Anne J. McNeil

Analyte-triggered gelation: initiating self-assembly via oxidation-induced planarization.

An alternative design strategy for inventing new triggered gelations was tested. Single crystal X-ray diffraction confirmed that an oxidation can convert a nonplanar dihydropyridine (1) that exhibits weak 1-D intermolecular interactions into a planar pyridine (2) that exhibits significant 1-D π-stacking interactions. Moreover, an electron-rich aryl ethynylene substituent was used to enhance the self-assembly process through donor−acceptor interactions. Pyridine 2 was discovered to form gels in mixtures of water with DMSO, alcohols, acetone, and DMF. Scanning electron microscopy revealed high-aspect-ratio fibers, and Raman spectroscopy confirmed that the π-stacking direction is coincident with the fiber axis. Gelation was induced at room temperature by adding a strong oxidant (cerium(IV) ammonium nitrate) to a solution of 1. A gel is also formed at room temperature when DMSO/H2O is added after the reaction of 1 with a weaker oxidant (nitric oxide).

Chain-Growth Polymerization of Aryl Grignards Initiated by a Stabilized NHC-Pd Precatalyst

An N-heterocyclic carbene-ligated palladium catalyst was discovered to mediate living, chain-growth polymerizations of both phenylene- and thiophene-based monomers. Polymerization of a fluorene-based monomer, on the other hand, did not proceed through a living, chain-growth pathway. Excitingly, block copolymerizations of phenylene and thiophene proceeded via a chain-growth pathway, regardless of the order of monomer addition. Although some chain termination was observed during these copolymerizations, this pathway could be minimized when the second monomer was added shortly after consumption of the first monomer. These results suggest that the catalyst resting-state at the end of polymerization is unstable. As a result, modifications to the NHC-scaffold or the 3-chloropyridine ligand will be necessary to generate an improved catalyst.

A general method for detecting protease activity via gelation and its application to artificial clotting

A modular system for detecting protease activity via enzyme-triggered gel formation is described. Protease-specific recognition sequences are utilized to achieve enzyme specificity. Artificial blood clotting is demonstrated by activating endogenous thrombin to trigger gelation in fibrinogen-deficient blood plasma.

Detecting a peroxide-based explosive via molecular gelation

A convenient and portable triacetone triperoxide (TATP) sensor was developed utilizing a thiol-to-disulfide oxidation to trigger a solution-to-gel phase transition. Using this method, TATP can be detected visually without any instrumentation.

Streamlined approach to a new gelator: inspiration from solid-state interactions for a mercury-induced gelation

A new gelator was discovered by identifying molecular scaffolds exhibiting 1D intermolecular interactions in the solid-state and synthesizing derivatives. Gelation can be triggered by adding Hg(OAc)(2) to a precursor molecule. The in situ gelation is selective for Hg(2+) over other metals.

Using polymeric additives to enhance molecular gelation: impact of poly(acrylic acid) on pyridine-based gelators

The effect of polymeric additives on molecular gelation was explored using poly(acrylic acid) and pyridine-based gelators. A significant reduction in the critical gel concentration (cgc) and an increase in gel strength were observed when the polymer was present during gel formation. Detailed studies revealed that the polymer is adsorbing onto the growing fibers, reducing the growth rates, and leading to thinner fibers. These and other morphological changes lead to improved gel properties by increasing the number of fiber–fiber entanglements. Several other polymers were briefly examined and these studies revealed that polymer structure is important. The polymer containing a complementary functional group relative to the gelator (e.g., H-bond donor/acceptor) provided the lowest cgc.

Accelerating Ni(II) precatalyst initiation using reactive ligands and its impact on chain-growth polymerizations.

Nickel(II) complexes with varying reactive ligands, which were designed to selectively accelerate the initiation rate without influencing the propagation rate in the chain-growth polymerization of π-conjugated monomers, were investigated. Precatalysts with electronically varied reacting groups led to faster initiation rates and narrower molecular weight distributions. Computational studies revealed that the reductive elimination rates are largely modulated by the ability of the two reacting arenes to stabilize the increasing electron density on the catalyst during reductive elimination. Overall, these studies provide insight into a key mechanistic step of cross-coupling reactions (reductive elimination) and highlight the importance of initiation in controlled chain-growth polymerizations.

Tools for Identifying Gelator Scaffolds and Solvents

Small molecule gelators are serendipitously discovered more often than they are designed. As a consequence, it has been challenging to develop applications based on the limited set of known materials. This synopsis highlights recent strategies to streamline the process of gelator discovery, with a focus on the role of unidirectional intermolecular interactions and solvation. We present these strategies as a series of tools that can be employed to help identify gelator scaffolds and solvents for gel formation. Overall, we suggest that this guided approach is more efficient than random derivatization and screening.

Evidence for a preferential intramolecular oxidative addition in Ni-catalyzed cross-coupling reactions and their impact on chain-growth polymerizations

Small molecule competition experiments were performed to determine whether Ni-catalyzed Kumada cross-coupling reactions proceed through an intramolecular oxidative addition. Indeed, preferential intramolecular oxidative addition was observed for all four complexes when stoichiometric quantities of competitive agent were present. At higher concentrations of competitive agent, the intramolecular pathway was still preferred when bidentate, electron-rich ligands were utilized, suggesting that these ligands promote the formation and reactivity of the key intermediate. To determine whether a similar pathway is involved in the polymerizations, (4-bromo-2,5-bis(hexyloxy)phenyl)magnesium bromide was polymerized in the presence and absence of competitive agent. The number-average molecular weights were lower and the molecular weight distributions were broadened substantially when competitive agent was present, consistent with the presence of competing intermolecular pathways. Because bidentate, electron-rich ligands suppressed these undesired intermolecular reactions, these ligands should lead to improved polymerization catalysts.

Effect of ligand electronic properties on precatalyst initiation and propagation in Ni-catalyzed cross-coupling polymerizations

The role of ligand-based electronic effects was investigated in the Ni-catalyzed polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride. The catalyst with the most electron-donating ligand outperformed the other catalysts by providing polymers with narrower molecular weight distributions. This result is attributed to both a suppression of competing reaction pathways (e.g., chain transfer and termination) as well as a relative acceleration of precatalyst initiation compared to propagation. Further studies revealed that, for all three catalysts, precatalyst initiation is slower than propagation, despite the fact that they exhibit the same rate-determining steps (i.e., reductive elimination) and have similar catalyst resting states. These results suggest that better control over the polymer molecular weight, end-functionality and sequence can be obtained with electron-rich catalysts, such as those described herein.