Primary processes of the archetypal model complex azido(porphinato)iron(III) from ultrafast vibrational-electronic spectroscopy.

Azidoiron complexes serve as valuable photochemical precursors for catalytically active species containing high-valent iron. In bioinorganic chemistry, azido(tetraphenylporphinato)iron(III), i.e., [FeIII(tpp)(N3)] with tpp = 5, 10, 15, 20-tetraphenylporphyrin-21, 23-diido, constitutes the archetypal model system that was used to access for the first time the terminal nitridoiron core, FeV ≡ N, in the biomimetic redox-non-innocent ligand environment. So far, the light-induced dynamics leading to the oxidation of the metal and the release of dinitrogen from the N3-ligand have only been studied for precursors featuring redox-innocent auxiliary ligands that simplify the electronic structure change accompanying the photo-transformation. Here, we monitored the primary events of the above paradigmatic complex, following its optical excitation in the ultraviolet-to-visible spectral range using femtosecond spectroscopy with probing in both the UV-vis and mid-infrared regions. Following ultrafast Soret-excitation at 400 nm, the complex relaxes to the lowest excited sextet state by a first internal conversion in less than 200 fs. The excited state then undergoes vibrational relaxation on a time scale of roughly 2 ps before internally converting yet again to recover the sextet electronic ground state within 19.5 ps. Spectroscopic evidence is obtained neither for a transient occupation of the energetically lowest metal-centered state, 41A1, nor for vibrational relaxation in the ground-state. The primary processes seen here are thus in contrast to those previously derived from ultrafast UV-pump/vis-probe and UV-pump/XANES-probe spectroscopies for the halide congener [FeIII(tpp)(Cl)]. Any photochemical transformation of the complex arises from two-photon-induced dynamics.

Ultrafast photogeneration of a metal-organic nitrene from 1,1'-diazidoferrocene.

Ferrocene and its derivatives have fascinated chemists for more than 70 years, not least due to the analogies with the properties of benzene. Despite these similarities, the obvious difference between benzene and ferrocene is the presence of an iron ion and hence the availability of d-orbitals for properties and reactivity. Phenylnitrene with its rich photochemistry can be considered an analogue of nitrenoferrocene. As with most organic and inorganic nitrenes, nitrenoferrocene can be obtained by irradiating the azide precursor. We study the photophysical and photochemical processes of dinitrogen release from 1,1'-diazidoferrocene to form 1-azido-1'-nitrenoferrocene with UV-pump-mid-IR-probe transient absorption spectroscopy and time-dependent density functional theory calculations including spin-orbit coupling. An intermediate with a bent azide moiety is identified that is pre-organised for dinitrogen release via a low-lying transition state. The photochemical decay paths on the singlet and triplet surfaces including the importance of spin-orbit coupling are discussed. We compare our findings with the processes discussed for photochemical dinitrogen activation and highlight implications for the photochemistry of azides more generally.

A Chiral Titanocene Complex as Regiodivergent Photoredox Catalyst: Synthetic Scope and Mechanism of Catalyst Generation.

We describe a combined synthetic, spectroscopic, and computational study of a chiral titanocene complex as a regiodivergent photoredox catalyst (PRC). With Kagan's complex catCl2 either monoprotected 1,3-diols or 1,4-diols can be obtained in high selectivity from a common epoxide substrate in a regiodivergent epoxide opening depending on which enantiomer of the catalyst is employed. Due to the catalyst-controlled regioselectivity of ring opening and the broader substrate scope, the PRC with catCl2 is also a highly attractive branching point for diversity-oriented synthesis. The photochemical processes of cat(NCS)2, a suitable model for catCl2, were probed by time-correlated single-photon counting. The photoexcited complex displays a thermally activated delayed fluorescence as a result of a singlet-triplet equilibration, S1 ⇄ T1, via intersystem crossing and recrossing. Its triplet state is quenched by electron transfer to the T1 state. Computational and cyclic voltammetry studies highlight the importance of our sulfonamide additive. By bonding to sulfonamide additives, chloride abstraction from [catCl2]- is facilitated, and catalyst deactivation by coordination of the sulfonamide group is circumvented.

Photoinduced Metallonitrene Formation by N2 Elimination from Azide Diradical Ligands.

