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

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; ΔE_q1 = 2.56 mm/s; ΔE_q2 = 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 (v_s) after treatment of 1 with ^l8O₂[v_s (Fe−^l6O-Fe) = 520 cm⁻¹; v_s (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.