Triazolylidene‐Iridium Complexes with a Pendant Pyridyl Group for Cooperative Metal–Ligand Induced Catalytic Dehydrogenation of Amines

• Published in: Chemistry : a European journal 2017-07, Vol.23 (37), p.8901-8911
• Main Authors: Valencia, Marta, Pereira, Ana, Müller‐Bunz, Helge, Belderraín, Tomás R., Pérez, Pedro J., Albrecht, Martin
• Format: Article
• Language: English
• Subjects: Activation; amine dehydrogenation; Amines; Catalysis; Catalytic activity; Chemistry; Dehydrogenation; Hydrogen; Hydrogen bonding; Hydrogen storage; Imines; Infrared radiation; Infrared spectroscopy; Iridium; Iridium compounds; Ligands; ligand–metal cooperativity; Metals; Protons; Selectivity; Spectroscopic analysis; triazolylidene; X-ray diffraction
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201700676

Two iridium(III) complexes containing a C,N‐bidentate pyridyl‐triazolylidene ligand were prepared that are structurally very similar but differ in their pendant substituent. Whereas complex 1 contains a non‐coordinating pyridyl unit, complex 2 has a phenyl group on the triazolylidene substituent. The presence of the basic pyridyl unit has distinct effects on the catalytic activity of the complex in the oxidative dehydrogenation of benzylic amines, inducing generally higher rates, higher selectivity towards formation of imines versus secondary amines, and notable quantities of tertiary amines when compared to the phenyl‐functionalized analogue. The role of the pyridyl functionality has been elucidated from a set of stoichiometric experiments, which demonstrate hydrogen bonding between the pendant pyridyl unit and the amine protons of the substrate. Such Npyr⋅⋅⋅H−N interactions are demonstrated by X‐ray diffraction analysis, 1H NMR, and IR spectroscopy, and suggest a pathway of substrate bond‐activation that involves concerted substrate binding through the Lewis acidic iridium center and the Lewis basic pyridyl site appended to the triazolylidene ligand, in agreement with ligand–metal cooperative substrate activation. Hang in there! A pendant pyridyl group in the C,N‐bidentate chelating carbene‐pyridyl complex 1 facilitates amine coordination to the iridium center and enhances catalytic activity of the metal center in amine dehydrogenation. Stoichiometric experiments provide evidence for hydrogen bonding of the amine substrate with the pendant pyridyl unit.