Communication: Direct evidence for sequential dissociation of gas-phase Fe(CO)5 via a singlet pathway upon excitation at 266 nm

Femtosecond Core-Level Spectroscopy Reveals Involvement of Triplet States in the Gas-Phase Photodissociation of Fe(CO)5

Excitation of iron pentacarbonyl [Fe(CO)5], a prototypical photocatalyst, at 266 nm causes the sequential loss of two CO ligands in the gas phase, creating catalytically active, unsaturated iron carbonyls. Despite numerous studies, major aspects of its ultrafast photochemistry remain unresolved because the early excited-state dynamics have so far eluded spectroscopic observation. This has led to the long-held assumption that ultrafast dissociation of gas-phase Fe(CO)5 proceeds exclusively on the singlet manifold. Herein, we present a combined experimental-theoretical study employing ultrafast extreme ultraviolet transient absorption spectroscopy near the Fe M2,3-edge, which features spectral evolution on 100 fs and 3 ps time scales, alongside high-level electronic structure theory, which enables characterization of the molecular geometries and electronic states involved in the ultrafast photodissociation of Fe(CO)5. We assign the 100 fs evolution to spectroscopic signatures associated with intertwined structural and electronic dynamics on the singlet metal-centered states during the first CO loss and the 3 ps evolution to the competing dissociation of Fe(CO)4 along the lowest singlet and triplet surfaces to form Fe(CO)3. Calculations of transient spectra in both singlet and triplet states as well as spin-orbit coupling constants along key structural pathways provide evidence for intersystem crossing to the triplet ground state of Fe(CO)4. Thus, our work presents the first spectroscopic detection of transient excited states during ultrafast photodissociation of gas-phase Fe(CO)5 and challenges the long-standing assumption that triplet states do not play a role in the ultrafast dynamics.

Simulating transient X-ray photoelectron spectra of Fe(CO)5 and its photodissociation products with multireference algebraic diagrammatic construction theory

Accurate simulations of transient X-ray photoelectron spectra (XPS) provide unique opportunities to bridge the gap between theory and experiment in understanding the photoactivated dynamics in molecules and materials. However, simulating X-ray photoelectron spectra along a photochemical reaction pathway is challenging as it requires accurate description of electronic structure incorporating core-hole screening, orbital relaxation, electron correlation, and spin-orbit coupling in excited states or at nonequilibrium ground-state geometries. In this work, we employ the recently developed multireference algebraic diagrammatic construction theory (MR-ADC) to investigate the core-ionized states and X-ray photoelectron spectra of Fe(CO)5 and its photodissociation products (Fe(CO)4, Fe(CO)3) following excitation with 266 nm light. The simulated transient Fe 3p and CO 3σ XPS spectra incorporating spin-orbit coupling and high-order electron correlation effects are shown to be in a good agreement with the experimental measurements by Leitner et al. [J. Chem. Phys., 2018, 149, 044307]. Our calculations suggest that core-hole screening, spin-orbit coupling, and ligand-field splitting effects are similarly important in reproducing the experimentally observed chemical shifts in transient Fe 3p XPS spectra of iron carbonyl complexes. Our results also demonstrate that the MR-ADC methods can be very useful in interpreting the transient XPS spectra of transition metal compounds.

The Role of Momentum Partitioning in Covariance Ion Imaging Analysis

We present results from a covariance ion imaging study, which employs extensive filtering, on the relationship between fragment momenta to gain deeper insight into photofragmentation dynamics. A new data analysis approach is introduced that considers the momentum partitioning between the fragments of the breakup of a molecular polycation to disentangle concurrent fragmentation channels, which yield the same ion species. We exploit this approach to examine the momentum exchange relationship between the products, which provides direct insight into the dynamics of molecular fragmentation. We apply these techniques to extensively characterize the dissociation of 1-iodopropane and 2-iodopropane dications prepared by site-selective ionization of the iodine atom using extreme ultraviolet intense femtosecond laser pulses with a photon energy of 95 eV. Our assignments are supported by classical simulations, using parameters largely obtained directly from the experimental data.

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

We investigated the photodissociation dynamics of CpCo(CO)2 (cyclopentadienylcobalt dicarbonyl) in metal-to-ligand charge transfer (MLCT) bands. By employing DFT calculations, the absorption band (210-240 nm) was characterized as a charge transfer from the Co center to the Cp (cyclopentadienyl, C5H5) ligand. Ion imaging was utilized to analyze the CO fragments and coordinatively unsaturated complexes (CpCoCO, CpCo, and CoC3H3) across the entire MLCT band. Measuring the production yields of individual unsaturated complexes as a function of photolysis wavelength by considering wavelength dependence indicated the involvement of several photochemical pathways: the first photodissociation and sequential dissociation of CpCo(CO)2, and the second photodissociation of unsaturated intermediates within the pulse duration of the photolysis laser. The recoil velocity shifts of CpCo and CoC3H3 were attributed to the onset of the sequential dissociation of CpCoCO. Evidence for the second photodissociation of CpCoCO was obtained through the matching of linear momenta between the CO(v = 0, 1) and CpCo fragments. The DFT calculations performed to determine the electronic structures and potential energy curves for photoinduced CO loss in CpCo(CO)2 and CpCoCO supported our interpretation of the experimental results. This study presents a practical approach to selectively detecting specific processes among the mixture of products and intermediates when photolyzing transition-metal carbonyls, as their concurrent generation is unavoidable in laser-based experiments.

