Polymorphic supramolecular isomers

We define polymorphic supramolecular isomers as supramolecular frameworks built from the same molecular components using the same interactions, i.e., supramolecular synthons (M-L, H-bonding, etc.), in all isomers giving rise to the same localized geometries but different framework topologies. To illustrate this terminology, we outline three examples of this class of isomerism. 

Two distinct polymorphic supramolecular isomers of [Ag(4-CNpy)2]Bp4 (4-CNpy = 4-cyanopyridine) have been reported. In both isomers, each Ag(I) cation is coordinated by two pyridyl and two nitrile donors. In the first isomer, the Ag(I) cation adopts a flattened tetrahedral environment in which both pyridyl and nitrile donors adopt conventional Ag—W bond lengths, [Ag-N(pyr), 2.270(6) A Ag-N(CN), 2.350(10) A]. Propagation of the polymer via the 4-cyanopyridine ligands from these tetrahedral nodes results in the formation of a three-dimensional diamondoid assay (Fig. la), which exhibits fourfold interpenetration. 

Figure 2 An example of molecular isomers (a), here the bridged and terminal forms of [Ir4(SCH2)3] [26] and a pair of supramolecular isomers (b), the orthorhombic a-Sg and the monoclinic y-Sg polymorphic modifications of crystalline sulfur [27].

Control over supramolecular isomers and polymorphs lies at the very heart of the concept of crystal engineering (i.e. design of solids). However, there is presently very little understanding concerning even the existence of supramolecular isomers, never mind how to control them. 

Molecular crystals can. in fact, be viewed as periodical supermolecules, in which a large number of molecules or ions interact via noncovalent interactions, generating collective physical and chemical properties.According to the same reasoning, polymorphic modifications of a molecular crystal, i.e.. different crystals of the same constituent molecule/ions, can be regarded as periodical supramolecular isomers, differing in distribution of the noncovalent bonds. 

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."... 

Supramolecular Materials Chemistry). Supramolecular isomerism is related to polymorphism in that, in both cases, the chemical constituents of two or more crystalline substances are identical. Indeed, polymorphs may generally be regarded as being supramolecular isomers of one another, although the reverse is not always true." Polymorphism of molecular crystals, in particular, remains an important phenomenon, especially in pharmaceuticals. However, although reports of polymorphs have increased significantly... 

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. 

Reports of conformational supramolecular isomers of organic materials include that of the compound 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile, 5, which has at least six polymorphs. The main difference in the six phases is the variation in the torsion angle between the thiophene and the o-nitroaniline moieties. Another example involves the two components C-methylcalix[4]resorcin-arene (6) and 4,4 -bipyridine (bipy), which can form three supramolecular isomers (7) in the presence of different... 

Different crystalline forms containing the same molecules or ions are called polymorphs. Polymorphism is of major industrial importance today.With the exception of review articles published by some of us in 1999/ the subject of organometallic polymorphism has never been reviewed. The investigation of crystal polymorphism requires a crystal-engineering approach the formation of different crystal forms implies that different sets of supramolecular interactions are established between the same building blocks. Under this viewpoint, therefore, polymorphs are crystal isomers. ... 

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