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Experimental and theoretical study of the influence of peripheral environment on magnetic properties of tetranuclear manganese skeleton in new representatives of calix[4]arene-containing [MnII2 MnIII2] clusters
Calix[4]arenes are versatile ligands, capable of supporting the formation of a wide variety of polymetallic clusters comprising 3d, 4f or 3d–4f metal ions. Calixarene-based metal ion fragments act as both bridging and structure capping moieties in these systems, and this behaviour is systematically extended upon moving to bis-calix[4]arene, a relatively new ligand in which two calix[4]arenes are tethered at the methylene bridge position. N,O-Ligands greatly influence cluster formation with bis-calix[4]arene, affording a remarkable mixed-valence [MnIV2MnIII10MnII8] cluster that displays coordination chemistry typical of each ligand type, but also new structure capping behaviour for the latter.
In the environment, the presence of toxic oxyanions such as Cr(VI) and As(V), especially in drinking water, creates serious hazards to human health. For efficient and selective detection of these species; novel calix[4]arene-based thiourea derivatives which obtained using p-tert-butylcalix[4]arene as starting material have been prepared from m-aniline or o-aniline or o-aminophenol in moderate yield. The structures of all new synthesized compounds obtained from these reactions were determined by using FTIR, ¹H, and ¹³C NMR spectroscopy. The complexing properties of calix[4]arene-based thiourea derivatives have been studied towards the As(V) and Cr(VI). It was found that calix[4]arene-based thiourea derivative 5,11,17,23-tetra-tert-butyl-25,27-bis(o-amino-phenylthioureido propoxy)-26,28-hydroxycalix[4]arene is an effective extractant for carrying HCr2O7²⁻ anions.
Using ab initio band structure and model calculations we studied magnetic
properties of one of the Mn$_4$ molecular magnets (Mn4(hmp)6), where two types
of the Mn ions exist: Mn3+ and Mn2+. The direct calculation of the exchange
constants in the GGA+U approximation shows that in contrast to a common belief
the strongest exchange coupling is not between two Mn3+ ions (J_{bb}), but
along two out of four exchange paths connecting Mn3+ and Mn2+ ions (J_{wb}).
The microscopic analysis performed within the perturbation theory allowed to
establish the mechanism for this largest ferromagnetic exchange constant. The
charge ordering of the Mn ions results in the situation when the energy of the
excited state in the exchange process is defined not by the large on-site
Coulomb repulsion U, but by much smaller energy V, which stabilizes the charge
ordered state. Together with strong Hund's rule coupling and specific orbital
order this leads to a large ferromagnetic exchange interaction for two out of
four Mn2+ --Mn3+ pairs.
The structure of tin zirconium trisulfide is of the NH4CdCl3 type with double columns of edge-sharing Zr octahedra. These columns are linked together by Sn atoms. Sn is coordinated to three S atoms at 2 x 2.619 (2) and 2.765 (2) angstrom; a fourth S atom is at 3.065 (2) angstrom. The zirconium coordination is approximately octahedral with six S atoms at 2.524 (2), 2 x 2.545 (2), 2.582 (2) and 2 x 2.592 (2) angstrom. Powder diffraction data were also collected; the JCPDS rue No. for tin zirconium sulfide is 44-1494.
A single configuration model containing nonorthogonal magnetic orbitals is developed to represent the important features of the antiferromagnetic state of a transition metal dimer. A state of mixed spin symmetry and lowered space symmetry is constructed which has both conceptual and practical computational value. Either unrestricted Hartree–Fock theory or spin polarized density functional theory, e.g., Xα theory, can be used to generate the mixed spin state wave function. The most important consequence of the theory is that the Heisenberg exchange coupling constant J can be calculated simply from the energies of the mixed spin state and the highest pure spin multiplet.
A new representative of calix[4]arene-containing tetranuclear manganese complexes of the [Mn-II Mn-2(III) (2)] type was obtained. According to the data of magnetoochemical studies, the complex exhibits properties of molecular magnet at the temperature below 5 K. Parameters of the exchange interaction and the activation energy were determined. The influence of the peripheral environment on the magnetic properties of the tetranuclear manganese framework in the structure of the complex was revealed.
The design, structural characterization and specific properties of (thia)calix[4]arene based supramolecular systems are considered.
Gut gemischt: Ein Grundzustandsspin von S=83/2 wurde für das gemischtvalente Manganaggregat [MnIII12MnII7(μ4-O)8(μ3,η1-N3)8(HL)12(MeCN)6]2+ nachgewiesen (H3L=2,6-Bis(hydroxymethyl)-4-methylphenol; der Kern ist in Polyederdarstellung gezeigt). Die Größe des Spins ist darauf zurückzuführen, dass alle Metallzentren ferromagnetisch koppeln.
