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1.
Inorg Chem ; 58(24): 16434-16444, 2019 Dec 16.
Artigo em Inglês | MEDLINE | ID: mdl-31749359

RESUMO

Accurate determination of the spin Hamiltonian parameters in transition-metal complexes with large zero-field splitting (ZFS) is an actual challenge in studying magnetic and spectroscopic properties of high-spin transition metal complexes. Recent critical papers have convincingly shown that previous determinations of these parameters, based only on the magnetic data, have low accuracy and reliability. A combination of X-band electron paramagnetic resonance (EPR) spectroscopy and SQUID magnetometry seems to be a more convincing and accurate approach. However, even in this case, the accuracy of the determination of the spin Hamiltonian parameters is strongly limited. In this work, we propose a purely spectroscopic approach, in which three complementary EPR spectroscopic techniques are used to unambiguously with high accuracy determine the spin Hamiltonian parameters for transition-metal complexes with S = 3/2. The applicability of this approach is demonstrated by analyzing the new quasi-octahedral high-spin Co(II) complex [Co(hfac)2(bpy)] (I). Along with the conventional X-band EPR spectroscopy, we also use such advanced techniques as multi-high-frequency EPR spectroscopy (MHF-EPR) and frequency-domain Fourier-transform THz-EPR (FD-FT THz-EPR). We demonstrate that the experimental data derived from the X-band and MHF-EPR EPR spectra allow determination of the g tensor (gx = 2.388, gy = 2.417, gz = 2.221) and the ZFS rhombicity parameter E/D = 0.158. The axial ZFS parameter D = 37.1 cm-1 is measured for I with the aid of FD-FT THZ-EPR spectroscopy, which is able to detect the high-energy EPR transition between the two Kramers doublets. CASSCF/NEVPT2 quantum-chemical calculations of magnetic parameters and magnetic direct current (dc) measurements are performed as well as testing options, and the results obtained in these ways are in good agreement with those derived using the proposed spectroscopic approach.

2.
Inorg Chem ; 57(4): 2013-2022, 2018 Feb 19.
Artigo em Inglês | MEDLINE | ID: mdl-29419288

RESUMO

Layered double hydroxides (LDHs) exhibit unparalleled anion exchange properties and the ability to be exfoliated into 2D nanosheets, which can be used as a building block to fabricate a wide variety of hybrid functional nanostructured materials. Still, if one wants to use LDHs as a magnetic building blocks in the design of complex architectures, the role played by the dipolar magnetic interactions in these layered materials needs to be understood. In this work, we synthesized and characterized a five-membered CoAl-LDH series with basal spacing ranging from 7.5 to 34 Å. A detailed experimental characterization allows us to conclude that the main factor governing the dipolar interactions between magnetic layers cannot be the interlayer spacing. Supporting theoretical modeling suggests instead a relevant role for spin correlation size, which, in the limit, is related to the lateral dimension of the layer. These results highlight the importance of cation ordering in the magnetic behavior of LDHs, and underpin the differences with homometallic-layered hydroxides.

3.
Phys Chem Chem Phys ; 19(38): 26098-26106, 2017 Oct 04.
Artigo em Inglês | MEDLINE | ID: mdl-28929159

RESUMO

In this paper, we propose a toy model to describe the magnetic coupling between the localized spins mediated by the itinerant electron in partially delocalized mixed-valence (MV) systems. This minimal model takes into account the key interactions that are common for all such systems, namely, electron transfer in the valence-delocalized moiety and magnetic exchange between the localized spins and the delocalized electrons. The proposed descriptive model is exactly solvable which allows us to qualitatively and quantitatively discuss the main features of the whole class of partially delocalized MV systems. In the case of relatively strong exchange coupling, the combined action of these two interactions is shown to give rise to a specific kind of double exchange coupling termed here as "external core" double exchange. In the opposite case of relatively strong electron transfer, the general Hamiltonian is shown to be reduced to the effective Hamiltonian of indirect exchange of the localized spins. We argue a possibility to efficiently control the magnetic coupling of the localized spins using an external electric field acting on the delocalized part of the system. Finally, we discuss the perspectives of the present model for molecular spintronics and spin qubits.

