Synthèse de molécules à haut spin anisotropes à ponts cyanure

Depuis la mise en évidence du phénomène de relaxation lente de l'aimantation du composé Mn12-ac dans les années 90, de nombreuses équipes se sont attelées à la synthèse de clusters présentant de telles propriétés. La méthode de synthèse "one-pot" la plus courante ne permet pourtant pas de contrôler l'état de spin fondamental et l'anisotropie de la molécule, responsables du phénomène. La méthode adoptée dans cette thèse est basée sur une synthèse par étapes et l'utilisation de ponts cyanure. Un choix judicieux des ligands a permis d'isoler des molécules, et d'avoir un contrôle non négligeable de la géométrie et de l'état de spin fondamental. Un complexe linéaire trinucléaire Cr2Ni de spin S = 4 ainsi qu'un complexe tétranucléaire de géométrie carrée Cr2Ni2 de spin S = 5 ont été isolés, mais ne présentent pas de comportement de molécule-aimant. L'anisotropie d'un complexe polynucléaire à ponts cyanure étant liée principalement à l'anisotropie locale des ions le constituant, l'étude de complexes mononucléaires de Ni(II) est donc fondamentale. Une série de complexes mononucléaires de Ni(II) de géométrie très distordue a été synthétisé et caractérisé par RPE à hauts champs et hautes fréquences et FDMRS, indiquant une anisotropie très élevée pour ce genre d'espèces, et contrôlée par le ligand. Trois complexes polynucléaires CrNi2 (S=7/2), CrNi3 (S=9/2) et Cr2Ni7 (2 spins S=7/2) ont été synthétisés à partir de ces précurseurs et étudiés par RPE. Ces espèces présentent une anisotropie élevée. Leur étude à basse température indique un phénomène d'effet tunnel magnétique en champ nul.Since the discovery of the slow relaxation of the magnetization of compound Mn12-ac, much groups in the molecular magnetism field have reported the synthesis of clusters showing such properties. The common "one -pot" synthetic method used for cluster synthesis doesn't allow much control of the spin ground state and magnetic anisotropy of the specie, those parameters being responsible for Single-Molecule Magnet behaviour.The method selected here is based on a stepwidse approach and the use of cyanide ligands as bridges.A judicious choice of the ligands allows to crystallize polynuclear bimetallic compounds, and to increase the control of the cluster's geometry and spin ground state. In that way, a trinuclear linear Cr2Ni complex (S=4) and a tetranuclear square Cr2Ni2 complex have been obtained, but don't behave like Single-Molecule Magnets.The anisotropy of a cyanide bridged polynuclear complex is closely related to the local anisotropy of its ions. The study of Ni(II) mononuclear complexes, and especially anisotropic ones, is thus crucial.A series of Ni(II) mononuclear complexes of very distorted geometry have been crystallized and studied by high fields and high frequencies EPR and FDMRS, indicating a huge anisotropy for that kind of compounds, which seems to be controled by the organic ligand.Three polynuclear complexes CrNi2 (S=7/2), CrNi3 (S=9/2) and Cr2Ni7 (2 spins S=7/2) have been obtained using the previous complexes as precursors and studied by EPR. Those species show important anisotropy parameters. Their low temperature study indicates a zero field magnetic tunnel effect phenomenon.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

A Tale of Two Complexes: Electro‐Assisted Oxidation of Thioanisole by an “O 2 Activator/Oxidizing Species” Tandem System of Non‐Heme Iron Complexes

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Chemoselective guest-triggered shaping of a polynuclear Cu II calix[6]complex into a molecular host

International audienceA new calix[6]arene scaffold bearing a tris-imidazole binding site at the small rim and three tetradentate aza ligands at the large rim was synthesized. The system binds three CuII ions at the large rim sites and is unable to bind a fourth one, which remains in solution. The charge repulsion between the complexes, together with the flexibility of the scaffold, disorganizes the small rim site for binding and prevents its use for host–guest studies. Although the presence of MeCN or DMF guests does not alter this state, the addition of a heptylamine guest, which further displays Brønsted basicity, restores its receptor ability by stabilizing the extra CuII ion at the tris-imidazole site with concomitant guest encapsulation and binding of an exo hydroxo ligand. This chemoselective nuclearity switch yields a tetranuclear complex in which the guest backbone is preorganized in front of three potentially reactive Cu(II) complexes, reminiscent of polynuclear CuII enzyme active sites

Playing with Magnetic Anisotropy in Hexacoordinated Mononuclear Ni(II) Complexes, An Interplay Between Symmetry and Geometry

International audienceThe magnetic anisotropy parameters of a hexacoordinate trigonally elongated Ni(II) complex with symmetry close to D3d are measured using field-dependent magnetization and High-Field and High-Frequency EPR spectroscopy (D = +2.95 cm-1 , |E/D| = 0.08 from EPR). Wavefunction based theoretical calculations reproduce fairly well the EPR experimental data and allows analysing the origin of the magnetic anisotropy of the complex. Calculations on model complexes allows getting insight into the origin of the large increase in the axial magnetic anisotropy (D) when the complex is brought to a prismatic geometry with a symmetry close to D3h.

