Probing 'spin-forbidden' oxygen-atom transfer: Gas-phase reactions of chromium-porphyrin complexes

Maria Elisa Crestoni, Simonetta Fornarini, Francesco Lanucara, Jeffrey J. Warren, James M. Mayer

    Research output: Contribution to journalArticlepeer-review

    Abstract

    Oxygen-atom transfer reactions of metalloporphyrin species play an important role in biochemical and synthetic oxidation reactions. An emerging theme in this chemistry is that spin-state changes can play important roles, and a 'two-state' reactivity model has been extensively applied especially in iron porphyrin systems. Herein we explore the gas-phase oxygen-atom transfer chemistry of meso-tetrakis(pentafluorophenyl) porphyrin (TPFPP) chromium complexes, as well as some other tetradentate macrocyclic ligands. Electrospray ionization in concert with Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry has been used to characterize and observe reactivity of the ionic species [(TPFPP)CrIII]+ (1) and [(TPFPP)CrVO] + (2). These are attractive systems to examine the effects of spin-state change on oxygenatom transfer because the d1 Cr V species are doublets, while the CrIII complexes have quartet ground states with high-lying doublet excited states. In the gas phase, [(TPFPP)CrIII]+ forms adducts with a variety of neutral donors, but O-atom transfer is only observed for NO2. Pyridine N-oxide adducts of 1 do yield 2 upon collision-induced dissociation (CID), but the ethylene oxide, DMSO, and TEMPO analogues do not. [(TPFPP)Cr VO]+ is shown by its reactivity and by CID experiments to be a terminal metal-oxo with a single, vacant coordination site. It also displays limited reaction chemistry, being deoxygenated only by the very potent reductant P(OMe)3. In general, [(TPFPP)CrVO]+ species are much less reactive than the Fe and Mn analogues. Thermochemical analysis of the reactions points toward the involvement of spin issues in the lower observed reactivity of the chromium complexes. © 2010 American Chemical Society.
    Original languageEnglish
    Pages (from-to)4336-4343
    Number of pages7
    JournalJournal of the American Chemical Society
    Volume132
    Issue number12
    DOIs
    Publication statusPublished - 31 Mar 2010

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