**1. Introduction: historical perspective**

The intrinsic awareness in structure-properties relationship of solids was firstly introduced on the study of minerals and inorganic compounds and has been preserved over time [1]. Within this frame, the first step on the polymorphism phenomenon was unknowingly given by Klaproth [2] in 1788 identifying three different crystalline phases of calcium carbonate (calcite, vaterite and aragonite). In 1819, Mitscherlich commenced his research on phosphates and arsenates of potassium (KH2PO4 and KH2AsO4) and established their complete morphological similarity, noting that *they crystallize in similar forms*. His subsequent results with the corresponding potassium salts (NH4H2PO4 and NH4H2AsO4) confirmed that observation and led him to pose that *there do exist bodies of dissimilar chemical composition having the same crystalline form*. He not only recovered the work of Whollaston with orthorhombic carbonates and sulfates of barium, strontium and lead, who already noticed this phenomenon in 1812 but also, extended to rhombohedral carbonates of calcium, magnesium, iron and manganese, and to sulfates of iron, copper, zinc, magnesium, nickel and cobalt. Curiously, he also evinced the basis of *seeding* at realizing that *having two substances both able to crystallize in various forms, the presence of one during the crystallization of the other will force the crystallization of the latter in the same form*. Then, he moved to Stockholm with Berzelius with whom he delved deeply into phosphates and arsenates and forged the concept of

*isomorphism*. Last but not least, he reported two distinct crystallized forms of sulfur in 1826 demonstrating *a clear case of an element which could be made to crystallize in two different systems of symmetry at will, by merely changing the crystallization conditions.* Therefore, Mitscherlich was identified as the one who took the first step towards the rise of *polymorphism* [3]. Shortly after, in 1824, the contradictory results garnered by Liebig [4] and Wöhler [5] during their research on silver fulminate and silver cyanate triggered the conflict which led them to be colleagues and to pose the following dilemma: *can two compounds with the same composition have different physical properties?*. Their results evinced two Ag salts with the same composition but different physical properties, which did not go unnoticed by Wöhler's master Berzelius, who merged these results with those of Mitscherlich. It is unclear who was the first to conceive the notion of *polymorphism* but this crucial period of 1820–1832 was mainly drawn by Mitscherlich [6–8] who shed light on this phenomenon, even though the concept was still vague. It was during these years when Berzelius [9] proposed the concept of *isomerism* (1831–1832) and it took until 1832 for Wöhler and Liebig [10] to report the first case of *polymorphism* in an organic compound. The awareness of *isomerism* set the beginning of structural chemistry, broadening the knowledge and understanding of organic structures.

Since observation was the essential tool to identify *polymorphism*, this research drastically changed with the accessibility and wide spreading use of the microscope, but it was not until 1839 when Frankenheim [11] introduced the first principles defining *polymorphism* and *a postriori*, Mallard [12] set the structural basis of *polymorphism* in 1876, relating differences in physical properties with different arrangements. One of the most remarkable contribution during this period was the "Rule of steps" or "Law of successive reactions" from Ostwald [13] in 1897. He pointed that during a succession of polymorphic forms, those appeared later are generally more stable. Despite not being considered a rule, it is still valid as a general observation. But the two major queries raised by Buerger and Bloom [14] in 1937 were still unanswered: what causes the formation of different phases of a substance and which factors determine them?.

The narrow link between crystallography and *polymorphism* was forged by Tamman [15] in 1926 and settled with the first polymorphic X-ray crystal structure determination of an organic compound, resorcinol, published by Robertson and Ubbelohde [16] in 1939. Despite this achievement, the next decades passed without a better understanding of *polymorphism,* being underrated until 1965 when McCrone [17] conducted a comprehensive study in which he defined a polymorph as: "*a solid crystalline phase of a given compound resulting from the possibility of at least two different arrangements of the molecules of that compound in the solid state*" and published a review in 1969 about the importance of such phenomenon in the pharmaceutical outlook [18]. Since the introduction of the term *allotrope* by Berzelius in 1841, *Polymorphism* had been taken with *allotropy* on the same meaning. But, it was not until the 1990s, when Sharma [19] and Reinke [20] set the differences between them: *polymorphism* occurs in chemical compounds while *allotropy* occurs in chemical elements. This work was crowned *a posteriori* by Dunitz's [21, 22] crystal description contribution.

In the 1970s, the works of Schmidt [23] and Paul and Curtin [24, 25] grounded the flourish of solid-state chemistry and precede the breakthrough of conceiving polymorphism. They served as inspiration to Bernstein and Desiraju, who laid the foundation for recent supramolecular chemistry. In 1978, Bernstein [26] changed the landscape of *polymorphism* by rationalizing the study of crystal packing forces on molecular conformations of polymorphs and later in 1990, together with Etter [27–30], applied their graph set descriptors and provide guidelines to understand polymorphic transformations [31]. Bernstein compiled most of this historical

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

results in his book "*Polymorphism in Molecular Crystals*", which the authors encourage reading [32]. Likewise, Desiraju achieved substantial progress in this field during his studies of structure-properties relationship of organic solids mainly of pharmaceutical interest, emphasizing and aiming its importance in this industry as reflected in his book "*Crystal Engineering: A Textbook*" [33]. Both channeled their polymorphism vision in terms of supramolecular chemistry.

During 1990s, computational chemistry went hand in hand, achieving methodologies capable to reproduce experimental results and enabling even crystal structure predictions. The first attainment was obtained by using Williams' software [34] that met the main handicap hitherto, the identification of lattice energy minimums. Subsequent years, many computational approaches were developed facing with computer-generated structures for prediction, to the extent that in 1999 a collaborative workshop held at the Cambridge Crystallographic Data Centre (CCDC) [35] brought together the benchmark computational groups of this period to provide an objective assessment of the possibilities of crystal structure prediction. The results gathered in this event were clearly summarized *a posteriori* in a paper published by Lommerse [36]. Further advances on prediction methods accuracy as well as the implementation of Density Functional Theory (*DFT*) and Machine Learning (*ML*) can be found in Spark's review [37].

Despite the basis of polymorphism were already defined at the end of the twentieth century, the increasing advances on X-ray diffraction techniques and crystallization methods afforded the determination and analysis of metal–organic structures, especially *coordination polymers*. *Polymorphism* of metal–organic complexes was still unexplored and these new class of materials, from which polymorphic structures grew exponentially, required classification and awareness. This impasse was encouraged by renowned researchers as Sharma [38], Ciani [39], Rogers [40] and Zaworotko [41]. In this regard, the same Zaworotko was who published a review in 2001 emphasizing the difference between *polymorphism* and *supramolecular isomerism* and underlined the link between them in organic and metal–organic networks [42].
