Mechanocatalysis

Using mechanical forces to control the rate and course of chemical reactions is an exciting, but only superficially explored area of research. Traditionally, chemists have used heat ? a source of random kinetic energy ? to help molecules surmount the kinetic barriers to product formation. Currently, catalysts are the chemists favorite tool to increase both reaction rate and product selectivity. When homogeneous catalysts are provided with polymeric substituents, the possibility arises to apply mechanical forces in a highly directional fashion because stress fields in the medium are transmitted to the catalyst by the polymer chains. In one of the conceptually most simple realizations of this principle, mechanical force is used to switch a transition metal catalyst from an inactive dormant state to its active state by mechanical removal of a ligand. On-off switching of catalyst activity by a mechanical trigger has many potential applications, including self-healing materials, immobilized catalysts switched by flow or stress, shear sensors, and surface polymerization induced by friction at the macroscopic as well as at the nanoscale. Recent work in the group of Sijbesma has demonstrated this principle to be very effective in two distinct catalytic systems based on polymeric coordination complexes of N-heterocyclic carbenes (NHCs), which are activated by means of ultrasound in solution. One system, a Ruthenium alkylidene complex, is activated by ultrasound to catalyze a ring closing metathesis reaction or to initiate ring opening polymerization. The other system comprises silver(I) NHC complexes, which liberate free carbenes as active transesterification catalysts. The aim of the current proposal is to enable the group to take full advantage of the unique position created with these findings, and to explore the full scope of catalyst activation by mechanical force. In order to achieve this goal, three topics are defined that will broaden the chemical basis of mechanocatalysis. In the first project, the initial findings in the area of mechanically activated metathesis catalysts will be elaborated, and new dormant catalysts will be developed with tuned activity and improved reversibility of dissociation. Methods will be developed to immobilize the dormant catalysts on surfaces for application in the third project. In the second project, the full scope of mechanochemical activation of NHC organocatalysts will be explored. Focus will be on increasing susceptibility to scission of the dormant catalyst by incorporation in a polymeric network, and on widening the range of chemical transformations, including exploratory studies of alternative NHC-metal complexes in which the liberated metal is the active catalyst. The third project will explore the engineering aspects of applying mechanical forces to the catalyst as effectively as possible. To this end alternatives to ultrasonication such as shear in the solid state will be investigated. Finally, in this project inroads will be made to mechanochemistry at the nanoscale by using AFM on catalyst covered surfaces to induce polymerization.