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Issue 39, 2012
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Structural, spectroscopic, and electrochemical properties of nonheme Fe(II)–hydroquinonate complexes: synthetic models of hydroquinone dioxygenases

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Abstract

Using the tris(3,5-diphenylpyrazol-1-yl)borate (Ph2Tp) supporting ligand, a series of mono- and dinuclear ferrous complexes containing hydroquinonate (HQate) ligands have been prepared and structurally characterized with X-ray crystallography. The monoiron(II) complexes serve as faithful mimics of the substrate-bound form of hydroquinone dioxygenases (HQDOs) – a family of nonheme Fe enzymes that catalyze the oxidative cleavage of 1,4-dihydroxybenzene units. Reflecting the variety of HQDO substrates, the synthetic complexes feature both mono- and bidentate HQate ligands. The bidentate HQates cleanly provide five-coordinate, high-spin Fe(II) complexes with the general formula [Fe(Ph2Tp)(HLX)] (1X), where HLX is a HQate(1-) ligand substituted at the 2-position with a benzimidazolyl (1A), acetyl (1B and 1C), or methoxy (1D) group. In contrast, the monodentate ligand 2,6-dimethylhydroquinone (H2LF) exhibited a greater tendency to bridge between two Fe(II) centers, resulting in formation of [Fe2(Ph2Tp)2(μ-LF)(MeCN)]·[2F(MeCN)]. However, addition of one equivalent of “free” pyrazole (Ph2pz) ligand provided the mononuclear complex, [Fe(Ph2Tp)(HLF)(Ph2pz)]·[1F(Ph2pz)], which is stabilized by an intramolecular hydrogen bond between the HLF and Ph2pz donors. Complex 1F(Ph2pz) represents the first crystallographically-characterized example of a monoiron complex bound to an untethered HQate ligand. The geometric and electronic structures of the Fe/HQate complexes were further probed with spectroscopic (UV-vis absorption, 1H NMR) and electrochemical methods. Cyclic voltammograms of complexes in the 1X series revealed an Fe-based oxidation between 0 and −300 mV (vs. Fc+/0), in addition to irreversible oxidation(s) of the HQate ligand at higher potentials. The one-electron oxidized species (1Xoxox) were examined with UV-vis absorption and electron paramagnetic resonance (EPR) spectroscopies.

Graphical abstract: Structural, spectroscopic, and electrochemical properties of nonheme Fe(ii)–hydroquinonate complexes: synthetic models of hydroquinone dioxygenases

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Publication details

The article was received on 09 Jul 2012, accepted on 07 Aug 2012 and first published on 09 Aug 2012


Article type: Paper
DOI: 10.1039/C2DT31504A
Citation: Dalton Trans., 2012,41, 12244-12253
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    Structural, spectroscopic, and electrochemical properties of nonheme Fe(II)–hydroquinonate complexes: synthetic models of hydroquinone dioxygenases

    A. E. Baum, H. Park, D. Wang, S. V. Lindeman and A. T. Fiedler, Dalton Trans., 2012, 41, 12244
    DOI: 10.1039/C2DT31504A

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