Generation of Highly Vibrationally Excited CO in Sequential Photodissociation of Iron Carbonyl Complexes

Abstract

Ultraviolet photochemistry of iron pentacarbonyl, Fe­(CO)5, was investigated with resonantly enhanced multiphoton ionization (REMPI) spectroscopy and ion imaging. The REMPI spectrum of CO photofragments, generated by ultraviolet irradiation of Fe­(CO)5, showed the generation in the highly vibrationally excited states with v = 11–15. Analysis of the band intensities observed in the 213–235 nm region indicated that the high-v CO generation was maximized at around 220 nm. Generation yields of the coordinatively unsaturated intermediates, Fe­(CO) n=1–4, were measured as a function of the photolysis wavelength using a nonresonant detection scheme. The yield spectrum of FeCO was correlated with that of the high-v CO fragments, suggesting high-v CO generation in the photodissociation of FeCO. The density functional theory calculations of the excited states of FeCO showed an intense photoabsorption to the metal-centered state near 220 nm. The theoretical results were consistent with the interpretation of FeCO + hν → Fe + high-v CO, which was experimentally indicated. The momentum distribution obtained from the velocity distributions of Fe, Fe­(CO)4, and CO fragments further supported that Fe is the counter-product of the high-v CO fragment. The present results provided selective observation of the photochemistry of the unsaturated iron carbonyl complexes, which has not been well elucidated in laser-based experiments because of the uncontrollable sequential photodissociation producing mixed Fe­(CO) n intermediates.

Virtual Issue on Photodissociation: From Fundamental Dynamics and Spectroscopy to Photochemistry in Planetary Atmospheres and in Space

“…Experimental strategies to study the intricate mechanisms of photodissociation often focus on isolated gas-phase molecules to avoid the complexities introduced by a surrounding environment such as a solvent. The preferred design for experiments of this type uses a molecular beam of internally cooled molecules, intersected by two pulsed laser beams to excite the molecules and probe the consequences of the excitation, and information-rich detection methods such as velocity map imaging (VMI), ,,,− H Rydberg atom photofragment translational spectroscopy (HRA PTS), , or photoelectron spectroscopy. , These approaches can be divided into two broad categories. Ultrafast time-resolved methods use femtosecond laser spectroscopy methods to observe the real-time changes in molecular structures and electronic states following the initial photoexcitation step .…”

“…Although most photodissociation studies focus on molecules that are stable in their ground states and are sufficiently volatile to be prepared in a molecular beam, photolytic or mass-spectrometric methods can also be used to prepare gaseous samples of free radicals, biradicals such as Criegee intermediates, or molecular cations. ,, The photodissociation dynamics of these reactive intermediates can then be explored using aforementioned techniques such as VMI, HRA PTS, and other time-of-flight mass spectrometry schemes. ,, Metal complexes can also be studied using these same methods, provided they have sufficient vapor pressure or can be transferred to the gas phase by laser ablation or desorption methods. Observation of fragmentation pathways, such as ligand detachment from metal complexes and measurement of kinetic energy release to the fragments, provides useful information about the nature of the bonding, the thermodynamics of ligand binding, and ligand dissociation pathways. ,, …”

“…Observation of fragmentation pathways, such as ligand detachment from metal complexes and measurement of kinetic energy release to the fragments, provides useful information about the nature of the bonding, the thermodynamics of ligand binding, and ligand dissociation pathways. 5,9,11…”

Primary and Secondary Processes in the Ultraviolet Photodissociation of CpCo(CO)2 (Cyclopentadienylcobalt Dicarbonyl)

“…In our previous study on Fe(CO) 5 photodissociation, we observed distinct variations in the ion images of unsaturated intermediates with respect to photolysis wavelengths, providing key insights into the involved. 22 Most heteroleptic carbonyl complexes exhibit MLCT bands that are relevant to charge transfer to individual ligands in the ultraviolet region. Tuning the photolysis wavelengths can enable the selection of excited states that involve specific charge-transferred ligands, provided that different charge distributions sufficiently modify the electronic energies.…”

“…We previously analyzed the ion imaging data combined with photolysis wavelength dependence during the photodissociation study of Fe(CO) 5 as a typical homoleptic carbonyl complex. 22 Here, we extended our application to similar experimental and analysis approaches to the photodissociation of CpCo(CO) 2 , in which the releasing CO ligands are limited to two. Another ligand of Cp (cyclopentadienyl, C 5 H 5 ) is coordinated to the Co center with the η 5 form, which is less dissociative than the CO ligands.…”