On the Oxidation of Octamethylenetetrathiafulvalene by CuBr 2 – Synthesis, Crystal Structure and Magnetic Properties of (OMTTF) 2 [Cu 4 Br 10 ]

The reaction of octamethylenetetrathiafulvalene (OMTTF) with excess CuBr 2 in tetrahydrofurane/acetonitrile yields black (OMTTF) 2 [Cu 4 Br 10 ] ( 1 ). The crystal structure determination shows the presence of OMTTF cations and tetranuclear bromidocuprate anions. The novel anion consists of four edge and corner sharing CuBr 4 tetrahedra, which are connected to a ring. The assignment of the ionic charges and oxidation states for the copper atoms is supported by the magnetic properties. 1 is antiferromagnetic with T N ≈ 30 K. The magnetic moment reaches 2.54 B.M., which indicates, together with the Curie–Weiss constant of –35 K, a coupling of the paramagnetic spins over the whole temperature region. The ionic charges of the salt‐like compound 1 are therefore (OMTTF 2+ ) 2 [(Cu + ) 2 (Cu 2+ ) 2 Br 10 ] 4– . The antiferromagnetism is explained by the coupling of the spins of two Cu 2+ ions in the anion with an exchange constant of J = –18 cm –1 . The Cu I and Cu II atoms are clearly distinguishable in the mixed valent anion. The OMTTF cation is not planar but exhibits an interplanar angle between the two central C 3 S 2 ring moieties of 15.3°, which is in accordance to the dicationic oxidation state.