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.

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).

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.

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.

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.

An Iron Complex with a Bent, O-Coordinated CO2 Ligand Discovered by Femtosecond Mid-Infrared Spectroscopy.

The activation of carbon dioxide by transition metals is widely recognized as a key step for utilizing this greenhouse gas as a renewable feedstock for the sustainable production of fine chemicals. However, the dynamics of CO2 binding and unbinding to and from the ligand sphere of a metal have never been observed in the time domain. The ferrioxalate anion is used in aqueous solution as a unique model system for these dynamics and femtosecond UV-pump mid-infrared-probe spectroscopy is applied to explore its photoinduced primary processes in a time-resolved fashion. Following optical excitation, a neutral CO2 molecule is expelled from the complex within about 500 fs to generate a highly intriguing pentacoordinate ferrous dioxalate that carries a bent carbon dioxide radical anion ligand, that is, a reductively activated form of CO2 , which is end-on-coordinated to the metal center by one of its two oxygen atoms.

Photochemical Kinetics of a Phosphine Oxide Free Radical Initiator from Femtosecond UV-Pump/Mid-IR-Probe Spectroscopy.

Femtosecond UV-pump/mid-infrared-probe spectroscopy was used to explore in detail the primary photochemical events of the free radical initiator, (2,4,6-trimethylbenzoyl)diphenylphosphine oxide, in liquid dichloromethane solution at room temperature. Following electronic excitation of its lowest excited singlet state, S1, the radical initiator undergoes an intersystem crossing to the triplet ground state, T1, with a time constant of 135 ps. A subsequent α-cleavage occurs from the triplet state with a time constant of 15 ps and yields a trimethylbenzoyl radical together with a diphenylphosphinoyl radical. Transient absorptions from the S1 and T1 states are observed that can be assigned to the P═O stretching mode and the symmetric in-plane deformation mode of the trimethylphenyl moiety of the radical initiator.

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.