Intramolecular Vibrational Energy Transfer in the Precatalyst [Mn(ppy)(CO) ₄ ] Tracked by Dual-Frequency 2D Infrared Spectroscopy

The thermal dissociation of carbon monoxide is a fundamental entry step to many catalytic cycles involving metal carbonyl (MCO) complexes as it reveals a vacant coordination site at the metal center, enabling substrate coordination. Overcoming the dissociation barrier requires sufficient accumulation of energy in vibrational modes with displacement vectors along the reaction coordinate (M-CO distance). Hence, understanding the energy transfer to and from these vibrations is essential in developing a detailed understanding of a precatalyst’s activation pathway. Here, the intramolecular vibrational energy redistribution (IVR) within the heteroleptic metal carbonyl complex [Mn(ppy)(CO)4] (1, ppy = cyclometalated 2-phenylpyridine) in dichloromethane solution has been studied using dual-frequency, two-dimensional infrared spectroscopy. The responses of three vibrational modes localized on the ppy-ligand were monitored following the photoexcitation of carbonyl ligand stretching modes. A rise of signal strength by a factor of 4.7 within the first 35 ps, followed by a decay within ca. 150 ps exemplify the IVR from the metal carbonyl to the organic moiety, and the intermolecular energy transfer (IET) to the solvent, respectively. Moreover, pronounced changes of the spectral shape within the first 10 ps indicate the population of distinct vibrationally excited states. Direct anharmonic coupling between the pumped and probed modes gives rise to the initial spectral features, which include an uncommon, negative anharmonic coupling. These features do not decay single-exponentially as expected, but rise during the first 23 ps instead. This finding is assigned to the population of low-frequency modes (<250 cm–1), which are predicted to have a similar coupling pattern toward the measured bands as the CO stretching modes, based on density functional theory (DFT) calculations. The stronger signals predominant at late waiting times have uniform anharmonic shifts of ca. −2.5 cm–1 arising from the coupling to medium-frequency modes (250–1200 cm–1), which are strongly localized on the ppy-ligand. Due to the distinct signal positions, the time-dependent populations of low- and medium-frequency modes can be evaluated independently. Rate constants for the rise of their populations were found to be 1/52 ps–1 and 1/58 ps–1, respectively, while the rate constants of depopulation via IET to the solvent are 1/19 ps–1 and 1/25 ps–1.