μ-Oxo-bridged iron(iii) complexes for the selective reduction of aromatic ketones catalyzed through base promoted in situ nanoparticle formation

Selective TH of aromatic ketones catalyzed by μ-oxo-diiron complexes and in situ NP formation in the presence of a base gave excellent yields (∼99–100%).

μ-Oxo-bridged diiron(iii) complexes of tripodal 4N ligands as catalysts for alkane hydroxylation reaction using m-CPBA as an oxidant: substrate vs. self hydroxylation

A series of non-heme μ-oxo-bridged dinuclear iron(iii) complexes of the type [Fe₂(μ-O)(L1–L6)₂Cl₂]Cl₂1–6 have been isolated and their catalytic activity towards oxidative transformation of alkanes into alcohols has been studied using m-choloroperbenzoic acid (m-CPBA) as an oxidant. All the complexes were characterized by CHN, electrochemical, and UV-visible spectroscopic techniques. The molecular structures of 2 and 5 have been determined successfully by single crystal X-ray diffraction analysis and both possesses octahedral coordination geometry and each iron atom is coordinated by four nitrogen atoms of the 4N ligand and a bridging oxygen. The sixth position of each octahedron is coordinated by a chloride ion. The (μ-oxo)diiron(iii) core is linear in 2 (Fe–O–Fe, 180.0°), whereas it is non-linear (Fe–O–Fe, 161°) in 5. All the diiron(iii) complexes show quasi-reversible one electron transfer in the cyclic voltammagram and catalyze the hydroxylation of alkanes like cyclohexane, adamantane with m-CPBA as an oxidant. In acetonitrile solution, adding excess m-CPBA to the diiron(iii) complex 2 without chloride ions leads to intramolecular hydroxylation reaction of the oxidant. Interestingly, 2 catalyzes alkane hydroxylation in the presence of chloride ions, but intramolecular hydroxylation in the absence of chloride ions. The observed selectivity for cyclohexane (A/K, 5–7) and adamantane (3°/2°, 9–18) suggests the involvement of high-valent iron–oxo species rather than freely diffusing radicals in the catalytic reaction. Moreover, 4 oxidizes (A/K, 7) cyclohexane very efficiently up to 513 TON while 5 oxidizes adamantane with good selectivity (3°/2°, 18) using m-CPBA as an oxidant. The electronic effects of ligand donors dictate the efficiency and selectivity of catalytic hydroxylation of alkanes.

The ligand stereoelectronic effect of diiron(iii) complexes determines the efficiency and selectivity of catalytic alkane hydroxylation with m-CPBA as an oxidant.

Oxidation of Organic Compounds with Peroxides Catalyzed by Polynuclear Metal Compounds

The review describes articles that provide data on the synthesis and study of the properties of catalysts for the oxidation of alkanes, olefins, and alcohols. These catalysts are polynuclear complexes of iron, copper, osmium, nickel, manganese, cobalt, vanadium. Such complexes for example are: [Fe2(HPTB)(m-OH)(NO3)2](NO3)2·CH3OH·2H2O, where HPTB-¼N,N,N0,N0-tetrakis(2-benzimidazolylmethyl)-2-hydroxo-1,3-diaminopropane; complex [(PhSiO1,5)6]2[CuO]4[NaO0.5]4[dppmO2]2, where dppm-1,1-bis(diphenylphosphino)methane; (2,3-η-1,4-diphenylbut-2-en-1,4-dione)undecacarbonyl triangulotriosmium; phenylsilsesquioxane [(PhSiO1.5)10(CoO)5(NaOH)]; bi- and tri-nuclear oxidovanadium(V) complexes [{VO(OEt)(EtOH)}2(L2)] and [{VO(OMe)(H2O)}3(L3)]·2H2O (L2 = bis(2-hydroxybenzylidene)terephthalohydrazide and L3 = tris(2-hydroxybenzylidene)benzene-1,3,5-tricarbohydrazide); [Mn2L2O3][PF6]2 (L = 1,4,7-trimethyl-1,4,7-triazacyclononane). For comparison, articles are introduced describing catalysts for the oxidation of alkanes and alcohols with peroxides, which are simple metal salts or mononuclear metal complexes. In many cases, polynuclear complexes exhibit higher activity compared to mononuclear complexes and exhibit increased regioselectivity, for example, in the oxidation of linear alkanes. The review contains a description of some of the mechanisms of catalytic reactions. Additionally presented are articles comparing the rates of oxidation of solvents and substrates under oxidizing conditions for various catalyst structures, which allows researchers to conclude about the nature of the oxidizing species. This review is focused on recent works, as well as review articles and own original studies of the authors.

