A Halide-Induced Redox-Switchable Catalyst for the Hydroboration of Terminal Alkenes

We describe the synthesis of two rhodium and iridium complexes featuring a di-NHC macrocyclic ligand, which incorporates both a diphenylene moiety and a naphthalenediimide (NDI) unit. We observed that the addition of fluoride or chloride resulted in substantial alterations to the steric and electronic properties of both complexes. Specifically, fluoride addition led to the reduction of the NDI unit through the formation of an (OH^-) NDI intermediate, while chloride produced a (Cl^-)···NDI adduct. Both adducts were confirmed by mass spectrometry. The impact of fluoride and chloride addition on the steric and electronic properties of the rhodium and iridium NDI-containing complexes was examined using spectroscopic and computational methods. The presence of either halide significantly enhanced the catalytic activity of the complexes in the hydroboration of terminal alkenes. Finally, we demonstrated that this catalytic enhancement is reversible, with the catalytic process being activated and deactivated by the sequential introduction of excess halide and NOBF₄. This observation reveals a rare example of a halide-induced redox-switchable catalytic (HIRSC) system.