Adamantane-like structures interpenetration

Figure 20.4.5 shows the crystal structure of [Cu l,4-C6H4(CN)2 2]BF4. The nodes are the four-connected Cu atoms, each being coordinated tetrahedrally by N=C-C6H4-C=N ligands as rods. The adamantane-like structure unit is shown in Fig. 20.4.5(a), and repetition of such units along a twofold axis leads to fivefold interpenetration. The remaining space is filled by the BF ions. 

Left) Two interpenetrating networks in the crystal structure of ice-VII. (Right) Two views of an adamantane-like structural unit. 

Cadmium cyanide, CdCCN), is analogous to Si02 with respect to the AB2 composition, the tetrahedral confiugration of A, the bridging behavior of B between a pair of A atoms, and the ability to build a three-dimensional framework in which cavities of molecular scale are formed. Cadmium caynide itself crystallizes in a cubic system of the anticuprite type, in which two identical fr-cristobalite-like frameworks interpenetrate each other without any cross-connection the cavity formed in one framework is filled by the other. When we replace one of the frameworks by appropriate guest molecules such as those of CCl, CCl CH, etc., we may obtain a novel clathrate structure with an adamantane-like cavity, as shown in Fig. 1 [1], Our results including those recently obtained are summarized in Table 1. 

Cd(CN)2, is isomorphous. To prevent the interpenetration, relatively large species must be hosted into the diamondoid network. Indeed, the mixed metal complex NMe4[Cu Zn CN)4] shows no interpenetration because the tetra-methylammonium cations, whose size fits to that of the adamantane-like cavity, occupy half of these cavities (both Cu and Zn ions are tetracoordinated). The interpenetration of neutral M(CN)2 nets (M = Zn, Cd) can be hindered by hosting neutral molecules, structurally similar to NMe4. The solution of this problem is obvious and elegant The crystallization of Cd(CN)2 in the presence of Me4C leads to a single diamond-like net. ... 

Figure 4. (a) Two interpenetrating adamantane units in the structure of Zn(CN)2. Circles represent Zn and connections between Zn atoms are provided by linear bridging cyanides, (b) A single cyclohexane-like Zn6(CN)6 ring catenated by six distinguishable rings. 

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