The ligand-centered radical complex (CoTPMA)2 -μ-bmtz(.-) (O3 SCF3 )3 ⋅CH3 CN (bmtz=3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine, TPMA=tris-(2-pyridylmethyl)amine) has been synthesized from the neutral ...bmtz precursor. Single-crystal X-ray diffraction studies have confirmed the presence of the ligand-centered radical. The Co(II) complex exhibits slow paramagnetic relaxation in an applied DC field with a barrier to spin reversal of 39 K. This behavior is a result of strong antiferromagnetic metal-radical coupling combined with positive axial and strong rhombic anisotropic contributions from the Co(II) ions.
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)∞ 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.
The ligand‐centered radical complex (CoTPMA)2‐μ‐bmtz.−(O3SCF3)3⋅CH3CN (bmtz=3,6‐bis(2′‐pyrimidyl)‐1,2,4,5‐tetrazine, TPMA=tris‐(2‐pyridylmethyl)amine) has been synthesized from the neutral bmtz ...precursor. Single‐crystal X‐ray diffraction studies have confirmed the presence of the ligand‐centered radical. The CoII complex exhibits slow paramagnetic relaxation in an applied DC field with a barrier to spin reversal of 39 K. This behavior is a result of strong antiferromagnetic metal–radical coupling combined with positive axial and strong rhombic anisotropic contributions from the CoII ions.
Radical coupling: The use of a radical bridging ligand to synthesize a dinuclear CoII model compound leads to strong magnetic exchange coupling and single‐molecule magnet behavior. Temperature dependence of the main components of the χT tensor indicate that Z is the hard axis of magnetization.
We report a new theoretical model that accounts for the unusual magnetic properties of the cyanide cluster {MnII(tmphen)23MnIII(CN)62} (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 ≈ 30 cm-1). The new model explains the recently discovered single-molecule magnet behavior of the {MnII(tmphen)23MnIII(CN)62} 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).
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.
A new semi-empirical magnetostructural correlation for exchange coupled dinuclear asymmetrically phenoxo-, alkoxo-, and hydroxo-bridged iron(III) complexes has been established. Based on an extensive ...comparison of the magnetic and structural parameters of 34 compounds, a strong correlation between the exchange coupling constant
J and the averaged iron–oxygen distance,
d, over the phenoxo-, alkoxo-, and hydroxo-bridge, could be demonstrated. This bridge provides much more efficient exchange pathway than every bridges over three atoms, e.g. acetate, phosphate, sulfate. The empirical relationship could be quantified in the form
J=−10
7
exp(−6.8
d). The magnitude of the FeO(X)Fe-bridging angle has no noticeable influence on the exchange coupling neither in case of complexes with identical nor in case of such with different bridging ligands.
A new semi-empirical magnetostructural correlation for exchange coupled dinuclear asymmetrically phenoxo-, alkoxo-, and hydroxo-bridged iron(III) complexes has been established. Further, three-atomic bridges as well as the bridging angles have no significant influence on the exchange coupling.
The aim of this communication is to probe the possibility of increasing the barrier for reversal of magnetization in the family of new cyano-bridged pentanuclear Mn(III)2Mn(II)3 clusters in which ...single molecule magnet behavior has been recently discovered. In this context, we analyze the global magnetic anisotropy arising from the unquenched orbital angular momenta of ground terms 3 T 1(t 2 4) of the two apical Mn(III) ions. The model takes into account the trigonal component of the crystal field, spin−orbit interaction in 3 T 1(t 2 4), and an isotropic exchange interaction between Mn(III) and Mn(II) ions. The height of the barrier is shown to be sensitive to the change of the trigonal field stabilizing orbital doublet 3 E, which carries the first-order orbital magnetic contribution and enhances with an increase of the trigonal field. This result is expected to be useful for the more rational design of new cyano-bridged SMMs with high blocking temperatures.
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.
Abstract
The ligand‐centered radical complex (CoTPMA)
2
‐μ‐bmtz
.−
(O
3
SCF
3
)
3
⋅
CH
3
CN (bmtz=3,6‐bis(2′‐pyrimidyl)‐1,2,4,5‐tetrazine, TPMA=tris‐(2‐pyridylmethyl)amine) has been synthesized from ...the neutral bmtz precursor. Single‐crystal X‐ray diffraction studies have confirmed the presence of the ligand‐centered radical. The Co
II
complex exhibits slow paramagnetic relaxation in an applied DC field with a barrier to spin reversal of 39 K. This behavior is a result of strong antiferromagnetic metal–radical coupling combined with positive axial and strong rhombic anisotropic contributions from the Co
II
ions.
An explanation is presented for the unusual magnetic behavior of two Re(II) compounds, Re(triphos)(CH3CN)3BF42 and Et4NRe(triphos)(CN)3, for which magnetic susceptibility data obtained on powder ...samples (2−300 K) suggest unusually strong temperature-independent paramagnetism (χTIP = (1.4−1.8) × 10-3 cm3 mol-1) and small low-temperature effective magnetic moments. A model is developed based on the jj-coupling scheme appropriate for description of the 5d shell of a Re(II) ion in a crystal field. The model accounts for a cubic crystal field, strong spin−orbit coupling, and a trigonal component of the crystal field produced by the ligand field acting on the Re(II) ions. The last two interactions act within the truncated basis containing eight lowest, strong cubic crystal field terms and result in the stabilization of two closely spaced Kramers doublets originating mainly from the 2T2( ) term. Efficient mixing of these Kramers doublets with those arising from 6A1( e2) and 4T1 (3T1)e terms is shown to result in the small low-temperature effective magnetic moments and anomalously strong temperature-independent paramagnetism, in accordance with the observed magnetic behavior. The model perfectly reproduces the χT vs T dependences over a wide temperature range (2−300 K), and the energy pattern obtained with the set of the best fit parameters provides a qualitative explanation for the observed light absorption and diffuse reflectance in a wide spectral area (200−2600 nm). The theoretical consideration predicts extremely strong anisotropy of the magnetic susceptibility and g-factors for both compounds with a C 3 easy axis of magnetization.
PDF