Carbonate-Bridged Lanthanoid Triangles: Single-Molecule Magnet Behavior, Inelastic Neutron Scattering, and Ab Initio Studies (original) (raw)
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Inorganic Chemistry, 2009
The first two families of polyoxometalate-based single-molecule magnets (SMMs) are reported here. Compounds of the general formula [Ln(W 5 O 18 ) 2 ] 9-(Ln III ) Tb, Dy, Ho, and Er) and [Ln(SiW 11 O 39 ) 2 ] 13-(Ln III ) Tb, Dy, Ho, Er, Tm, and Yb) have been magnetically characterized with static and dynamic measurements. Slow relaxation of the magnetization, typically associated with SMM-like behavior, was observed for [Ln(W 5 O 18 ) 2 ] 9-(Ln III ) Ho and Er) and [Ln(SiW 11 O 39 ) 2 ] 13-(Ln III ) Dy, Ho, Er, and Yb). Among them, only the [Er(W 5 O 18 ) 2 ] 9derivative exhibited such a behavior above 2 K with an energy barrier for the reversal of the magnetization of 55 K. For a deep understanding of the appearance of slow relaxation of the magnetization in these types of mononuclear complexes, the ligandfield parameters and the splitting of the J ground-state multiplet of the lanthanide ions have been also estimated. Caneschi, A.; Cornia, A.; de Biani, F. F.; Gatteschi, D.; Sangregorio, C.; Sessoli, R.; Sorace, L. J. Am. Chem. Soc. 1999, 121, 5302-5310. (d) Barra, A. L.; Caneschi, A.; Gatteschi, D.; Goldberg, D. P.; Sessoli, R. J. Solid State Chem. 1999, 145, 484-487. (e) Yoo, J.; Brechin, E. K.; Yamaguchi, A.; Nakano, M.; Huffman, J. C.; Maniero, A. L.; Brunel, L. C.; Awaga, K.; Ishimoto, H.; Christou, G.; Hendrickson, D. N.
Inorganic chemistry, 2017
Inelastic neutron scattering (INS) has been used to investigate the crystal field (CF) magnetic excitations of the analogs of the most representative lanthanoid-polyoxometalate single-molecule magnet family: Na9[Ln(W5O18)2] (Ln = Nd, Tb, Ho, Er). Ab initio complete active space self-consistent field/restricted active space state interaction calculations, extended also to the Dy analog, show good agreement with the experimentally determined low-lying CF levels, with accuracy better in most cases than that reported for approaches based only on simultaneous fitting to CF models of magnetic or spectroscopic data for isostructural Ln families. In this work we demonstrate the power of a combined spectroscopic and computational approach. Inelastic neutron scattering has provided direct access to CF levels, which together with the magnetometry data, were employed to benchmark the ab initio results. The ab initio determined wave functions corresponding to the CF levels were in turn employed ...
Chemical Science, 2013
The use of an amino-pyridyl substituted b-diketone, N-(2-pyridyl)-ketoacetamide (paaH), has allowed for the isolation of two new families of isostructural mononuclear lanthanide complexes with general , Tb (2), Dy (3), Ho (4), Er (5) and Y (6)) and [Ln(paaH*) 2 (NO 3 ) 2 (MeOH)][NO 3 ] (Ln ¼ Tb , Dy (8), Ho (9) and Er ). The dysprosium members of each family (3 and 8) show interesting slow magnetic relaxation features. Compound 3 displays Single Molecule Magnet (SMM) behaviour in zero DC field with an energy barrier to thermal relaxation of E a ¼ 177 cm À1 ) with s 0 ¼ 2.5(8) Â 10 À7 s, while compound 8 shows slow relaxation of the magnetization under an optimum DC field of 0.2 T with an energy barrier to thermal relaxation of E a ¼ 64 K (44 cm À1 ) with s 0 ¼ 6.2 Â 10 À7 s. Ab initio multiconfigurational calculations of the Complete Active Space type have been employed to elucidate the electronic and magnetic structure of the lowlying energy levels of compounds 2-5 and 8. The orientation of the anisotropic magnetic moments for compounds 2-5 are rationalized using a clear and succinct, chemically intuitive method based on the electrostatic repulsion of the aspherical electron density distributions of the lanthanides. † Electronic supplementary information (ESI) available: Experimental, X-ray crystallography, magnetism, computational details and references. CCDC 914681-914691. For ESI and crystallographic data in CIF or other electronic format see
Chemistry - A European Journal, 2002
The ground-state properties of a Co II 3 moiety encapsulated in a polyoxometalate anion were investigated by combining measurements of specific heat, magnetic susceptibility, and low-temperature magnetization with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na 12 [Co 3 W-(D 2 O) 2 (ZnW 9 O 34 ) 2 ] ¥ 40 D 2 O (Co 3 ). The ferromagnetic Co 3 O 14 cluster core consists of three octahedrally oxo-coordinated Co II ions. According to the singleion anisotropy and spin ± orbit coupling of the octahedral Co II ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co 3 spin cluster is anisotropic and is expressed as
A Polynuclear Lanthanide Single-Molecule Magnet with a Record Anisotropic Barrier
Angewandte Chemie International Edition, 2009
