Veacheslav Vieru

A Stable Pentagonal Bipyramidal Dy(III) Single-Ion Magnet with a Record Magnetization Reversal Barrier over 1000 K

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.

Significant Enhancement of Energy Barriers in Dinuclear Dysprosium Single-Molecule Magnets Through Electron-Withdrawing Effects

The effect of electron-withdrawing ligands on the energy barriers of Single-Molecule Magnets (SMMs) is investigated. By introducing highly electron-withdrawing atoms on targeted ligands, the energy barrier was significantly enhanced. The structural and magnetic properties of five novel SMMs based on a dinuclear {Dy2} phenoxo-bridged motif are explored and compared with a previously studied {Dy2} SMM (1). All complexes share the formula [Dy2(valdien)2(L)2]·solvent, where H2valdien = N1,N3-bis(3-methoxysalicylidene) diethylenetriamine, the terminal ligand L = NO3(-) (1), CH3COO(-) (2), ClCH2COO(-) (3), Cl2CHCOO(-) (4), CH3COCHCOCH3(-) (5), CF3COCHCOCF3(-) (6), and solvent = 0.5 MeOH (4), 2 CH2Cl2 (5). Systematic increase of the barrier was observed for all complexes with the most drastic increase seen in 6 when the acac ligand of 5 was fluorinated resulting in a 7-fold enhancement of the anisotropic barrier. Ab initio calculations reveal more axial g tensors as well as higher energy first excited Kramers doublets in 4 and 6 leading to higher energy barriers for those complexes.

A {CrIII2DyIII2} Single‐Molecule Magnet: Enhancing the Blocking Temperature through 3d Magnetic Exchange

It would therefore be of greatsignificance to develop this class of materials such that thisbehavior can be observed at substantially higher temper-atures, thus greatly improving the application prospects. Thiscan potentially be achieved by increasing the anisotropyassociated with the ground state, which results in longerrelaxation times. Increases in the magnetic-anisotropy barrierhave recently been achieved by the use of lanthanide ions,which possess large single-ion magnetic anisotropies. This hasresultedinthedevelopmentofseveralmono-andpolynuclearcompounds displaying energy barriers to magnetic reversal ofupto938 K,

Influence of Guest Exchange on the Magnetization Dynamics of Dilanthanide Single-Molecule-Magnet Nodes within a Metal–Organic Framework†

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.

Influence of the Ligand Field on Slow Magnetization Relaxation versus Spin Crossover in Mononuclear Cobalt Complexes

The electronic and magnetic properties of the complexes [Co(terpy)Cl2 ] (1), [Co(terpy)(NCS)2 ] (2), and [Co(terpy)2 ](NCS)2 (3) were investigated. The coordination environment around Co(II) in 1 and 2 leads to a high-spin complex at low temperature and single-molecule magnet properties with multiple relaxation pathways. Changing the ligand field and geometry with an additional terpy ligand leads to spin-crossover behavior in 3 with a gradual transition from high spin to low spin.

Modifying the properties of 4f single-ion magnets by peripheral ligand functionalisation

We study the ligand-field splittings and magnetic properties of three ErIII single-ion magnets which differ in the peripheral ligand sphere but exhibit similar first coordination spheres by inelastic neutron scattering (INS) and SQUID magnetometry. The INS spectra of the three compounds are profoundly different pointing at a strong response of the magnetic behavior to minor structural changes, as they are e.g. encountered when depositing molecules on surfaces. The observation of several magnetic excitations within the J = 15/2 ground multiplet together with single-crystal magnetic measurements allows for the extraction of the sign and magnitude of all symmetry-allowed Stevens parameters. The parameter values and the energy spectrum derived from INS are compared to the results of state-of-the-art ab initio CASSCF calculations. Temperature-dependent alternating current (ac) susceptibility measurements suggest that the magnetisation relaxation in the investigated temperature range of 1.9 K < T < 5 K is dominated by quantum tunnelling of magnetisation and two-phonon Raman processes. The possibility of observing electron paramagnetic resonance transitions between the ground-state doublet states, which can be suppressed in perfectly axial single-ion magnets, renders the studied systems interesting as representations of quantum bits.

