To see if the ammonium carboxylate salt 4 acts as an intermediate before the amide bond formation, milling of 1 and 2 in 2:1 ratio at 190 °C was stopped after 3 min of milling. Indeed, PXRD analysis confirmed the presence of 4 and ascertain it as a reaction intermediate. Furthermore, single crystals of 4 which we used to collect SCXRD data for crystal structure solution were grown directly in a milling jar by cooling the reaction mixture after hightemperature ball milling (Table , entry 4).