Preventing Pd–NHC bond cleavage and switching from nano-scale to molecular catalytic systems: amines and temperature as catalyst activators

Many reactions catalyzed by Pd complexes with N-heterocyclic carbene (NHC) ligands are performed in the presence of amines which usually act as coupling reagents or mild bases. However, amines can react with Pd/NHC complexes in a number of ways: enhancing molecular catalysis, causing the catalyst deactivation or triggering the ligandless modes of catalysis by producing NHC-free active palladium species. This study gains insight into conditions required for the efficient use of amines as activators of molecular Pd/NHC catalysis and preventing the undesirable reductive cleavage of the Pd–NHC bond in catalytic systems. Reactions of Pd/NHC complexes with various amines within a temperature range of 25–140 °C and thermal stability of the resulting amino-complexes are examined. The results indicate the major influence of the amine structure and reaction temperature on the catalyst transformation. In particular, thermal decomposition of Pd/NHC complexes with aliphatic amine ligands predominantly leads to reductive Pd–NHC bond cleavage, while deprotonation of the complexes with primary and secondary aliphatic amine ligands in the presence of strong bases at 25–60 °C promotes the activation of molecular Pd/NHC catalysis. Efficient Pd–PEPPSI complex–amine systems suitable for strong-base-promoted C–S cross-coupling reactions between aryl halides and thiols are suggested on the basis of these findings.