Non-heme Imine-based iron complexes as catalysts for oxidative processes

Non-heme iron complexes are emerging as powerful and versatile catalysts in several oxidative transformations. The most investigated iron complex structures are based on aminopyridine ligands, but a number of imine-based ligands have been also tested. In this review a collection of recent results obtained in oxidation catalysis with non-heme imine-based iron complexes is presented. Their catalytic performances in C-H, C=C and -S- oxidation are spread over a wide range of efficiency, going from very low to quite high. Such performances are discussed, whenever possible, in light of the operating reaction mechanisms and of catalyst stability. In order to facilitate the discussion, an initial survey of the most useful mechanistic tools widely applied to distinguish a metal-based oxidation from a radical-chain process is also reported. Imine-based catalysts are divided into two classes: (i) salen-Fe complexes, and (ii) imine-Fe complexes. In some cases clues for free-radical oxidation mechanisms have been reported while in other cases evidence for metal-based mechanisms has been collected. The preferred mechanistic pathway is shown to be a function of catalyst structure and features. Interestingly, some imine-based iron complexes are able to perform stereospecific oxidation reactions, demonstrating that the imine functionality can be incorporated in ligands designed for oxidation catalysis. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Direct structural and mechanistic insights into fast bimolecular chemical reactions in solution through a coupled XAS/UV-Vis multivariate statistical analysis

In this work, we obtain detailed mechanistic and structural information on bimolecular chemical reactions occurring in solution on the second to millisecond time scales through the combination of a statistical, multivariate and theoretical analysis of time-resolved coupled X-ray Absorption Spectroscopy (XAS) and UV-Vis data. We apply this innovative method to investigate the sulfoxidation of p-cyanothioanisole and p-methoxythioanisole by the nonheme FeIV oxo complex [N4Py·FeIV(O)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) in acetonitrile at room temperature. By employing statistical and multivariate techniques we determine the number of key chemical species involved along the reaction paths and derive spectral and concentration profiles for the reaction intermediates. From the quantitative analysis of the XAS spectra we obtain accurate structural information for all reaction intermediates and provide the first structural characterization in solution of complex [N4Py·FeIII(OH)]2+. The employed strategy is promising for the spectroscopic characterization of transient species formed in redox reactions. © The Royal Society of Chemistry

Activation of C-H bonds by a nonheme iron(iv)-oxo complex: mechanistic evidence through a coupled EDXAS/UV-Vis multivariate analysis

The understanding of reactive processes involving organic substrates is crucial to chemical knowledge and requires multidisciplinary efforts for its advancement. Herein, we apply a combined multivariate, statistical and theoretical analysis of coupled time-resolved X-ray absorption (XAS)/UV-Vis data to obtain detailed mechanistic information for on the C-H bond activation of 9,10-dihydroanthracene (DHA) and diphenylmethane (Ph2CH2) by the nonheme FeIV-oxo complex [N4Py·FeIV(O)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) in CH3CN at room temperature. Within this approach, we determine the number of key chemical species present in the reaction mixtures and derive spectral and concentration profiles for the reaction intermediates. From the quantitative analysis of the XAS spectra the transient intermediate species are structurally determined. As a result, it is suggested that, while DHA is oxidized by [N4Py·FeIV(O)]2+ with a hydrogen atom transfer-electron transfer (HAT-ET) mechanism, Ph2CH2 is oxidized by the nonheme iron-oxo complex through a HAT-radical dissociation pathway. In the latter process, we prove that the intermediate FeIII complex [N4Py·FeIII(OH)]2+ is not able to oxidize the diphenylmethyl radical and we provide its structural characterization in solution. The employed combined experimental and theoretical strategy is promising for the spectroscopic characterization of transient intermediates as well as for the mechanistic investigation of redox chemical transformations on the second to millisecond time scales. This journal i

Hydrogen atom transfer (HAT) processes promoted by the quinolinimide-N-oxyl radical: a kinetic and theoretical study

A kinetic study of the hydrogen atom transfer (HAT) reactions from a series of organic compounds to the quinolinimide-N-oxyl radical (QINO) was performed in CH3CN. The HAT rate constants are significantly higher than those observed with the phthalimide- N-oxyl radical (PINO) as a result of enthalpic and polar effects due to the presence of the N-heteroaromatic ring in QINO. The relevance of polar effects is supported by theoretical calculations conducted for the reactions of the two N-oxyl radicals with toluene, which indicate that the HAT process is characterized by a significant degree of charge transfer permitted by the π-stacking that occurs between the toluene and the N-oxyl aromatic rings in the transition state structures. An increase in the HAT reactivity of QINO was observed in the presence of 0.15 M HClO4 and 0.15 M Mg(ClO4)2 due to the protonation or complexation with the Lewis acid of the pyridine nitrogen that leads to a further decrease in the electron density in the N-oxyl radical. These results fully support the use of N-hydroxyquinolinimide as a convenient substitute for N-hydroxyphthalimide in the catalytic aerobic oxidations of aliphatic hydrocarbons characterized by relatively high C–H bond dissociation energies

