Frank W. Fowler

Single-Crystal-to-Single-Crystal Topochemical Polymerizations by Design

The polymerization of simple conjugated dienes has long been of interest: polydienes occur throughout Nature, and polyisoprene and its analogues form the basis of entire industries. In contrast, the polymers of similar small conjugated compounds, diacetylenes, trienes, and triacetylenes, are either unknown or laboratory curiosities. For 40 years, the only viable synthetic method for the 1,4-polymerization of a diacetylene was a topochemical polymerization in a condensed phase. But such an approach is hit or miss: if the diacetylene monomers have a solid-state structure preorganized at distances matching the repeat distance in the final polymer, then thermal or photochemical energy can bring about the polymerization. However, most monomers lack the proper structural parameters and simply do not react. As discussed in this Account, we have developed a supramolecular host-guest strategy that imposes the necessary structural parameters upon a diacetylene monomer that otherwise does not polymerize. We apply this strategy in the synthesis of new types of conjugated polymers made from diacetylenes, triacetylenes, and trienes. To implement the host-guest strategy, we chose a host that would self-assemble into a supramolecular structure with the requisite intermolecular spacing. For diacetylenes, the ideal spacing is 4.9 A, and the oxalamides, which routinely crystallize with a spacing of 5 A, make ideal host molecules. We chose specific oxalamide host substituents that bind to the diacetylene guest molecule, typically through hydrogen bonding. We have focused upon the single-crystal-to-single-crystal polymerizations, allowing us to obtain and characterize the polymers in perfect crystalline form and to define and better understand the reaction trajectories. We have prepared several new classes of polydiacetylenes using this strategy, including the first terminal polydiacetylenes and an aryl-substituted diacetylene. Interestingly, to prepare poly(diiododiacetylene), we used halogen bonds to bind the host and guest. The simplest polydiacetylene known, poly(diiododiacetylene), lacks the side chains that complicate the structures of similar previous polymers. Future studies should provide insights into the role of such side chains in conjugated materials. We further demonstrated the strength of the host-guest strategy by moving from the polydiacetylenes to the polytriacetylenes. Although the structural requirements for a triacetylene polymerization had been stated decades ago, no one had ever found a triacetylene with the requisite spacing of 7.4 A. We designed a series of pyridine-substituted vinylogous amide hosts to achieve this spacing. Cocrystallization of these host molecules with a triacetylene dicarboxylic acid gave us the desired structure. Using thermal annealing, we completed the synthesis of the triacetylene polymer.

Preparation and Structure of a Tubular Addition Polymer: A True Synthetic Nanotube

The structure of a synthetic nanotube prepared by the solid-state polymerization of a stacked column of diacetylene-based macrocycles has been determined. A polyether macrocycle monomer with two parallel diacetylene functionalities was prepared. Its crystal structure revealed that the compound crystallizes with structural parameters suitable for topochemical polymerization. Slow annealing of a single crystal for 35 days brought about a single-crystal-to-single-crystal polymerization resulting in the first experimentally determined structure of a tubular addition polymer.

A rational design of molecular materials

Supramolecular chemistry was used to solve two practical problems, the preparation of layered polydiacetylenes and the first ever 1,6-polymerization of a triacetylene to give a polytriacetylene Copyright

Pressure-Induced Polymerization of Diiodobutadiyne in Assembled Cocrystals

Diiodobutadiyne forms cocrystals with bis(pyridyl)oxalamides in which the diyne alignment is near the ideal parameters for topochemical polymerization to the ordered conjugated polymer, poly(diiododiacetylene) (PIDA). Nonetheless, previous efforts to induce polymerization in these samples via heat or irradiation were unsuccessful. We report here the successful ordered polymerization of diiodobutadiyne in these cocrystals, by subjecting them to high external pressure (0.3-10 GPa). At the lower end of the pressure range, the samples contain primarily monomer, as demonstrated by X-ray diffraction studies, but some polymerization does occur, leading to a pronounced color change from colorless to blue and to the development of intense Raman peaks at 962, 1394, and 2055 cm-1, corresponding to the poly(diacetylene). At higher pressures, the samples turn black and contain primarily polymer, as determined by solid-state NMR and Raman spectroscopy. Both density functional theory calculations (B3LYP/LanL2DZ) and comparisons to authentic samples of PIDA have confirmed the data analysis.

