Eva Rentschler

Magnetic anisotropy of the antiferromagnetic ring [Cr8F8Piv16].

A new tetragonal (P42(1)2) crystalline form of [Cr8F8Piv16] (HPiv = pivalic acid, trimethyl acetic acid) is reported. The ring-shaped molecules, which are aligned in a parallel fashion in the unit cell, form almost perfectly planar, regular octagons. The interaction between the CrIII ions is antiferromagnetic (J = 12 cm(-1)) which results in a S = 0 spin ground state. The low-lying spin excited states were investigated by cantilever torque magnetometry (CTM) and high-frequency EPR (HFEPR). The compound shows hard-axis anisotropy. The axial zero-field splitting (ZFS) parameters of the first two spin excited states (S = 1 and S = 2, respectively) are D1 = 1.59(3) cm(-1) or 1.63 cm(-1) (from CTM and HFEPR, respectively) and D2 = 0.37 cm(-1) (from HFEPR). The dipolar contributions to the ZFS of the S = 1 and S = 2 spin states were calculated with the point dipolar approximation. These contributions proved to be less than the combined single-ion contributions. Angular overlap model calculations that used parameters obtained from the electronic absorption spectrum, showed that the unique axis of the single-ion ZFS is at an angle of 19.3(1) degrees with respect to the ring axis. The excellent agreement between the experimental and the theoretical results show the validity of the used methods for the analysis of the magnetic anisotropy in antiferromagnetic CrIII rings.

A new dinuclear unsymmetric copper(II) complex as model for the active site of catechol oxidase

Abstract The crystal structure, magnetic, redox and spectroscopic properties of a novel unsymmetrical dinuclear copper(II) complex, prepared by the reaction between copper(II) perchlorate, sodium acetate and the unsymmetrical, binucleating ligand HTPPNOL, where HTPPNOL is N,N,N′-tris-(2-pyridylmethyl)-1,3-diaminopropan-2-ol, is reported. HTPPNOL (1 equiv.) reacted with 1 equiv. of copper(II) ion, in methanol, and produced the mononuclear copper complex [Cu(TPPNOL)](ClO4)(BPh4) (1). On the other hand, the reaction of 1 equiv. of HTPPNOL with 2 equiv. each of copper (II) ion and acetate, in methanol, produced the dinuclear complex [Cu2(TPPNOL)(OOCCH3)](ClO4)2 (2), whose structure has been determined by X-ray diffraction. In complex 2, as a result of the inherent asymmetry of the ligand HTPPNOL, one copper ion is five-coordinated (distorted trigonal-bipyramidal) while the other copper is four-coordinated (distorted square-planar). Then, as a result of the presence of distinct geometries for the metal centres, complex 2 exhibits a ferromagnetic coupling (J=+25.41 cm−1). Titration experiments carried out on the dinuclear complex suggest a pKa=8.0, which was related to the aquo/hydroxo equilibrium. Complex 2 is able to oxidise 3,5-di-tert-butylcatechol to the respective o-quinone. The oxidation reaction was studied by following the appearance of the quinone spectrophotometrically, at pH 8.0 and 25xa0°C.

Syntheses, Structures, and Magnetic Properties of Diphenoxo-Bridged CuIILnIII and NiII(Low-Spin)LnIII Compounds Derived from a Compartmental Ligand (Ln = Ce−Yb)

