Redox and acid–base properties of asymmetric non-heme (hydr)oxo-bridged diiron complexes

Abstract

The diiron unit is commonly found as the active site in enzymes that catalyze important biological transformations. Two μ-(hydr)oxo-diiron(iii) complexes with the ligands 2,2'-(2-methyl-2-(pyridine-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)diphenol (H2L) and 2,2'-(2-methyl-2(pyridine-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)bis(4-nitrophenol) (H2L(NO2)), namely [(FeL)2(μ-O)] () and [(FeL(NO2))2(μ-OH)]ClO4 () were synthesized and characterized. In the solid state, both structures are asymmetric, with unsupported (hydr)oxo bridges. Intramolecular hydrogen bonding of the ligand NH groups to the phenolate O atoms hold the diiron cores in a bent configuration (Fe-O-Fe angle of 143.7° for and 140.1° for ). A new phenolate bridged diferrous complex, [(FeL)2] (), was synthesized and characterized. Upon exposure to air the diferrous complex is oxidized to the diferric . Cyclic voltammetry at different scan rates and chemical reduction of [(FeL)2(μ-OH)]BPh4 () with cobaltocene revealed disproportionation followed by proton transfer, and a mixed-valence species could not be trapped. Subsequent exposure to molecular oxygen results in the formation of . Electrochemical studies of indicate easier reduction of the diiron(iii/iii) to the mixed-valence state than for . The protonation of by benzoic acid to form [(FeL)2(μ-OH)](+) only changes the Fe-O-Fe angle by 5° (from 143.7° to 138.6°), and the pKa of the hydroxo bridge is estimated to be about 20.4. We attribute this high pKa partly to stabilization of the benzoate by hydrogen bonding to the ligand's amine proton. Magnetic susceptibility studies on solid samples of and yielded values of the antiferromagnetic exchange coupling constants, J, for these S = 5/2 dimers of -13.1 cm(-1) and -87.5 cm(-1), respectively, typical of such unsupported hydroxo- and oxo-bridges.

Diiron oxo reactivity in a weak-field environment

“…In Solomon's studies, enhanced basicity is observed with a more acute Fe–O–Fe angle due to a reduction in the Fe–O orbital overlap, thereby resulting in pronounced electron density on the bridging oxide ligand when examining an isovalent series 3,42. Further comparison with other diiron μ-oxo complexes corroborates this relationship, as a diiron( iii ) μ-oxo synthesized by Houser and coworkers10 (∠Fe–O–Fe 143.71(10)°, p K a 21.3(1)) is several orders of magnitude more basic than 6 (∠Fe–O–Fe 167.1(3)°, p K a –1.8(6)) (Table 1); whereas an example by Grapperhaus and coworkers43 with similar core metrics to 6 (∠Fe–O–Fe 168.47(13)°, p K a 6.1(3)) is less basic than the Houser example. The Grapperhaus example is more basic than 6 , which can be attributed to differences in coordination geometry; the tetrahedral iron sites in 6 are influenced more by the enhanced covalency to the oxo moiety (due to the increased iron electrophilicity) than the octahedrally coordinated iron sites in the Grapperhaus example 43…”

“…Synthetic model complexes featuring a μ-oxo or μ-hydroxo bridge have been prepared in order to create functional mimics of these enzymes 7–9. In particular, the interconversion between hydroxo- and oxo-bridged states has been of interest as it is proposed to play an important role during the catalytic cycle of metalloenzymes 10,11. While there are many synthetic examples of diferric Fe–O–Fe units,12–18 few mixed-valent Fe II –O–Fe III complexes and Fe II –O–Fe II complexes have been reported 19–22.…”

X-ray absorption spectroscopic characterization of the diferric-peroxo intermediate of human deoxyhypusine hydroxylase in the presence of its substrate eIF5a

“…We propose that the 2.11-Å shell consists of the three protein-derived ligands as well as a terminal solvent ligand, while the 1.95-Å shell can be reasonably assigned to hydroxo ligands. One of the latter scatterers very likely corresponds to the μ -OH bridge found in hDOHH- R , as analogous bridges in synthetic diferric complexes have Fe III -( μ -OH) distances between 1.94 Å and 2.02 Å [55, 60, 84–87]. The other scatterer could, in principle, be assigned to a second hydroxo bridge, but such an Fe III 2 ( μ -OH) 2 core should give rise to an Fe•••Fe distance much shorter than the 3.41-Å separation found for hDOHH- D and hDOHH- D•S .…”

“…The 2.15-Å shell likely consists of two histidine ligands and a terminal water ligand based on bond metrics discussed earlier. The 1.98-Å shell would comprise a terminal carboxylate ligand, the hydroxo bridge (1.94 – 2.02 Å) [55, 60, 85–87, 97], and the proximal oxygen of a μ -1,2-peroxo ligand (1.86 – 1.94 Å) [52, 70–72]. These results compare well to the parameters found by Suzuki and co-workers in the crystal structure of [Fe III 2 ( μ -OH)( μ -1,2-O 2 )(L) 2 ] + (L = bis(6-methylpyridyl-2-methyl)-3-aminopropionate), which has an Fe•••Fe distance of 3.396 Å and average Fe–O and Fe–N distances of 1.95 Å and 2.21 Å, respectively [85].…”

The synthesis, characterisation and application of iron(iii)–acetate complexes for cyclic carbonate formation and the polymerisation of lactide

“…This method has been previously been used to make a µ-oxo-bridged Fe(III)-salen complex by Webster and co-workers, 60 along with other examples of preparation and application of this complex in the literature. [61][62][63][64][65][66][67][68][69][70][71] However, with the range of salalen, salan and salen ligands (X) [72][73][74][75][76] used in this study, the Fe(X)OAc complex was consistently isolated (Scheme 1). All complexes were recrystallised or washed with cold ethanol and characterised by High-Resolution Mass Spectrometry (HR-MS), elemental analysis and Infra-Red spectroscopy (FT-IR).…”