Magnetically bistable cobalt-dioxolene complexes with a tetradentate N-donor base.

Synthesis and magnetic characterization of a family of cobalt-dioxolene complexes [(Me2TPA)Co(36-DBCat)] (1), [(Me2TPA)Co(36-DBCat)](PF6) (2) and [(Me2TPA)Co(diox-(OMe)3)](BPh4) (3) (Me2TPA = bis(6-methyl-2-pyridyl)methyl-(2-pyridylmethyl)amine; 36-DBCat = dianion of 3,6-di-tert-butylcatechol; diox-(OMe)3 - 2,5-di-tert-butyl-3,3,4-trimethoxy-6-oxocyclohexa-1,4-dienolate) is reported. The neutral complex 1 is found to form hexa- (CoO2N4, 1a) and pentacoordinated (CoO2N3, 1b) isomers. Variable temperature single crystal X-ray diffraction analysis of 1a and 1b clearly indicates the presence of the high-spin divalent metal ion and the dianionic catecholate form of the dioxolene ligand. Oxidation of 1 by ferrocenium hexafluorophosphate results in the formation of the ionic octahedral complex 2, demonstrating thermally induced valence-tautomeric transition (ls-CoIII-36-DBCat ⇄ hs-CoII-36-DBSQ) in the solid state with T1/2 = 175 K (36-DBSQ = radical-anionic semiquinonate form of the redox-ligand). In contrast, aerial oxidation of 1 is accompanied by changes in the structure of dioxolene resulting in oxocyclohexadienolate ligand and the formation of an ionic complex of high-spin divalent cobalt (3). Compounds 1a, 1b, and 3 are found to demonstrate a field-induced single-ion magnet behavior. The analysis of the electronic structures of 1, 2 and 3 with the aid of DFT and SA-CASSCF/NEVPT2 calculations is also given.

Field-induced single-ion magnet behaviour of a hexacoordinated Co(ii) complex with easy-axis-type magnetic anisotropy.

A coordination compound with the composition [CoLCl2]·H2O (L = bis-condensation product of diacetyl and 2-hydrazinyl-4,6-dimethylpyrimidine) was synthesized, in which the Co(ii) ion was hexacoordinated. Under applied DC fields, this compound exhibited single-ion magnet behavior. Two relaxation processes were observed when increasing the applied magnetic field from 1000 to 3200 Oe. The first relaxation (high-frequency) was observed both at 1000 Oe and 3200 Oe, while the second relaxation was only registered under a field of 3200 Oe at low frequencies (<1 Hz) and low temperatures (<5 K). Modeling of the magnetic DC properties using the Griffith Hamiltonian accompanied by quantum chemical calculations revealed easy-axis-type magnetic anisotropy with weak rhombic contributions.

Enhancing the blocking temperature in single-molecule magnets by incorporating 3d-5d exchange interactions.

We report the first single-molecule magnet (SMM) to incorporate the [Os(CN)(6)](3-) moiety. The compound (1) has a trimeric, cyanide-bridged Mn(III)-Os(III)-Mn(III) skeleton in which Mn(III) designates a [Mn(5-Brsalen)(MeOH)](+) unit (5-Brsalen=N,N'-ethylenebis(5-bromosalicylideneiminato)). X-ray crystallographic experiments reveal that 1 is isostructural with the Mn(III)-Fe(III)-Mn(III) analogue (2). Both compounds exhibit a frequency-dependent out-of-phase χ''(T) alternating current (ac) susceptibility signal that is suggestive of SMM behaviour. From the Arrhenius expression, the effective barrier for 1 is found to be Δ(eff)/k(B)=19 K (τ(0)=5.0×10(-7) s; k(B)=Boltzmann constant), whereas only the onset (1.5 kHz, 1.8 K) of χ''(T) is observed for 2, thus indicating a higher blocking temperature for 1. The strong spin-orbit coupling present in Os(III) isolates the E'(1g(1/2))(O(h)*) Kramers doublet that exhibits orbital contributions to the single-ion anisotropy. Magnetic susceptibility and inelastic neutron-scattering measurements reveal that substitution of [Fe(CN)(6)](3-) by the [Os(CN)(6)](3-) anion results in larger ferromagnetic, anisotropic exchange interactions going from quasi-Ising exchange interactions in 2 to pure Ising exchange for 1 with J(parallel)(MnOs)=-30.6 cm(-1). The combination of diffuse magnetic orbitals and the Ising-type exchange interaction effectively contributes to a higher blocking temperature. This result is in accordance with theoretical predictions and paves the way for the design of a new generation of SMMs with enhanced SMM properties.

Highly anisotropic exchange interactions in a trigonal bipyramidal cyanide-bridged Ni(II)3Os(III)2 cluster.

