Pierre Rabu

Synthesis, structure and magnetism of homologous series of polycrystalline cobalt alkane mono- and dicarboxylate soaps.

Carboxylate-bridged chain complexes of Co(II) (the diaquacobalt(II) mono- and ,-dialkanoates) form two homologous series of layered compounds which have been fully characterised both structurally and magnetically. The crystal structures of two selected members, [Co[CH3(CH2)10COO]2(H2O)2] and [Co[CH3(CH2)18COO]2(H2O)2], have been solved by X-ray powder diffraction and selected-area electron diffraction methods, and refined by the Rietveld technique. Crystal data: monoclinic, P 2(1)/a; a=9.688(1), b=7.5495(9), c=37.281(5) A, =96.70(3) primary, Z=4; and monoclinic, P 2(1)/a; a=9.7260(7), b=7.5477(7), c=57.53(1) A, =94.66(4) primary, Z=4, respectively. Their isomorphous structures contain layers of octahedral diaquacobalt(II) ions bonded to two chemically inequivalent alkanoates, one chelating and one bridging two Co atoms about 6.3 A apart, thus confirming the rare anti-anti conformation mode of the -RCOO groups recently proposed for diaquacobalt(II) ,-dodecanedioate. Extensive magnetic characterisation allowed estimation of the feeble antiferromagnetic coupling, which is weaker in the mono- than in the dialkanoate series.

Metal-Peptide Frameworks (MPFs) : Bioinspired Metal Organic Frameworks

Chiral metal-organic frameworks (MOFs) have attracted a growing interest for their potential use in energy technologies, asymmetric catalysis, chiral separation, and on a more basic level, the creation of new topologies in inorganic materials. The current paper is the first report on a peptide-based MOF, a metal peptide framework (MPF), constructed from an oligovaline peptide family developed earlier by our group (Mantion, A.; et al. Macromol. Biosci. 2007, 7, 208). We have used a simple oligopeptide, Z-(L-Val)2-L-Glu(OH)-OH, to grow porous copper and calcium MPFs. The MPFs form thanks to the self-assembling properties of the peptide and specific metal-peptide and metal-ammonia interactions. They are stable up to ca. 250 degrees C and have some internal porosity, which makes them a promising prototype for the further development of MPFs.

Copper(II) and cobalt(II) dicarboxylate-based layered magnets: influence of π electron ligands on the long range magnetic ordering

Abstract We discuss the structure and magnetic properties of multilayer compounds M 2 (OH) 2 A· z H 2 O, where M=Co(II) or Cu(II) and A is a dicarboxylate anion. They consist of metal hydroxide-based layers connected through difunctional species (A), which, depending on their nature (alkane-, alkene-dioate or terephthalate) act as pillars or electronic connectors. Either metamagnetism or ferromagnetism is observed, with a huge coercive field of 5.9 T at 4 K in the case of the cobalt(II) hydroxyterephthalate. The aim is to show that π electron species connecting ferromagnetic layers may induce unexpected magnetic properties. High-frequency EPR experiments (95 GHz) and magnetic measurements are reported, giving information on the local structure and the exchange couplings.

Hybrid organic–inorganic multilayer materials: influence of π electrons as magnetic media in a series of bridged-layer compounds M2(OH)4−xAx/2 (M=Cu(II) or Co(II), A=dicarboxylate anion)

Abstract We report the preparation, structure and magnetic properties of a series of multilayer copper(II) hydroxy-dicarboxylates. These are prepared by anion-exchange, starting from copper(II) hydroxy-acetate Cu 2 (OH) 3 (OAc)·H 2 O. The acetate ion is substituted for n- methylene dicarboxylates O 2 CXCO 2 2− , with X=(CH 2 ) n and n =1 to 8. Similar exchange reactions are carried out with unsaturated dicarboxylate anions (X=(CH) n , with n =2, 4 and X=C 4 H 6 ). The compounds, of general formulation Cu 2 (OH) 4− x (XC 2 O 4 ) x /2 . · z H 2 O, exhibit a layered structure with a step-like variation of the basal spacing, according to n parity. IR spectroscopy indicates that the carboxylate functions are linked to the metal ions, each aliphatic chain bridging adjacent hydroxide layers. Only compounds with n =2, 4 and 8 show a net magnetic moment within copper(II) layers. For the short chain anions, an antiferromagnetic order occurs at low temperature, with a metamagnetic transition in low field, except for the unsaturated dicarboxylate Cu 2 (OH) 1.92 (C 4 H 6 C 2 O 4 ) 1.04 ·0.22H 2 O which exhibits a long-range ferromagnetic order at T C =13 K. For the long-chain anion, X=C 8 H 16 , a ferrimagnetic 3d order is stabilized below T C =17 K. On the basis of structural and magnetic findings, a comparison is made between the saturated and unsaturated aliphatic chain compounds. This study provides new insight on the influence of π electrons on the ordering between magnetic layers. Finally, preliminary results for the cobalt(II) trans -hexenedioate analog indicate that a 3d ferromagnet is obtained below T C =56.5 K, thus emphasizing the key role of the in-plane spin–spin correlations on the ordering temperature.

