Light-driven N 2 cleavage into molecular nitrides is an attractive strategy for synthetic nitrogen fixation. However, suitable platforms are rare. Furthermore, the development of catalytic protocols via this elementary step suffers from poor understanding of N–N photosplitting within dinitrogen complexes, as well as of the thermochemical and kinetic framework for coupled follow-up chemistry. We here present a tungsten pincer platform, which undergoes fully reversible, thermal N 2 splitting and reverse nitride coupling, allowing for experimental derivation of thermodynamic and kinetic parameters of the N–N cleavage step. Selective N–N splitting was also obtained photolytically. DFT computations allocate the productive excitations within the {WNNW} core. Transient absorption spectroscopy shows ultrafast repopulation of the electronic ground state. Comparison with ground-state kinetics and resonance Raman data support a pathway for N–N photosplitting via a nonstatistically vibrationally excited ground state that benefits from vibronically coupled structural distortion of the core. Nitride carbonylation and release are demonstrated within a full synthetic cycle for trimethylsilylcyanate formation directly from N 2 and CO.

Herein, we present a detailed study on the photophysical properties and the excited state reactivity of two mononuclear Re(CO) 3 complexes with imdazol-pyridine ligands equipped with and without a local proton source, [Re(CO) 3 LCl], where for 1:yl)pyridine. Time-resolved IR and UV/vis spectroscopy revealed that excitation of 1 and 2 is followed by population of the triplet excited state within <100 fs, where structural and vibrational relaxation to the T 1 equilibrium structure is observed on the picosecond time scale. The T 1 state can be viewed as a MLCT state as all ν(CO) features in the transient infrared (TRIR) spectra are shifted to higher wavenumbers upon excitation, which is indicative for a decreasing Re → CO π-backdonation. The T 1 states have considerably long lifetimes at room temperature of 160 ns for 1 and 430 ns for 2 in dmf and they can be successfully quenched by the sacrificial electron donors triethanolamine (TEOA) and 1,3-dimethyl-2-phenyl-2,3-dihydro-1Hbenzo[d]imidazole (BIH). The quenching rates are 2 orders of magnitude larger for BIH than for TEOA, as the latter reaction is endergonic. However, both species are not active in the photochemical CO 2 -to-CO conversion. We rationalize this for 2 by the low steady-state concentration of the initial reduction product, [Re(CO) 3 LCl] − , which ejects chloride rather fast. Thus, the second, homogeneous electron transfer process between [Re(CO) 3 LCl] − and [Re(CO) 3 L(solvent)] forming the active species [Re(CO) 3 L] − , has a very low probability and decomposition pathways come to the fore. 1 decomposes under irradiation in the presence of BIH or TEOA forming the initial photoproduct 3. We tentatively assume that the ligand in 3 is deprotonated and switches from a N,N-to a N,O − -coordination mode. This indicates that in the excited state the Re−N bond is cleaved quite easily, as this decomposition pathway has not been observed under electrochemical conditions.

Molecular systems combining light harvesting and charge storage are receiving great attention in the context of, for example, artificial photosynthesis and solar fuel generation. As part of ongoing efforts to develop new concepts for photoinduced proton-coupled electron transfer (PCET) reactivities, we report a cyclometallated iridium(III) complex [Ir(ppy) 2 ( S−S bpy)](PF 6 ) ([1]PF 6 ) equipped with our previously developed sulfurated bipyridine ligand S−S bpy. A new one-step synthetic protocol for S−S bpy is developed, starting from commercially available 2,2′bipyridine, which significantly facilitates the use of this ligand. [1] + features a two-electron reduction with potential inversion (|E 1 | > | E 2 |) at moderate potentials (E 1 = −1.12, E 2 = −1.11 V versus. Fc +/0 at 253 K), leading to a dithiolate species [1] − . Protonation with weak acids allows for determination of pK a = 23.5 in MeCN for the S−H•••S − unit of [1H]. The driving forces for both the H atom and the hydride transfer are calculated to be ∼60 kcal mol −1 and verified experimentally by reaction with a suitable H atom and a hydride acceptor, demonstrating the ability of [1] + to serve as a versatile PCET reagent, albeit with limited thermal stability. In MeCN solution, an orange emission for [1]PF 6 from a triplet-excited state was found. Density functional calculations and ultrafast absorption spectroscopy are used to give insight into the excited-state dynamics of the complex and suggest a significantly stretched S−S bond for the lowest triplet-state T 1 . The structural responsiveness of the disulfide unit is proposed to open an effective relaxation channel toward the ground state, explaining the unexpectedly short lifetime of [1] + . These insights as well as the quantitative ground-state thermochemistry data provide valuable information for the use of S−S bpy-functionalized complexes and their disulfide-/dithiol-directed PCET reactivity.

A [Pd2L4] coordination cage, assembled from electron-rich phenothiazine-based ligands and encapsulating an electron- deficient anthraquinone-based disulfonate guest, is reported. Upon excitation at 400 nm, transient absorption spectroscopy unveils photoinduced electron...