Homo- and Heterobimetallic Pd-, Ag-, and Ni-Hybrid Sale-Bis-NHC Complexes

salen ligands and N-heterocyclic carbenes (NHC) are among the most abundant structural ligand motifs for metal catalysis. In this article we present a modular approach to chiral ligands merging both ligand motifs for the preparation of bimetallic catalysts, in which one metal (M-1) is coordinated by the salen moiety and the other metal (M-2) binds to two NHCs. After selective complexation of M-1 = Pd-II, Ni-II ion into the salen N2O2 coordination site, heterobimetallic M-1/Ag(I) complexes were synthesized which could be further utilized for the preparation of homo- and heterobimetallic M-1/Pd-II complexes by an oxidative transmetalation to Pd(0) as a key step of the catalyst synthesis. The structures of the bimetallic complexes and the intermetallic distances strongly depend on the counterions of M-2 = Pd-II, as revealed by X-ray and UV-vis studies. In the absence of an anionic ligand with suitable Lewis basicity the salen oxygen atoms serve as bridging ligands for both metals. A preliminary investigation into catalysis showed that the complexes are capable of catalyzing the 1,4-addition of oxindoles to 2-nitrostyrene.