We report the synthesis, characterization, and solution chemistry of a series of new Fe(II) complexes based on the tetradentate ligand N-methyl-N,N'-bis(2-pyridyl-methyl)-1,2-diaminoethane or the pentadentate ones N,N',N'-tris(2-pyridyl-methyl)-1,2-diaminoethane and N,N',N'-tris(2-pyridyl-methyl)-1,3-diaminopropane, modified by propynyl or methoxyphenyltriazolyl groups on the amino functions. Six of these complexes are characterized by X-ray crystallography. In particular, two of them exhibit an hexadentate coordination environment around Fe(II) with two amino, three pyridyl, and one triazolyl groups. UV-visible and cyclic voltammetry experiments of acetonitrile solutions of the complexes allow to deduce accurately the structure of all Fe(II) species in equilibrium. The stability of the complexes could be ranked as follows: [L(5)Fe(II)-py](2+) > [L(5)Fe(II)-Cl](+) > [L(5)Fe(II)-triazolyl](2+) > [L(5)Fe(II)-(NCMe)](2+), where L(5) designates a pentadentate coordination sphere composed of the two amines of ethanediamine and three pyridines. For complexes based on propanediamine, the hierarchy determined is [L(5)Fe(II)-Cl](+) > [L(5)Fe(II)(OTf)](+) > [L(5)Fe(II)-(NCMe)](2+), and no ligand exchange could be evidenced for [L(5)Fe(II)-triazolyl](2+). Reactivity of the [L(5)Fe(II)-triazolyl](2+) complexes with hydrogen peroxide and PhIO is similar to the one of the parent complexes that lack this peculiar group, that is, generation of Fe(III)(OOH) and Fe(IV)(O), respectively. Accordingly, the ability of these complexes at catalyzing the oxidation of small organic molecules by these oxidants follows the tendencies of their previously reported counterparts. Noteworthy is the remarkable cyclooctene epoxidation activity by these complexes in the presence of PhIO.

An original mechanistic study of the reaction of [(L)FeII]2+ (L = TPEN) with dioxygen has been carried out by cyclic voltammetry. Electrochemical data of intermediates [(L)FeIV(O)]2+, [(L)FeIII(OOH)]2+ and [(L)FeIII(OO)]+ are reported. Reaction mechanism between this FeII complex and O2 under reductive conditions is determined.

A novel DNA‐based hybrid catalyst comprised of salmon testes DNA and an iron(III) complex of a cationic meso‐tetrakis(N‐alkylpyridyl)porphyrin was developed. When the N‐methyl substituents were placed at the ortho position with respect to the porphyrin ring, high reactivity in catalytic carbene‐transfer reactions was observed under mild conditions, as demonstrated in the catalytic enantioselective cyclopropanation of styrene derivatives with ethyl diazoacetate (EDA) as the carbene precursor. A remarkable feature of this catalytic system is the large DNA‐induced rate acceleration observed in this reaction and the related dimerization of EDA. It is proposed that high effective molarity of all components of the reaction in or near the DNA is one of the key contributors to this unique reactivity. This study demonstrates that the concept of DNA‐based asymmetric catalysis can be expanded into the realm of organometallic chemistry.

The piano-stool configuration combined with N-heterocyclic carbene (NHC) ligation constitutes an attractive scaffold for employing iron in catalysis. Here, we have expanded this scaffold by installing a pentamethyl cyclopentadienyl (Cp*) ligand as a strong electron donor compared to the traditionally used unsubstituted cyclopentadiene (Cp). Moreover, decarboxylation is introduced as a method to prepare these iron(II) NHC complexes, which avoids the isolation of air-sensitive free carbenes. In addition to the Cp/ Cp* variation, the complexes have been systematically modulated at the NHC scaffold, the NHC wingtip groups, and the ancillary ligands in order to identify critical factors that govern the catalytic activity of the iron center in the hydrosilylation of aldehydes. These modulations reveal the importance of steric tailoring and optimization of electron density for high catalytic performance. The data demonstrate a critical role of the NHC scaffold with triazolylidenes imparting consistently higher activity than imidazolylidenes and a correlation between catalytic activity and steric rather than electronic factors. Moreover, the implementation of steric bulk is strongly dependent on the nature of the NHC and severely limited by the Cp* iron precursor. The best performing catalytic systems reach turnover frequencies, TOF max 's, of up to 360 h −1 at 60 °C. Mechanistic investigations by 1 H NMR and in situ IR spectroscopies indicate a catalyst activation that involves CO release and aldehyde coordination to the [Fe(Cp)(NHC)I] fragment.

The interaction of a number of first-row transition-metal ions with a 2,2′-bipyridyl alanine (bpyA) unit incorporated into the lactococcal multidrug resistance regulator (LmrR) scaffold is reported. The composition of the active site is shown to influence binding affinities. In the case of Fe(II), we demonstrate the need of additional ligating residues, in particular those containing carboxylate groups, in the vicinity of the binding site. Moreover, stabilization of di-tert-butylsemiquinone radical (DTB-SQ) in water was achieved by binding to the designed metalloproteins, which resulted in the radical being shielded from the aqueous environment. This allowed the first characterization of the radical semiquinone in water by resonance Raman spectroscopy.

Metal coordination complexes can display interesting biological activity, as illustrated by the bleomycins (BLMs), a family of natural antibiotics that when coordinated to a redox-active metal ion, show antitumor activity. Yet, which metal ion is required for the activity in cells is still subject to debate. In this study, we described how different metal ions affect the intracellular behavior and activity of the synthetic BLM-mimic N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine (N4Py). Our study shows that a mixture of iron(II), copper(II), and zinc(II) complexes can be generated when N4Py is added to cell cultures but that the metal ion can also be exchanged by other metal ions present in cells. Moreover, the combination of chemical data, together with the performed biological experiments, shows that the active complex causing oxidative damage to cells is the FeII-N4Py complex and not per se the metal complex that was initially added to the cell culture medium. Finally, it is proposed that the high activity observed upon the addition of the free N4Py ligand is the result of a combination of scavenging of biologically relevant metals and oxidative damage caused by the iron(II) complex.

The reactivity and selectivity of non-hemeF e II complexes as oxidationc atalysts can be substantially modified by alteration of the ligand backbone or introductiono f various substituents. In comparison with the hexadentate ligand2 )h as am ethyl group on two of the four picolyl positions. Fe II complexation by 2Me L 6 2 yields two diastereomericc omplexes with very similars tructures, which only differ in the axial/equatorial positions occupied by the methylated pyridylg roups.I ns olution, these two isomers exhibit different magnetic behaviors. Whereas one isomer exhibits temperature-dependent spin-state conversion betweent he S = 0a nd S = 2s tates, the other is more reluctant towards this spin-statee quilibrium and is essentially diamagnetic at room temperature. Their catalytic properties for the oxidation of anisole by H 2 O 2 are very different and correlate with their magnetic properties,w hich reflect their lability/inertness. These different properties most likely depend on the different steric constraintso ft he methylated pyridyl groups in the two complexes.

Pincer-type tridentate pyridyl bis(pyridylidene amide) (pyPYA2) ligand systems were coordinated to the Earth-abundant first row transition metals nickel, cobalt and zinc. A one-pot synthesis in water/ethanol afforded octahedral homoleptic bis-PYA...