Novel ET-Coordinated Copper(I) Complexes: Syntheses, Structures, and Physical Properties (ET = BEDT-TTF = Bis(ethylenedithio)tetrathiafulvalene)

New molecular charge-transfer complexes of bis(ethylenedithio)tetrathiafulvalene (ET), (ET)Cu2Br4 (1), (ET)2Cu6Br10 (2), (ET)2[Cu4Br6ET] (3), (ET)2Cu2Br4 (4), (ET)2Cu3Br7(H2O) (5), and (ET)2Cu6Br10(H2O)2 (6), have been synthesized
by diffusing reaction of ET and CuIIBr2. Their crystal structures and physical properties have been investigated.
X-ray analyses revealed that ET molecules coordinated to the copper ions with the sulfur atoms of the ethylenedithio
groups in all compounds. The Cu−S distances are found in the range 2.268(5)−2.417(8) Å, being close to the
typical CuI−S coordination bond distances. Strong d−π interactions between d-electrons of the copper ions and
π-electrons of the ET molecules can be expected through the Cu−S coordination bonds. ET molecules behave as
trans-bidentate ligands bonding to two different copper ions in 1 and 3, as cis-bidentate ligands in 2, 5, and 6, and
as monodentate ligands in 4. In the crystal structure of 3, there are two types of ET molecules in the crystal
structure, where the first type is a trans-bidentate ligand and the second forms a stacking structure by itself.
Compounds 1, 2, 4, and 6 show semiconducting behavior down to low temperature (σRT = 1.6 × 10-2 S cm-1 and
Ea = 122 meV for 1, σRT = 2.1 S cm-1 and Ea = 21 meV for 2, σRT = 5.4 × 10-4 S cm-1 and Ea = 239 meV
for 4, and σRT = 5.1 × 10-2 S cm-1 and Ea = 207 meV for 6). On the other hand, in 3, a metal-like region is
observed along the b-axis and c-axis, due to the contribution of stacked ET molecules, and a metal−semiconductor
transition occurs at 280 and 270 K, respectively. Also, 5 exhibits metallic conductivity in the temperature ranges
240−300 and 200−300 K along the b-axis and c-axis, respectively, despite the oxidation state of ET with 2+.