Structural supramolecular isomers

Scheme 5. Structural supramolecular isomers for self-assembly of T-shaped nodes.

We define structural supramolecular isomers as supramolecular frameworks built from the same molecular components using different supramolecular interactions. Examples of structural supramolecular isomerism in coordination polymers are relatively rare, which probably reflects the requirement of multiple interactive modes being present in a single building-block, ligand, or hydrogen-bonding moiety. 

Both compounds with the general formula [Cu (tetrenH2)]4[W (CN)8]4 10H2O, obtained under different pH conditions, are examples of structural supramolecular isomers. The components of the network remain the same, but they have a different superstructure and hence are effectively different compounds. The preference for one isomer over the other can be attributed to the substitutional reactivity of the [Cu(tetren)] precursor at different pH. Therefore, adjusting the pH of the reaction mixture can be an effective way of controlling the struetural topology of these assemblies. 

Structural supramolecular isomers Isomers where although the components of the network remain the same, a different network is generated. 

The protein folding, notorious for an astronomic number of possible conformations, is only an example of the multiple minima problem, inherently connected to all applications of theory to structural chemistry (isomers, supramolecular structures etc.). The multiple minima problem is also virtually ubiquitous in other sciences, and whenever a mathematical description is used, the situation is encountered more and more often. Despite the complexity of the protein folding, remarkable achievements in the prediction of the 3D structure of globular proteins are possible nowadays. 

Ag(bipy)(N03)]n generates another type of supramolecular isomer for self-assembly of T-shaped components. It self-assembles into linear Ag-bipy chains that cross-link via Ag-Ag bonds. This particular 3D structure has been described as a Lincoln Log type structure and exhibits a threefold level of interpenetration that is open enough to facilitate ion exchange of the loosely bound nitrate anions.47 ... 

Figure 9.26 (a) zig-zag and helical supramolecular isomers of ID chains made up of a linear spacer (light grey) and a cis metal centre (black) (reprinted with permission from Section Key Reference 2001 American Chemical Society), (b) chiral helical structure of [Ni(bpy)(benzoate)2(MeOH)2]. 

In some Instances, more than one structuie may be possible for a given set of molecular components, and such structures can be described as supramolecular isomers For example, three forms of [Co2(N03)4 I,2-Z /5-(4-pyridyI)ethane 3]n ha e been structurally characterized with different network structures This results from the conformational freedom of I,2-Z /y(4-pyridyI)ethane (Fig 4c), which unlike 4,4 -bipy can adopt gauche or anti conformations... 

The term "supramolecular isomerism" was first used by Zaworotko to describe distinct forms of highly related coordination polymer materials. This is complicated by the observation that supramolecular isomerism for a given network system is commonly combined with a variation in guest solvent molecules within the extended structure. Variation of guest molecules within a framework does not, of course, define new supramolecular isomers of the framework if the latter is unchanged. In a recent review, Zaworotko et stated that supramolecular isomerism is closely related to the well-documented subject of polymorphism in crystalline solids." Zaworotko defined supramolecular isomerism in this context as "the existence of more than one type of network superstructure for the same molecular building blocks" and related the phenomenon "to structural isomerism at the molecular level."... 

Fig. 1 Class I supramolecular isomers of [Ag(4-CNpy)2]BF4l in which the flexible Ag(I) coordination geometry leads to three-dimensional diamondoid (a) or two-dimensional (4.4) grid structures.

The complex [Cu(SCN)(dpt)]. [dpt = 2,4-bis(4-pyT-idyl)-1.3.5-triazine] exhibits structural supramolecular isomerism, with the two isomers adopting either a three-dimensional lattice A or a one-dimensional ribbon polymer Isomer A exhibits N3S binding via two dpt... 

Reaction of Co(N03)2 with 4,4 -bipy in the presence of pyridine and benzene templates leads to the formation of a further structural motif.In this case, a three-dimensional lattice (Fig. 5c) is observed which again is a supramolecular Isomer C of the ladder structure. In this structure, the T-shaped connections are arranged such that pairs are oriented at 180 with respect to each other and then at 90 to the adjacent pair of connections. This affords a complicated three-dimensional lattice that is triply interpenetrated, leaving channels that are occupied by guest benzene solvent molecules. 

