Hetero- and Homobimetallic Complexes Bridged by a Bis(NHC) Ligand: Synthesis via Selective Sequential Metalation and Catalytic Applications in Tandem Organic Transformations

A (bis)azolium salt [L1-H-2]Br-2 (5), synthesized following multistep procedures, was realized to be a suitable platform for accessing the bis(NHC) ligand supported heterobimetallic Ir-III-M (M = Pd-II/Au-I) 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*)Cl-2](2) selectively yielded a monometallic Ir-III complex 6, which was further metalated using Pd(OAc)(2)/NaOAc to afford the heterobimetallic Ir-III-Pd-II 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 Ir-III-Au-I complex 8. Further, the related homobimetallic Ir-III and Pd-II complexes 9 and 10, respectively, have also been synthesized directly from [L1-H-2]Br-2. 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 Ir-III-Pd-II 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.

Automated summary

A bis(azolium salt [L1-H-2]Br-2 was synthesized as a suitable platform for heterobimetallic Ir-III-M (M = Pd-II/Au-I) complexes. The heterobimetallic Ir-III-Pd-II complex 7 showed better catalytic activity in one-pot tandem organic transformations compared to the corresponding homobimetallic counterparts. The study suggests some sort of cooperativity between the connected Ir and Pd metal centers via a single ligand framework.