Quantum tunneling and quantum phase interference in a [Mn(II)2Mn(III)2] single-molecule magnet

[Mn4(hmp)6(H2O)2(NO3)2](NO3)2.2.5H2O (1) has been synthesized from the reaction of 2-hydroxymethylpyridine (Hhmp) with Mn(NO3)2.4H2O in the presence of tetraethylammonium hydroxide. 1 crystallizes in the triclinic P space group with two crystallographically independent centrosymmetrical [Mn4(hmp)6(H2O)2(NO3)2]2+ complexes in the packing structure. Four Mn ions are arranged in a double-cuboidal fashion where outer Mn2+ are heptacoordinated and inner Mn3+ are hexacoordinated. dc magnetic measurements show that both Mn2+...Mn3+ and Mn3+...Mn3+ interactions are ferromagnetic with J(wb)/k(B) = +0.80(5) K, and J(bb)/k(B) = +7.1(1) K, respectively, leading to an S(T) = 9 ground state. Combined ac and dc measurements reveal the single-molecule magnet (SMM) behavior of 1 with both thermally activated and ground-state tunneling regimes, including quantum phase interference. In the thermally activated regime, the characteristic relaxation time (tau) of the system follows an Arrhenius law with tau0 = 6.7 x 10(-)(9) s and delta(eff)/k(B) = 20.9 K. Below 0.34 K, tau saturates indicating that the quantum tunneling of the magnetization becomes the dominant relaxation process as expected for SMMs. Down to 0.04 K, field dependence of the magnetization measured using the mu-SQUID technique shows the presence of very weak inter-SMM interactions (zJ'/k(B) approximately -1.5 x 10(-3) K) and allows an estimation of D/k(B) at -0.35 K. Quantum phase interference has been used to confirm the D value and to estimate the transverse anisotropic parameter to E/k(B) = +0.083 K and the ground-state tunnel splitting delta(LZ) = 3 x 10(-7) K at H(trans) = 0 Oe. These results rationalize the observed tunneling time (tau(QTM)) and the effective energy barrier (delta(eff)).     

Synthesis, structural and magnetic properties of a series of copper(II) complexes containing a monocarboxylated perchlorotriphenylmethyl radical as a coordinating open-shell ligand

A series of complexes of copper(II)-containing a perchlorotriphenylmethyl radical functionalized with a carboxylic group as a new ligand is reported. The compounds [Cu(PTMMC)(2)(L)(3)](PTMMC = (tetradecachloro-4-carboxytriphenyl)methyl radical; L =(1) H(2)O, (2) pyrimidine and ethanol or (3) pyridine), [Cu(2)(PTMMC)(2)(MeCOO)(2)(H(2)O)(2)](4) and [Cu(HPTMMC)(2)(L)(3)](HPTMMC =alpha-H-(tetradecachlorotriphenyl)methane-4-carboxylic acid; L = pyridine)(5) were structurally characterized. In complexes 1, 2, 3, and 5, the copper(II) ion is coordinated to two PTMMC (or HPTMMC) units in a slightly distorted square planar surrounding, while 4 shows a paddle-wheel copper(II) dimer structure, where each Cu metal ion has four O atoms of different carboxylate groups, two of them belonging to two PTMMC radicals. The copper(II)-radical exchange couplings are antiferromagnetic for complexes 1, 2 and 3. A linear three-spin model was applied to complexes 1, 2 and 3 to give J/k(B)=-24.9, -15.0 and -20.7 K, respectively. Magnetic properties of 4 show that it is one of the scarce examples of a spin-frustrated system composed of organic radicals and metal ions. In this case, experimental data were fitted to a magnetic model based on a symmetrical butterfly arrangement to give a copper(II)-copper(II) exchange coupling of J/k(B)=-350.0 K and a copper(II)-radical exchange coupling of J/k(B)=-21.3 K, similar to that observed for the copper(II)-radical interactions in complexes, and.     

Magneto-structural xorrelations in 2D and 3D extended structures of manganese(II)-malonate systems

