Reversible Bimetallic Inhibition to Modulate Selectivity During Catalysis

Bimetallic complexes have demonstrated a great ability
to enhance
the activity of monometallic systems for bond activation and catalysis.
In this work, we explore the opposite approach: using a second metal
to passivate the activity of another by reversible bimetallic inhibition.
To do so we have synthesized a family of nine electrophilic gold complexes
of formula Au(PR3)(NTf2) ([NTf2]− = [N(SO2CF3)2]−) that can act as inhibitors in the semihydrogenation
of terminal and internal alkynes catalyzed by the iconic iridium Vaska
complex IrCl(CO)(PPh3)2. This behavior parallels
the well-known passivation effect of lead over palladium in the heterogeneous
Lindlard catalyst. Most gold fragments, except for the most hindered,
form metal-only Lewis pairs upon combination with iridium, which have
been fully characterized and exhibit distinct dative Ir → Au
bonds. When applied to alkyne hydrogenation, these bimetallic structures
have a clear tendency toward olefin formation, while the monometallic
catalyst unselectively leads to overreduction products. Our computational
studies not only provide a feasible mechanism for the Ir-only system,
but also evince the active role of gold in passivating iridium by
reversibly forming heterobimetallic structures that lead to enhanced
selectivity.