Xavier Solans

Unique Single-Atom Binding of Pseudohalogeno Ligands to Four Metal Ions Induced by Their Trapping into High-Nuclearity Cages

μ4 end-on coordination (through the N atom) of the pseudohalogeno ligands X- =N3- and NCO- has been observed in the isostructural nonanuclear CoII cages with the general formula [Co9 {(2-C5 H4 N)2 CO2 }4 (O2 CMe)8 X2 ]; this mode is imposed by the trapping of anions X- into cavities formed inside the cage. The change of X- modulates the magnetic behavior of the cage from antiferromagnetic coupling to a ferrimagnetic high-spin ground state.

Three new dinuclear copper(II) complexes with [Cu(μ1,3-N3)2Cu]2+ and [Cu(μ1,1-N3)2Cu]2+ asymmetrical cores: syntheses, structures and magnetic behaviour

Three new dinuclear copper(II) complexes, derived from end-to-end and end-on azido bridging ligands and triamine derivatives, have been synthesized and their crystal structures have been determined by X-ray diffraction methods. These are the dinuclear end-to-end compounds, [Cu2(μ1,3-N3)2(Et3dien)2](ClO4)2 (1) and [Cu2(μ1,3-N3)2(Medpt)2](ClO4)2 (2), and the dinuclear end-on compound [Cu2(μ1,1-N3)2(Medien)2](ClO4)2 (3) (Et3dien is triethyldiethylenetriamine, Medpt is methyldipropylenetriamine and Medien is methyldiethylenetriamine). The [Cu(μ-N3)2Cu]2+ cores are asymmetrical in all 3 compounds. Variable temperature magnetic susceptibility data were collected and fitted to the appropriate equation derived from the Hamiltonian H = −JS1S2. 1 shows unusual ferromagnetic interactions with J = 9 cm−1 through the end-to-end azido bridges while 2 shows an antiferromagnetic interaction through the end-to-end azido bridges with the highest reported absolute magnitude of J = −105 cm−1. The magnetic data of 1 and 2 have been correlated with the Addison parameter, τ, and mainly with the Cu–N3–Cu torsion angle, Δ. 3 shows an unusual antiferromagnetic interaction with J = −16.8 cm−1 through the end-on azido bridges.

SPIN-CROSSOVER IN THE FE(ABPT)2(NCX)2 (X = S, SE) SYSTEM : STRUCTURAL, MAGNETIC, CALORIMETRIC AND PHOTOMAGNETIC STUDIES

The compounds [Fe(abpt) 2 (NCS) 2 ] ( 1 ) and [Fe(abpt) 2 (NCSe) 2 ] ( 2 ) with abpt=4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole have been synthesized. The X-ray structures have been determined at 293 K. 1 and 2 are isostructural and crystallize in the monoclinic space group P 2 1 / n with Z =2, a =8.538(8), b =10.246(8), c =16.45(2) A, β =93.98(9)° for 1 and a =8.623(2), b =10.243(3), c =16.585(3) A, β =93.19(2)° for 2 . In both complexes, the coordination core has a similar pseudo-octahedral geometry with the NCS − ( 1 ) and NCSe − ( 2 ) groups in the trans -position. Variable-temperature magnetic susceptibility data give evidence for a low-spin (LS)↔high-spin (HS) conversion centered at T 1/2 around 180 and 224 K for 1 and 2 , respectively. The spin conversion takes place gradually, without hysteresis. The enthalpy and entropy changes associated with the spin conversion are evaluated from the DSC measurements: Δ H =5.8±0.5 ( 1 ) and 8.6±0.8 kJ mol −1 ( 2 ); Δ S =32.5±3 ( 1 ) and 38±4 J mol −1 K −1 ( 2 ). At 10 K the light-induced excited spin state trapping (LIESST) effect has been observed within the SQUID magnetometer cavity, by irradiating powder samples with a Kr + laser coupled to an optical fiber. The critical LIESST temperatures T liesst are around 40 and 32 K for 1 and 2 , respectively. The magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed a light-induced thermal hysteresis (LITH) effect. The HS→LS relaxation kinetics have been investigated in the temperature range 6–40 K. A thermally activated mechanism at elevated temperatures and a nearly temperature independent relaxation behavior at low temperatures can be observed for 1 . The very fast relaxation precludes the estimation of the kinetic parameters for 2 at temperatures higher than 10 K.