4.
Inorg Chem ; 56(16): 9547-9554, 2017 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-28771336

RESUMO

As part of the search for systems in which control of quantum entanglement can be achieved, here we consider the paramagnetic mixed valence polyoxometalate K2Na6[GeV14O40]·10H2O in which two electrons are delocalized over the 14 vanadium ions. Applying a homogeneous electric field can induce an antiferromagnetic coupling between the two delocalized electronic spins that behave independently in the absence of the field. On the basis of the proposed theoretical model, we show that the external field can be used to generate controllable quantum entanglement between the two electronic spins traveling over a vanadium network of mixed valence polyoxoanion [GeV14O40]8-. Within a simplified two-level picture of the energy pattern of the electronic pair based on the previous ab initio analysis, we evaluate the temperature and field dependencies of concurrence and thus indicate that the entanglement can be controlled via the temperature, magnitude, and orientation of the electric field with respect to molecular axes of [GeV14O40]8-.

5.
Inorg Chem ; 55(18): 9361-7, 2016 Sep 19.
Artigo em Inglês | MEDLINE | ID: mdl-27583664

RESUMO

The synthesis and magnetostructural characterization of [Fe(III)3(µ3-O)(H2O)3[Fe(II)(bppCOOH)(bppCOO)]6](ClO4)13·(CH3)2CO)6·(solvate) (2) are reported. This compound is obtained as a secondary product during synthesis of the mononuclear complex [Fe(II)(bppCOOH)2](ClO4)2 (1). The single-crystal X-ray diffraction structure of 2 shows that it contains the nonanuclear cluster of the formula [Fe(III)3(µ3-O)(H2O)3[Fe(II)(bppCOOH)(bppCOO)]6](13+), which is formed by a central Fe(III)3O core coordinated to six partially deprotonated [Fe(II)(bppCOOH)(bppCOO)](+) complexes. Raman spectroscopy studies on single crystals of 1 and 2 have been performed to elucidate the spin and oxidation states of iron in 2. These studies and magnetic characterization indicate that most of the iron(II) complexes of 2 remain in the low-spin (LS) state and present a gradual and incomplete spin crossover above 300 K. On the other hand, the Fe(III) trimer shows the expected antiferromagnetic behavior. From the structural point of view, 2 represents the first example in which bppCOO(-) acts as a bridging ligand, thus forming a polynuclear magnetic complex.

6.
J Chem Theory Comput ; 12(8): 3545-60, 2016 Aug 09.
Artigo em Inglês | MEDLINE | ID: mdl-27398679

RESUMO

In this article, we focus on the electron-vibrational problem of the tetrameric mixed-valence (MV) complexes proposed for implementation as four-dot molecular quantum cellular automata (mQCA).1 Although the adiabatic approximation explored in ref 2 is an appropriate tool for the qualitative analysis of the basic characteristics of mQCA, like vibronic trapping of the electrons encoding binary information and cell-cell response, it loses its accuracy providing moderate vibronic coupling and fails in the description of the discrete pattern of the vibronic levels. Therefore, a precise solution of the quantum-mechanical vibronic problem is of primary importance for the evaluation of the shapes of the electron transfer optical absorption bands and quantitative analysis of the main parameters of tetrameric quantum cells. Here, we go beyond the Born-Oppenheimer paradigm and present a solution of the quantum-mechanical pseudo Jahn-Teller (JT) vibronic problem in bielectronic MV species (exemplified by the tetra-ruthenium complexes) based on the recently developed symmetry-assisted approach.3,4 The mathematical approach to the vibronic eigenproblem takes into consideration the point symmetry basis, and therefore, the total matrix of the JT Hamiltonian is blocked to the maximum extent. The submatrices correspond to the irreducible representations (irreps) of the point group. With this tool, we also extend the theory of the mQCA cell beyond the limit of prevailing Coulomb repulsion in the electronic pair (adopted in ref 2), and therefore, the general pseudo-JT problems for spin-singlet ((1)B1g, 2(1)A1g, (1)B2g, (1)Eu) ⊗ (b1g + eu) and spin-triplet states ((3)A2g, (3)B1g, 2(3)Eu) ⊗ (b1g + eu) in a square-planar bielectronic system are solved. The obtained symmetry-adapted electron-vibrational functions are employed for the calculation of the profiles (shape functions) of the charge transfer absorption bands in the tetrameric MV complexes and for the discussion of the magnetic properties.

