Self-Assembled Coordination Cages for Catalysis and Proton Conduction

Biological systems construct varieties of self-assembled architectures with incredible elegance and precession utilizing proteins as subunits to accomplish widespread functions. Inspired by natural systems, construction of artificial model systems with such sophistication and delicacy has become an intriguing field of research over the last two decades using so-called self-assembly process. Judiciously selected complementary building units encoded with specific chemical and structural information can be self-assembled into pre-programmed abiological architectures in a manner similar to biological self-assembly. In this regard, kinetically labile metal-ligand coordination has become an efficient and powerful protocol for the construction of highly intricate structures with specific topology and functionality due to its simple design principle, high bond enthalpy, and predictable directionality. Two-component self-assembly is very widely used methodology and easy to monitor. Recently, multi-component self-assembly has come up as an alternative and effective pathway to achieve complex architectures connecting more than two components in a single step. However, formation of selective single product from multicomponents is entropically unfavorable. Only a very few 3D architectures have been known, that are obtained from a mixture of ditopic and tri- or tetratopic donors with metal acceptors with or without employing templates. Development of template-free multicomponent architectures is still in its infancy. Strong tendency of Pd(II)/Pt(II) to attain square-planar geometry around the metal center and kinetically labile nature of Pd(II)/Pd(II)-N(pyridine) bonds made them chemists’ favourite to engineer desired supramolecular coordination architectures with structural resemblance to Platonic or Archimedean solids by employing symmetrical pyridyl donors due to their predictable directionality. In case of poly-imidazole donors, free rotation of C-N bond connecting imidazole and phenyl ring allows various dispositions of the donating nitrogen with respect to the aromatic backbone, and therefore, the structural topology of the architectures, made of poly-imidazole ligands becomes much more interesting as compared to symmetrical Platonic or Archimedean solids. The physico-chemical properties of self-assembled coordination cages depend on the structures of the complexes. Presence of large internal cavity surrounded by aromatic core, provides an excellent environment for the encapsulation of varieties of guest molecule or as nano-reactors for different organic transformations. Structural investigation in terms of packing interactions, solvent molecules, intermolecular channels can sometimes determine the property of such self-assembled materials as well. Presence of acidic water as well as H-bonded 3D-networks of water molecules in molecular pockets make them potential material for proton conduction. In addition, metal-ligand coordination offers opportunity to introduce new functionality through pre-synthetic modification of the building constituents to influence the property of the supramolecular systems. Incorporation of unsaturated ethynyl functionality attached to the heavy transition metal is expected to exhibit efficient luminescence due to the facile metal to ligand charge transfer (MLCT). Hence, the final assemblies can be employed as chemosensors for electron-deficient nitroaromatics, which are the chemical signature of many of the commercially available explosives. The present investigation is focused on design and construction of discrete, nanoscopic coordination cages with unusual structural topology employing mainly imidazole-based donors with Pd(II)/Pt(II) acceptors and their applications in catalysis, chemosensing, and proton conduction. CHAPTER 1 of the thesis provides a general introduction to self-assembly focusing on the importance and advantages of metal-ligand directional bonding approach towards the construction of supramolecular architectures with various structural topologies. This chapter also includes a brief review on the applications of such coordination cages in various fields especially as ‘molecular flask’ for the observation of unique chemical phenomena and unusual reactions. Part A of CHAPTER 2 describes the synthesis of a new hollow Pd6 water soluble cage [{(tmen)Pd}6(timb)4](NO3)12 (1) via two-component self-assembly of a triimidazole donor and 90° Pd(II) acceptor [tmen = N,N,N’,N’-tetramethylethylenediamine, timb = 1,3,5-tris(1-imidazolyl)benzene]. The