Takafumi Kitazawa

Spin-crossover behaviour of the coordination polymer FeII(C5H5N)2NiII(CN)4

The iron(II) spin state of Fe(C5H5N)2Ni(CN)4 changes from paramagnetic to diamagnetic when the temperature is lowered, while that of Fe(NH3)2Ni(CN)4·2C6H6 does not.

57Fe Mössbauer spectra of FeNi(CN)4 layered host aniline clathrates

Abstract We measured the 57 Fe Mossbauer spectra of three types of aniline clathrate materials with a two-dimensional layered host consisting of iron(II)tetracyanonickelate(II). The formation of the three aniline clathrate materials depends on the pH of the mother aqueous solution containing equimolar amounts of (NH 4 ) 2 Fe(SO 4 ) 2 · 6H 2 O and K 2 [Ni(CN) 4 ] · H 2 O. The spectra for the three clathrate materials indicate that all the iron atoms are in iron(II) high spin states. In the spectrum of material I obtained from the relatively high pH (8–10) solution, there exists one kind of doublet with quadrupole splitting( QS ) of 1.44mm/s. Material I is previously known as Fe(NH 3 ) 2 Ni(CN) 4 · 2C 6 H 5 NH 2 . In material III , obtained from the relatively low pH (5–6) solution, the quadrupole splitting of 2.42 mm/s is larger than that of I . The Mossbauer data i.r. and elemental analysis for III suggest that the probable composition is Fe(H 2 O) 2 Ni(CN) 4 · [ mH 2 O · nC 6 H 5 NH 2 ]. Material II obtained from the middle range of pH (6–8), has at least two kinds of iron(II) sites, I type and III type, respectively.

Luminescence tuning of imidazole-based lanthanide(III) complexes [Ln = Sm, Eu, Gd, Tb, Dy]

To tune the lanthanide luminescence in related molecular structures, we synthesized and characterized a series of lanthanide complexes with imidazole-based ligands: two tripodal ligands, tris{[2-{(1-methylimidazol-2-yl)methylidene}amino]ethyl}amine (Me(3)L), and tris{[2-{(imidazol-4-yl)methylidene}amino]ethyl}amine (H(3)L), and the dipodal ligand bis{[2-{(imidazol-4-yl)methylidene}amino]ethyl}amine (H(2)L). The general formulas are [Ln(Me(3)L)(H(2)O)(2)](NO(3))(3)·3H(2)O (Ln = 3+ lanthanide ion: Sm (1), Eu (2), Gd (3), Tb (4), and Dy (5)), [Ln(H(3)L)(NO(3))](NO(3))(2)·MeOH (Ln(3+) = Sm (6), Eu (7), Gd (8), Tb (9), and Dy (10)), and [Ln(H(2)L)(NO(3))(2)(MeOH)](NO(3))·MeOH (Ln(3+) = Sm (11), Eu (12), Gd (13), Tb (14), and Dy (15)). Each lanthanide ion is 9-coordinate in the complexes with the Me(3)L and H(3)L ligands and 10-coordinate in the complexes with the H(2)L ligand, in which counter anion and solvent molecules are also coordinated. The complexes show a screw arrangement of ligands around the lanthanide ions, and their enantiomorphs form racemate crystals. Luminescence studies have been carried out on the solid and solution-state samples. The triplet energy levels of Me(3)L, H(3)L, and H(2)L are 21 000, 22 700, and 23 000 cm(-1), respectively, which were determined from the phosphorescence spectra of their Gd(3+) complexes. The Me(3)L ligand is an effective sensitizer for Sm(3+) and Eu(3+) ions. Efficient luminescence of Sm(3+), Eu(3+), Tb(3+), and Dy(3+) ions was observed in complexes with the H(3)L and H(2)L ligands. Ligand modification by changing imidazole groups alters their triplet energy, and results in different sensitizing ability towards lanthanide ions.

