Jian H. Zhang

A pyrazole ligand yielding both chloro-bridged dinuclear and tetranuclear copper(II) compounds. The crystal and molecular structure of bis[μ-chloro-chloro(3,4-dimethyl-5-phenylpyrazole)(4,5-dimethyl-3-phenylpyrazole)copper(II)] and of (μ4-oxo)hexakis(μ-chloro)tetrakis(3,4-dimethyl-5-phenylpyrazole)tetracopper(II)

Abstract The reaction of CuCl2·2H2O and 3(5),4-dimethyl-5(3)-phenylpyrazole (hereafter Hdmppz) gives green crystals that analyze as CuCl2(C11H12N2)2 (1). The green filtrate produces brown crystals of composition Cu4OCl6(C11H12N2)4 (2). The crystal structure of both compounds was determined from single-crystal X-ray data. Crystals of bis[μ-chloro-chloro-(3,4-dimethyl-5-phenylpyrazole)-(4,5-dimethyl-3-phenylpyrazole)copper(II)] (1) are triclinic, space group P 1 ; a = 9.071(1), b = 11.008(1), c = 11.358(1) A, α = 93.42(1), β = 97.50(1), γ = 96.16(1)°, Z = 1. The dinuclear unit is located on a crystallographic inversion center. The copper ion is five-coordinated by two pyrazole nitrogen atoms and one bridging and one non-bridging chloride anion. The coordination is best described as a distorted square pyramid with the long bridging chloride as the top. Surprisingly the two pyrazole ligands are coordinating through different nitrogen atoms, one through the nitrogen atom close to the phenyl substituent and the other through the nitrogen close to the methyl substituent. Crystals of (μ4-oxo)hexakis(μ-chloro)tetrakis(4,5-dimethyl-3-phenyl-pyrazole)tetra-copper(II) (2) are monoclinic, space group I2/a; a = 20.836(1), b = 11.161(1), c = 22.996(1) A, β = 104.54(1)°, Z = 4. The copper ions are coordinated by five ligands in a trigonal-bipyramidal arrangement. The pyrazole nitrogen and the oxo-anion form the two tops of the bipyramid and the chlorides form the basal plane. Magnetic susceptibility measurements of 1 and 2 show that there is considerable antiferromagnetic interaction in these compounds as can be expected. For 1 the experimental data could be fitted to a model for a dinuclear compound with 2J as the singlet triplet splitting and zJ′ the inter-dinuclear exchange. The best fit was found for g = 2.13, J/k = −5.8 K and zJ′/k = -0.22 K. For 2 a model was used with two different exchange parameters Ja and Jb. The best fit was obtained for g = 2.11 and Ja/k = −51 K and Jb/k= −33 K.

Magnetic and electrical properties of a new chromium telluride phase: CrTe2

Abstract A new material, CrTe2, is prepared by using a rapid precipitation metathesis reaction between the Zintl material K2Te2 and CrCl2 in solution. The d.c. conductivity of this material exhibits semiconductivity and is dominated by the variable range hopping mechanism over a wide temperature region. This material undergoes a transition to a spin glass state at 18 K, that is characterized by d.c. susceptibility measurement as well as the behavior of the remanent at magnetization fields in the range 0–5 Tesla.

Diiron(III) complexes of some relevance to the purple acid phosphatases

Abstract Three diiron(III) complexes of the tetradentate tripodal ligand N-(-ohydroxybenzyl)-N,N-bis(2-pyridylmethyl)amine (HDP), [Fe2(HDP)2O(O2CPh)BPh4 (1), [Fe2(HDP)2O{O2P(OPh)2}]BPh4 (2), and [Fe2(HDP)2{O2(OPh)2}2](BPh4)2 (3), have been synthesized as models for the active site of the purple acid phosphtes. Single crystals of 1 ( P2 1 2 1 2 1 , a=17.728(3) A , b=18.204(4) A c=19.3383(4) A , Z=4, V=6255 A 3 ) and 3 ( P 1 , a=14.316(11) A , b=15.136(17) A , c=13.303(6) A , α=97.04(7)°, β=104.48(5)°, γ=115.51(8)°, Z=1, V=2430(6) A 3 ) were obtained and subjected to X-ray diffraction analysis. Complex 1 has a (μ-oxo)(μ-benzoato)Diiron(III) core, while complex 3 has a bis(μ-phosphato)diiron(III) core, the tetradentate HDP completing the coordination spere about each iron center in both complexes. Due to differences in their core structures, the FeFe separations in 1 and 3 are 3.217(11) A and 4.819(1) A, respectively. Complex 2 is presumed to have a structure analogous to 1 with phosphate replacing the benzoate bridge. Both 1 and 2 exhibit strong atiferromagnetic coupling due to the presence of the oxo bridge (J=−111 and −96 cm−1, respectively; H=−2JS1·S2). Interestingly, the iron(III) centers in 3 are not coupled at all; 3 exhibits Curie behavior throughout the entire temperature range studied (6–300 K) with a magnetic moment commensurate with two high-spin iron(III) centers. The three complexes exhibit visible absorption maxima at 522, 516, and 605 nm, respectively, arising from phenolate-to-iron(III) charge transfer transitions. The shifts in the λmax values can be rationalized on the basis of the Lewis acidities of the respective iron(III) centers. These are compared with those of the purple acid phosphatases.

Magnetic Characterization of 4,4′-(Butadiyne-1,4-Diyl)-Bis-(2,2,6,6-Tetramethyl-4-Hydroxy-Piperidin-1-Oxyl) and its Products on Thermal Treatment

Abstract The magnetic properties of the title compound have been measured with a superconducting squid susceptometer before and after thermal treatment. The ferromagnetism that has been previously reported by other authors to occur in the title compound following thermal treatment has not been observed. This material (α-phase) and a similar polymorph (β-phase) are shown to exhibit normal (Curie-Weiss) behavior before and after heat treatment and have a maximum ferromagnetic contribution corresponding to 5 ppm iron contamination.

Thiocyanato and azido oxo-bridged chromium(III) dimers with aromatic diimine ligands

The spectroscopic, magnetic and electrochemical characteristics of four [{Cr(N–N)2L}2O]2+ dimers are reported, where N–N = 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen) and L–= NCS– or N–3. Electronic spectra are dominated by strong, diimine ligand-centred π–π* transitions, red-shifted on co-ordination to the chromium(III) centres. Strong antiferromagnetic coupling in [{Cr(bipy)2(NCS)}2O][ClO4]2·H2O (J=–247 cm–1) and [{Cr(phen)2(NCS)}2O][ClO4]2·H2O (J=–271 cm–1) indicates a diamagnetic ground state arising from Cr(dπ)–O(pπ)–Cr(dπ) bonding within near-linear CrOCr units. An upper limit of 0 on pKa for both [{Cr(N–N)2(NCS)}2OH]3+ complexes reveals substantial bipy- and phen-promoted CrOCr π-bonding strength enhancements relative to the previously characterized [{Cr(tpma)(NCS)}2O]2+ dimer [tpma = tris(2-pyridylmethyl)amine]. Oxidative and reductive cyclic voltammograms of the µ-O2– dimers have been determined in an effort to identify the highest occupied and lowest unoccupied molecular orbitals. Both magnetic and electrochemical findings suggest the splitting of formally non-bonding b2g(highest occupied) and b1u(lowest unoccupied) levels by virtue of π interactions with the diimine ligands.