Heterobimetallic Complexes Bridged by an Unsymmetrical Bis(NHC) Ligand: Study of Enhanced Catalytic Activity in Tandem Transformations and Understanding of Cooperativity between the Metal Centers

Abstract

The bis(azolium) salt [L1-H₂ ]Br₂ was found to serve as a suitable platform for accessing the heterobimetallic Ir^III -M (M=Pd^II /Au^I ) and Pd^II -Ir^III complexes. Initially, selective mono-metalation of [L1-H₂ ]Br₂ yielded an orthometalated Ir^III - or non-orthometalated Pd^II -complex. Sequential metalation of the mono-Ir^III complex resulted in the formation of heterobimetallic Ir^III -Pd^II /Au^I complexes. Similarly, a distinct heterobimetallic Pd^II -Ir^III complex was synthesized starting from the mono-Pd^II complex. Further, the corresponding homobimetallic Ir^III -Ir^III and Pd^II -Pd^II complexes were directly obtained from [L1-H₂ ]Br₂ . Additionally, monometallic Pd^II and Ir^III analogues were synthesized from [L2-H]Br and [L3-H]Br, respectively. The heterobimetallic Ir^III -Pd^II and Pd^II -Ir^III complexes were then evaluated as catalysts in various one-pot tandem catalytic reactions in which they demonstrated superior activity than the mixtures of both their corresponding homobimetallic Ir^III -Ir^III /Pd^II -Pd^II and monometallic Ir^III /Pd^II counterparts, under the constant concentrations of metal centers. Moreover, while comparing complexes Ir^III -Pd^II and Pd^II -Ir^III , the former exhibits higher activity in all the studied reactions. All these findings suggest the presence of some form of cooperativity between the two metal centers (Ir and Pd) connected by a single ligand framework in Ir^III -Pd^II and Pd^II -Ir^III complex, with Ir^III -Pd^II displaying better cooperativity that has been validated by electrochemical, NMR, and DFT studies.

Keywords: N-heterocyclic carbenes; heterobimetallic complexes; organometallic chemistry; synergistic cooperation; tandem catalysis.