This article is a part of our efforts to control the magnetic anisotropy in cyanide-based exchange-coupled systems with the eventual goal to obtain single-molecule magnets with higher blocking temperatures. We give the theoretical interpretation of the magnetic properties of the new pentanuclear complex {[Ni(II)(tmphen)(2)](3)[Os(III)(CN)(6)](2)} x 6 CH(3)CN (Ni(II)(3)Os(III)(2) cluster). Because the system contains the heavy Os(III) ions, spin-orbit coupling considerably exceeds the contributions from the low-symmetry crystal field and exchange coupling. The magnetic properties of the Ni(II)(3)Os(III)(2) cluster are described in the framework of a highly anisotropic pseudo-spin Hamiltonian that corresponds to the limit of strong spin-orbital coupling and takes into account the complex molecular structure. The model provides a good fit to the experimental data and allows the conclusion that the trigonal axis of the bipyramidal Ni(II)(3)Os(III)(2) cluster is a hard axis of magnetization. This explains the fact that in contrast with the isostructural trigonal bipyramidal Mn(III)(2)Mn(II)(3) cluster, the Ni(II)(3)Os(III)(2) system does not exhibit the single-molecule magnetic behavior.

Single-ion anisotropy and exchange interactions in the cyano-bridged trimers MnIII2MIII(CN)6 (MIII = Co, Cr, Fe) species incorporating [Mn(5-Brsalen)]+ units: an inelastic neutron scattering and magnetic susceptibility study.

The electronic structures of the compounds K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)M(III)(CN)(6)].2H(2)O (M(III) = Co(III), Cr(III), Fe(III)) have been determined by inelastic neutron scattering (INS) and magnetic susceptibility studies, revealing the manganese(III) single-ion anisotropy and exchange interactions that define the low-lying states of the Mn-M(III)-Mn trimeric units. Despite the presence of an antiferromagnetic intertrimer interaction, the experimental evidence supports the classification of both the Cr(III) and Fe(III) compounds as single-molecule magnets. The value of 17(2) cm(-1) established from AC susceptibility measurements for a spin-reversal barrier of K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)Cr(CN)(6)].2H(2)O may be readily rationalized in terms of the energy level diagram determined directly by INS. AC susceptibility measurements on samples of K[(5-Brsalen)(2)(H(2)O)(2)-Mn(2)Fe(CN)(6)].2H(2)O are contrary to those previously reported, exhibiting but the onset of peaks below temperatures of 1.8 K at oscillating frequencies in the range of 100-800 Hz. INS measurements reveal an anisotropic ferromagnetic manganese(III)-iron(III) exchange interaction, in accordance with theoretical expectations based on the unquenched orbital angular momentum of the [Fe(CN)(6)](3-) anion, giving rise to an M(s) approximately +/-9/2 ground state, isolated by approximately 11.5 cm(-1) from the higher-lying levels. The reported INS and magnetic data should now serve as a benchmark against which theoretical models that aim to inter-relate the electronic and molecular structure of molecular magnets should be tested.

A highly anisotropic cobalt(II)-based single-chain magnet: exploration of spin canting in an antiferromagnetic array.

In this article we report for the first time experimental details concerning the synthesis and full characterization (including the single-crystal X-ray structure) of the spin-canted zigzag-chain compound [Co(H2L)(H2O)]infinity [L = 4-Me-C6H4-CH2N(CPO3H2)2], which contains antiferromagnetically coupled, highly magnetically anisotropic Co(II) ions with unquenched orbital angular momenta, and we also propose a new model to explain the single-chain magnet behavior of this compound. The model takes into account (1) the tetragonal crystal field and the spin-orbit interaction acting on each Co(II) ion, (2) the antiferromagnetic Heisenberg exchange between neighboring Co(II) ions, and (3) the tilting of the tetragonal axes of the neighboring Co units in the zigzag structure. We show that the tilting of the anisotropy axes gives rise to spin canting and consequently to a nonvanishing magnetization for the compound. In the case of a strong tetragonal field that stabilizes the orbital doublet of Co(II), the effective pseudo-spin-1/2 Hamiltonian describing the interaction between the Co ions in their ground Kramers doublet states is shown to be of the Ising type. An analytical expression for the static magnetic susceptibility of the infinite spin-canted chain is obtained. The model provides an excellent fit to the experimental data on both the static and dynamic magnetic properties of the chain.

Origin of the single chain magnet behavior of the Co(H2L)(H2O) compound with a 1D structure.

The paper is aimed at the elucidation of the main factors responsible for the single-chain magnet behavior of the cobalt(II) disphosphonate compound Co(H2L)(H2O) with a 1D structure. The model takes into account the spin-orbit interaction, the axial component of the octahedral crystal field acting on the ground-state cubic 4T1 terms of the Co(II) ions, the antiferromagnetic exchange interaction between Co(II) ions as well as the difference in the crystallographic positions of these ions. The conditions that favor the single-chain magnet behavior based on spin canting in a 1D chain containing inequivalent Co(II) centers are discussed. The peculiarities of this behavior in chains containing orbitally degenerate ions are revealed. The qualitative explanation of the experimental data is given.

Isotropic magnetic exchange between anisotropic Yb(III) ions. Study of Cs3Yb2Cl9 and Cs3Yb2Br9 crystals.