7.
J Chem Phys ; 143(13): 134307, 2015 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-26450314

RESUMO

Our interest in this article is prompted by the vibronic problem of charge polarized states in the four-dot molecular quantum cellular automata (mQCA), a paradigm for nanoelectronics, in which binary information is encoded in charge configuration of the mQCA cell. Here, we report the evaluation of the electronic levels and adiabatic potentials of mixed-valence (MV) tetra-ruthenium (2Ru(ii) + 2Ru(iii)) derivatives (assembled as two coupled Creutz-Taube complexes) for which molecular implementations of quantum cellular automata (QCA) was proposed. The cell based on this molecule includes two holes shared among four spinless sites and correspondingly we employ the model which takes into account the two relevant electron transfer processes (through the side and through the diagonal of the square) as well as the difference in Coulomb energies for different instant positions of localization of the hole pair. The combined Jahn-Teller (JT) and pseudo JT vibronic coupling is treated within the conventional Piepho-Krauzs-Schatz model adapted to a bi-electronic MV species with the square-planar topology. The adiabatic potentials are evaluated for the low lying Coulomb levels in which the antipodal sites are occupied, the case just actual for utilization in mQCA. The conditions for the vibronic self-trapping in spin-singlet and spin-triplet states are revealed in terms of the two actual transfer pathways parameters and the strength of the vibronic coupling. Spin related effects in degrees of the localization which are found for spin-singlet and spin-triplet states are discussed. The polarization of the cell is evaluated and we demonstrate how the partial delocalization caused by the joint action of the vibronic coupling and electron transfer processes influences polarization of a four-dot cell. The results obtained within the adiabatic approach are compared with those based on the numerical solution of the dynamic vibronic problem. Finally, the Coulomb interaction between the cells is considered and the influence of the vibronic coupling on the shape on the non-linear cell-cell response function is revealed.

8.
J Chem Phys ; 139(4): 044304, 2013 Jul 28.
Artigo em Inglês | MEDLINE | ID: mdl-23901977

RESUMO

We propose a microscopic analytical approach to the description of the low-temperature dissipative intracluster electron transfer dynamics in centrosymmetric one-electron mixed-valence (MV) dimers. The dissipative system (bath) is supposed to consist of the acoustic phonons of the crystal surrounding that are coupled to the delocalized electron(s) of a MV dimer. Although the concept of the bath is the spin-boson model is more generic, the present consideration is relevant, for example, to a MV bi-center impurity in an ionic crystal. The model allows us to develop an approximate microscopic approach within which the relaxation processes are explicitly taken into account without additional assumption regarding spectral function of the bath. It is assumed that initially the extra electron is localized on a certain center and then the time-dependent localization probability (averaged value of the electron dipole moment) is evaluated with the emphasis on the damping of the amplitude of the Rabi oscillations. The approach assumes the following conditions: (i) the vibrational spectrum of the crystal does not show the presence of local modes; (ii) the itinerant electron is weakly coupled to the long-waves acoustic phonons which is peculiar to fully delocalized Robin and Day class III MV systems; (iii) the Debye energy ℏωD exceeds the electronic resonance energy gap 2ß (ß is the electron transfer parameter). We have demonstrated that the dissipation in this case is super-ohmic with the low-frequency spectral function J(ω) ∝ ω(5). The time dependences of the localization probabilities show nearly picosecond damped oscillations. The longitudinal relaxation time T1 has been shown to be two times shorter than the decoherence time T2 thus giving the upper bound for T2, T2 ≤ 2T1.

