Iron(II) α-Aminopyridine Complexes and Their Catalytic Activity in Oxidation Reactions: A Comparative Study of Activity and Ligand Decomposition

Ferrocenophanium Stability and Catalysis

Ferrocenium catalysis is a vibrant research area, and an increasing number of ferrocenium-catalyzed processes have been reported in the recent years. However, the ferrocenium cation is not very stable in solution, which may potentially hamper catalytic applications. In an effort to stabilize ferrocenium-type architectures by inserting a bridge between the cyclopentadienyl rings, we investigated two ferrocenophanium (or ansa-ferrocenium) cations with respect to their stability and catalytic activity in propargylic substitution reactions. One of the ferrocenophanium complexes was characterized by single crystal X-ray diffraction. Cyclic voltammetry experiments of the ferrocenophane parent compounds were performed in the absence and presence of alcohol nucleophiles, and the stability of the cations in solution was judged based on the reversibility of the electron transfer. The experiments revealed a moderate stabilizing effect of the bridge, albeit the effect is not very pronounced or straightforward. Catalytic propargylic substitution test reactions revealed decreased activity of the ferrocenophanium cations compared to the ferrocenium cation. It appears that the somewhat stabilized ferrocenophanium cations show decreased catalytic activity.

S. Paqui M, Amorim AL, P. de Paula C, Rocha MC, Malavazi I, Cunha A, S. Santana F, Ribeiro RR, Gariani RA, Mendes SR, Xavier FR. Polypyridyl iron(iii) complexes containing long alkyl chains: synthesis, characterization, DFT calculations and biological activity

A bis(picolyl)amine ligand was functionalized with a long alkyl chain, and two iron(iii) complexes were prepared and their biological activity was presented.

Iron-Catalyzed Oxidation of 1-Phenylethanol and Glycerol With Hydrogen Peroxide in Water Medium: Effect of the Nitrogen Ligand on Catalytic Activity and Selectivity

The iron(II) complexes [Fe(bpy)₃](OTf)₂ (bpy = 2,2'-bipyridine; OTf = CF₃SO₃) (1) and [Fe(bpydeg)₃](OTf)₂ (bpydeg = N⁴,N⁴-bis(2-(2-methoxyethoxy)ethyl) [2,2'-bipyridine]-4,4'-dicarboxamide) (2), the latter being a newly synthesized ligand, were employed as catalyst precursors for the oxidation of 1-phenylethanol with hydrogen peroxide in water, using either microwave or conventional heating. With the same oxidant and medium the oxidation of glycerol was also explored in the presence of 1 and 2, as well as of two similar iron(II) complexes bearing tridentate ligands, i.e., [Fe(terpy)₂](OTf)₂ (terpy = 2, 6-di(2-pyridyl)pyridine) (3) and [Fe(bpa)₂](OTf)₂ (bpa = bis(2-pyridinylmethyl)amine) (4): in most reactions the major product formed was formic acid, although with careful tuning of the experimental conditions significant amounts of dihydroxyacetone were obtained. Addition of heterocyclic amino acids (e.g., picolinic acid) increased the reaction yields of most catalytic reactions. The effect of such additives on the evolution of the catalyst precursors was studied by spectroscopic (NMR, UV-visible) and ESI-MS techniques.

Iron-Catalyzed Intramolecular Amination of Aliphatic C-H Bonds of Sulfamate Esters with High Reactivity and Chemoselectivity

It is challenging to develop simple and low cost catalytic systems while maintaining high reactivity and selectivity. An iron-catalyzed intramolecular C-H amination of sulfamate esters using simple and cheap ligands is reported with general substrate scope (31 examples, up to 95% yield). The addition of second ligand, bipyridine, is able to accelerate the reaction and increase the yield. The ready availability of these iron catalysts provides a promising approach to selective introduction of nitrogen into hydrocarbon feedstock.

Spectroscopic investigation and direct comparison of the reactivities of iron pyridyl oxidation catalysts

The growing interest in green chemistry has fueled attention to the development and characterization of effective iron complex oxidation catalysts. A number of iron complexes are known to catalyze the oxidation of organic substrates utilizing peroxides as the oxidant. Their development is complicated by a lack of direct comparison of the reactivities of the iron complexes. To begin to correlate reactivity with structural elements, we compare the reactivities of a series of iron pyridyl complexes toward a single dye substrate, malachite green (MG), for which colorless oxidation products are established. Complexes with tetradentate, nitrogen-based ligands with cis open coordination sites were found to be the most reactive. While some complexes reflect sensitivity to different peroxides, others are similarly reactive with either H₂O₂ or tBuOOH, which suggests some mechanistic distinctions. [Fe(S,S-PDP)(CH₃CN)₂](SbF₆)₂ and [Fe(OTf)₂(tpa)] transition under the oxidative reaction conditions to a single intermediate at a rate that exceeds dye degradation (PDP=bis(pyridin-2-ylmethyl) bipyrrolidine; tpa=tris(2-pyridylmethyl)amine). For the less reactive [Fe(OTf)₂(dpa)] (dpa=dipicolylamine), this reaction occurs on a timescale similar to that of MG oxidation. Thus, the spectroscopic method presented herein provides information about the efficiency and mechanism of iron catalyzed oxidation reactions as well as about potential oxidative catalyst decomposition and chemical changes of the catalyst before or during the oxidation reaction.

