EFFECTS OF TEMPERATURE ON THE CRYSTAL AND MOLECULAR-STRUCTURE OF THE MIXED-VALENCE LINEAR-CHAIN [PT(EN)2][PT(EN)2X2](CLO4)4 (X = CL, BR)

The crystal structures of the pure monoclinic and orthorhombic phases of [Pt(en)2][Pt(en)2Cl2](ClO4)4 (PtCl) and [Pt(en)2][Pt(en)2Br2](ClO4)4 (PtBr), with en = 1,2-diaminoethane, were determined at several temperatures. The monoclinic phase of PtCl has space group P2(1)/m, with a = 7.972(3) angstrom, b = 10.874(5) angstrom, c = 8.500(4) angstrom, beta = 108.91(4)-degrees, Z = 2, d = 2.619 g/cm3, and T = -20-degrees-C. The monoclinic phase of PtBr crystallizes in space group P2(1)/m, with a = 7.959(2) angstrom, b = 10.957(2) angstrom, c = 8.529(2) angstrom, beta = 109.32(3)-degrees, Z = 2, d = 2.811 g/CM3, and T = 25-degrees-C. The orthorhombic phase of PtCl has space group Ibam, with a = 13.594(4) angstrom, b = 9.657(5) angstrom, c = 10.855(4) angstrom, Z = 4, d = 2.562 g/cm3, and T = 25-degrees-C. The orthorhombic phase of PtBr crystallizes in space group Ibam, with a = 13.671(5) angstrom, b = 9.680(3) angstrom, c = 10.994(3) angstrom, Z = 4, d = 2.712 g/CM3, and T = 40-degrees-C. The monoclinic phases of PtCl and PtBr were also determined at T = -70 and -93-degrees-C, respectively. The lattice constants for PtCl(-70-degrees-C) are a = 7.947(4) angstrom, b = 10.871(5) angstrom, c = 8.445(5) angstrom, and beta = 108.79(4)-degrees, and the lattice constants for PtBr(-93-degrees-C) are a = 7.912(2) angstrom, b = 10.900(2) angstrom, c = 8.387(2) angstrom, and beta = 109.02(3)-degrees. The corresponding PtCl and PtBr phases are isostructural. These systems contain chains composed of alternating, octahedral Pt(en)2X2 and square planar (4 + 2) Pt(en)2 species; the axial halides of the octahedral species occupy semicoordinate axial sites of the square planar species. This motif results in alternating Pt-X distances along the chain, resulting in a charge density wave (CDW). The chains are linked together via a weak hydrogen-bonding network involving the NH2 hydrogens of the ethylenediamine ligands and the perchlorate counterions. The major structural differences between the phases are related to changes in the hydrogen bonding brought on by increased thermal motions of the perchlorate ions as the temperature is raised. These changes in hydrogen bonding alter crystal packing and incur subtle changes in PtX chain geometry and CDW strength. Raman spectroscopy measurements show a direct correlation between the frequency of the fundamental X-Pt-X stretch and the strength of the CDW.