Guillaume Rogez

Iron Polyoxometalate Single‐Molecule Magnets

Iron sandwich on a tungstate bun: Two new polyoxotungstates with paramagnetic iron(III) heteroatoms (see structure, W blue, Fe yellow, O red) possess S=15/2 and S=5 ground states. Both compounds are single-molecule magnets, and the hexairon species shows large hysteresis (see picture) and quantum tunneling effects at low temperature. Electrochemical studies indicate that these species are stable in solution for a wide range of pH values.

Core–Multishell Magnetic Coordination Nanoparticles: Toward Multifunctionality on the Nanoscale

Three-dimensional Prussian Blue analogues (PBAs) and related cyano-bridged coordination networks have been at the forefront of the field of molecular magnetism for more than a decade because of the extraordinary variety of their physical properties (electrochromism, ferromagnetism, photomagnetism, piezomagnetism, spin crossover), which opens up prospects for original functional materials. The large metal–metal distance ( 5 ) across the cyano bridge leads to relatively large porosity, which may play a role in hydrogen storage, ion selection, catalysis, and sensors. 14] One important issue is the effect of size reduction on the physical and chemical behavior of cyano-bridged coordination networks and their possible application as molecule-based components in devices. 16] A unique way to take advantage of the physical behavior of PBAs stemming from their rich electronic properties and porosity is to synthesize multishell nanoparticles such that a single particle consists of a core of a given network surrounded by shells of networks that may contain other functionalities. We report here the design of core–multishell nanocrystals thanks to the stabilization of surfactant-free particles in water. Epitaxial growth of different shells on various charged cores is demonstrated, and the thickness of the shells can be fine-tuned. The synergy between the different components is illustrated with one selected magnetic core–shell system. During the last few years, several groups have attempted to establish chemical routes that allow the stabilization of coordination (or metal–organic) nanoparticles of various face-centered-cubic PBAs of the general formula AxM [M’(CN)6](2+x)/3, where A is an alkali-metal cation and M II and M’ are transition-metal ions (see the Supporting Information). Generally, a chemical agent (organic or inorganic) is used during the synthetic process to control the growth of the particles, preclude their aggregation, and ensure their dispersion in different solvents. However, the presence of such protective agents weakens, in most cases, the surface reactivity of the particles and their electronic coupling with other objects, consequently decreasing their multifunctional potential. This can be avoided by the stabilization in solution of surfactant-free nanoparticles. We have recently shown that such electrostatic stabilization can be achieved in the case of the Cs[NiCr(CN)6] network leading to quasi-monodisperse particles with a size of 6.5 nm in diameter. The stabilization of surfactant-free nanoparticles makes it possible to perform coordination chemistry on the particles surface and opens the possibility of the epitaxial growth of one or several shells on the preexisting cores in solution. Thus, the key requirement for the preparation of pure core–shell nanoparticles is 1) stabilization in solution of well-defined crystalline surfactant-free charged nanoparticles and 2) prevention of the side nucleation of the shell by controlling the addition rate and the concentration of the components. Inorganic multishell particles have been prepared on oxides, sulfides, and metallic cores; some interesting examples of shape control have been reported by epitaxial growth seed-mediated procedures involving surfactants. 34] However, this is the first example of coremultishell particles based on coordination networks. The general procedure for the simple growth process on the charged cores present in solution is straightforward and thus feasible on a large scale: a dilute solution containing the divalent metal salt (M(H2O)6Cl2) and CsCl, and another containing the hexacyanometalate(III) salt are added dropwise (1 mL s ) to a stirred solution containing the core particles. The thickness of the growing shell is finely controlled by adjusting the amount of material added in solution (see the Supporting Information). As the growth process occurs, the solution is diluted in order to avoid aggregation that may occur because of the increase of the ionic force. To show the versatility and the efficiency of this approach, we report the preparation and the characterization of surfactant-free Cs[FeCr(CN)6] and Cs [CoCr(CN)6] nanoparticles as well as the design of core–(multi)shell particles of three different systems: 1) bicomponent particles made of a shell of CoII[CrIII(CN)6]2=3 on top of the Cs[FeCr(CN)6] core (denoted CsFeCr@CoCr), 2) tricomponent particles made of two different shells of Cs[FeCr(CN)6] and then Cs [NiCr(CN)6] grown on [*] Dr. L. Catala, Dr. D. Brinzei, Y. Prado, Prof. T. Mallah Institut de Chimie Mol culaire et des Mat riaux d’Orsay Universit Paris-Sud 11, 91405 Orsay (France) Fax: (+ 33)1-6915-4754 E-mail: laurecatala@icmo.u-psud.fr mallah@icmo.u-psud.fr

