Correlation between thermal stability and molecular and crystal structure in a series of potassium–dibenzo-18-crown-6 solid compounds

Abstract

The compounds [K(NO₃)L](L = 6,7,9,10,17,18,20,21-octahydrodibenzo[b,k][1,4,7,10,13,16]-hexaoxacyclo-octadecin) and [K(Hpht)L](Hpht = hydrogenphthalate) have been studied by differential scanning calorimetry and single-crystal X-ray analyses. Both the complexes undergo, upon melting, a partial dissociation into their components. The enthalpy of melting of [K(NO₃)L] is fairly high (14.6 kcal mol^–1) and equal, within experimental error, to the enthalpy of melting of the free ligand, L. The entropy of melting of the complex reflects an increased stiffening in the complexed crown ether molecule with respect to the free ligand. Crystals of [K(NO₃)L] and of [K(Hpht)L] are monoclinic, space group P2₁/c with a= 15.054 4(7), b= 20.575(2), c= 14.259(l)Å, β= 97.655(5)°, Z= 8; and orthorhombic, space group P2₁2₁2₁ with a= 15.990(2), b= 17.915(1), c= 9.314(1)Å, Z= 4 respectively. Refinement of the atomic parameters by least squares gave a final R factor of 0.0544 (R′= 0.0590) and of 0.0622 (R′= 0.0735) respectively. Both the structures consist of independent complex molecules {two in an asymmetric unit for [K(NO₃)L]} in which the potassium ion is surrounded by eight oxygen donor atoms, six from L and two from the anions. The nitrate ion acts as a symmetrically bonded bidentate ligand [K ⋯ O 2.832(4), 2.836(4) and 2.815(4), 2.790(4)Å] while Hpht is asymmetrically bonded [K ⋯ O 2.729(9), 3.168(8)Å]. The potassium ions are pulled 0.631(1) and 0.465(1)Å from the least-squares plane through the six oxygen donors of L, towards the nitrate ion, and 0.592 Å toward the hydrogenphthalate ion. The comparison of these complexes with the NCS derivative shows the existence of a correlation between their thermal stability and the displacement of the potassium ion from the donor ring.