Single Molecule Magnets

An unprecedentedly large cryogenic magnetocaloric effect is observed in gadolinium acetate tetrahydrate (see picture, Gd pink, O red, C gray). The change in its magnetic entropy is ascribed to the high magnetic density combined with dominant ferromagnetism. For the first time in a molecular complex, direct measurements of the magnetocaloric effect corroborate indirect estimates based on heat capacity and magnetization.

Single-molecule magnets (SMMs) with one transition-metal ion often rely on unusual geometry as a source of magnetically anisotropic ground state. Here we report a cobalt(II) cage complex with a trigonal prism geometry showing single ion magnet behavior with very high Orbach relaxation barrier of 152 cm–1. This, to our knowledge, is the largest reported relaxation barrier for a cobalt-based mononuclear SMM. The trigonal prismatic coordination provided by the macrocyclic ligand gives intrinsically more stable molecular species than previously reported SMMs, thus making this type of cage complexes more amendable to possible functionalization that will boost their magnetic anisotropy even further.

The homoleptic mononuclear compound [Co(bpp-COOMe)2](ClO4)2 (1)(bpp-COOMe = methyl 2,6-di(pyrazol-1-yl)pyridine-4-carboxylate) crystallizes in the monoclinic C2/c space group, and the cobalt(II) ion possesses a pseudo-octahedral environment given by the two mer-coordinated tridentate ligands. Direct-current magnetic data, single-crystal torque magnetometry, and EPR measurements disclosed the easy-axis nature of this cobalt(II) complex, which shows single-molecule magnet behavior when a static field is applied in alternating-current susceptibility measurements. Diamagnetic dilution in the zinc(II) analogue [Zn(bpp-COOMe)2](ClO4)2 (2) afforded the derivative [Zn0.95Co0.05(bpp-COOMe)2](ClO4)2 (3), which exhibits slow relaxation of magnetization even in zero field thanks to the reduction of dipolar interactions. Theoretical calculations confirmed the overall electronic structure and the magnetic scenario of the compound as drawn by experimental data, thus confirming the spin-phonon Raman relaxation mechanism, and a direct quantum tunneling in the ground state as the most plausible relaxation pathway in zero field.

Single-molecule magnets (SMMs) exhibit a magnetic bistability due to the presence of an anisotropy barrier
that, at low temperature, opposes the reversal of the molecular giant spin leading to a memory effect.
Equally interesting are one-dimensional molecular materials exhibiting slow relaxation of the magnetization
due to the anisotropic exchange interaction along the chain.
This experiment was the continuation of a previous one (HE-3911) on the detection of the magnetochiral effect
with hard X-ray on molecular magnetic helices. The observation of a strong magnetochiral effect in the
Cobalt(II) derivative with a much weaker effect in the Manganese(II) is the first comparative investigation in
this field, and is to be published very soon. This time, we investigated chiral magnetic chains with biaxial
symmetry exhibiting spin-canted arrangement of the magnetic moments. The two compounds we wanted to
investigate comprise either lanthanide(III) anisotropic ions or Jahn-Teller distorted manganese(III).

Compound [Cr5(tpda)4Cl2] (H2tpda = N2,N6-di(pyridin-2-yl)pyridine-2,6-diamine), an Extended Metal Atom Chain complex featuring two quadruply-bonded {Cr2} units, exhibits field-induced slow relaxation of its magnetization arising from the terminal chromium(II) ion and provides the first example of a chromium(II)-based Single-Molecule Magnet.

