On the Oxidation of Octamethylenetetrathiafulvalene by CuBr2 - Synthesis, Crystal Structure and Magnetic Properties of (OMTTF)2-[Cu4Br10]

The reaction of octamethylenetetrathiafulvalene (OMTTF) with excess CuBr2 in tetrahydrofurane/acetonitrile yields black (OMTTF)(2)[Cu4Br10] (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 CuBr4 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 approximate to 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 (OMTTF2+)(2)[(Cu+)(2)(Cu2+)(2)Br-10](4-). The antiferromagnetism is explained by the Coupling of the spins of two Cu2+ 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 C3S2, ring moieties of 15.3 degrees, which is in accordance to the dicationic oxidation state.