Synthesis and reactions of 1-Azirines

Publisher Summary Azirine is the term used to describe a three-membered heterocycle containing one nitrogen atom and one double bond. There are two isomeric azirines (1 and 2), which have been designated by Chemical Abstracts and “The Ring Index,” as 1H- and 2H-azirine, respectively. The 2-azirine ring system is of theoretical interest since it is a cyclic conjugated structure containing 477 electrons and is predicted by Huckels rule not to be stabilized by cyclic delocalization. Electronically it is analogous to cyclobutadiene. The chapter discusses about synthesis of 1-azirines and neber reaction and related reactions, the preparation and decomposition of vinyl azides, photolysis and pyrolysis of isoxazoles, and some miscellaneous 1 -azirine synthesis. Reactions of 1-azirines in case of electrophilic reagents, nucleophilic reagents, and cycloadditions are also studied. It is clear from this review that our knowledge of the azirine ring system is still in its infancy. Although a considerable number of 1-azirines have been prepared in a very short time, discovery of new and refinement of old synthetic techniques are clearly needed. Finally, the 1-azirine ring system is potentially a very valuable starting point for the preparation of new and unusual heterocyclic compounds. Already a number of very interesting aziridines have been prepared and 1 -azirines were key intermediates for the preparation of the 1 -azabicyclo[2.1 .0]pentane and 1-azabicyclo[l.l .0]butane ring systems.

The application of the 2-amino-4-pyrimidones to supramolecular synthesis

Abstract A strategy for preparation of two dimensional layered compounds is introduced. This strategy explores the possibility of using the 2-amino-4( H )-pyrimidone ring system as a host for dicarboxylic acids guests. This strategy is successful for the orientation of adipic acid into a two dimensional β-network with a molecular repeat distance of 6.5 A.

The Diels-Alder reaction of 1-azadienes. The effect of an α-cyano substituent

Abstract N-Acyl-α-cyano-1-azadienes have been observed to be reactive and anti selective dienes in the intramolecular hetero Diels-Alder reaction.

Silver coordination and hydrogen bonds : A study of competing forces

Abstract A series of six bipyridyl ligands, amino-methylpyridine ureas, and oxalamides have been synthesized and complexes with silver salts have been prepared. The 3-pyridyl ligands form silver complexes with a 2:1 ligand to metal ratio: Ag(3u)2BF4, Ag(3o)2BF4, and Ag(3o)2NO3. In these three complexes the ligands form hydrogen-bonded one-dimensional α-networks in accordance with an anticipated design. Each silver cation is four coordinate, bringing four of the α-networks together to form a channeled solid. The anions, NO3− or BF4−, occupy the channels. The 4-pyridyl and 2-pyridyl ligands form silver complexes with a 1:1 ligand-to-metal ratio. The silver cations of the 4-pyridyl ligand complexes, Ag(4u)NO3·H2O and Ag(4o)NO3, have linear two coordination geometries. The 2-pyridyl urea complex, Ag(2u)BF4, features a three-coordinate silver cation with the third bond coming from the oxygen atom of a neighboring urea functionality. This results in a unique ladder structure. The 2-pyridyl oxalamide structure, Ag(2u)BF4, has two-coordinate silver cations and is helical, with a hydrogen bond between neighboring oxalamide functionalities.

Design of molecular solids: utility of the hydroxyl functionality as a predictable design element

In order to design a new molecular solid one needs a library of functional groups that will self assemble into predictable structural patterns. To test the utility of the hydroxyl group for this purpose, a series of ureas and oxalamides with side-arm substituents containing the hydroxyl functionality were prepared. X-Ray crystallographic analysis of five such compounds revealed a variety of structural features indicating that the hydroxyl group alone was not a predictable design element. However, when a pair of co-crystals of dihydroxyl substituted oxalamides with dipyridyl compounds were prepared, supramolecular structures were prepared consistent with predictions. This suggests that the hydroxyl group coupled to a pyridine hydrogen-bond acceptor is a useful design element for the preparation of molecular networks.

What have We Learned about Topochemical Diacetylene Polymerizations

A series of five polydiacetylenes have been synthesized via topochemical polymerizations of the corresponding monomers. The requisite structural parameters for the polymerization were achieved by the use of a host–guest co-crystal strategy. The atom trajectories of the polymerization reactions are discussed.