Structural, spectroscopic, and electron paramagnetic resonance studies on two solid phases of bis-dipropylammonium tetrachlorocuprate(II) (DPACC)

Abstract

Bis-dipropylammonium tetrachlorocuprate(II) exists in two known crystalline phases. Below 330 K (β phase) the crystal is orthorhombic Pbca with cell constants (at 293(1) K) a = 13.029(2) Å, b = 15.252(3) Å, c = 20.997(2) Å, V = 4175.5(9) Å3 The crystal undergoes a phase transition at 330 K to the α phase, which is monoclinic P21/n with cell constants (at 342(1) K) a = 13.277(4) Å, b = 10.280(3) Å, c = 16.362(13) Å, β = 104.31(5)°, V = 2164(2) Å3. The β phase consists of discrete, distorted tetrahedral CuCl42− anions, which are surrounded by dipropylammonium cations. Hydrogen bonding between the anions and cations results in a two-dimensional network of linked anions coplanar with the c-glide plane of the unit cell. The terminal carbon of one of the dipropylammonium chains assumes the higher energy gauche conformation while all other carbon and nitrogen atoms are found in the anti conformation. The local geometry of the α phase is very similar to that of the β phase. Powder electron paramagnetic resonance measurements in both phases confirm the persistence of the β phase anion geometry in the α phase. The disorder and increased thermal motion of the alkyl groups of the cation account for observed differences between the crystal structures of the two phases. This phase transition is similar to those found in long chain monoalkylammonium metal halide layer compounds that involve "melting" of the alkyl chain. Near infrared – visible spectra at 77 and 295 K show two distinct peaks at 12 700 and 9800 cm−1 in the first case and a very broad band maximizing at 11 200 cm−1 in the latter case. The highest-energy d-d transition correlates well with those of other CuCl42− anions of varying tetragonal distortion from tetrahedral geometry. The present data are used in conjunction with earlier data to develop the correlation between the highest-energy d-d transition and the distortion from tetrahedral symmetry.