Metal nanocrystals have been prepared in onium salts using supercritical carbon dioxide assisted synthesis. During the process, metal salts precursors are reduced by H-2 in an organic solvent free media, the onium salts allowing both the stabilization of the nanostructured material and the preparation of an air and moisture stable powder made of embedded metal nanocrystals. Those catalysts proved to be highly effective in the hydroboration of alkyne process. (C) 2014 Elsevier B.V. All rights reserved.