Synthesis And Utility Of Bis-Urea Macrocycles As Nanoreactors And As Ligands For Metal Organic Materials

“Supramolecular chemistry” powered by non-covalent interactive forces forms the crux in the area of host-guest chemistry. Supramolecular assemblies often have different chemical and physical properties than that of its individual molecular entities and are used to develop novel functional materials. Our expertise involves making functional materials from macrocycles, which contain two urea groups and two rigid C shaped spacer groups. These individual macrocyclic components can self-assemble through hydrogen bonding and other non-covalent interactions to form porous supramolecular assemblies that can be used as confined reaction environments and as ligands to synthesize novel metal organic materials. This dissertation focuses on studying the self-assembly, and the utility of three bis-urea macrocyclic systems, namely phenylethynylene, pyridine-phenylethylene, and bipyridine. My major research effort focuses on the scope and applications of the phenylethynylene bis-urea and its pyridine counterpart pyridine-phenylethylene macrocycles as confined environments for studying the absorption and diffusion of guests and investigating their reactivity in confinement. The second research project is based on bipyridine bis-urea macrocycle, which is a great candidate to study the architectures formed by interplay of metal ligand coordination and hydrogen bonds in the presence of suitable metallic guests. This dissertation consists of six chapters. The introductory chapter is devoted to discuss the structure and reactivity of organic solid-state host-guest systems as reaction media to carryout photoreactions. The work described in chapters two and three has been focused on our efforts to use phenylethynylene bis-urea as a nanoreactor to modulate [2+2] photodimerization of series of benzopyrones. We went beyond studying dimerizations with the reactor built from pyridine-phenylethylene bisurea where we were able to facilitate photoinduced polymerization reactions of isoprene which is detailed in chapter four. Chapter five describes the structure, electrochemistry and photophysical properties of an exo di-ruthenium complex synthesized using the bipyridine bis-urea macrocycle. It extends to a description of its application as a photosensitizer to carryout electronically mismatched Diels-Alder reaction of isoprene and trans-anethole using visible light. The chapter six reports the solid state structures and subsequent Hirshfeld surface analysis of 6-substituted chromones, which were used as guest molecules in chapter three

Crystalline Bis-urea Nanochannel Architectures Tailored for Single-File Diffusion Studies

Urea is a versatile building block that can be modified to self-assemble into a multitude of structures. One-dimensional nanochannels with zigzag architecture and cross-sectional dimensions of only ∼3.7 Å × 4.8 Å are formed by the columnar assembly of phenyl ether bis-urea macrocycles. Nanochannels formed by phenylethynylene bis-urea macrocycles have a round cross-section with a diameter of ∼9.0 Å. This work compares the Xe atom packing and diffusion inside the crystalline channels of these two bis-ureas using hyperpolarized Xe-129 NMR. The elliptical channel structure of the phenyl ether bis-urea macrocycle produces a Xe-129 powder pattern line shape characteristic of an asymmetric chemical shift tensor with shifts extending to well over 300 ppm with respect to the bulk gas, reflecting extreme confinement of the Xe atom. The wider channels formed by phenylethynylene bis-urea, in contrast, present an isotropic dynamically average electronic environment. Completely different diffusion dynamics are revealed in the two bis-ureas using hyperpolarized spin-tracer exchange NMR. Thus, a simple replacement of phenyl ether with phenylethynylene as the rigid linker unit results in a transition from single-file to Fickian diffusion dynamics. Self-assembled bis-urea macrocycles are found to be highly suitable materials for fundamental molecular transport studies on micrometer length scales

Synergistic effects of hydrogen and halogen bonding in co-crystals of dipyridylureas and diiodotetrafluorobenzenes

<p>Herein, we investigate co-crystallization of three linear co-formers that contain urea and pyridyl groups with three regioisomers of diiodotetrafluorobenzene (DITFB) to afford eleven co-crystals. The linear <i>o</i>-<i>, m</i>-<i>,</i> and <i>p</i>- dipyridylureas vary distance and geometry between the urea carbonyl oxygen and two pyridyl nitrogen acceptors, while the donors consist of urea NH groups and the activated halides in DITFB. Electrostatic potential calculations suggest that the <i>o</i>-dipyridylurea co-former presents two significantly different acceptors. In comparison, the acceptors in the <i>m</i>- and <i>p</i>-dipyridylurea co-formers display electrostatic potentials within 5–6 kJ/mol and should be competitive, potentially leading to altered assembly motifs. Overall, ten of the co-crystals consistently display the urea assembly motif as the best acceptor/donor pair. Seven structures were obtained as the predicted 1:1 ratio with halogen bonding interactions linking ditopic halogen bond donors and the pyridyl units through N···I interactions ranging from 78.4 to 83.1% of the van der Waals radii. Modified structures were more likely when there was a structural mismatch with the geometrically challenging <i>o</i>-DITFB donor and <i>m</i>- or <i>p</i>-dipyridylurea co-former. The majority of the co-crystal structures (10/11) demonstrated fully satisfied hydrogen and halogen bonding interactions suggesting that these synthons can be used synergistically to generate complex solid-state structures.</p

Applications of a Bis-Urea Phenylethynylene Self-Assembled Nanoreactor for [2 + 2] Photodimerizations

Confined environments can be used to alter the selectivity of a reaction by influencing the organization of the reactants, altering the mobility of trapped molecules, facilitating one reaction pathway or selectively stabilizing the products. This manuscript utilizes a series of potentially photoreactive guests to interrogate the utility of the one-dimensional nanochannels of a porous host to absorb and facilitate the reaction of encapsulated guests. The host is a columnar self-assembled phenylethynylene bis-urea macrocycle, which absorbs guests, including coumarin, 6-methyl coumarin, 7-methyl coumarin, 7-methoxy coumarin, acenaphthylene, <i>cis</i>-stilbene, <i>trans-</i>stilbene, and <i>trans</i>-β-methylstyrene to afford crystalline inclusion complexes. We examine the structure of the host:guest complexes using powder X-ray diffraction, which suggests that they are well-ordered highly crystalline materials. Investigations using solid-state cross-polarized magic angle spinning ¹³C­{¹H}­CP-MAS NMR spectroscopy indicate that the guests are mobile relative to the host. Upon UV-irradiation, we observed selective photodimerization reactions for coumarin, 6-methyl coumarin, 7-methyl coumarin, and acenaphthylene, while the other substrates were unreactive even under prolonged UV-irradiation. Grand Canonical Monte Carlo simulations suggest that the reactive guests were close paired and preorganized in configurations that facilitate the photodimerization with high selectivity while the unreactive guests did not exhibit similar close pairing. A greater understanding of the factors that control diffusion and reaction in confinement could lead to the development of better catalysts