Reaction of the unsymmetrical phenol ligand 2-((bis(2-pyridylmethyl)amino)methyl)-6-(((2-pyridylmethyl)benzylamino)methyl)-4-methylphenol (HL-Bn) or its 2,6-dichlorobenzyl analogue (HL-BnCl(2)) with Fe(H(2)O)(6)(ClO(4))(2) in the presence of disodium m-phenylenedipropionate (Na(2)(mpdp)) followed by exposure to atmosphere affords the diiron(II,III) complexes [Fe(2)(L-Bn)(mpdp)(H(2)O)](ClO(4))(2) and [Fe(2)(L-BnCl(2))(mpdp)(CH(3)OH)](ClO(4))(2), respectively. The latter complex has been characterized by X-ray crystallography. It crystallizes in the monoclinic system, space group P2(1)/n, with a = 13.3095(14) A, b = 20.1073(19) A, c = 19.4997(19) A, alpha = 90 degrees, beta = 94.471(2) degrees, gamma = 90 degrees, V = 5202.6(9) A(3), and Z = 4. The structure of the compound is very similar to that of [Fe(2)(L-Bn)(mpdp)(H(2)O)](BPh(4))(2) determined earlier, except for the replacement of a water by a methanol on the ferrous site. Magnetic measurements of [Fe(2)(L-Bn)(mpdp)(H(2)O)](BPh(4))(2) reveal that the two high-spin Fe ions are moderately antiferromagnetically coupled (J = -3.2(2) cm(-)(1)). Upon dissolution in acetonitrile the terminal ligand on the ferrous site is replaced by a solvent molecule. The acetonitrile-water exchange has been investigated by various spectroscopic techniques (UV-visible, NMR, Mössbauer) and electrochemistry. The substitution of acetonitrile by water is clearly evidenced by Mössbauer spectroscopy by a reduction of the quadrupole splitting value from 3.14 to 2.41 mm/s. In addition, it causes a 210 mV downshift of the oxidation potential of the ferrous site and a similar reduction of the stability domain of the mixed-valence state. Exhaustive electrolysis of a solution of [Fe(2)(L-Bn)(mpdp)(H(2)O)](2+) shows that the aqua diferric species is not stable and undergoes a chemical reaction which can be partly reversed by reduction to the mixed-valent state. This and other electrochemical observations suggest that upon oxidation of the diiron center to the diferric state the aqua ligand is deprotonated to a hydroxo. This hypothesis is supported by Mössbauer spectroscopy. Indeed, this species possesses a large quadrupole splitting value (DeltaE(Q) >or= 1.0 mm.s(-)(1)) similar to that of analogous complexes with a terminal phenolate ligand. This study illustrates the drastic effects of aqua ligand exchange and deprotonation on the electronic structure and redox potentials of diiron centers.

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The efficient integration of binding, catalysis, and multiple turnovers remains a challenge in building enzyme models. We report that systematic derivatization of polyethylene imine (PEI) with alkyl (C(2)-C(12)), benzyl, and guanidinium groups gives rise to catalysts ('synzymes') with rate accelerations (k(cat)/k(uncat)) of up to 10(4) for the intramolecular transesterification of 2-hydroxypropyl-p-nitrophenyl phosphate, HPNP, in the absence of metal. The synzymes exhibit saturation kinetics (K(M) approximately 250 microM, k(cat) approximately 0.5 min(-1)) and up to 2340 turnovers per polymer molecule. Catalysis can be specifically and competitively inhibited by anionic and hydrophobic small molecules. The efficacy of catalysis is determined by the PEI derivatization pattern. The derivatization reagents exert a synergistic effect, i.e., their combinations increase catalysis by more than the sum of each single modification. The pH-rate profile for k(cat)/K(M) is bell shaped with a maximum at pH 7.85 and can be explained as a combination of two effects that both have to be operative for optimal activity: K(M) increases at high pH due to deprotonation of PEI amines that bind the anionic substrate and kcat decreases as the availability of hydroxide decreases at low pH. Thus, catalysis is based on substrate binding by positively charged amine groups and the presence of hydroxide ion in active sites in an environment that is tuned for efficient catalysis. Inhibition studies suggest that the basis of catalysis and multiple turnovers is differential molecular recognition of the doubly negatively charged transition state (over singly charged ground state and product): this contributes a factor of at least 5-10-fold to catalysis and product release.

