Sheet network

We are investigating the template formation of 2-D and 3-D metal-di-cyanamide anionic networks, for instance of type Mn(dca)3, by use of [M(N,N)3]n+ cations such as [M(2,2 -bipy)3]2+. A hexagonal sheet network was formed in [Fen(2,2 -bipy)3][Fen(dca)3]2 in which the cations fitted beautifully within the hexagonal windows. The cation remained LS between 4-300 K [66], Attempts to make the Con(2,2 -bipy)32+ analogue unfortunately led to dissociation of dca and bipy and formation of a zig-zag chain structure in the weakly-coupled HS complex [Con(dca)2(2,2 -bipy)2]n. The complex [Fen(propyl-tetrazole)6]2+, which has a very sharp SCO transition [42], unfortunately did not yield a network product. 

In the search for new organic conducting materials, x-ray structure determinations of fused 1,2,5-selenadiazoles and 1,2,5-thiadiazoles have been carried out. Compounds (25a) and (25b) are planar molecules possessing a two-fold axis and mirror plane. Short C=N S and C N Se contacts connect each molecule in the crystal with four neighbors, forming a virtually planar sheet network (Figure 7), The sheets stack to form infinite layers with face-to-face overlapping between... 

Figure 7 Section of the sheet network of the condensed 1,2,5-selenadiazole (25b) showing C=N- Se...

It is useful as a point of departure, to briefly describe the basic crystal lattice common to phyllosilicates. The elementary character is the SiO tetrahedral linkage of an essentially two-dimensional, hexagonally symmetric, network. One side of this "sheet network is coordinated with other cation-oxygen complexes joined by an important component of covalent bonding while the other is coordinated by essentially ionic bonding or van der Waals type bonds. The key to phyllosilicate structures is the oxygen network which determines the shape and extent of the structure. 

Figure 6 Room-temperature crystal packing of 0-(BEDT-TTF)2I3 (a) loose intrastack packing of BEDT-TTF molecules (ds s > 3.60 A) and I3 anions (b) corrugated sheet network of short (ds s < 3.60 A) interstack S—S interactions. Only S atoms of the BEDT-TTF molecules are given. (From Ref. 64.)...

Possibly the most important structural feature that has been revealed from crystallographic studies performed at two temperatures (298 and 125 K) is the existence of an infinite sheet network (32) of Se-Se interactions as shown in Fig. 6. At room temperature the intermolecular intra- and inferstack Se-Se distances are all similar and have values of 3.9-4.9 A, compared to the van der Waals radius sum for the selenium atom (52) of 4.0 A. However, as the temperature is lowered (298 - 125 K) rather unusual changes occur, viz. the ratio of the decrease in the interstack mfrastack Se-Se distances is not unity but is approximately 2 1 (32, 40). Thus, the distances between the chains shown in Fig. 6 decrease, on the average, by twice as much as the distances between TMTSF molecules in each stack. This most certainly leads to increased interchain bonding and electronic delocalization through the selenium atom network as the temperature is decreased (42). 

Fig. 6. Representation of the selenium atom infinite sheet network in (TMTSF)2X salts. This network is the main pathway for electrical conduction. Lines connect selenium atoms that are closer than the van der Waals radius sum (Se Se) of 4.0 A. Full lines show in.trasta.ck, broken lines, inter stack distances dltds, and d9.

However, significant differences occur between the (TMTSF)2X and (ET)2X structures which are important to point out. Compared to the TMTSF salts the ET donor molecules are far from planar and do not stack in the same zig-zag array as shown in Fig. 5, but clearly in a more complicated array. And a striking difference is observed between the sulfur atom corrugated sheet network (56) of (ET)2X (X = Re04- or Br04 ) (Fig. 13) and the selenium atom sheet network in (TMTSF)2X (Fig. 6). 

Fig. 13. A stereoview of the short (< 3.60 A) intermolecular interstack S-S interactions in (ET)2Re04 and (ET)2Br04 which form a two-dimensional corrugated sheet network (56). This network, which is the principal pathway for electrical conduction, is much different from that observed in (TMTSF)2X salts, but similar to the network of interstack S-S interactions observed in ET2(C104)(TCE)0 5 (59).

Fig. 15. Stereoview of the corrugated sheet network of short inferstack S S interactions (faint lines) between nonplanar and nonparallel ET molecules in / -(ET)2I3 at 125 K (90% ellipsoids). For clarity, only the sulfur atoms of ET are shown.

Fig. 17. Stereoview of the novel sandwich or layered structure of / -(ET)2IBr2 composed of alternating two-dimensional sheets of linear (Br-I-Br) anions, between which a corrugated sheet network of short interstack S S interactions is inserted. Only the S atoms of the ET molecules are shown in the network and light lines indicate the interstack (ds s < 3.60 A) interactions. The —CH2 groups at ET protrude from both ends of the molecule (directly out of the plane of the page) and grasp the X3 " anions in a pincer hold. Therefore, by varying the length of the X3 " anion the interstack S S distances can be directly altered (108).

Figure 4.28 Common two-dimensional sheet networks (a) a (4,4)-net (or 4". 6 in Schlafli notation) (b and c) (6,3)-nets (Schlafli, 6 ) using differently shaped nodes.

Figure 9.3 The infinite sheet network with 12-member rings bound together vertically by hydrogen bonds between the carboxyl gorups attached to each sheet [36].

Fig. 20 (A) Molecular structure of polymeric [NaMg(CH2SiMe3)3loo 27 showing the contents of the asymmetric unit. (B) Section of the two-dimensional sheet network of 27. (C) Section of the extended polymeric framework of [NaMg(HMDS)2("Bu)]oo 28.

Subclasses also exist and the most useful ones are probably the neso-, soro-, cyclo-, ino-, phyllo-, and tecto-silicates which give some indication of the state of polymerization of SiOt tetrahedra into rings, chains, bands, sheets, networks, and so forth. 

In analogy to the early work by Bailar, iron polymers with linear or two-dimensional sheet network stmctures such as 12 have been postulated to form when Fe + is reacted with 2,5-dihydroxybenzoquinone in basic media. - Each iron center is also coordinated to two water molecules. 

Stimulated by an interest in the sheet network structure proposed for the 1 1 complex between melamine and isocyanuric acid (Figure 4) (45), we chose to study cocrystals of derivatives of melamine (M) and barbituric acid (B). These components often form 1 1 cocrystals (eq 1). These crystals are interesting for two reasons. First,... 

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