Two polymeric malonato-bridged manganese(II) complexes of formula [Mn(mal)(H(2)O)(2)](n)() (1) and [Mn(2)(mal)(2)(4,4'-bipy)(H(2)O)(2)](n)() (2) have been synthesized and characterized (mal = malonate dianion; 4,4'-bipy = 4,4'-bipyridine). The crystal structure of complex 1 was already known. Complex 2 crystallizes in monoclinic space group P2(1)/n, Z = 2, with unit cell parameters of a = 7.2974(10) A, b = 18.7715(10) A, c = 7.514(3) A, and beta = 91.743(12) degrees. The structure determination reveals that the complex [Mn(2)(mal)(2)(4,4'-bipy)(H(2)O)(2)](n)() (2) is a 3D network being composed of Mn-malonate sheets which are pillared by bidentate 4,4'-bipy spacer forming small voids. The Mn-Mn distances through Mn-mu-(O3-C8-O4)-Mn, Mn-mu(O1-C6-O2)-Mn, and Mn-mu-4,4'-bipy-Mn bridges are 5.561, 5.410, and 11.723 A, respectively. The magnetic behaviors of complexes 1 and 2 in the temperature range 300-2 K are very close, corresponding to a weak antiferromagnetic coupling. The magnetic pathways of complex 1 are through two Mn-O-C-O-Mn with anti-anti conformation and two Mn-O-C-O-Mn with syn-anti conformations and in complex 2 through all Mn-O-C-O-Mn with syn-anti conformations. Both syn-anti and anti-anti conformations create weak antiferromagnetic coupling, and the susceptibility data are fitted by the expansion series of Lines and the Curély formula for an S = 5/2 antiferromagnetic quadratic layer, based on the exchange Hamiltonian H = -Sigma(nn)()JS(i)()S(j)(). The best fit is given by the superexchange parameters J = -0.32 cm(-)(1) and g = 2.00 for complex 1 and J = -0.14 cm(-)(1), J(inter) = -0.031 cm(-)(1), and g = 2.00 for complex 2. Finally, in both the complexes there is a magnetic pathway Mn-O-C-C-C-O-Mn, and this pathway through the three carbon atoms of the malonato-bridging ligand could be considered negligible.     

Manganese(II) and copper(II) hexafluoroacetylacetonate 1:1 complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine: regiochemical parity analysis for exchange behavior of complexes between radicals and paramagnetic cations

Mn(hfac)(2) and Cu(hfac)(2) form coordination complexes with 5-(4-[N-tert-butyl-N-aminoxyl]phenyl)pyrimidine, PyrimPh-NIT. (Mn[PyrimPh-NIT](hfac)(2))(2) and (Cu[PyrimPh-NIT](hfac)(2))(2), 1 and 2, respectively, are cyclic M(2)L(2) dimers that exhibit strong exchange coupling between the coordinated paramagnetic dication (M) and nitroxide (NIT) unit. The M-NIT exchange is strongly antiferromagnetic (AFM) in 1 and strongly ferromagnetic (FM) in 2. Magnetic susceptibility measurements for 1 were fitted to an AFM spin pairing model with J/k = -0.25 K between Mn-NIT spin sites units. Complex 2 also exhibits AFM spin pairing between S = 1 Cu-NIT spin units that is somewhat field dependent at low temperature. The fit of corrected paramagnetic susceptibility chi(T) to an AFM spin pairing model at 200 Oe yields J/k = (-)3.8 K, quite similar to earlier measurements at 1000 Oe yielding J/k = (-)5.0 K. At 1.40 K, the magnetization of 2 does not approach saturation until somewhat above 170 kOe, giving an S-shaped curve; at 0.55 K, the magnetization curve shows steps characteristic of field-induced crossover between the S = 0 ground state and excited spin states. From the steps in the 0.55 K data, we estimate J/k = (-)3.8-4.0 K for 2, in good agreement with the analysis of chi(T).     

Antiferromagnetic three-dimensional order induced by carboxylate bridges in a two-dimensional network of [Cu3(dcp)2(H2O)4] trimers

A new Cu(II) complex, [Cu(3)(dcp)(2)(H(2)O)(4)](n), with the ligand 3,5-pyrazoledicarboxylic acid monohydrate (H(3)dcp) has been prepared by hydrothermal synthesis, and it crystallizes in the monoclinic space group P2(1)/c with a = 11.633(2) A, b = 9.6005(14) A, c = 6.9230(17) A, beta = 106.01(2) degrees, and Z = 2. In the solid state structure of [Cu(3)(dcp)(2)(H(2)O)(4)](n), trinuclear [Cu(3)(dcp)(2)(H(2)O)(4)] repeating units in which two dcp(3-) ligands chelate the three Cu(II) ions with the central Cu(II) ion, Cu(1) (on an inversion center), link to form infinite 2D sheets via syn-anti equatorial-equatorial carboxylate bridges between Cu(2) atoms in adjacent trimers. These layers are further linked by syn-anti axial-equatorial carboxylate bridging between Cu(1) atoms in adjacent sheets resulting in the formation of a crystallographic 3D network. A detailed analysis of the magnetic properties of [Cu(3)(dcp)(2)(H(2)O)(4)](n) reveals that the dcp(3-) ligand acts to link Cu(II) centers in three different ways with coupling constants orders of magnitude apart in value. In the high temperature region above 50 K, the dominant interaction is strongly antiferromagnetic (J/k(B) = -32 K) within the trimer units mediated by the pyrazolate bridges. Below 20 K, the trimer motif can be modeled as an S = 1/2 unit. These units are coupled to their neighbors by a ferromagnetic interaction mediated by the syn-anti equatorial-equatorial carboxylate bridge. This interaction has been estimated at J(2D)/k(B) = +2.8 K on the basis of a 2D square lattice Heisenberg model. Finally, below 3.2 K a weak antiferromagnetic coupling (J(3D)/k(B) = -0.1 K) which is mediated by the syn-anti axial-equatorial carboxylate bridges between the 2D layers becomes relevant to describe the magnetic (T, H) phase diagram of this material.     

