No abstract

The observation of ferro-and ferrimagnetic ordering at relatively high and even ambient temperatures has been established for several hexacyanometalate-based materials possessing the Prussian blue structure type.[1] High Curie (critical) temperature (T c ) magnets may find applications in many areas, including magnetic shielding and memory storage devices. [2] Examples of these materials with high ) reported for (1).[1e] The n CN of (4) and ( [5b] were not detected from IR studies. Upon brief exposure of (4) and (5) as powders to air, as reported for (2) and (3), [1h,i] V=O forms within the lattice, resulting in the appearance of the n V=O (978 cm ±1 for (4), and 974 cm ±1 for (5)) band as well as a shift of the strong n CN absorption to 2170 cm ±1 for (4) and 2174 cm ±1 for (5). Nonetheless, it retains its magnetic behavior. The powder X-ray diffraction patterns of (4) and (5) are nearly featureless, so the materials are essentially amorphous. This was also observed for (1)[1e] and it is assumed that the local structure (cubic) and unit cell parameter (a = 10.66 ) determined by extended X-ray absorption fine structure (EXAFS) for (1) [1e] are similar in (4) and (5). The measured density of (4), 1.652(5) Mg m ±3 , is in good agreement with the calculated density of 1.636 Mg m ±3 based on the above unit cell parameter (a = 10.66 ) and the composition as determined below. Based upon the observed density the calculated a is 10.62 . In contrast, the measured density of (5) The compositions of (4) and (5) were analyzed by X-ray photoelectron spectroscopy (XPS), energy dispersive spectrometry (EDS), and thermogravimetric analysis (TGA). The results of the XPS studies were used to determine the stoichiometry of elements and the mixed oxidation states of the vanadium ions. An average formula of K 0.50 V

The reaction of [M II (OH 2 ) 6 ](NO 3 ) 2 (M ) Co, Ni) and [N(CN) 2 ]leads to formation of isomorphous M[N(CN) 2 ] 2 [M ) Co (2a), Ni (3)], respectively, while the reaction of [Co II (OH 2 ) 6 ]-(NO 3 ) 2 in 1% pyridine (py) solution with [N(CN) 2 ]leads to the formation of Co[N(CN) 2 ] 2 py 2 . The structure of 2a, R-Co[N(CN) 2 ] 2 , was determined from the Rietveld analyses of both powder X-ray (synchrotron) and neutron as well as single-crystal X-ray diffraction data, whereas the structure of 3 was determined from the Rietveld analysis of powder neutron diffraction data. 2a (pink) and 3 (light blue) belong to the orthorhombic space group Pnnm with Z ) 2 [2a (single crystal): a ) 5.9985(15), b ) 7.0711(18), c ) 7.4140(19) Å, V ) 314.47(14) Å 3 , R 1 ) 0.027. 3 (powder neutron diffraction): a ) 5.97357(25), b ) 7.03196(28), c ) 7.29424(22) Å, V ) 306.40(3) Å 3 , χ 2 ) 1.650]. Thermolysis of Co[N(CN) 2 ] 2 py 2 leads to intensely blue β-Co II [N(CN) 2 ] 2 , 2b. The M in 2a and 3 are six-coordinate and bound to six different µ 3 -bonded [N(CN) 2 ]ligands, forming a rutile-like 3-D framework. Both M II sites are slightly tetragonally elongated, with average axial M-N distances (295 K) of 2.161(5) (2a) and 2.137(2) Å ( 3) and average equatorial M-N distances of 2.091(4) (2a) and 2.051(1) Å (3), and each M II is coordinated to four equatorial [N(CN) 2 ]ligands that bridge between two adjacent M II sites, generating ribbon-like 1-D chains that propagate along the c-axis. Adjacent chains pack out-of-registry, with the central N2s bridging to M II ions of adjacent chains, forming a 3-D network. [N(CN) 2 ]has pseudo-C 2v symmetry with average C1tN1 distances of 1.158 (2a) and 1.159 Å (3) at room temperature and C1-N2 distances of 1.315 (2a) and 1.313 Å (3). The structure of 2b has not yet been elucidated, but on the basis of the color and magnetic properties, it is thought to be comprised of tetrahedral Co(II). The shortest M‚‚‚M separations are 5.936 (2a) and 5.881 Å (3) at room temperature via neutron diffraction studies. The susceptibility for 2a, 2b, and 3 can be fit by the Curie-Weiss expression with g ) 2.60, θ ) 9 K (T > 50 K); g ) 2.27, θ ) -7 K (T > 60 K); and g ) 2.20, θ ) 21 K (T > 50 K), respectively. The observed room-temperature effective moments of 5.13, 4.37, and 3.17 µ B , respectively, exceed the spin-only moments as expected for these metal ions, but are consistent with octahedral Co(II), tetrahedral Co(II), and octahedral Ni-(II), respectively. Ferromagnetic behavior is suggested for 2a and 3 from the 5 K, 5.5 T saturation magnetizations of 14 000 (2a) and 11 900 emu Oe/mol (3), hysteresis loops with coercive fields, H cr , of 800 (2a), 680 (2b), and 7000 Oe (3) at 2 K, field-cooled and zero-fieldcooled low-field M(T) data showing magnetic ordering below essentially magnetic fieldindependent bifurcation temperatures of 9.2 (2a) and 20.6 K (3), and observations of both in-phase, χ′(T), and out-of-phase, χ′′(T), components of the ac susceptibility maxima slightly below the bifurcation temperatures. The...

