Iridium(III) and Rhodium(III) Complexes With Imidazo[1,5-a]pyridine-Based Cyclometalating Ligands: Synthesis, Photophysical and Electrochemical Properties, and Catalytic Activities Toward the [4 + 2] Cycloaddition Reactions of Tertiary Anilines and Maleimides

Eighteen cationic iridium(III) (PC1–PC9) and rhodium(III) (PC10–PC18) complexes with a general formula [M(ppy)₂(N^N)]⁺ (ppy = 2-phenylpyridine; N^N = imidazo[1,5-a]pyridine-based ancillary ligands bearing different halogen atoms) were synthesized and characterized. The photophysical and electrochemical properties and catalytic activities of these complexes were investigated, and their properties were supported by density functional theory (DFT) calculations. The aims of this work were to study the influences of changes in the metal centers, the replacement of the pyridine ring by the pyrazine ring, the extension of the conjugation systems, and the attachment of different halogen atoms at the backbone of imidazo[1,5-a]pyridine on the photophysical, electrochemical, and catalytic properties of PC1–PC18. It is found that the variation in the metal centers affected these properties the most. The experimental and calculated results revealed that the photophysical and electrochemical behaviors of iridium(III) complexes and rhodium(III) compounds are different. Upon photoexcitation, the iridium(III) complexes exhibited intense and long-lived (6.96–13.03 μs) green to orange–red luminescence in acetonitrile at 298 K, and the emissions originated from ³ILCT transitions. The rhodium(III) complexes showed blue to green emissions with shorter excited-state lifetimes (2.87–4.94 μs) than those of the iridium(III) complexes in acetonitrile. The emissions of the rhodium(III) complexes were attributed to ³MLCT or mixed ³MLCT/³ILCT/³LLCT transitions. All the complexes are catalytically active for the [4 + 2] cycloaddition reactions of tertiary anilines with maleimides, producing a series of tetrahydroquinoline derivatives.