Zero-Field SMM Behavior Triggered by Magnetic Exchange Interactions and a Collinear Arrangement of Local Anisotropy Axes in a Linear Co3 II Complex

A new linear trinuclear Co(II)3 complex with a formula of [{Co(μ-L)}2Co] has been prepared by self-assembly of Co(II) ions and the N3O3-tripodal Schiff base ligand H3L, which is obtained from the condensation of 1,1,1-tris(aminomethyl)ethane and salicylaldehyde. Single X-ray diffraction shows that this compound is centrosymmetric with triple-phenolate bridging groups connecting neighboring Co(II) ions, leading to a paddle-wheel-like structure with a pseudo-C3 axis lying in the Co−Co−Co direction. The Co(II) ions at both ends of the Co(II)3 molecule exhibit distorted trigonal prismatic CoN3O3 geometry, whereas the Co(II) at the middle presents an elongated trigonal antiprismatic CoO6 geometry. The combined analysis of the magnetic data and theoretical calculations reveal strong easy-axis agnetic anisotropy for both types of Co(II) ions (|D| values higher than 115 cm−1) with the local anisotropic axes lying on the pseudo-C3 axis of the molecule. The magnetic exchange interaction between the middle and ends Co(II) ions, extracted by using either a Hamiltonian accounting for the isotropic magnetic coupling and ZFS or the Lines’ model, was found to be medium to strong and antiferromagnetic in nature, whereas the interaction between the external Co(II) ions is weak antiferromagnetic. Interestingly, the compound exhibits slow relaxation of magnetization and open hysteresis at zero field and therefore SMM behavior. The significant magnetic exchange coupling found for [{Co(μ-L)}2Co] is mainly responsible for the quenching of QTM, which combined with the easy-axis local anisotropy of the CoII ions and the ollinearity of their local anisotropy axes with the pseudo-C3 axis favors the observation of SMM behavior at zero field

The Magnetoelastic Distortion of Multiferroic BiFeO3_3 in the Canted Antiferromagnetic State

Using THz spectroscopy, we show that the spin-wave spectrum of multiferroic BiFeO$_3$ in its high-field canted antiferromagnetic state is well described by a spin model that violates rhombohedral symmetry. We demonstrate that the monoclinic distortion of the canted antiferromagnetic state is induced by the single-ion magnetoelastic coupling between the lattice and the two nearly anti-parallel spins. The revised spin model for BiFeO$_3$ contains two new single-ion anisotropy terms that violate rhombohedral symmetry and depend on the direction of the magnetic field.Comment: 28 pages (main & supplementary), 2 figures (main article), 15 figures (supplementary material

Incorporation of CrIII into a Keggin polyoxometalate as a chemical strategy to stabilize a labile {CrIIIO4} tetrahedral conformation and promote unattended single-ion magnet properties

Polyoxometalates (POMs) provide rigid and highly symmetric coordination sites and can be used as a strategy for the stabilization of magnetic ions. Herein, we report a new member of the Keggin archetype, the Cr-centered Keggin anion [α- CrW12O40]5− (CrW12), with the unusual tetrahedral coordination of CrIII reported for the first time in POMs conferring unattended magnetic properties. POM chemistry has recently presented excellent examples of single-molecule and single-ion magnets (SMMs and SIMs) as well as molecular spin qubits; however, the majority of POM-based SIMs reported to date contain lanthanoid ions. CrW12, as the first example of a chromium(III) SIM, exhibits slow relaxation of magnetization and quantum tunneling with a single- ion magnetic behavior even above 10 K with an energy barrier for the reversal of the magnetization of 3.0 K. The first 3d-metal SIM based on a nonlacunary Keggin anion is the foundation for a new research area in POM chemistry

Dinuclear Co(II)Y(III)vs. tetranuclear CoII2YIII2 complexes: the effect of increasing molecular size on magnetic anisotropy and relaxation dynamics

A new CoII2YIII2 complex with the formula [{Co(-L)Y(NO3)}2(-CO3)2]·2CH3OH·2H2O (where H2L = N,N',N'-trimethyl-N,N'-bis(2-hydroxy-3-methoxy-5-methylbenzyl) diethylenetriamine has been prepared and its structure solved by single-crystal X-Ray diffraction. The tetranuclear structure is formed by the connection of two [Co(μ-L)Y(NO3)] dinuclear units through two carbonate bridging ligands, which exhibit a tetradentate coordination mode. The CoII ion exhibits a slightly trigonally distorted CoN3O3 coordination environment. From direct-current magnetic data a large and positive axial anisotropy parameter was extracted (D = +82.62 cm-1) and its sign unambiguously confirmed by HFEPR spectra and ab initio calculations. The extracted D value is rather larger than those previously reported for the analogous CoIIYIII dinuclear complexes, which agrees with the fact that the CoII ion in the CoII2YIII2 complex exhibits the lower distortion from the octahedral geometry in this family of CoIInYIIIn complexes. Dynamic ac magnetic measurements show that the reported compound presents field-induced slow relaxation for magnetization reversal, through a combination of direct and Raman processes below and above 4 K, respectively. Magnetic measurements on the diluted magnetic counterpart (Zn/Co = 10/1) show the persistence of these processes, pointing out their single-ion origin. The Raman relaxation process for the Co2Y2 complex is faster that those observed for the CoY dinuclear counterparts. This fact and the existence of the persistent direct process at low temperature could be because the former molecule is larger and flexible than the latter ones

A Neutrally Charged Trimethylmanganese(III) Complex: Synthesis, Characterization, and Disproportionation Chemistry

