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

Salen and NHC ligands are mond the most important ligands for homogenous 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 intermtallic distances suitable for cooperative catalysis, chiral macrocyclic hybrid ligands featuring a salen and two linked NHC donor moieties have been prepared inthe 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 Pd-2(dba)(3). X-ray crystal structure analyses for Ni/Ag-2 and Pd/Ag-2 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 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 alpha-cyanoacetate are reported.