Photochemical C-H activation reactions mediated by transition metal complexes often proceed via the formation of σ-complexes. In these intermediates, the metal center coordinates to a C-H σ-bond, and metal-ligand donation and back-donation interactions ultimately lead to C-H bond cleavage. Because metal-alkane σ-complexes are weakly bond and short-lived, current experimental methods provide limited access to the transient electronic-structure effects that control their reactivity. Here, we photochemically prepared three different types of Rh-alkane σ-complexes in solution and demonstrate how optical pump and X-ray probe spectroscopy gives access to their decisive valence-electron interactions. With femtosecond-resolution and Rh specific X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Rh L3-edge we access the orbital interactions between the Rh centers, the ancillary ligands and the alkane C-H σ-bonds of the Rh-alkane σ-complexes. Supported by theoretical calculations, we identify spectral fingerprints of Rh-alkane donation and back-donation interactions and find trends for the reactivity of the σ-complexes toward C-H activation. We uncover the particular importance of specific occupied molecular orbitals with specific symmetry in modulating reactivity by providing, channeling and directing electron density via the metal to and from the C-H bond. Elucidating the electronic factors that facilitate C-H bond activation provides a basis for future ligand substitutions aimed at enhancing the reactivity of metal-alkane σ-complexes with enhanced efficiency of the activation step.

Transition-metal complexes are central for catalysis, material sciences and biochemistry, owing to their diverse non-radiative decay pathways driven by femtosecond-scale metal-ligand charge transfer. Resonant double-core-hole (DCH) ion-yield spectroscopy enables direct observation of ultrafast electronic and nuclear dynamics of such complexes with a direct coupling between strongly localized, transient charges and the chemically active valence electrons. Yet, previous DCH investigations were confined to atoms and small molecules at the K -edge. Here, we extend resonant DCH ion-yield spectroscopy to the L -edge of the transition-metal carbonyl Fe(CO) 5 in the gas phase to investigate fragmentation dynamics and sequential DCH ionization of the Fe center. Using intense femtosecond X-ray pulses, we observe signatures of multiply charged Fe cations, up to Fe 15+ . Comparison with atomic-level ionization-pathway modeling indicates a photon-energy range between 740eV and 780eV in which cations originating from core-excited DCH states dominate. Competing DCH channels, (i) resonant excitation following ionization or (ii) resonant-resonant excitation, govern the ionization and subsequent relaxation cascade. These channels enable charge build-up far beyond the single-core-hole (SCH) limit and induce fluence-dependent depletion of lower-charge fragments. Our results establish L -edge DCH resonance spectroscopy as a powerful approach for probing ultrafast charge redistribution and fragmentation in transition-metal complexes.

The electronic structure of iridium carbonyl complexes is known to be fundamental to their ability to activate alkane C-H bonds following UV photolysis. Here, we investigate three prototypical iridium complexes with different ancillary ligands using valence-to-core resonant inelastic X-ray scattering measurements at the Ir L3-edge in combination with optical absorption spectroscopy and calculations based on time-dependent density functional theory. We characterize experimentally how the nature and degree of metal-ligand hybridization impact valence-excited state energetics as well as how changes in ionic vs. covalent metal-ligand interactions for different ancillary ligands modulate charge densities at the central metal atom. The selectivity of our methods to the valence-excited state manifold allows us to observe and quantify shifts in the d-d and charge-transfer manifold of excited-states, which are both thought to influence the yield of photochemical C-H bond activation. Our combined experimental and theoretical study of this series of iridium complexes reveals the interplay of ligand structure, metal-ligand bonding or covalency and valence-excited state landscape, which allows to deduce a general understanding of how these properties impact photochemical pathways and reactivity in C-H activation and other photocatalytic applications.

The structural dynamics of metal carbonyls are central to processes ranging from catalysis to organometallic synthesis. Here we investigate the photodissociation of a prototypical transition metal carbonyl, Fe(CO)5, using mega-electron-volt ultrafast electron diffraction. By separately tracking structural evolution along the axial and equatorial directions, we provide an atomistic, angle-resolved view of the nuclear motions preceding CO dissociation and infer key features of the excited-state potential energy surface from the experimental observations. We further show that vibrational coupling before reaching the conical intersection facilitates the loss of a random carbonyl ligand via the Berry pseudorotation mechanism.