Controlling the oxidation of bis-tridentate cobalt(ii) complexes having bis(2-pyridylalkyl)amines: ligand vs. metal oxidation

Two bis-tridentate chelated cobalt(ii) complexes, which differ in the ligand structure by a methylene group, activate molecular oxygen (O₂), and give different oxidation products.

C-H Bond Oxidation Catalyzed by an Imine-Based Iron Complex: A Mechanistic Insight

A family of imine-based nonheme iron(II) complexes (LX)2Fe(OTf)2 has been prepared, characterized, and employed as C-H oxidation catalysts. Ligands LX (X = 1, 2, 3, and 4) stand for tridentate imine ligands resulting from spontaneous condensation of 2-pycolyl-amine and 4-substituted-2-picolyl aldehydes. Fast and quantitative formation of the complex occurs just upon mixing aldehyde, amine, and Fe(OTf)2 in a 2:2:1 ratio in acetonitrile solution. The solid-state structures of (L1)2Fe(OTf)(ClO4) and (L3)2Fe(OTf)2 are reported, showing a low-spin octahedral iron center, with the ligands arranged in a meridional fashion. (1)H NMR analyses indicate that the solid-state structure and spin state is retained in solution. These analyses also show the presence of an amine-imine tautomeric equilibrium. (LX)2Fe(OTf)2 efficiently catalyze the oxidation of alkyl C-H bonds employing H2O2 as a terminal oxidant. Manipulation of the electronic properties of the imine ligand has only a minor impact on efficiency and selectivity of the oxidative process. A mechanistic study is presented, providing evidence that C-H oxidations are metal-based. Reactions occur with stereoretention at the hydroxylated carbon and selectively at tertiary over secondary C-H bonds. Isotopic labeling analyses show that H2O2 is the dominant origin of the oxygen atoms inserted in the oxygenated product. Experimental evidence is provided that reactions involve initial oxidation of the complexes to the ferric state, and it is proposed that a ligand arm dissociates to enable hydrogen peroxide binding and activation. Selectivity patterns and isotopic labeling studies strongly suggest that activation of hydrogen peroxide occurs by heterolytic O-O cleavage, without the assistance of a cis-binding water or alkyl carboxylic acid. The sum of these observations provides sound evidence that controlled activation of H2O2 at (LX)2Fe(OTf)2 differs from that occurring in biomimetic iron catalysts described to date.

Highly efficient oxidation of alcohols catalyzed by a porphyrin-inspired manganese complex

A novel strategy for catalytic oxidation of a variety of benzylic, allylic, propargylic, and aliphatic alcohols to the corresponding aldehydes or ketones has been successfully developed.

Catalysis and molecular magnetism of dinuclear iron(III) complexes with N-(2-pyridylmethyl)-iminodiethanol/-ate

The reaction of N-(2-pyridylmethyl)iminodiethanol (H2pmide) and Fe(NO3)3·9H2O in MeOH led to the formation of a dimeric iron(III) complex, [(Hpmide)Fe(NO3)]2(NO3)2·2CH3OH (1). Its anion-exchanged form, [(pmide)Fe(N3)]2 (2), was prepared by the reaction of 1and NaN3 in MeOH, during which the Hpmide ligand of 1 was also deprotonated. These compounds were investigated by single crystal X-ray diffraction and magnetochemistry. In complex 1, one iron(III) ion was bonded with a mono-deprotonated Hpmide ligand and a nitrate ion. The two iron(III) ions within the dinuclear unit were connected by two ethoxy groups with an inversion center. In 2, one iron(III) ion was coordinated with a deprotonated pmide ligand and an azide ion. The Fe(pmide)(N3) unit was related by symmetry through an inversion center. Both 1 and 2 efficiently catalyzed the oxidation of a variety of alcohols under mild conditions. The oxidation mechanism was proposed to involve an Fe(IV)=O intermediate as the major reactive species and an Fe(V)=O intermediate as a minor oxidant. Evidence for this proposal was derived from reactivity and Hammett studies, KIE (kH/kD) values, and the use of MPPH (2-methyl-1-phenylprop-2-yl hydroperoxide) as a mechanistic probe. Both compounds had significant antiferromagnetic interactions between the iron(III) ions via the oxygen atoms. 1 showed a strong antiferromagnetic interaction within the Fe(III) dimer, while 2 had a weak antiferromagnetic coupling within the Fe(III) dimer.

Hydrocarbon oxidation catalyzed by a cheap nonheme imine-based iron(ii) complex

An imine-based iron complex, prepared in situ from iron(ii), pyridine-2-carbaldehyde and 2-aminomethylpyridine, effectively catalyzes hydrocarbon oxidation at low loadings.

Chemoselective, iron(II)-catalyzed oxidation of a variety of secondary alcohols over primary alcohols utilizing H2O2 as the oxidant

A mild, iron-based catalyst system is presented that selectively oxidizes secondary alcohols to the corresponding hydroxy ketones in the presence of primary alcohols within 15 minutes at room temperature, utilizing H2O2 as the oxidant.