Tandem Oligomerization-Hydrogenation Using Brønsted Acidic Iridium Hydride Catalysts

Area of Science:

• Organometallic Chemistry
• Catalysis

Background:

• Cationic iridium complexes with bulky phosphine ligands are explored for catalytic applications.
• The behavior of tris-[3,5-bis-(trifluoromethyl)-phenyl]-phosphine ligand in iridium complexes is investigated.

Purpose of the Study:

• To investigate the unexpected formation of acidic metal hydrides from cationic iridium complexes.
• To elucidate the mechanism of hydrogenation catalysis involving net-dihydrogen heterolysis.
• To understand the competition between olefin reduction and cationic oligomerization.

Main Methods:

• Synthesis and characterization of cationic iridium complexes.
• Catalytic studies involving hydrogenation and oligomerization reactions.
• Mechanistic investigations using isotopic labeling and kinetic studies.

Main Results:

• Cationic iridium complexes with tris-[3,5-bis-(trifluoromethyl)-phenyl]-phosphine unexpectedly yield acidic metal hydrides.
• Net-dihydrogen heterolysis facilitates hydrogenation catalysis via an ionic mechanism.
• A tandem oligomerization/hydrogenation sequence produces isooctane (2,4,4-trimethylpentane) from isobutene.
• Mechanistic evidence supports the generation of neutral iridium hydrides through proton transfer from a cationic hydride precursor to an olefin.

Conclusions:

• The study reveals a novel pathway for ionic hydrogenation catalysis mediated by cationic iridium complexes.
• The formation of acidic metal hydrides is a key feature of these systems.
• The observed tandem reaction highlights the potential for selective synthesis of valuable products like isooctane.