Synthesis and Catalytic Applications of Heterobimetallic Carbene Complexes Obtained via Sequential Metalation of Two Bisazolium Salts

Abstract

A simple sequential metalation approach starting from the imidazolium/benzimidazolium salt 4(I)2 yielded the heterobimetallic RhIII/M (M = PdII, IrI , AuI , RuII) complexes [6]− [9]. Alternatively, a symmetrical 1,3-imidazolium substituted benzene was used for the preparation of the heterobimetallic M′/PdII (M′ = RhIII, IrIII) complexes [12] and [13]. The versatile stepwise approach used for the preparation of complexes [6]−[9] involved the deprotonation reaction of the bisazolium salt 4(I)2 in the presence of [RhCp*(Cl)2]2 to afford the monometallic complex [5]I featuring a chelating coordinated bidentate CNHC^Cphenyl ligand. Complex [5]I was reacted with Ag2O to give a nonisolated RhIII/AgI complex which in a subsequent transmetalation reaction yielded the heterobimetallic RhIII/M bisNHC complexes (M = PdII [6], IrI [7], AuI [8], RuII [9]). Similarly, heterobimetallic M′/PdII bis-NHC complexes [12] (M′ = RhIII) and [13] (M′ = IrIII) have been prepared from a symmetrical bisazolium salt by generating first the monometallic M′ complexes followed by a transmetalation reaction of the in situ generated M′/AgI complexes with [Pd(dmba)(μ-Cl)]2. The RhIII/PdII complexes [6] and [12] and the IrIII/PdII complex [13] were used as catalysts for two orthogonal tandem reactions, namely, the Suzuki−Miyaura coupling/transfer hydrogenation and the Suzuki−Miyaura-coupling/α-alkylation of ketones. The catalytic activity of the heterobimetallic complexes was compared to mixtures of the related monometallic analogues [14]−[17], with the heterobimetallic complexes generally showing a higher catalytic activity. In addition, nBuOH was found to play a dual role as an alkylating and reducing agent in the Suzuki− Miyaura coupling/α-alkylation of ketones.