14925-96-3

An iron(II) complex with a pyridine-containing 14-membered macrocyclic (PyMAC) ligand L1 (L1 = 2,7,12-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1] heptadeca-1(17),13,15-triene), 1, was prepared and characterized. Complex 1 contains low-spin iron(II) in a pseudo-octahedral geometry as determined by X-ray crystallography. Magnetic susceptibility measurements (298 K, Evans method) and Moessbauer spectroscopy (90 K, δ = 0.50(2) mm/s, ΔEQ = 0.78(2) mm/s) confirmed that the low-spin configuration of Fe(II) is retained in liquid and frozen acetonitrile solutions. Cyclic voltammetry revealed a reversible one-electron oxidation/reduction of the iron center in 1, with E1/2(FeIII/FeII) = 0.49 V vs Fc+/Fc, a value very similar to the half-wave potentials of related macrocyclic complexes. Complex 1 catalyzed the epoxidation of cyclooctene and other olefins with H2O2. Low-temperature stopped-flow kinetic studies demonstrated the formation of an iron(IV)-oxo intermediate in the reaction of 1 with H2O2 and concomitant partial ligand oxidation. A soluble iodine(V) oxidant, isopropyl 2-iodoxybenzoate, was found to be an excellent oxygen atom donor for generating Fe(IV)-oxo intermediates for additional spectroscopic (UV-vis in CH3CN: λmax = 705 nm, ε ≈ 240 M-1 cm-1; Moessbauer: δ = 0.03(2) mm/s, ΔEQ = 2.00(2) mm/s) and kinetic studies. The electrophilic character of the (L1)FeIV=O intermediate was established in rapid (k2 = 26.5 M-1 s-1 for oxidation of PPh3 at 0 °C), associative (ΔH a = 53 kJ/mol, ΔS a = -25 J/K mol) oxidation of substituted triarylphosphines (electron-donating substituents increased the reaction rate, with a negative value of Hammet's parameter ρ = -1.05). Similar double-mixing kinetic experiments demonstrated somewhat slower (k2 = 0.17 M-1 s-1 at 0 °C), clean, second-order oxidation of cyclooctene into epoxide with preformed (L1)FeIV=O that could be generated from (L1)FeII and H2O2 or isopropyl 2-iodoxybenzoate. Independently determined rates of ferryl(IV) formation and its subsequent reaction with cyclooctene confirmed that the Fe(IV)-oxo species, (L1)FeIV=O, is a kinetically competent intermediate for cyclooctene epoxidation with H2O2 at room temperature. Partial ligand oxidation of (L1)FeIV=O occurs over time in oxidative media, reducing the oxidizing ability of the ferryl species; the macrocyclic nature of the ligand is retained, resulting in ferryl(IV) complexes with Schiff base PyMACs. NH-groups of the PyMAC ligand assist the oxygen atom transfer from ferryl(IV) intermediates to olefin substrates.

Big cis-ster: The use of an (R,R)-bipyrrolidine backbone with two α-methylpyridine pendant arms affords a tetradentate N4 ligand that coordinates an iron center with cis-α topology (see picture; Fe purple, C gray, N blue, O red, S yellow, F green). This complex catalyzes the reaction between H2O2 and cis-2-heptene to afford a cis-diol product in very high enantioselectivity. (Figure Presented)

Linear tetradentate N2Py2 ligands can coordinate to an octahedral FeII center in three possible topologies (cis-α, cis-β, and trans). While for the N,N'-bis(2-pyridylmethyl)-l,2- diaminoethane (bpmen) complex, only the cis-α topology has been observed, for N,N'-bis(2-pyridylmethyl)-1,2-diamino-cyclohexane (bpmcn) both cis-α and cis-β isomers have been reported. To date, no facile interconversion between cis-α and cis-β topologies has been observed for iron(II) complexes even at high temperatures. However, this work provides evidence for facile interconversion in solution of cis-α, cis-β, and trans topologies for [Fe(bpmpn)X2] (bpmpn = N, N'-bis(2-pyridylmethyl)-1,3- diaminopropane; X = triflate, CH3CN) complexes. As reported previously, the catalytic behavior of cis-α and cis-β isomers of [Fe(bpmcn)(OTf)2] with respect to olefin oxidation depends dramatically on the geometry adopted by the iron complex. To establish a general pattern of the catalysis/ topology dependence, this work presents an extended comparison of the catalytic behavior for oxidation of olefins of a family of [Fe(N2py2)] complexes that present different topologies. 18O labeling experiments provide evidence for a complex mechanistic land-scape in which several pathways should be considered. Complexes with a trans topology catalyze only non-water-assisted epoxidation. In contrast, complexes with a cis-α topology, such as [Fe(bpmen)X2] and [Fe(α-bpmcn)-(OTf)2], can catalyze both epoxidation and cis-dihydroxylation through a water-assisted mechanism. Surprisingly, [Fe(bpmpn)X2] and [Fe(β-bpmcn)-(OTf)2] catalyze epoxidation via a water-assisted pathway and cis-dihydroxylation via a non-water-assisted mechanism, a result that requires two independent and distinct oxidants.

Previous studies have classified a series of nonheme iron catalysts for olefin cis-dihydroxylation by H2O2 into two groups. Complex 1, [(TPA)Fe(OTf)2], representative of Class A catalysts, forms a low-spin FeIII-OOH intermediate that gives rise to a high-valent FeV(=O)OH oxidant. The preference of this catalyst for electron-rich olefins demonstrates its electrophilic character. On the other hand, complex 2, [(6-Me3-TPA)Fe(OTf)2], representative of Class B catalysts, prefers instead to oxidize electron-deficient olefins, suggesting an oxidant with nucleophilic character. It is suggested that such a nucleophilic oxidant may be the high-spin FeIII-OOH intermediate derived from 2 or the FeIV(=O)(?OH) species derived therefrom. Copyright

Efficient and highly stereoselective oxidation of olefins to cis-diols as the major product is obtained by using biomimetic non-heme FeII catalysts in combination with H2O2.

The diiron-substituted silicotungstate γ-SiW10{Fe3+(OH2}2O 386- (schematically shown) is an effective catalyst for the oxygenation of alkenes in homogeneous reaction media with 1 atm of molecular oxygen. For example, a selectivity for cyclooctene oxide of 98% and a turnover number of 10000 were achieved in the epoxidation of cyclooctene. The catalyst is stable under the reaction conditions, and its ability to use molecular oxygen raises the prospect of using it in industrial epoxidation processes.