Targeted synthesis of a CrIII–O–VV core oxo-bridged complex: spectroscopic, magnetic and electrical properties

The results of vibrational, electronic, structural, thermal, magnetic and impedance spectroscopy studies are presented in the first reported compound with a Cr^III–O–V^V bridge.

Activation of a Non-Heme FeIII -OOH by a Second FeIII to Hydroxylate Strong C-H Bonds: Possible Implications for Soluble Methane Monooxygenase

Non-heme iron oxygenases contain either monoiron or diiron active sites, and the role of the second iron in the latter enzymes is a topic of particular interest, especially for soluble methane monooxygenase (sMMO). Herein we report the activation of a non-heme FeIII -OOH intermediate in a synthetic monoiron system using FeIII (OTf)3 to form a high-valent oxidant capable of effecting cyclohexane and benzene hydroxylation within seconds at -40 °C. Our results show that the second iron acts as a Lewis acid to activate the iron-hydroperoxo intermediate, leading to the formation of a powerful FeV =O oxidant-a possible role for the second iron in sMMO.

Di- and Tetrairon(III) μ-Oxido Complexes of an N3S-Donor Ligand: Catalyst Precursors for Alkene Oxidations

The new di- and tetranuclear Fe(III) μ-oxido complexes [Fe₄(μ-O)₄(PTEBIA)₄](CF₃SO₃)₄(CH₃CN)₂] (1a), [Fe₂(μ-O)Cl₂(PTEBIA)₂](CF₃SO₃)₂ (1b), and [Fe₂(μ-O)(HCOO)₂(PTEBIA)₂](ClO₄)₂ (MeOH) (2) were prepared from the sulfur-containing ligand (2-((2,4-dimethylphenyl)thio)-N,N-bis ((1-methyl-benzimidazol-2-yl)methyl)ethanamine (PTEBIA). The tetrairon complex 1a features four μ-oxido bridges, while in dinuclear 1b, the sulfur moiety of the ligand occupies one of the six coordination sites of each Fe(III) ion with a long Fe-S distance of 2.814(6) Å. In 2, two Fe(III) centers are bridged by one oxido and two formate units, the latter likely formed by methanol oxidation. Complexes 1a and 1b show broad sulfur-to-iron charge transfer bands around 400–430 nm at room temperature, consistent with mononuclear structures featuring Fe-S interactions. In contrast, acetonitrile solutions of 2 display a sulfur-to-iron charge transfer band only at low temperature (228 K) upon addition of H₂O₂/CH₃COOH, with an absorption maximum at 410 nm. Homogeneous oxidative catalytic activity was observed for 1a and 1b using H₂O₂ as oxidant, but with low product selectivity. High valent iron-oxo intermediates could not be detected by UV-vis spectroscopy or ESI mass spectrometry. Rather, evidence suggest preferential ligand oxidation, in line with the relatively low selectivity and catalytic activity observed in the reactions.

Recent Advances in Homogeneous Metal-Catalyzed Aerobic C–H Oxidation of Benzylic Compounds

Csp3–H oxidation of benzylic methylene compounds is an established strategy for the synthesis of aromatic ketones, esters, and amides. The need for more sustainable oxidizers has encouraged researchers to explore the use of molecular oxygen. In particular, homogeneous metal-catalyzed aerobic oxidation of benzylic methylenes has attracted much attention. This account summarizes the development of this oxidative strategy in the last two decades, examining key factors such as reaction yields, substrate:catalyst ratio, substrate scope, selectivity over other oxidation byproducts, and reaction conditions including solvents and temperature. Finally, several mechanistic proposals to explain the observed results will be discussed.

Heterometallic CuIIFeIII and CuIIMnIII alkoxo-bridged complexes revealing a rare hexanuclear M6(μ-X)7(μ3-X)2 molecular core

The novel hexanuclear complexes [Cu4Fe2(OH)(Piv)4(tBuDea)4Cl]·0.5CH3CN (1) and [Cu4Mn2(OH)(Piv)4(tBuDea)4Cl] (2) were prepared through one-pot self-assembly reactions of copper powder and iron(ii) or manganese(ii) chloride with N-tert-butyldiethanolamine (H2tBuDea) and pivalic acid (HPiv) in acetonitrile. Crystallographic studies revealed the uncommon molecular core type M6(μ-X)7(μ3-X)2 in 1 and 2, which can be viewed as a combination of two trimetallic M3(μ-X)2(μ3-X) fragments joined by three bridging atoms. The analysis and classification of the hexanuclear complexes having a M3(μ-X)2(μ3-X) moiety as a core forming fragment using data from the Cambridge Structural Database (CSD) were performed. Variable-temperature (1.8-300 K) magnetic susceptibility measurements of 1 showed a decrease of the effective magnetic moment value at low temperature, indicative of antiferromagnetic coupling between the magnetic centres (JFe-Cu/hc = -6.9 cm-1, JCu-Cu/hc = -4.1 cm-1, JFe-Fe/hc = -24.2 cm-1). Complex 1 acts as a catalyst in the reaction of mild oxidation of cyclohexane with H2O2, showing the yields of products, cyclohexanol and cyclohexanone, up to 17% using pyrazinecarboxylic acid as a promoter. In the oxidation of cis-1,2-dimethylcyclohexane with m-chloroperoxybenzoic acid (m-CPBA), 70% of retention of stereoconfiguration was observed for tertiary alcohols. Compound 1 also catalyses the amidation of cyclohexane with benzamide. In all three catalytic reactions the by-products were investigated in detail and discussed.

