Structure and Ion Exchange Properties of a New Cobalt Borate with a Tunnel Structure “Templated” by Na⁺

The new anhydrous borate Na2Co2B12O21 has been synthesized by flux methods and studied by single-crystal X-ray diffraction (space group I2/a with a = 17.1447(15) Å, b = 4.5530(5) Å, c = 19.4408(15) Å, β = 103.212(5)°, V = 1477.4(2) Å3, Z = 4). Refinement of its structure reveals it is the first metaborate exhibiting a tunnel network, with internal dimensions of 4.5 × 8.8 Å2. Further single-crystal diffraction studies show that the Na+ ions within the tunnels are exchangeable with Li+ along with the absorption of water molecules to form Li2(H2O)2Co2B12O21, making this compound a unique non-siliceous zeotype.