Serendipity and Design in the Generation of New Coordination Polymers:  An Extensive Series of Highly Symmetrical Guanidinium-Templated, Carbonate-Based Networks with the Sodalite Topology

The serendipitous discovery of a 3D [Cu(CO₃)₂^2-]_n network with the topology of the 4²6⁴ sodalite net in [Cu₆(CO₃)₁₂(CH₆N₃)₈]·K₄·8H₂O paved the way for the deliberate engineering of an extensive series of structurally related guanidinium-templated metal carbonates of composition [M₆(CO₃)₁₂(CH₆N₃)₈]Na₃[N(CH₃)₄]·xH₂O, where the divalent metal M in the framework may be Mg, Mn, Fe, Co, Ni, Cu, Zn, or Cd. A closely related crystalline material with a [Ca(CO₃)₂^2-]_n sodalite-like framework, but containing K⁺ rather than Na⁺, of composition [Ca₆(CO₃)₁₂(CH₆N₃)₈]K₃[N(CH₃)₄]·3H₂O was also isolated. All of these compounds were obtained under the simplest possible conditions from aqueous solution at room temperature, and their structures were determined by single-crystal X-ray diffraction. Pairs of guanidinium cations are associated with the hexagonal windows of the sodalite cages, alkali-metal cations are associated with their square windows, and N(CH₃)₄⁺ ions are located at their centers. Structures fall into two classes depending on the metal, M^II, in the framework. One type, the BC type (Im3̄m), comprising the compounds for which M²⁺ = Ca²⁺, Mn²⁺, Cu²⁺, and Cd²⁺, has a body-centered cubic unit cell, while the second type, the FC type (Fd3̄c), for which M²⁺ = Mg²⁺, Fe²⁺, Co²⁺, Ni²⁺, and Zn²⁺, has a face-centered cubic unit cell with edges on the order of twice those of the BC structural type. The metal M in the BC structures has four close carbonate oxygen donors and four other more distant ones, while M in the FC structures has an octahedral environment consisting of two bidentate chelating carbonate ligands and two cis monodentate carbonate ligands.