Stop‐frame amination: A very efficient amine transfer reaction is mediated by a biomimetic diiron complex, giving both single and double insertions of a tosylamine group into a pendant benzyl group of the ligand (see structure). The monoinsertion product has been shown by X‐ray crystallography and other physical methods to be a tosylanilinato diiron(III) complex.

Metal-catalyzed nitrene transfer reactions arouse intense interest as clean and efficient procedures for amine synthesis. Efficient Rh- and Ru-based catalysts exist but Fe alternatives are actively pursued. However, reactive iron imido species can be very short-lived and getting evidence of their occurrence in efficient nitrene-transfer reactions is an important challenge. We recently reported that a diiron(III,II) complex is a very efficient nitrene-transfer catalyst to various substrates. We describe herein how, by combining desorption electrospray ionization mass spectrometry, quantitative chemical quench experiments, and DFT calculations, we obtained conclusive evidence for the occurrence of an {Fe(III) Fe(IV) NTosyl} intermediate that is very active in H-abstraction and nitrene-transfer reactions. DFT calculations revealed a strong radical character of the tosyl nitrogen atom in very low-lying electronic configurations of the Fe(IV) ion which are likely to confer its high reactivity.

The development of iron catalysts for carbon-heteroatom bond formation, which has attracted strong interest in the context of green chemistry and nitrene transfer, has emerged as the most promising way to versatile amine synthetic processes. A diiron system was previously developed that proved efficient in catalytic sulfimidations and aziridinations thanks to an Fe Fe active species. To deal with more demanding benzylic and aliphatic substrates, the catalyst was found to activate itself to a Fe Fe L active species able to catalyze aliphatic amination. Extensive DFT calculations show that this activation event drastically enhances the electron affinity of the active species to match the substrates requirements. Overall this process consists in a redox self-adaptation of the catalyst to the substrate needs.

Nature has developed a large variety of metalloenzymes capable of catalyzing selective oxidation reactions through the reductive activation of dioxygen at mono-or dinuclear iron active sites. [1][2][3][4] Among these metalloenzymes, cytochromes P 450 (cytP 450 ), with a mononuclear iron active site, are the most widely known and studied, whereas soluble methane monooxygenase (MMO), which contains a dinuclear iron center, is under intensive investigation.[5] Both systems oxidize a variety of organic substrates by the use of dioxygen and electrons provided by NAD(P)H to give the desired products and water. Much progress has been made in understanding the structure/function relationship of these metalloenzymes on the basis of synthetic systems, [6][7][8] and key intermediates, such as high-valent iron-oxo [9,10] and ironperoxo species [5] have been identified and characterized. However, in most of these synthetic models, the high-valent iron-oxo or iron-peroxo species were generated by the addition of chemical oxidants, such as PhIO, H 2 O 2 , or O 2 , in the presence of electron and proton donors. Gray and coworkers were the first to photogenerate different intermediates of cytP 450 through a photosensitizer-cytP 450 assembly in the presence or the absence of electron scavengers.

Development of artificial systems, capable of delivering electrons to metal-based catalysts for the reductive activation of dioxygen, has been proven very difficult for decades, constituting a major scientific lock for the elaboration of environmentally friendly oxidation processes. Here we demonstrate that the incorporation of a flavin mononucleotide (FMN) in a water-soluble polymer, bearing a locally hydrophobic microenvironment, allows the efficient reduction of the FMN by NADH. This supramolecular entity is then capable of catalysing a very fast single-electron reduction of manganese(III) porphyrin by splitting the electron pair issued from NADH. This is fully reminiscent of the activity of natural reductases such as the cytochrome P450 reductases with kinetic parameters, which are three orders of magnitude faster compared with other artificial systems. Finally, we show as a proof of concept that the reduced manganese porphyrin activates dioxygen and catalyses the oxidation of organic substrates in water.