Characterized cis-FeV(O)(OH) intermediate mimics enzymatic oxidations in the gas phase.

FeV(O)(OH) species have long been proposed to play a key role in a wide range of biomimetic and enzymatic oxidations, including as intermediates in arene dihydroxylation catalyzed by Rieske oxygenases. However, the inability to accumulate these intermediates in solution has thus far prevented their spectroscopic and chemical characterization. Thus, we use gas-phase ion spectroscopy and reactivity analysis to characterize the highly reactive [FeV(O)(OH)(5tips3tpa)]2+ (32+) complex. The results show that 32+ hydroxylates C-H bonds via a rebound mechanism involving two different ligands at the Fe center and dihydroxylates olefins and arenes. Hence, this study provides a direct evidence of FeV(O)(OH) species in non-heme iron catalysis. Furthermore, the reactivity of 32+ accounts for the unique behavior of Rieske oxygenases. The use of gas-phase ion characterization allows us to address issues related to highly reactive intermediates that other methods are unable to solve in the context of catalysis and enzymology.

Trapping Iron(III)-Oxo Species at the Boundary of the "Oxo Wall": Insights into the Nature of the Fe(III)-O Bond.

Terminal non-heme iron(IV)-oxo compounds are among the most powerful and best studied oxidants of strong C-H bonds. In contrast to the increasing number of such complexes (>80 thus far), corresponding one-electron-reduced derivatives are much rarer and presumably less stable, and only two iron(III)-oxo complexes have been characterized to date, both of which are stabilized by hydrogen-bonding interactions. Herein we have employed gas-phase techniques to generate and identify a series of terminal iron(III)-oxo complexes, all without built-in hydrogen bonding. Some of these complexes exhibit ∼70 cm-1 decrease in ν(Fe-O) frequencies expected for a half-order decrease in bond order upon one-electron reduction to an S = 5/2 center, while others have ν(Fe-O) frequencies essentially unchanged from those of their parent iron(IV)-oxo complexes. The latter result suggests that the added electron does not occupy a d orbital with Fe═O antibonding character, requiring an S = 3/2 spin assignment for the nascent FeIII-O- species. In the latter cases, water is found to hydrogen bond to the FeIII-O- unit, resulting in a change from quartet to sextet spin state. Reactivity studies also demonstrate the extraordinary basicity of these iron(III)-oxo complexes. Our observations show that metal-oxo species at the boundary of the "Oxo Wall" are accessible, and the data provide a lead to detect iron(III)-oxo intermediates in biological and biomimetic reactions.

Transforming hemithioindigo from a two-way to a one-way molecular photoswitch by isolation in the gas phase.

Hemithioindigo compounds are attractive two-way molecular photoswitches combining stilbene and thioindigo parts connected by a C-C double bond. In solution, these photoswitches have been well studied. This study presents the investigation of a hemithioindigo derivative in the gas phase. Visible absorption spectra, measured by standard (visPD) and helium-tagging visible photodissociation (He-visPD) techniques were used to unravel absorption characteristics at the level of isolated molecules at 3 Kelvin. Comparison between the Z and E isomers shows a quite distinctive behavior upon visible light absorption. The Z isomer readily undergoes Z → E conversion in the gas phase, as evidenced by the changes in the helium-tagging infrared photodissociation (He-IRPD) spectra. Surprisingly, visible light excitation of the E isomer does not lead to efficient E → Z isomerization unlike in solution. Instead, the ions relax back to their ground state. Influencing the microenvironment of the E isomer by complexation with the highly polar betaine zwitterion resulted in absorption changes, albeit without activating the photoswitching process. Hence, isolation in the gas phase transforms hemithioindigo into a one-way molecular photoswitch. Furthermore, the combination of He-visPD and IRPD spectroscopies proved to be an excellent method for studying photochemical processes such as the double-bond isomerization in the gas phase.

Detection of Indistinct Fe-N Stretching Bands in Iron(V) Nitrides by Photodissociation Spectroscopy.

