SYNTHESIS AND CHARACTERIZATION OF THE PSEUDO-1-D MIXED-VALENCE (ET4-(N)ME(N)N)CU2X4 SALTS - PINNED CHARGE-DENSITY WAVE SYSTEMS EXHIBITING INTERVALENCE CHARGE-TRANSFER

A series of mixed-valence Cu(I)/Cu(II) linear chain compounds exhibiting pinned charge density waves have been synthesized. The compounds have stoichiometry ACuICuIIX4 with X = Cl or Br and A = tetraethylammonium (TEA), triethylmethylammonium (TEM), or diethyldimethylammonium (DEM). The crystal structures of thc bromide analogues have been determined. The (TEM)Cu2Br4 salt is tetragonal, with space group P4(2)/ncm, a = 10.812 (1) angstrom, c = 13.234 (2) angstrom, V = 1546.9 (4) angstrom3, Z = 4, d(x) = 2.42 g/cm3, and R = 0.0647. The (DEM)Cu2Br4 salt is also tetragonal, with space group P4(2)/ncm, a = 10.523 (1) angstrom, c = 13.1630 (9) angstrom, V = 1457.5 (4) angstrom3, Z = 4, d(x) = 2.50 g/cm3, and R = 0.0974. The (TEA)Cu2Br4 salt is orthorhombic, with space group P2(1)2(1)2(1), a = 11.084 (2) angstrom, b = 19.696 (3) angstrom, c = 21.847 (2) angstrom, V = 4770 (1) angstrom3, Z = 12, d(x) = 2.41 g/cm3, and R = 0.0358. These structures reveal infinite chains of alternating Cu(I)X4(3-) and Cu(II)X4(2-) tetrahedra (distorted) sharing edges, thc chains being effectively isolated by bulky tetraalkylammonium cations. Although the valence states appear to be localized, with a 0.1 angstrom difference in thc Cu(I)-X and Cu(II)-X bond lengths, several characterization techniques indicate Robin and Day class II behavior. The data also support a Peierls 4k(f) distorted charge density wave state for these chains. All salts show an intervalence charge-transfer band in the visible region of the spectrum. Conductivity measurements indicate semiconductive behavior. Temperature-dependent magnetic susceptibility measurements on (TEA)Cu2Br4 reveal weak antiferromagnetic coupling, which is attributed to short Br...Br contacts between Cu(II)X4(2-) species within the chains. The above physical properties are rationalized in light of previous EHMO band calculations.