Boosting selective oxidation of cyclohexane over a metal–organic framework by hydrophobicity engineering of pore walls

Hydrophobicity engineering for an iron-porphyrinic metal–organic framework has been developed to greatly improve the catalytic performance toward cyclohexane oxidation.

Stereoselective oxidation of alkanes with m-CPBA as an oxidant and cobalt complex with isoindole-based ligands as catalysts

Coordination compound of cobalt catalyses hydroxylation of inert C–H bonds with 98% retention of stereoconfiguration of alkane skeleton.

Solid state π–π stacking and higher order dimensional crystal packing, reactivity, and electrochemical behaviour of salphenazine actinide and transition metal complexes

Here, we describe and compare the synthesis, solution characterization, and solid-state behaviour of the salphenazine ligand and metal complexes.

Molecular iron complexes as catalysts for selective C–H bond oxygenation reactions

This feature article summarises recent developments in homogeneous C–H bond oxygenation catalysed by molecular iron complexes.

Self-enhanced electrogenerated chemiluminescence of ruthenium(ii) complexes conjugated with Schiff bases

Self-enhanced electrochemiluminescence based on ruthenium complexes conjugated with Schiff bases is studied and used for co-reactant free metal anion recognition.

Synthesis and structure of iron(III) complexes of amine-bis(phenolate) ligands

The synthesis and structures of four new iron(III) amine-bis(phenolate) complexes are reported. Reaction of anhydrous FeCl3 with the diprotonated tridentate ligand isopropyl-N,N-bis(2-methylene-4-t-butyl-6-methylphenol) (H2L1) and NEt3 produces the trigonal bipyramidal iron(III) complex [NEt3H]+ [FeCl2L1]– (1). The reaction of FeBr3 with the sodium or lithium salts, Na2L1 and Li2L2, results in the formation of FeBr2L1H (2) and FeBr2L2H (3), tetrahedral iron(III) complexes possessing two bromide ligands and quaternized ammonium fragments. A trigonal bipyramidal FeIII hydroxido-bridged dimer, [Fe(μ-OH)L2]2 (4), was also isolated during the synthesis of 3. Single-crystal X-ray molecular structures have been determined for complexes 1–4 and H2L2.

Monomeric Ti(IV) homopiperazine complexes and their exploitation for the ring opening polymerisation of rac-lactide

BACKGROUND

The area of biodegradable/sustainable polymers is one of increasing importance in the 21st Century due to their positive environmental characteristics. Lewis acidic metal centres are currently one of the most popular choices for the initiator for the polymerisation. Thus, in this paper we report the synthesis and characterisation of a series of monometallic homopiperazine Ti(IV) complexes where we have systematically varied the sterics of the phenol moieties.

RESULTS

When the ortho substituent of the ligand is either a Me, tBu or amyl then the β-cis isomer is isolated exclusively in the solid-state. Nevertheless, in solution multiple isomers are clearly observed from analysis of the NMR spectra. However, when the ortho substituent is an H-atom then the trans-isomer is formed in the solid-state and solely in solution. The complexes have been screened for the polymerisation of rac-lactide in solution and under the industrially preferred melt conditions. Narrow molecular weight material (PDI 1.07 - 1.23) is formed under melt conditions with controlled molecular weights.

CONCLUSIONS

Six new Ti(IV) complexes are presented which are highly active for the polymerisation. In all cases atactic polymer is prepared with predictable molecular weight control. This shows the potential applicability of Ti(IV) to initiate the polymerisations.

Synthesis of ortho-acylphenols through the palladium-catalyzed ketone-directed hydroxylation of arenes

Ketone in charge: a formal ketone-directed palladium-catalyzed ortho-hydroxylation of arenes has been developed as an effective approach to access o-acylphenols from simple arylketones. A Pd-catalyzed oxidative ortho-carbonylation reaction using ketone directing groups to access a ketal-lactone motif is also demonstrated. The ubiquity and versatile nature of ketones make these methods attractive. BTI=PhI(TFA)(2); DCE=1,2-dichloroethane.