9.
Inorg Chem ; 51(2): 985-1001, 2012 Jan 16.
Artigo em Inglês | MEDLINE | ID: mdl-22220521

RESUMO

Two new trinuclear copper(II) complexes, [Cu(3)(µ(3)-OH)(daat)(Hdat)(2)(ClO(4))(2)(H(2)O)(3)](ClO(4))(2)·2H(2)O (1) and [Cu(3)(µ(3)-OH)(aaat)(3)(H(2)O)(3)](ClO(4))(2)·3H(2)O (2) (daat = 3,5-diacetylamino-1,2,4-triazolate, Hdat = 3,5-diamino-1,2,4-triazole, and aaat = 3-acetylamino-5-amino-1,2,4-triazolate), have been prepared from 1,2,4-triazole derivatives and structurally characterized by X-ray crystallography. The structures of 1 and 2 consist of cationic trinuclear copper(II) complexes with a Cu(3)OH core held by three N,N-triazole bridges between each pair of copper(II) atoms. The copper atoms are five-coordinate with distorted square-pyramidal geometries. The magnetic properties of 1 and 2 and those of five other related 1,2,4-triazolato tricopper(II) complexes with the same triangular structure (3-7) (whose crystal structures were already reported) have been investigated in the temperature range of 1.9-300 K. The formulas of 3-7 are [Cu(3)(µ(3)-OH)(aaat)(3)(H(2)O)(3)](NO(3))(2)·H(2)O (3), {[Cu(3)(µ(3)-OH)(aat)(3)(µ(3)-SO(4))]·6H(2)O}(n) (4), and [Cu(3)(µ(3)-OH)(aat)(3)A(H(2)O)(2)]A·xH(2)O [A = NO(3)(-) (5), CF(3)SO(3)(-) (6), or ClO(4)(-) (7); x = 0 or 2] (aat =3-acetylamino-1,2,4-triazolate). The magnetic and electron paramagnetic resonance (EPR) data have been analyzed by using the following isotropic and antisymmetric exchange Hamiltonian: H = -J[S(1)S(2) + S(2)S(3)] - j[S(1)S(3)] + G[S(1) × S(2) + S(2) × S(3) + S(1) × S(3)]. 1-7 exhibit strong antiferromagnetic coupling (values for both -J and -j in the range of 210-142 cm(-1)) and antisymmetric exchange (G varying from to 27 to 36 cm(-1)). At low temperatures, their EPR spectra display high-field (g < 2.0) signals indicating that the triangles present symmetry lower than equilateral and that the antisymmetric exchange is operative. A magneto-structural study showing a lineal correlation between the Cu-O-Cu angle of the Cu(3)OH core and the isotropic exchange parameters (J and j) has been conducted. Moreover, a model based on Moriya's theory that allows the prediction of the occurrence of antisymmetric exchange in the tricopper(II) triangles, via analysis of the overlap between the ground and excited states of the local Cu(II) ions, has been proposed. In addition, analytical expressions for evaluating both the isotropic and antisymmetric exchange parameters from the experimental magnetic susceptibility data of triangular complexes with local spins (S) of (1)/(2), (3)/(2), or (5)/(2) have been purposely derived. Finally, the magnetic and EPR results of this work are discussed and compared with those of other tricopper(II) triangles reported in the literature.


Assuntos
Cobre/química , Compostos Organometálicos/química , Cristalografia por Raios X , Espectroscopia de Ressonância de Spin Eletrônica , Ligantes , Magnetismo , Modelos Moleculares , Estrutura Molecular , Temperatura , Triazóis/química
10.
Chemistry ; 17(46): 13032-43, 2011 Nov 11.
Artigo em Inglês | MEDLINE | ID: mdl-22167882

RESUMO

Three high-nuclearity Ni-substituted polyoxotungstates (POTs)--[Ni(enMe)2(H2O)2]2[Ni(H2O)6]2[Ni(enMe)2][Ni(H2O)2]1.5[HNi20X4W34(OH)4O136(H2O)6(enMe)8]·11 H2O (3), [Ni(en)2(H2O)]2[H8Ni21X4W34(OH)4O136(en)10(H2O)5]·22 H2O (4), and [Ni(enMe)2]2[H6Ni22X4W34(OH)4O136(H2O)6(enMe)10]·18 H2O (5), in which en = ethylenediamine, enMe = 1,2-diaminopropane, X = 0.5 P+0.5 Ge--were made under hydrothermal conditions and characterized by IR spectroscopy, elemental analysis, thermogravimetric analysis, powder X-ray diffraction, and single-crystal X-ray diffraction. The structures of 3­5 can be viewed as novel derivatives of [H6Ni20P4W34(OH)4O136(enMe)8(H2O)6]·12 H 2O (1) and [Ni(en)2(H2O)]2[H8Ni20P4W34(OH)4O136(en)9(H2O)4]·16 H 2O (2), which both contain 20 nickel ions per structural unit. Compound 3 is the first example of a 1D cluster chain constructed from Ni20-substituted polyanions [HNi20X4 W34(OH)4O136(H2O)6(enMe)8]11− and [Ni(enMe)2]2+ bridges. Compound 4 is a novel cluster­organic chain built by Ni21-substituted polyanions [H8Ni21X4W34(OH)4O136(en)10(H2O)5]4− and en molecule bridges. Compound 5 is a discrete POT with 22 Ni centers, and is not only the largest nickel-substituted POT, but also contains the highest number of nickel ions in one polyanion to date. Magnetic measurements illustrate that overall ferromagnetic interactions exist in 1­5. The magnetic behavior of 1 and 2 was theoretically simulated by the MAGPACK magnetic program package.