Growth of β-KGd1-xNdx(WO4)2 single crystals in K2W2O7 solvents

Abstract High-temperature solutions of the K2W2O7-KGd1 − xNdx(WO4)2 system have been investigated and β-KGd1 − xNdx(WO4)2 (KGW:Nd) single crystals were grown. The concentration and temperature regions of the KGW:Nd phase as well as the thermal dependence of their solubility for x = 0, 0.05, 0.10, 0.25, 0.50 and 1.00 were determined. These results were supported by X-ray and cell parameters data. EPMA analyses were used to determine the Nd3+ distribution coefficient for different initial compositions of the solution. The obtained data suggested that K2W2O7 is a suitable solvent for KGW:Nd single crystal growth. Transmission measurements have also been included as a first approach to the optical behaviour of the growth crystals.

Crystal structure of strontium hexaferrite SrFe12O19

The room temperature structure of SrFe/sub 12/O/sub 19/ hexagonal ferrite has been refined from X-ray single-crystal data. Structural isomorphism of the title compound with the hexagonal ferrite BaFe/sub 12/O/sub 19/ has been demonstrated. In particular, it is corroborated that the bipyramidal Fe ions have a fast diffusional motion within a quasiharmonic double-well potential as in BaFe/sub 12/O/sub 19/. Some structural differences within the R-block are found among Ba and Sr magnetoplumbites which may account for the different dynamic, magnetic, and crystallochemical behavior of both compounds.

Structure, crystal growth and physical anisotropy of KYb(WO4)2, a new laser matrix

The crystal structure of monoclinic KYb(WO4)2 (KYbW) crystals has been refined (in space group C2/c) at room temperature by using single-crystal X-ray diffraction data. KYbW undoped crystals were grown by the TSSG (top-seeded-solution growth) slow-cooling method. The crystals show {110}, {\bar{1}11}, {010} and {310} faces, which basically define their habit. The linear thermal expansion tensor has been determined and the principal axis with maximum thermal expansion (\alpha ^{\,\prime}_{33} = 16.68 × 10−6 K−1), {\bf X}^{\,\prime}_{3}, was located 12° from the c axis. Its principal {\bf X}^{\,\prime}_{1}, {\bf X}^{\,\prime}_{2} and {\bf X}^{\,\prime}_{3} axes are [302], [010] and [106] directions, respectively, in the crystallographic system. The optical tensor has been studied at λ = 632.8 nm at room temperature; two principal axes, Ng and Nm, are located in the ac plane, while the other, Np, is parallel to [010]. The principal axis with maximum refractive index (ng = 2.45), Ng, was located 19° from the c axis.

Two new nickel(II) cubane compounds derived from pyridine-2-methoxide (Pym): {Ni4(Pym)4Cl4(CH3OH)4} and {Ni4(Pym)4(N3)4(CH3OH)4}. Crystal structures and magnetic properties

Abstract The pyridine-2-methoxide anion (Pym) allows the two new nickel(II) cubane compounds: {Ni 4 (Pym) 4 Cl 4 (CH 3 OH) 4 } ( 1 ) and {Ni 4 (Pym) 4 (N 3 ) 4 (CH 3 OH) 4 } ( 2 ), to be easily synthesized. 1 and 2 are structurally characterised. Complexes 1 and 2 crystallise in the tetragonal system. 1 and 2 have similar structures that consist of tetrameric cubane complexes with the alkoxide oxygen atoms of the pyridine-2-methoxide ligand and octahedrally co-ordinated nickel(II) atoms placed alternately at the corners of the cube. Magnetic susceptibility data, measured from 2 to 260 K show ferromagnetic behaviour for 1 and 2 . These data were fitted by using the equation derived from the Hamiltonian H =−2 J 1 ( S 1 S 2 + S 3 S 4 )−2 J 2 ( S 1 S 3 + S 1 S 4 + S 2 S 3 + S 2 S 4 ) including the Zero Field Splitting for the ground state S =4, giving the parameters J 1 =0.43, J 2 =5.28, D =−0.47 cm −1 , g =2.11 for 1 and J 1 =9.78, J 2 =2.42, D =−0.10 cm −1 , g =2.21 for 2 .