Photoinduced ligand-exchange dynamics in molybdenum hexacarbonyl have been studied using time-resolved IR and X-ray spectroscopy. We find that the energies of the unoccupied molybdenum 4d-derived orbitals that are accessed through X-ray transitions correlate with red-shifts in CO stretching modes. This provides complementary electronic structure information from the metal and the ligand perspective.

The excited state photophysics of heteroatomic organic molecules are determined by the interplay between internal conversion and intersystem crossing. Experimental techniques which can access both orbital and spin character of the electronic excited states in detail are crucial for a mechanistic understanding of these processes. In this work, we demonstrate the required sensitivity to excited-state orbital and spin character with femtosecond time-resolved resonant inelastic x-ray scattering (RIXS). We apply the method to probe the excited state photophysics of pyrazine in aqueous solution at the nitrogen K-edge. We clearly separate the dynamics of two singlet and two triplet excited states and identify a new mechanism for the intersystem crossing of pyrazine involving the dark Au(nπ*) state.

Soft X-ray absorption spectroscopy of first row transition elements at their respective L-edges provides important information about the oxidation and spin states of the metal centers. However, the associated sample damage in radiation-sensitive samples substantially alters the electronic and chemical structures of redox-active metal centers. Here, we measure the soft X-ray spectrum of the model MnIII(acac)3 complex containing a redox-active MnIII metal center in an octahedral environment with a superconducting transition-edge sensor detector. To reduce the secondary damage resulting primarily from the diffusion of radicals and electrons, the spectra are collected at 30 K and 80 K on solid samples. Starting from the first scan, we detect the contribution of X-ray induced sample damage leading to a change in the MnII intensity. However, at low temperatures, particularly at 30 K, we do not observe a gradual increase in the radiation damage with successive scans with the X-ray beam at the same spot. At our estimated dose of 90 kGy, we find 62% of MnIII(acac)3 is still intact at 30 K. However, at room temperature, we see a gradual increase in radiation damage with increasing numbers of scans at the same spot, which is consistent with the possibility of increased diffusion rates of secondary radicals and electrons as noted in other studies.

Coordinatively unsaturated transition-metal complexes are employed as active catalysts in a wide range of homogeneous chemical reactions, including C-H bond activation. Because of their high reactivity and resulting affinity to rapidly bind substrate molecules, they are generally challenging to detect and characterize when prepared photochemically. Here, we generate the 16-electron cationic complex [CpFe(CO)2]+ (Cp = cyclopentadienyl) in the gas phase using electrospray ionization and probe its electronic structure with x-ray absorption spectroscopy at the Fe L-edge. Supported by multiconfigurational spectrum calculations, the distinct L-edge absorption profile of [CpFe(CO)2]+ reveals direct access to a low-lying unoccupied Fe 3d-derived orbital, which is characteristic of the unsaturated coordination of the complex and of its pronounced affinity to bind even notoriously unreactive moieties such as alkanes.

The reactivity towards C-H bond activation of alkanes with transition metals is determined by the ability of the metal to donate and withdraw electron density in due proportion. Manipulating this reactivity in a controlled way is difficult, because the hypothesized metal-alkane charge-transfer interactions are challenging to access experimentally. Using time-resolved X-ray spectroscopy, we track the charge-transfer interactions in a C-H activating Rh complex and reveal changes in oxidation state as well as valence-orbital energies and character from femtosecond Rh-alkane bond formation to nanosecond C-H bond cleavage. Our X-ray spectroscopic signatures reflect how alkane-to- metal donation determines metal-alkane bond stability and how metal-to-alkane back-donation facilitates C-H bond cleavage by oxidative addition. The ability to dissect charge-transfer interactions on an orbital-level provides new opportunities for manipulating reactivity for C-H activation with transition metals.

