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1.
J Phys Chem Lett ; : 1508-1515, 2020 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-31994400

RESUMO

A homoleptic gadolinium(III) complex with the smallest helicene-type ligand, 1,10-phenanthroline-N,N'-dioxide (phendo) [Gd(phendo)4](NO3)3·xMeOH (phendo = 1,10-phenanthroline-N,N'-dioxide, MeOH = methanol), shows slow relaxation of the magnetization characteristic for Single Ion Magnets (SIM), despite negligible magnetic anisotropy, confirmed by ab initio calculations. Solid state dilution magnetic and EPR studies reveal that the magnetization dynamics of the [Gd(phendo)4]3+ cation is controlled mainly by a Raman process. Pulsed EPR experiments demonstrate long phase memory times (up to 2.7 µs at 5 K), enabling the detection of Rabi oscillations at 20 K, which confirms coherent control of its spin state.

2.
Inorg Chem ; 58(1): 756-768, 2019 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-30562002

RESUMO

Reaction of 2-hydroxy3-methoxybenzaldehyde ( o-vanillin) with 1,1,1-tris(aminomethyl)ethane, Me-C(CH2NH2)3, or with N, N', N''-trimethylphosphorothioic trihydrazide, P(S)[NMe-NH2]3, yields two tripodal LH3 and L1H3 ligands which are able to give cationic heterotrinuclear [LCoGdCoL]+ or [L1CoGdCoL1]+ complexes. The CoII ions are coordinated to these deprotonated ligands in the inner N3O3 site, while the GdIII ion is linked to three deprotonated phenoxo oxygen atoms of two anionic [LCo]- or [L1Co]- units. Air oxidation of these trinuclear complexes does not yield complexes associating CoIII and GdIII ions. With the first ligand, the structurally characterized resulting complex is the neutral mononuclear LCoIII compound, while in the second case, oxidation of the CoII ions turned out to be impossible. The [L1CoLnCoL1]+ complexes behave as single-molecule magnets with effective energy barriers for the reversal of magnetization varying from Ueff = 51.3 K, τo = 2 × 10-6 s for the yttrium complex to Ueff = 29.5, 29.4, 27.4 K and τo = 1.3 × 10-7, 1.47 × 10-7, 1.50 × 10-7 s for the gadolinium ones, depending on the used counteranions. The energy decrease is compensated by the suppression of quantum tunneling of magnetization in absence of applied field, thanks to the introduction of a ferromagnetic Co-Gd interaction. Current work also shows that uncritical use of conventional spin Hamiltonians, based on quenched orbital momenta, can be misleading and that ab initio calculations are indispensable for establishing the picture of real magnetic interaction. Ab initio calculations show that the CoII sites in the investigated compounds possess large unquenched orbital moments due to the first-order spin-orbit coupling resulting in strongly axial magnetic anisotropy. Although the CoII ions are not axial enough for showing slow relaxation of magnetization by themselves, blocking barriers of exchange type are obtained thanks to the exchange interaction with GdIII ions.

