Structural, Spectroscopic, and Electrochemical Properties of Nonheme Fe(II)-Hydroquinonate Complexes: Synthetic Models of Hydroquinone Dioxygenases

Authors

Amanda E. Baum, Marquette UniversityFollow
Heaweon Park, Marquette UniversityFollow
Denan Wang, Marquette UniversityFollow
Sergey Lindeman, Marquette UniversityFollow
Adam T. Fiedler, Marquette UniversityFollow

Document Type

Article

Publication Date

10-2012

Source Publication

Dalton Transactions

Source ISSN

1477-9226

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)(HL^X)] (1X), where HL^X 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 (H₂L^F) exhibited a greater tendency to bridge between two Fe(II) centers, resulting in formation of [Fe₂(^Ph2Tp)₂(μ-L^F)(MeCN)]·[2F(MeCN)]. However, addition of one equivalent of “free” pyrazole (^Ph2pz) ligand provided the mononuclear complex, [Fe(^Ph2Tp)(HL^F)(^Ph2pz)]·[1F(^Ph2pz)], which is stabilized by an intramolecular hydrogen bond between the HL^F 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 (1X_oxox) were examined with UV-vis absorption and electron paramagnetic resonance (EPR) spectroscopies.

Comments

Accepted version. Dalton Transactions, Vol. 41, No. 39 (October 2012): 12244-12253. DOI. © 2012 Royal Society of Chemistry. Used with permission.

Recommended Citation

Baum, Amanda E.; Park, Heaweon; Wang, Denan; Lindeman, Sergey; and Fiedler, Adam T., "Structural, Spectroscopic, and Electrochemical Properties of Nonheme Fe(II)-Hydroquinonate Complexes: Synthetic Models of Hydroquinone Dioxygenases" (2012). Chemistry Faculty Research and Publications. 377.
https://epublications.marquette.edu/chem_fac/377