Syntheses, characterization, and magnetic properties of a series of diphenoxo-bridged discrete dinuclear M(II)Ln(III) complexes (M = Cu or Ni, Ln = Ce-Yb) derived from the compartmental Schiff base ligand, H(2)L, obtained on condensation of 3-ethoxysalicylaldehyde with trans-1,2-diaminocyclohexane, are described. Single crystal X-ray structures of eight Cu(II)Ln(III) compounds (Ln = Ce (1), Pr (2), Nd (3), Sm (4), Tb (7), Ho (9), Er (10), and Yb (12)) and three Ni(II)Ln(III) (Ln = Ce (13), Sm (16), and Gd (18)) compounds have been determined. Considering the previously reported structure of the Cu(II)Gd(III) (6) compound (Eur. J. Inorg. Chem. 2005, 1500), a total of twelve structures are discussed/compared in this study. Four types of composition are observed in the Cu(II)Ln(III) complexes: [Cu(II)LLn(III) (NO(3))(3)(H(2)O)] (1-3: Ln = Ce-Nd), [Cu(II)LSm(III)(NO(3))(3)]·CH(3)COCH(3) (4), [Cu(II)(H(2)O)LLn(III)(NO(3))(3)] (5: Ln = Eu; 6: Ln = Gd), and [Cu(II)LLn(III)(NO(3))(3)] (4A: Ln = Sm; 7-12: Ln = Tb-Yb). On the other hand, the Ni(II)Ln(III) complexes are characterized to have two types of composition: [Ni(II)LLn(III)(H(2)O)(NO(3))(3)] (13-15: Ln = Ce-Nd) and [Ni(II)LLn(III)(NO(3))(3)]·0.5CH(3)COCH(3) (16-24: Ln = Sm-Yb). Among twelve X-ray structures, seven belong to three different isomorphous sets (Cu(II)Ce(III) (1), Cu(II)Pr(III) (2), Cu(II)Nd(III) (3), and Ni(II)Ce(III) (13); Cu(II)Tb(III) (7), Cu(II)Ho(III) (9), Cu(II)Er(III) (10), and Cu(II)Yb(III) (12); Ni(II)Sm(III) (16) and Ni(II)Gd(III) (18)), whereas space group/unit cell parameters of two others (Cu(II)Sm(III) (4) and Cu(II)Gd(III) (6)) are of different types. The lanthanide(III) centers in Cu(II)Ce(III) (1), Cu(II)Pr(III) (2), Cu(II)Nd(III) (3), and Ni(II)Ce(III) (13) complexes are eleven-coordinated, while the lanthanide(III) centers in other compounds are ten-coordinated. As evidenced from the dihedral angle (δ) between the CuO(phenoxo)(2) and LnO(phenoxo)(2) planes, variation in the extent of planarity of the bridging moiety in the Cu(II)Ln(III) compounds takes place; the ranges of δ values are 0.8-6.2° in the 4f(1-7) analogues and 17.6-19.1° in the 4f(8-13) analogues. The Cu(II)Gd(III) (6) compound exhibits ferromagnetic interaction (Eur. J. Inorg. Chem. 2005, 1500). The nature of the magnetic exchange interaction in the Cu(II)Ln(III) complexes has been understood by utilizing the empirical approach; the Ni(II)Ln(III) complexes have been used as references. The metal centers in the Eu(III) complex are uncorrelated, while other 4f(1-6) analogues (Ce(III), Pr(III), Nd(III), and Sm(III)) exhibit antiferromagnetic interaction. Among the higher analogues (4f(7-13)), only Yb(III) exhibits antiferromagnetic interaction, while interaction in other analogues (Gd(III), Tb(III), Dy(III), Ho(III), Er(III), and Tm(III)) is ferromagnetic. An important aspect of the present study is the measurement of the magnetic susceptibility of the unblocked samples as well as on blocking the samples with grease to avoid powder reorientation, if any. Comparison of the two sets of data reveals significant difference in some cases.

DC and AC conductivity and dielectric properties of the metal-radical compound: Aqua[bis(2-dimethylaminomethyl-4-NIT-phenolato)]copper(II)

Abstract The AC and DC conductivity and dielectric properties of the crystalline metal-radical compound aqua[bis(2-dimethylaminomethyl-4-NIT-phenolato)]copper(II) have been investigated. The DC electrical measurements show that the compound is a typical semiconductor with moderate activation energy E a =0.56 eV and at room conductivity σ 25 =1.38×10 −6 S cm −1 as its electrical conductivity increases with increasing temperature. The AC conductivity of the sample is found to be proportional to ωs. The temperature dependence of both the AC conductivity and the frequency exponent s is reasonably well interpreted in terms of the correlated barrier hopping model. The dielectric properties have been investigated as a function of frequency and temperature. Values of the dielectric constant e′ and dielectric loss e″ were found to decrease with frequency and increase with temperature. The effect of different temperatures on the AC conductivity and dielectric properties was also investigated. The AC conductivity and values of the dielectric constant increase with increasing temperature.