The Quest for Nanoscale Magnets: The example of [Mn12] Single Molecule Magnets

Recent advances on the organization and characterization of [Mn12] single molecule magnets (SMMs) on a surface or in 3D are reviewed. By using nonconventional techniques such as X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM), it is shown that [Mn12]-based SMMs deposited on a surface lose their SMM behavior, even though the molecules seem to be structurally undamaged. A new approach is reported to get high-density information-storage devices, based on the 3D assembling of SMMs in a liquid crystalline phase. The 3D nanostructure exhibits the anisotropic character of the SMMs, thus opening the way to address micrometric volumes by two photon absorption using the pump-probe technique. We present recent developments such as µ-SQUID, magneto-optical Kerr effect (MOKE), or magneto-optical circular dichroism (MOCD), which enable the characterization of SMM nanostructures with exceptional sensitivity. Further, the spin-polarized version of the STM under ultrahigh vacuum is shown to be the key tool for addressing not only single molecule magnets, but also magnetic nano-objects.

A stable hybrid bisphosphonate polyoxometalate single-molecule magnet.

The long history of polyoxometalate (POM) chemistry initially featured the characterization of diamagnetic species. However, the last few decades have seen the development of several families of paramagnetic POM molecules that display a diverse range of structures and properties. These entities can be of nanosize scale, as exemplified by the discovery of the spin-frustrated Keplerate clusters of general formula {(Mo)Mo5}12M30 (M=Fe , Cr, V). Although of smaller size than these molybdenum compounds, high-nuclearity polyoxovanadate mixed-valent species have also been characterized. The unexpectedly strong magnetic couplings observed in a number of these systems have been studied in detail both experimentally and theoretically. The third— and most extended—magnetic POM family is that of polyoxotungstates. Large clusters in which vacant polyoxotungstate ligands encapsulate 3d or 4f metals have been obtained, and most of these systems are purely inorganic. In particular, very few hybrid polynuclear 3d magnetic polyoxotungstates synthesized under mild conditions have been characterized to date. Moreover, to the best of our knowledge, no general method for the preparation of hybrid, soluble, and stable polyoxotungstates encapsulating magnetic clusters has been proposed. This contrasts with the case of the diamagnetic polyoxotungstates, for which numerous triol-functionalized, organosilyl and organostannic derivatives have been reported. Hybrid molybdenum-based POM chemistry has also been widely developed, but mainly {Mo6} Lindqvist and Anderson-type systems encapsulating a maximum of one 3d cation have been used as a platform for connecting various organic substrates. The establishment of a method allowing the rational synthesis of stable hybrid POMs incorporating several divalent or trivalent 3d metal centers thus remains a challenge. Such materials would be of interest in various fields as they would permit the covalent grafting of POMs containing 3d clusters to a range of organic substrates or surfaces, and thus the elaboration of new functional materials. For example, the covalent connection of photoactive organic groups to multi-electrochromic 3d-substituted inorganic entities may afford optical materials with new properties. Alternatively, Co-substituted polyoxotungstates have been shown to be efficient photocatalysts, which may in itself motivate the search for related hybrid materials. Clearly, hybrid 3d magnetic polyoxotungstates are also interesting in the molecular magnetism field. Since 2008, it is known that 3dor 4f-substituted polyoxotungstates can behave as single molecule magnets (SMMs). Moreover, deposition of POM SMMs on single-wall carbon nanotubes (SWCNTs) has provided evidence that individual POM molecules present a slow relaxation of the magnetization, a crucial point with respect to the potential use of such nanocomposites as information storage devices. However, in such a system, no control of the SWCNT/POM interface is possible. This is due to the purely inorganic nature of the deposited POM—a characteristic of all the 3d POM SMMs species reported to date. Indeed, the reinforcement of the interactions between the two components via covalent grafting or p–p stacking together with the control of the POM/substrate distance obviously implies the elaboration of hybrid POM SMM systems. Herein, we report the characterization of the polyanion [{(B-a-PW9O34)Co3(OH) ACHTUNGTRENNUNG(H2O)2ACHTUNGTRENNUNG(O3PC(O) ACHTUNGTRENNUNG(C3H6NH3)PO3)}2Co] 14 (1), which is built of a heptanuclear Co core sandwiched by two {PW9} units and connected to two bisphosphonato ligands, each possessing a functionalizable alkyl ammonium arm. The stability of this hybrid unit has been unambiguously assessed by using P and H NMR spectroscopy and electrochemical techniques. Single-crystal studies performed on an array of micro-SQUIDs have revealed that the ferromagnetic species 1 exhibits SMM behavior. [a] Dr. H. El Moll, Dr. A. Dolbecq, Dr. J. Marrot, G. Rousseau, Dr. M. Haouas, Dr. F. Taulelle, Prof. P. Mialane Institut Lavoisier de Versailles UMR 8180, Universit de Versailles Saint-Quentin 45 Avenue des Etats-Unis, 78035 Versailles Cedex (France) Fax: (+33)1-39-25-43-81 E-mail : mialane@chimie.uvsq.fr [b] Dr. G. Rogez Institut de Physique et de Chimie des Mat riaux de Strasbourg (IPCMS) UMR 7504, CNRS Universit de Strasbourg 23 rue du Lœss, BP 43, 67034 Strasbourg Cedex 2 (France) [c] Dr. W. Wernsdorfer Institut N el, CNRS et Universit Joseph Fourier BP 166, F-38042 Grenoble Cedex 9 (France) [d] Dr. B. Keita Laboratoire de Chimie Physique Groupe d’Electrochimie et de Photo lectrochimie, UMR 8000, CNRS Universit Paris-Sud, B timent 350, 91405 Orsay cedex (France) E-mail : bineta.keita@lcp.u-psud.fr Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201200140.

