Isurika R. Fernando

Redox Control of the Binding Modes of an Organic Receptor

The modulation of noncovalent bonding interactions by redox processes is a central theme in the fundamental understanding of biological systems as well as being ripe for exploitation in supramolecular science. In the context of host-guest systems, we demonstrate in this article how the formation of inclusion complexes can be controlled by manipulating the redox potential of a cyclophane. The four-electron reduction of cyclobis(paraquat-p-phenylene) to its neutral form results in altering its binding properties while heralding a significant change in its stereoelectronic behavior. Quantum mechanics calculations provide the energetics for the formation of the inclusion complexes between the cyclophane in its various redox states with a variety of guest molecules, ranging from electron-poor to electron-rich. The electron-donating properties displayed by the cyclophane were investigated by probing the interaction of this host with electron-poor guests, and the formation of inclusion complexes was confirmed by single-crystal X-ray diffraction analysis. The dramatic change in the binding mode depending on the redox state of the cyclophane leads to (i) aromatic donor-acceptor interactions in its fully oxidized form and (ii) van der Waals interactions when the cyclophane is fully reduced. These findings lay the foundation for the potential use of this class of cyclophane in various arenas, all the way from molecular electronics to catalysis, by virtue of its electronic properties. The extension of the concept presented herein into the realm of mechanically interlocked molecules will lead to the investigation of novel structures with redox control being expressed over the relative geometries of their components.

Single-color pseudorotaxane-based temperature sensing

Colored pseudorotaxane solutions can be used to assess temperature changes over large temperature windows. The color intensity of our novel pseudorotaxane systems decreases gradually from −50 to +50 °C with no shift in absorption maximum, making these and similar pseudorotaxanes attractive candidates for single-wavelength colorimetric temperature sensors.

Sliding-Ring Catenanes

Template-directed protocols provide a routine approach to the synthesis of mechanically interlocked molecules (MIMs), in which the mechanical bonds are stabilized by a wide variety of weak interactions. In this Article, we describe a strategy for the preparation of neutral [2]catenanes with sliding interlocked electron-rich rings, starting from two degenerate donor-acceptor [2]catenanes, consisting of a tetracationic cyclobis(paraquat-p-phenylene) cyclophane (CBPQT(4+)) and crown ethers containing either (i) hydroquinone (HQ) or (ii) 1,5-dioxynaphthalene (DNP) recognition units and carrying out four-electron reductions of the cyclophane components to their neutral forms. The donor-acceptor interactions between the CBPQT(4+) ring and both HQ and DNP units present in the crown ethers that stabilize the [2]catenanes are weakened upon reduction of the cyclophane components to their radical cationic states and are all but absent in their fully reduced states. Characterization in solution performed by UV-vis, EPR, and NMR spectroscopic probes reveals that changes in the redox properties of the [2]catenanes result in a substantial decrease of the energy barriers for the circumrotation and pirouetting motions of the interlocked rings, which glide freely through one another in the neutral states. The solid-state structures of the fully reduced catenanes reveal profound changes in the relative dispositions of the interlocked rings, with the glycol chains of the crown ethers residing in the cavities of the neutral CBPQT(0) rings. Quantum mechanical investigations of the energy levels associated with the four different oxidation states of the catenanes support this interpretation. Catenanes and rotaxanes with sliding rings are expected to display unique properties.

Mapping the supramolecular chemistry of pyrazole-4-sulfonate: layered inorganic–organic networks with Zn2+, Cd2+, Ag+, Na+ and NH4+, and their use in copper anticorrosion protective films

Five compounds based on the versatile pyrazole-4-sulfonate anion (4-SO3-pzH = L−) were synthesized by the reaction of ligand HL with ZnO, CdCO3, Ag2O, NaOH and NH3, respectively. Crystals of Zn(4-SO3-pzH)2(H2O)2, Cd(4-SO3-pzH)2(H2O)2, Ag(4-SO3-pzH), Na(4-SO3-pzH)(H2O) and NH4(4-SO3-pzH) were obtained from aqueous solutions upon evaporation, and were characterized by single-crystal X-ray diffraction, IR and NMR spectroscopy, thermogravimetric analysis and copper corrosion inhibition experiments. We found that the non-isomorphous, 3-dimensional inorganic–organic layered solid state structure of these compounds is determined by an intricate interplay between the size, charge and coordination preference of the cation, and an extended lattice of hydrogen bonds and aromatic interactions. Ligand L− incorporates a host of different binding capabilities (metal coordination through the pyrazole N-atom and/or the sulfonate O-atom, hydrogen-bonding both as donor and acceptor, π–π and C–H⋯π interactions). Thin films formed by these complexes on copper metal surfaces were studied by optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. ZnL2, CdL2, AgL and NH4L, in addition to the free ligand HL, were tested as corrosion inhibitors on copper metal surfaces at three different pH values (2, 3 and 4), and the corrosion rates were quantified. Significant corrosion protection was observed with all compounds at pH 4 and 3.