MAGNETIC materials of mesoscopic dimensions (a few to many thousands of atoms) may exhibit novel and useful properties such as giant magnetostriction, magnetoresistivity and magnetocaloric effects14. Such materials also allow one to study the transition from molecular to bulk-like magnetic behaviour. One approach for preparing mesoscopic magnetic materials is to fragment bulk ferromagnets; a more controllable method is to take a 'bottom-up' approach, using chemistry to grow well defined clusters of metal ions5,6. Lis7 has described a twelve-ion manganese cluster in which eight of the Mn ions are in the +3 oxidation state (spin S=2) and four are in the +4 state (S=3/2). These ions are magnetically coupled to give an S=10 ground state8, giving rise to unusual magnetic relaxation properties8,9. Here we report that the magnetization of the Mn12 cluster is highly anisotropic and that the magnetization relaxation time becomes very long below a temperature of 4 K, giving rise to pronounced hysteresis. This behaviour is not, however, strictly analogous to that of a bulk ferromagnet, in which magnetization hysteresis results from the motion of domain walls. In principle, a bistable magnetic unit of this sort could act as a data storage device.
A first principles calculation of cluster exchange coupling constants (i.e., Jab in the total spin Hamiltonian Hex = -2 Σatom pairs,a,bJabSa·Sb) is attempted within the framework of the standard SCF-Χα-SW method. The results for the triply Cl bridged dimer Mo2Cl93- in the salts Cs3Mo2Cl9 (d(Mo-Mo) = 2.655 Å) and K3Mo2Cl9 (d(Mo-Mo) = 2.53 Å) are Jab-(calcd) = -355 and -1268 cm-1, respectively. The corresponding experimental values are -421 and -556 cm-1. From the calculated electronic structure of Mo2Cl93-, the exchange coupling mechanism is seen to be purely direct metal-metal interaction with no superexchange. The presence of Mo-Mo bonding which resides in a predominantly Mo-bridging Cl orbital, and is quite distinct from the coupling of the magnetic electrons, is also revealed by the calculations. It is concluded that, while there is much room for improvement, the standard SCF-Χα-SW method is useful in studying exchange coupling in clusters.
ORCA is a general-purpose quantum chemistry program package that features virtually all modern electronic structure methods (density functional theory, many-body perturbation and coupled cluster theories, and multireference and semiempirical methods). It is designed with the aim of generality, extendibility, efficiency, and user friendliness. Its main field of application is larger molecules, transition metal complexes, and their spectroscopic properties. ORCA uses standard Gaussian basis functions and is fully parallelized. The article provides an overview of its current possibilities and documents its efficiency. © 2011 John Wiley & Sons, Ltd.
Starting with spin polarized determinants for an antiferromagnetic transition metal dimer and spin projected states obtained from them, we show that both superexchange coupling and ligand spin polarization contribute to the Heisenberg coupling constant J describing a ladder of spin states. A single broken symmetry UHF calculation when combined with an independent calculation for the high-spin state yields J from the equation, E(Smax) − EB = − S2maxJ. Ligand spin polarization affects the average value of both spin-independent one-electron operators like the electron density at the metal site and spin-dependent operators such as A tensors at metal and ligand sites. The theoretical analysis here shows a number of new physical effects and generalizes the results of our previous paper which dealt solely with the effects of superexchange coupling. The limitations of this approach are also examined. A more comprehensive theory is readily formulated for computational purposes, but closed form equations as a function of S are very complicated. In general, the spin hamiltonian is not of Heisenberg form when all ligand spin polarization terms are included. However, the major ligand spin polarization terms and the superexchange terms are of Heisenberg form. Both of these are included in the broken symmetry approach.
Ab initio calculations of effective exchange interactions between spins are performed for H–H, H–He–H and a simplified model of binuclear manganese oxide, Mn2O2, by using the spin-unrestricted Hartree–Fock (UHF), spin-polarized density functional (DFT) and UHF + DFT hybrid methods. The scopes and limitations of these broken-symmetry approaches are discussed in relation to several computational schemes of effective exchange integrals (Jab). The natural orbitals (UNO or DNO) of the UHF, DFT and hybrid DFT solutions for magnetic clusters are used for interpretation of the superexchange interactions in Mn2O2 complexes.
The self consistent field multiple scattering Xα model has been used to calculate the electronic structure and the magnetic coupling constant, J, of planar and pseudotetrahedral [Cu2Cl6]2- complexes with the aim of exploiting the actual possibilities of the model to describe the spectral (electronic and EPR) and magnetic properties of transition-metal polynuclear complexes. The observed variation of J with the degree of tetrahedral distortion has been quantitatively reproduced. The 19000-cm-1 feature characteristic of the electronic spectra of hexachlorodicuprate(II) complexes is computed in the present calculations as a chlorine-to-metal charge transfer.
In the cone conformation calix[4]arenes possess lower-rim polyphenolic pockets that are ideal for the complexation of various transition-metal centres. Reaction of these molecules with manganese salts in the presence of an appropriate base (and in some cases co-ligand) results in the formation of a family of calixarene-supported [Mn(III)(2)Mn(II)(2)] clusters that behave as single-molecule magnets (SMMs). Variation in the alkyl groups present at the upper-rim of the cone allows for the expression of a degree of control over the self-assembly of these SMM building blocks, whilst retaining the general magnetic properties. The presence of various different ligands around the periphery of the magnetic core has some effect over the extended self-assembly of these SMMs.