We report for the first time infrared spectra of three non-heme pseudo-octahedral iron(V) nitride complexes with assigned Fe-N stretching vibrations. The intensities of the Fe-N bands in two of the complexes are extremely weak. Their detection was enabled by the high resolution and sensitivity of the experiments performed at 3 K for isolated complexes in the gas phase. Multireference CASPT2 calculations revealed that the Fe-N bond in the ground doublet state is influenced by two low-lying excited doublet states. In particular, configuration interaction between the ground and the second excited state leads to avoided crossing of their potential energy surfaces along the Fe-N coordinate, which thus affects the ground-state Fe-N stretching frequency and intensity. Therefore, DFT calculated Fe-N stretching frequency strongly depends on the amount of Hartree-Fock exchange potential. As a result, by tuning the amount of Hartree-Fock exchange potential in the B3LYP functional, it was possible to obtain theoretical spectra perfectly consistent with the experimental data. The theory shows that the intensity of the Fe-N stretching vibration can almost vanish due to strong coupling with other stretching modes of the ligands.

Spin-State-Controlled Photodissociation of Iron(III) Azide to an Iron(V) Nitride Complex.

The generation of iron(V) nitride complexes, which are targets of biomimetic chemistry, is reported. Temperature-dependent ion spectroscopy shows that this reaction is governed by the spin-state population of their iron(III) azide precursors and can be tuned by temperature. The complex [(MePy2 TACN)Fe(N3 )]2+ (MePy2 TACN=N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane) exists as a mixture of sextet and doublet spin states at 300 K, whereas only the doublet state is populated at 3 K. Photofragmentation of the sextet state complex leads to the reduction of the iron center. The doublet state complex photodissociates to the desired iron(V) nitride complex. To generalize these findings, we show results for complexes with cyclam-based ligands.

Chasing the Evasive Fe═O Stretch and the Spin State of the Iron(IV)-Oxo Complexes by Photodissociation Spectroscopy.

We demonstrate the application of infrared photodissocation spectroscopy for determination of the Fe═O stretching frequencies of high-valent iron(IV)-oxo complexes [(L)Fe(O)(X)]2+/+ (L = TMC, N4Py, PyTACN, and X = CH3CN, CF3SO3, ClO4, CF3COO, NO3, N3). We show that the values determined by resonance Raman spectroscopy in acetonitrile solutions are on average 9 cm-1 red-shifted with respect to unbiased gas-phase values. Furthermore, we show the assignment of the spin state of the complexes based on the vibrational modes of a coordinated anion and compare reactivities of various iron(IV)-oxo complexes generated as dications or monocations (bearing an anionic ligand). The coordinated anions can drastically affect the reactivity of the complex and should be taken into account when comparing reactivities of complexes bearing different ligands. Comparison of reactivities of [(PyTACN)Fe(O)(X)]+ generated in different spin states and bearing different anionic ligands X revealed that the nature of anion influences the reactivity more than the spin state. The triflate and perchlorate ligands tend to stabilize the quintet state of [(PyTACN)Fe(O)(X)]+, whereas trifluoroacetate and nitrate stabilize the triplet state of the complex.

Infrared and Visible Photodissociation Spectra of Rhodamine Ions at 3 K in the Gas Phase.

Helium-tagging predissociation spectroscopy in the visible spectral range (He@VisPD) is shown for Rhodamine 123, Rhodamine 110, and Rhodamine 110's silver salt (silver carboxylate). It is shown that the spectra reflect single-photon absorption. The helium-tagged ions are in the ground vibrational state, and the He@VisPD spectra feature the Franck-Condon envelopes for the excitation to the first excited singlet state that agree very well with theoretical simulations. The S0 → S1 excitation energies are 2.712 ± 0.006 eV for Rhodamine 123, 2.700 ± 0.006 eV for Rhodamine 110, and 2.751 ± 0.006 eV for the silver salt of Rhodamine 110. The determined energies can be slightly blue-shifted due to the binding energy of helium. The Rhodamine ions were also characterized by helium-tagging infrared photodissociation spectroscopy. The distinctive spectral features of the individual derivatives are described and the spectra are compared to the classical solid-state IR spectra.