Patrick Dubourdeaux

A Diiron(III,IV) Imido Species Very Active in Nitrene‐Transfer Reactions

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.

Reversible (de)protonation-induced valence inversion in mixed-valent diiron(II,III) complexes.

The coupling of electron and proton transfers is currently under intense scrutiny. This Communication reports a new kind of proton-coupled electron transfer within a homodinuclear first-row transition-metal complex. The triply-bridged complex [Fe(III)(μ-OPh)(μ(2)-mpdp)Fe(II)(NH(2)Bn)] (1; mpdp(2-) = m-phenylenedipropionate) bearing a terminal aminobenzyl ligand can be reversibly deprotonated to the anilinate complex 2 whose core [Fe(II)(μ-OPh)(μ(2)-mpdp)Fe(III)(NHBn)] features an inversion of the iron valences. This observation is supported by a combination of UV-visible, (1)H NMR, and Mössbauer spectroscopic studies.

Preparation and crystal structure of trans-dihydroxo-[tetrakis(2,6-dichlorophenyl)porphinato]ruthenium(IV)·2toluene

Abstract Trans-dihydroxo-[tetrakis(2,6-dichlorophenyl)porphinato]ruthenium(IV) ([Ru(OH)2(TDCPP)]) was prepared by meta-chloroperbenzoic acid oxidation of [Ru(CO)(TDCPP)] in dichloromethane-toluene, and its crystal structure is reported. Crystal data for [Ru(OH)2(TDCPP)]·2toluene:C44H22N4O2Cl8Ru·2C7H8, orthorhombic, space group Pbca a = 13.149(1), b = 19.893(2), c = 21.093(2)A, U = 55.17.3(2) A3, Z = 4. The short axial RuO bond distance, 1.790(7) A, is in the range expected for a double Ru(IV)-oxygen bond. Both hydroxo ligands are approximately located in the mean plane of two opposite dichlorophenyl groups. Full-matrix least-squares refinement of positional and thermal parameters, using 2368 unique reflections with F > 2.5 σ (F) led to R(F) = 0.063; Rw = 0.066.

Phosphoester Hydrolysis: The Incoming Substrate Turns the Bridging Hydroxido Nucleophile into a Terminal One

Identifying the active nucleophile in hydrolysis reactions catalyzed by binuclear hydrolases is a recurrent problem and a matter of intense debate. We report on the phosphate ester hydrolysis by a Fe(III)Fe(II) complex of a binucleating ligand. This complex presents activities in the range of those observed for similar biomimetic compounds in the literature. The specific electronic properties of the Fe(III)Fe(II) complex allowed us to use (1)H NMR and Mössbauer spectroscopies to investigate the nature of the various species present in the solution in the pH range of 5-10. Both techniques showed that the hydrolysis activity is associated to a μ-hydroxido Fe(III)Fe(II) species. Further (1)H NMR experiments show that binding of anions or the substrate changes this bonding mode suggesting that a terminal hydroxide is the likely nucleophile in these hydrolysis reactions. This view is further supported by the structure determination of the hydrolysis product.

Redox Self‐Adaptation of a Nitrene Transfer Catalyst to the Substrate Needs

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 FeIII FeIV active species. To deal with more demanding benzylic and aliphatic substrates, the catalyst was found to activate itself to a FeIII FeIV 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.

A diiron complex mediates an intramolecular aliphatic hydroxylation by various oxygen donors

In the presence of hydrogen peroxide, m-chloroperbenzoic acid or an iodosyl arene, the tert-butyl group of the ligand H(L-t-Bu) in the complex [Fe2(L-t-Bu)(mpdp)]2+ is quantitatively hydroxylated to a butanolate terminally bound to one iron in [Fe2(L-t-Bu - H + O)(mpdp)]2+, and mass spectrometry experiments indicate that the reaction proceeds according to different mechanisms.

