Lehn defines the concept of Supramolecular chemistry as 'chemistry beyond the molecule'; chemistry of the intermolecular bond that includes the functions and structures of the entities resulting from the bonding between multiple chemical species. According to Vogtle and Alfter (1991), supramolecular chemistry, contrary to molecular chemistry, is based on intermolecular bonding or interactions, while the later is predominantly based on the covalent bonding of atoms. In other words, supramolecular chemistry deals with the bonding between two or more building blocks that are bound together by means of intermolecular bonds.
The aforesaid concept bears on the organized entities of higher complexity resulting from the interaction between two or more chemical species that bind together by intermolecular forces." Self-recognition and self-assembly processes represent the basic operational components underpinning supramolecular chemistry, in which interactions are mainly non-covalent in nature, such as van der Waals, iconic, hydrogen-bonding, or coordinative interactions. Therefore, these bondings are weaker and normally reversible in nature as against conventional covalent bonds (Schubert, Hormeier, & Newkome, 2006).
The increasing amount of study into this area of chemistry is due to the growing demand for smaller devices for their use in nanotechnology, stated by the National Nanotechnology Initiative in the United States as “the understanding and control of matter at dimensions of roughly 1 – 100 nanometers, where unique phenomena enable novel applications”.
Metal-ligand coordination is among the most important interactions used in supramolecular chemistry. Chelate complexes play a prominent role in this field, which are derived from N-heteroaromatic ligands, largely based on 2,2'-bipyridine and 2,2':6',2"-terpyridine and have become an ever-growing synthetic and structural frontier (Housecroft & Pfaltz, 2007).
The chemistry of 2,2':6',2"terpyridines (designated as terpyridine or tpy) is much younger as compared to that of 2,2'-bipyridines. Terpyridine was first isolated in the early 1930 by Morgan and Burstall. In this experiment pyridine was heated at 340 °C with anhydrous Fe [Cl.sub.3] within an autoclave (50 atm) for 36 hours. Furthermore, the parent terpyridine was isolated along with innumerable other N-containing products. Subsequently it was found that when Fe (II) ions were mixed with a solution of terpyridine compounds, a purple color was generated that gave the first indication of metal complex formation. Since then, the chemistry of terpyridine remained a curiosity for nearly six decades, when its unique properties were employed to construct supramolecular assemblies. The amount of publications and literature pertaining to terpyridine has risen dramatically; a trend that is anticipated to continue, since it is a crucial structural element in newly designed constructs based on metallo-polymers and crystal engineering.
The terpyridine molecule comprises of three nitrogen atoms and thus can function as a tridentate ligand. It has been broadly studied as a prominent complexing ligand for a wide array of transition metal ions. Moreover, advances in the design and synthesis of tailored terpyridine derivatives have given rise to the ever-growing potential applications of terpyridine. The common features of terpyridine metal complexes include the unique redox and photophysical properties that are largely based on the electronic effect of the substituents. Hence, terpyridine complexes play a pivotal role in photochemistry for the construction of luminescent devices or as sensitizers for light-to-electricity conversion. Furthermore, ditopic terpyridinyl units may produce polymetallic species which in turn may be utilized in the preparation of electrochemical or luminescent sensors.
Terpyridine ligands have also been applied in asymmetric catalysis. One of the most interesting application involving newer supramolecular architectures is the generation of "mixed complexes", wherein two terpyridine ligands which are differently functionalized are aligned into a single transition metal ion. Another interesting field for new terpyridine compounds is their unique application in supramolecular chemistry. In regards to the current study, the formation of supramolecular terpyridine including dendrimers may be highlighted. Grid-like supramolecular structures are formed from layer-by-layer self-assembly of extended terpyridine compounds/ complexes across graphite surfaces. In additiona, terpyridines, integrated into macromolecules, enable the formation of well-structured supramolecular polymer architectures, thereby allowing for “switching" within chemical and physical properties of materials (Housecroft & Pfaltz, 2007).
Everyscience.com (2004) defines chelating ligands as polydentate ligands that are capable of forming a ring which includes the metal atom. The resulting complex is termed as a chelate complex. Chelating ligands usually bind in several arrangements, applying different degrees of strain into the ring formed. However, the distance of separation between the two donor atoms seen within the chelating ligand, as well as the degree of strain introduced into the ring in the complex is typically defined in terms of the bite distance.