C-S Bond Cleavage in Aromatic Sulfide Radical Cations

The results of our recent studies of the structural effects on the C-S bond fragmentation process of aromatic sulfur radical cations are reported

Photo- and Radiation Chemical Studies of Lignin Model Compounds

The basic mechanistic aspects of the photo- and radiation chemistry of lignin model compounds (LMCs) are discussed with respect to important processes related to lignin degradation. Several reactions occur after direct irradiation, photosensitized or radiation chemically induced oxidation of LMCs. Direct irradiation studies on LMCs have provided supportive evidence for the involvement of hydrogen abstraction reactions from phenols, beta -cleavage of substituted alpha -aryloxyacetophenones and cleavage of ketyl radicals (formed by photoreduction of aromatic ketones or hydrogen abstraction from arylglycerol beta -aryl ethers) in the photoyellowing of lignin rich pulps. Photosensitized and radiation chemically induced generation of reactive oxygen species and their reaction with LMCs are reviewed. The side-chain reactivity of LMC radical cations, generated by radiation chemical means, is also discussed in relation with the enzymatic degradation of lignin

Spectral Properties and Reactivity of Diarylmethanol Radical Cations in Aqueous Solution. Evidence for Intramolecular Charge Resonance

Spectral properties and reactivities of ring-methoxylated diarylinethane and diarylmethanol radical cations, generated in aqueous solution by pulse and gamma-radiolysis and by the one-electron chemical oxidant potassium 12-tungstocobalt(III)ate, have been studied, The radical cations display three oands in the UV, visible, and vis-NIR regions of the spectrum. The vis-NIR band is assigned to an intramolecular charge resonance interaction (CR) between the neutral donor and charged acceptor rings, as indicated by the observation that the relative intensity of the vis-NIR band compared to that of the UV and visible bands does not increase with increasing substrate concentration and that the position and intensity of this band is influenced by the ring-substitution pattern. In acidic solution (pH 4), monomethoxylated diarylmethanol radical Cations 1a(.+)1e(.+) decay by C(a)-H deprotonation [k (1.7-1.9) x 10(4) s(-1)] through the intermediacy of a ketyl radical, which is further oxidized in the reaction medium to give the corresponding benzophenones, as evidenced by both time-resolved spectroscopic and product studies. With the dimethoxylated radical cation 2(.+) C(a)-H deprotonation is instead significantly slower (k = 6.7 x 102 s 1). In basic solution, 1a(.+)-1e(.+) undergo (-)OH-induced deprotonation from the alpha-OH group with k(OH) approximate to1.4 x 10(10) M (1) S(-1), leading to a ketyl radical anion, which is oxidized in the reaction medium to the corresponding benzophenone

SIDE-CHAIN OXIDATION OF BENZYLTRIMETHYLSILANES BY IODOSYLBENZENE IN THE PRESENCE OF IRON AND MANGANESE PORPHYRINS

Benzyltrimethylsilanes react with iodosylbenzene in the presence of either iron(III) or manganese(III) tetrakis(pentafluorophenyl)porphyrin (TFPPM, with M = Fe, Mn) to give alpha-hydroxybenzyltrimethylsilanes, which are then rapidly converted into the corresponding benzaldehydes in the reaction medium. In these reactions the active species is the metal-oxo complex, TFPPM(V)=O, formed by iodosylbenzene and TFPPM. A relative reactivity study for a series of ring substituted benzyltrimethylsilanes has shown that when M = Fe, the reaction is quite selective (rho = -1.85), with the m-MeO substituent exhibiting a much higher reactivity than expected. When M = Mn, a lower rho value (-1.15) is observed and no anomalous reactivity is found with the m-MeO group. These result suggest that the side-chain hydroxylation of benzyltrimethylsilane by TFPPMn(V)=O occurs by the well known hydrogen atom transfer mechanism. For the corresponding reactions induced by TFPPFe(V)=O a coupled proton/electron transfer mechanism, which might involve the formation of a charge-transfer complex, seems more likely