Magnets synthesized from molecules have contributed to the renaissance in the study of magnetic materials. Three-dimensional network solids exhibiting magnetic ordering have been made from several first-row metal ions and bridging unsaturated cyanide, tricyanomethanide, and/or dicyanamide ligands. These materials possess several different structural motifs, and the shorter the bridge, the stronger the interaction (i.e., -Ctriple bondN- > -Ntriple bondC-N- >> Ntriple bondC-N-Ctriple bondN- = Ntriple bondC-C-Ctriple bondN-). Cyanide additionally has the ability to discriminate between C- and N-bonding to form ordered heterobimetallic magnets, and the strong coupling can lead to ferro- or ferrimagnetic ordering substantially above room temperature. Tricoordination of tricyanomethanide results in spin-frustrated systems, which possess interpenetrating rutile-like networks. In contrast, single rutile-like frameworks are formed by mu(3)-bonded dicyanamide, which leads to ferromagnetics and weak ferromagnetics.

The supramolecular chemistry and crystal structures of five bis(imidazolium 2,6-pyridinedicarboxylate)M(II) trihydrate complexes, where M ) Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , or Zn 2+ (1-5, respectively), are reported. These complexes serve as supramolecular building blocks that self-assemble when crystallized to generate a single, well-defined, predictable structure in the solid state. 2,6-Pyridinedicarboxylate anions and imidazolium cations form strong ionic hydrogen bonds that dominate crystal packing in compounds 1-5 by forming twodimensional networks, or layers of molecules. This layer motif serves as a platform with which to control and predict molecular packing by design for engineering the structures of crystals. Moreover, compounds 1-5 create a robust organic host lattice that accommodates five different transition metals without significantly altering molecular packing. Growth of crystals from solutions that contain two or more different metal complexes produces mixed crystals in which mixtures of the different metal complexes are incorporated in the same relative molar ratio present in solution. Epitaxial growth of crystals from one metal complex on the surface of a seed crystal that contains a second metal complex generates composite crystals in which the different metal complexes are segregated into different regions of the crystals. Compounds 1-5 form crystalline solids that represent a new class of modular materials in which the organic ligands serve as a structural component that defines a single packing arrangement that persists over a range of structures, and in which the metal serves as an interchangeable component with which to vary the physical properties of the material.

The dynamic susceptibility study of photoinduced magnetism in a molecule-based magnet, K1-2xCo1+x[Fe(CN)(6)];yH(2)O (0.2

No abstract

No abstract