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 °C. Single crystals have been grown by annealing of Na₇Cu₃O₈, in the presence of Na₂O₂, at 450 °C for 2000 h in silver crucibles, which were sealed in glass ampoules under dried Ar. According to the X-ray analysis of the crystal structure ( , Z = 1, a = 5.5891(2), b = 6.0945(2), c = 7.8890(3) Å, α = 110.059(2), β = 108.669(2), γ = 90.237(2)°) a new Cu₃O₈^7- 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.