Synthesis, crystal structure, and magnetic properties of two new Cu(II) complexes with end-to-end azido bridging ligands

Two new copper(II) complexes, derived from an end-to-end azido linker and two different tridentate Schiff base ligands, have been synthesised and their crystal structures determined by X-ray diffraction methods. They are the single azido-bridged [Cu(L1xa0−xa0H)(μ-1,3-N3)] nn(H2O)2n (1) chain and the dinuclear [Cu2(L2)2(μ-1,3-N3)2(ClO4)2] (2) where L1 and L2 are two n different tridentate Schiff bases obtained by condensation of salicylaldehyde with 4-(2-aminoethyl)morpholine and n of pyridine-2-aldehyde with N,N-dimethylpropane-1,3-diamine, respectively. Structural results show that each copper(II) centre in 1 is in a square pyramidal geometry while in 2 the geometry of copper(II) is octahedral with one coordinated n perchlorate in both copper(II) complexes. Variable temperature magnetic susceptibility data in the n range of 300–2 K for 1 reveal the existence of intrachain weak antiferromagnetic interactions through the end-to-end azido coupler, while the data for 2 n show the existence of unusual intradimer ferromagnetic interactions through the end-to-end azido pathway, which almost always leads to an anti-parallel spin orientation. Fitting of the data for 1 using the Bonner–Fisher model for an antiferromagnetically coupled chain of S nxa0=xa01/2 local spins resulted in the parameters J nxa0=xa0 n−4.3 cm−1, g nxa0=xa02.1 and R nxa0=xa04.0xa0×xa010−4. Temperature dependent susceptibility data for 2 were fitted using the Bleaney–Bowers expression for a Cu(II) dimer, which n led to the parameters J nxa0=xa0 n+2.4 cm−1, g nxa0=xa02.11 and ρ nxa0=xa00.003.

Structural and Magnetic Characterization of a μ-1,5-Dicyanamide-Bridged Iron Basic Carboxylate [Fe3O(O2C(CH3)3)6] 1D Chain

We are reporting an unprecedented example of a mu-1,5-dicyanamide (dca)-bridged iron basic carboxylate, [Fe3O(O2C(CH3)3)6], 1D chain. As revealed from X-ray determination, the Fe3O cores are arranged in a zigzag configuration along the chain and strictly aligned in the same plane. The chains are well-isolated by the bulky tert-butyl groups. Magnetic measurements showed that the Fe3O units are weakly antiferromagnetically coupled (J = -0.6 cm(-1)) through the dca ligand while possessing a well-isolated S = 1/2 spin ground state arising from competing antiferromagnetic interactions.

Syntheses, Structures, Magnetic Properties, and Density Functional Theory Magneto-Structural Correlations of Bis(μ-phenoxo) and Bis(μ-phenoxo)-μ-acetate/Bis(μ-phenoxo)-bis(μ-acetate) Dinuclear FeIIINiII Compounds

The bis(μ-phenoxo) Fe(III)Ni(II) compound [Fe(III)(N3)2LNi(II)(H2O)(CH3CN)](ClO4) (1) and the bis(μ-phenoxo)-μ-acetate/bis(μ-phenoxo)-bis(μ-acetate) Fe(III)Ni(II) compound {[Fe(III)(OAc)LNi(II)(H2O)(μ-OAc)](0.6)·[Fe(III)LNi(II)(μ-OAc)2](0.4)}(ClO4)·1.1H2O (2) have been synthesized from the Robson type tetraiminodiphenol macrocyclic ligand H2L, which is the [2 + 2] condensation product of 4-methyl-2,6-diformylphenol and 2,2-dimethyl-1,3-diaminopropane. Single-crystal X-ray structures of both compounds have been determined. The cationic part of the dinuclear compound 2 is a cocrystal of the two species [Fe(III)(OAc)LNi(II)(H2O)(μ-OAc)](+) (2A) and [Fe(III)LNi(II)(μ-OAc)2](+) (2B) with weights of 60% of the former and 40% of the latter. While 2A is a triply bridged bis(μ-phenoxo)-μ-acetate system, 2B is a quadruply bridged bis(μ-phenoxo)-bis(μ-acetate) system. Variable-temperature (2-300 K) magnetic studies reveal antiferromagnetic interaction in 1 and ferromagnetic interaction in 2 with J values of -3.14 and 7.36 cm(-1), respectively (H = -2JS1·S2). Broken-symmetry density functional calculations of exchange interaction have been performed on complexes 1 and 2 and also on previously published related compounds, providing good numerical estimates of J values in comparison to experiments. The electronic origin of the difference in magnetic behavior of 1 and 2 has been well understood from MO analyses and computed overlap integrals of BS empty orbitals. The role of acetate and thus its complementarity/countercomplementarity effect on the magnetic properties of diphenoxo-bridged Fe(III)Ni(II) compounds have been determined on computing J values of model compounds by replacing bridging acetate and nonbridging acetate ligand(s) by water ligands in the model compounds derived from 2A,B. The DFT calculations have also been extended to develop several magneto-structural correlations in these types of complexes, and the correlations focus on the role of Fe-O-Ni bridge angle, average Fe/Ni-O bridge distance, Fe-O-Ni-O dihedral angle, and out-of-plane shift of the phenoxo group.