The synthesis and properties of an unusual, neutrally charged and volatile <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylethylenediamine trimethyl manganese­(III) complex, (TMEDA)­MnMe₃, are described, along with its facile disproportionation to the corresponding Mn­(II) and Mn­(IV) complexes. Characterization by single-crystal XRD, UV–vis spectroscopy, high-frequency and -field EPR (HFEPR), magnetic susceptibility, and density functional theory (DFT) computations indicate that the (TMEDA)­MnMe₃ electronic structure can be described as largely square pyramidal Mn­(III) centered. The paucity of manganese­(III) polyalkyls and the simplicity and reactivity of this compound implicate it as a potentially useful synthetic building block

.Single-Ion Magnetic Behaviour in an Iron(III) Porphyrin Complex: A Dichotomy Between High Spin and 5/2-3/2 Spin Admixture

International audienceA mononuclear iron(III) porphyrin compound exhibiting unexpectedly slow magnetic relaxation, which is a characteristic of single-ion magnet behaviour, is reported. This behaviour originates from the close proximity (approximate to 550 cm(-1)) of the intermediate-spinS=3/2 excited states to the high-spinS=5/2 ground state. More quantitatively, although the ground state is mostlyS=5/2, a spin-admixture model evidences a sizable contribution (approximate to 15 %) ofS=3/2 to the ground state, which as a consequence experiences large and positive axial anisotropy (D=+19.2 cm(-1)). Frequency-domain EPR spectroscopy allowed them(S)= |+/- 1/2&RightAngleBracket;->|+/- 3/2&RightAngleBracket; transitions to be directly accessed, and thus the very large zero-field splitting in this 3d(5)system to be unambiguously measured. Other experimental results including magnetisation, Mossbauer, and field-domain EPR studies are consistent with this model, which is also supported by theoretical calculations

Double exchange in a mixed-valent octanuclear iron cluster, [Fe8(μ4-O)4(μ-4-Cl-pz)12Cl4]−

International audienc

Analysis of magnetic anisotropy and the role of magnetic dilution in triggering Single-Molecule Magnet (SMM) behavior in a family of (CoYIII)-Y-II dinuclear complexes with easy-plane anisotropy

Three new closely related (CoYIII)-Y-II complexes of general formula [Co(mu-L)(mu-X)Y(NO3)(2)] (X- = NO3- 1, benzoate 2, or 9-anthracenecarboxylato 3) have been prepared with the compartmental ligand N,N',N ''-trimethyl-N,N ''-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H2L). In these complexes, Co-II and Y-III are triply bridged by two phenoxide groups belonging to the di-deprotonated ligand (L2-) and one ancillary anion X-. The change of the ancillary bridging group connecting Co-II and Y-III ions induces small differences in the trigonally distorted CoN3O3 coordination sphere with a concomitant tuning of the magnetic anisotropy and intermolecular interactions. Direct current magnetic, high-frequency and -field EPR (HFEPR), frequency domain Fourier transform THz electron paramagnetic resonance (FD-FT THz-EPR) measurements, and ab initio theoretical calculations demonstrate that Co-II ions in compounds 1-3 have large and positive D values (approximate to 50 cm(-1)), which decrease with increasing the distortion of the pseudo-octahedral Co-II coordination sphere. Dynamic ac magnetic susceptibility measurements indicate that compound 1 exhibits field-induced single-molecule magnet (SMM) behavior, whereas compounds 2 and 3 only display this behavior when they are magnetically diluted with diamagnetic Zn-II (Zn/Co = 10:1). In view of this, it is always advisable to use magnetically diluted complexes, in which intermolecular interactions and quantum tunneling of magnetism (QTM) would be at least partly suppressed, so that 'hidden single-ion magnet (SIM)' behavior could emerge. Field-and temperature-dependence of the relaxation times indicate the prevalence of the Raman process in all these complexes above approximately 3 K

Spectroscopic and Computational Studies of Spin States of Iron(IV) Nitrido and Imido Complexes

High-oxidation-state metal complexes with multiply bonded ligands are of great interest for both their reactivity as well as their fundamental bonding properties. This paper reports a combined spectroscopic and theoretical investigation into the effect of the apical multiply bonded ligand on the spin-state preferences of threefold symmetric iron(IV) complexes with tris(carbene) donor ligands. Specifically, singlet (S = 0) nitrido [{PhB(Im$^R$)$_3$}FeN], R = $^t$Bu (1), Mes (mesityl, 2) and the related triplet (S = 1) imido complexes, [{PhB(Im$^R$)$_3$}Fe(NR′)]$^+$, R = Mes, R′ = 1-adamantyl (3), $^t$Bu (4), were investigated by electronic absorption and Mössbauer effect spectroscopies. For comparison, two other Fe(IV) nitrido complexes, [(TIMEN$^{Ar}$)FeN]$^+$ (TIMEN$^{Ar}$ = tris[2-(3-aryl-imidazol-2-ylidene)ethyl]amine; Ar = Xyl (xylyl), Mes), were investigated by $^57$Fe Mössbauer spectroscopy, including applied-field measurements. The paramagnetic imido complexes 3 and 4 were also studied by magnetic susceptibility measurements (for 3) and paramagnetic resonance spectroscopy: high-frequency and -field electron paramagnetic resonance (for 3 and 4) and frequency-domain Fourier-transform (FD-FT) terahertz electron paramagnetic resonance (for 3), which reveal their zero-field splitting parameters. Experimentally correlated theoretical studies comprising ligand-field theory and quantum chemical theory, the latter including both density functional theory and ab initio methods, reveal the key role played by the Fe 3d$_z ^2$ (a$_1$) orbital in these systems: the nature of its interaction with the nitrido or imido ligand dictates the spin-state preference of the complex. The ability to tune the spin state through the energy and nature of a single orbital has general relevance to the factors controlling spin states in complexes with applicability as single molecule devices