Transition-metal nitrides/nitrenes are highly promising reagents for catalytic nitrogen-atom-transfer reactivity. They are typically prepared in situ upon optically induced N2 elimination from azido precursors. A full exploitation of their catalytic potential, however, requires in-depth knowledge of the primary photo-induced processes and the structural/electronic factors mediating the N2 loss with birth of the terminal metal-nitrogen core. Using femtosecond infrared spectroscopy, we elucidate here the primary molecular-level mechanisms responsible for the formation of a unique platinum(II) nitrene with a triplet ground state from a closed-shell platinum(II) azide precursor. The spectroscopic data in combination with quantum-chemical calculations provide compelling evidence that product formation requires the initial occupation of a singlet excited state with an anionic azide diradical ligand that is bound to a low-spin d8 -configured PtII ion. Subsequent intersystem crossing generates the Pt-bound triplet azide diradical, which smoothly evolves into the triplet nitrene via N2 loss in a near barrierless adiabatic dissociation. Our data highlight the importance of the productive, N2 -releasing state possessing azide ππ* character as a design principle for accessing efficient N-atom-transfer catalysts.

Photolysis of an Archetypal Model Complex. Photooxidation Versus Photoreduction of Azido(porphinato)iron(III).

Azidoporphinatoiron(III) ([1]) is an archetypal model complex for the photochemical generation of nitridoiron(V) complexes via cleavage of dinitrogen. So far, this process has only been studied with continuous irradiation in thin films under cryogenic conditions or in frozen solutions. In addition, the photooxidation from iron(III) to iron(V) competes with photoreduction to iron(II) via cleavage of an azidyl radical. The quantum yields of both pathways remained hitherto undisclosed. Here, we investigated the photolysis of this model complex in room temperature liquid solution using stationary and time-resolved infrared spectroscopy. The two reaction pathways are unambiguously identified in quenching studies and their quantum yields are accurately determined. Nitridoporphinatoiron(V) ([2]) exhibits N-atom-2-electron-transfer reactivity toward tert-butyl isonitrile and forms a carbodiimido species. In the presence of tert-butyl isonitrile, the two products of the photoreduction pathway react to cationic diisonitriloporphinatoiron(III) and azide anions, which in turn combine to reform [1] and the quencher.

Observing the Entry Events of a Titanium-Based Photoredox Catalytic Cycle in Real Time.

Titanium-based catalysis in single electron transfer (SET) steps has evolved into a versatile approach for the synthesis of fine chemicals and first attempts have recently been made to enhance its sustainability by merging it with photo-redox (PR) catalysis. Here, we explore the photochemical principles of all-Ti-based SET-PR-catalysis, i.e. in the absence of a precious metal PR-co-catalyst. By combining time-resolved emission with ultraviolet-pump/mid-infrared-probe (UV/MIR) spectroscopy on femtosecond-to-microsecond time scales we quantify the dynamics of the critical events of entry into the catalytic cycle; namely, the singlet-triplet interconversion of the do-it-all titanocene(IV) PR-catalyst and its one-electron reduction by a sacrificial amine electron donor. The results highlight the importance of the PR-catalyst's singlet-triplet gap as a design guide for future improvements.

Ultrafast Dynamics of Photochemical Nitrile Imine Formation.

The chemical reactivity of nitrile imines is of great utility in organic synthesis with applications rapidly expanding into the materials and life sciences. Yet, our understanding of the electronic and molecular structures of nitrile imines remains incomplete and the elementary mechanism of their photoinduced generation is entirely unknown. Here, femtosecond infrared spectroscopy after 266 nm-excitation of 2,5-diphenyltetrazole has been carried out to temporally resolve the formation and structural relaxation dynamics of the nascent diphenylnitrile imine in liquid solution under ambient conditions. The infrared-spectroscopic evolution is interpreted by an initial sequence of intersystem crossings within 250 fs followed by the cleavage of N2 with formation of a structurally relaxed nitrile imine on the adiabatic ground-state singlet surface within a few tens of picoseconds. The infrared spectrum supports the notion of a "floppy" nitrile imine molecule whose equilibrium character ranges from fully propargylic to fully allenic in the room temperature liquid solution.

