Talal Mallah

Spin-crossover coordination nanoparticles.

Spin-crossover coordination nanoparticles of the cyanide-bridged three-dimensional network Fe(pyrazine){Pt(CN) 4} were prepared at three different sizes using a microemulsion. The 14 nm particles present a transition centered around 265 K with a hysteresis of 6 K.

Universal Theoretical Approach to Extract Anisotropic Spin Hamiltonians

Monometallic Ni(II) and Co(II) complexes with large magnetic anisotropy are studied using correlated wave function based ab initio calculations. Based on the effective Hamiltonian theory, we propose a scheme to extract both the parameters of the zero-field splitting (ZFS) tensor and the magnetic anisotropy axes. Contrarily to the usual theoretical procedure of extraction, the method presented here determines the sign and the magnitude of the ZFS parameters in any circumstances. While the energy levels provide enough information to extract the ZFS parameters in Ni(II) complexes, additional information contained in the wave functions must be used to extract the ZFS parameters of Co(II) complexes. The effective Hamiltonian procedure also enables us to confirm the validity of the standard model Hamiltonian to produce the magnetic anisotropy of monometallic complexes. The calculated ZFS parameters are in good agreement with high-field, high-frequency electron paramagnetic resonance spectroscopy and frequency domain magnetic resonance spectroscopy data. A methodological analysis of the results shows that the ligand-to-metal charge transfer configurations must be introduced in the reference space to obtain quantitative agreement with the experimental estimates of the ZFS parameters.

Structural and Luminescent Properties of Micro- and Nanosized Particles of Lanthanide Terephthalate Coordination Polymers

Reaction in water between rare earth ions (Ln = Y, La-Tm, except Pm) and the sodium salt of terephthalic acid leads to a family of lanthanide-based coordination polymers of general formula [Ln2(C8H4O4)3(H2O)4] n with Ln = La-Tm or Y. The isostructurality of the compounds with the previously reported Tb-containing polymer is ascertained on the basis of their X-ray powder diffraction diagrams. The coordination water molecules can be reversibly removed without destroying the crystal structure for compounds involving one of the lighter lanthanide ions (La-Eu). For compounds involving one of the heavier lanthanide ions (Tb-Tm) or yttrium, a structural change occurs during the drying process. X-ray diffraction data show this new anhydrous phase corresponding to the linking of pairs of Er(III) ions through mu-carboxylate bridges. Porosity profiles calculated for the anhydrous phases of Tb(III) and Er(III) show the presence of channels with very small sections. The luminescent properties of all the compounds have been recorded and the two most luminescent polymers, namely, the europium- and the terbium-containing ones, have been studied in more detail. Tb(III)-containing compounds display large quantum yields, up to 43%. Polyvinylpyrrolidone nanoparticles doped with [Ln2(C8H4O4)3(H2O)4] n (Ln = Eu, Tb, Er) have also been synthesized and characterized. The encapsulation of the coordination polymers results in somewhat reduced luminescence intensities and lifetime, but the nanoparticles can be dispersed in water and remain unchanged in this medium for more than 20 h.

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

Ising-type magnetic anisotropy and single molecule magnet behaviour in mononuclear trigonal bipyramidal Co(II) complexes

The magnetic anisotropy of two pentacoordinate trigonal bipyramidal (C3v symmetry) Co(II) complexes, [Co(Me6tren)Cl]ClO4 (1) and [Co(Me6tren)Br]Br (2), was investigated and analysed by magnetic studies, high field multifrequency electron paramagnetic resonance (EPR) and ab initio calculations. Negative D parameters expressing an Ising-type anisotropy (easy axis of magnetization) were found experimentally for both complexes. Calculations led to D values very close to the experimental ones, which allows a robust rationalisation of the magnetic anisotropy in these complexes. The wavefunctions of the ground and the first four excited states reveal that they are strongly multideterminantal i.e. linear combinations of several determinants. The most important contribution to the spin orbit coupling between the ground and lowest excited states stabilizes the largest MS = ±3/2 components of the S = 3/2 state and therefore brings a large negative contribution to D. The analysis of the difference between the magnitudes of the anisotropy of the two complexes led to the conclusion that a large Ising anisotropy is preferred when weak σ-donating ligands are in the equatorial plane and strong π-donating ones are in axial positions; thus providing an efficient tool to chemists to predict the magnetic anisotropy in these types of complexes. The investigation of the magnetic behaviour of a single crystal of 1 by micro-SQUID shows, as expected, the presence of an easy axis of magnetization. The magnetic behaviour is consistent with quantum tunnelling of the magnetization mediated by intermolecular three-dimensional antiferromagnetic exchange interactions. Upon dilution of the Co(II) molecules in the isostructural Zn(II) compound, a blocking of the magnetization below 2 K is demonstrated; it results in an opening of the magnetization hysteresis loop in zero applied magnetic field.