The stringlike complex [Fe4(tpda)3Cl2] (2; H2tpda = N2,N6-bis(pyridin-2-yl)pyridine-2,6-diamine) was obtained as the first homometallic extended metal atom chain based on iron(II) and oligo-α-pyridylamido ligands. The synthesis was performed under strictly anaerobic and anhydrous conditions using dimesityliron, [Fe2(Mes)4] (1; HMes = mesitylene), as both an iron source and a deprotonating agent for H2tpda. The four lined-up iron(II) ions in the structure of 2 (Fe···Fe = 2.94−2.99 Å, Fe···Fe···Fe = 171.7−168.8°) are wrapped by three doubly deprotonated twisted ligands, and the chain is capped at its termini by two chloride ions. The spectroscopic and electronic properties of 2 were investigated in dichloromethane by UV−vis−NIR absorption spectroscopy, 1H NMR spectroscopy, and cyclic voltammetry. The electrochemical measurements showed four fully resolved, quasi-reversible one-electron-redox processes, implying that 2 can adopt five oxidation states in a potential window of only 0.8 V. Direct current (dc) magnetic measurements indicate dominant ferromagnetic coupling at room temperature, although the ground state is only weakly magnetic. On the basis of density functional theory and angular overlap model calculations, this magnetic behavior was explained as being due to two pairs of ferromagnetically coupled iron(II) ions (J = −21 cm−1 using JS ̂ i ·S ̂ j convention) weakly antiferromagnetically coupled with each other. Alternating-current susceptibility data in the presence of a 2 kOe dc field and at frequencies up to 1.5 kHz revealed the onset of slow magnetic relaxation below 2.8 K, with the estimated energy barrier Ueff/kB = 10.1(1.3) K.

The synthetic approaches to design hexanuclear Lanthanide-based single molecule magnets are demonstrated in detail. Extensive analysis of the reported hexanuclear systems provides vital insight for controlling the relaxation dynamics of Ln(III)-based SMMS. The assembly of hexanuclear lanthanide-based SMMs employing several ligands is summarized. Favorable structural units could be exploited as motifs in the formation of novel Ln(III)-based SMMs, slow magnetic relaxation feature being the principal focus for them. This review will span a number of hexanuclear homometallic 4f-complexes, which have been critical in the understanding of the manner in which the lanthanide centers in a complex interact magnetically. Heterometallic 3d-4f complexes which also present hexanuclearity are purposefully evaded to merely streamline and simplify the discussion in a much focused manner. Hexanuclear lanthanide model has particularly been the recent focus of attention to study and comprehend the magnetic coupling in 4f systems. This is to draw imperative conclusions concerning the most effective super-exchange pathways permitting the most efficient intracomplex interactions by demonstrating the research developed in the last few years on the fascinating field of Ln 6-based SMMs with precise emphasis on how recent studies tend to address the issue of relaxation dynamics in these systems from synthetic point of view.

The European Research Council awarded, in the frame of the program IDEAS,an Advanced Grant to Roberta Sessoli. The funded research project, started in January 2011, is centred on the study of molecularnanomagnets, also known as Single Molecule Magnets (SMMs), i.e. a classof molecules that at low temperature exhibit magnetic hysteresis ofpure molecular origin and not related to a cooperative effect.
In this project novel phenomena originated by the combination of SMMs with conducting and magnetic substrates in hybrid structures will beexplored using an interdisciplinary approach that starts from the design and synthesis of SMMs and includes their assembling on surfacefrom solution or in high vacuum as well as the tuning of the interaction with conducting, and magnetic substrates through chemicaltuning of SMMs. We will focus on the quantum dynamics of the magnetization that seems particularly affected by the interaction witha magnetic substrate opening perspectives for novel spintronic devices and for an unexplored strategy to increase the blocking temperature of SMMs.