Synthesis and characterization of mixed ligand complexes of copper with nalidixic acid and (NN) donors. Crystal structure of [Cu(Phen)(Nal)-(H2O)]NO3·3H2O

Abstract The synthesis of two new mixed ligand complexes of Cu(II), [Cu(Phen)(Nal)]+ and [Cu(Bipy)-(Nal)]+, is presented (Nal= nalidixate). The results obtained confirm those suggested in a previous paper: the nalidixic acid (I) binds to metal ions by the 3-carboxylate and 4-oxo groups in the solid state. The complex [Cu(Phen)(Nal)(H2O)]NO3·3H2O is triclinic with a = 16.654(3), b = 11.130(2), c = 7.918(2) A, α = 98.53(2), β = 112.12(2), γ = 89.94(2)°, V = 1342.2(8) A3, P I - , Dx = 1.551 g cm−3, Z = 2, F(000) = 650, λ(Mo Kα) = 0.71069 A, μ(Mo Kα) = 9.20 cm−1, 288 K.

Assembly of cyclopalladated units: synthesis, characterisation, X-ray crystal structure and study of the reactivity of the tetrametallic cyclopalladated complex [Pd{C6H4CHN(C6H42-O)}]4·2CHCl3

The reaction of the equimolar amounts of the Schiff base C 6 H 4 CHN(C 6 H 4 2-OH) ( 1 ) and palladium(II) acetate in refluxing methanol for 2 h produces [Pd{C 6 H 4 CHN(C 6 H 4 2-O)}] 4 ( 2 )·2CHCl 3 . The X-ray crystal structure of 2 ·2CHCl 3 reveals that it contains a central non-planar eight-membered ring “Pd 4 O 4 ” formed by the self-assembly of four cyclopalladated fragments in which the ligand behaves as a [C, N, O] 2− terdentate ligand and the oxygen atoms act as bridges between the monomeric units. The reaction of 2 with triphenylphosphine or 1,1′-bis(diphenylphosphino)ferrocene (dppf) produces the opening of the central “Pd 4 O 4 ” core and the formation of the monomeric derivative [Pd{C 6 H 4 CHN(C 6 H 4 2-O)}(PPh 3 )] ( 3 )·CH 2 Cl 2 and the trimetallic complex [{Pd[C 6 H 4 CHN(C 6 H 4 2-O)]} 2 (μ-dppf)] ( 4 ), respectively. In 3 and 4 , the ligand also behaves as a dianionic and terdentate [C, N, O] 2− group, thus indicating that the PdO bond exhibits remarkable stability. The X-ray crystal structure of 3 ·CH 2 Cl 2 confirmed the mode of binding of the Schiff base and a trans arrangement between the imine nitrogen and the PPh 3 ligand. A comparative study of the spectrochemical properties of compounds 2 – 4 is also reported.

Benzoate as terminal ligand in the defective double-cubane, tetranuclear cobalt(II) complex [Co4(N3)2(O2CPh)2{(py)2C(OH)O}4].2DMF with simultaneous μ1,1-azido and μ-o bridges [(py)2C(OH)O = the monoanion of the hydrated, gem-diol form of di-2-pyridyl ketone]

The reaction of Co(O2CPh)2 with di-2-pyridyl ketone, (py)2CO, and NaN3 in DMF allows isolation of the tetranuclear compound [Co4(N3)2(O2CPh)2{(py)2C(OH)O}4]/2DMF (2) instead of the expected nonanuclear cage [Co9(N3)2(O2CPh)8{(py)2CO2}4]. The Xray diffraction analysis reveals a defective double-cubane, tetrameric entity in which the Co II atoms are linked by h 1 ,m2-N3 azide ligand and two kinds of O-bridges. The molar magnetic susceptibility measurements of 2 in the 2 � /300 K range indicate bulk ferromagnetic coupling. Some synthetic aspects of the Co II /(py)2CO/N3 � reaction system are also discussed. # 2002 Elsevier