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Hetero- And Homobimetallic Complexes Bridged by a Bis(NHC) Ligand: Synthesis via Selective Sequential Metalation and Catalytic Applications in Tandem Organic Transformations
Published in ACS Publications
A (bis)azolium salt [L1-H2]Br2 (5), synthesized following multistep procedures, was realized to be a suitable platform for accessing the bis(NHC) ligand supported heterobimetallic IrIII-M (M = PdII/AuI) complexes via a sequential metalation strategy for their potential catalytic applications in one-pot tandem organic transformations. First, the reaction of 5 with 0.5 equiv of [Ir(Cp*)Cl2]2 selectively yielded a monometallic IrIII complex 6, which was further metalated using Pd(OAc)2/NaOAc to afford the heterobimetallic IrIII-PdII complex 7. On the other hand, complex 6 was reacted with Ag2O, followed by transmetalation with [Au(SMe2)Cl] in a one-pot manner, to yield the IrIII-AuI complex 8. Further, the related homobimetallic IrIII and PdII complexes 9 and 10, respectively, have also been synthesized directly from [L1-H2]Br2. All the homo/heterobimetallic complexes have been well-characterized by multinuclear NMR spectroscopy, ESI-mass spectrometry, and via single-crystal X-ray diffraction studies of the complexes 7, 8, and 10. The heterobimetallic IrIII-PdII complex 7 has been tested as a catalyst for three one-pot tandem catalytic reactions: (a) Suzuki-Miyaura coupling and transfer hydrogenation of ketones, (b) hydrodefluorination and transfer hydrogenation of ketones, and (c) hydrodehalogenation and transfer hydrogenation of imines. Importantly, the catalytic activity of heterobimetallic complex 7 in the above-mentioned reactions was found to be better than the mixture of their corresponding homobimetallic counterparts 9 and 10, keeping the concentration of the metal centers constant. These observations affirm some sort of cooperativity between the two metal centers (Ir and Pd) connected via a single ligand frame in 7 when catalytic activity is concerned, which thus constitutes a superior catalytic system than that of the cases where two separate metal complexes (hence, the two metal centers are not connected by a single ligand framework) are used. © 2021 American Chemical Society.
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PublisherACS Publications
Open AccessNo