Photochemically prepared transition-metal complexes are known to be effective at cleaving the strong C-H bonds of organic molecules in room temperature solutions. There is also ample theoretical evidence that the bidirectional charge-transfer between an incoming alkane C-H group and the transition metal is the decisive interaction in the C-H activation reaction. What is missing, however, are experimental methods to directly probe these interactions in order to reveal what determines reactivity of intermediates and the rate of the reaction. Here, we propose metal specific and time-resolved valence-to-core resonant inelastic X-ray scattering (VtC-RIXS) at the transition metal L-edge as a method to provide a full account of the evolution of metal-alkane interactions during transition-metal mediated C-H activation reactions. For the model system cyclopentadienyl rhodium dicarbonyl (CpRh(CO)2), we demonstrate with a combination of experiment and quantum chemical simulation how the Rh-centered valence-excited final states probed with VtC-RIXS directly reflect changes in donation and back-donation between the alkane C-H group and the transition metal as the reaction proceeds via its intermediates. Following the initial photo-triggered CO dissociation, we find substantial reduction in charge donation onto the metal and the resulting stabilization of metal-centered states as the alkane coordinates to the Rh center in a σ-complex intermediate. C-H bond cleavage in the final oxidative addition step is instead characterized by a substantial increase in back-donation as the new Rh-hydrogen and Rh-carbon bonds are formed. We benchmark and validate our simulations against experimental steady-state measurements. With our study, we predict the key spectral fingerprints for future time-resolved experiments of C-H activation reactions with CpRh(CO)2 and related compounds.

We propose, using simulated RIXS signatures, a way of spectroscopically accessing the “two-way” charge transfer interactions between the C–H bond and the metal during C–H activation, thereby verifying and motivating orbital correlation diagrams.

Photodissociation of ironpentacarbonyl in solution generates transient species in different electronic states, which we have studied theoretically. From ab initio molecular dynamics simulations in ethanol solution, the closed-shell parent compound Fe(CO)5 is found to interact weakly with the solvent, whereas the irontetracarbonyl (Fe(CO)4) species, formed after photodissociation, has a strongly spin-dependent behavior. It coordinates a solvent molecule tightly in the singlet state and weakly in the triplet state. From the simulations, we have gained insight into intersystem crossing in solvated irontetracarbonyl, based on the distinct structural differences induced by the change in multiplitity. Alternative forms of coordination between Fe(CO)4 and functional groups of the ethanol molecule are simulated and a quantum chemical investigation of the energy landscape for the coordinated irontetracarbonyl give information about the interconversion of different transient species in solution. Furthermore, insights from the simulations, in which we find evidence of a solvent exchange mechanism, challenge the previously proposed mechanism of chain walking for undercoordinated metal carbonyls in solution.

C-H bond activation reactions with transition metals typically proceed via the formation of alkane 𝜎-complexes, where an alkane C-H 𝜎-bond binds to the metal. Due to the weak nature of metal-alkane bonds, 𝜎-complexes are challenging to characterize experimentally. Here, we photochemically prepare the model 𝜎-complex Cr(CO)5-alkane from Cr(CO)6 in octane solution and characterize the nature of its metal-ligand bonding interactions. Using femtosecond optical absorption spectroscopy, we find photo-induced CO dissociation from Cr(CO)6 to occur within the 100 fs time-resolution of the experiment. Rapid geminate recombination by a fraction of molecules is found to occur with a time constant of 150 fs. The formation of bare Cr(CO)5 in its singlet ground state is followed by complexation of an octane molecule from solution with a time constant of 8.2 ps. Picosecond X-ray absorption spectroscopy at the Cr L-edge and O K-edge provides unique information on the electronic structure of the Cr(CO)5-alkane 𝜎-complex both from the metal and ligand perspectives. We find substantial destabilization of the lowest unoccupied molecular orbital upon coordination of the C-H bond to the undercoordinated Cr center in the Cr(CO)5-alkane 𝜎-complex, accompanied with rehybridization between metal and ligand orbitals. Our study demonstrates the value of combining optical and X-ray spectroscopic methods as complementary tools to study the properties of alkane 𝜎-complexes as the decisive intermediates in C-H bond activation reactions.

Measured and calculated time-resolved photoelectron spectra and excited-state molecular dynamics simulations of photoexcited gas-phase molecules Fe(CO)5 and Cr(CO)6 are presented. Samples were excited with 266 nm pump pulses and probed with 23 eV photons from a femtosecond high-order harmonic generation source. Photoelectron intensities are seen to blue-shift as a function of time from binding energies characteristic of bound electronic excited states via dissociated-state energies toward the energies of the dissociated species for both Fe(CO)5 and Cr(CO)6, but differences are apparent. The excited-state and dissociation dynamics are found to be faster in Cr(CO)6 because the repopulation from bound excited to dissociative excited states is faster. This may be due to stronger coupling between bound and dissociative states in Cr(CO)6, a notion supported by the observation that the manifolds of bound and dissociative states overlap in a narrow energy range in this system.

