Reaction of (μ-Oxo)diiron(III) Core with CO₂ in N-Methylimidazole:  Formation of Mono(μ-carboxylato)(μ-oxo)diiron(III) Complexes with N-Methylimidazole as Ligands

Abstract

Several iron(III) complexes with N-methylimidazole (N-MeIm) as the ligand have been synthesized by using N-MeIm as the solvent. Under anaerobic conditions, [Fe(N-MeIm)(6)](ClO(4))(3) (1) reacts with stoichiometric amounts of water in N-MeIm to afford the (mu-oxo)diiron(III) complex, [Fe(2)(mu-O)(N-MeIm)(10)](ClO(4))(4) (3). Exposure of a solution of 3 in N-MeIm to stoichiometric and excess CO(2) gives rise to the (mu-oxo)(mu-carboxylato)diiron(III) species [Fe(2)(mu-O)(mu-HCO(2))(N-MeIm)(8)](ClO(4))(3) (4) and the methyl carbonate complex [Fe(2)(mu-O)(mu-CH(3)OCO(2))(N-MeIm)(8)](ClO(4))(3) (5), respectively. Formation of the formato-bridged complex 4 upon fixation of CO(2) by 3 in N-MeIm is unprecedentated. Methyl transfer from N-MeIm to a bicarbonato-bridged (mu-oxo)diiron(III) intermediate appears to give rise to 5. Complex 3 is a good starting material for the synthesis of (mu-oxo)mono(mu-carboxylato)diiron(III) species [Fe(2)(mu-O)(mu-RCO(2))(N-MeIm)(8)](ClO(4))(3) (where R = H (4), CH(3) (6), or C(6)H(5) (7)); addition of the respective carboxylate ligand in stoichiometric amount to a solution of 3 in N-MeIm affords these complexes in high yields. Attempts to add a third bridge to complexes 4, 6, and 7 to form the (mu-oxo)bis(mu-carboxylato)diiron(III) species result in the isolation of the previously known triiron(III) mu-eta(3)-oxo clusters [[Fe(mu-RCO(2))(2)(N-MeIm)](3)O](ClO(4)) (8). The structures of 3, 4, 6, and 7 allow one, for the first time, to inspect the various features of the [Fe(2)(mu-O)(mu-RCO(2))](3+) moiety with no strain from the ligand framework.

Synthesis and Characterization of Carboxylate-Rich Complexes Having the {Fe2(μ-OH)2(μ-O2CR)}3+ and {Fe2(μ-O)(μ-O2CR)}3+ Cores of O2-Dependent Diiron Enzymes

“…The two asymmetric iron atoms have a distorted octahedral geometry and are linked by only one carboxylate and one oxo group. The Fe···Fe distance (3.3429(9) Å) of 4 is similar to the 3.3−3.4 Å found in the (μ-oxo)(μ-carboxylato)diiron(III) core of RNR-R2 4 and slightly longer than those of reported {Fe 2 (μ-O)(μ-O 2 CR)} 3+ complexes with N-rich coordination environments. − The remaining coordination sites of 4 are completed by three Hdmpz and three carboxylate ligands, again generating carboxylate-rich metal coordination environments for the two iron atoms. The 4.2 K Mössbauer spectrum of 2 exhibits a quadrupole doublet with δ = 0.51 (2) mm/s, Δ E Q = 1.26 (2) mm/s, and Γ = 0.30 (2) mm/s (Figure S7).…”

Probing the Mechanism for 2,4′-Dihydroxyacetophenone Dioxygenase Using Biomimetic Iron Complexes

“…In general, the bond distances are quite similar to those in the iron­(II) complexes reflecting a geometry that is closer to octahedral. The softer nature of the imidazolyl ligands overall is presumably responsible for the bond distances (Fe–N Avg = 2.140 Å) which are comparable to other hexacoordinate iron­(III) complexes bonded to imidazole nitrogens. − …”

Synthetic Models for Non-Heme Carboxylate-Bridged Diiron Metalloproteins: Strategies and Tactics

“…For example, the iron(III) compound [Fe(MeIm) 6 ] 3+ converts to a (μ-oxo)diiron(III) complex upon reaction with water in MeIm. The resulting [Fe 2 (μ-O)(MeIm) 10 ] 4+ cation, when exposed to CO 2 or following addition of a carboxylate ligand, afforded the methyl carbonate [Fe 2 (μ-O)(μ-O 2 COCH 3 )(MeIm) 6 ] 3+ or the (μ-oxo)mono(μ-carboxylato)diiron(III) [Fe 2 (μ-O)(μ-O 2 CR)(MeIm) 6 ] 3+ (R = H, Me, Ph) species, respectively …”