Stereospecific Oxidation of Alkanes Catalyzed by Fe2(μ-Carboxylato) Complexes,which Model Some of the Structural Features of the Active Center of Methane Monooxygenase

Binuclear -oxalate iron complexes (1 and 2) that model the Fe₂(-COO) nucleus of the methane monooxygenase active center catalyze the stereospecific transfer of an oxygen atom from H₂O₂ to C–H bonds in alkanes. Due to the framework nature of the ligand, the binuclear structure of the iron complex is retained in the solution. These data reliably confirm the fact that the catalytic reaction occurs at a binuclear center via a mechanism with the participation of both iron atoms.     

Synthesis and Catalytic Activity of the Fe(II) and Fe(III) Complexes with a New Polydentate Ligand Containing an Amide Donor

The iron complexes [Fe²⁺(tpcaH)(MeCN)₂](ClO₄)₂ (I) and [Fe₂O(tpca)₂(H₂O)₂](ClO₄)₂ (II) based on the new potentially tetradentate ligand bis(2-pyridyl)methyl-2-pyridylcarboxamide (tpcaH) with the amide donor group inside the molecule were synthesized. The composition of the complexes was confirmed by elemental and mass spectrometric analyses and various spectral methods. Compounds I and II exhibited good catalytic activity in the stereospecific oxidation of alkanes by hydrogen peroxide.     

Tetranuclear iron(III) complexes of an octadentate pyridine-carboxylate ligand and their catalytic activity in alkane oxidation by hydrogen peroxide

Reaction of the octadentate ligand 2,6-bis{3-[N,N-di(2-pyridylmethyl)amino]propoxy}benzoic acid (LH) with Fe(ClO4)3 leads to the formation of the tetranuclear complexes [Fe4(mu-O)2(LH)2(ClCH2-CO2)4](ClO4)4 (1), [{Fe2(mu-O)L(R-CO2)}2](ClO4)4 (2 R = C6H5-, 3 R = CH3-, 4, R = ClCH2-). The crystal structures of complexes 1 and 2 reveal that they consist of two Fe(III)2(mu-O)(mu-RCO2)2 cores that are linked via the two LH/L ligands to give a "dimer of dimers" structure. Complex assumes a helical shape, with protonated carboxylic acid moieties of the two ligands forming a hydrogen-bonded pair at the center of the cation. In complexes 2, 3 and 4, central carboxylates of the two ligands bridge the iron ions in each of the two Fe2O units, with an interdimer iron-iron separation of approximately 10 A and an intradimer separation of approximately 3.1 A. The second carboxylate bridge within the Fe2O units is defined by exogenous benzoate (2), acetate (3) or chloroacetate (4) ligands. The aqua complex [{Fe2(mu-O)L(H2O)2}2](ClO4)6 (5) is proposed to have a similar structure, but with the exogenous bridging carboxylates replaced by two terminal water ligands. These complexes exhibit electronic and M?ssbauer spectral features that are similar to those of (mu-oxo)diiron(III) proteins as well as other related (mu-oxo)bis(mu-carboxylato)diiron(III) complexes. This similarity shows that these properties are not significantly affected by the nature of the bridging exogenous carboxylate, and that the octadentate framework ligand is essential in stabilizing the "dimer of dimers" structure. This structural feature remains in highly diluted solution (10(-5) M) as evidenced by electrospray ionization mass-spectroscopy (ES MS). Cyclic voltammetric studies of complexes 2 and 5 showed two irreversible two-electron reductions, indicating that the two Fe2O units of the tetranuclear complexes behave as distinct redox entities. Complexes 2, 3 and, especially, the aqua complex 5 are active alkane oxidation catalysts. Catalytic reactions carried out with alkane substrate molecules and hydrogen peroxide predominantly gave alcohols. High stereospecificity in the oxidation of cis-1,2-dimethylcyclohexane supports the metal-based molecular mechanism of O-insertion into C-H bonds postulated for non-heme iron enzymes such as methane monooxygenase.     

New phenanthroline iron complexes: Synthesis and catalytic activity in alkane oxidation with hydrogen peroxide

New binuclear iron complexes with labile coordination sites, [Fe₂OL₄(H₂O)₂](ClO₄)₄ where L stands for hydrophobic phenanthroline (phen) bearing electronegative and lipophilic substituents (Cl, Br, C₂H₅, C₆H₅, C₆H₁₃), are synthesized. The stereospecificity, alcohol/ketone ratio, regioselectivity, and substituent effect in the hydroxylation of alkanes (cyclohexane, adamantane, cis-1,2-dimethylcyclohexane) with hydrogen peroxide catalyzed by these complexes are studied. The said parameters, as well as a decrease in the alkane oxidation regioselectivity with increasing hydrogen peroxide concentration, are similar to the respective parameters for mononuclear iron complexes of tetradentate ligands. The results confirm that regardless of the nature of chelating ligands, the same mechanism operates in both cases, namely, the transfer of an oxygen atom with the participation of ferryl intermediates.