AMES, Iowa - When chemists want to combine two or more organic materials, ordinarily they use a solvent to carry out a reaction that results in the desired compound. Researchers at the U.S. Department of Energy's Ames Laboratory have found a way to combine organic materials in solid state without the use of solvents. This revolutionary solvent-free process means that environmentally harmful solvents, such as benzene, dichloromethane and others, could be removed from many of the chemical processes used to produce millions of consumer and industrial products. Organic materials have stable molecular crystal structures that usually keep them from reacting when they're in a solid state. Solvents break down these crystal structures so the dissolved materials can be combined to form new compounds. But the process leaves behind a contaminated solvent that often can't be reused, creating disposal problems.
"Most of these solvents pose serious risks to health and the environment and are costly to decontaminate and dispose of," Ames Laboratory senior scientist Vitalij Pecharsky said. "So if you could produce organic materials without using solvents, it would have a great impact on both materials science and chemistry."
The discovery, to be announced in the June issue of the Journal of the American Chemical Society, uses high-energy ball-milling, a well-known process for producing and modifying metal alloys. Materials to be processed are placed in a hardened steel vial along with steel balls. The vial is vigorously shaken and mechanical energy transferred into the system alters the crystallinity of the solids and provides mass transfer, eventually combining the materials into new compounds. Earlier findings by Ames Lab researchers were also published in the March issue of Chemical Communications.
To test their theory, Pecharsky and Ames Lab colleagues Viktor Balema, Jerzy Wiench, and Marek Pruski turned to well-known and well-documented chemical transformations of organophosphorus compounds, including the reaction discovered by Nobel Prize-winner Georg Wittig. The Wittig reaction is used to transform phosphorus ylides and aldehydes or ketones into unsaturated hydrocarbons - irreplaceable building blocks in the preparation of numerous organic materials and pharmaceuticals.
Traditionally, phosphorus ylides are generated by treating phosphonium salts with strong bases. And phosphonium salts are obtained from organic phosphines and alkyl halogenides, again in solution. "I would never have believed that solvents could be excluded from all these reactions if I hadn't done it myself " said Ames Laboratory associate scientist Viktor Balema.
A number of solid phosphonium salts where prepared during high-energy ball-milling of triphenylphosphine with solid organic bromides. They were further used for mechanically induced preparation of several stable phosphorus ylides, or in the solvent-free Wittig reaction. The latter was also performed by ball-milling of commercially available phosphonium salts, solid aldehydes or ketones, and anhydrous potassium carbonate (K2CO3) without a solvent.
The occurrence of these reactions during mechanical processing was confirmed in a truly interdisciplinary fashion by combining techniques usually used independently. X-ray diffraction revealed the changes in the structure, differential thermal analysis provided thermodynamic information, and solid-state nuclear magnetic resonance (or NMR) was used for identifying the materials.
"Solid-state NMR spectroscopy is the primary tool that my group uses for the studies of materials and chemical reactions," said Pruski. "NMR's usefulness for probing the structure of materials strongly depends upon the ability to obtain the high-resolution spectra, which serve as fingerprints of the physico-chemical surroundings of the studied atoms." Pruski and Wiench are also interested in the development of new spectroscopic tools in the area of NMR. "However, in the case of mechanically induced reactions, the well-established technique of magic angle spinning (MAS) NMR proved most useful," Wiench said. "We were able to monitor the progress of these reactions and identify their products in a very straightforward manner."
Nearly all of the discovered transformations, previously performed exclusively in a solution, were found to be exceptionally efficient and selective in the solid state. Furthermore, they can be carried out consecutively, or as "one-pot" processes, when components required for performing several different processes are ball-milled together in the same vial. "Remarkably, a 'one-pot' Wittig-type reaction between phosphines, organic halogenides, aldehydes or ketones, and a base is impossible in a solution, but it has been successfully carried out in a mill without a solvent," said Pecharsky.
Another striking demonstration of the fundamental importance for both materials and chemical sciences, is the generation of non-stabilized triphenylmethylenephosphorane during ball-milling of solid (methyl)triphenylphosphonium bromide and anhydrous potassium carbonate.
"Generally, the preparation of this ylide in a solution requires very strong bases since the strength of K2CO3 is not sufficient for removing the hydrogen ion from the methyl group in the phosphonium salt," said Balema.
A provisional patent application has been filed for the process. In the meantime, the group is working to further explore mechanically driven solid-state processes such as solvent-free transformations of transition metal complexes and supramolecular design.
The research is funded by the Department of Energy's Office of Basic Energy Sciences Divisions of Materials Sciences and Chemical Sciences. Ames Laboratory is operated for the DOE by Iowa State University. The Lab conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.
Materials provided by Ames Laboratory. Note: Content may be edited for style and length.
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