Synthesis and crystal structure of Na7Cu3O8, containing a new type of oxocuprate(III) oligoanion

Na7Cu3O8 was prepared through oxidation of a Na3CuO2/NaCuO mixture (2:1) in dried oxygen at 450 degreesC. Single crystals have been grown by annealing of Na7Cu3O8, in the presence of Na2O2, at 450 degreesC for 2000 h in silver crucibles, which were sealed in glass ampoules under dried At. According to the X-ray analysis of the crystal structure (P (1) over bar, Z = 1, a = 5.5891(2), b = 6.0945(2), c = 7.8890(3) Angstrom, alpha = 110.059(2), beta = 108.669(2), gamma = 90.237(2)degrees) a new Cu3O87- oxocuprate anion, consisting of three edge sharing CuO4 squares, is the prominent structural feature. These anions are aligned parallel to the space diagonal of the unit cell and can be regarded as infinite chains from which every fourth copper atom has been removed. This new representative of an oxocuprate(III) anion gives support to the expectation that the gap between dimeric and infinite edge sharing units of square planar cuprate anions can be closed, in principle.