Photochemistry of transition metal carbonyls

The purpose of this Tutorial Review is to outline the fundamental photochemistry of metal carbonyls, and to show how the advances in technology have increased our understanding of the detailed mechanisms, particularly how relatively simple experiments can provide deep understanding of complex problems. We recall some important early experiments that demonstrate the key principles underlying current research, concentrating on the binary carbonyls and selected substituted metal carbonyls. At each stage, we illustrate with examples from recent applications. This review first considers the detection of photochemical intermediates in three environments: glasses and matrices; gas phase; solution. It is followed by an examination of the theory underpinning these observations. In the final two sections, we briefly address applications to the characterization and behaviour of complexes with very labile ligands such as N₂, H₂ and alkanes, concentrating on key mechanistic points, and also describe some principles and examples of photocatalysis.

Ultrafast infrared transient absorption spectroscopy of gas-phase Ni(CO)4 photodissociation at 261 nm

We employ ultrafast mid-infrared transient absorption spectroscopy to probe the rapid loss of carbonyl ligands from gas-phase nickel tetracarbonyl following ultraviolet photoexcitation at 261 nm. Here, nickel tetracarbonyl undergoes prompt dissociation to produce nickel tricarbonyl in a singlet excited state; this electronically excited tricarbonyl loses another CO group over tens of picoseconds. Our results also suggest the presence of a parallel, concerted dissociation mechanism to produce nickel dicarbonyl in a triplet excited state, which likely dissociates to nickel monocarbonyl. Mechanisms for the formation of these photoproducts in multiple electronic excited states are theoretically predicted with one-dimensional cuts through the potential energy surfaces and computation of spin–orbit coupling constants using equation of motion coupled cluster methods (EOM-CC) and coupled cluster theory with single and double excitations (CCSD). Bond dissociation energies are calculated with CCSD, and anharmonic frequencies of ground and excited state species are computed using density functional theory (DFT) and time-dependent density functional theory (TD-DFT).

A localized view on molecular dissociation via electron-ion partial covariance

Inner-shell photoelectron spectroscopy provides an element-specific probe of molecular structure, as core-electron binding energies are sensitive to the chemical environment. Short-wavelength femtosecond light sources, such as Free-Electron Lasers (FELs), even enable time-resolved site-specific investigations of molecular photochemistry. Here, we study the ultraviolet photodissociation of the prototypical chiral molecule 1-iodo-2-methylbutane, probed by extreme-ultraviolet (XUV) pulses from the Free-electron LASer in Hamburg (FLASH) through the ultrafast evolution of the iodine 4d binding energy. Methodologically, we employ electron-ion partial covariance imaging as a technique to isolate otherwise elusive features in a two-dimensional photoelectron spectrum arising from different photofragmentation pathways. The experimental and theoretical results for the time-resolved electron spectra of the 4d3/2 and 4d5/2 atomic and molecular levels that are disentangled by this method provide a key step towards studying structural and chemical changes from a specific spectator site.

Photoinduced bond oscillations in ironpentacarbonyl give delayed synchronous bursts of carbonmonoxide release

Early excited state dynamics in the photodissociation of transition metal carbonyls determines the chemical nature of short-lived catalytically active reaction intermediates. However, time-resolved experiments have not yet revealed mechanistic details in the sub-picosecond regime. Hence, in this study the photoexcitation of ironpentacarbonyl Fe(CO)₅ is simulated with semi-classical excited state molecular dynamics. We find that the bright metal-to-ligand charge-transfer (MLCT) transition induces synchronous Fe-C oscillations in the trigonal bipyramidal complex leading to periodically reoccurring release of predominantly axial CO. Metaphorically the photoactivated Fe(CO)₅ acts as a CO geyser, as a result of dynamics in the potential energy landscape of the axial Fe-C distances and non-adiabatic transitions between manifolds of bound MLCT and dissociative metal-centered (MC) excited states. The predominant release of axial CO ligands and delayed release of equatorial CO ligands are explained in a unified mechanism based on the σ^*(Fe-C) anti-bonding character of the receiving orbital in the dissociative MC states.

The photodissociation of transition metal carbonyls is involved in catalysis and synthetic processes. Here the authors, using semi-classical excited state molecular dynamics, observe details of the early stage dynamics in the photodissociation of Fe(CO)₅, including synchronous bursts of CO at periodic intervals of 90 femtoseconds.

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

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.

