SYNTHESIS AND STRUCTURAL STUDIES OF SUPRAMOLECULES USING N-HETEROCYCLIC LIGANDS

Abstract

Design and supramolecular synthesis of the functional solid materials through noncovalent interaction could lead the prospect to find new supramolecular synthons which triggers the construction of potential intriguing architecture. It results in enhancing the physical properties of the bulk material without chemically modifying themselves. Well known N-heterocyclic compounds such as pyrazole and pyridyl based ligands have attracted the researchers for the synthesis of new functional solid materials. The propensity of pyrazole and pyridine to acquire hydrogen from the acidic component has allowed these molecules to interact with H-donor as well as acceptor moieties in the multi-component system. Moreover, these molecules are potential ditopic ligands, which display diversity of coordination modes in the formation of metal complexes and distinct Supramolecular Organic Frameworks (SOFs) with different dimensionality (0D, 1D, 2D and 3D). In the realm of crystal engineering, the component with both proton donor and acceptor might display an array of non-covalent interactions. In this context, the phosphonic acid which is less exploited as compare to carboxylic acid, possesses tetrahedral geometry with two ionisable hydrogen atoms and one acceptor atom has been employed in this thesis. Both coformers with Hdonor and H-acceptor in N-heterocyclic ligands and phosphonic acid may lead to intriguing and potential architecture. The first chapter of the thesis includes the brief introduction and literature survey regarding crystal engineering involved in the system acquiring N-heterocyclic ditopic ligand, phosphonic acid and metal complexes representing CO2 adsorption followed by the brief description of the goal behind the presented work. In second chapter of the thesis, a family of bispyrazole-phosphonate was synthesized and assessed the competitive hydrogen bonding interactions between different phosphonic acids possessing diverse functional group on it. The main aim for this work is to obtain the degree of control and reliability of the non-covalent interactions involved between the resulted series of bispyrazole-phosphonate system. Overall fourteen molecular complexes were obtained from two conformational flexible and one rigid bispyrazole separated by different groups present between two complementary pyrazoles i.e., 3,3',5,5'-tetramethyl-1H,1'H-4,4'-bipyrazole (BPz), methylenebis(3,5-dimethylpyrazole) (MBPz) and 1,2-bis(3,5-dimethyl-1H-pyrazol-4-yl)diazene, 6 (BPaz) with variety of phosphonic acids i.e., phenylphosphonic acid (H2PPA), phosphonoacetic acid (H2PAA), 3-phosphonopropionic acid (H2PPRA), tert-butylphosphonic acid (H2tBPA), etidronic acid (H4EA), (1,4-phenylenebis(methylene))diphosphonic acid (H4DPA) via synthon approach, among which twelve were salts and two were the co-crystals. X-ray analysis revealed that all compounds result in different supramolecular architecture with the formation of cyclic hydrogen bond motif as a structural motif. It was significant to note that instead of presence of carboxylic group on phosphonic acid in some salts, the synthon formation was observed merely by the phosphonic group and no synthon was formed by carboxylic group. However, besides the dimer formation, the most interesting feature of the supramolecular network of phosphonate salts of pyrazole is the formation of trimeric and extended trimeric (tetrameric) heterosynthons. Furthermore, the analysis of theoretically obtained interaction and synthon energy in the solid state for all the compounds deduced that the stability for salt involving the (1,4- phenylenebis(methylene))diphosphonic acid (DPA) is largest among all the salts. It was also pointed out that the occurrence of particular synthon having the higher stability is more. On the other hand, the thermal study revealed that the phosphonate salts of pyrazole analogues are stable upto 280 ºC which might be due to the presence of more strong interactions. As a consequence, it was concluded that the stability of all the compounds observed from DFT level energy was in agreement with the stability provided by TG analysis. Hence, the present study provides an enhanced understanding about the effects of substituents on the resulted supramolecular assembly and ended in the designing of new synthons. The third chapter of the thesis deals with the formation of the molecular salts composed of (anthracen-9-ylmethyl)phosphonic acid (H2APa) and 3,3',5,5'-tetramethyl-1H,1'H- 4,4'-bipyrazole (BPz) and an assessment of formation of their several psuedopolymorphs (solvates) and ternary complexes was carried out. In other words, further enhancing the understanding of potential structural effect of different solvent molecules on phosphonicpyrazole system was studied. To gain the profound knowledge of consistency of the noncovalent interactions, numerous solvates were synthesized to control the deprotonation of ionisable proton in presence of particular solvent molecule. Interestingly, all resulted in a salt formation with four type of different frameworks revealing host-guest hydrogen bonded assemblies where APaBPz assembled into a host and diversity of solvent molecules such as tetrahydrofuran (THF), dioxane, benzene, thiophene, nitromethane, nitrobenzene, thiophenol and 7 toluene remains inside the cavity as the guests. It is worth noting that all solvates retained the acid