The solvation dynamics for deoxygenated and oxygenated Vaska's complex, bis(triphenylphosphene) iridium(I) carbonyl chloride, (deoxy-VC and oxy-VC) were characterized using twodimensional 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 dÀπ* 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 dÀπ orbitals. 2D-IR spectra were analyzed using the inverse centerline slope (CLS) as a representative of the normalized frequencyÀfrequency 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.

The vibrational solvatochromism of bis(triphenylphosphine) iridium(I) carbonyl chloride (Vaska's complex, VC) was investigated by FTIR spectroscopy. The carbonyl stretching frequency (ν(CO)) was measured in 16 different organic solvents with a wide range of Lewis acidities for VC and its dioxygen (VC-O(2)), hydride (VC-H(2)), iodide (VC-I(2)), bromide (VC-Br(2)), and sulfide (VC-S(X)) adducts. The ν(CO) of the VC-O(2) complex was sensitive to the solvent electrophilicity, whereas minimal correlation was found for VC and the other adducts. The stretching frequency of the trans-O(2) ligand on VC-O(2) was measured to be anticorrelated with ν(CO), supporting a model in which this ligand indirectly affects the carbonyl frequency by modulating the extent of metal-to-CO back-bonding. The ν(CO) values obtained from DFT calculations on VC adducts with solvent continua and explicit hydrogen bonds were used to aid the interpretations of the experimental results. The O(2) ligand is more susceptible to stronger specific solvent interactions and it binds in a fundamentally different mode from the monatomic ligands, providing a more direct communication channel with those metal d-orbitals that have the appropriate symmetry to back-bond into the carbonyl π*-orbital.

A vibrational pump-probe and FTIR study was performed on two different adducts of Vaska's complex in two different sets of binary solvent mixtures. The carbonyl vibrational mode in the oxygen adduct exhibits solvatochromic shifts of ~10 cm(-1) in either benzyl alcohol or chloroform relative to benzene-d6, whereas this vibration is nearly unchanged for the iodine adduct for the same three solvents. The width and center frequency of the carbonyl stretch for each adduct are compared to its vibrational lifetime in binary mixtures of benzene-d6 with either benzyl alcohol or chloroform. In neat solvents, the trends in line width, frequency, and vibrational lifetime are consistent for the two adducts, but complex relationships emerge when the trends in each property are compared as a function of mixed solvent composition. ν(CO) is more sensitive to the solvation environment around the trans ligand, whereas the line width and lifetime depend on the environment around the CO group itself. The carbonyl frequency and width vary nonlinearly across the two binary solvent series, indicating preferential solvation. In contrast, the vibrational lifetime changes linearly with solvent composition and is correlated with the mole fraction of chloroform but anticorrelated with the mole fraction of benzyl alcohol. The results are explained by differences in the densities of solvent modes that affect intermolecular relaxation of the carbonyl mode.

Linear absorption spectroscopy of the iridium-bound carbonyl on an iodated adduct of Vaska's complex has shown that the mean vibrational frequency is insensitive to solvation by a broad range of solvents, while the spectral line width changes significantly. The spectral broadening is more significant in chloroform than benzyl alcohol, which is puzzling considering that benzyl alcohol is more polar. In this study, 2D-IR spectroscopy was performed on this vibrational mode to dissect the linear line shape into its homogeneous and inhomogeneous contributions in binary solvent mixtures of either chloroform or benzyl alcohol in d6-benzene. The full frequency-frequency correlation function was determined, including the homogeneous line width and fast spectral diffusion. We find that the frequency fluctuation magnitudes show the most notable changes in chloroform mixtures, while the time constants for spectral diffusion change more dramatically in benzyl alcohol mixtures. Nonetheless, we conclude that the frequency fluctuation magnitudes in both solvent mixtures most clearly explain the differences in their linear line widths. The homogeneous contributions were found to either stay the same or decrease as the more polar solvent was added to d6-benzene, thereby implicating inhomogeneous dynamics as the dominant broadening mechanism.