Molecular paramagnetic semiconductor: crystal structures and magnetic and conducting properties of the Ni(dmit)(2) salts of 6-oxoverdazyl radical cations (dmit = 1,3-dithiol-2-thione-4,5-dithiolate)

Four kinds of 1:1 and 1:3 salts of 3-[4-(trimethylammonio)phenyl]-1,5-diphenyl-6-oxoverdazyl radical cation ([1](+)) and its mono- and dimethyl derivatives ([2](+) and [3](+)) with Ni(dmit)(2) anions (dmit = 1,3-dithiol-2-thione-4,5-dithiolate) ([1](+)[Ni(dmit)(2)](-) (4), [2](+)[Ni(dmit)(2)](-) (5), [3](+)[Ni(dmit)(2)](-) (6), and [1](+)[Ni(dmit)(2)](3)(-) (7)) have been prepared, and the magnetic susceptibilities (chi(M)) have been measured between 1.8 and 300 K. The chi(M) values of salts 5 and 7 can be well reproduced by the sum of the contributions from (i). a Curie-Weiss system with a Curie constant of 0.376 (K emu)/mol and negative Weiss constants (THETAV;) of -0.4 and -1.7 K and (ii). a dimer system with strong negative exchange interactions of 2J/k(B) = -354 and -258 K, respectively. The dimer formations in Ni(dmit)(2) anions have been ascertained by the crystal structure analyses of salts 4-6. In salts 4 and 6, Ni(dmit)(2) dimer molecules are sandwiched between two verdazyl cations, indicating the formation of a linear tetramer in 4 and 6. The magnetic susceptibility data for salts 4 and 6 have been fitted to a linear tetramer model using an end exchange interaction of 2J(1)/k(B) = -600 K and a central interaction of 2J(2)/k(B) = -280 K for 4 and 2J(1)/k(B) = -30 K and 2J(2)/k(B) = -580 K for 6, respectively. The results of the temperature dependence of the g(T) value in salts 4-6 obtained by ESR measurement also support the above analyses. The 1:1 salts 4-6 are insulators. On the other hand, the conductivity of the 1:3 salt 7 at 20 degrees C was sigma = 0.10 S cm(-)(1) with an activation energy E(A) = 0.099 eV, showing the semiconductor property. Salt 7 is a new molecular paramagnetic semiconductor.     

The relaxation of OH (v=1) and OD (v=1) by H2O and D2O at temperatures from 251 to 390 K

We report rate coefficients for the relaxation of OH(v=1) and OD(v=1) by H2O and D2O as a function of temperature between 251 and 390 K. All four rate coefficients exhibit a negative dependence on temperature. In Arrhenius form, the rate coefficients for relaxation (in units of 10(-12) cm3 molecule-1 s-1) can be expressed as: for OH(v=1)+H2O between 263 and 390 K: k=(2.4+/-0.9) exp((460+/-115)/T); for OH(v=1)+D2O between 256 and 371 K: k=(0.49+/-0.16) exp((610+/-90)/T); for OD(v=1)+H2O between 251 and 371 K: k=(0.92+/-0.16) exp((485+/-48)/T); for OD(v=1)+D2O between 253 and 366 K: k=(2.57+/-0.09) exp((342+/-10)/T). Rate coefficients at (297+/-1 K) are also reported for the relaxation of OH(v=2) by D2O and the relaxation of OD(v=2) by H2O and D2O. The results are discussed in terms of a mechanism involving the formation of hydrogen-bonded complexes in which intramolecular vibrational energy redistribution can occur at rates competitive with re-dissociation to the initial collision partners in their original vibrational states. New ab initio calculations on the H2O-HO system have been performed which, inter alia, yield vibrational frequencies for all four complexes: H2O-HO, D2O-HO, H2O-DO and D2O-DO. These data are then employed, adapting a formalism due to Troe (J. Troe, J. Chem. Phys., 1977, 66, 4758), in order to estimate the rates of intramolecular energy transfer from the OH (OD) vibration to other modes in the complexes in order to explain the measured relaxation rates-assuming that relaxation proceeds via the hydrogen-bonded complexes.     

