Matrix and Polymer Soft-Landing Isolation of Selected Acids with Pyridine and Poly(4-vinylpyridine): A Comparative Infrared Spectroscopic Study of Hydrogen Bonding

Hydrogen bonding plays a key role in the formation of nanostructures, as it is the “glue” between layers that are built by the layer-by-layer technique. Poly(4-vinylpyridine) (PVP) is one of the most commonly used polymers in these sandwich-structured films, often in conjunction with poly(carboxylic acid)s such as poly(acrylic acid) in the PVP/PAA interpolymer complex. In addition, PVP is commonly used as a polymer matrix for embedding semiconductor nanoparticles. In this study, hydrogen-bonded complexes of water, formic acid, and pentachlorocyclopropane, with pyridine in a traditional matrix isolation experiment and PVP in a novel “polymer soft-landing” isolation experiment, have been characterized for the first time at 16 K. Changes in vibrational modes of the proton donor species and in some cases pyridine modes provided ample evidence for complex formation. In the case of water and pentachlorocyclopropane, the matrix and polymer soft-landing results were quite similar, whereas formic acid formed a significantly different complex with pyridine in the argon matrix than with the pyridine ring on the PVP polymer. This work demonstrates clearly the benefit of using both the conventional matrix isolation technique and our polymer soft-landing variation in tandem to probe the structure of these complexes and thus elucidate the nature of the C–H···N, C–H···OC, and O–H···N linkages

Allosteric Regulation of Supramolecular Oligomerization and Catalytic Activity via Coordination-Based Control of Competitive Hydrogen-Bonding Events

Herein, we demonstrate that the activity of a hydrogen-bond-donating (HBD) catalyst embedded within a coordination framework can be allosterically regulated in situ by controlling oligomerization via simple changes in coordination chemistry at distal Pt­(II) nodes. Using the halide-induced ligand rearrangement reaction (HILR), a heteroligated Pt­(II) triple-decker complex, which contains a catalytically active diphenylene squaramide moiety and two hydrogen-bond-accepting (HBA) ester moieties, was synthesized. The HBD and HBA moieties were functionalized with hemilabile ligands of differing chelating strengths, allowing one to assemble them around Pt­(II) nodes in a heteroligated fashion. Due to the hemilabile nature of the ligands, the resulting complex can be interconverted between a flexible, semiopen state and a rigid, fully closed state in situ and reversibly. FT-IR spectroscopy, ¹H DOSY, and ¹H NMR spectroscopy titration studies were used to demonstrate that, in the semiopen state, intermolecular hydrogen-bonding between the HBD and HBA moieties drives oligomerization of the complex and prevents substrate recognition by the catalyst. In the rigid, fully closed state, these interactions are prevented by steric and geometric constraints. Thus, the diphenylene squaramide moiety is able to catalyze a Friedel–Crafts reaction in the fully closed state, while the semiopen state shows no reactivity. This work demonstrates that controlling catalytic activity by regulating aggregation through supramolecular conformational changes, a common approach in Nature, can be applied to man-made catalytic frameworks that are relevant to materials synthesis, as well as the detection and amplification of small molecules

A Multi-State, Allosterically-Regulated Molecular Receptor With Switchable Selectivity

A biomimetic, ion-regulated molecular receptor was synthesized via the Weak-Link Approach (WLA). This structure features both a calix[4]­arene moiety which serves as a molecular recognition unit and an activity regulator composed of hemilabile phosphine alkyl thioether ligands (P,S) chelated to a Pt­(II) center. The host–guest properties of the ion-regulated receptor were found to be highly dependent upon the coordination of the Pt­(II) center, which is controlled through the reversible coordination of small molecule effectors. The environment at the regulatory site dictates the charge and the structural conformation of the entire assembly resulting in three accessible binding configurations: one closed, inactive state and two open, active states. One of the active states, the semiopen state, recognizes a neutral guest molecule, while the other, the fully open state, recognizes a cationic guest molecule. Job plots and ¹H NMR spectroscopy titrations were used to study the formation of these inclusion complexes, the receptor binding modes, and the receptor binding affinities (<i>K</i>_a) in solution. Single crystal X-ray diffraction studies provided insight into the solid-state structures of the receptor when complexed with each guest molecule. The dipole moments and electrostatic potential maps of the structures were generated via DFT calculations at the B97D/LANL2DZ level of theory. Finally, we describe the reversible capture and release of guests by switching the receptor between the closed and semiopen configurations via elemental anion and small molecule effectors

Boron-Dipyrromethene-Functionalized Hemilabile Ligands as “Turn-On” Fluorescent Probes for Coordination Changes in Weak-Link Approach Complexes

Herein we report a new class of hemilabile ligands with boron-dipyrromethene (Bodipy) fluorophores that, when complexed to Pt­(II), can signal changes in coordination mode through changes in their fluorescence. The ligands consist of phosphino-amine or phosphino-thioether coordinating moieties linked to the Bodipy’s meso carbon via a phenylene spacer. Interestingly, this new class of ligands can be used to signal both ligand displacement and chelation reactions in a fluorescence “turn-on” fashion through the choice of weakly binding heteroatom in the hemilabile moiety, generating up to 10-fold fluorescence intensity increases. The Pt­(II) center influences the Bodipy emission efficiency by regulating photoinduced electron transfer between the fluorophore and its meso substituent. The rates at which the excited Bodipy-species generate singlet oxygen upon excitation suggest that the heavy Pt­(II) center also influences Bodipy’s emission efficiency by affecting intersystem crossing from the Bodipy excited singlet to excited triplet states. This signaling strategy provides a quantitative read-out for changes in coordination mode and potentially will enable the design of new molecular systems for sensing and signal amplification

Modulation of Electronics and Thermal Stabilities of Photochromic Phosphino–Aminoazobenzene Derivatives in Weak-Link Approach Coordination Complexes

A series of d⁸ transition-metal (Pt­(II) and Pd­(II)) coordination complexes incorporating phosphine-functionalized aminoazobenzene derivatives as hemilabile phosphino–amine (P,N) ligands were synthesized and studied as model weak-link approach (WLA) photoresponsive constructs. The optical and photochemical properties of these complexes were found to be highly influenced by various tunable parameters in WLA systems, which include type of metal, coordination mode, type of ancillary ligand, solvent, and outer-sphere counteranions. In dichloromethane, reversible chelation and partial displacement of the P,N coordinating moieties allow for toggling between aminoazobenzene- or pseudostilbene- and azobenzene-type derivatives. The reversible switching between electronic states of azobenzene can be controlled through either addition or extraction of chloride counterions and is readily visualized in the separation between π–π* and n−π* bands in the complexes’ electronic spectra. In acetonitrile solution, the WLA variables inherent to semiopen complexes have a significant impact on the half-lives of the corresponding <i>cis</i> isomers, allowing one to tune their half-lives from 20 to 21000 s, while maintaining photoisomerization behaviors with visible light. Therefore, one can significantly increase the thermal stability of a <i>cis</i>-aminoazobenzene derivative to the extent that single crystals for X-ray diffraction analysis can be grown for the first time, uncovering an unprecedented edge-to-face arrangement of the phenyl rings in the <i>cis</i> isomer. Overall, the azobenzene-functionalized model complexes shed light on the design parameters relevant for photocontrolled WLA molecular switches, as well as offer new ways of tuning the properties of azobenzene-based, photoresponsive materials