Assuntos
Níquel/química , Compostos de Tungstênio/síntese química , Cristalografia por Raios X , Conformação Molecular , Estrutura Molecular , Espectrofotometria Infravermelho , Compostos de Tungstênio/química
13.
Inorg Chem ; 48(7): 3027-38, 2009 Apr 06.
Artigo em Inglês | MEDLINE | ID: mdl-19275219

RESUMO

The use of the mononuclear species (NBu(4))(2)[Re(IV)Cl(4)(ox)] (NBu(4)(+) = tetra-n-butylammonium cation; ox = oxalate dianion) as a ligand toward fully solvated divalent first-row transition-metal ions affords the tetranuclear complexes (NBu(4))(4)[{Re(IV)Cl(4)(mu-ox)}(3)M(II)] with M = Mn (1), Fe (2), Co (3), Ni (4), and Cu (5). Their structure is made up of discrete [{ReCl(4)(mu-ox)}(3)M](4-) anions and bulky NBu(4)(+) cations. The complexes 2-5 crystallize in the triclinic system with space group P1; 2 and 5 as well as 3 and 4 are isostructural. The Re and M atoms exhibit somewhat distorted ReCl(4)O(2) and MO(6) octahedral surroundings, with the oxalate groups adopting the bis-bidentate bridging mode. Magnetic susceptibility measurements on polycrystalline samples of 1-5 in the temperature range 1.9-300 K show the occurrence of intramolecular antiferromagnetic [J = -1.30 cm(-1) (1)] and ferromagnetic couplings [J = +1.62 (2), +3.0 (3), +16.3 (4), and +4.64 cm(-1) (5)], with the Hamiltonian being defined as H = -J[S(M)(S(Re1) + S(Re2) + S(Re3))]. Compound 4 is the first example of an oxalato-bridged heterometallic species that behaves as a single-molecule magnet with a ground-state spin S = (11)/(2) and D = -0.8(1) cm(-1), as shown by the study of its static and dynamic magnetic properties and a high-frequency electron paramagnetic resonance study on polycrystalline samples together with detailed micro-SQUID measurements on single crystals.

14.
Inorg Chem ; 47(23): 11314-23, 2008 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-18937442

RESUMO

Complexes [Fe(Hhbi)(2)(NO(3))].2EtOH (1.2EtOH) and [Fe(2)(mu-OH)(2)(Hhbi)(4)].2H(2)O.8EtOH (2.2H(2)O.8EtOH) crystallize in the orthorhombic Fdd2 and P4(2)2(1)2 space groups, respectively (Hhbi(-) = the monoanion of 2-(2'-hydroxyphenyl benzimidazole). Complex 1 exhibits paramagnetic relaxation as evidenced by Mossbauer spectroscopy, and significant axial zero-field splitting (1.5 cm(1)

Assuntos
Ferro/química , Compostos Organometálicos/química , Benzimidazóis/química , Cristalografia por Raios X , Espectroscopia de Ressonância de Spin Eletrônica , Magnetismo , Espectrofotometria Infravermelho , Espectroscopia de Mossbauer , Termogravimetria
15.
Chemistry ; 14(8): 2514-26, 2008.
Artigo em Inglês | MEDLINE | ID: mdl-18213674

