Reversible Bimetallic\nInhibition to Modulate Selectivity\nDuring Catalysis

Bimetallic complexes have demonstrated a great ability\nto enhance\nthe activity of monometallic systems for bond activation and catalysis.\nIn this work, we explore the opposite approach: using a second metal\nto passivate the activity of another by reversible bimetallic inhibition.\nTo do so we have synthesized a family of nine electrophilic gold complexes\nof formula Au(PR<sub>3</sub>)(NTf<sub>2</sub>) ([NTf<sub>2</sub>]<sup>−</sup> = [N(SO<sub>2</sub>CF<sub>3</sub>)<sub>2</sub>]<sup>−</sup>) that can act as inhibitors in the semihydrogenation\nof terminal and internal alkynes catalyzed by the iconic iridium Vaska\ncomplex IrCl(CO)(PPh<sub>3</sub>)<sub>2</sub>. This behavior parallels\nthe well-known passivation effect of lead over palladium in the heterogeneous\nLindlard catalyst. Most gold fragments, except for the most hindered,\nform metal-only Lewis pairs upon combination with iridium, which have\nbeen fully characterized and exhibit distinct dative Ir → Au\nbonds. When applied to alkyne hydrogenation, these bimetallic structures\nhave a clear tendency toward olefin formation, while the monometallic\ncatalyst unselectively leads to overreduction products. Our computational\nstudies not only provide a feasible mechanism for the Ir-only system,\nbut also evince the active role of gold in passivating iridium by\nreversibly forming heterobimetallic structures that lead to enhanced\nselectivity.