Solvent versus temperature control over the infrared band shape and position in Fe(CO)3(η(4)-ligand) complexes

The solute-solvent interactions between Fe(CO)3(η(4)-cyclooctatetraene) (FeCOT) and 27 solvents were examined by infrared (IR) spectroscopy. The observed change in band shape and position of the carbonyl bands as a function of solvent was found to be very similar to that previously observed in temperature-dependent IR experiments of Fe(CO)3(η(4)-norborndiene) (FeNBD). While for FeNBD the change in band shape results from dynamic exchange of carbonyl ligands, temperature-dependent IR experiments in ethyl acetate show that the observed changes are not a result of carbonyl ligand site exchange for FeCOT. We therefore concluded that the solvent dependence of the IR spectra must be a consequence of a static solute-solvent interaction. We find that the linear solvation energy model (J. Am. Chem. Soc. 1977, 99, 6027-6038; Chem. Soc. Rev. 1993, 22, 409-416) provides a satisfactory account for the spectral changes due to the solvent. From this model, we are able to conclude that the solute-solvent interactions of this system are influenced by the solvent's polarizability and hydrogen bonding acidity. We also observed interdependence between the change in fwhm and band positions for all three carbonyl bands, which brings us to the conclusion that the observed changes in the IR carbonyl band shape of FeCOT are a consequence of the solute-solvent interactions, rather than any solvent friction effects. This implies that care must be taken to separate the effects of chemical dynamics and solvatochromism when examining IR spectra of molecules suspected of exhibiting dynamically broadened vibrational spectra.