dimer even with the change in functional group present on the solvent molecule which substantiates that it is a robust synthon for the formation of the host-guest assembly. Besides, variety of new synthons was also observed. Notably, the bulky and aromatic moiety of the phosphonic acid with conformational flexibility changes its conformation in small magnitude depending upon the upcoming guest and probably be the reason for the different host-guest assemblies. Hence, X-ray data analysis showed the formation of ··· and C–H··· interactions were also responsible for the change in framework. Few ternary supramolecules were also produced from phenol, 2-nitrophenol, 4-nitrophenol and p-cresol with the guest molecule resided inside the cavity. However in presence of water molecule, the hydrated APaBPz system involves water molecules as the constituent of the framework; hence no such host-guest assembly is resulted. Therefore, the thermal study showed that the hydrated APaBPz system was found to be more stable solvate which decomposes at higher temperature. Additionally, a 0D Cu(II) based molecular complex, [Cu2( 2-C15H12PO3)2(2,2'-BPy)2(H2O)2]((NO3)2.X) was also envisioned using newly synthesized ethyl hydrogen (anthracen-9-ylmethyl)phosphonate and 2,2'-bipyridine as an auxillary base in presence of triethylamine. Single crystal X-ray characterized metal complex revealed that the dinuclear assembly is comprised of two CuN2O3 square pyramidal at the two opposite corner connected through bridged CPO3 tetrahedra of the phosphonate. However, the - electron cloud of 2,2'-bipyridine and phosphonate ligands in the dinuclear assembly are chiefly accountable for the extension of 0D to 3D structure with a 1D channel. It is interested to note that the resulted metal complex undergo an important aspect of solid state transformation (crystal-amorphous-crystal transformation) which led to the formation of Cu2( 2-C15H12PO3)2(2,2'- BPy)2(NO3)2].MeOH), neutral coordination complex from cationic coordination complex on external stimuli. Hence, the transformation of cationic metal complex to neutral metal complex based on anthracene-phosphonomonoester may show tunable luminescent behavior. Besides pyrazole ligands, some pyridyl based ditopic ligands such as 2,2'-bipyridine, 4,4'-bipyridine, 1,10-phenanthroline and 4,2':6',4''-terpyridine were used for deducing its supramolecular behavior with tripodal phosphonic acid i.e., ((2,4,6-trimethylbenzene-1,3,5- triyl)tris(methylene))triphosphonic acid in fourth chapter. Overall total five different packing motifs were observed, mainly defined by the columns of tripodal phosphonic acid in all these resulted salts. Remarkably, this phosphonic acid showed two different conformations in the salts 8 depending upon the ditopic ligands used. On the other hand, X-ray data analysis disclosed that the two of the resulted salts showed an important feature of polymorphism in which one of the polymorph displays hydrogen bonded chain formed by triphosphonic and water via O–H O interaction whereas in another polymorph, hydrogen bond is found between triphosphonic acid and water molecules ensuing in a continuous chain of water and one of the OH of the phosphonic acid. After analysis, it has been predicted that the latter complex permits the continuous proton transfer in the infinite chain of phosphonic and water molecules; hence it was presumed that it might show proton conductivity. Moreover, the resulted salts acquiring 4,4'-bipyridine, 1,10- phenanthroline showed the tendency to transform into another solid complex via SCSC transformation. Furthermore, some 2D MOFs were also ascertained with the newly synthesized conformationally flexible ditopic pyridine based ligand which may have appealing structural properties for the generation of soft porous materials, hence, eight isostructural 2D MOFs were successfully synthesized with variation in anions (NO3 -, Cl-, Br-, I-, SCN-, ClO4 -) coordinated to the metal ion M(II) (M = Cd2+, Co2+, Zn2+, Eu2+). Depending upon their particular size, the tuned pore size of MOFs can be used as the absorber for selective gases. Single crystal X-ray diffraction study reveals that all metal complexes possess similar structural features such as metal-coordination environments, network topologies, ··· interactions and exhibits a 2D uninodal of 44-sql topology with slightly tuned cavity. All complexes except the complex with ClO4 - anion exhibited [[M(dmpt)2(A)2]·X] as the secondary building unit (SBU), building the 2D layer which is extended into three-dimensional (3D) framework. On the other hand, the effect of weak interactions involved in the structural dynamics of the systems has also been studied. While the MOF with ClO4 - anion displayed the extended supramolecular large rectangular cavities of dimension 13.9 X 6.4 Å2, accommodated by uncoordinated ClO4 - anions, CHCl3 molecule and coordinated DMSO molecule from Cd(II). Moreover, gas adsorption study of these complexes demonstrated that the resulted complexes showed the selective adsorption of CO2 gas over N2 gas. Albeit, the CO2 uptake shown by these compounds is modest but comparatively the complexes with anion NO3 - and SCN- shows the higher CO2 uptake. The material and reagents, synthetic procedures, experimental details, some spectroscopic measurements for all the compounds including the salts, co-crystals and metal complexes reported in chapter 2-4 have been provided in chapter five of the thesis.