Molecular metamagnet [Ni(4ImNNH)(2)(NO(3))(2)] (4ImNNH = 4-imidazolyl nitronyl nitroxide) and the related compounds showing supramolecular H-bonding interactions

A new chelating radical ligand 4ImNNH (2-(4-imidazolyl)-4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide) was prepared, and complexation with divalent transition metal salts gave complexes, [M(4ImNNH)(2)X(2)], which showed intermolecular ferromagnetic interaction in high probability (7 out of 10 paramagnetic compounds investigated here). The nitrate complexes (X = NO(3); M = Mn (1), Co (2), Ni (3), Cu (4)) crystallize isomorphously in monoclinic space group P2(1)/a. The equatorial positions are occupied with two 4ImNNH chelates and the nitrate oxygen atoms are located at the axial positions. Magnetic measurements revealed that the intramolecular exchange couplings in 1, 2, and 4 were antiferromagnetic, while that in 3 was ferromagnetic with 2J/k(B) = +85 K, where the spin Hamiltonian is defined as H = -2J(S(1).S(2) + S(2).S(3)) based on the molecular structures determined as the linear radical-metal-radical triads. The intramolecular ferromagnetic interaction in 3 is interpreted in terms of orthogonality between the radical pi and metal dsigma orbitals. Compounds 1-3 exhibited intermolecular ferromagnetic interaction ascribable to a two-dimensional hydrogen bond network parallel to the crystallographic ab plane. Complex 3 became an antiferromagnet below 3.4 K and exhibited a metamagnetic transition on applying a magnetic field of 5.5 kOe at 1.8 K. The complexes prepared from metal halides, [M(4ImNNH)(2)X(2)] (X = Cl, Br; M = Mn, Co, Ni, Cu), showed intramolecular antiferromagnetic interactions, which are successfully analyzed based on the radical-metal-radical system. The crystal structures determined here on 1-4, [Mn(4ImNNH)(2)Cl(2)], and [Cu(4ImNNH)(2)Br(2)] always have intermolecular hydrogen bonds of H(imidazole).X(axial ligand)-M, where X = NO(3), Cl, Br. This interaction seems to play an important role in molecular packing and presumably also in magnetic coupling.     

Structures and magnetic behavior of 1-, 2-, and 3D coordination polymers in the Cu(II)-dicyanamide-pyrimidine family

Using aqueous conditions, three new coordination polymers containing Cu(2+) cations, dicyanamide (dca) anions, and pyrimidine (pym) were isolated and structurally and magnetically characterized. Comprising the bulk of the product yield, Cu(dca)(2)(pym)(2), 1, crystallizes in the monoclinic space group P2(1)/c with a = 7.3569(5) A, b = 13.4482(9) A, c = 7.4559(5) A, beta = 98.984(3) degrees, and V = 728.6(1) A and forms linear 1D chains. The second compound, Cu(dca)(NO(3))(pym)(H(2)O), 2, is also monoclinic, P2(1)/n, with a = 7.6475(3) A, b = 12.2422(5) A, c = 11.0286(4) A, beta = 106.585(2) degrees, and V = 989.6(1) A(3). A 2D network structure consisting of both bridging mu-dca and pym ligands is formed while the NO(3)(-) and H(2)O are axially bonded to the Cu center. Cu(3)(dca)(6)(pym)(2).0.75H(2)O, 3, is triclinic, Ponemacr;, with a = 7.7439(4) A, b = 9.3388(5) A, c = 10.1779(5) A, alpha = 86.014(2) degrees, beta = 88.505(2) degrees, gamma = 73.623(2) degrees, and V = 704.46(9) A(3). The structure of 3 is quite unique in that [Cu(3)(pym)(2)](6+) trimers are interconnected via mu-dca ligands affording a complex 3D self-penetrating framework. Magnetically, 1 exhibits extremely weak exchange interactions along the Cu-(dca)(2)-Cu ribbons while 2 and 3 display very strong magnetic couplings mediated by the mu-bonded pym ligands. Moreover, 2 shows a broad maximum in chi(T) at 40 K and behaves as a uniform 1D antiferromagnetic chain with g = 2.09(1), J/k(B) = -42.6(1) K, and TIP = -66 x 10(-)(6) emu/mol. An S = (1)/(2) trimer model that includes intertrimer interactions successfully described the magnetic behavior of 3, yielding g = 2.10(1), J/k(B) = -69.4(5) K, theta = -0.28(3) K, and TIP = -180 x 10(-)(6) emu/mol. It is found that mu-bonded dca and pym ligands mediate very weak and very strong exchange interactions, respectively, between Cu(2+) centers.     