assembly was successfully crystallized with a hydrophilic dianionic benzoquinone derivative (formed in situ by the decomposition of DDQ) as [{(tmen)Pd}6(timb)4](NO3)10()2(H2O)18 (3), and a hydrophobic sterically demanding aromatic aldehyde as [{(tmen)Pd}6(timb)4](NO3)12{()4a}2(H2O)27 (5a) [where 2H2 = 2,3-dichloro-5,6-dihydroxycyclohexa-2,5diene-1,4-dione, 4a = 1-pyrenecarboxaldehyde,  = exohedral and  = endohedral] to confirm the hydrophobic nature of the cavity. Experiments were carried out to show that the hydrophobic confined nanospace of the cage (1) catalyses the Knoevenagel condensation of a series of different aromatic monoaldehydes with active methylene compounds in ‘green’ aqueous medium. The Knoevenagel condensation reaction is basically a dehydration reaction because water is a by-product. So the presence of water should, in principle, promote the backward reaction as per Le Chatelier’s principle. In general, these reactions with organic substrates are not performed in water. However, difficulty has been overcome using hydrophobic cavity of the cage. It has also been established that the cavity of the cage also enhances the rate of Diels-Alder reaction of 9-hydroxymethylanthracene with N-phenylmaleimide/N-cyclohexylmaleimide. Figure 1. Catalytic Knoevenagel condensation and Diels-Alder reaction using hydrophobic cavity of the cage (1) in aqueous medium. Part B of CHAPTER 2 reports unique three-component self-assembly incorporating both tri- and tetra-topic donors. Until now, a very few 3D-architectures have been known that are obtained from self-assembly of ditopic and tri- or tetratopic donors with metal acceptors. Scheme 1. Three-component self-assembly of a Pd7 cage (1) from cis-blocked Pd(II) 90° acceptor (M), tri-imidazole (timb) and tetra-imidazole (tim) donors. Self-assembled multicomponent discrete architecture composed of both tri- and tetra-topic donors is yet to be reported due to difficulty in prediction of the final structure from the mixture of ligands having multiple donor sites. The first example of self-sorted Pd7 molecular boat [{(tmen)Pd}7(timb)2(tim)2](NO3)14(H2O)20 (1) [tmen = N,N,N’,N’-tetramethylethylenediamine, timb = 1,3,5-tris(1-imidazolyl)-benzene, tim = 1,2,4,5-tetrakis(1-imidazolyl)benzene] was synthesized via three-component self-assembly of cis-(tmen)Pd(NO3)2, tetra- (tim) and tri-topic donors (timb) in a 7:2:2 ratio. The cavity of this cage was also utilized as a nanoreactor for catalytic Knoevenagel condensations of a series of aromatic aldehydes with 1,3-dimethylbarbituric acid (e) and Meldrum’s acid (f) in aqueous media. CHAPTER 3 presents the results of an investigation on how simple variation of length and coordination mode of linear donors can self-discriminate into markedly different complex architectures, from Pd8 molecular swing [{(tmen)Pd}8(tim)2(bpy)4](NO3)16 (1) or [{(tmen)Pd}8(tim)2(stt)5](NO3)6 (2) to Pd6 molecular boat [{(tmen)Pd}6(tim)2(bpe/dpe/pin/dpb)2](NO3)12, (3/4/5/6). Also by enhancing denticity [bidentate to tridentate (ptp)] as well as introducing asymmetry, they self-sort into Pd7 molecular tent [{(tmen)Pd}7(tim)2(ptp)2](NO3)14 (7) by employing it in a self-assembly of cis-(tmen)Pd(NO3)2 and tetraimidazole (tim) donor [where tmen = N,N,N’,N’-tetramethylethylenediamine, bpy = 4,4’-bipyridyl, stt = sodium terephthalate, bpe = trans-1,2-bis(4-pyridyl)ethylene, dpe = 1,2-di(pyridin-4-yl)ethane, pin = N-(pyridin-4-yl)isonicotinamide, dpb = 1,4-di(pyridin-4-yl)benzene, ptp = 6'-(pyridin-4-yl)-3,4':2',4''-terpyridine, and tim = 1,2,4,5-tetrakis(1- imidazolyl)benzene]. In these cases, control of the geometrical principles and stereo-electronic preferences of the building units allowed the formation of such intricate architectures. Some of these assemblies represent first examples of such types of structures, and their formation would not be anticipated by taking into account only the geometry of the donor and acceptor building units. In addition to their direct structural confirmation using single crystal X-ray diffraction analysis, propensity of the assemblies (1 and 3) to form inclusion complexes with large guest like C60 in solution was also demonstrated by fluorescence quenching experiment. The high KSV values for both the assemblies 1 (1.0 × 10-5 M-1) and 2 (1.6 × 10-6 M-1) with C60 indicated the propensity of these assemblies to form complexes with C60 in solution. Furthermore, inspection of crystal packing of other five complexes (2 and 4 - 7) revealed the presence of water molecules H-bonded with NO3– (O-H···O=N) and 3D H-bonded networks of water in the intermolecular