Functional Materials from Self-Assembled Bis-urea Macrocycles

Abstract

CONSPECTUS: This Account highlights the work from our laboratories on bis-urea macrocycles constructed from two C-shaped spacers and two urea groups. These simple molecular units assembled with high fidelity into columnar structures guided by the three-centered urea hydrogen bonding motif and aryl stacking interactions. Individual columns are aligned and closely packed together to afford functional and homogeneous microporous crystals. This approach allows for precise and rational control over the dimensions of the columnar structure simply by changing the small molecular unit. When the macrocyclic unit lacks a cavity, columnar assembly gives strong pillars. Strong pillars with external functional groups such as basic lone pairs can expand like clays to accept guests between the pillars. Macrocycles that contain sizable interior cavities assemble into porous molecular crystals with aligned, well-defined columnar pores that are accessible to gases and guests. Herein, we examine the optimal design of the macrocyclic unit that leads to columnar assembly in high fidelity and probe the feasibility of incorporating a second functional group within the macrocycles. The porous molecular crystals prepared through the self-assembly of bis-urea macrocycles display surface areas similar to zeolites but lower than MOFs. Their simple one-dimensional channels are well-suited for studying binding, investigating transport, diffusion and exchange, and monitoring the effects of encapsulation on reaction mechanism and product distribution. Guests that complement the size, shape, and polarity of the channels can be absorbed into these porous crystals with repeatable stoichiometry to form solid host-guest complexes. Heating or extraction with an organic solvent enables desorption or removal of the guest and subsequent recovery of the solid host. Further, these porous crystals can be used as containers for the selective [2 + 2] cycloadditions of small enones such as 2-cyclohexenone or 3-methyl-cyclopentenone, while larger hosts bind and facilitate the photodimerization of coumarin. When the host framework incorporates benzophenone, a triplet sensitizer, UV-irradiation in the presence of oxygen efficiently generates singlet oxygen. Complexes of this host were employed to influence the selectivity of photooxidations of 2-methyl-2-butene and cumene with singlet oxygen. Small systematic changes in the channel and bound reactants should enable systematic evaluation of the effects of channel dimensions, guest dimensions, and channel-guest interactions on the processes of absorption, diffusion, and reaction of guests within these nanochannels. Such studies could help in the development of new materials for separations, gas storage, and catalysis.

Modulating the reactivity of chromone and its derivatives through encapsulation in a self-assembled phenylethynylene bis-urea host

“…These 'nanoreactors' provide confined environments to induce selectivity, modulate the reaction pathway, and potentially catalyze the reaction [8]. Our group studies how the photolysis of small organic molecules is altered and influenced by the encapsulation within the cylindrical channels of stable, porous, crystalline hosts [9]. These hosts are formed through the supramolecular assembly of bis-urea macrocycles, such as the phenylethynylene bis-urea 1, which selfassembles into columns that contain guest accessible channels of $0.9 nm diameter (Fig.…”

Thiourea binding with carboxylic acid promoted cationic ring-opening polymerization

“…These anion receptors are based on neutral [17] bi-or multi-H-bond donor compounds, which can host important classes of oxyanions or halide anions through H-bonding. Well-known hydrogen bond donor (HBD) compounds, such as (thio)ureas [18][19][20][21], (macrocyclic)amides [22], squarimides [23], and calixarenes [24], can recognize anions like phosphates, sulfates, carboxylates, and cyanide, and worked as hydrogen bonding catalysts 10…”

Structural analysis of bis-bispidine tetraazamacrocycle: Long-range weak interactions in a channeled organic crystal

“…Synthesis of 3,7-bis(bromoacetyl)-9,9-diethoxy-3,7-diazabicyclo[3.3.1] nonane (5) To a solution of N-Boc-N 0 -allylbispidinone 3 (2.50 g, 8.92 mmol) in anhydrous 1,2-dichloroethane (20 mL) was added 1-chloroethyl chloroformate (1.45 mL, 13.4 mmol) at ambient temperature under N 2 . The solution was stirred at 70°C for 1 h before the solvent was removed under high vacuum (0.05 Torr).…”

“…In addition to their functions, the structures of these materials have been studied extensively in an effort to elucidate the building processes as well as to develop novel crystals [1][2][3][4][5]. A common approach to such materials is through self-assembly of rigid macrocycles by stacking via non-covalent interactions.…”