Giant Ising-Type Magnetic Anisotropy in Trigonal Bipyramidal Ni(II) Complexes: Experiment and Theory

This paper reports the experimental and theoretical investigations of two trigonal bipyramidal Ni(II) complexes, [Ni(Me(6)tren)Cl](ClO(4)) (1) and [Ni(Me(6)tren)Br](Br) (2). High-field, high-frequency electron paramagnetic resonance spectroscopy performed on a single crystal of 1 shows a giant uniaxial magnetic anisotropy with an experimental D(expt) value (energy difference between the M(s) = ± 1 and M(s) = 0 components of the ground spin state S = 1) estimated to be between -120 and -180 cm(-1). The theoretical study shows that, for an ideally trigonal Ni(II) complex, the orbital degeneracy leads to a first-order spin-orbit coupling that results in a splitting of the M(s) = ± 1 and M(s) = 0 components of approximately -600 cm(-1). Despite the Jahn-Teller distortion that removes the ground term degeneracy and reduces the effects of the first-order spin-orbit interaction, the D value remains very large. A good agreement between theoretical and experimental results (theoretical D(theor) between -100 and -200 cm(-1)) is obtained.

Origin of the Magnetic Anisotropy in Heptacoordinate Ni-II and Co-II Complexes

The nature and magnitude of the magnetic anisotropy of heptacoordinate mononuclear Ni(II) and Co(II) complexes were investigated by a combination of experiment and ab initio calculations. The zero-field splitting (ZFS) parameters D of [Ni(H(2)DAPBH)(H(2)O)(2)](NO(3))(2)⋅2 H(2)O (1) and [Co(H(2)DAPBH)(H(2)O)(NO(3))](NO(3)) [2; H(2)DAPBH = 2,6-diacetylpyridine bis- (benzoyl hydrazone)] were determined by means of magnetization measurements and high-field high-frequency EPR spectroscopy. The negative D value, and hence an easy axis of magnetization, found for the Ni(II) complex indicates stabilization of the highest M(S) value of the S = 1 ground spin state, while a large and positive D value, and hence an easy plane of magnetization, found for Co(II) indicates stabilization of the M(S) = ±1/2 sublevels of the S = 3/2 spin state. Ab initio calculations were performed to rationalize the magnitude and the sign of D, by elucidating the chemical parameters that govern the magnitude of the anisotropy in these complexes. The negative D value for the Ni(II) complex is due largely to a first excited triplet state that is close in energy to the ground state. This relatively small energy gap between the ground and the first excited state is the result of a small energy difference between the d(xy) and d(x(2)-y(2)) orbitals owing to the pseudo-pentagonal-bipyramidal symmetry of the complex. For Co(II), all of the excited states contribute to a positive D value, which accounts for the large magnitude of the anisotropy for this complex.

Magnetic Bistability of Individual Single-Molecule Magnets Grafted on Single-Wall Carbon Nanotubes†

A POM to remember: Hexanuclear Fe(III) polyoxometalate (POM) single-molecule magnets (see structure) can be noncovalently assembled on the surface of single-wall carbon nanotubes. Complementary characterization techniques (see TEM image and magnetic hysteresis loops) demonstrate the integrity and bistability of the individual molecules, which could be used to construct single-molecule memory devices.

A chromium(III) nickel(II) cyanide-bridged ferromagnetic layered structure with corrugated sheets

An infinite stair-like layered ferromagnetic system is obtained from the reaction of K3[CrIII(CN)6] with [NiII-(cyclam)][ClO4]2(cyclam = 1,4,8,11-tetraazacyclotetradecane); a quasi-two-dimensional magnetic behaviour is suggested by the absence of three-dimensional magnetic order down to 2 K.

Photomagnetic nanorods of the Mo(CN)8Cu2 coordination network

Nanorods of the photomagnetic coordination network Mo(CN)8Cu2 coated with polyvinylpyrrolidone were prepared and exhibit an enhanced effect upon irradiation when compared to the bulk.