Bimetallic N‑Heterocyclic Carbene Rh(I) Complexes: Probing the Cooperative Effect for the Catalyzed Hydroelementation of Alkynes

A series of bimetallic N-heterocyclic
carbene Rh­(I) complexes (13, 14, 16, 18, 19, and 21) were prepared,
in which two metal
fragments are linked to a central organic scaffold. These complexes
were investigated as catalysts for the hydroelementation of alkynes,
and their efficiency as catalysts was compared to that of a series
of monometallic analogues (12, 15, 17, and 20). To determine the catalyst properties
that result in a cooperative, bimetallic enhancement of the reaction
rate, systematic variation of the intermetallic distance and the ligand
donor properties of the bimetallic complexes was explored. Two related
catalyzed reactions were investigated: the dihydroalkoxylation and
hydrosilylation of alkynes. Nanoparticle formation was observed during
the hydrosilylation reaction using complexes 17–19.