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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 Fe IV oxo complex [N4Py·Fe IV (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·Fe III (OH)] 2+ . The employed strategy is promising for the spectroscopic characterization of transient species formed in redox reactions.

Precise control of substrate positioning and orientation (its proximity to the reactive unit) is often invoked to rationalize the superior enzymatic reaction rates and selectivities when compared to synthetic models. Artificial nonheme iron (IV) oxo (Fe(IV)=O) complexes react with C(sp3)−H bonds via a biomimetic Hydrogen Atom Transfer/Hydroxyl Rebound mechanism, but rates, site‐selectivity and even hydroxyl rebound efficiency (ligand rebound versus substrate radical diffusion) are smaller than in oxygenases. Herein, we quantitatively analyze how substrate binding modulates nonheme Fe(IV)=O reactivity by comparing rates and outcomes of C−H oxidation by a pair of Fe(IV)=O complexes that share the same first coordination sphere but only one contains a crown ether receptor that recognizes the substrate. Substrate binding makes the reaction intramolecular, exhibiting Michaelis–Menten kinetics and increased reaction rates. In addition, C−H oxidation occurs with high site selectivity for remote sites. Analysis of Effective Molarity reveals that the system operates at its maximal theoretical capability for the oxidation of these remote sites. Remarkably, substrate positioning also affects Hydroxyl Rebound, whose efficiency only increases on the sites placed in proximity by recognition. Overall, these observations provide evidence that supramolecular control of substrate positioning can effectively modulate the reactivity of oxygenases and its models. [ABSTRACT FROM AUTHOR]

The chemoselectivity between S‐oxidation and hydrogen atom transfer (HAT) from C−H bonds has been investigated in the oxidations of a series of aryl sulfides, alkyl aromatic compounds and benzylic alcohols promoted by the iron(IV)‐oxo complex [(N4Py)FeIV(O)]2+ (N4Py: N,N‐bis(2‐pyridylmethyl)‐N‐bis(2‐pyridyl)‐methylamine) either alone or in the presence of the N‐hydroxyphthalimide (NHPI) mediator via kinetic and product studies. Kinetic analyses indicate a generally higher reactivity of [(N4Py)FeIV(O)]2+ for S‐oxidation process while HAT is favored in the reactions promoted by phthalimide‐N‐oxyl radical (PINO) deriving from NHPI oxidation. Product analysis in intermolecular competitive oxidations confirms the kinetic results with sulfoxides obtained as major products in the oxidation promoted by [(N4Py)FeIV(O)]2+. Conversely, when NHPI is employed as a mediator, significant differences in terms of chemoselectivity are observed, and HAT‐derived products are obtained in higher yields which translate into an inversion of selectivity in the case of the substrates containing activated C−H bonds like diphenylmethane, triphenylmethane and benzylic alcohols. A similar change of chemoselectivity is also observed in the oxidation of aromatic substrates containing both a sulfur atom and α to OH benzylic C−H bonds, with the sulfoxide product more abundant in the absence of NHPI and carbonyl products prevailing with the [(N4Py)FeIV(O)]2+/NHPI system. [ABSTRACT FROM AUTHOR]

Herein we report that an imine‐based iron complex decorated with crown‐ether moieties self‐assembles in CH3CN solution from three building blocks (namely Fe(CH3CN)2(OTf)2, 2‐picolylamine and aldehyde 6a) added in the proper molar ratio (1:2:2, respectively). Aldehyde 6a, endowed with a benzocrown‐ether receptor, can be prepared in two steps from commercial materials with high yields (79 %) and no need of chromatographic separations. The self‐assembled complex is highly stable in CH3CN solution. The complex has been tested as catalyst for the oxidation of aromatic compounds endowed with primary ammonium anchoring group with H2O2. An unexpected yet moderate selectivity for remote meta over ortho hydroxylation was obtained, which is mainly due to the steric hindrance of the catalyst. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. ¹H 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. [ABSTRACT FROM AUTHOR]