Interpenetrating 6-connected nets

Interpenetrating 3D Nets Each Containing Two Types of Nodes with Different Connectivities... 

Figure 34. Two interpenetrating 3,5-connected nets in the structure of [Ag(tcm)(dabco>]. Circles in order of decreasing size represent Ag, N, and C, respectively.

We have supposed that in any system all the points together form one connected net, that is, it is possible to travel along links from any point to any other. There are some very interesting structures in which this is not possible, namely, those consisting of two or more interpenetrating (interlocking) structures (Fig. 3.17). 

Luckily, wc do not have to deal with all those the number of known molecular based four-connected nets is much lower. In the study by Ockwigel al. 28 nets were found containing tetrahedral or approximately tetrahedral nodes, three containing tetrahedral and square planar nodes, and four containing square planar nodes, [6] and in their investigation of interpenetration Blatov el al. found 14 different four-connected nets [7J. 

Figure 7.24 The teb net is unique among the four-connected nets in that the smallest ring is greater than a 7-ring. It is also self-interpenetrating.

A number of additional compounds with three- and four-connected nets can be found in the recent literature, [19-22] and a number of interpenetrating structures have been listed by Proserpio et al. [14] Recently Wang et al. reported on the deliberate self-assembly of triangles, squares, and tetrahedral [23]. The RCSR database currently includes some 30 different three- and four-connected nets [24]. The Ge3N4 net described by Wells will probably also be made in the future. 

The conclusion we can draw from this is that if a net is self dual, then it should be easy for one or more identical nets to interpenetrate. The concept of using dual nets in the analysis of interpenetration is still rather recent, but seems promising, see for example the discussion of a number of four-connected nets in a recent article by Delgado Friedrichs, O Keeffe and Yaghi [21],... 

A useful concept in the consideration of framework structures and, in particular, in the consideration of their various modes of interpenetration, is that of a net. A net is a collection of points with some clearly defined connectivity. We shall be concerned with infinite nets and shall refer to the interconnected points as nodes and to the connecting units either as connecting rods or simply as connections or rods. For a fuller account of nets the reader is referred to monographs by Wells [2]. 

Some general points about interpenetrating networks can be illustrated by the example of Zn(CN)2, which was structurally characterized over half a century ago [3]. It consists of two independent diamond-like nets with the 66-a topology, in which zinc provides the tetrahedral nodes and cyanide provides linear connections between nodes. These two equivalent but independent nets interpenetrate as shown in Figure 4, such that the nodes of one net are located at the centers of the... 

Figure 10. Parallel interpenetrating (4,4) nets in the structure of [Cd(4-pic)2 Ag(CN)2 2] (4-pic) where 4-pic, i.e. 4-methylpyridine, is pendant and is omitted. Cd therefore is the apparently 4-connecting (actually 6-coordinate) center shown here. Each Cd is connected to four others by bridging Ag(CN)I- Circles in order of decreasing size represent Cd, Ag and CN.

Figure 14. Schematic representation of the two parallel interpenetrating (4,4) nets in [Mn43](C104)2 showing the polyrotaxane-polycate-nane associations. 6-connecting nodes are provided by 6-coordinate Mn and node-to-node connections by ligand 4.

Figure 17. Inclined interpenetration (actually perpendicular) of two (4,4) nets in [M(4,4 -bipy)2(H20)2](SiF6). 4-Connecting nodes are provided by M which has two pendant trans H20 ligands not shown here and connections between nodes are provided by 4,4 -bipy.

Figure 25. (a) Three interpenetrating (10,3)-b nets with T-shaped 3-connecting nodes in the structure of [Ag(bipy)]N03. Circles represent Ag atoms. Long connections are via bridging bipy units and short connections are direct Ag... Ag interactions, (b) An idealized version of the mode of interpenetration of three (10,3)-b nets, of the type seen in [Ag(bipy)]N03. 

In Na[Ti2(PS4)3] the Tirv centers act as 3-connecting nodes, being chelated by three bidentate PS4- ligands which in turn act as /i2-ligands [29], generating an extended net with the (8,3)-c topology [1]. Two of these then interpenetrate [1]. 

Quartz, which is chiral, has a 6482 topology (Wells uses the designation 6482-b, reserving 6482-a for the more symmetrical NbO net discussed below). In the structures of [M Au(CN)2 2], M is Zn or Co, M provides the 4-connected centers and Au(CN)2 acts as an approximately linear bridging group to generate quartz-like nets [31]. Six such nets, all of the same hand, then interpenetrate [31]. 

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