Heterobimetallic Carbene Complexes Bearing Cyclometalated IrIII/RhIII and Mixed NHC∧Py/PPh3 Coordinated PdII Centers: Structures and Tandem Catalysis

Heterobimetallic complexes bearing NHC donor ligands are gaining immense popularity in organometallic chemistry and tandem catalysis. It is known that the NHC reacts with Pd-II in the presence of pyridine to yield PEPPSI type complexes and the NHC ligands having ortho-C-H proton easily orthometalate to Ir-III or Rh-III centers. Combining these two methodologies in a stepwise fashion, we present here a series of heterobimetallic Ir-III-Pd-II and Rh-III-Pd-II complexes from a dicarbene donor ligand featuring cyclometalated Ir-III or Rh-III and mixed NHC perpendicular to Py /PPh3 coordinated Pd-II centers. All the heterobimetallic complexes have been structurally characterized by X-ray crystallographic analysis. The heterobimetallic complexes featuring mixed NHC perpendicular to PPh3 coordinated Pd-II centers show better activity in tandem Suzuki-Miyaura/transfer hydrogenation reactions compared to both, the heterobimetallic complexes possessing PEPPSI type Pd(II)centers, and the equimolar mixture of their mononuclear Pd-II and Rh-III or Ir-III counterparts. The heterobimetallic complex featuring cyclometalated Ir-III and mixed NHC perpendicular to PPh3 coordinated Pd-II center shows excellent selectivity for 4-biphenylmethanol (isolated yield: 92 %) in tandem catalysis.

Automated summary

Heterobimetallic complexes with NHC donor ligands show promising catalytic activity and selectivity in tandem catalysis, making them potential candidates for various organic transformations.