Synthesis and Characterization of Fe(II) β-Diketonato Complexes with Relevance to Acetylacetone Dioxygenase: Insights into the Electronic Properties of the 3-Histidine Facial Triad
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American Chemical Society
A series of high-spin iron(II) β-diketonato complexes have been prepared and characterized with the intent of modeling the substrate-bound form of the enzyme acetylacetone dioxygenase (Dke1). The Dke1 active site features an Fe(II) center coordinated by three histidine residues in a facial geometry—a departure from the standard 2-histidine-1-carboxylate (2H1C) facial triad dominant among nonheme monoiron enzymes. The deprotonated β-diketone substrate binds to the Fe center in a bidentate fashion. To better understand the implications of subtle changes in coordination environment for the electronic structures of nonheme Fe active sites, synthetic models were prepared with three different supporting ligands (LN3): the anionic Me2Tp and Ph2Tp ligands (R2Tp = hydrotris(pyrazol-1-yl)borate substituted with R-groups at the 3- and 5-pyrazole positions) and the neutral PhTIP ligand (PhTIP = tris(2-phenylimidazol-4-yl)phosphine). The resulting [(LN3)Fe(acacX)]0/+ complexes (acacX = substituted β-diketonates) were analyzed with a combination of experimental and computational methods, namely, X-ray crystallography, cyclic voltammetry, spectroscopic techniques (UV–vis absorption and 1H NMR), and density functional theory (DFT). X-ray diffraction results for complexes with the Me2Tp ligand revealed six-coordinate Fe(II) centers with a bound MeCN molecule, while structures of the Ph2Tp and PhTIP complexes generally exhibited five-coordinate geometries. Each [(LN3)Fe(acacX)]0/+ complex displays two broad absorption features in the visible region that arise from Fe(II)→acacX charge transfer and acacX-based transitions, consistent with UV–vis data reported for Dke1. These absorption bands, along with the Fe redox potentials, are highly sensitive to the identity of LN3 and substitution of the β-diketonates. By interpreting the experimental results in conjunction with DFT calculations, detailed electronic-structure descriptions of the complexes have been obtained, with implications for our understanding of the Dke1 active site.