Vibrational relaxation of carbon dioxide in water.

The dynamics of vibrational relaxation of carbon dioxide in water has been studied using femtosecond mid-infrared pump-probe spectroscopy with excitation of the anti-symmetric stretching (ν3) fundamental state of the solute. The relaxation dynamics were recorded at a constant pressure of 500 bars and in the temperature range between 300 and 600 K, thereby covering the liquid-to-near-critical region of the solvent. The excited state of the ν3-mode is deactivated in two competing pathways: (i) direct relaxation to the ground state with resonant transfer of the excess vibrational energy into the bending-librational continuum of the water solvent and (ii) relaxation to the bending fundamental state with transfer into the intramolecular bending mode of H2O. The rate of pathway (i) decreases with increasing temperature, from ∼1/(9 ps) at 300 K to ∼(1/16 ps) at 600 K and obeys Fermi's golden rule strictly, provided that the spectral density of energy-accepting solvent states is derived from the stationary infrared absorption profile of H2O. The rate of pathway (ii) is 1/(23 ps) and assumed to be temperature-independent within our data analysis. Finally, the bending fundamental of CO2 can also relax to the ground state by resonantly transferring the remaining excess energy to the librational fundamentals of the solvent.

Ultrafast "end-on"-to-"side-on" binding-mode isomerization of an iron-carbon dioxide complex.

Carbon dioxide (CO2) binding by transition metals is a captivating phenomenon with a tremendous impact in environmental science and technology, most notably, for establishing circular economies based on greenhouse gas emissions. The molecular and electronic structures of coordination compounds containing CO2 can be studied in great detail using photochemical precursors bearing the photolabile oxalato-ligand. Here, we study the photoinduced elementary dynamics of the ferric complex, [FeIII(cyclam)(C2O4)]+, in dimethyl sulfoxide solution using femtosecond mid-infrared spectroscopy following oxalate-to-iron charge transfer excitation with 266 nm pulses. The pump-probe response in the ν3-region of carbon dioxide gives unequivocal evidence that a CO2-molecule is detached from the metal within only 500 fs and with a primary quantum yield of 38%. Simultaneously, a primary ferrous product is formed that carries a carbon dioxide radical anion ligand absorbing at 1649 cm-1, which is linked to the metal in a bent-O-"end-on" fashion. This primary ηO,bent1-product is formed with substantial excess vibrational energy, which relaxes on a time scale of several picoseconds. Prior to full thermalization, however, a fraction of the ferrous primary product can structurally isomerize at a rate of 1/(3.5 ps) to a secondary ηCO2-product absorbing at 1727 cm-1, which features a bent carbon dioxide ligand that is linked to the metal in a "side-on" fashion. The ηO,bent1-to-ηCO2 isomerization requires an intersystem crossing from the sextet to the quartet state, which rationalizes a partial trapping of the system in the metastable bent-O-"end-on" geometry. Finally, a fraction (62%) of the initially photoexcited complexes can return without structural changes to the parent's electronic ground state, but dressed with excess kinetic energy, which relaxes again on a time scale of several picoseconds.

Photo-Initiated Cobalt-Catalyzed Radical Olefin Hydrogenation.

Outer-sphere radical hydrogenation of olefins proceeds via stepwise hydrogen atom transfer (HAT) from transition metal hydride species to the substrate. Typical catalysts exhibit M-H bonds that are either too weak to efficiently activate H2 or too strong to reduce unactivated olefins. This contribution evaluates an alternative approach, that starts from a square-planar cobalt(II) hydride complex. Photoactivation results in Co-H bond homolysis. The three-coordinate cobalt(I) photoproduct binds H2 to give a dihydrogen complex, which is a strong hydrogen atom donor, enabling the stepwise hydrogenation of both styrenes and unactivated aliphatic olefins with H2 via HAT.

Intramolecular O-H⋯S hydrogen bonding in threefold symmetry: Line broadening dynamics from ultrafast 2DIR-spectroscopy and ab initio calculations.