Modulation of slow magnetic relaxation by tuning magnetic exchange in {Cr2Dy2} single molecule magnets

A new series of heterometallic 3d–4f single molecule magnets (SMMs) of general formula [CrIII2DyIII2(OMe)2(RN{(CH2)2OH}2)2(acac)4(NO3)2] (R = Me, Et, nBu) is reported, displaying slow relaxation of the magnetization and magnetic hysteresis with non-zero coercive fields. Dynamic magnetic susceptibility experiments show that the three complexes possess anisotropy barriers of 34, 37 and 41 K (24, 29 and 26 cm−1); of similar magnitude to their {CoIII2DyIII2} counterparts. The replacement of the diamagnetic CoIII for paramagnetic CrIII ions results in significantly longer relaxation times, as observed via M(H) hysteresis at low temperatures, absent for the CoIII complexes. The present complexes are also compared to those of a similar CrIII–DyIII complex of formula [CrIII2DyIII2(OMe)2(O2CPh)4(mdea)2(NO3)2] (mdeaH2 = N-methyldiethanolamine), which displays SMM behaviour with a larger anisotropy barrier of 77 K (∼54 cm−1) and even longer relaxation times. We show that the long relaxation times compared to the CoIII analogues are due to the significant magnetic exchange interactions between the CrIII and DyIII ions, resulting in the suppression of quantum tunnelling of the magnetization (QTM) and leading to a multilevel relaxation barrier. The height of the relaxation barrier in these CrIII systems is then shown to be directly related to the strength of the exchange interactions between the CrIII and DyIII ions, showing a clear route towards enhancing the slow magnetic relaxation of coupled CrIII–DyIII systems.

Synthesis and Magnetic Properties of a New Family of Macrocyclic MII3LnIII Complexes: Insights into the Effect of Subtle Chemical Modification on Single-Molecule Magnet Behavior

Thirteen tetranuclear mixed-metal complexes of the hexaimine macrocycle (L(Pr))(6-) have been prepared in a one-pot 3:1:3:3 reaction of copper(II) acetate hydrate, the appropriate lanthanide(III) nitrate hydrate, 1,4-diformyl-2,3-dihydroxybenzene (1), and 1,3-diaminopropane. The resulting family of copper(II)-lanthanide(III) macrocyclic complexes has the general formula Cu(II)(3)Ln(III)(L(Pr))(NO(3))(3)·solvents (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Tb, Ho, Er, Tm, or Yb). X-ray crystal structure determinations carried out on [Cu(3)Ce(L(Pr))(NO(3))(3)(MeOH)(3)] and [Cu(3)Dy(L(Pr))(NO(3))(3)(MeOH)(3)] confirmed that the large Ln(III) ion is bound in the central O(6) site and the three square pyramidal Cu(II) ions in the outer N(2)O(2) sites (apical donor either nitrate anion or methanol molecule) of the Schiff base macrocycle. Only the structurally characterized Cu(3)Tb complex, reported earlier, is a single-molecule magnet (SMM): the other 12 complexes do not exhibit an out-of-phase ac susceptibility signal or hysteresis of magnetization in a dc field. Ab initio calculations allowed us to rationalize the observed magnetic properties, including the significant impact of subtle chemical modification on SMM behavior. Broken-symmetry density functional theory (BS-DFT) calculations show there is a subtle structural balance as to whether the Cu···Cu exchange coupling is ferro- or antiferromagnetic. Of the family of 13 magnetically characterized tetranuclear Cu(II)(3)Ln(III) macrocyclic complexes prepared, only the Tb(III) complex is an SMM: the theoretical reasons for this are discussed.

Square-Planar Ruthenium(II) Complexes: Control of Spin State by Pincer Ligand Functionalization

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.

Determination of magnetic anisotropy in a multinuclear Tb III -based single-molecule magnet†

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.