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

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.