The dynamics of intramolecular hydrogen-bonding involving sulfur atoms as acceptors is studied using two-dimensional infrared (2DIR) spectroscopy. The molecular system is a tertiary alcohol whose donating hydroxy group is embedded in a hydrogen-bond potential with torsional C3-symmetry about the carbon-oxygen bond. The linear and 2DIR-spectra recorded in the OH-stretching region of the alcohol can be simulated very well using Kubo's line shape theory based on the cumulant expansion for evaluating the linear and nonlinear optical response functions. The correlation function for OH-stretching frequency fluctuations reveals an ultrafast component decaying with a time constant of 700 fs, which is in line with the apparent decay of the center line slopes averaged over absorption and bleach/emission signals. In addition, a quasi-static inhomogeneity is detected, which prevents the 2DIR line shape to fully homogenize within the observation window of 4 ps. The experimental data were then analyzed in more detail using a full ab initio approach that merges time-dependent structural information from classical molecular dynamics (MD) simulations with an OH-stretching frequency map derived from density functional theory (DFT). The latter method was also used to obtain a complementary transition dipole map to account for non-Condon effects. The 2DIR-spectra obtained from the MD/DFT method are in good agreement with the experimental data at early waiting delays, thereby corroborating an assignment of the fast decay of the correlation function to the dynamics of hydrogen-bond breakage and formation.

Spin-Controlled Binding of Carbon Dioxide by an Iron Center: Insights from Ultrafast Mid-Infrared Spectroscopy.

The influence of the spin on the mode of binding between carbon dioxide (CO2 ) and a transition-metal (TM) center is an entirely open question. Herein, we use an iron(III) oxalato complex with nearly vanishing doublet-sextet gap, and its ultrafast photolysis, to generate TM-CO2 bonding patterns and determine their structure in situ by femtosecond mid-infrared spectroscopy. The formation of the nascent TM-CO2 species according to [L4 FeIII (C2 O4 )]+ + hν → [L4 Fe(CO2 )]+ + CO2 , with L4 =cyclam, is evidenced by the coincident appearance of the characteristic asymmetric stretching absorption of the CO2 -ligand between 1600 cm-1 and 1800 cm-1 and that of the free CO2 -co-fragment near 2337 cm-1 . On the high-spin surface (S=5/2), the product complex features a bent carbon dioxide radical anion ligand that is O-"end-on"-bound to the metal. In contrast, on the intermediate-spin and low-spin surfaces, the product exhibits a "side-on"-bound, bent carbon dioxide ligand that has either a partial open-shell (for S=3/2) or fully closed-shell character (for S=1/2).

Probing the primary processes of a triazido-cobalt(III) complex with femtosecond vibrational and electronic spectroscopies. Photochemical selectivity and multi-state reactivity.

The elementary dynamics following 355 nm-excitation of the complex, mer-[Co(dien)(N3)3], were studied in liquid dimethyl sulfoxide (DMSO) solution using femtosecond-ultraviolet-pump/mid-infrared-to-near-ultraviolet probe spectroscopy in conjunction with electronic structure calculations based on density functional theory. Following the initial N3--to-Co charge transfer excitation, the parent complex undergoes an ultrafast metal-to-ligand back electron transfer (BET) within 2 ps thereby populating a metal-centered singlet excited state, 1MC, which can either repopulate the electronic ground state or cleave an azido ligand from the ligand sphere surrounding the metal center. From the asymptotic ground-state bleaching signal after 1 ns, a primary quantum yield for ligand loss of ca. 13% is estimated. The IR-spectrum of the product demonstrates that the photodissociation occurs selectively from the equatorial binding site thereby leading exclusively to the solvolysis product, mer-trans-[Co(dien)(N3)2(DMSO)]+, which features the solvent ligand in the equatorial coordination plane and the azides in the two axial positions. The remarkable photochemical selectivity is traced back to the initial BET and the nature of the intermediate state, 1MC, whose electronic structure entails occupancy of the σ-antibonding d(x2-y2)-orbital. A stereochemical scrambling at the stage of the primary penta-coordinated diazido product is kinetically inhibited on the singlet surface by an energy barrier of roughly 27 kJ mol-1. Primary penta-coordinated products that may be born on the triplet surface are funneled to their singlet ground-state preferentially from geometries with trans-oriented azido ligands thereby also preventing a stereochemical isomerization that could possibly arise from an intersystem crossing.

