Investigations of Thermally Controlled Mechanochemical Milling Reactions

Mechanochemical milling reactions have received much attention recently as a green and highly efficient path toward various relevant materials. Control over the fundamental reaction parameters in the milling procedure, such as temperature and pressure of the reactor, is still in its infancy, and the vast majority of milling reactions are done by controlling just the basic parameters such as frequency and milling media weight. We demonstrate here how milling under controlled, prolonged, and variable heating programs accomplished in a new milling reactor introduces a new level of mechanochemical reactivity beyond what can be achieved by conventional mechanochemical or solution procedures and also reduces the time and energy costs of the milling process. The methodology is demonstrated on four varied systems: C–C bond-forming Knoevenagel condensation, selective C–N bond formation for amide/urea synthesis, selective double-imine condensation, and solid-state formation of an archetypal open metal-organic framework, MOF-74. The potential of this methodology is best demonstrated on the one-pot selective synthesis of four complex products containing combinations of amide, amine, or urea functionalities from the same and simple acyl azide and diamine reactants. Principal control over this enhanced reactivity and selectivity stemmed from the application of specific heating regimes to mechanochemical processing accomplished by a new, in-house developed mechanochemical reactor. As even a moderate increase in temperature strongly affects the selectivity and the rate of mechanochemical reactions, the results presented are in line with recent challenges of the accepted theories of mechanochemical reactivity.