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Reactivity of a Cobalt(III)–Hydroperoxo Complex in Electrophilic Reactions

Bongki Shin†, Kyle D. Sutherlin‡, Takehiro Ohta§, Takashi Ogura§, Edward I. Solomon*‡∥, and Jaeheung Cho*†

† Department of Emerging Materials Science, DGIST, Daegu 42988, Korea

‡ Department of Chemistry, Stanford University, Stanford, California 94305, United States

§ Picobiology Institute, Graduate School of Life Science, University of Hyogo, RSC-UH LP Center, Hyogo 679-5148, Japan

∥ Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, California 94025, United States

Inorg. Chem., Article ASAP

DOI: 10.1021/acs.inorgchem.6b02288

Publication Date (Web): November 15, 2016

Copyright © 2016 American Chemical Society

*E-mail: edward.solomon@stanford.edu., *E-mail: jaeheung@dgist.ac.kr.

Synopsis

A cobalt(III)−hydroperoxo complex was prepared by the protonation of a cobalt(III)−peroxo complex. Reactivity studies reveal that the cobalt(III)−hydroperoxo complex is capable of conducting oxygen atom transfer with an electrophilic character. Alternatively, the cobalt(III)−hydroperoxo complex does not perform hydrogen atom transfer reactions, while analogous low-spin Fe(III)−hydroperoxo complexes are capable of this reactivity. Density functional theory calculations indicate that this lack of reactivity is due to the high free energy cost of O−O bond homolysis.

http://pubs.acs.org/doi/abs/10.1021/acs.inorgchem.6b02288

Abstract

The reactivity of mononuclear metal–hydroperoxo adducts has fascinated researchers in many areas due to their diverse biological and catalytic processes. In this study, a mononuclear cobalt(III)–peroxo complex bearing a tetradentate macrocyclic ligand, [CoIII(Me3-TPADP)(O2)]+(Me3-TPADP = 3,6,9-trimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane), was prepared by reacting [CoII(Me3-TPADP)(CH3CN)2]2+ with H2O2 in the presence of triethylamine. Upon protonation, the cobalt(III)–peroxo intermediate was converted into a cobalt(III)–hydroperoxo complex, [CoIII(Me3-TPADP)(O2H)(CH3CN)]2+. The mononuclear cobalt(III)–peroxo and −hydroperoxo intermediates were characterized by a variety of physicochemical methods. Results of electrospray ionization mass spectrometry clearly show the transformation of the intermediates: the peak at m/z 339.2 assignable to the cobalt(III)–peroxo species disappears with concomitant growth of the peak at m/z 190.7 corresponding to the cobalt(III)–hydroperoxo complex (with bound CH3CN). Isotope labeling experiments further support the existence of the cobalt(III)–peroxo and −hydroperoxo complexes. In particular, the O–O bond stretching frequency of the cobalt(III)–hydroperoxo complex was determined to be 851 cm–1 for 16O2H samples (803 cm–1 for 18O2H samples), and its Co–O vibrational energy was observed at 571 cm–1 for 16O2H samples (551 cm–1 for 18O2H samples; 568 cm–1 for 16O22H samples) by resonance Raman spectroscopy. Reactivity studies performed with the cobalt(III)–peroxo and −hydroperoxo complexes in organic functionalizations reveal that the latter is capable of conducting oxygen atom transfer with an electrophilic character, whereas the former exhibits no oxygen atom transfer reactivity under the same reaction conditions. Alternatively, the cobalt(III)–hydroperoxo complex does not perform hydrogen atom transfer reactions, while analogous low-spin Fe(III)–hydroperoxo complexes are capable of this reactivity. Density functional theory calculations indicate that this lack of reactivity is due to the high free energy cost of O–O bond homolysis that would be required to produce the hypothetical Co(IV)–oxo product.