Vibrational Relaxation Dynamics of an Azido-Cobalt(II) Complex from Femtosecond UV-Pump/MIR-Probe Spectroscopy and Model Simulations with Ab Initio Anharmonic Couplings.

Vibrational energy relaxation is of critical importance for the light-controlled reactivity of transition-metal complexes. In time-resolved optical spectroscopies, it gives rise to pronounced spectral redistributions with complex band shifts and thus to nonexponential kinetics, all of which are very difficult to quantify. Here we study the vibrational relaxation dynamics of a pentacoordinated azido-cobalt(II) complex in liquid solution following its ultrafast charge-transfer excitation in the near-ultraviolet (UV). The complex is photochemically remarkably stable and returns within the experimental time resolution back to its quartet electronic ground state via internal conversion. The nonadiabatic transition effectively instantaneously converts the entire photon energy into kinetic energy of the vibrational degrees of freedom. The ensuing relaxation dynamics of the vibrationally highly excited complex are monitored as a function of time using femtosecond mid-infrared (MIR) spectroscopy in the antisymmetric stretching region of the azido ligand and occur on a time scale of a few tens of picoseconds. The dynamic evolution of the MIR spectrum due to vibrational cooling of the complex can be understood quantitatively within the framework of an anharmonic coupling model, which relies on an ab initio intramolecular cubic/quartic force field from density functional theory combined with second-order vibrational perturbation theory. The simulations suggest that the primary internal conversion preferentially dumps the excess energy into the low-frequency bending modes of the azido ligand, whereas its high-frequency stretching modes are barely affected by the initial nonadiabatic transition. Surprisingly, the two bending vibrations appear to relax independently of one another, each with its own characteristic cooling time.

Vibrations tell the tale. A time-resolved mid-infrared perspective of the photochemistry of iron complexes.

Owing to its superb structure-specificity, time-resolved infrared spectroscopy (TR-IR) has emerged in recent years as one of the most powerful tools for the exploration of photochemical dynamics in solution. It can be carried out in two distinct modes of operation: real-time Fourier-transform spectroscopy following flash photolysis or purely laser-based pump-probe spectroscopy. These two approaches combine to access nearly seamlessly a time window ranging from around hundred femtoseconds all the way up to hundreds of seconds. In this article, we highlight the most recent applications of these techniques in temporally resolving the multiscalar photochemical dynamics of iron complexes. The processes of interest here entail ligand dissociations or fragmentations, which lead to the formation of unique Fe-containing intermediates with remarkable electronic structure, exceptional molecular geometry, or unprecedented chemical reactivity. Thus, TR-IR can provide a novel unexpected view into the realm of iron chemistry including such fascinating topics like high-valent iron, ferracyclic compounds, or carbon dioxide binding to iron centers.

Time-resolved Fourier-transform infrared spectroscopy reveals the hidden bimolecular process of the ferrioxalate actinometer.

Step-scan Fourier-transform infrared spectroscopy was used to monitor the photochemical reactions following the 266 nm-photolysis of aqueous ferrioxalate solutions on microsecond-to-millisecond time scales. Together with most recent observations from ultrafast infrared spectroscopy the reported results finally disclose the full molecular-level mechanism of a photochemical system that is widely known as the Hatchard-Parker actinometer.

Molecular and electronic structure of an azidocobalt(iii) complex derived from X-ray crystallography, linear spectroscopy and quantum chemical calculations.

The photochemistry of transition-metal azides is remarkably complex and can involve multiple competing pathways leading to different fragmentation patterns. Therefore, an in-depth study of such rich photochemistry requires a thorough prior understanding of the molecular and electronic structures of these complexes. To this end, stationary (i.e. linear) spectroscopies in the ultraviolet-to-visible (UV/vis) and the mid-infrared (MIR) spectral regions are most often employed. Here, we investigate the electronic and vibrational spectroscopies of the cationic diazidocobalt(iii) complex, trans-[Co(cyclam)(N3)2]+, in liquid dimethyl sulfoxide (DMSO) solution and interpret the experimental data in terms of detailed quantum chemical calculations. The X-ray crystallography reveals a Ci-symmetric molecular structure of the complex whereas in liquid solution, evidence for symmetry breakage with loss of the inversion center of the ligand sphere is found from both, the UV/vis and MIR-data. This interpretation is fully corroborated by a stereochemical and conformational analysis of the complex using ab initio calculations involving nuclear degrees of freedom of both, the equatorial cyclam ancillary ligand and the two axial azido ligands.