Second-sphere effects on H 2 O 2 activation by non-heme Fe II complexes: role of a phenol group in the [H 2 O 2 ]-dependent accumulation of Fe IV O vs. Fe III OOH

International audienceRedox metalloenzymes achieve very selective oxidation reactions under mild conditions using O2 or H2O2 as oxidants and release harmless side-products like water. Their oxidation selectivity is intrinsically linked to the control of the oxidizing species generated during the catalytic cycle. To do so, a second coordination sphere is used in order to create a pull effect during the activation of O2 or H2O2, thus ensuring a heterolytic O–O bond cleavage. Herein, we report the synthesis and study of a new non-heme FeII complex bearing a pentaazadentate first coordination sphere and a pendant phenol group. Its reaction with H2O2 generates the classical FeIIIOOH species at high H2O2 loading. But at low H2O2 concentrations, an FeIVO species is generated instead. The formation of the latter is directly related to the presence of the 2nd sphere phenol group. Kinetic, variable temperature and labelling studies support the involvement of the attached phenol as a second coordination sphere moiety (weak acid) during H2O2 activation. Our results suggest a direct FeII → FeIVO conversion directed by the 2nd sphere phenol via the protonation of the distal O atom of the FeII/H2O2 adduct leading to a heterolytic O–O bond cleavage

Hydroxylation of Aromatics by H 2 O 2 Catalyzed by Mononuclear Non‐heme Iron Complexes: Role of Triazole Hemilability in Substrate‐Induced Bifurcation of the H 2 O 2 Activation Mechanism

International audienceRieske dioxygenases are metalloenzymes capable of achieving cis-dihydroxylation of aromatics under mild conditions using O2 and a source of electrons. The intermediate responsible for this reactivity is proposed to be a cis-Fe V (O)(OH) moiety. Molecular models allow generating Fe III (OOH) species with H2O2, to yield a Fe V (O)(OH) species with tetradentate ligands, or {Fe IV (O)/OH•} pairs with pentadentate ones. We have designed a new pentadentate ligand mtL4 2 bearing a labile triazole, to generate an "in-between" situation. Two iron complexes [(mtL4 2)FeCl](PF6) and [(mtL4 2)Fe(OTf)2]) were obtained and their reactivity studied towards aromatic substrates in the presence of H2O2. Spectroscopic and kinetic studies reflect that triazole is bound at the Fe II state, but decoordinates in the Fe III (OOH). The resulting [(mtL4 2)Fe III (OOH)(MeCN)] 2+ then lies on a bifurcated decay pathway (end-on homolytic vs side-on heterolytic) depending on the addition of aromatic substrate: in the absence of substrate, it follows a side-on pathway leading to a putative (N4)Fe V (O)(OH), while in the presence of aromatics it switches to an end-on homolytic pathway yielding a {(N5)Fe IV (O);OH•} reactive species, through recoordination of triazole This switch significantly impacts the reaction regioselectivity

Heterolytic O−O bond cleavage upon single electron transfer to a nonheme Fe(III)−OOH complex

International audienceThe one-electron reduction of the nonheme iron(III)-hydroperoxo complex, [Fe$^{III}$(OOH)(L$_5$$^2$)]$^{2+}$ (L$_5$$^2$=N-methyl-N,N’,N’-tris(2-pyridylmethyl)ethane-1,2-diamine), carried out at −70 °C results in the release of dioxygen and in the formation of [Fe$^{II}$(OH)(L$_5$$^2$)]$^+$ following a bimolecular process. This reaction can be performed either with cobaltocene as chemical reductant, or electrochemically. These experimental observations are consistent with the disproportionation of the hydroperoxo group in the putative Fe$^{II}$(OOH) intermediate generated upon reduction of the Fe$^{III}$(OOH) starting complex. One plausible mechanistic scenario is that this disproportionation reaction follows an O−O heterolytic cleavage pathway via a Fe$^{IV}$-oxo specie

Base-controlled mechanistic divergence between iron(IV)-oxo and iron(III)-hydroperoxo in the H2O2 activation by a nonheme iron(II) complex

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Glycoligands Tuning the Magnetic Anisotropy of NiII Complexes

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