Homo- and Heterobimetallic Pd–, Ag–, and Ni–Hybrid Salen–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
(M1) is coordinated by the salen moiety and the other metal
(M2) binds to two NHCs. After selective complexation of
M1 = PdII, NiII ion into the salen
N2O2 coordination site, heterobimetallic M1/Ag­(I) complexes were synthesized which could be further utilized
for the preparation of homo- and heterobimetallic M1/PdII 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 M2 = PdII, 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.