2)

Abstract

An inclusion compound of constant composition [Ni(DBM)2Py2]·2(CH3COCH3) is synthesized, Ni = Ni(II), DBM is dibenzoylmethanate-anion (C6H5CO)2CH−, Py is pyridine. The compound and its dissociation products are examined by structural and thermal analysis, as well as vapor pressure measurements. The crystal structure of the clathrate is solved in the monoclinic space group P21/n (temperature 173 K, a = 11.8617(9) A, b = 10.0096(6) A, c = 17.2895(9) A, β = 96.72(1)°, V = 2038.7(2) A3, Z = 2, final R 1 = 0.032). The host molecule [Ni(DBM)2Py2] is an uncharged octahedral complex, the central Ni(II) atom being surrounded by two DBM-anions in the equatorial plane and two terminal pyridines in the axial positions. Pairs of guest molecules are located in the voids of the molecular crystal. Calculated packing coefficients of the clathrate and the stable form of the host complex are 0.685(2) and 0.668(1) at 173 K, respectively. Isochoric melting of the clathrate has incongruent nature and occurs at 58–60°C to yield the solid phase of the host complex, while in air the complex decomposes in gaseous acetone and a metastable form of the host (apohost). The collapse of the metastable form of [Ni(DBM)2Py2] to the stable one takes place at 131°C with the release of 11 kJ/mol. The equilibrium vapor pressure of acetone over the clathrate is measured with the spoon gauge technique in a temperature interval from 292 K to 310 K (at 298 K the pressure is 0.48P 0, P 0 is the saturated vapor pressure of liquid acetone at this temperature). The experimental dependence logP − 1/T is used to derive thermodynamic parameters of the process of clathrate dissociation 1/2[Ni(DBM)2Py2]· 2(CH3COCH3)(s) = 1/2[Ni(DBM)2Py2](s) + CH3COCH3(gas): ΔH av 0 = 53±3 kJ/mol, ΔS av 0 = 160±10 J/(mol·K), ΔG 298 0 = 4.74±0.07 kJ/mol.