The preparation, X-ray structure, and detailed physical characterization are presented for a new type of single-molecule magnet [Mn4(O2CMe)2(pdmH)6](ClO4)2 (1). Complex 1.2MeCN.Et2O crystallizes in the triclinic space group P1, with cell dimensions at 130 K of a = 11.914(3) A, b = 15.347(4) A, c = 9.660(3) A, alpha = 104.58(1) degree, beta = 93.42(1) degree, gamma = 106.06(1) degree, and Z = 1. The cation lies on an inversion center and consists of a planar Mn4 rhombus that is mixed-valent, MnIII2MnII2. The pdmH- ligands (pdmH2 is pyridine-2,6-dimethanol) function as either bidentate or tridentate ligands. The bridging between Mn atoms is established by either a deprotonated oxygen atom of a pdmH- ligand or an acetate ligand. The solvated complex readily loses all acetonitrile and ether solvate molecules to give complex 1, which with time becomes hydrated to give 1.2.5H2O. Direct current and alternating current magnetic susceptibility data are given for 1 and 1.2.5H2O and indicate that the desolvated complex has a S = 8 ground state, whereas the hydrated 1.2.5H2O has a S = 9 ground state. Ferromagnetic interactions between MnIII-MnII and MnIII-MnIII pairs result in parallel spin alignments of the S = 5/2 MnII and S = 2 MnIII ions. High-frequency EPR spectra were run for complex 1.2.5H2O at frequencies of 218, 328, and 436 GHz in the 4.5-30 K range. A magnetic-field-oriented polycrystallite sample was employed. Fine structure is clearly seen in this parallel-field EPR spectrum. The transition fields were least-squares-fit to give g = 1.99, D = -0.451 K, and B4 degrees = 2.94 x 10(-5) K for the S = 9 ground state of 1.2.5H2O. A molecule with a large-spin ground state with D < 0 can function as a single-molecule magnet, as detected by techniques such as ac magnetic susceptibility. Out-of-phase ac signals (chi'' M) were seen for complexes 1 and 1.2.5H2O to show that these complexes are single-molecule magnets. A sample of 1 was studied by ac susceptibility in the 0.4-6.4 K range with the ac field oscillating at frequencies in the 1.1-1000 Hz range. A single peak in chi'' M vs temperature plots was seen for each frequency; the temperature of the chi'' M peak varies from 2.03 K at 995 Hz to 1.16 K at 1.1 Hz. Magnetization relaxation rates were evaluated in this way. An Arrhenius plot gave an activation energy of 17.3 K, which, as expected, is less than the 22.4 K value calculated for the thermodynamic barrier for magnetization direction reversal for an S = 8 complex with D = -0.35 K. The 1.2.5H2O complex with an S = 9 ground state has its chi'' M peaks at higher temperatures.
The capability of the density functional broken symmetry approach for the calculation of various EPR parameters of exchange coupled metal clusters is demonstrated by studying the experimentally well-investigated [Mn(III)Mn(IV)(mu-O)(2)(mu-OAc)DTNE](2+) complex. Geometry optimizations of the complex in its broken symmetry and high spin states yielded structures with two distinct manganese sites and geometrical parameters in good agreement with the X-ray structure. Exchange coupling constants were calculated from the energy differences between the high spin and broken symmetry states using the Heisenberg spin Hamiltonian. Very good agreement between theory and experiment was achieved with the B3LYP hybrid functional. The g-tensor calculations were performed employing the coupled perturbed Kohn-Sham equations. A strategy for the computation of g-tensor site values is presented and provides single-site g-tensors that are in good agreement with the expectations for Mn(III) and Mn(IV), respectively. Spin projection gave the g-tensor of the coupled manganese complex in very good agreement with the experimental results. Complete (55)Mn hyperfine tensors, including spin-orbit contributions, were calculated and spin-projected. The source of anisotropy in this system could be traced back to the Mn(III) ion in line with the experimental results. The isotropic manganese hyperfine coupling constants were underestimated by factors between 1.4 and 2.5. It is shown that this deficiency is systematic in character and not anchored in the broken symmetry approach. Nuclear quadrupole splitting of the (55)Mn nuclei is shown to be small in this system. In addition, (14)N and (1)H ligand hyperfine data were calculated and compared well with the experimental results. The quality of the extended point-dipole model was demonstrated in application to (1)H anisotropic hyperfine coupling constants.
Mixing it up: A ground spin state of S = 83/2 is found in the mixed-valent manganese aggregate [MnIII12MnII7(μ4-O)8(μ3,η1- N3)8(HL)12(MeCN)6]2+ (H3L = 2,6-bis(hydroxymethyl)-4-methylphenol; see polyhedral representation of the core) and arises through ferromagnetic coupling of all the metal centers. (Figure Presented).
Structural distortion in a [Mn-6] complex switches the magnetic exchange from antiferro- to ferromagnetic, resulting in a single-molecule magnet with a record anisotropy barrier.
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