Binuclear copper(II) complexes of a new sulphur-containing binucleating ligand: structural and physicochemical properties

The new ligand HL [2,6-bis{4-(benzimidazol-2-yl)-2-thiabutyl}-4-methylphenol] has been prepared. Its reaction with various copper(II) salts afforded a series of complexes 1–6 of general formula Cu2L(X)Y2(X = Y = Cl, 1; X = Y = Br, 2; X = OH, Y = CIO4, 3; X = Y = NO3, 4; X = N3, Y = NO3, 5; X = N3, Y = CIO4, 6). The molecular structure of 6 was determined by X-ray diffraction analysis. It exists as a discrete, dinuclear species in the solid state. The two copper sites are related by a symmetry axis and bridged by the phenoxo oxygen and a nitrogen from the µ-1,1-azide. Each copper is in a square-pyramidal environment, the base of which is constituted by an oxygen, a sulphur and a nitrogen from the organic ligand and the bridging azide nitrogen, while a perchlorato oxygen occupies the apical site. All compounds exhibit a high antiferromagnetic coupling which probably results from sulphur ligation. They exchange electrons at moderately positive potentials which reflects the balance between the hard phenoxo oxygen and the soft nitrogen and sulphur donors provided by the ligand.

Cis/Trans Isomerizations in Diiron Complexes Involving Aniline or Anilide Ligands

We have recently reported a deprotonation-induced valence inversion within a phenoxido-bridged mixed-valent diiron(II,III) complex. The initial aniline coordinated to the Fe(II) site reacts with triethylamine, and the resulting complex contains an anilide ligand coordinated to the Fe(III) ion. The behavior of these complexes in acetonitrile is indeed more intricate. Owing to the very distinctive spectroscopic signatures of the complexes, the conjunction of NMR, Mössbauer, and UV-visible absorption spectroscopies allows one to evidence two isomerization reactions, one involving the aniline linked to Fe(II) and the other the anilide on Fe(III). Theoretical calculations sustain this conclusion. Aniline in the cis position versus the bridging phenoxide is shown to be the most stable isomer while the anilide trans to the phenoxido bridge is favored. The trans isomer of the aniline complex is more acidic than the cis one by 1 pKa unit. Isomerization of the anilide complex is 10 times faster than the analogous isomerization of the aniline complex. Both reactions are proposed to proceed through a unique mechanism. This is the first time that such isomerization reactions are evidenced in dinuclear complexes.

A μ-oxo diiron complex of a new binucleating tetrapyridyl ligand

Abstract The new hexadentate ligand 2,11-bis-{N,N-[bis-(2-methylpyridyl)amino]methyl}benzo[c]phenanthrene has been designed to hold and stabilize a (μ-oxo)diiron(III) unit. Its synthesis and that of a (μ-oxo)(bis-μ-car☐ylato)diiron(III) complex are described. The iron compound is characterized by several pysicochemical techniques: UV-Vis spectroscopy, magnetism, electrochemistry and mass spectrometry. The latter technique shows the binuclear (as opposed to tetranuclear) nature of the compound.

Deprotonation in Mixed-Valent Diiron(II,III) Complexes with Aniline or Benzimidazole Ligands

We have previously investigated cis/trans isomerization processes in phenoxido-bridged mixed-valent Fe(II)Fe(III) complexes that contain either one aniline or one anilide ligand. In this work, we compare the properties of similar complexes bearing one terminal protic ligand, either aniline or 1H-benzimidazole. Whatever the ligand, (1)H NMR spectroscopy clearly evidences that the complexes are present in CH3CN as a mixture of cis- and trans-isomers in a close to 1:1 ratio. We show here that addition of NEt3 indeed allows the deprotonation of these ligands, the resulting complexes bearing either anilide or benzimidazolide that are coordinated to the ferric site. The latter are singular examples of a high-spin ferric ion coordinated to a benzimidazolide ligand. Whereas the trans-isomer of the anilide complex is the overwhelming species, benzimidazolide species are mixtures of cis- and trans-isomers in equal proportions. Moreover, cyclic voltammametry studies show that Fe(III)Fe(III) complexes with 1H-benzimidazole are more stable than their aniline counterparts, whereas the reverse is observed for the deprotonated species.