Metal-Ligand Bonding Strength of Fluoro-Substituted Cyclometalated Iridium(III) Complexes from Raman and Infrared Spectra

The Raman and infrared spectra of fac-tris(2-phenylpyrazolato-N, C-2')iridium (Ir(ppz)(3)), bis[(4,6-difluorophenyl)-pyridinato-N,C-2'] (picolinato)-iridium (FIrpic), fac-tris [1-(4-fluorophenyl)-3-methylimidazolin-2-ylidene-C,C-2']-iridium [Ir,(fpmi)(3)], and fac-tris[1-(2,4-difluorophenyl)-3-methylimidazolin-2-ylidene-C,C-2']iridium [Ir(dfpmi)(3)] were recorded, and complete vibrational analyses were performed. Quantum-chemical calculations using the density-functional theory methods B3LYP and B3PW91 were employed to obtain the vibrational wavenumbers and spectral intensities. The optimized geometries and vibrational wavenumbers agree with the experimental data, indicating that the calculations yielded accurate geometries for the iridium complexes. Based on comparison of the spectra of their analogues Ir(ppy)(3) and ligands and theoretical calculations, we assigned the vibrational wavenumbers for the metal-ligand modes: metal-ligand stretching bands v(Ir-C) are at 270/321 cm(-1) for Ir(ppz)(3), 321 cm(-1) for Firpic, 332 cm(-1) (imidazole) and 286/303 cm(-1) (phenyl) for Ir(fpmi)(3), and 377 cm(-1) (imidazole) and 283/299 cm(-1) (phenyl) for Ir(dfpmi)(3) and metal-ligand stretching bands v(Ir-N) are at 229/243 cm(-1) for Ir(ppz)(3) and 267 cm(-1) (picolinate) and 282 (phenyl) cm(-1) for FIrpic. We observed greater vibrational wavenumbers for the metal ligand bands in the fluoro complexes, indicating that the electron-withdrawing group F increases the strength of the bond from the ligand to iridium.