3.
Chemistry ; 24(62): 16652-16661, 2018 Nov 07.
Artigo em Inglês | MEDLINE | ID: mdl-30107056

RESUMO

The magnetism and magnetization blocking of a series of [Fe2 Dy2 (OH)2 (teaH)2 (RC6 H4 COO)6 ] complexes was investigated, in which teaH3 =triethanolamine and R=meta-CN (1), para-CN (2), meta-CH3  (3), para-NO2  (4) and para-CH3  (5), by combining ab initio calculations and EPR measurements. The results of broken-symmetry DFT calculations show that in all compounds the Fe-Fe exchange interaction is antiferromagnetic and stronger by far than the Fe-Dy and Dy-Dy interactions. As a result, the lowest two exchange doublets probed by EPR spectroscopy mostly originate from the Ising interaction of the dysprosium ions in all compounds. A correct quantitative description of the splitting of these two doublets requires, however, an explicit account of the Fe-Dy and Fe-Fe interactions. Due to the inversion symmetry of the complexes, the doublets under consideration are described by a collinear Ising exchange interaction. This picture is also supported by the EPR spectra, which could be simulated with parameters close to those extracted from the calculations. The magneto-structural analysis shows an increase of the antiferromagnetic Fe-Fe exchange interaction with increasing Fe-O-Fe angle and Fe-Fe distance. For the Dy-Fe interaction, the opposite tendency is observed, that is, a decrease of antiferromagnetic exchange coupling with increasing Dy-O-Fe angle and Dy-Fe distance. The transversal g factors extracted from the ab initio calculations have values in the range of 0.01-0.2, testifying to the lack of high axiality of the ground state of the dysprosium ions. This explains the lack/poor single-molecule magnetic behavior of this series of compounds at the investigated temperatures of a few Kelvin. Due to a very small gap (fractions of a wavenumber) between the ground and first-excited exchange doublet, relaxation takes place by magnetic moment reversal at individual dysprosium sites in the considered temperature domain.

4.
Angew Chem Int Ed Engl ; 57(27): 8164-8169, 2018 07 02.
Artigo em Inglês | MEDLINE | ID: mdl-29734485

RESUMO

A rare example of an organometallic terbium single-ion magnet is reported. A Tb3+ -[1]ferrocenophane complex displays a larger barrier to magnetization reversal than its isostructural Dy3+ analogue, which is reminiscent of trends observed for lanthanide-bis-phthalocyanine complexes. Detailed ab initio calculations support the experimental observations and suggest a significantly larger ground-state stabilization for the non-Kramers ion Tb3+ in the Tb complex than for the Kramers-ion Dy3+ in the Dy complex.

5.
Chemistry ; 24(23): 6079-6086, 2018 Apr 20.
Artigo em Inglês | MEDLINE | ID: mdl-29388717

RESUMO

Increasing both the energy barrier for magnetization reversal and the coercive field of the hysteresis loop are significant challenges in the field of single-molecule magnets (SMMs). Coordination geometries of lanthanide ions and magnetic interactions between lanthanide ions are both important for guiding the magnetic behavior of SMMs. We report a high energy barrier of 657 K (457 cm-1 ) in a diamagnetic-ion-diluted lanthanide chain compound with a constrained bisphenoid symmetry (D2d ); this energy barrier is substantially higher than the barrier of 567 K (394 cm-1 ) of the non-diluted chain compound with intrachain ferromagnetic interactions. Although intrachain magnetic interaction lowers the energy barrier for magnetization reversal, it can greatly enhance the coercive fields and zero-field remanence of the hysteresis loops, which is crucial for the rational design of high-performance SMMs. Factors related to the coordination sphere of the lanthanide center, which govern the high magnetic relaxation barriers through the second excited Kramer's doublets and the magnetic interactions that affect the hysteresis loops, were revealed through ab initio calculations.

6.
Dalton Trans ; 46(3): 638-642, 2017 Jan 17.
Artigo em Inglês | MEDLINE | ID: mdl-27991621

RESUMO

Herein we explored the transitions of two magnetic interaction states (antiferromagnetic or ferromagnetic) upon structural variations in two dinuclear Dy(iii) complexes.

