Effects of a Bridging Dicarboxylate Ligand on the Synthesis and Physical Properties of (μ-Oxo)bis(μ-carboxylato)diiron(III) Complexes

Robert H. Beer, William B. Tolman, Simon G. Bott, Stephen J. Lippard

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The molecule m-phenylenedipropionic acid (H2MPDP), in which two carboxylates are part of the same bridging unit, has been used in the synthesis of diiron(III) oxo protein analogues containing the (μ-oxo)bis(μ-carboxylato)diiron(III) core. The di-carboxylate ligand MPDP2- facilitates the self-assembly synthesis of [Fe2O(MPDP){HB(pz)3}2] and [Fe2O(MPDP)B2Cl2], complexes having terminal bidentate, B, dipyridyl or bis(1-methylimidazol-2-yl)phenylmethoxymethane (BIPhMe, a polyimidazole nitrogen donor), and monodentate chloride ligands. The structural features and physical properties of [Fe2O(MPDP){HB(pz)3}2] and the two [Fe2O(MPDP)B2Cl2] complexes, elucidated in single-crystal X-ray diffraction studies, are very similar to those of the acetate analogue, [Fe2O(O2CCH3)2(HB(pz)3}2], indicating that the coordinated MPDP2- ligand preserves the integrity of the (μ-oxo)bis(μ-carboxylato)diiron(III) core. The multidentate ligand does not impart unusual stability to the [Fe2O(MPDP)]2+ core, however, since decomposition to form [Fe{HB(pz)3}2] occurred during attempts to reduce [Fe2O(MPDP){HB(pz)3)2] electrochemically. Small differences between the spectroscopic properties of [Fe2O(MPDP){HB(pz)3}2] and [Fe2O(MPDP)B2Cl2] complexes can be accounted for by changes in terminal ligation. The coordination of the BIPhMe imidazole nitrogen atoms to the (μ-oxo)bis(μ-carboxylato)diiron(III) center affords a more accurate model of the histidine ligation found in the proteins and more stable complexes than the dipyridyl counterparts. Chloride ion is coordinated cis to the oxo bridge in both [Fe2O-(MPDP)B2Cl2] complexes, the position where exogenous ligands and dioxygen bind in the invertebrate respiratory protein hemerythrin (Hr). The presence of this labile ligand facilitates terminal ligand exchange reactions. Crystal data: [Fe2O-(MPDP){HB(pz)3}2],5CHCl3, triclinic, [formula omited], a = 12.878 (1) Å, b = 20.301 (1) Å, c = 11.906 (1) Å, α = 96.28 (l)°, β = 106.96 (1)°, γ = 74.33 (l)°, Z = 2; [Fe2O(MPDP)(4,4′-Me2bpy)2Cl2]-2CH3CN, triclinic, [formula omited], a = 10.941 (1) Å, b = 11.328 (1) Å, c = 18.781 (2) Å, a = 100.34 (l)°, β = 95.66 (l)°, y = 115.56 (1)°, Z = 2; [Fe2O(MPDP)(BIPhMe)2Cl2]-3CHCl3-CH3CN, triclinic, [formula omited], a = 15.446 (2) Å, b = 16.077 (2) Å, c = 12.453 (2) Å, a = 95.41 (1)°, β = 100.68 (1)°, γ = 93.17 (1)°, Z = 2.

Original languageEnglish (US)
Pages (from-to)2082-2092
Number of pages11
JournalInorganic Chemistry
Issue number9
StatePublished - May 1 1991


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