A unique series of dinuclear transition metal–polyradical complexes with a m-phenylenediamine spacer and their catalytic reactivity

A series of dinuclear transition metal complexes with either six or four iminosemiquinone radicals, in which the metal centres are separated by a distance of approximately 6.8 A, together with their catalytic reactivity is reported.

Copper complexes with mono- and bidentate-bridging nitronyl nitroxide-substituted benzoate ligands

Carboxylate-bridged copper complexes [Cu2(μ-L)4(S)2] 1 and 2 (S=solvent) with pendant nitronyl nitroxide (NIT) radical substituents have been prepared by ligand exchange reaction from copper(II) acetate with 4- and 3-carboxyphenyl-NIT (HL1 and HL2), respectively. Further reaction of 1 with pyridine leads to a monomeric copper complex [Cu(L1)2(py)2(H2O)] (3). Crystal structure determinations of 1 and 2 show a dimeric structure with Cu⋯Cu distances of 2.629(1) and 2.645(3) A. Four nitronyl nitroxide ligands L1 or L2 are coordinated to the dicopper core. Magnetic susceptibility measurements on powdered samples reveal dominant exchange interactions between the two copper centers with JCu–Cu=−150 cm−1 (1, S=H2O, dmso), and JCu–Cu=–140 cm−1 (2b, S=CH3CN). Weaker intermolecular interactions were accounted for with Weiss constants. The benzoic acid group in L1 and L2 does not mediate effectively an exchange interaction between the pendant nitronyl nitroxide and the copper ions coordinated to the carboxylate group. These experimental findings can be rationalized by DFT calculation of the spin density in L1 and L2, which shows the unpaired electron to be localized on the NO-groups with little contribution from the phenyl ring and the spin density on the carboxylate group to be negligible.

A Series of MIICuII3 Stars (M = Mn, Ni, Cu, Zn) Exhibiting Unusual Magnetic Properties

The work in this report describes the syntheses, electrospray ionization mass spectromtery, structures, and experimental and density functional theoretical (DFT) magnetic properties of four tetrametallic stars of composition [M(II)(Cu(II)L)3](ClO4)2 (1, M = Mn; 2, M = Ni; 3, M = Cu; 4, M = Zn) derived from a single-compartment Schiff base ligand, N,N-bis(salicylidene)-1,4-butanediamine (H2L), which is the [2 + 1] condensation product of salicylaldehyde and 1,4-diaminobutane. The central metal ion (Mn(II), Ni(II), Cu(II), or Zn(II)) is linked with two μ2-phenoxo bridges of each of the three [Cu(II)L] moieties, and thus the central metal ion is encapsulated in between three [Cu(II)L] units. The title compounds are rare or sole examples of stars having these metal-ion combinations. In the cases of 1, 3, and 4, the four metal ions form a centered isosceles triangle, while the four metal ions in 2 form a centered equilateral triangle. Both the variable-temperature magnetic susceptibility and variable-field magnetization (at 2-10 K) of 1-3 have been measured and simulated contemporaneously. While the Mn(II)Cu(II)3 compound 1 exhibits ferromagnetic interaction with J = 1.02 cm(-1), the Ni(II)Cu(II)3 compound 2 and Cu(II)Cu(II)3 compound 3 exhibit antiferromagnetic interaction with J = -3.53 and -35.5 cm(-1), respectively. Variable-temperature magnetic susceptibility data of the Zn(II)Cu(II)3 compound 4 indicate very weak antiferromagnetic interaction of -1.4 cm(-1), as expected. On the basis of known correlations, the magnetic properties of 1-3 are unusual; it seems that ferromagnetic interaction in 1 and weak/moderate antiferromagnetic interaction in 2 and 3 are possibly related to the distorted coordination environment of the peripheral copper(II) centers (intermediate between square-planar and tetrahedral). DFT calculations have been done to elucidate the magnetic properties. The DFT-computed J values are quantitatively (for 1) or qualitatively (for 2 and 3) matched well with the experimental values. Spin densities and magnetic orbitals (natural bond orbitals) correspond well with the trend of observed/computed magnetic exchange interactions.