RESUMO

Complexes [Fe9(X)2-(O2CMe)8{(2-py)2CO2}4] (X(-)=OH(-) (1), N3(-) (2), and NCO(-) (3)) have been prepared by a route previously employed for the synthesis of analogous Co(9) and Ni(9) complexes, involving hydroxide substitution by pseudohalides (N3(-), NCO(-)). As indicated by DC magnetic susceptibility measurements, this substitution induced higher ferromagnetic couplings in complexes 2 and 3, leading to higher ground spin states compared to that of 1. Variable-field experiments have shown that the ground state is not well isolated from excited states, as a result of which it cannot be unambiguously determined. AC susceptometry has revealed out-of-phase signals, which suggests that these complexes exhibit a slow relaxation of magnetization that follows Arrhenius behavior, as observed in single-molecule magnets, with energy barriers of 41 K for 2 (tau 0=3.4 x 10(-12) s) and 44 K for 3 (tau 0=2.0 x 10(-11) s). Slow magnetic relaxation has also been observed by zero-field 57Fe Mössbauer spectroscopy. Characteristic integer-spin electron paramagnetic resonance (EPR) signals have been observed at X-band for 1, whereas 2 and 3 were found to be EPR-silent at this frequency. 1H NMR spectrometry in CD3CN has shown that complexes 1-3 are stable in solution.

16.
Chemistry ; 13(12): 3525-36, 2007.
Artigo em Inglês | MEDLINE | ID: mdl-17226868

RESUMO

Five Co(II) silicotungstate complexes are reported. The centrosymmetric heptanuclear compound K(20)[{(B-beta-SiW(9)O(33)(OH))(beta-SiW(8)O(29)(OH)(2))Co(3)(H(2)O)}(2)Co(H(2)O)(2)]47 H(2)O (1) consists of two {(B-beta-SiW(9)O(33)(OH))(beta-SiW(8)O(29)(OH)(2))Co(3)(H(2)O)} units connected by a {CoO(4)(H(2)O)(2)} group. In the chiral species K(7)[Co(1.5)(H(2)O)(7))][(gamma-SiW(10)O(36))(beta-SiW(8)O(30)(OH))Co(4)(OH)(H(2)O)(7)]36 H(2)O (2), a {gamma-SiW(10)O(36)} and a {beta-SiW(8)O(30)(OH)} unit enclose a mononuclear {CoO(4)(H(2)O)(2)} group and a {Co(3)O(7)(OH)(H(2)O)(5)} fragment. The two trinuclear Co(II) clusters present in 1 enclose a mu(4)-O atom, while in 2 a mu(3)-OH bridging group connects the three paramagnetic centers of the trinuclear unit, inducing significantly larger Co-L-Co (L=mu(4)-O (1), mu(3)-OH (2)) bridging angles in 2 (theta(av(Co-L-Co))=99.1 degrees ) than in 1 (theta(av(Co-L-Co))=92.8 degrees ). Weaker ferromagnetic interactions were found in 2 than in 1, in agreement with larger Co-L-Co angles in 2. The electrochemistry of 1 was studied in detail. The two chemically reversible redox couples observed in the positive potential domain were attributed to the redox processes of Co(II) centers, and indicated that two types of Co(II) centers in the structure were oxidized in separate waves. Redox activity of the seventh Co(II) center was not detected. Preliminary experiments indicated that 1 catalyzes the reduction of nitrite and NO. Remarkably, a reversible interaction exists with NO or related species. The hybrid tetranuclear complexes K(5)Na(3)[(A-alpha-SiW(9)O(34))Co(4)(OH)(3)(CH(3)COO)(3)]18 H(2)O (3) and K(5)Na(3)[(A-alpha-SiW(9)O(34))Co(4)(OH)(N(3))(2)(CH(3)COO)(3)]18 H(2)O (4) were characterized: in both, a tetrahedral {Co(4)(L(1))(L(2))(2)(CH(3)COO)(3)} (3: L(1)=L(2)=OH; 4: L(1)=OH, L(2)=N(3)) unit capped the [A-alpha-SiW(9)O(34)](10-) trivacant polyanion. The octanuclear complex K(8)Na(8)[(A-alpha-SiW(9)O(34))(2)Co(8)(OH)(6)(H(2)O)(2)(CO(3))(3)]52 H(2)O (5), containing two {Co(4)O(9)(OH)(3)(H(2)O)} units, was also obtained. Compounds 2, 3, 4, and 5 were less stable than 1, but their partial electrochemical characterization was possible; the electronic effect expected for 3 and 4 was observed.

