On the Photoabsorption by Permanganate Ions in Vacuo and the Role of a Single Water Molecule. New Experimental Benchmarks for Electronic Structure Theory

We report electronic spectra of mass‐selected MnO₄^? and MnO₄^??H₂O using electronic photodissociation spectroscopy. Bare MnO₄^? fragments by formation of MnO₃^? and MnO₂^?, while the hydrated complex predominantly decays by loss of the water molecule. The band in the visible spectral region shows a well‐resolved vibrational progression consistent with the excitation of a Mn? O stretching mode. The presence of a single water molecule does not significantly perturb the spectrum of MnO₄^?. Comparison with the UV/Vis absorption spectrum of permanganate in aqueous solution shows that complete hydration causes a small blueshift, while theoretical models including a dielectric medium have predicted a redshift. The experimental data can be used as benchmarks for electronic structure theory methods, which usually predict electronic spectra in the absence of a chemical environment.     

Transformation of a Cp*–Iridium(III) Precatalyst for Water Oxidation when Exposed to Oxidative Stress

The reaction of [Cp*Ir(bzpy)NO₃] ( 1 ; bzpy=2‐benzoylpyridine, Cp*=pentamethylcyclopentadienyl anion), a competent water‐oxidation catalyst, with several oxidants (H₂O₂, NaIO₄, cerium ammonium nitrate (CAN)) was studied to intercept and characterize possible intermediates of the oxidative transformation. NMR spectroscopy and ESI‐MS techniques provided evidence for the formation of many species that all had the intact Ir–bzpy moiety and a gradually more oxidized Cp* ligand. Initially, an oxygen atom is trapped in between two carbon atoms of Cp* and iridium, which gives an oxygen–Ir coordinated epoxide, whereas the remaining three carbon atoms of Cp* are involved in a η³ interaction with iridium ( 2 a ). Formal addition of H₂O to 2 a or H₂O₂ to 1 leads to 2 b , in which a double MeCOH functionalization of Cp* is present with one MeCOH engaged in an interaction with iridium. The structure of 2 b was unambiguously determined in the solid state and in solution by X‐ray single‐crystal diffractometry and advanced NMR spectroscopic techniques, respectively. Further oxidation led to the opening of Cp* and transformation of the diol into a diketone with one carbonyl coordinated at the metal ( 2 c ). A η³ interaction between the three non‐oxygenated carbons of “ex‐Cp*” and iridium is also present in both 2 b and 2 c . Isolated 2 b and mixtures of 2 a – c species were tested in water‐oxidation catalysis by using CAN as sacrificial oxidant. They showed substantially the same activity than 1 (turnover frequency values ranged from 9 to 14 min^?1).