Negatively charged pi-(C60-)2 dimer with biradical state at room temperature

A negatively charged pi-(C60-)2 dimer bonded by two single bonds was found in the ionic multicomponent complex {(MDABCO+).CoIITMPP}2.(C60-)2.(C6H4Cl2)2.5.(C6H5CN)1.5 (1). In contrast to the previously described diamagnetic sigma-(C60-)2 dimer, the negatively charged pi-dimer has a biradical state at room temperature: (C60*-)2 (S = 1). The behavior of spins in this dimer can be described by a model with a singlet ground state (S = 0) and a close lying excited triplet (S = 1) state with the energy gap of 2|JAF| = 70 +/- 2 cm-1. On the whole, complex 1 shows a strong antiferromagnetic interaction of spins with a Weiss constant of -34 K.     

Co3(RL)2(hfac)6 ladder complex of 5-[4-(N-tert-butyl-N-aminoxyl)phenyl]pyrimidine

5-[4-(N-tert-butyl-N-aminoxyl)phenyl]pyridimine (4NITPhPyrim = RL) forms a 1-D ladder polymer complex with Co(hfac)2 of stoichiometry Co3(RL)2(hfac)6, having antiparallel [Co(II)RL]n linear chains (rails) that are cross-linked by Pyrim-Co(hfac)2-Pyrim rungs. The magnetic behavior above 100 K is consistent with contributions from one high-spin Co(II) ion (the cross-link, S = 3/2) plus two Co-ON units with strongly antiferromagnetic (AFM) metal-radical exchange (each S = 1). The chiT data show an AFM downturn as the temperature drops. Assuming weak exchange along chain portions of the polymer due to poor spin polarization across the phenyl-pyrimidine bond in RL, a linear three-spin (S = 1, 3/2, and 1) fit to the T > 18 K data yields an AFM cross-linker (rung) effective exchange of J(CL)/k = (-)5.3 K = (-)3.7 cm(-)(1). Superexchange (sigma-orbital overlap) is a likely mechanism for the effective AFM exchange between CoON and Co spin sites in the three-spin groupings.     

Copper(II) azide complexes of aliphatic and aromatic amine based tridentate ligands: novel structure, spectroscopy, and magnetic properties

Copper(II) azide complexes of three tridentate ligands namely 2,6-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L), 2,6-(pyrazol-1-ylmethyl)pyridine (L'), and dipropylenetriamine (dpt) yield three kinds of complexes with different azide-binding modes. The ligand L forms two end-on-end (mu-1,3) diazido-bridged binuclear complexes, [CuL(mu-N(3))](2)(ClO(4))(2) (1) and [CuL(mu-N(3))(ClO(4))](2).2CH(3)CN (2), and L' forms a perchlorato-bridged quasi-one-dimensional chain complex, [CuL'(N(3))(ClO(4))](n)() (3) with monodentate azide coordination. The ligation of dipropylenetriamine (dpt) gives a end-on (mu-1,1) diazido-bridged binuclear copper complex [Cu(dpt)(mu-N(3))](2)(ClO(4))(2) (4). The crystal and molecular structures of these complexes have been solved. Variable-temperature EPR results of 1 and 2 are identical and indicate the presence of both ferromagnetic and antiferromagnetic interactions within the dimer, the former dominating at low temperatures and the latter at high temperatures. The unusual temperature-dependent magnetic moment and EPR spectra of this dimer reveal the presence of temperature-dependent population of two triplet states, one being caused by antiferromagnetic and the other by ferromagnetic interaction, the former transforming to the latter on cooling. While the interaction of ground spin doublets of the two metal centers gives rise to a ferromagnetic coupling of J(g) = 90.73 cm(-1), the other coupling of J(e) = -185.64 cm(-1) is suggested to be caused by the interaction between an electron in one metal center and an electron from the azide of the other monomer by excitation of a d-electron to the empty ligand orbital. The ferromagnetic state is energetically favored by 104.39 cm(-1). Compound 3 exhibits axial spectra at room temperature and 77 K, and variable-temperature magnetic susceptibility data indicate that the copper centers form a weakly antiferromagnetic one-dimensional chain with J = -0.11 cm(-1). In the case of 4, the unique presence of two nonidentical dimeric units with different bond lengths and bond angles within the unit cell as inferred by crystal structure is proved by single-crystal EPR spectroscopy.     