In continuation of our work on supramolecular architectures of single-molecule magnets (SMMs) as a promising strategy in developing their magnetic performance, in this paper we report the synthesis and single crystal X-ray structure of the centered triangular tetrairon(III) SMM, [Fe4(PhpPy)2(dpm)6], Fe4 (Hdpm = dipivaloylmethane, H3PhpPy = 2-(hydroxymethyl)-2-(4-(pyridine-4-yl)phenyl)propane-1,3-diol), and its assembly in the coordination polymer {[Fe4(PhpPy)2(dpm)6Ag](ClO4)}n, Fe4Ag, upon reaction with silver(I) perchlorate. Thanks to the presence of the pyridyl rings on the two tripodal ligands, Fe4 behaves as divergent ditopic linker, and due to the Fe4:AgClO4 1:1 ratio, Fe4Ag probably possesses a linear arrangement in which silver(I) ions are linearly coordinated by two nitrogen atoms, forming 1D chains whose positive charge is balanced by the perchlorate anions. The stabilization of such a polymeric structure can be ascribed to the long distance between the two donor nitrogen atoms (23.4 Å) and their donor power. Fe4Ag shows slow relaxation of the magnetization which follows a thermally activated process with Ueff/kB = 11.17(18) K, τ0 = 2.24(17) 10−7 s in zero field, and Ueff/kB = 14.49(5) K, τ0 = 3.88(8) 10−7 s in 1-kOe applied field, in line with what reported for tetrairon(III) SMMs acting as building blocks in polymeric structures.

Single-molecule magnets (SMMs) have been extensively studied in the last 10–15 years due to the importance of their promising application for future molecular electronics (super-high-density storage of information , quantum computers, molecular spintronics) [1–5]. Single-molecule magnets are paramagnetic molecules capable of fixing magnetization [4]. This effect is observed in high-spin molecules for which the energy spectrum of low-lying spin levels (hereinafter, the spin spectrum) has the following specific features: (a) the ground level is represented by two degenerate quantum states with the magnetic moment projections ± M and (b) the ground level is separated by a considerable energy gap from the excited levels. In particular, such a spectrum is generated due to splitting of the ground spin multiplet S in a zero magnetic field for high-spin molecules with axial symmetry and negative magnetic anisotropy D. Since the splitting of the (2 S + 1)-fold degenerate ground spin multiplet S is given by D (M s is the spin projection), a double-well potential is formed at D < 0, its minima corresponding to the quantum states with spin projections M s = + S and M s = – S [4]. These minima are separated by the energy barrier U , which enters into the exponent of the Arrhenius temperature dependence τ = τ 0 exp of the paramag-netic relaxation time τ of a cluster. Formally, the magnetic anisotropy D is determined by the relationship U = | D | S 2. The barrier U = | D | S 2 blocks tunneling transitions between the spin states M s = + S and M s = – S and, thus, fixes the magnetization of the molecule.

The experimental investigation of the molecular magnetic anisotropy in crystals in which the magnetic centers are symmetry related, but do not have a parallel orientation has been approached by using torque magnetometry.  A single crystal of the orthorhombic organometallic Cp*ErCOT [Cp*=pentamethylcyclopentadiene anion (C5 Me5 (-​) )​; COT=cyclooctatetraenedianion (C8 H8 (2-​) )​] single-​molecule magnet, characterized by the presence of two nonparallel families of molecules in the crystal, has been investigated above its blocking temperature.  The results confirm an Ising-​type anisotropy with the easy direction pointing along the pseudosymmetry axis of the complex, as previously suggested by out-​of-​equilibrium angular-​resolved magnetometry.  The use of torque magnetometry, not requiring the presence of magnetic hysteresis, proves to be even more powerful for these purposes than standard single-​crystal magnetometry.  Furthermore, exploiting the sensitivity and versatility of this technique, magnetic anisotropy has been investigated up to 150 K, providing additional information on the crystal-​field splitting of the ground J multiplet of the Er(III) ion.

