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.

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.

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 ).

We have performed a systematic investigation of the influence of the basis set on relative spin-state energies for a number of iron compounds. In principle, with an infinitely large basis set, both Slater-type orbital (STO) and Gaussian-type orbital (GTO) series should converge to the same final answer, which is indeed what we observe for both vertical and relaxed spin-state splittings. However, we see throughout the paper that the STO basis sets give consistent and rapidly converging results, while the convergence with respect to the basis set size is much slower for the GTO basis sets. For example, the large GTO basis sets that give good results for the vertical spin-state splittings of compounds 1-3 (6-311+G**, Ahlrichs VTZ2D2P) fail for the relaxed spin-state splittings of compound 4 (where 1 is Fe-(PyPepS)2 (PyPepSH 2 = N-(2-mercaptophenyl)-2-pyridinecarboxamide), 2 is Fe(tsalen)Cl (tsalen = N, N'-ethylenebis-(thiosalicylideneiminato)), 3 is Fe(N(CH2-o-C6H4S) 3)(1-Me-imidazole), and 4 is FeFHOH). Very demanding GTO basis sets like Dunning's correlation-consistent (cc-pVTZ, cc-pVQZ) basis sets are needed to achieve good results for these relaxed spin states. The use of popular (Pople-type) GTO, effective core potentials basis set (ECPB), or mixed ECPB(Fe):GTO(rest) basis sets is shown to lead to substantial deviations (2-10 kcal/mol, 14-24 kcal/mol for 3-21G), in particular for the high spin states that are typically placed at too low energy. Moreover, the use of an effective core potential in the ECPB basis sets results in spin-state splittings that are systematically different from the STO-GTO results.