Solvation Dynamics of Vaska’s Complex by 2D-IR Spectroscopy

Abstract

The solvation dynamics for deoxygenated and oxygenated Vaska’s complex, bis(triphenylphosphene) iridium(I) carbonyl chloride, (deoxy-VC and oxy-VC) were characterized using two-dimensional infrared (2D-IR) spectroscopy in d 6 -benzene, chloroform, and DMF. The iridium-bound carbonyl was used as a probe of the static and dynamic chemical environments in each solvent system. The linear IR spectra of the complexes were consistent with CO frequency modulation through d−π* backbonding interactions. The deoxy-VC center frequencies were insensitive to the solvent type, but those of oxy-VC were sensitive to the surrounding solvent, presumably due to the indirect influence of the dioxygen ligand on the carbonyl vibrational frequency. The vibrational lifetimes of the VC carbonyls were consistent with intramolecular relaxation through the metal d–π orbitals. 2D-IR spectra were analyzed using the inverse centerline slope (CLS) as a representative of the normalized frequency–frequency correlation function. Multiexponential fits to the CLS decays revealed solvation dynamics on several time scales, ranging from a few to tens of picoseconds, with a shift of the relative proportion of the slower dynamics for the oxygenated complexes. The measured dynamics were compared to previously determined oxidative addition rate constants to hypothesize the potential role of solvent shell fluctuations in the overall reaction rate.