Unimolecular Double Photoionization-Induced Processes in Iron Pentacarbonyl

The dissociations of nascent Fe(CO)₅⁺⁺ ions created by 40.81 eV photoionization of iron pentacarbonyl have been examined using threefold and fourfold electron–ion coincidence measurements. The energies and forms of the ions have been explored by high-level calculations, revealing several new structures. The most stable form of Fe(CO)₅⁺⁺ has a quite different geometry from that of the neutral molecule. The dissociation pattern can be modeled as a sequence of CO evaporations followed by two-body charge separations. Each Fe(CO)_n⁺⁺ (n = 1–4) dication is stable in a restricted energy range; as its internal energy increases, it first ejects a neutral CO, then loses CO⁺ by charge separation at higher energy. In the initial stages, charge-retaining CO evaporations dominate over charge separation, but the latter become more competitive as the number of residual CO ligands decreases. At energies where ionization is mainly from the CO ligands, new Fe–C and C–C bonds are created by a mechanism which might be relevant to catalysis by Fe.

Dissociations of nascent Fe(CO)₅⁺⁺ ions by sequential CO evaporations, leading (in restricted energy ranges) to stable Fe(CO)_n⁺⁺ (n = 1−4) dicationic species. At energies where ionization is mainly from the CO ligands, new Fe−C and C−C bonds are created by a mechanism which might be relevant to catalysis by Fe.

Ultraviolet photodissociation of gas-phase iron pentacarbonyl probed with ultrafast infrared spectroscopy

It is well known that ultraviolet photoexcitation of iron pentacarbonyl results in rapid loss of carbonyl ligands leading to the formation of coordinatively unsaturated iron carbonyl compounds. We employ ultrafast mid-infrared transient absorption spectroscopy to probe the photodissociation dynamics of gas-phase iron pentacarbonyl following ultraviolet excitation at 265 and 199 nm. After photoexcitation at 265 nm, our results show evidence for sequential dissociation of iron pentacarbonyl to form iron tricarbonyl via a short-lived iron tetracarbonyl intermediate. Photodissociation at 199 nm results in the prompt production of Fe(CO)₃ within 0.25 ps via several energetically accessible pathways. An additional 15 ps time constant extracted from the data is tentatively assigned to intersystem crossing to the triplet manifold of iron tricarbonyl or iron dicarbonyl. Mechanisms for formation of iron tetracarbonyl, iron tricarbonyl, and iron dicarbonyl are proposed and theoretically validated with one-dimensional cuts through the potential energy surface as well as bond dissociation energies. Ground state calculations are computed at the CCSD(T) level of theory and excited states are computed with EOM-EE-CCSD(dT).

Primary and Secondary Loss of CO and NO Ligands in the Ultraviolet Photodissociation of the Heteroleptic Co(CO)3NO Complex

The photodissociation dynamics of the heteroleptic Co(CO)₃NO complex were investigated in the metal-to-carbonyl (CO) ligand charge-transfer band to compare the reactivity of the CO and nitrosyl (NO) ligands. The final state distributions of both the CO and NO fragments were measured using resonance-enhanced multiphoton ionization (REMPI) spectroscopy and velocity-map ion-imaging. The primary CO photofragment was differentiated from the secondary fragments of the subsequent unimolecular decomposition of coordinatively unsaturated intermediates by comparing the momentum distributions. The internal energy of the Co(CO)₂NO intermediate was sufficiently high (≥348 kJ/mol) to be generated in the electronic excited state, indicating the occurrence of the primary CO elimination on an excited state. The NO fragments exhibited two velocity components. The analysis of the final state distributions suggested that the higher- and lower-kinetic-energy components originated from the direct primary elimination and sequential elimination, respectively. The direct photoelimination through a transiently bent ligand conformation was illustrated on the basis of a two-dimensional REMPI approach and time-dependent density functional theory calculations. The present results of both ligands demonstrate the correlation between elimination mechanisms and possible ligand conformations in the electronic excited state.

Chemical interactions and dynamics with femtosecond X-ray spectroscopy and the role of X-ray free-electron lasers

X-ray free-electron lasers with intense, tuneable and short-pulse X-ray radiation are transformative tools for the investigation of transition-metal complexes and metalloproteins. This becomes apparent in particular when combining the experimental observables from X-ray spectroscopy with modern theoretical tools for calculations of electronic structures and X-ray spectra from first principles. The combination gives new insights into how charge and spin densities change in chemical reactions and how they determine reactivity. This is demonstrated for the investigations of structural dynamics with metal K-edge absorption spectroscopy, spin states in excited-state dynamics with metal 3p-3d exchange interactions, the frontier-orbital interactions in dissociation and substitution reactions with metal-specific X-ray spectroscopy, and studies of metal oxidation states with femtosecond pulses for 'probe-before-destroy' spectroscopy. The role of X-ray free-electron lasers is addressed with thoughts about how they enable 'bringing back together' different aspects of the same problem and this is thought to go beyond a conventional review paper where these aspects are formulated in italic font type in a prequel, an interlude and in a sequel. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.