Metal-binding studies of linear rigid-axle [2]pseudorotaxanes with in situ generated anionic metal halide complexes

A systematic study of metal–pseudorotaxane binding, using the linear N,N′-bis(4-pyridyl)-4,4′-bipyridinium axle (PyBP2+) and aromatic crown ethers bis(1,5-naphtho)-32-crown-8 (BN32C8), bis(1,5-naphtho)-38-crown-10 (BN38C10) and bis-para-phenylene-34-crown-10 (BPP34C10), is presented. The three corresponding [2]pseudorotaxanes, including the novel, fully characterized (visible absorption and nuclear magnetic resonance spectra, association constant, electrochemistry, crystal structure) [PyBP/BPP34C10]2+ system, were each reacted with MX2 (M = Zn2+, Cd2+, Hg2+; X = Cl−, Br−, I−). Of the twenty-seven different pseudorotaxane/MX2 combinations explored, fifteen yielded single-crystals containing a pseudorotaxane unit, and were characterized by X-ray diffraction; in eight of those crystals metal–pseudorotaxane binding was observed. The results reflect the lower association constant of [PyBP/BPP34C10]2+ (170 M−1) than the corresponding ones of [PyBP/BN32C8]2+ (870 M−1) and [PyBP/BN38C10]2+ (420 M−1) in solution, where pseudorotaxanes are in equilibrium with the corresponding free axle and wheel components. In all cases, the isolated crystals contain anionic metal halide species, such as singly charged MX3− (M = Zn, Cd; X = Cl, Br, I) or doubly charged CdX42− (X = Br, I) and Hg2X62− (X = Cl, Br, I). Ordered 3D-arrays of perfectly aligned pseudorotaxane units are found in the dichroic mercury-based crystals. Our study demonstrates that although specific intermolecular interactions (collectively called crystal packing forces), may occasionally interfere with the crystallization of metal–pseudorotaxane complexes based on pseudorotaxanes with low association constants, the use of pseudorotaxanes instead of synthetically more challenging rotaxanes is a promising approach for the construction of metal–organic rotaxane frameworks (MORFs).

X-Shaped Oligomeric Pyromellitimide Polyradicals

The synthesis of stable organic polyradicals is important for the development of magnetic materials. Herein we report the synthesis, isolation, and characterization of a series of X-shaped pyromellitimide (PI) oligomers (Xn-R, n = 2-4, R = Hex or Ph) linked together by single C-C bonds between their benzenoid cores. We hypothesize that these oligomers might form high-spin states in their reduced forms because of the nearly orthogonal conformations adopted by their PI units. 1H and 13C nuclear magnetic resonance (NMR) spectroscopies confirmed the isolation of the dimeric, trimeric, and tetrameric homologues. X-ray crystallography shows that X2-Ph crystallizes into a densely packed superstructure, despite the criss-crossed conformations adopted by the molecules. Electrochemical experiments, carried out on the oligomers Xn-Hex, reveal that the reductions of the PI units occur at multiple distinct potentials, highlighting the weak electronic coupling between the adjacent redox centers. Finally, the chemically generated radical anion and polyanion states, Xn-Hex•- and Xn-Hexn(•-), respectively, were probed extensively by UV-vis-NIR absorption, EPR, and electron nuclear double resonance (ENDOR) spectroscopies. The ENDOR spectra of the radical monoanions Xn-Hex•- reveal that the unpaired electron is largely localized on a single PI unit. Further reductions of Xn-Hex•- yield EPR signals (in frozen solutions) that can be assigned to spin-spin interactions in X2-Hex2(•-), X3-Hex3(•-), and X4-Hex4(•-). Taken together, these findings demonstrate that directly linking the benzene rings of PIs with a single C-C bond is a viable method for generating stabilized high-spin organic anionic polyradicals.