Dedicated to Professor Annie K. Powell on the occasion of her 50th birthday Single-molecule magnets (SMMs) continue to be an attractive research field because of their unique and intriguing properties and potential applications in high-density data storage technologies and molecular spintronics. [1] The anisotropic barrier (U) of an SMM is derived from a combination of an appreciable spin ground state (S) and uniaxial Ising-like magneto-anisotropy (D). [2] The magnet-like behavior can be observed by slow relaxation of the magnetization below the blocking temperature. Since the discovery of SMMs in the early 1990s, this assumption has formed the basis for the understanding of the origin of the anisotropic barrier. However, in recent years the development of novel lanthanide-only SMMs that challenge and defy this theory pose a number of questions: [3] How can slow relaxation of the magnetization be observed in a nonmagnetic state complex? Why are large energy barriers seen for mononuclear lanthanide(III) complexes? To answer such important questions, it is vital to investigate novel SMMs with high magnetoanisotropy for which the influence of the large negative D value could result in higher anisotropic barriers. Clearly lanthanide-based polynuclear systems are an important avenue to explore in the pursuit of SMMs with higher anisotropic barriers, because of the strong spin-orbit coupling commonly observed in 4f systems. [3] However, lanthanide-only SMMs are rare. [3, The majority of reported SMMs have been prepared with transition-metal ions, [2] although the recent application of a mixed transition-metal/ lanthanide strategy also yielded many structurally and magnetically interesting systems. [6] The scarcity of lanthanide-only SMMs results from the difficulty in promoting magnetic interactions between the lanthanide ions. The interactions can, however, be enhanced by overlapping bridging ligand orbitals. In addition, fast quantum tunneling of the magnetization (QTM), which is common for lanthanide systems, generally prevents the isolation of SMMs with high anisotropic energy barriers.
Molecular Magnets Based on Homometallic Hexanuclear Lanthanide(III) Complexes
Inorganic Chemistry, 2014
The reaction of lanthanide(III) chloride salts (Gd(III), Dy(III), Tb(III), and Ho(III)) with the hetero donor chelating ligand N′-(2-hydroxy-3-methoxybenzylidene)-6-(hydroxymethyl)picolinohydrazide (LH 3) in the presence of triethylamine afforded the hexanuclear Ln(III) complexes [{Ln 6 (L) 2 (LH) 2 }(μ 3-OH) 4 ][MeOH] p [H 2 O] q [Cl] 4 •xH 2 O• yCH 3 OH (1, Ln = Gd(III), p = 4, q = 4, x = 8, y = 2; 2, Ln = Dy(III), p = 2, q = 6, x = 8, y = 4; 3, Ln = Tb(III), p = 2, q = 6, x = 10, y = 4; 4, Ln = Ho(III), p = 2, q = 6, x = 10, y = 2). Xray diffraction studies revealed that these compounds possess a hexanuclear [Ln 6 (OH) 4 ] 14+ core consisting of four fused [Ln 3 (OH)] 8+ subunits. Both static (dc) and dynamic (ac) magnetic properties of 1−4 have been studied. Single-molecule magnetic behavior has been observed in compound 2 with an effective energy barrier and relaxation time pre-exponential parameters of Δ/k B = 46.2 K and τ 0 = 2.85 × 10 −7 s, respectively. ■ EXPERIMENTAL SECTION Reagents and General Procedures. Solvents and other general reagents used in this work were purified according to standard procedures. 13 Pyridine-2,6-dicarboxylic acid, sodium borohydride, DyCl
Dalton Transactions, 2014
The lanthanide(III) cyanoacetate complexes of the formula {[Ln 2 (CNCH 2 COO) 6 (H 2 O) 4 ]·2H 2 O} n , where Ln = Eu (1), Gd (2), Nd (3), have been prepared and characterized by X-ray diffraction analysis. Complexes 1 and 2 are isostructural and differ from the binding scheme of the neodymium compound 3, structurally described earlier. In all cases, the cyano group of the cyanoacetate ligand is not coordinated to the lanthanide cation. The carboxylic groups exhibit different binding modes: 2-bidentate-chelating, 2-bidentate and 2-tridentate-chelating bridging for 1 and 2, and 4-bidentate and 2-tridentate-chelating bridging for the complex 3. The Eu compound 1 shows field induced paramagnetism, as expected for a non-magnetic ground state with mixing from higher states. Combining the dc magnetization and luminescence measurements the spin-orbit coupling constant λ = 343 ± 4 cm −1 was found, averaged over the two different sites for Eu in the lattice. In the Gd complex 2, a crystal field splitting of D/k B = −0.11 ± 0.01 K has been found for the S = 7/2 multiplet of the Gd(III) ion. No slow relaxation at H = 0 is observed because the low anisotropy barrier allows fast spin reversal through classical processes. The application of an external magnetic field induces two slow relaxation processes. It is argued that the first relaxation rate is caused by the resonant phonon trapping (RPT) mechanism, while the second, slower relaxation rate is due to the lifting of the Kramers degeneracy on the ground state. For compound 3 heat capacity and dc susceptibility measurements indicate that at very low temperatures the ground state Kramers doublet has strong single ion anisotropy. The energy to the next excited doublet Δ ZFS /k B = 104 K has been calculated by ab initio calculation methods. The g* tensor has also been calculated, showing that it has predominant anisotropy along the z-axis, and there is an important transversal component. At H = 0 quantum tunnelling is an effective mechanism in producing a fast relaxation to equilibrium at temperatures above 1.8 K.