On the Missed Palladium in Dibenzylidenacetone Complexes

The palladium complexes with dibenzylidenacetone (dba) ligand (Pd 2 (dba) 3 or Pd(dba) 2 ) are used as precursors of Pd(0) for cross coupling reaction (Heck, Suzuki, Stille and Buckwald coupling reactions) in combination with phosphine ligands. The real tridimensional structure of these palladium catalyst is dubious and in particular the difference between Pd(dba) 3 , Pd(dba) 2 and Pd 2 (dba) 3 is not so clear. 1 Moreover, some catalytic tests showed that the activity of Pd(dba) 2 and Pd 2 (dba) 3 were almost the same 2 and the difference can be due to the quantity of dba present in the reaction medium that can be involve in the catalytic cycle. 3 We found, using XRD, NMR, IR and theoretical calculation, that the tridimensional supramolecular structural arrangement of the different form of palladium complex with dba is the same: it is formed by three molecules of dba with two site for the coordination of the metal as reported for Pd 2 (dba) 3.1,4 The two coordination have different energy and the site with higher energy could be only partially occupied. In conclusion, we can propose a unified formulation for all palladium dba catalysts.