Iridium(III) Bis-tridentate Complexes with 6-(5-Trifluoromethylpyrazol-3-yl)-2,2′-bipyridine Chelating Ligands: Synthesis, Characterization, and Photophysical Properties

A novel family of iridium(III) bis-tridentate complexes are reported that contain the deprotonated, N∧N∧N-coordinated 6-(5-trifluoromethylpyrazol-3-yl)-2,2′-bipyridine (fpbpyH) ligand. The monocationic homoleptic iridium(III) complex [Ir(fpbpy)2](PF6) (1) was first prepared by treatment of IrCl3·3H2O with 2.2 equiv of fpbpyH in degassed ethylene glycol at 196 °C and structurally characterized by single-crystal X-ray crystallography. The reaction of Ir(tpy)Cl3 with an equimolar amount of fpbpyH under comparable conditions generated a dicationic heteroleptic iridium(III) complex, [Ir(fpbpy)(tpy)](PF6)2 (2), featuring one 2,2′:6′2′′-terpyridine (tpy) in place of one fpbpy. The charge-neutral heteroleptic complex [Ir(fpbpy)(dppy)] (3) (dppyH2 = 2,6-diphenylpyridine) was also afforded successfully via a solvent-free method, giving a complex with one dppy bound to the Ir(III) ion through two carbon atoms for the cyclometalation reaction. Complexes 1−3 are emissive in acetonitrile solution at ambient temperature, perhaps best assigned to the MLCT character, mixed with intraligand charge transfer (ILCT) transition inside fpbpy and ligand-to-ligand charge transfer (LLCT) π(fpbpy) → π*(tpy) or π(dppy) → π*(fpbpy) transition, respectively, which are supported by DFT calculations. It is noteworthy that the photoluminescence of 3 displays a significant red-shifting compared to those of 1 and 2 due to the large reduction of the HOMO−LUMO energy gap as a consequence of the introduction of the more electron-donating and strong ligand-field cyclometalate dppy chelate.