Structural Effects of Sodium Cations in Polynuclear, Multicubane‐Type Mixed Na–Ni Complexes

We have now obtained unusualmixed nickel/sodium polynuclear complexes and identifiedremarkablestructural effects ofthe sodiumcations associatedwith the reagent used to deprotonate the alcohol function. Inaddition to their intrinsic novelty, such observations arerelevant to the possible in situ formation of unexpected andpossibly overlooked active species in catalytic reactions.The dinuclear complex [Ni(m-Cl)(HL)]

Spontaneous stabilization and isolation of dispersible bimetallic coordination nanoparticles of CsxNi[Cr(CN)6]y

The spontaneous stabilization of 6.5 nm bimetallic Ni–Cr cyanide bridged nanoparticles was achieved in water. Dispersible particles were recovered from solution using two different coating agents. The magnetic studies of the particles in the powder form show either a superparamagnetic or a spin glass like behaviour depending on the isolation technique used.

An approach to chiral magnets using α-hydroxycarboxylates

Carboxylate-bridged complexes of transition metals, MII = MnII, FeII, CoII, NiII, CuII, were synthesized by reaction of MII salts with (R)-mandelic acid ((R)-2-hydroxyphenylacetic acid), methoxymandelic acid (racemic) (2-methoxyphenylacetic acid) and (R)-malic acid ((R)-2-hydroxybutanedioic acid) under hydrothermal conditions. A wide variety of structures has been obtained, with various dimensionalities: 3D networks when the malic diacid is used, 2D networks with the mandelic acid, 1D isolated chains organised in planes for the Co and Cu compounds with MeOmandelic acid and monomers for the Ni analogue. These complexes have been fully characterized structurally and magnetically. During the reaction process, the (R)-mandelic ligand undergoes a racemisation reaction but the compounds obtained with (R)-malic acid are chiral and exhibit metal-centred circular dichroism. Extensive magnetic characterization of all compounds indicates rather weak coupling interactions between paramagnetic centres linked through carboxylate bridges. Curie-like paramagnetic, antiferromagnetic, ferromagnetic or weak ferromagnetic behaviour was observed and is discussed on the basis of the structural features. The bimetallic compounds Mn0.63Co0.37(R)-malate monohydrate (12) and Mn0.79Ni0.21(R)-malate monohydrate (13) are new examples of chiral magnets.

Supramolecular Organization and Magnetic Properties of Mesogen-Hybridized Mixed-Valent Manganese Single Molecule Magnets [MnIII8MnIV4O12(Lx,y,z-CB)16(H2O)4]

