Macrocyclic Salen–Bis-NHC Hybrid Ligands and Their Application to the Synthesis of Enantiopure Bi- and Trimetallic Complexes

Salen and NHC ligands are among the most important ligands for homogeneous catalysis. We have recently reported bimetallic complexes, in which both motifs have been merged for the first time. However, the intermetallic distances, which play a crucial role for cooperative bimetallic catalysis, were probably not appropriate in these first-generation hybrid catalysts. To generate heterobimetallic salen/NHC hybrid complexes with intermetallic distances suitable for cooperative catalysis, chiral macrocyclic hybrid ligands featuring a salen and two linked NHC donor moieties have been prepared in the present study. For the ligand formation, chiral enantiopure diamines as well as chiral enantiopure bisimidazoles were employed, and a matched/mismatched situation was found depending on the configuration of both chirality sources. Regioselective complexation of Zn­(II), Ni­(II), and Pd­(II) by the salen N2O2 coordination sphere was efficiently accomplished. Subsequent coordination of the NHC units was achieved for Ag­(I), Cu­(I), Au­(I), and Pd­(II), in the latter case by oxidative transmetalation with Pd2(dba)3. X-ray crystal structure analyses for Ni/Ag2 and Pd/Ag2 complexes show strongly puckered macrocycles, in which one of the NHC-bound Ag­(I) centers is in close proximity to the salen-bound Ni­(II) or Pd­(II) centers and in which this Ag­(I) apparently interacts with both salen O-donor atoms. Preliminary data for the 1,4-addition of an oxindole to a nitroolefin and for the Conia-ene reaction of an α-cyanoacetate are reported.