Chelating ligands are ligands that are attached to a central metal ion by means of bonds from two or more donor atoms. In other words, chelating ligands are those polydentate ligands that use the metal ion to form a ring including. The resulting complex formed is termed as a chelate complex.
Furthermore, chelating ligands can be arranged and bound in different formations, applying different levels of stran into the ring formed. “Five and six membered rings are often favoured with saturated C and N based ligands, as in the example, as the tetrahedral angles may be preserved, and with unsaturated ligands as electron delocalization may be possible” (Everyscience.com).
Using a chelating ligand, such as a terpyridine, offers stability and control to a given resulting supramolecular structure. Various types of metals in various oxidation states can be used to form complexes of 2, 2’:6’, 2”- terpyridine ligands, providing a specific level of versatility to these systems. For instance, the three nitrogen donors on each tpy form strong associations with the metal ion. On applying the 4'- substitution, no chirality arises from the MN6core, in contrast to systems that use mono-substituted bpy ligands. Indeed, this explains that the resulting structure is predictable, which is one of the most important features of such type of synthesis. Figure 1.1 shows the 2,2':6',2"-terpyridine ligand with ring atom numbering scheme.
Figure 1.1: 2,2':6',2"-Terpyridine Ligands
The molecule 2,2':6',2"-terpyridine (tpy) was among the 20 products of the reaction between pyridine and anhydrous iron(III) chloride, devised by Morgan and Burstall.
Since then, the synthesis has been developed to give better yields and more specific results. Apart from the field of supramolecular chemistry, terpyridine ligands can be applied to several other fields. Common examples of application of the ligands include: preparation of luminescent materials and other potential uses as luminescent protein labels.
Figure 1.2: The original Kröhnke 2,2':4',3"-terpyridine synthesis.
In recent times, there have been several types of synthetic routes to create substituted terpyridines. Generally, these are based on two different techniques, namely, ring coupling and ring closure. A good case of the ring closure method is the Kröhnke (1976) synthesis, as indicated in schematic 1.2. This technique is illustrated by Kröhnke in their research study for the synthesis of 2,2':4',3"-terpyridine. The second technique is based on the direct coupling of the pyridine rings. However, there is certain degree of inefficiency in this method, since it produces less than 10% yield of terpyridine, on average.
An effective technique for directly forming a hydroxy-substituted terpyridine makes use of acetone and 2-acetylpyridine as the main reactants to produce 4'-hydroxy- 2,2':6',2"-terpyridine or HO-tpy.15 “This can then be used as the starting point for further functionalisation of the terpyridine ligand” (Housecroft, & Pfaltz, 2007). Furthermore, this technique becomes more efficient for the synthesis of building blocks for supramolecular self assembly as the resulting ligand here contains the nitrogen atoms arranged in such a way so as to enable the coordination to an octahedral metal ion. Nonetheless, the original Kröhnke synthesis sees a different positioning of the donor atoms so that all three of them cannot interact or bond simultaneously to the metal ion. Figure 1.5 shows the direct formation of hydroxy-substituted terpyridine by the ring closure method
Figure 1.3: Ring closure to generate 4'-substituted tpy directly
The hydroxyl group offers a means for attaching all types of other functionalities to the terpyridine ligand itself which makes it an efficient functional group to include. The use of a base produces a nucleophile to which substitution reactions can be applied. Substituted terpyridine ligands has been widely employed as a building block in supramolecular chemistry as these ligands are capable of readily coordinating to a broad variety of transition metal ions and can be substituted with several functionalities. When 4'-Substituted tpy coordinates with an octahedral metal centre generates a rigid linear moiety which in turn can generate molecular rods and wires.
Everyscience.com. (2004). Classification of Ligands. Retrieved from http://www.everyscience.com/Chemistry/Inorganic/Reactions_of_Metal_Complexes/b.1114.php
Housecroft, C. E., & Pfaltz, A. (2007). Metal complexes of Alkyne-Functionalised 2, 2’:6’, 2”-Terpyridine Ligands. Retrieved from http://edoc.unibas.ch/561/1/DissB_7855.pdf
Kröhnke, F. (1976). Synthesis, 1-24.
Schubert, U. S., Hormeier, H., & Newkome, G. R. (2006). Modern Terpyridine chemistry. Weinheim: John Wiley & Sons. http://www.dmmserver.com/DialABook/978/352/731/9783527314751.html
Vogtle, F. & Alfter, F. (1991). Supramolecular chemistry: An introduction. Michigan: Wiley Publishers.