More than a Solvent: Donor–Acceptor Complexes of Ionic Liquids and Electron Acceptors

The applicability of room-temperature ionic liquids (RTILs) as inert solvents is generally based on their electrochemical window. We herein show that this concept has its limitations if RTILs are exposed to an oxidizing environment in the presence of light. Acetonitrile solutions of RTILs with 1-methyl-3-ethylimidazolium as cation and five different anions, including thiocyanate (SCN^–) and dicyanamide (DCA^–), were investigated. Upon addition of organic electron acceptors to solutions of RTILs with SCN^– or DCA^–, charge-transfer (CT) absorption bands due to the formation of donor–acceptor complexes between the anion and the electron acceptor were observed. Time-resolved measurements from the femtosecond to the microsecond regimes were used to investigate the nature and the excited-state dynamics of these complexes upon excitation in the CT band. We show that even though the RTILs are seemingly inert according to their electrochemical properties, the dicyanamide and thiocyanate based RTILs can actively participate in photochemical reactions in oxidizing environments and therefore differ from the behavior expected for an inert solvent. This has not only important implications for the long-term stability of RTIL-based systems but can also lead to misinterpretation of photochemical studies in these solvents

Structural and Kinetic Studies of Intermediates of a Biomimetic Diiron Proton-Reduction Catalyst

One-electron reduction and subsequent protonation of a biomimetic proton-reduction catalyst [FeFe­(μ-pdt)­(CO)₆] (pdt = propanedithiolate), <b>1</b>, were investigated by UV–vis and IR spectroscopy on a nano- to microsecond time scale. The study aimed to provide further insight into the proton-reduction cycle of this [FeFe]-hydrogenase model complex, which with its prototypical alkyldithiolate-bridged diiron core is widely employed as a molecular, precious metal-free catalyst for sustainable H₂ generation. The one-electron-reduced catalyst was obtained transiently by electron transfer from photogenerated [Ru­(dmb)₃]⁺ in the absence of proton sources or in the presence of acids (dichloro- or trichloroacetic acid or tosylic acid). The reduced catalyst and its protonation product were observed in real time by UV–vis and IR spectroscopy, leading to their structural characterization and providing kinetic data on the electron and proton transfer reactions. <b>1</b> features an intact (μ²,κ²-pdt)­(μ-H)­Fe₂ core in the reduced, <b>1^–</b>, and reduced-protonated states, <b>1H</b>, in contrast to the Fe–S bond cleavage upon the reduction of [FeFe­(bdt)­(CO)₆], <b>2</b>, with a benzenedithiolate bridge. The driving-force dependence of the rate constants for the protonation of <b>1^–</b> (<i>k</i>_pt = 7.0 × 10⁵, 1.3 × 10⁷, and 7.0 × 10⁷ M^–1 s^–1 for the three acids used in this study) suggests a reorganization energy >1 eV and indicates that hydride complex <b>1H</b> is formed by direct protonation of the Fe–Fe bond. The protonation of <b>1^–</b> is sufficiently fast even with the weaker acids, which excludes a rate-limiting role in light-driven H₂ formation under typical conditions