Catalyst stability determines the catalytic activity of non-heme iron catalysts in the oxidation of alkanes

A series of iron(II) bis(triflate) complexes [Fe(L)(OTf)2] containing linear tetradentate bis(pyridylmethyl)diamine ligands with a range of ligand backbones has been prepared. The backbone of the ligand series has been varied from a two-carbon linkage [ethylene (1), 4,5-dichlorophenylene (2) and cyclohexyl (3)] to a three-carbon [propyl (4)) and a four-carbon linkage (butyl (5)]. The coordination geometries of these complexes have been investigated in the solid state by X-ray crystallography and in solution by 1H and 19F NMR spectroscopy. Due to the labile nature of high-spin iron(II) complexes in solution, dynamic equilibria of complexes with different coordination geometries (cis-α, cis-β and trans) are observed with ligands 2–5. In these cases, the geometry observed in the solid state does not necessarily represent the only or even the major geometry present in solution. The ligand field strength in the various complexes has been investigated by variable temperature magnetic moment measurements and UV-vis spectroscopy. The strongest ligand field is observed with the most rigid ligands 1 and 2, which generate complexes [Fe(L)(OTf)2] with a cis-α coordination geometry and the corresponding complexes [Fe(L)(CH3CN)2]2+ display spin crossover behaviour. The catalytic properties of the complexes for the oxidation of cyclohexane, using hydrogen peroxide as the oxidant, have been investigated. An increased flexibility in the ligand results in a weaker ligand field, which increases the lability of the complexes. The activity and selectivity of the catalysts appear to be related to the strength of the ligand field and the stability of the catalyst in the oxidising environmen

A Systematic Account on Aromatic Hydroxylation by a Cytochrome P450 Model Compound I:A Low-Pressure Mass Spectrometry and Computational Study

Cytochrome P450 enzymes are heme containing mono-oxygenases that mainly react through oxygen atom transfer. Specific features of substrate and oxidant that determine the reaction rate constant for oxygen atom transfer are still poorly understood and, therefore, we did a systematic gas-phase study on reactions by iron(IV)-oxo porphyrin cation radical structures with arenes. We present here the first results obtained by using Fourier transform-ion cyclotron resonance mass spectrometry and provide rate constants and product distributions for the assayed reactions. Product distributions and kinetic isotope effect studies implicate a rate determining aromatic hydroxylation reaction that correlates with the ionization energy of the substrate and no evidence of aliphatic hydroxylation products is observed. To further understand the details of the reaction mechanism, a computational study on a model complex was performed. These studies confirm the experimental hypothesis of dominant aromatic over aliphatic hydroxylation and show that the lack of an axial ligand affects the aliphatic pathways. Moreover, a two parabola valence bond model is used to rationalize the rate constant and identify key properties of the oxidant and substrate that drive the reaction. In particular, the work shows that aromatic hydroxylation rates correlate with the ionization energy of the substrate as well as with the electron affinity of the oxidant

Towards robust alkane oxidation catalysts: electronic variations in non-heme iron(II) complexes and their effect in catalytic alkane oxidation

A series of non-heme iron(II) bis(triflate) complexes containing linear and tripodal tetradentate ligands has been prepared. Electron withdrawing and electron donating substituents in the para position of the pyridine ligands as well as the effect of pyrazine versus pyridine and sulfur or oxygen donors instead of nitrogen donors have been investigated. The electronic effects induced by these substituents influence the strength of the ligand field. UV-vis spectroscopy and magnetic susceptibility studies have been used to quantify these effects and VT 1H and 19F NMR spectroscopy as well as X-ray diffraction have been used to elucidate structural and geometrical aspects of these complexes. The catalytic properties of the iron(II) complexes as catalysts for the oxidation of cyclohexane with hydrogen peroxide have been evaluated. In the strongly oxidising environment required to oxidise alkanes, catalyst stability determines the overall catalytic efficiency of a given catalyst, which can be related to the ligand field strength and the basicity of the ligand and its propensity to undergo oxidation

The Role of Nonheme Transition Metal-Oxo, -Peroxo, and -Superoxo Intermediates in Enzyme Catalysis and Reactions of Bioinspired Complexes

Transition metals are common cofactors in enzymes and enable catalysis to take place via reaction barriers that are accessible at room temperature. Oxygen-activating metalloenzymes are versatile species in Nature involved in vital processes ranging from biodegradation to biosynthesis. Since oxygen-activating intermediates are not readily amenable to experimental study, research has started to focus on biomimetic model systems that have the active site coordination sphere and structural features, but react in solution. In our research group, we have been involved in computational modeling of heme and nonheme iron dioxygenases as well as biomimetic models of these complexes. In this contribution, an overview is given on recent results of the characterization and reactivity patterns of metal-oxo, metal-peroxo, and metal-superoxo complexes. In particular, in recent studies attempts were made to trap and characterize the short-lived oxygen-bound intermediate in the catalytic cycle of cysteine dioxygenase. Many suggested structures could be ruled out by theoretical considerations, yet these also provided suggestions of possible candidates for the experimentally observed spectra. In addition, we review recent studies on the nonheme iron(III)-hydroperoxo species and how its reactivity patterns with arenes are dramatically different from those found for heme iron(III)-hydroperoxo species. In the final two sections there is a description, with illustrations, of a series of computational studies on manganese(V)-oxo and side-on manganese(III)-peroxo moieties that identify a unique spin-state reactivity pattern with a surprising product distribution.</p