Self-Assembly Of Discrete Molecular Architectures : Design, Synthesis And Characterization

Abstract

Stepwise covalent synthesis of large molecules is often time consuming and laborious and thus generally ends in a low yield of the target product. It is also difficult to achieve a large desired product where the controlling force is a non-directional weak interaction. Instead, by utilizing stronger metal-ligand directional coordination bonding approach, one can easily prepare the desired large molecules using appropriate molecular units. Further attractive feature of this approach is the incorporation of functional groups into final structures to make the assemblies functional. It is found that symmetrical polypyridyl and rigid linkers have been used widely in the construction of finite supramolecules of Pd (II) and Pt(II). Flexible linkers are rarely used since they are less predictable in self-assembly and have a tendency to form undesired polymer. However, flexible linkers may generate pseudo rigid assemblies that can distort their shapes to obtain a more thermodynamically stable conformation for host-guest interactions. Similarly, use of non-symmetric or ambidentate linkers is not explored much. These linkers may generate a mixture of several linkage isomeric products and thus difficult to monitor the reaction. Moreover, isolation of these products in pure form is also a challenging task. On the other hand, recent research revealed that porous polyacetylene organic compounds are suitable sensors for the detection of electron deficient nitroaromatics, which are the chemical signatures of many commercial explosives. Possibility of discrete supramolecules as sensors for these explosives is very less studied. The main thrusts of the present investigation are to incorporate flexible and nonsymmetrical linkers in the construction of finite discrete assemblies of Pd/Pt; and to design appropriate π-electron rich supramolecules as sensors for the detection of electron deficient nitroaromatics. Chapter 1 of this thesis gives a brief introduction to the supramolecular chemistry. It also gives a brief introduction to the design principle of metal-ligand coordination driven selfassembly approach towards the generation of large architectures. Chapter 2 reports the synthesis of a series of two-dimensional supramolecular architectures via coordination driven self-assembly of Pt/Pd containing ditopic acceptors and non-symmetrical donor ligands. The use of non-symmetrical donor ligands in coordination driven self-assembly is a challenging task because they may generate a mixture of isomers due to different connectivity of the non-symmetric (ambidentate) linkers. But in all the cases exclusive formation of a single linkage isomer was established. Na-nicotinate was treated with [cis-(dppf)Pd(OTf)2] to yield [(dppf)3Pd3(L3)](CF3SO3)3(H2O)2(MeOH)7(Et2O) as the single linkage isomeric triangle. An analogous treatment using Na-isonicotinate instead of Na-nicotinate yielded a mixture of single isomeric square and triangle with the later one as the major product in solution. Further extension of this study using cis-(tmen)Pd(NO3)2 instead of [cis-(dppf)Pd(OTf)2] also showed the formation of a mixture of square and triangle [tmen = N,N,N’,N’- tetramethylethane-1,2-diamine]. Surprisingly, in both the cases square was the product which was crystallized exclusively in solid state though triangle was the major component in solution. The square-triangle equilibria in both the cases were studied by diffusion ordered NMR spectroscopy (DOSY) and variable temperature multinuclear NMR. Moreover, this chapter reports the incorporation of amide functionality into a Pt(II) nanoscopic molecular rectangle via self-assembly of an organometallic “clip” and a non-symmetric amide ligand. Chapter 3 presents synthesis of several metallamacrocycles via coordination driven selfassembly using Pd/Pt-P bonding interaction as driving force instead of much widely used Pd/Pt-N bonding interaction. It is also established that Pd/Pt-P bonding interaction is indeed better than the widely used Pd/Pt-N interaction. Several macrocycles were also synthesized by the combination of several Pd containing 90° angular subunits and a bisimidazole ditopic flexible donor. In this case also the bonding interaction between the imidazole and Pd(II) was found to be stronger than the interaction between pyridyl donor and Pd(II). Chapter 4 describes synthesis of several new Pt2 and Pt3 shape selective organometallic linkers incorporating ethynyl functionality. The Pt2 molecular clip was assembled with several linear dipyridyl linkers to prepare a series of molecular rectangles. In one case N, N’-bis(4-pyridylidene)ethylenediamine was used as donor to create a N4 pocket in the macrocycle. This rectangle was fluorescent in nature and showed efficient fluorescence quenching in solution upon binding of hard transition metal ions (Fe3+, Cu2+ and Ni2+) into the N4 pocket. The non-responsive nature of the fluorescence quenching upon addition of soft metal ions (Zn2+ and Cd2+) containing d10 configuration makes it an interesting example of sensor for transition metal ions. The Pt3 linkers were used in combination with organic clip-type linkers to prepare a series of molecular prisms by [2 + 3] self-assembly (Scheme 1). Incorporation of ethynyl functionality helped to make the resulting supramolecules π-electron rich and luminescent in nature. Possibility of these supramolecules as sensors for the detection of electron deficient nitroaromatics (TNT and picric acid), which are the chemical signatures of explosives has been explored. A complementary approach was also used to prepare trigonal prism using organic tritopic donor and the Pt2 molecular clip. Chapter 5 presents the design and self-assembly of two new flexible supramolecular nanoballs. These assemblies incorporate two flexible tritopic amide/ester based building blocks and were prepared in excellent yields (96-97%) via coordination driven selfassembly. The first one was resulted from the reaction of four equivalents of a new tritopic ester ligand N, N', N''-tris(4-pyridylmethyl) trimesic ester with three equivalents of C4 symmetric Pd(NO3)2. The second analogous structure was obtained by the selfassembly of the flexible N, N', N''-tris(3-pyridylmethyl)trimesic amide and Pd(NO3)2. The assemblies were characterized with multinuclear NMR spectroscopy, electrospray ionization mass spectroscopy, elemental analysis and TGA. The ester based ball showed the inclusion of NEt4 + in solution. This chapter also describes the exclusive formation of a Pt(II) trigonalbipyramidal (TBP) cage upon the treatment of a Pt(II) 90° acceptor with a new tripodal flexible ligand containing ester functionality. The formation of Pt(II) TBP cage in this case is due to the flexibility of the donor arms of the ligand due to the presence of flexible ester functional group. In continuation of this work, a rigid tripodal ligand 1,1,1-tris(4-pyridyl)COOR with an ester cap [where R = Ph-CH(C2H5)] was assembled with cis-(PEt3)2Pt(OTf)2 to yield a somewhat unusual double-square cage by [4 + 6] self-assembly.