Two-Step Thermal Spin Transition and LIESST Relaxation of the Polymeric Spin-Crossover Compounds Fe(X-py)2[Ag(CN)2]2 (X=H, 3-methyl, 4-methyl, 3,4-dimethyl, 3-Cl)†

In the series of polymeric spin-crossover compounds Fe(X-py)(2)[Ag(CN)(2))](2) (py=pyridine, X=H, 3-Cl, 3-methyl, 4-methyl, 3,4-dimethyl), magnetic and calorimetric measurements have revealed that the conversion from the high-spin (HS) to the low-spin (LS) state occurs by two-step transitions for three out of five members of the family (X=H, 4-methyl, and X=3,4-dimethyl). The two other compounds (X=3-Cl and 3-methyl) show respectively an incomplete spin transition and no transition at all, the latter remaining in the HS state in the whole temperature range. The spin-crossover behaviour of the compound undergoing two-step transitions is well described by a thermodynamic model that considers both steps. Calculations with this model show low cooperativity in this type of systems. Reflectivity and photomagnetic experiments reveal that all of the compounds except that with X=3-methyl undergo light-induced excited spin state trapping (LIESST) at low temperatures. Isothermal HS-to-LS relaxation curves at different temperatures support the low-cooperativity character by following an exponential decay law, although in the thermally activated regime and for aX=H and X=3,4-dimethyl the behaviour is well described by a double exponential function in accordance with the two-step thermal spin transition. The thermodynamic parameters determined from this isothermal analysis were used for simulation of thermal relaxation curves, which nicely reproduce the experimental data.

Unexpected isotope effect on the spin transition of the coordination polymer Fe(C5H5N)2[Ni(CN)4]

The two-dimensional coordination polymer spin-crossover compound Fe(pyridine)2[Ni(CN)4] (1) and its isotope substituted analogues Fe(pyridine-D5)2[Ni(CN)4] (2) and Fe(pyridine-15N)2[Ni(CN)4] (3) have been synthesised and the isotope effect on the thermal spin-crossover behaviour has been studied using magnetic susceptibility, 57Fe Mossbauer, Raman and calorimetric techniques. All these methods confirmed that upon isotope substitution—contrary to previous observations—the spin transition temperature shifted downwards from 202 to 194 K. The theoretical analysis of spectroscopic data has revealed that this shift was a result of a subtle balance between different vibrational and electronic factors.

Silica-mimetic polymorphism of the Cd(CN)2 host lattice depending on the guest G in Cd(CN)2·xG clathrates

The single-crystal structures have been determined for the Cd(CN)2 host clathrates Cd(CN)2·xBun2O·yH2O (x, y≈ 0.5)1, Cd(CN)2·0.5Bui2O 2a, Cd(CN)2·0.5(PriCH2CH2)2O 2b, Cd(CN)2·PriCl 2o, Cd(CN)2·CHCl2CH2Cl 3a, Cd(CN)2·PriBr 3b and Cd(CN)2·PriCN 4, prepared in order to mimic the polymorphism of SiO2 by Cd(CN)2. The hexagonal P63/mmc host lattice of 1 is isostructural with the high-temperature form of tridymite, Accommodating the dibutyl ether guest in the channel cavity extending along the c axis and the water molecule is hydrogen bonded to the ether in the cage neighbouring the channel. The cubic Fdmm host lattice of 2 in the high-temperature cristobalite structure provides two neighbouring tetrahedral cavities for the respective alkyl ether guests in 2a and 2b; the structure of 2o is the same. The cubic Fd3m lattice is transformed into the tetragonal P41212 one in 3 and 4, similar to the deformation from high-to low-temperature cristobalite, the lattice of 4(Z= 8) being more distorted than those of 3a and 3b(Z= 4). The polymorphic behaviour of these Cd(CN)2 lattices is discussed in terms of the geometry and function of the guest molecules.

Magnetic Properties and Structures of the α- and δ-Phases of p-NPNN

We have investigated the crystal structures and magnetic properties of the α- and δ-phases of an organic radical p -NPNN ( p -nitrophenyl α-nitronyl nitroxide), which yielded the first organic bulk ferromagnet, β- p -NPNN (the β-phase). The results are compared with those of the β- and γ-phases, which show distinct magnetic ordering due to intermolecular ferromagnetic couplings. Common structural features are found in the four phases. Unlike the other phases, the α-phase exhibits a weak antiferromagnetic interaction. This is attributable to phenyl–phenyl overlapping found only in the α-phase. The crystal structure of δ- p -NPNN contains a packing motif similar to that of the γ-phase. Though the δ-phase exhibits ferromagnetic interactions, it shows no magnetic ordering down to 0.46 K. It is suggested that ferromagnetic two-leg spin ladders are formed in the δ-phase, instead of a two-dimensional network as in γ- p -NPNN. This low dimensionality explains the absence of a magnetic transition.