Hybrid organic-inorganic layered compounds prepared by anion exchange reaction: correlation between structure and magnetic properties

The synthesis, structure and magnetic properties of a series of hybrid organic-inorganic copper(II) compounds are reported. Anion exchange reactions were used to prepare layered materials made of copper(II)-based inorganic layers separated by long chain anions (n-alkyl sulfates or n-alkyl carboxylates). Contrary to classical intercalated compounds, the exchanged anion is coordinated to the metal ion. Different packing modes of the n-alkyl chains (mono- or bi-layers) are evidenced, depending on the bridging functions. The magnetic properties have been related to the nature of the anionic spacer and the basal spacing. When ferromagnetic in-plane interactions dominate, 3D ferromagnetic ordering is usually observed for large spacing, due to a dipolar coupling effect. For unsaturated aliphatic chains, π electrons are shown to reinforce the interlayer exchange coupling, giving a ferromagnetic ground-state.

Magnetic properties of a new 3D-cobalt phosphonate: Co3(O3PC2H4CO2)2

The magnetic properties of a new cobalt(II) phosphonate have been investigated. The compound is isostructural with the pillared layered metal phosphonates M 3 (O 3 PC 2 H 4 CO 2 ) 2 , M=Zn, Mn. In this structure, the magnetic transition metal ions are arranged within layers formed by the interconnection of CoO 4 tetrahedra and CoO 6 octahedra. The ac and dc magnetic susceptibilities and magnetization measurements show that Co 3 (O 3 PC 2 H 4 CO 2 ) 2 is a canted antiferromagnet at T N =15.5 K. Below T N , the hysteresis cycles exhibit an S shape related to a spin flop effect. The specific heat measurements indicate that large short-range correlations are present above T N in relation with the layered character of the structure.

Isolation of the Large {Actinide}38 Poly-oxo Cluster with Uranium

By controlling the water content, a new poly-oxo-metalate species containing 38 uranium centers has been solvothermally synthesized in the presence of benzoic acid in tetrahydrofuran (THF). The {U38} motif contains a distorted UO2 core of fluorite type, stabilized by benzoate and THF molecules. This compound is analogous to the {Pu38} motif and was characterized by X-ray photoelectron spectroscopy and magnetic analyses.

Long range ferromagnetism in tunable cobalt(II) layered compounds up to 25 Å apart

Abstract We discuss the structure-magnetic property correlations for a series of layered cobalt(II) compounds, derived from the parent host Co(OH) 2 . Intercalation takes place by exchange of OH − by organic or inorganic anions resulting in interlayer spacing between 4.65 and 25.4 A. For all the compounds, ferromagnetic in-plane interaction dominate the magnetic behavior at high temperature, but long range order occurs at low temperature; 3d antiferromagnetic order is observed for small basal spacing and ferromagnetic for large spacing. The nature of the magnetic interaction is discussed briefly.

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.

Uniform Metal (Hydr)Oxide Particles from Water/Ionic Liquid Precursor (ILP) Mixtures

We have recently shown that the hydrated ionic liquid tetrabutylammonium hydroxide (TBAH) is an efficient ionic liquid precursor (ILP) for the fabrication of ZnO/carbohydrate materials (D. Mumalo-Djokic, W. B. Stern, A. Taubert, Cryst. Growth Des. 2008, 8, 330). The current paper shows that ZnO is just one example out of the large group of technologically important metal (hydr)oxides that can be made using TBAH. Simply by using different metal acetates as precursors in TBAH, it is possible to make a wide variety of metal (hydr)oxides with well-defined size, morphology, and chemical composition. It is also possible to dope metal oxide particles or to synthesize mixed metal oxide particles, and therefore to control properties like magnetism.