7.
Inorg Chem ; 55(23): 12158-12171, 2016 Dec 05.
Artigo em Inglês | MEDLINE | ID: mdl-27934418

RESUMO

The polynuclear compounds containing anisotropic metal ions often exhibit efficient barriers for blocking of magnetization at fairly arbitrary geometries. However, at variance with mononuclear complexes, which usually become single-molecule magnets (SMM) under the sole requirement of a highly axial crystal field at the metal ion, the factors influencing the SMM behavior in polynuclear complexes, especially, with weakly axial magnetic ions, still remain largely unrevealed. As an attempt to clarify these conditions, we present here the synthesis, crystal structures, magnetic behavior, and ab initio calculations for a new series of NiII-LnIII-WV trimetallics, [(CN)7W(CN)Ni(H2O)(valpn)Ln(H2O)4]·H2O (Ln = Y 1, Eu 2, Gd 3, Tb 4, Dy 5, Lu 6). The surprising finding is the absence of the magnetic blockage even for compounds involving strongly anisotropic DyIII and TbIII metal ions. This is well explained by ab initio calculations showing relatively large transversal components of the g-tensor in the ground exchange Kramers doublets of 1 and 4 and large intrinsic tunneling gaps in the ground exchange doublets of 3 and 5. In order to get more insight into this behavior, another series of earlier reported compounds with the same trinuclear [WVNiIILnIII] core structure, [(CN)7W(CN)Ni(dmf)(valdmpn)Ln(dmf)4]·H2O (Ln = GdIII 7, TbIII 8a, DyIII 9, HoIII 10), [(CN)7W(CN)Ni(H2O)(valdmpn)Tb(dmf)2.5(H2O)1.5]·H2O·0.5dmf 8b, and [(CN)7W(CN)Ni(H2O)(valdmpn)Er(dmf)3(H2O)1]·H2O·0.5dmf 11, has been also investigated theoretically. In this series, only 8b exhibits SMM behavior which is confirmed by the present ab initio calculations. An important feature for the entire series is the strong ferromagnetic coupling between Ni(II) and W(V), which is due to an almost perfect trigonal dodecahedron geometry of the octacyano wolframate fragment. The reason why only 8b is an SMM is explained by positive zero-field splitting on the nickel site, precluding magnetization blocking in complexes with fewer axial Ln ions. Further analysis has shown that, in the absence of ZFS on Ni ion, all compounds in the two series (except those containing Y and Gd) would be SMMs. The same situation arises for perfectly axial ZFS on Ni(II) with the main anisotropy axis parallel to the main magnetic axis of Ln(III) ions. In all other cases the ZFS on Ni(II) will worsen the SMM properties. The general conclusion is that the design of efficient SMMs on the basis of such complexes should involve isotropic or weekly anisotropic metal ions, such as Mn(II), Fe(III), etc., along with strongly axial lanthanides.

8.
Dalton Trans ; 45(42): 16556-16560, 2016 Oct 25.
Artigo em Inglês | MEDLINE | ID: mdl-27242133

RESUMO

The synthesis, structure and magnetic properties of the HAN-bridged tri-dysprosium complex [{(thd)3Dy}3HAN] (1) are described. The complex is an SMM that shows two relaxation processes owing to the presence of two geometrically distinct Dy3+ sites in 1. Ab initio calculations reveal that the magnetic ground state of 1 is characterized by magnetic frustration.

9.
J Am Chem Soc ; 138(16): 5441-50, 2016 04 27.
Artigo em Inglês | MEDLINE | ID: mdl-27054904

RESUMO

Single-molecule magnets (SMMs) with a large spin reversal barrier have been recognized to exhibit slow magnetic relaxation that can lead to a magnetic hysteresis loop. Synthesis of highly stable SMMs with both large energy barriers and significantly slow relaxation times is challenging. Here, we report two highly stable and neutral Dy(III) classical coordination compounds with pentagonal bipyramidal local geometry that exhibit SMM behavior. Weak intermolecular interactions in the undiluted single crystals are first observed for mononuclear lanthanide SMMs by micro-SQUID measurements. The investigation of magnetic relaxation reveals the thermally activated quantum tunneling of magnetization through the third excited Kramers doublet, owing to the increased axial magnetic anisotropy and weaker transverse magnetic anisotropy. As a result, pronounced magnetic hysteresis loops up to 14 K are observed, and the effective energy barrier (Ueff = 1025 K) for relaxation of magnetization reached a breakthrough among the SMMs.