17.
Inorg Chem ; 45(8): 3240-8, 2006 Apr 17.
Artigo em Inglês | MEDLINE | ID: mdl-16602780

RESUMO

A new iron(III) phosphite templated by ethylenediamine has been synthesized using solvothermal conditions under autogenous pressure. The (C2N2H10)[Fe(HPO3)F3] compound has been characterized by single-crystal X-ray diffraction data and spectroscopic and magnetic techniques. The crystal structure is formed by chains extended along the c axis and surrounded by ethylenediammonium cations. A study by diffuse-reflectance spectroscopy has been performed, and the calculated Dq, B, and C parameters for the Fe(III) cations are 1030, 720, and 3080 cm(-1), respectively. The Mössbauer spectrum at room temperature is characteristic of Fe(III) ions. The electron spin resonance (ESR) spectra carried out at different temperatures show isotropic signals with a g value of 2.00(1). The thermal evolution of the intensity of the ESR signals indicates the existence of antiferromagnetic interactions for the Fe(III) phase. The magnetic susceptibility data of the Cr(III) and V(III) compounds show antiferromagnetic couplings. The J-exchange parameters of the Fe(III) and Cr(III) compounds have been calculated by using a model for a triangular spin ladder chain. The values are J1 = -1.63(1) K and J2 = -0.87(2) K with g = 2.02 for the Fe(III) phase and J(1) = -0.56(2) K and J2 = -0.40(2) K with g = 1.99 for the Cr(III) compound. In the case of the V(III) phase, the fit has been performed considering a linear chain with the magnetic parameters D = 2.5 cm(-1) and J = -1.15(1) K.

18.
Inorg Chem ; 45(5): 2317-26, 2006 Mar 06.
Artigo em Inglês | MEDLINE | ID: mdl-16499398

RESUMO

The use of salicylaldehyde oxime (H2salox) in iron(III) carboxylate chemistry has yielded two new hexanuclear compounds [Fe6(mu3-O)2(O2CPh)10(salox)2(L)2].xMeCN.yH2O [L = MeCONH2, x = 6, y = 0 (1); L = H2O, x = 2, y = 3 (2)]. Compound 1 crystallizes in the triclinic space group P with (at 25 degrees C) a = 13.210(8) A, b = 13.87(1) A, c = 17.04(1) A, alpha = 105.79(2) degrees , beta = 96.72(2) degrees , gamma = 116.69(2) degrees , V = 2578.17(2) A(3), and Z = 1. Compound 2 crystallizes in the monoclinic space group C2/c with (at 25 degrees C) a = 21.81(1) A, b = 17.93(1) A, c = 27.72(1) A, beta = 111.70(2) degrees , V = 10070(10) A(3), and Z = 4. Complexes 1 and 2 contain the [Fe6(mu3-O)2(mu2-OR)2]12+ core and can be considered as two [Fe3(mu3-O)] triangular subunits linked by two mu2-oximato O atoms of the salox2- ligands, which show the less common mu3:eta1:eta2:eta1 coordination mode. The benzoato ligands are coordinated through the usual syn,syn-mu2:eta1:eta1 mode. The terminal MeCONH2 ligand in 1 is the hydrolysis product of the acetonitrile solvent in the presence of the metal ions. Mössbauer spectra from powdered samples of 2 give rise to two well-resolved doublets with an average isomer shift consistent with that of high-spin Fe(III) ions. The two doublets, at an approximate 1:2 ratio, are characterized by different quadrupole splittings and are assigned to the nonequivalent Fe(III) ions of the cluster. Magnetic measurements of 2 in the 2-300 K temperature range reveal antiferromagnetic interactions between the Fe(III) ions, stabilizing an S = 0 ground state. NMR relaxation data have been used to investigate the energy separation between the low-lying states, and the results are in agreement with the susceptibility data.