Accurate computations of experimental observables are essential for interpreting the high information content held within x-ray spectra. However, for complicated systems this can be difficult, a challenge compounded when dynamics becomes important owing to the large number of calculations required to capture the time-evolving observable. While machine learning architectures have been shown to represent a promising approach for rapidly predicting spectral lineshapes, achieving simultaneously accurate and sufficiently comprehensive training data is challenging. Herein, we introduce Δ-learning for x-ray spectroscopy. Instead of directly learning the structure-spectrum relationship, the Δ-model learns the structure dependent difference between a higher and lower level of theory. Consequently, once developed these models can be used to translate spectral shapes obtained from lower levels of theory to mimic those corresponding to higher levels of theory. Ultimately, this achieves accurate simulations with a much reduced computational burden as only the lower level of theory is computed, while the model can instantaneously transform this to a spectrum equivalent to a higher level of theory. Our present model, demonstrated herein, learns the difference between TDDFT(BLYP) and TDDFT(B3LYP) spectra. Its effectiveness is illustrated using simulations of Rh L3-edge spectra tracking the C–H activation of octane by a cyclopentadienyl rhodium carbonyl complex.

Transition metal reactivity toward carbon–hydrogen (C–H) bonds hinges on the interplay of electron donation and withdrawal at the metal center. Manipulating this reactivity in a controlled way is difficult because the hypothesized metal-alkane charge-transfer interactions are challenging to access experimentally. Using time-resolved x-ray spectroscopy, we track the charge-transfer interactions during C–H activation of octane by a cyclopentadienyl rhodium carbonyl complex. Changes in oxidation state as well as valence-orbital energies and character emerge in the data on a femtosecond to nanosecond timescale. The x-ray spectroscopic signatures reflect how alkane-to-metal donation determines metal-alkane complex stability and how metal-to-alkane back-donation facilitates C–H bond cleavage by oxidative addition. The ability to dissect charge-transfer interactions on an orbital level provides opportunities for manipulating C–H reactivity at transition metals.

Wie weit reicht der Einfluss eines Protons in Wasser? Röntgenspektroskopie enthüllt – Schicht für Schicht – die elektronische Struktur von Wassermolekülen bei Hydratation eines Protons. Es ergibt sich eine strukturelle Hierarchie, in der das Proton stark mit den drei nächsten Wassermolekülen interagiert und einen hybridisierten H7O3+-Kern formt, während die erste Hydrathülle durch das elektrische Feld der positiven Protonenladung beeinflusst wird. Weitere Solvathüllen enthalten reguläres Wasser, wie Ehud Pines, Philippe Wernet, Michael Odelius, Erik T. J. Nibbering et al. in ihrem Forschungsartikel berichten (e202211066).

The 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 the mechanistic details in the sub-picosecond regime. Hence in this study, the photoexcitation of ironpentacarbonyl Fe(CO)5 has been simulated by semi-classical surface-hopping excited state molecular dynamics based on time-dependent density functional theory. We find that the bright metal-to-ligand charge-transfer (MLCT) transition induces 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, which we explain in terms 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 a delayed release of equatorial CO ligands are explained in an unified mechanism based on the (sigma*)(Fe-C) anti-bonding character of the receiving orbital in the dissociative MC states both in the Franck-Condon region and at distorted geometries.

We determine how the transient electronic structure changes of imidazole base upon proton transfer from a naphthol photoacid in aqueous solution can be locally monitored with ultrafast nitrogen K-edge spectroscopy.

Photoacids show a strong increase in acidity in the first electronic excited state, enabling real-time studies of proton transfer in acid-base reactions, proton transport in energy storage devices and biomolecular sensor protein systems. Several explanations have been proposed for what determines photoacidity, ranging from variations in solvation free energy to changes in electronic structure occurring along the four stages of the Förster cycle. Here we use picosecond nitrogen K-edge spectroscopy to monitor the electronic structure changes of the proton donating group in a protonated aromatic amine photoacid in solution upon photoexcitation and subsequent proton transfer dynamics. Probing core-to-valence transitions locally at the amine functional group and with orbital specificity, we clearly reveal pronounced electronic structure, dipole moment and energetic changes on the conjugate photobase side. This result paves the way for a detailed electronic structural characterization of the photoacidity phenomenon.