**4. Supramolecular chemistry of ferrocenyl ureas and thioureas**

#### **4.1 Supramolecular chemistry**

Supramolecular chemistry [44] focuses on the design and synthesis of "Supramolecular Entities" [45], i.e., compound elements held together by noncovalent connections including hydrogen bonding, bonds with halogens, forces of coordination, or π-π connections (**Figure 7**) [46]. Research in supramolecular chemistry and crystal engineering is principally centered around host-guest arrangements, binding of anion and cation, coordination polymers, developments of self-assembly networks, biological simulators, gels, fibers, liquid polymers, crystals, and other various types of materials [47].

Supramolecular chemistry objects to the considerations of interactions between molecules and packing patterns in molecular crystals and, consequently, usage of information spawned for potential novel material designing gathered with targeted structures and proficient characteristics [48–50]. From this objective, one can consider assembling molecular crystals having a multitude of noncovalent interactions among which a prominent position is occupied by H-bonds owing to their noticeable directionalities and reasonably high strength [51]. Hydrogen bonds are characteristically much weaker in comparison to covalent bonds though and hence have minimum predictability, which often destabilizes the crystal designing process utilizing these interactions [52].

While considering the molecular synthesis, synthetic schemes are confidently planned by researchers for molecules comprising very complex framework. In synthesizing crystalline organic solids, the term engineering can be invoked infrequently in its true sense. Taking from supramolecular structure to design a crystal

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

*Molecular structure of 1-(3-bromobenzoyl)-3-(4-ferrocenylphenyl)urea.*

characteristics [34] (**Figure 4**).

**3.2 Ferrocenyl thioureas**

of derivative compounds [40].

depicted in (**Figure 4**). These N-H groups are modified to supplement target anion and minimal intramolecular H-bonding to observe strong and selective binding

Over the past few years, assortment of urea-based hosts comprising one or more than one urea subunits is premeditated and tested for anion recognition and for being capable of sensing [35]. New perceptions into characteristics of interactions between urea and anionic moiety providing structural measures for considered designing of novel anion-selective receptors which contain two or additional urea binding groups have also been discovered in recent times [36]. On the other hand, there are few instances of ferrocenyl urea derivatives as redox active anionophores [37]. The molecular structure of a representative ferrocenyl urea derivative is presented in **Figure 5** [38].

Replacement of an oxygen atom in urea moiety by a sulfur atom results in thiourea formation, the characteristics of which are significantly diverged than those of urea due to the variation in electronegative character among sulfur and oxygen atoms [39]. Thiourea-based compounds and complexes have also been explored for several biological activities because of the thio-carbonyl group, which influences biochemical activity by lipophilic or hydrophilic character and electronic properties

**54**

**Figure 5.**

**Figure 4.**

*Ferrocenyl urea [34].*

**Figure 7.** *Noncovalent interactions in supramolecular synthesis.*

structure, the whole process considers experiential observations and a posteriori structure of crystal and analytical measurement [53].