Mn2(saltmen)2Ni(pao)2(L)2](A)2 with L=pyridine, 4-picoline, 4-tert-butylpyridine, N-methylimidazole and A=ClO4-, BF4-, PF6-, ReO4-: a family of single-chain magnets

A series of single-chain magnets, [Mn2(saltmen)2Ni(pao)2(L)2](A)2 (saltmen(2-)=N,N'-(1,1,2,2-tetramethylethylene) bis(salicylideneiminate), pao-=pyridine-2-aldoximate; A-=ClO4- with L=4-picoline; 2, 4-tert-butylpyridine; 3, N-methylimidazole; 4, and L=pyridine with A-=BF4-; 5, PF6-; 6, ReO4-; 7), was prepared by reactions between MnIII dimer units, i.e., [Mn2(saltmen)2(H2O)2](A)2 (A-=ClO4-, BF4-, PF6-) or Mn2(saltmen)2(ReO4)2, and NiII monomeric units, i.e., Ni(pao)2(L)2, in methanol/water media. The crystal structures of 4, 6, and 7 were established by single-crystal X-ray crystallography. These three compounds are isostructural with [Mn2(saltmen)2Ni(pao)2(py)2](ClO4)2 (1) (Clérac, R.; Miyasaka, H.; Yamashita, M.; Coulon, C. J. Am. Chem. Soc. 2002, 124, 12837) and crystallize in monoclinic space group C2/c. The linear arrangement of MnIII dimer units and NiII building blocks leads to an alternating chain having a repeating unit, [-(O)2-Mn-ON-Ni-NO-Mn-]. The chains are well separated with the nearest interchain intermetallic distance of 10.36 A for 4, 10.51 A for 6, and 10.30 A for 7, and there is no significant pi-pi interchain interaction between ligands. The void space between the chains is occupied by counteranions, which control the three-dimensional organization of the chains. The X-ray diffraction analysis (XRD) on a powder sample was also performed for all compounds. The XRD patterns for 1, 2, and 4-7 are very similar, emphasizing the isostructural nature of these materials although they have individually slight different interchain distances. Inversely, the XRD pattern for 3 reveals a completely different shape being indicative of the peculiar crystal packing compared to the others. Nevertheless, the one-dimensional nature of the structure is also kept in 3 as indicated by magnetic measurements. The whole family of compounds exhibits quasi-identical magnetic behavior compared to that described for 1. Above 30 K, the heterometallic chain can be described as an assembly of antiferromagnetically coupled Mn...Ni....Mn trimers (via oximate bridge, -24.2 K相似文献   

Ferromagnetic exchange interactions for Cu6(12+) and Mn6(12+) hexagons sandwiched by two B-alpha-[XW9O33]9- (X = As(III) and Sb(III)) ligands in D3d-symmetric polyoxotungstates

Two novel Cu6 and Mn6 hexagon sandwiched polyoxometalates, [(CuCl)6(AsW9O33)2]12- (1a) and [(MnCl)6(SbW9O33)2]12- (2a), have been synthesized and characterized by X-ray single-crystal analysis and magnetic measurements. These complexes are D3d symmetric and were isolated as [n-BuNH3]+ salts from aqueous solutions: (n-BuNH3)12[(CuCl)6(AsW9O33)2].6H2O (1), rhombohedral, R, a = 20.33(1) A, c = 26.35(2) A, Z = 3 and (n-BuNH3)12[(MnCl)6(SbW9O33)2].6H2O (2). Six Cu (or Mn) atoms, each of which shows 5-fold coordination, make an approximately equatorial hexagon with a first-neighboring Cu...Cu (Mn...Mn) distance of 2.913(2) A (3.248(1) A) and a Cu-O-Cu (Mn-O-Mn) bond angle of 94.5(2) degrees (100.4(2) degrees ). The magnetic behavior investigated by magnetic susceptibility measurements shows the ferromagnetic exchange interactions with J/k = +12.7 K (J/hc = +8.82 cm(-1)) and a S = 3 ground state for 1 and J/k = +0.20 K (J/hc = +0.14 cm(-1)) and a S = 15 ground state for 2, when only J refers to the isotropic magnetic-exchange interactions for first-neighbor atoms of the approximately equilateral Cu6(12+) and Mn6(12+) hexagons. The single-crystal ESR spectroscopy of 1 under the orientation of the magnetic field along a Cu6 hexagon's nearly 6-fold axis equal to the c axis on the variation of temperature supports the S = 3 ground state and allows an estimate of the zero-field (fine-structure) energy separation between Sz = 0 and Sz = +/-1 of D = -0.182 K to be obtained.     

From an S(T) = 3 single-molecule magnet to diamagnetic ground state depending on the molecular packing of Mn(III)salen-type dimers decorated by N,N'-dicyano-1,4-naphthoquinonediiminate radicals

