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Chasing the Evasive Fe=O Stretch and the Spin State of the Iron(IV)-Oxo Complexes by Photodissociation Spectroscopy Erik Andris, Rafael Navrátil, Juraj Jasik, Terencio Thibault, Martin Srnec, Miquel Costas, and Jana Roithova J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12291 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017
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Journal of the American Chemical Society
Chasing the Evasive Fe=O Stretch and the Spin State of the Iron(IV)-Oxo Complexes by Photodissociation Spectroscopy Erik Andris,‡ Rafael Navrátil,‡ Juraj Jašík,‡ Terencio Thibault,‡ Martin Srnec,⊥* Miquel Costas,†* and Jana Roithová‡* ‡
Department of Organic Chemistry, Faculty of Science, Charles University, Hlavova 2030/8, 12843 Prague 2 (Czech Republic). † Departament de Quimica and Institute of Computational Chemistry and Catalysis (IQCC), University of Girona, Campus Montilivi, Girona 17071 (Spain). ⊥
J. Heyrovsky Institute of Physical Chemistry of the CAS, v. v. i., Dolejškova 2155/3, 182 23 Prague 8 (Czech Republic).
ABSTRACT: 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 the 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.
ed ions urged the development of novel techniques that combine MS with other spectroscopic methods such as infrared (IR) spectroscopy.14
INTRODUCTION Non-heme high-valent iron-oxo chemistry is a rapidly evolving field with interdisciplinary connections between organic, inorganic and theoretical chemistry. The importance of nonheme iron-oxo compounds in many enzymatic reactions1 has been appreciated mainly after the seminal works of Bollinger and Krebs.2 The first X-Ray structure characterization of a non-heme iron(IV)-oxo complex from the laboratories of Nam and Que enabled development of synthetic models that to a great extent facilitated our understanding of these reactive species and their function.3-5 Characterization of the active iron centers currently relies mainly on UV/VIS, Mössbauer, resonance Raman (rR), EPR and X-Ray spectroscopic techniques. As first demonstrated by Que and Nam,6 resonance Raman spectroscopy usually provides the most direct evidence for the Fe=O moiety.5a,7,8 The rR method takes advantage of resonance enhancement and thus relies on the presence of intense electronic absorption features. These are, however, not always present.9,10 In addition, they have limitations when dealing with complex mixtures. As a result, minor though important - species may stay unnoticed. As numerously reported, mass spectrometry is an ideal tool for detection of reactive species present in low concentrations (dynamic range of a standard mass spectrometer is ~10⁴-10⁶).11-13 The requirement of detailed structural information on the detect-
Our approach, helium tagging infrared photodissociation spectroscopy (IRPD), consists in trapping the ions of interest in a cryogenic ion trap (~ 3 K) filled with helium. The ions are collisionally cooled and form weakly bound complexes with helium atoms. The complexes, irradiated by a tunable IR beam, detach helium if they absorb a photon. Hence, we obtain IR spectra as a dependence of the depletion of the helium complexes on the IR wavenumber.15 Herein, we report a set of IRPD spectra of representative non-heme iron(IV)-oxo complexes with a special emphasis on the Fe=O stretching vibration. The interface of electrospray ionization (ESI) serves to transfer the complexes from their acetonitrile solution to the gas phase. Assignment of vibrational bands to the Fe=O stretching modes is based on the 18O labeling, which has been done by the oxygen atom exchange with H₂18O (Chart 1).16
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filled with a gaseous reagent at 313 Kelvin and
