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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Übergangsmetallnitride/nitrene sind vielversprechende Reagenzien für katalytischen Stickstoffatomtransfer, die in situ durch optisch induzierte N2‐Eliminierung aus Azidkomplexen zugänglich sind. Um ihr synthetisches Potenzial für die Katalyse voll ausschöpfen zu können, ist ein grundlegendes Verständnis der primären photoinduzierten Prozesse und der stereoelektronischen Faktoren, welche die N2‐Dissoziation kontrollieren, unerlässlich. Mit Hilfe von Femtosekunden‐Infrarotspektroskopie klären wir hier die primären molekularen Mechanismen auf, die für die Bildung eines einzigartigen Platin(II)nitrens mit Triplett‐Grundzustand aus einem geschlossenschaligen Platin(II)azidkomplex verantwortlich sind. Die spektroskopischen Daten in Kombination mit quantenchemischen Rechnungen ergeben einen konsistenten Mechanismus: Produktbildung erfordert die anfängliche Besetzung eines angeregten Singulett‐Zustands mit anionischen Azid‐Diradikal‐Liganden, der an ein low‐spin d8‐konfiguriertes PtII‐Ion gebunden ist. Anschließende Interkombination erzeugt das an Platin gebundene Triplett‐Aziddiradikal, welches sich durch N2‐Verlust in einer nahezu barrierelosen, adiabatischen Dissoziation nahtlos in das Triplett‐Nitren umwandelt. Unsere Daten unterstreichen die Bedeutung des produktiven, N2‐freisetzenden Zustands mit Azid ππ* Charakter als Konstruktionsprinzip für den Zugang zu effizienten Stickstoffatomtransfer‐Katalysatoren. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The elementary light-triggered dynamics of the transition metal complex, trans-[CoIII(acac)2(N3)(py)] (trans-[1]), was investigated using femtosecond mid-infrared (mIR) and UV-Vis spectroscopies following ultrafast acetylacetonate-to-cobalt charge-transfer excitation at 266 nm. The spectro-temporal response is indicative of an ultrafast return to the electronic ground state via a sequential intramolecular electron transfer. The locally excited ligand-to-metal charge transfer state, 1LMCTacac, decays within 200 fs via azide-to-acac electron transfer to another charge-transfer state, 1LMCTazide, that features an azidyl radical ligand attached to a cobalt(II) center. A subsequent cobalt-to azidyl electron transfer occurring within 1.8 ps generates an excited metal centered state, 1MC, thereby re-establishing the fully closed-shell character of the anionic azido and acac ligands. The fully relaxed electronic ground state of trans-[1] is then recovered via internal conversion and vibrational energy relaxation on a 10 ps-time scale. [ABSTRACT FROM AUTHOR]

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., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

Transition-metal nitrides/nitrenes are highly promising reagents for catalytic nitrogen-atom-transfer reactivity. They are typically prepared in situ upon optically induced N 2 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 N 2 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 d 8 -configured Pt II ion. Subsequent intersystem crossing generates the Pt-bound triplet azide diradical, which smoothly evolves into the triplet nitrene via N 2 loss in a near barrierless adiabatic dissociation. Our data highlight the importance of the productive, N 2 -releasing state possessing azide ππ* character as a design principle for accessing efficient N-atom-transfer catalysts., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

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., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

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 H 2 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 H 2 to give a dihydrogen complex, which is a strong hydrogen atom donor, enabling the stepwise hydrogenation of both styrenes and unactivated aliphatic olefins with H 2 via HAT., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)