Carboxylate-Bridged Diiron(II) Complexes: Synthesis, Characterization, and O2-Reactivity of Models for the Reduced Diiron Centers in Methane Monooxygenase and Ribonucleotide Reductase

Abstract

Diiron(II) complexes with an oxygen-rich coordination environment were assembled with the dinucleating dicarboxylate ligands m-xylylenediamine bis(Kemp's triacid)imide (H2XDK) and the more soluble analogue m-xylylenediamine bis(propyl Kemp's triacid)imide (H2PXDK). X-ray crystallographic analysis revealed that, in most of the complexes, only one monodentate N-donor ligand is bound to each iron(II) ion. In addition to XDK and PXDK, a variety of other ligands bridge the dimetallic core including chloride, fluoride, triflate, or carboxylate. The tris(carboxylate-bridged) complexes [Fe2(μ-XDK)(μ-O2CPh)(ImH)2(O2CPh)(MeOH)] (3) and [Fe2(μ-O2CC(CH3)3)(μ-PXDK)(N-MeIm)2(O2CC(CH3)3)] (4) have the same ligand composition as the diiron(II) cores in the hydroxylase component of methane monooxygenase (MMO) and in the R2 protein of ribonucleotide reductase (RNR). The coordination environments of the two iron centers in 3 and 4 are inequivalent, with one iron being 6-coordinate and the other being 4-coordinate. The bridging benzoate and pivalate ligands have an unusual coordination mode with a Fe−O−C bond angle close to 180°. Fits of magnetic susceptibility data collected for several of the complexes between 300 and 4 K indicated that the two iron(II) centers are weakly antiferromagnetically coupled with a coupling constant which does not depend on the nature of the bridging ligands. Mössbauer spectra of polycrystalline and frozen THF solution samples of 4 exhibited two overlapping doublets of equal intensity, reflecting the different coordination environments of the two iron(II) ions. The nearly identical Mössbauer parameters for the solid and frozen solution samples indicate that the dinuclear core remains intact upon dissolution. Stopped-flow studies of the reaction of 3 and 4 with O2 in THF showed the rapid formation (kψ ≈ 74 s-1 for 3 at 202.5 K and kψ ≈ 300 s-1 for 4 at 197 K) of colored intermediates with broad absorption maxima near 660 and 670 nm, respectively. These values are characteristic of peroxo-to-iron charge-transfer bands and similar to that observed for the (μ-peroxo)diiron(III) intermediate (Hperoxo) in the MMO reaction cycle.