Identification of the vibrational modes in the far-infrared spectra of ruthenium carbonyl clusters and the effect of gold substitution

Abstract

High-quality far-IR absorption spectra for a series of ligated atomically precise clusters containing Ru₃, Ru₄, and AuRu₃ metal cores have been observed using synchrotron radiation, the latter two for the first time. The experimental spectra are compared with predicted IR spectra obtained following complete geometric optimization of the full cluster, including all ligands, using DFT. We find strong correlations between the experimental and predicted transitions for the low-frequency, low-intensity metal core vibrations as well as the higher frequency and intensity metal-ligand vibrations. The metal core vibrational bands appear at 150 cm⁻¹ for Ru₃(CO)₁₂, and 153 and 170 cm⁻¹ for H₄Ru₄(CO)₁₂, while for the bimetallic Ru₃(μ-AuPPh₃)(μ-Cl)(CO)₁₀ cluster these are shifted to 177 and 299 cm⁻¹ as a result of significant restructuring of the metal core and changes in chemical composition. The computationally predicted IR spectra also reveal the expected atomic motions giving rise to the intense peaks of metal-ligand vibrations at ca. 590 cm⁻¹ for Ru₃, 580 cm⁻¹ for Ru₄, and 560 cm⁻¹ for AuRu₃. The obtained correlations allow an unambiguous identification of the key vibrational modes in the experimental far-IR spectra of these clusters for the first time.