Single molecule magnets (SMM) may be considered for the construction of future integrated nanodevices, provided however that some degree of ordering is imparted to these molecules (surfaces nanostructuration). Combining such nanoobjects with liquid-crystalline orderings to control their assembly and to potentially address them individually therefore appears as one promising strategy. Four mesomorphic, mixed-valent [Mn(III)(8)Mn(IV)(4)O(12)(L(x,y,z-CB))(16)(H(2)O)(4)] SMM, differing in the number of liquid-crystalline promoters, (L(x,y,z-CB)), were synthesized, and their self-organizing and magnetic properties were investigated. The influence of the peripheral modifications, and precisely how supramolecular ordering and magnetic properties may be affected by the evolution of the proto-mesogenic cyanobiphenyl-based ligands substitution pattern, was explored. Small-angle X-ray scattering studies revealed that all of the hybridized clusters self-organize into room-temperature bilayer smectic phases, mandated by the specific mesogenic functionalization and that the polymetallic cores are further organized according to a short-range pseudo-2D lattice with hexagonal and/or square symmetry. All mesomorphous hybridized dodecamanganese complexes still behave as SMM: they exhibit blocking of the magnetization at about 2.6 K as evidenced by the occurrence of frequency-dependent out-of-phase ac susceptibility signals as well as an opening of the hysteresis cycle with coercive fields varying between 0.13 and 0.6 T, depending on the surface ligands topology. Comparison of the magnetic properties within this series reveals intricate correlations between the structural features of the mesomorphous molecule magnet (i.e., symmetry of the ligands substitution patterns, molecular conformation, average intercluster distances, and respective inclination) with respect to the relative proportion of slow- and fast-relaxing species and the absolute values of the coercive fields.

Tunable Synthesis of Prussian Blue in Exponentially Growing Polyelectrolyte Multilayer Films

Polyelectrolyte multilayer (PEM) films have become very popular for surface functionalization and the design of functional architectures such as hollow polyelectrolyte capsules. It is known that properties such as permeability to small ionic solutes are strongly dependent on the buildup regime of the PEM films. This permeability can be modified by tuning the ionization degree of the polycations or polyanions, provided the film is made from weak polyelectrolytes. In most previous investigations, this was achieved by playing on the solution pH either during the film buildup or by a postbuildup pH modification. Herein we investigate the functionalization of poly(allylamine hydrochloride)/poly(glutamic acid) (PAH/PGA) multilayers by ferrocyanide and Prussian Blue (PB). We demonstrate that dynamic exchange processes between the film and polyelectrolyte solutions containing one of the component polyelectrolyte allow one to modify its Donnan potential and, as a consequence, the amount of ferrocyanide anions able to be retained in the PAH/PGA film. This ability of the film to be a tunable reservoir of ferrocyanide anions is then used to produce a composite film containing PB particles obtained by a single precipitation reaction with a solution containing Fe(3+) cations in contact with the film. The presence of PB in the PEM films then provides magnetic as well as electrochemical properties to the whole architecture.

New layered organic-inorganic magnets incorporating azo dyes

New hybrid organic/inorganic magnets have been synthesized by insertion of stilbazolium-carboxylate and/or -sulfonate dyes into cobalt and copper layered hydroxides. Co2(OH)3.57(MR)0.43·1.88 H2O (MR⊂Co (1)) (MRNa: Methyl Red sodium salt: sodium 2-[4-(Dimethylamino)phenylazo]benzoate), Co2(OH)3.27(MO)0.73·2.94 H2O (MO⊂Co (2)) (MONa: Methyl Orange sodium salt: sodium 4-[4-(Dimethylamino)phenylazo]benzenesulfonate), Co2(OH)3.45(OrangeIV)0.55·2.92 H2O (OrangeIV⊂Co (3)) (OrangeIVNa: OrangeIV sodium salt: sodium 4-[4-(Anilino)phenylazo]benzenesulfonate), and Co2(OH)3.46(MY10)0.27·2.34 H2O (MY10⊂Co (4)) (MY10Na2: Mordant Yellow 10 disodium salt: sodium 3-[4-(sodium sulfonate)phenylazo]-6-hydroxybenzoate) have been synthesized by anionic exchange in alkaline media starting from Co2(OH)3(OAc)·H2O. Cu2(OH)3.23(MR)0.77·2.92 H2O (MR⊂Cu (5)), Cu2(OH)3.29(MO)0.71·2.31 H2O (MO⊂Cu (6)), and Cu2(OH)3.39(OrangeIV)0.61·1.80 H2O (OrangeIV⊂Cu (7)) were obtained starting from the pre-intercalated copper hydroxide Cu2(OH)3(DS) (DS−: dodecylsulfate). This is the first time pre-intercalation is used for the insertion of molecules within the interlamellar space of simple layered hydroxides. The hybrid copper layered hydroxides show ferrimagnetic behaviour with ordering at very low temperature (below 1.8 K). The cobalt analogues are ferrimagnets with ordering temperatures ranging from 6.9 K to 18 K.