10.
Sci Rep ; 6: 24046, 2016 Apr 18.
Artigo em Inglês | MEDLINE | ID: mdl-27087470

RESUMO

Combining strong magnetic anisotropy with strong exchange interaction is a long standing goal in the design of quantum magnets. The lanthanide complexes, while exhibiting a very strong ionic anisotropy, usually display a weak exchange coupling, amounting to only a few wavenumbers. Recently, an isostructural series of mixed (Ln = Gd, Tb, Dy, Ho, Er) have been reported, in which the exchange splitting is estimated to reach hundreds wavenumbers. The microscopic mechanism governing the unusual exchange interaction in these compounds is revealed here by combining detailed modeling with density-functional theory and ab initio calculations. We find it to be basically kinetic and highly complex, involving non-negligible contributions up to seventh power of total angular momentum of each lanthanide site. The performed analysis also elucidates the origin of magnetization blocking in these compounds. Contrary to general expectations the latter is not always favored by strong exchange interaction.

11.
Chemistry ; 22(15): 5309-18, 2016 Apr 04.
Artigo em Inglês | MEDLINE | ID: mdl-26918833

RESUMO

A dinuclear Co(II) complex (1) featuring unprecedented anodic and cathodic switches for single-molecule magnet (SMM) activity has been recently investigated (J. Am. Chem. Soc. 2013, 135, 14670). The presence of sandwiched radicals in different oxidation states of this compound mediates magnetic coupling between the high-spin (S=3/2) cobalt ions, which gives rise to SMM activity in both the oxidized ([1(OEt2)](+)) and reduced ([1](-)) states. This feature represents the first example of a SMM exhibiting fully reversible, dual ON/OFF switchability. Here we apply ab initio and broken-symmetry DFT calculations to elucidate the mechanisms responsible for magnetic properties and magnetization blocking in these compounds. It is found that due to the strong delocalization of the magnetic molecular orbital, there is a strong antiferromagnetic interaction between the radical and cobalt ions. The lack of high axiality of the cobalt centres explains why these compounds possess slow relaxation of magnetization only in an applied dc magnetic field.

12.
Chem Sci ; 7(3): 2128-2137, 2016 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-29899940

RESUMO

The structures and magnetic properties of the arsenic- and selenium-ligated dysprosium single-molecule magnets (SMMs) [Cp'3Dy(AsH2Mes)] (3-Dy), [(η5-Cp'2Dy){µ-As(H)Mes}]3 (4-Dy), [Li(thf)4]2[(η5-Cp'2Dy)3(µ3-AsMes)3Li] ([Li(thf)4]2[5-Dy]), and [(η5-Cp'2Dy){µ-SeMes}]3 (6-Dy) are described. The arsenic-ligated complexes 4-Dy and 5-Dy are the first SMMs to feature ligands with metalloid elements as the donor atoms. The arsenide-ligated complex 4-Dy and the selenolate-ligated complex 6-Dy show large anisotropy barriers in the region of 250 cm-1 in zero d.c. field, increasing to 300 cm-1 upon 5% magnetic dilution. Theoretical studies reveal that thermal relaxation in these SMMs occurs via the second-excited Kramers' doublet. In contrast, the arsinidene-ligated SMM 5-Dy gives a much smaller barrier of 23 cm-1, increasing to 35 cm-1 upon dilution. The field-dependence of the magnetization for 4-Dy and 5-Dy at 1.8 K show unusual plateaus around 10 kOe, which is due to the dominance of arsenic-mediated exchange over the dipolar exchange. The effects of the exchange interactions are more pronounced in 5-Dy, which is a consequence of a small but significant increase in the covalent contribution to the predominantly ionic dysprosium-arsenic bonds. Whereas the magnetically non-dilute dysprosium SMMs show only very narrow magnetization versus field hysteresis loops at 1.8 K, the impact of magnetic dilution is dramatic, with butterfly-shaped loops being observed up to 5.4 K in the case of 4-Dy. Our findings suggest that ligands with heavier p-block element donor atoms have considerable potential to be developed more widely for applications in molecular magnetism.