pockets. Interestingly, the present complexes (2 and 4 - 7) show high conductivity across low-humidity range at ambient temperature and achieve a conductivity of ~10-3 Scm-1 at 75% relative humidity and 296 K. These supra-molecular architectures represent a new generation of discrete materials that display high proton conductivity under ambient conditions with activation energy comparable to that of Nafion. Scheme 2. Exclusive formation of Pd8 molecular swings (1 and 2), Pd6 molecular boats (3-6), and Pd7 molecular tent (7) via self-sorting. CHAPTER 4 presents self-selection by synergistic effect of morphological information and coordination ability of the ligands through specific coordination interactional algorithms within dynamic supramolecular systems involving a tetratopic Pd(II) acceptor and three different pyridine- and imidazole-based donors (La - Lc) [La = 1,3-bis((E)-2-(pyridin-3-yl)vinyl)benzene, Lb = 1,3-di(1H-imidazol-1-yl)benzene, and Lc = tris(4-(1H-imidazol-1-yl)phenyl)amine]. Three different cages, ‘paddle wheel’ cluster Pd2(La)4(NO3)4 (2a), molecular barrel Pd3(Lb)6(NO3)6 (2b) and molecular sphere Pd6(Lc)8(NO3)12 (2c) were first synthesized via two-component self-assembly of a tetratopic Pd(II) acceptor (1) and individual pyridine- and imidazole-based donors (La - Lc). When all the four components were allowed to interact in a complex reaction mixture, only one out of three cages was isolated. The inherent dynamic nature of the kinetically labile coordination bond allows constitutional adaptation through component exchange in the competition experiment involving multiple constituents to self-organize into specific combination and thereby, achieve the thermodynamically most stable assembly. The preferential binding affinity towards a particular partner was also established by transforming a non-preferred cage to a preferred cage by the interaction with the appropriate ligand and thus, this represents the first examples of two-step cage-to-cage transformation through constitutional evolution of Figure 2. Cage-to-cage transformation from non-preferred cage to preferred cage upon treatment with appropriate ligand; and Nyquist plots of the complexes (2b and 2c) under 98% RH condition and ambient temparature. dynamic systems induced by both coordination ability and geometry of the ligand. Moreover, computational study further supported the fact that coordination interaction of imidazole moiety to Pd(II) is enthalpically more preferred compared to pyridine which drives the selection process. In addition, analysis of crystal packing of both the complexes (2b and 2c) indicated the presence of strong H-bonds between NO3- and water molecules; as well as H-bonded 3D-networks of water. Interestingly, both the complexes exhibit promising proton conductivity (10-5 to ca. 10-3 S cm-1) at ambient temperature under relative humidity of ~98% with low activation energy. CHAPTER 5 covers design and synthesis of new organometallic building block 1,3,5-tris(4-trans-Pt(PEt3)2I(ethynyl)phenyl)benzene (1) incorporating Pt-ethynyl functionality and [2 + 3] self-assembly of its nitrate analogue 1,3,5-tris(4-trans-Pt(PEt3)2(ONO2)(ethynyl)phenyl)benzene (2) with “clip” type bidentate donors (L1 – L3) separately afforded three trigonal prismatic architectures (3a – 3c), respectively (Scheme 3), Scheme 3. Schematic presentation of three different donors (L1 – L3) and a new planar tritopic acceptor (2) and their [3 + 2] self-assembly into trigonal prismatic architectures (3a - 3c). [L1 = N1,N3-di(pyridin-3-yl)isophthalamide; L2 = 1,3-bis((E)-2-(pyridin-3-yl)vinyl)benzene; L3 = 1,3-bis(pyridin-3-ylethynyl)benzene]. All these prisms were characterized and their shapes/sizes are predicted through geometry optimization employing molecular mechanics universal force field (MMUFF) simulation. The extended -conjugation including the presence of Pt-ethynyl functionality make them electron rich as well as luminescent in nature. As expected, cages 3b and 3c exhibit fluorescent quenching in solution upon addition of picric acid [PA], which is a common constituent of many explosives. Interestingly, the non-responsive nature of fluorescent intensity towards other electron-deficient nitro-aromatic explosives (NAEs) makes them promising selective sensors for PA with a detection limit deep down to ppb. Complexes 3b – c represent the first examples of molecular metallocages as selective sensors for picric acid. Furthermore, solid-state quenching of fluorescent intensity of the thin film of 3b upon exposure to saturated vapor of picric acid draws special attention for infield application