Oxo-transfer chemistry mediated by iron underpins many biological processes and today is emerging as synthetically very important for the catalytic oxidation of C-H and C=C moieties that are hard to activate conventionally. Despite the vast amount of research in this area, experimental characterization of the reactive species under catalytic conditions is very limited, although a Fe(V)=O moiety was postulated. Here we show, using variable-temperature mass spectrometry, the generation of a Fe(V)=O species within a synthetic non-haem complex at -40 °C and its reaction with an olefin. Also, with isotopic labelling we were able both to follow oxygen-atom transfer from H(2)O(2)/H(2)O through Fe(V)=O to the products and to probe the reactivity as a function of temperature. This study pioneers the implementation of variable-temperature mass spectrometry to investigate reactive intermediates.

An analytical set of field-induced coordinates ͑FICs͒ is defined. It is shown that, instead of 3N Ϫ6 normal coordinates, a relatively small number of FICs is sufficient to describe the vibrational polarizability and hyperpolarizabilities due to nuclear relaxation. The fact that the number of FICs does not depend upon the size of the molecule leads to computational advantages. A method is provided for separating anharmonic contributions from harmonic contributions as well as effective mechanical from electrical anharmonicity. Hartree-Fock calculations on a dozen representative conjugated molecules illustrate the procedures and indicate that anharmonicity can be very important. Other potential applications including the determination of zero-point vibrational averaging corrections are noted.

A nonheme iron catalyst catalyzed stereoselective oxidation of alkanes with H2O2 with remarkable efficiency and exhibiting an unprecedented high incorporation of water into the oxidized products. The present results challenge the canonical description of oxygenases, the standard oxo-hydroxo tautomerism that applies to heme systems and serves as a precedent for alternative pathways for the oxidation of hydrocarbons at nonheme iron oxygenases.

Electrides are ionic compounds thus far appearing in the solid state, where the anionic part is constituted by isolated electrons. We herein provide an unambiguous computational means to distinguish electrides from similar species, proving the existence of some electrides in the gas phase. We also put forward a recipe to design new electrides.

A detailed mechanistic study of the hydroxylation of alkane C-H bonds using H2O2 by a family of mononuclear non heme iron catalysts with the formula [Fe(II)(CF3SO3)2(L)] is described, in which L is a tetradentate ligand containing a triazacyclononane tripod and a pyridine ring bearing different substituents at the α and γ positions, which tune the electronic or steric properties of the corresponding iron complexes. Two inequivalent cis-labile exchangeable sites, occupied by triflate ions, complete the octahedral iron coordination sphere. The C-H hydroxylation mediated by this family of complexes takes place with retention of configuration. Oxygen atoms from water are incorporated into hydroxylated products and the extent of this incorporation depends in a systematic manner on the nature of the catalyst, and the substrate. Mechanistic probes and isotopic analyses, in combination with detailed density functional theory (DFT) calculations, provide strong evidence that C-H hydroxylation is performed by highly electrophilic [Fe(V)(O)(OH)L] species through a concerted asynchronous mechanism, involving homolytic breakage of the C-H bond, followed by rebound of the hydroxyl ligand. The [Fe(V)(O)(OH)L] species can exist in two tautomeric forms, differing in the position of oxo and hydroxide ligands. Isotopic-labeling analysis shows that the relative reactivities of the two tautomeric forms are sensitively affected by the α substituent of the pyridine, and this reactivity behavior is rationalized by computational methods.

The spectroscopic and chemical characterization of a new synthetic non-heme iron(IV)-oxo species [Fe IV (O)( Me,H Pytacn)(S)] 2+ (2, Me,H Pytacn = 1-(2′-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane, S = CH 3 CN or H 2 O) is described. 2 has been prepared by reaction of [Fe II (CF 3 SO 3 ) 2 ( Me,H Pytacn)] (1) with peracetic acid. Complex 2 bears a tetradentate N 4 ligand that leaves two cis-sites available for binding an oxo group and a second external ligand but, unlike related iron(IV)-oxo of tetradentate ligands, it is remarkably stable at room temperature (t 1/2 > 2h at 288 K). Its ability to exchange the oxygen atom of the oxo ligand with water has been analyzed in detail by means of kinetic studies, and a mechanism has been proposed on the basis of DFT calculations. Hydrogen-atom abstraction from C-H bonds and oxygen atom transfer to sulfides by 2 have also been studied. Despite its thermal stability, 2 proves to be a very powerful oxidant that is capable of breaking the strong C-H bond of cyclohexane (BDE = 99.3 kcal·mol −1 ).

Getting in tune: Systematic tuning of the electronic properties of modular non-heme iron coordination complexes can be used to extract important information on the reaction mechanism and intermediates, which, in turn, help to explain the activity of these systems as water oxidation catalysts.

The encapsulation of metal clusters in endohedral metallofullerenes (EMFs) takes place in cages that in most cases are far from being the most stable isomer in the corresponding hollow fullerenes. There exist several possible explanations for the choice of the hosting cages in EMFs, although the final reasons are actually not totally well understood. Moreover, the reactivity and regioselectivity of (endohedral metallo)fullerenes have in the past decade been shown to be generally dependent on a number of factors, such as the size of the fullerene cage, the type of cluster that is being encapsulated, and the number of electrons that are transferred formally from the cluster to the fullerene cage. Different rationalizations of the observed trends had been proposed, based on bond lengths, pyramidalization angles, shape and energies of (un)occupied orbitals, deformation energies of the cages, or separation distances between the pentagon rings. Recently, in our group we proposed that the quest for the maximum aromaticity (maximum aromaticity criterion) determines the most suitable hosting carbon cage for a given metallic cluster (i.e. EMF stabilization), including those cases where the IPR rule is not fulfilled. Moreover, we suggested that local aromaticity plays a determining role in the reactivity of EMFs, which can be used as a criterion for understanding and predicting the regioselectivity of different reactions such as Diels-Alder cycloadditions or Bingel-Hirsch reactions. This review highlights different aspects of the aromaticity of fullerenes and EMFs, starting from how this can be measured and ending by how it can be used to rationalize and predict their molecular structure and reactivity.