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Roberta Sessoli

Publications and source records attributed to Roberta Sessoli.

At least 19 recordsLinked to original sources
PubMed2007

Effects of 3d-4f magnetic exchange interactions on the dynamics of the magnetization of Dy(III)-M(II)-Dy(III) trinuclear clusters.

[{Dy(hfac)(3)}(2){Fe(bpca)(2)}] x CHCl(3) ([Dy(2)Fe]) and [{Dy(hfac)(3)}(2){Ni(bpca)(2)}]CHCl(3) ([Dy(2)Ni]) (in which hfac(-)=1,1,1,5,5,5-hexafluoroacetylacetonate and bpca(-)=bis(2-pyridylcarbonyl)amine anion) were synthesized and characterized. Single-crystal X-ray diffraction shows that [Dy(2)Fe] and [Dy(2)Ni] are linear trinuclear complexes. Static magnetic susceptibility measurements reveal a weak ferromagnetic exchange interaction between Ni(II) and Dy(III) ions in [Dy(2)Ni], whereas the use of the diamagnetic Fe(II) ion leads to the absence of magnetic exchange interaction in [Dy(2)Fe]. Dynamic susceptibility measurements show a thermally activated behavior with the energy barrier of 9.7 and 4.9 K for the [Dy(2)Fe] and [Dy(2)Ni] complexes, respectively. A surprising negative effect of the ferromagnetic exchange interaction has been found and has been attributed to the structural conformation of these trinuclear complexes.

Journal Article↗
PubMed2006

Synthesis and characterization of mixed-valent manganese phosphonate cage complexes.

The reaction of phenylphosphonic acid (PhPO(3)H(2)) with the mixed-valent basic oxo-centered manganese triangle [Mn(3)O(O(2)CCMe(3))(6)(py)(3)] (1; where py=pyridine) in the presence of a suitable base gives four different manganese clusters depending on the identity of the base. The syntheses and structural characterization of [Mn(18)(mu(3)-O)(8)(PhPO(3))(14)(O(2)CCMe(3))(12)(py)(6)(H(2)O)(2)] (2), [Mn(7)(mu(3)-O)(3)(O(3)PPh)(3)(O(2)CCMe(3))(8)(py)(3)] (3), [Mn(9)Na(mu(3)-O)(4)(mu(4)-O)(2)(O(3)PPh)(2)(O(2)CCMe(3))(12)(H(2)O)(2)(H(2)O)(0.67)(Py)(0.33)] (4), and [Mn(13)(mu(3)-O)(8)(OMe)(8)(O(3)PPh)(4)(O(2)CCMe(3))(10)] (5) are described. Complexes 4 and 5 are homovalent Mn(III) cages, while 2 and 3 contain divalent, trivalent, and/or tetravalent ions. All the manganese centers are valence-localized, the octahedral Mn(III) sites being recognizable by marked Jahn-Teller distortions. The magnetic properties of compounds 2-5 have been investigated in the polycrystalline state by magnetic susceptibility and high-field magnetization measurements, which reveal that spin ground states vary from 0< or =S > or =8. AC susceptibility measurements performed on 4 and 5, in the 1.6-10.0 K ranges show the presence of out of AC susceptibility signal (chi(M)'') for 4, and an effective energy barrier (U(eff)) for the re-orientation of the magnetization is found to be 17 K, but for 5, the chi(M)'' maximum is found to be below 1.5 K.

Binding Sites↗
PubMed2006

Heterotopic helicand for designing heterometallic helicates.

A new diazine tetratopic helicand, H4L, is obtained from 3-formylsalicylic acid and hydrazine. The reaction between H4L and cobalt(II) perchlorate, iron(III) perchlorate, and sodium carbonate leads to triple-stranded tetranuclear anionic helicates, [L3Co(II)2Fe(III)2]2-, which are connected through Na ions, resulting in chiral coordination polymers, [L3Na2Fe2Co2(H2O)4(EtOH)2].3H2O.

Aldehydes↗
PubMed2006

A family of rare-earth-based single chain magnets: playing with anisotropy.

The first family of rare-earth-based single chain magnets is presented. Compounds of general formula [M(hfac)3(NITPhOPh)], where M = Eu, Gd, Tb, Dy, Ho, Er, or Yb, and PhOPh is the nitronyl-nitroxide radical (2,4'-benzoxo-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), have been structurally characterized and found to be isostructural. The characterization of both static and dynamic magnetic properties of the whole family is reported. Dy, Tb, and Ho compounds display slow relaxation of the magnetization, and ac susceptibility shows a thermally activated regime with energy barriers of 69, 45, and 34 K for Dy, Tb, and Ho compounds, respectively, while only a frequency-dependent susceptibility is observed for Er below 2.0 K. In Gd and Yb derivatives, antiferromagnetic interactions dominate. The pre-exponential factors differ by about 4 orders of magnitude. Finite size effects, due to naturally occurring defects, affect the static and dynamic properties of the compounds differently.

Journal Article↗
PubMed2006

Tuning anisotropy barriers in a family of tetrairon(III) single-molecule magnets with an S = 5 ground state.