Assuntos
Compostos de Ferro/química , Química Inorgânica/métodos , Cristalografia por Raios X , Compostos de Ferro/síntese química , Espectroscopia de Ressonância Magnética , Magnetismo , Estrutura Molecular , Espectroscopia de Mossbauer
19.
Dalton Trans ; (11): 2031-7, 2005 Jun 07.
Artigo em Inglês | MEDLINE | ID: mdl-15909055

RESUMO

N,N'-Bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L) is a versatile receptor able to adapt to the coordinative preferences of different metal cation guests. With first-row transition metal ions, L tends to form binuclear complexes but, depending on the nature of the particular metal ion, the structure of the binuclear complex may be very different. Herein we report a study of the structure and magnetic properties of the corresponding nickel(II) and cobalt(II) complexes. The X-ray crystal structure of the nickel complex (1), with formula [Ni2(L)(CH3CN)4](ClO4)4.CH3CN, shows that this compound presents a symmetric coordination environment with L adopting an anti arrangement. Each Ni(II) ion is six-coordinate in a distorted octahedral environment, and both metal ions are quite far from each other. On the other hand, the X-ray crystal structure of the cobalt complex (2), with formula [Co(L)(mu-OH)Co(CH3CN)](ClO4)3, reveals a rather different structure. Coordination number asymmetry is found: one of the Co(II) is five-coordinate in a distorted trigonal-bipyramidal coordination environment, while the second Co(II) ion is six-coordinate in a distorted octahedral arrangement. Now L adopts a syn arrangement and a hydroxide group acts as a bridge between both cobalt ions. This hydroxo-bridged Co(II) binuclear complex shows structural features that mimic the active site of methionine aminopeptidases. The magnetic properties of 1 and 2 have been investigated in the temperature range 2.0-300 K. Whereas 1 displays a Curie law except for temperatures below 50 K where zero-field splitting of the S= 1 ground state is observed, antiferromagnetic exchange in the singular asymmetric binuclear Co(II) complex 2 has been observed. This magnetic behaviour has been fitted considering first-order spin-orbit coupling in the assumed axially distorted octahedral site and totally quenched orbital contribution in the five-coordinate site in which zero-field splitting of the S= 3/2 ground state is operative.


Assuntos
Compostos Aza/química , Cobalto/química , Éteres de Coroa/química , Éteres de Coroa/síntese química , Magnetismo , Níquel/química , Compostos Organometálicos/química , Cátions , Cristalografia por Raios X , Modelos Moleculares , Temperatura
20.
Inorg Chem ; 44(10): 3389-95, 2005 May 16.
Artigo em Inglês | MEDLINE | ID: mdl-15877418

RESUMO

The magnetic exchange interactions in a C0(3)(11) moiety encapsulated in Na(17) [(NaOH(2))Co(3)(H(2)O)(P(2)W(15)O(56))(2)] (NaCo(3)) were studied by a combination of magnetic measurements (magnetic susceptibility and low-temperature magnetization), with a detailed Inelastic Neutron Scattering (INS) investigation. The novel structure of the salt was determined by X-ray crystallography. The ferromagnetic Co(3)O(14) triangular cluster core consists of three octahedrally oxo-coordinated Co(II) ions sharing edges. According to the single-ion anisotropy and spin-orbit coupling usually assumed for octahedral Co(II) ions, the appropiate exchange Hamiltonian to describe the ground-state properties of the isosceles triangular Co(3) spin cluster is anisotropic and is expressed as H = - 2sigma(alpha)(=)(x,y,z)(J(alpha)(12)S(1alpha)S(2alpha) + J(alpha)(23)S(2alpha)S(3alpha) + J(alpha)(13)S(1alpha)S(3alpha)), where J(alpha) are the components of the exchange interactions between the Co(II) ions. To reproduce the INS data, nonparallel anisotropic exchange tensors needed to be introduced, which were directly connected to the molecular symmetry of the complex. The following range of parameters (value +/- 0.5 cm(-1)) was found to reproduce all experimental information while taking magnetostructural relations into account: J(x)(12) = J(y)(13) = 8.6 cm(-1); J(y)(12) = J(x)(13) = 1.4 cm(-1); J(z)(12) = J(z)(13) = 10.0 cm(-1); J(x)(23) = J(y)(23) = 6.5 cm(-1) and = 3.4 cm(-1).

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