Models of the Reduced Forms of Polyiron-Oxo Proteins: An Asymmetric, Triply Carboxylate Bridged Diiron(II) Complex and Its Reaction with Dioxygen

William B. Tolman, Shuncheng Liu, James G. Bentsen, Stephen J. Lippard

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Abstract

A diiron(II) model compound having features relevant to the active sites of the reduced forms of the polyiron-oxo proteins hemerythrin (Hr), ribonucleotide reductase (RR), and methane monooxygenase (MMO) has been prepared and characterized by elemental analysis, X-ray crystallography, magnetic susceptibility measurements, and Mössbauer, UV-vis, IR, 1H NMR, and EPR spectroscopy. The X-ray crystal structure of the complex, [Fe2(O2CH)4(BIPhMe)2] (1) (BIPhMe = 2,2′-bis(1-methylimidazolyl)phenylmethoxymethane), reveals that the ferrous ions are bridged by one monodentate and two bidentate (syn-syn) bridging formates, a novel arrangement among diiron compounds. Moreover, the molecule has both five- and six-coordinate iron atoms, an asymmetry that belies its simple formula. Thus, one iron atom is octahedral, with two imidazoles from a BIPhMe ligand and a terminal formate completing its coordination sphere, and the other contains only a BIPhMe in addition to the bridging ligands. The latter metal ion has a distorted trigonal-bipyramidal geometry owing to the presence of a weak interaction (Fe•••O = 2.787 (3) Å) with the “dangling” oxygen of the monodentate bridging formate. The inequivalence of the iron atoms in 1 is further manifest by the presence of two overlapping quadrupole doublets in its Mössbauer spectrum (δ1 = 1.26 mm/s; δ2 = 1.25 mm/s; ΔEq1 = 2.56 mm/s; ΔEq2 = 3.30 mm/s). Solid-state magnetic susceptibility measurements acquired between 6 and 300 K indicate the absence of significant magnetic exchange interaction in 1. Consistent with its asymmetric structure and lack of magnetic coupling, large dipolar contributions to the isotropic shifts of 1 are observed in its 1H NMR spectrum. Analysis of the latter was aided by using the diamagnetic shift calibrant [Zn(BIPhMe)Cl2], which was further characterized by elemental analysis, X-ray crystallography, and IR spectroscopy. An unusually shaped integer spin signal at g ~ 16 appears in the EPR spectrum of 1. In a reaction relevant to the functional activity of RR and MMO and reminiscent of oxidative processes previously characterized in heme-iron chemistry, exposure of 1 to O2 afforded [Fe2O(O2CH)4(BIPhMe)2]•H2O (2•H2O), which was characterized by elemental analysis, X-ray crystallography, magnetic susceptibility measurements, and by Mössbauer, UV-vis, IR, 1H NMR, and resonance Raman spectroscopy. The physical properties of 2 are consistent with the presence of its (µ-oxo)bis(µ-carboxylato)diiron(III) core and are similar to those reported for Hr and other complexes with such bridging units. Proof that the source of the oxo bridge in 2 is dioxygen and not adventitious water was obtained by resonance Raman spectroscopic monitoring of its symmetric Fe-O-Fe vibration (vs) after treatment of 1 with l8O2[vs (Fe−l6O-Fe) = 520 cm−1; vs (Fe−18O-Fe) = 502 cm−1]. Both oxygen atoms of O2 are incorporated into product, as indicated by manometric measurements of O2 uptake revealing consumption of 0.6 (1) equiv per mol of 1. EPR spectra of solutions of 1 briefly exposed to O2 and then quickly frozen exhibit a signal with features at g = 1.84 and 1.94 arising from a mixed valent Fe(II)Fe(III) species, a possible intermediate in the conversion of 1 to 2. Acquisition of EPR power saturation data allowed estimation of an antiferromagnetic exchange coupling constant (J) for this species of −31 (2) cm−1, a value similar to those determined for the mixed valent form of MMO. A mechanism for the oxidation of 1 consistent with these observations and reminiscent of reaction schemes proposed for porphyrin-Fe(II) oxidations is suggested.

Original languageEnglish (US)
Pages (from-to)152-164
Number of pages13
JournalJournal of the American Chemical Society
Volume113
Issue number1
DOIs
StatePublished - Jan 1 1991

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