**2. Types and borders: from organic to metal–organic**

During the early stages of polymorphism, the lack of crystallographic tools conditioned the understanding and therefore, classification of the different polymorphic forms, but the exponential advances in this field provided perhaps too many concurrent data to bring it together and form a unique mindset. It was not until 1965 when McCrone [17] gathered the knowledge hitherto and set a more excluding definition of polymorphism. The inclusion or not of hydrated forms and solvates have been discussed since the rise of polymorphism, but be that as it may, he ventured to propose their exclusion from polymorphic forms and to avoid the use of *pseudopolymorphism* to define them. This assertion was underpinned by Bernstein [32] in 2002, even if actually it is a wide spread term, probably because of its acceptance by the pharmaceutical industry [43] from its regulatory and patentability point of view. Since the aim of this chapter is not about *pseudopolymorphs* we therefore, decided to exclude this term to refer to solvates and hydrates, which can lead to undesirable misunderstandings. In 2012, Desiraju, Karpinski, Thaper and Zaworotko [44] defined *cocrystals* as *solids that are crystalline single-phase materials composed of two or more different molecular and/or ionic compounds generally in a stoichiometric ratio which are neither solvates nor simple salts*. As well as *salts* that are *any of numerous compounds that result from replacement of part or all of the acid hydrogen of an acid by a metal or a radical acting like a metal;* 

*an ionic or electrovalent crystalline solid*. Instead, *solvates* have one component, commonly a solvent, which is liquid by itself and *hydrates* are a particular case of *solvates*, containing water as solvent. A deeper discussion on *pseudopolymorphism* can be found in references [45–47].

Until the beginning of the twenty-first century, the polymorphs' classification was settled from the perspective of crystal packing forces and introduced *conformational* and *packing polymorphism* to sort the examples hitherto. *Coordination polymers* are infinite repeating coordination entities composed of organic molecules serving as linkers and metal ions as nodes [48]. Their main classification is based on dimensionality which is the number of directions in which the array is extended. Therefore, they can be divided into one-, two- or three-dimensional. The rise of this field befell in 1961 when Bailar [49] firstly introduced the term *coordination polymers*. Distinction between polymorphs and other forms as solvates, hydrates or cocrystals in organic structures was well defined, but in the case of metal–organic structures, in particular *coordination polymers,* this classification was not sufficient.

Numerous structures which were not truly polymorphs (solvent molecules were present in the lattice), but neither were solvates, started to rise at the end of the twentieth century. This new domain required the borders to be clarified and established. Yet, these rigid structures were thought to be less disposed to suffer structural variations but some examples were gathered in works of Janiak [50] with Zn(II) poly(pyrazolyl)borates, Ciani [39] with Ag(I) and 4-cyanopyridine, Zaworotko [41, 51] with Co(II) and pyridyl containing linkers, Ripmeester [52] with Cu(II) and a diketone and Rogers [40] with Hg(II) and tetrapyridylporphyrines. Thus, *supramolecular isomerism* was invoked to merge those different types of structures (mainly coordination polymers) assembled from identical building blocks. In this impasse, Zaworotko [42] strongly contributed to shed light upon this ambiguity in his review of 2001. He defined *supramolecular isomerism* as "…*the existence of more than one type of network superstructure for the same molecular building blocks…"* and pointed that polymorphs are a particular case of supramolecular isomers "…*polymorphs can therefore be regarded as being supramolecular isomers of one another but the reverse is not necessarily the case*". This assertion should be clear before classifying metal–organic structures.

Henceforth, *supramolecular isomerism* was divided into *structural* (regarding the formation of different networks *inter alia* ladder, brickwall, 3D frame, herringbone, bilayer or Lincoln logs), *conformational* (relying on the flexibility of the ligands) and a new class named *catenane* (promoted by interpenetration), being polymorphic forms a certain condition of them. A schematic representation highlighting such division is depicted in **Figure 1**. At that time, *structural isomerism* was focused on the particular case of *architectural isomers* [41], understood as variations of the connectivity of the ligands between two structures, sharing the same composition as a result of ligand conformations [53]. In particular, to those cases in which the accommodation of different solvent guest molecules promoted the change in the spatial disposition of the organic linkers.

In this direction, Robin and Fromm [48] in 2006 described *supramolecular isomerism* as the ability of a substance to arrange into one or several network superstructures by different molecular or supramolecular assemblies with the inherent condition that organic linkers and metal ion remain the same and this metal ion retains an equal coordination sphere. They also reported several examples of a new subclass of structural supramolecular isomers named as *ring opening*. Recently, Zhou [54] has provided the specific term *framework isomers* to define supramolecular isomerism in MOFs, in which solvent occluded molecules are trivial.

*Polymorphism and Supramolecular Isomerism: The Impasse of Coordination Polymers DOI: http://dx.doi.org/10.5772/intechopen.96930*

#### **Figure 1.**

*Schematic representation of the structural relationship between the different architectures among organic and metal–organic compounds.*
