Cationic Dirhodium Complexes Bridged by 2‑Phosphinopyridines Having an Exquisitely Positioned Axial Shielding Group: A Molecular Design for Enhancing the Catalytic Activity of the Dirhodium Core

This
report describes a strategy to create highly electrophilic
dirhodium catalysts. The electrophilicity of lantern-type dirhodium
complexes is generally decreased by the coordination of a ligand to
the axial site, which often causes a reduction in the catalytic activity.
We designed and synthesized a series of cationic dirhodium complexes
bridged by 2-diarylphosphinopyridines having a bulky 2,4,6-triisopropylphenyl
(Tip) group that can prevent the attack of external molecules to the
closest axial site. Theoretical calculations indicated that the Tip
group weakly interacts with the axial site but hardly reduces the
electrophilicity of the dirhodium core. The complexes served as excellent
catalyst precursors for the dehydrogenative silylation of alcohols
using hydrosilanes under mild conditions and a low metal loading,
producing the silyl ethers in higher yields in comparison to conventional
dirhodium complexes.