Two manganese(III) tetradentate Schiff-base dimers to which N,N'-dicyano-1,4-naphthoquinonediiminate (DCNNQI) radicals are attached have been selectively synthesized by varying the solvents used in the reactions: [Mn2(5-MeOsaltmen)2(DCNNQI)2].MeOH (1) and [Mn2(5-MeOsaltmen)(2)(DCNNQI)(2)] x 2CH2Cl2.2CH3CN (2) [5-MeOsaltmen2- = N,N'-(1,1,2,2-tetramethylethylene)bis(5-methoxysalicylideneiminate)]. These two complexes share the same molecular core, [(DCNNQI.-)-Mn(III)-(O)2-Mn(III)-(DCNNQI.-)], where -(O)2- is a biphenolate bridge in the out-of-plane dimerized [Mn(2)(5-MeOsaltmen)2]2+ moiety. However, their packing arrangements are completely different. Whereas complex 1 is found to be relatively isolated, strong intermolecular dimerization of the DCNNQI moieties (with the nearest contact being approximately 3.0 A) is observed in 2, forming a one-dimensional chain of [-Mn(III)-(O)2-Mn(III)-(DCNNQI.-)2-](infinity). The magnetic susceptibility of 1 can be modeled with an [S = 1/2, 2, 2, 1/2] four-spin system including strong antiferromagnetic Mn(III)/DCNNQI radical coupling (J(Mn/rad)/kB = -23 K) and ferromagnetic Mn(III)/Mn(III) coupling through the biphenolate bridge (J(Mn/Mn)/kB = +2.0 K). These interactions lead to an ST = 3 ground state that possesses significant uniaxial anisotropy (D(S=3)/kB = -2.1 K). Low-temperature ac and dc magnetic data of 1 reveal its single-molecule magnet behavior with quantum tunneling of the magnetization. By contrast, 2 possesses the diamagnetic ground state induced by dominating Mn(III)-Mn(III) antiferromagnetic interactions mediated by the diamagnetic DCNNQI dimers and/or pi-pi contact along the b axis.     

Mn(dca)2(pym)2 and Mn(dca)2(pym)(H2O) {dca = dicyanamide; pym = pyrimidine}: New coordination polymers exhibiting 1- and 2-D topologies

Two new coordination polymers comprised of Mn(2+), N(CN)(2)(-) (dicyanamide, herein denoted dca) and pyrimidine (pym) have been synthesized and structurally and magnetically characterized. Mn(dca)(2)(pym)(2), , crystallizes in the orthorhombic space group Pbcn and forms trans bi-bridged Mn-(micro(1,5)-dca)(2)-Mn ribbons that extend along the b-axis of the unit cell. Two terminally bonded pym ligands are trans-coordinated to the Mn center. Adjacent chains interdigitate in an undulating fashion presumably due to a templating effect imposed by the pym ligands where N-atoms of consecutive pym rings stack parallel to the chain axis. Mn(dca)(2)(pym)(H(2)O), , which crystallizes in the monoclinic space group P2(1)/c, has a unique interdigitated 2D network that consists of double-bridged [Mn(2)(dca)(2)(pym)(2)(H(2)O)(2)](2+)"dimers" that are connected via single-bridging dca ligands. Each MnN(5)O octahedron is comprised of a coordinated H(2)O, a monodentate pym ligand, and four micro(1,5)-bridging dca anions. The magnetic data for were fitted to a uniform antiferromagnetic chain model which yielded J/k(B) = -0.34(1) K. In contrast, is best described as an alternating chain owing to the presence of both single- and double-bridging dca anions; a least-squares fit afforded J(a) = -0.43(1) and J(b) = -0.20(2) K, respectively. A transition to long-range magnetic ordering was observed for below approximately 2.4 K in addition to a field-induced spin flop transition at 15.6 kOe (1.7 K).     

Crystallography and magnetism of two 1-(4-nitroxylphenyl)pyrroles

Stable radicals 1-(4-(N-tert-butyl-N-aminoxyl)phenyl)pyrrole (BNPP) and 1-(4-(N-[para-methoxyphenyl]-N-aminoxyl)phenyl)pyrrole (MNPP) were synthesized and characterized by crystallography and magnetism. BNPP crystals exhibit 1-D chains of intermolecular nitroxide NO to nitroxide CH3 contacts, but polycrystalline magnetic susceptibility measurements show quite small antiferromagnetic (AFM) exchange interactions. MNPP shows stronger AFM exchange interactions that appear to be associated with a 2-D planar mesh of crystallographic nitroxide to nitroxide (N)O...N(O) contacts of 4.0-4.2 A. The AFM behavior of MNPP can be fitted to a 2-D square planar Heisenberg antiferromagnetic exchange model with J/k = (-)0.78 +/- 0.04 K and mean field constant theta = (-)0.77 +/- 0.12 K.     