Despite the prevalent belief about a strong anisotropy of the magnetic exchange in rare-earth compounds, Cs3Yb2Cl9 and Cs3Yb2Br9 crystals are found to exhibit fully isotropic exchange coupling between Yb3+ ions. In this article, we attempt to reveal the physical origin of this surprising feature. Our theoretical consideration is based on a model of the kinetic exchange between two octahedrally coordinated Yb3+ ions in their ground Kramers doublet states. It is shown that a mechanism of kinetic exchange involving intercenter electron hopping between 4f orbitals of two Yb3+ ions in a face-shared binuclear unit results in fully isotropic antiferromagnetic exchange coupling, while a mechanism in which the electron jumps from the 4f to the 5d orbital gives rise to a highly anisotropic interaction. Comparison of these results with the experimental data along with qualitative arguments regarding the relative significance of these two contributions to the overall exchange indicate that, in face-shared Yb3+ binuclear units, the 4f <--> 4f mechanism plays a dominant role.

Role of the orbitally degenerate Mn(III) ions in the single-molecule magnet behavior of the cyanide cluster ([MnII(tmphen)2]3[Mn(III)(CN)6]2) (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline).

We report a new theoretical model that accounts for the unusual magnetic properties of the cyanide cluster ([MnII(tmphen)2]3[MnIII(CN)6]2) (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline). The model takes into account (1) the spin-orbit interaction, (2) the trigonal component of the crystal field acting on the ground-state cubic (3)T(1) terms of the apical Mn(III) ions, and (3) the isotropic contribution to the exchange interaction between Mn(III) and Mn(II) ions. The ground state of the cluster was shown to be the state with the total angular momentum projection |M(J)| = 15/2; the energies of the low-lying levels obtained from this treatment increase with decreasing |M(J)| values, a situation that leads to a barrier for the reversal of magnetization (U(eff) approximately 30 cm(-1)). The new model explains the recently discovered single-molecule magnet behavior of the ([MnII(tmphen)2]3[MnIII(CN)6]2)in contrast to the traditional approach that takes into account only the ground-state spin (S) and a negative zero-field splitting parameter (D(S) < 0).

Microscopic approach to the pseudo-spin-1/2 Hamiltonian for Kramers doublets in exchange coupled Co(II) pairs.

The microscopic theory of the magnetically anisotropic effective pseudo-spin-(1)/(2) Hamiltonian for a pair of Co(2+) ions is reported. In the framework of the second-order perturbation approach, the analytical expressions are found for the components of the tensor of the exchange interaction, g-tensor, and the factor in the temperature independent paramagnetic contribution. The parameters of the Hamiltonian are expressed in terms of the basic intra- and intercenter parameters of the pair, namely, the spin-orbit coupling constant, orbital reduction factor, exchange integral, and low symmetry crystal field parameters including axial and rhombic terms.

Novel chiral three-dimensional iron(III) compound exhibiting magnetic ordering at T(c) = 40 K.

The preparation and crystal structure determination of the iron(III) compound of formula [(NH(4))(2)[Fe(2)O(ox)(2)Cl(2)].2H(2)O](n) (1) (ox = oxalate dianion) are reported here. Complex 1 crystallizes in the orthorhombic system, space group Fdd2, with a = 14.956(7) A, b = 23.671(9) A, c = 9.026(4) A, and Z = 8. The structure of complex 1 consists of the chiral anionic three-dimensional network [Fe(2)O(ox)(2)Cl(2)](2-) where the iron(III) ions are connected by single oxo and bisbidentate oxalato groups. The metal-metal separations through these bridging ligands are 3.384(2) and 5.496(2) A, respectively. Ammonium cations and crystallization water molecules are located in the helical pseudohexagonal tunnels defined by iron atoms. The longest iron-iron distance in the pseudohexagonal tunnel is 15.778(2) A whereas the shortest one is 8.734(2) A. The iron atoms are hexacoordinated: a terminal chloro ligand and five oxygen atoms, that of the oxo group and four from two cis coordinated oxalate ligands, build a distorted octahedral environment around the metal atom. The Fe-O(oxo) bond distance [1.825(2) A] is significantly shorter than the Fe(III)-O(ox) [average value 2.103(4) A] and Fe(III)-Cl bond distances [2.314(2) A]. Magnetic susceptibility measurements of 1 in the temperature range 2.0-300 K reveal the occurrence of a susceptibility maximum at 195 K and a transition toward a magnetically ordered state in the lower temperature region with T(c) = 40 K. The strong antiferromagnetic coupling through the oxo bridge (J = -46.4 cm(-1), the Hamiltonian being H = -JS(A).S(B)) accounts for the susceptibility maximum whereas a weak spin canting of approximately 0.3 degrees due to the antisymmetric magnetic exchange within the chiral three-dimensional network is responsible for the magnetic ordering. The values of coercive field (H(c)) and remnant magnetization (M(r)) obtained from the hysteresis loop of 1 at 5 K are 4000 G and 0.016 micro(B).