13.
Angew Chem Int Ed Engl ; 54(34): 9861-5, 2015 Aug 17.
Artigo em Inglês | MEDLINE | ID: mdl-26119180

RESUMO

Multitopic organic linkers can provide a means to organize metal cluster nodes in a regular three-dimensional array. Herein, we show that isonicotinic acid N-oxide (HINO) serves as the linker in the formation of a metal-organic framework featuring Dy2 single-molecule magnets as nodes. Importantly, guest solvent exchange induces a reversible single-crystal to single-crystal transformation between the phases Dy2(INO)4(NO3)2⋅2 solvent (solvent=DMF (Dy2-DMF), CH3CN (Dy2-CH3CN)), thereby switching the effective magnetic relaxation barrier (determined by ac magnetic susceptibility measurements) between a negligible value for Dy2-DMF and 76 cm(-1) for Dy2-CH3CN. Ab initio calculations indicate that this difference arises not from a significant change in the intrinsic relaxation barrier of the Dy2 nodes, but rather from a slowing of the relaxation rate of incoherent quantum tunneling of the magnetization by two orders of magnitude.

14.
Inorg Chem ; 54(12): 5784-94, 2015 Jun 15.
Artigo em Inglês | MEDLINE | ID: mdl-26020445

RESUMO

The unique enantiopure {[Λ-Co(II)((R)-mpm)2]3[W(V)(CN)8]2}·9H2O [(R)-1] and {[Δ-Co(II)((S)-mpm)2]3[W(V)(CN)8]2}·9H2O [(S)-1], where mpm = α-methylpyridinemethanol, magnetic spongelike materials, crystallizing in the chiral P21 space group, are constructed of cyanido-bridged {Co3W2} trigonal bipyramids with three cis-[Co(II)(mpm)2(µ-NC)2] moieties in equatorial sites and two [W(V)(CN)8](3-) units in apical positions. The arrangement of {Co3W2} clusters in the crystal lattice is controlled by interactions with crystallization H2O molecules, resulting in two independent hydrogen-bonding systems: the first weaving along open channels in the a direction (weakly bonded H2O) and the second closed in the cages formed by the surrounding [W(CN)8](3-) and mpm fragments (strongly bonded H2O). The strong optical activity of (R)- and (S)-1 together with continuous chirality measure (CCM) analysis confirms the chirality transfer from enantiopure (R)- and (S)-mpm to [Co(mpm)2(µ-NC)2] units, a cyanido-bridged skeleton, and to the whole crystal lattice. Magnetic properties confronted with ab initio calculations prove the ferromagnetic couplings within Co(II)-NC-W(V) linkages inside {Co3W2} molecules, accompanied by weak antiferromagnetic intermolecular interactions. The reversible removal of weakly bonded H2O above 50 °C induces the structural phase transition 1 ⇄ 1deh and strongly affects the magnetic characteristics. The observed changes can be interpreted in terms of the combined effects of the decreasing strength of ferromagnetic Co(II)-W(V) coupling and the increasing role of antiferromagnetic intermolecular correlation, both connected with dehydration-induced structural modifications in the clusters' core and supramolecular network of 1.

15.
Chem Commun (Camb) ; 51(52): 10373-6, 2015 Jul 04.
Artigo em Inglês | MEDLINE | ID: mdl-25990868

RESUMO

Magneto-structural relationships were studied experimentally and theoretically for two enantiomorphic tetranuclear [CuTb]2 SMMs. For the first time, the determination of the magnetic anisotropy axis of an individual magnetic ion, Tb(3+), was achieved in a polynuclear Tb(3+)-based SMM.