57Fe Mössbauer spectroscopic and magnetic study of a spin-crossover polymer complex, Fe(3-chloropyridine)2Ni(CN)4

The coordination polymer Fe(3-chloropyridine)2Ni(CN)4 (2) has been prepared by a method similar to that for Fe(pyridine)2Ni(CN)4 (1). The complex (2) has been characterized by57Fe Mössbauer spectroscopy and a SQUID technique.57Fe Mössbauer and magnetic susceptibility data show that complex (2) exhibits spin-crossover behavior. The spin transition of (2) occurs between 120 and 80 K with very small hysteresis or without hysteresis. The temperature range of the spin transition in (2) is lower than that in (1). A residual high spin iron(II) fraction is observed at low temperatures in (2), being different from (1). SQUID data also show that samples treated differently yield different spin transition curves.

Hofmann-H2O-type and Hofmann-H2O-Td-type host structures accomodating 1,4-dioxane: crystal structures of trans-bis (morpholine-N) cadmium(II) tetracyanonickelate(II), trans-diaquacadmium(II) tetracyanonickelate(II)-(1,4-dioxane)() and trans-diaquacadmium(II) tetracyanocadmate(II) (1,4-dioxane)()

Abstract In order to stabilise six-membered alicyclic ether guests through a hydrogen bond with an H2O ligand in the host, Hofmann-H2O-type [Cd(H2O)2Ni(CN)4] and Hofmann-H2O-Td-type [Cd(H2O)2Cd(CN)4] hosts were examined for accomodating 1,4-dioxane and morpholine as probable guests. The single crystal structures of the eventually obtained morpholine-ligated complex [Cd(C4H9NO)2Ni(CN)4] (1) and 1,4-dioxane-guest clathrates, [Cd(H2O)2Ni(CN)4]·2C4H8O2 (2) and [Cd(H2O)2Cd(CN)4]·2C4 H8O2 (3), are closely related in topology with previously known [Cd(py)2Ni(CN)4], [Cd(NH3)2Ni(CN)4]·2C4H8O2 and [Cd(NH3)2Cd(CN)4] ·2C6H6, respectively, but involve hydrogen bonds between the morpholine ligands in 1, and between the aqua ligand of the host and the guest 1,4-dioxane in 2 and 3. Complex 1 crystallises in the orthorhombic system, Pmna, a = 7.985(1), b = 6.695(2), c = 14.314(2) A , Z = 2, R = 0.048 for 1240 reflections; clathrate 2: monoclinic, P2/m, a = 7.3980(8), b = 7.6454(6), c = 8.0639(9) A , β = 93.703(9)°, Z = 1, R = 0.041 for 1040; 3: monoclinic, C2/c, a = 12.054(5), b = 11.334(6), c = 15.249(2) A , β = 91.62(2)°, Z = 4, R = 0.043 for 2129. The structure of a Hofmann-H2O-type hydrate [Fe(H2O)2Ni(CN)4]·4H2O, orthorhombic Pnma, a = 12.200(2), b = 14.028(2), c = 7.2327(9) A , Z = 4, R = 0.042 for 710, is also reported.

Mineralomimetic chemistry of cyanometallates

Abstract The structural similarity between Cdx(CN)y and SixOy in the AxBy composition, A taking tetrahedral positions linked successively with B to build up multi-dimensional structures such as 1D-chains, 2D-layers, or 3D-lattices, can be utilized in developing mineral-like but unprecedented inclusion or supramolecular structures using cadmium cyanide and cyanocadmate moieties as building blocks. Examples have been demonstrated with single crystal structures for such inclusion structures as silica-mimetic Cd(CN)2 host clathrates, clay-mimetic 2D-layer and zeolite-mimetic 3D-lattice cyanocadmate host inclusion compounds.