Ni(II) and hs-Fe(II) complexes of a paramagnetic thiazyl ligand, and decomposition products of the iron complex, including an Fe(III) tetramer

Synthesis and structural, magnetic and electrochemical characterization of the Ni(hfac) 2(pyDTDA) and the Fe(hfac) 2(pyDTDA) complexes are reported (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato-; pyDTDA = 4-(2'-pyridyl)-1,2,3,5-dithiadiazolyl). Unlike the previously reported Mn(II) and Cu(II) complexes, but similar to the Co(II) complex, the Ni(II) and Fe(II) complexes are not dimerized in the solid state, allowing for magnetic coupling between the metal ion and paramagnetic ligand to be readily obtained from solid state magnetic measurements: Ni complex, J/k B = +132(1) K, using H = -2 J{ S Ni. S Rad} and g Ni = 2.04(2) and g Rad = 1.99(2); Fe complex, J/k B = -60.3(3) K, using H = -2 J{ S Fe. S Rad} and g av = 2.11(2). The iron complex is unusually unstable. A thermal decomposition product is isolated wherein the coordinated pyDTDA ligand appears to have been transformed into a coordinated 2-(2'-pyridyl)-4,6-bis(trifluoromethyl)pyrimidine. The iron complex also yields a solution decomposition product in the presence of air that is best described as an oxygen bridged iron(III) tetramer with two hfac ligands on each of three iron atoms and two oxidized pyDTDA ligands chelated on the fourth.     

Synthesis, structure, and magnetic ordering of layered (2-D) V-based tris(oxalato)metalates

The reaction of K3[M(III)(ox)3].3H2O [M = V (1), Cr; ox = oxalate], Mn(II)/V(II), and [N(n-Bu)4]Br in water leads to the isolation of 2-D V-based coordination polymers, [[N(n-Bu)4][Mn(II)V(III)(ox)3]]n (2), [[N(n-Bu)4][V(II)Cr(III)(ox)3]]n (3), [[N(n-Bu)4][V(II)V(III)(ox)3]]n (4), and an intermediate in the formation of 4, [[N(n-Bu)4][V(II)V(III)(ox)3(H2O)2]]n.2.5H2O (4a), while 1-D [V(II)(ox)(H2O)2]n (5) is obtained by using Na2ox and [V(OH2)6]SO4 in water. The structures of 1-5 have been investigated by single crystal and/or powder X-ray crystallography. In 1, V(III) is coordinated with three oxalate dianions as an approximately D3 symmetric, trigonally distorted octahedron. 1 is paramagnetic [mu(eff) = 2.68 mu(B) at 300 K, D = 3.84 cm(-1) (D/k(B) = 5.53 K), theta = -1.11 K, and g = 1.895], indicating an S = 1 ground state. 2 exhibits intralayer ferromagnetic coupling below 20 K, but does not magnetically order above 2 K, and 3 shows a strong antiferromagnetic interaction between V(II), S = 3/2 and Cr(III), S = 3/2 ions (theta = -116 K) within the 2-D layers. 4 and 4a magnetically order as ferrimagnets at T(c)'s, taken as the onset of magnetization, of 11 and 30 K, respectively. The 2 K remanent magnetizations are 2440 and 2230 emu.Oe mol(-1) and the coercive fields are 1460 and 4060 Oe for 4 and 4a, respectively. Both 4 and 4a clearly show frequency dependence, indicative of spin-glass-like behavior. The glass transition temperatures were at 6.3 and 27 K, respectively, for 4 and 4a. 1-D 5 exhibits antiferromagnetic coupling of -4.94 cm(-1) (H = -2Jsigma(i=1)n.S(i-1) - gmu(B)sigma(i=0)(n)H.S(i)) between the V(II) ions.     

Molecular magnetic properties of two-copper(II) containing complexes [Cu(II) (1-phenylamidino-O-n-butylurea) en (H2O)]2(2+) and [Cu(II) sulphato-mono (1-phenylamidino-O-methylurea)]2. An EPR study

Electron paramagnetic resonance (EPR) investigations were conducted on [Cu(II) (1-phenylamidino-O-n-butylurea) en (H2O)]2(2+) (1) and [Cu(II) sulphato-mono (1-phenylamidino-O-methylurea)]2 (2) respectively, in the temperature range 300-77K. Fine structure characteristics of S = 1 system, was observed in both complexes with zero field splitting of 0.0525 and 0.0225 cm(-1), respectively, suggesting the formation of dimeric complexes. The presence of the half-field signal (DeltaMs= +/-2), in the complex 1, further confirmed the formation of dimer. The temperature dependence of EPR signal intensity has given evidence for the ferromagnetic (FM) coupling between the two Cu2+ ions. The isotropic exchange interaction constants J, were evaluated from this and were found out to be approximately 57 and approximately 27 cm(-1), respectively, for the complexes 1 and 2. The photoacoustic spectra of these complexes had shown a band around 26,400 cm(-1) characteristic of metal-metal bonding giving an independent support for the existence of dimeric Cu2+ species. The high magnetic moment values at room temperature for complex 1 (2.68 microB) and complex 2 (2.00 microB), obtained from the magnetic susceptibility measurements, support the formation of ferromagnetically coupled Cu2+ dimers.