Fig. 6 Class III supramolecular isomers of [Co2(N03)4(l,2-bis(pyrid-4-yl)ethane)3]. j(guest), (a) isomer exhibiting ligands with both gauche and anti ligand conformations to afford a one-dimensional polymer (b) isomer again containing both gauche and anti ligand conformations, but in this case, forming a two-dimensional bilayer structure. (View this art in color at WWW. dekke r. com.)...

A first study that illustrates the potential of this approach was the preparation of an unprecedented example of genuine supramolecular isomers which exhibits different solid-state reactivity. " The combination of MnCla, NaNCS and frans-l,2-bis(2-pyridyl)ethene (2,2 -bpe) in a mixture of solvents water/ethanol, using two molar metal-bipyridine ratios (1 1 and 1 2), leads to two structurally related 2D networks as single phases based on the same H-bonded metal complex, respectively. 

Supramolecular isomers comprise network structures that differ from one another, but the network and, ideally, the entire crystal have identical chemical compositions (Figure 1). Occasionally, crystals containing different solvent or guest molecules may form. Such systems are not, strictly, supramolecular isomers, but the term may still be of some use in describing the framework that is making up the host structure. ... 

Figure 1 Supramolecular isomers have the same chemical composition but differ in their network structure. (Reproduced from Ref. 5. Royal Society of Chemistry, 2009.)...

While some instances of supramolecular isomerism are immediately and visually apparent, it is often difficult to describe the structures briefly enough to be useful, while still fully enough to be complete. A number of topological approaches have been described over the years as aids in the description of extended structures. Both graph set and network analyses are useful for this purpose. As many supramolecular isomers can be described as 2D or 3D nets, they are amenable to an analysis of their network topology, as described fully in another chapter in this volume (see Network and Graph Set Analysis, Supramolecular Materials Chemistry). ... 

The compound l,3,5-tris(4-cyanobenzoyl)benzene, 2, provides an example of an organic compound which displays both polymorphism and structural isomerism in its crystal forms. Recrystalhzation of 2 from acetone/water gave two concomitant polymorphs (forms A and B), illustrated in Figure 4. Each polymorph exists in a distinct network structure, held together by C-H- O hydrogen bonds. In form A, these generate a honeycomb network while in form B, a ladder network is produced. As both networks contain the identical molecular building blocks, they are supramolecular isomers of each other. 

The authors proposed that a concentration effect is responsible for the formation of these supramolecular isomers. Both isomers exist in the same system, but after total diffusion only 3A remains, suggesting that this is the thermodynamically stable product and that 3B can be considered a kinetic prodnct. The same authors later showed that the ring is the favored isomer when the perchlorate ion is replaced by nitrate or PFe". In the latter case, using AgPFe as starting material produced either the [Ag(oddc)]2 (PF6)2 THF solvate metallacycle or a 2D polycatenated structure with composition [Ag(oddc)2] (PFe), which, although not an isomer of 3A, retains some of its features (Figure 6). 

Luminescence is one such property which has yielded to the study of supramolecular isomers. The absorption and emission spectra of the supramolecular isomers of [Ni(4-bpd)2(NCS)2], 13, where 4-bpd = l,4-bis(4-pyridyl)-2,3-diaza-l,3-butadiene, showed distinct differences which could be attributed to the differing conformations of the bpd ligand in the square-grid framework versus the 6 8 framework structures." Both isomers also... 

FIGURE 3.8 Eight metal-organic supramolecular isomers formed with the flexible ligand TA with transition metals Zn, Cd, Co, Cu, and Mg. All the isomers showed the rigid framework while structure A (TetZB) showed flexible framework towards guest sorption [21, 22, 47], Py = pyrazine, DB = l,4-diazabicyclo[2.2.2]octane, and BP = bipyridine. (The stmctures reproduced from Motkuri et al. in referenced RSC and ACS journals, with permission). 

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