Molecular electronics offers unique scientific and technological possibilities, resulting from both the nanometre scale of the devices and their reproducible chemical complexity. Two fundamental yet different effects, with no classical analogue, have been demonstrated experimentally in single-molecule junctions: quantum interference due to competing electron transport pathways, and the Kondo effect due to entanglement from strong electronic interactions. Here we unify these phenomena, showing that transport through a spin-degenerate molecule can be either enhanced or blocked by Kondo correlations, depending on molecular structure, contacting geometry and applied gate voltages. An exact framework is developed, in terms of which the quantum interference properties of interacting molecular junctions can be systematically studied and understood. We prove that an exact Kondo-mediated conductance node results from destructive interference in exchange-cotunneling. Nonstandard temperature dependences and gate-tunable conductance peaks/ nodes are demonstrated for prototypical molecular junctions, illustrating the intricate interplay of quantum effects beyond the single-orbital paradigm.

Magneto-chiral dichroism (MχD) is a non-reciprocal, i. e. directional, effect observed in magnetised chiral systems featuring an unbalanced absorption of unpolarised light depending on the direction of the magnetisation. Despite the fundamental interest in a phenomenon breaking both parity and time reversal symmetries, MχD is one of the least investigated aspects of light-matter interaction because of the weakness of the effect in most reported experiments. Here we have exploited the element selectivity of hard X-ray radiation to investigate the magneto-chiral properties of enentiopure crsytals of two isostructural molecular helicoidal chains comprising Cobalt(II) and Manganese (II) ions, respectively. A strong magneto-chiral dichroism, with Kuhn asymmetry of the order of a few percent, has been observed in the Cobalt chain system, while it is practically absent for the Manganese derivative. The spectral features of the XMχD signal differ significantly from the natural and magnetic ...

A series of Ni4 cubane complexes with the composition [Ni(hmp)(ROH)Cl]4 complexes 1−4 where R) −CH3 (complex 1), −CH2CH3 (complex 2), −CH2CH2(C4H9) (complex 3), −CH2CH2CH2(C6H11) (complex 4), hmp- is the anion of 2-hydroxymethylpyridine, t-Buhmp- is the anion of 4-tert-butyl-2-hydroxymethylpyridine, and dmb is 3,3-dimethyl- 1-butanol] and [Ni(hmp)(dmb)Br]4 (complex 5) and [Ni(t-Buhmp)(dmb)Cl]4 (complex 6) were prepared. All six complexes were characterized by dc magnetic susceptibility data to be ferromagnetically coupled to give an S ) 4 ground state with significant magnetoanisotropy (D ≈ −0.6 cm-1). Magnetization hysteresis measurements carried out on single crystals of complexes 1−6 establish the single-molecule magnet (SMM) behavior of these complexes. The exchange bias observed in the magnetization hysteresis loops of complexes 1 and 2 is dramatically decreased to zero in complex 3, where the bulky dmb ligand is employed. Fast tunneling of magnetization is observed for the high-symmetry (S4 site symmetry) Ni4 complexes in the crystal of complex 3, and the tunneling rate can even be enhanced by destroying the S4 site symmetry, as is the case for complex 4, where there are two crystallographically different Ni4 molecules, one with C2 and the other with C1 site symmetry. Magnetic ordering temperatures due to intermolecular dipolar and magnetic exchange interactions were determined by means of very low-temperature ac susceptibility measurements; complex 1 orders at 1100 mK, complex 3 at 290 mK, complex 4 at ∼80 mK, and complex 6 at <50 mK. This confirms that bulkier ligands correspond to more isolated molecules, and therefore, magnetic ordering occurs at lower temperatures for those complexes with the bulkiest ligands.

The use of triethanolamine (teaH3) in 3d/4f chemistry produces the enneanuclear cluster compound [CuII5GdIII4O2(OMe)4(teaH)4(O2CC(CH3)3)2(NO3)4]·2MeOH·2Et2O (1·2MeOH·2Et2O) whose molecular structure comprises a series of vertex- and face-sharing {GdIIICuII3} tetrahedra. Magnetic studies reveal a large number of spin states populated even at the lowest temperatures investigated. Combined with the high magnetic isotropy, this enables 1 to be an excellent magnetic refrigerant for low temperature applications.