Photoinduced Dynamics of a Diazidocobalt(III) Complex Studied by Femtosecond UV-Pump/IR-to-Vis-Probe Spectroscopy.

The photochemistry of the cationic diazidocobalt(III) complex, trans-[Co(cyclam)(N3)2]+, following its ligand-to-metal charge transfer (LMCT) excitation is studied in liquid dimethyl sulfoxide (DMSO) solution using femtosecond spectroscopy with detection in a very broad spectral region covering the near-ultraviolet (near-UV) all the way to the mid-infrared (MIR), thereby enabling a combined probing of electronic and vibrational degrees of freedom of the dynamically evolving system. The initially prepared singlet LMCT-state decays, via the metal-centered singlet excited state, 1MC(1Eg), into the triplet ground state, 3MC (3Eg/3A1g), on a time scale shorter than 25 ps. During this time period, the vibrational spectrum demonstrates uniquely that the nature of the complex changes from a monoazidocobalt(II) species bearing a neutral azide radical ligand immediately after photon absorption to a metal-centered open-shell diazidocobalt(III) species. At the same time, the 3MC state is characterized by a very strong absorption band centered at 710 nm, which can be assigned to a transition to the triplet LMCT state. The 1LMCT lifetime is about 2 ps, whereas that of the excited state, 1MC, is defined by the primary intersystem crossing time of 6 ps. The ensuing intersystem recrossing from 3MC to the parent's singlet ground state, 1A1g, occurs with a rate of 1/(110 ps). The mid-infrared pump-probe spectrum after 1 ns, gives evidence for a heterolytic Co-N bond fission with a quantum yield of ∼5%, leading to free azide anions and the monoazido species, trans-[Co(cyclam)(N3)(OSMe2)]+, featuring an oxygen-bound DMSO ligand in its coordination sphere.

Probing the band gap of liquid ammonia with femtosecond multiphoton ionization spectroscopy.

The electronic band gap, i.e. the energy difference between the top of the valence band and the bottom of the conduction band, is widely recognized as the key property characterizing the electronic structure of bulk liquids and liquid solvents like water or ammonia. Here, the band gap of liquid ammonia at 270 K and 300 bar was studied with 2-photon ionization spectroscopy using the solvated electron primary yield as a near-infrared action-spectroscopic probe. The experimentally determined escape probability, which is the fraction of solvated electrons that is able to avoid geminate recombination within the first nanosecond after ionization, was used to extract a value of -(1.27 ± 0.03) eV for the vertical electron affinity of the liquid.

Femtosecond infrared spectroscopy reveals the primary events of the ferrioxalate actinometer.

Chemical actinometry is an indispensable analytical tool in preparative photochemistry that allows for a precise measurement of radiant fluxes inside photoreactors. An actinometer thus enables an absolute determination of the quantum yield of a photochemical reaction of interest. The "gold standard" of chemical actinometry in liquid systems is the Hatchard-Parker actinometer, i.e. an aqueous solution of potassium trisoxalatoferrate(iii), which is based on the light-induced net transformation of ferric into ferrous oxalate complexes. Although the absolute photochemical quantum yield for this fundamental standard system has been accurately known for many years, the underlying molecular-level mechanisms and time scales associated with a photoreduction of the ferrioxalate actinometer remained so far largely obscured. Here, we use femtosecond mid-infrared spectroscopy combined with ultrafast laser photolysis to obtain unique structural-dynamical information associated with the primary light-triggered processes thereby finally providing the missing quantitative molecular-level foundations that ultimately justify a utilization of aqueous ferrioxalate as a true photochemical standard. Following photon absorption by the octahedral parent complex, an ultrafast decarboxylation occurs within 500 fs, which generates a penta-coordinated ferrous dioxalate that carries a bent carbon dioxide radical anion ligand in an "end-on" O-coordinated fashion. This unique intermediate structurally isomerizes on a tens of picoseconds time scale and subsequently loses a CO2˙--ligand to form a square-planar bisoxalatoferrate(ii) on a hundreds of picoseconds time scale.