Tetrairon(III) Single-Molecule Magnets (SMMs) with a propeller-like structure exhibit tuneable magnetic anisotropy barriers in both height and shape. The clusters [Fe4(L1)2(dpm)6] (1), [Fe4(L2)2(dpm)6] (2), [Fe4(L3)2(dpm)6].Et2O (3.Et2O), and [Fe4(OEt)3(L4)(dpm)6] (4) have been prepared by reaction of [Fe4(OMe)6(dpm)6] (5) with tripodal ligands R-C(CH2OH)3 (H3L1, R = Me; H3L2, R = CH2Br; H3L3, R = Ph; H3L4, R = tBu; Hdpm = dipivaloylmethane). The iron(III) ions exhibit a centered-triangular topology and are linked by six alkoxo bridges, which propagate antiferromagnetic interactions resulting in an S = 5 ground spin state. Single crystals of 4 reproducibly contain at least two geometric isomers. From high-frequency EPR studies, the axial zero-field splitting parameter (D) is invariably negative, as found in 5 (D = -0.21 cm(-1)) and amounts to -0.445 cm(-1) in 1, -0.432 cm(-1) in 2, -0.42 cm(-1) in 3.Et2O, and -0.27 cm(-1) in 4 (dominant isomer). The anisotropy barrier Ueff determined by AC magnetic susceptibility measurements is Ueff/kB = 17.0 K in 1, 16.6 K in 2, 15.6 K in 3.Et2O, 5.95 K in 4, and 3.5 K in 5. Both |D| and U(eff) are found to increase with increasing helical pitch of the Fe(O2Fe)3 core. The fourth-order longitudinal anisotropy parameter B4(0), which affects the shape of the anisotropy barrier, concomitantly changes from positive in 1 ("compressed parabola") to negative in 5 ("stretched parabola"). With the aid of spin Hamiltonian calculations the observed trends have been attributed to fine modulation of single-ion anisotropies induced by a change of helical pitch.

Journal Article↗
PubMed2006

A family of ferro- and antiferromagnetically coupled decametallic chromium(III) wheels.

The synthesis and crystal structures of a family of decametallic Cr(III) "molecular wheels" are reported, namely [Cr10(OR)20(O2CR')10] [R' = Me, R = Me (1), Et (2); R' = Et, R = Me (3), Et (4); R' = CMe3, R = Me (5), Et (6)]. Magnetic studies on 1-6 reveal a remarkable dependence of the magnetic behaviour on the nature of R. In each pair of complexes with a common carboxylate (R') the nearest neighbour CrCr magnetic exchange coupling is more antiferromagnetic for the ethoxide-bridged (R = Et) cluster than for the methoxide analogue. In complexes 2, 4 and 6 the overall coupling is weakly antiferromagnetic resulting in diamagnetic (S = 0) ground states for the cluster, whilst in 1 and 5 it is weakly ferromagnetic thus resulting in very high-spin ground states. This ground state has been probed directly in the perdeuterated version of 1 ([D]1) by inelastic neutron scattering experiments, and these support the S = 15 ground state expected for ferromagnetic coupling of ten Cr(III) ions, and they also indicate that a single J-value model is inadequate. The ground state of 5 is large but not well defined. The trends in J on changing R are further supported by density functional calculations on 1-6, which are in excellent agreement with experiment. The very large changes in the nature of the ground state between 1 and 2, and 5 and 6 are the result of relatively small changes in J that happen to cross J = 0, hence changing the sign of J.

Journal Article↗
PubMed2005

Isolated single-molecule magnets on native gold.

The incorporation of thioether groups in the structure of a Mn12 single-molecule magnet, [Mn12(O12)(L)16(H2O)4] with L = 4-(methylthio)benzoate, is a successful route to the deposition of well-separated clusters on native gold surfaces and to the addressing of individual molecules by scanning tunnelling microscopy.

Journal Article↗
PubMed2005

Advances in single-molecule magnet surface patterning through microcontact printing.

We present an implementation of strategies to deposit single-molecule magnets (SMMs) using microcontact printing microCP). We describe different approaches of microCP to print stripes of a sulfur-functionalized dodecamanganese (III, IV) cluster on gold surfaces. Comparison by atomic force microscopy profile analysis of the patterned structures confirms the formation of a chemically stable single layer of SMMs. Images based on chemical contrast, obtained by time-of-flight secondary ion mass spectrometry, confirm the patterned structure.

Crystallization↗
PubMed2004

Ferromagnetic ordering in bis(benzene)chromium(+) Bis(4,5-dimethoxybenzene-1,2-dithiolato)nickel(-).

The compound bis(benzene)chromium(+) bis(4,5-dimethoxy-1,2-benzenedithiolato)nickel(-), [Cr(bz)(2)](+)[Ni(dmox)(2)](-), was prepared as well-defined crystals by a metathesis reaction. X-ray single-crystal structure confirms the expected symmetries of the cations (D6h) and anions (D2h). The Ni-S bond length is typical for a monoanionic bis(arene)-1,2-dithiolato nickel complex thus confirming the ascribed charges. The structure reveals interpenetrating cation and anion sublattices where all ions are well separated from other ions of the same charge and with almost perpendicular symmetry planes of cations and anions. Above ca. 10 K the compound is paramagnetic and follows the Curie-Weiss law for S = 1/2 on each ion with a Weiss temperature of 8.5 K. Magnetic susceptibility and magnetization studies reveal the transition to a ferromagnetically ordered phase at 3.4 K. The ferromagnetic interaction is likely to occur through short contacts between anion heteroatoms (O and Ni) and cation H atoms, forming a 2-D network. A third dimension is possible through somewhat longer interionic S-H contacts. The Curie-Weiss law behavior can be modeled for 1-D chains with a ferromagnetic interaction of J = 11.6 K.

Journal Article↗