16.
Chemistry ; 21(2): 579-89, 2015 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-25359612

RESUMO

Functionalization of the PNP pincer ligand backbone allows for a comparison of the dialkyl amido, vinyl alkyl amido, and divinyl amido ruthenium(II) pincer complex series [RuCl{N(CH2 CH2 PtBu2 )2 }], [RuCl{N(CHCHPtBu2 )(CH2 CH2 PtBu2 )}], and [RuCl{N(CHCHPtBu2 )2 }], in which the ruthenium(II) ions are in the extremely rare square-planar coordination geometry. Whereas the dialkylamido complex adopts an electronic singlet (S=0) ground state and energetically low-lying triplet (S=1) state, the vinyl alkyl amido and the divinyl amido complexes exhibit unusual triplet (S=1) ground states as confirmed by experimental and computational examination. However, essentially non-magnetic ground states arise for the two intermediate-spin complexes owing to unusually large zero-field splitting (D>+200 cm(-1) ). The change in ground state electronic configuration is attributed to tailored pincer ligand-to-metal π-donation within the PNP ligand series.


Assuntos
Complexos de Coordenação/química , Rutênio/química , Alquilação , Amidas/química , Butadienos/química , Espectroscopia de Ressonância Magnética , Modelos Moleculares , Compostos de Vinila/química
17.
Chem Commun (Camb) ; 51(11): 2044-7, 2015 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-25536910

RESUMO

We report the synthesis, structure and magnetic properties of the first 4d-4f single-molecule magnet. The complex [Ru(III)2Dy(III)2(OMe)2(O2CPh)4(mdea)2(NO3)2] displays a butterfly type core, with an anisotropy barrier of 10.7 cm(-1). Ab initio and DFT calculations provide insight into the observed magnetic behaviour.

18.
Inorg Chem ; 53(18): 9785-99, 2014 Sep 15.
Artigo em Inglês | MEDLINE | ID: mdl-25170649

RESUMO

The 3,5-dichlorobenzoate anion, L(-), serves as a bridging ligand and 2,2'-bipyridine, bipy, as a terminal bidentate ligand to yield, through hydrothermal syntheses, the tetranuclear clusters Dy2Co2L10(bipy)2, 1, and Ln2Ni2L10(bipy)2, where Ln is the trivalent La, 2, Gd, 3, Tb, 4, Dy, 5, or Ho, 6, ion. Single-crystal X-ray diffraction reveals that the six complexes are all isomorphous with the monoclinic P21/c space group and with lattice parameters that decrease with the lanthanide contraction. The two cobalt(II) or nickel(II) and two Ln(III) cations are linked by the 10 L(-) anions to generate Dy2Co2 or Ln2Ni2 3d-4f cationic heteronuclear clusters with a slightly bent Co···Dy···Dy···Co or Ni···Ln···Ln···Ni arrangement. Direct current magnetic susceptibility studies reveal that the complexes are essentially paramagnetic, with room-temperature χ(M)T values close to the expected values for two cobalt(II) or nickel(II) and two Ln(III) cations. The temperature dependence of χ(M)T for 1 and 5 is well reproduced by ab initio calculations with the inclusion of weak magnetic exchange between the cobalt(II) or nickel(II) and a dysprosium(III) and between two dysprosium(III) ions. The calculated magnetic exchange parameters are J(Dy-Co) = 0.2 cm(-1) and J(Dy-Dy) = 0.02 cm(-1) for 1 and J(Dy-Ni) = -0.2 cm(-1) and J(Dy-Dy) = 0.03 cm(-1) for 5. Alternating current magnetic susceptibility studies reveal that 1 and 5 exhibit slow magnetic relaxation with effective energy barriers, Ueff, for the reversal of the magnetization for 1 of 82(2) cm(-1) in a 0 Oe dc bias field and 79.4(5) cm(-1) in a 1000 Oe dc bias field and, for 5, 73(1) cm(-1) in a 0 dc bias field; the calculated energies of 66.1(1) and 61.0(1) cm(-1) for the first excited spin-orbit state of dysprosium(III) in 1 and 5 agree rather well with these effective energy barriers. The entire Arrhenius plots of the logarithm of τ, the relaxation rate of the magnetization in 1 and 5, have been fit with contributions from quantum tunneling, direct Raman scattering, and Orbach thermal processes. The observation of a low-temperature magnetization reversal mechanism in 5 but not in 1 may be understood through the calculated exchange energy spectrum in their ground state.

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