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A Caneschi

Publications and source records attributed to A Caneschi.

17 recordsLinked to original sources
PubMed2005

Polynuclear lanthanide hydroxo complexes: new chemical precursors for coordination polymers.

The synthesis of hexanuclear lanthanide hydroxo complexes by controlled hydrolysis led to polymorphic compounds. The hexanuclear entities crystallize in four different ways that depend on the extent of their hydration. The four structures can be described as hexanuclear lanthanide entities with formula [Ln(6)(mu(6)-O)(mu(3)-OH)(8)(NO(3))(6)(H(2)O)(12)](2+). Two additional NO(3)(-) ions intercalate between the hexanuclear entities in order to ensure the electroneutrality of the crystal structure. Some crystallization water molecules fill the intermolecular space. The three first families of compounds (1-3) exhibit crystal structures that have previously been reported. The fourth family of compounds (4) is described here for the first time. Its chemical formula is [Ln(6)(mu(6)-O)(mu(3)-OH)(8)(NO(3))(6)(H(2)O)(12)](NO(3))(2).2H(2)O (Ln = Gd, Er, and Y). In this paper, the chemical and thermal stabilities of the hexanuclear lanthanide compounds are reported together with the magnetic properties of the Gd(III)-containing species. To use these entities as precursors for new materials, the substitution of the nitrato groups by chloride ions has been studied. Two byproduct compounds have so been obtained: The first (compound 5) is a nitrato/chloride hexanuclear compound of chemical formula [Er(6)(mu(6)-O)(mu(3)-OH)(8)(NO(3))(6)(H(2)O)(12)](NO(3))Cl.2H(2)O. The second one (compound 6) is a polymeric compound in which the hexanuclear entities are linked by an unexpected and original N(2)O(4) bridge. Its chemical formula is [Er(6)(mu(6)-O)(mu(3)-OH)(8)(NO(3))(4)(H(2)O)(11)(OH)(ONONO(2))]Cl(3).2H(2)O. Its crystal structure can be described as the juxtaposition of chainlike molecular motifs. To the best of our knowledge, this is the first example of a coordination polymer synthesized from an isolated polylanthanide hydroxo complex.

Journal Article↗
PubMed2005

Combined charge and spin density experimental study of the yttrium(III) semiquinonato complex Y(HBPz3)2(DTBSQ) and DFT calculations.

High-resolution X-ray diffraction and polarized neutron diffraction experiments have been performed on the Y-semiquinonate complex, Y(HBPz3)2(DTBSQ), in order to determine the charge and spin densities in the paramagnetic ground state, S = (1/2). The aim of these combined studies is to bring new insights to the antiferromagnetic coupling mechanism between the semiquinonate radical and the rare earth ion in the isomorphous Gd(HBPz3)2(DTBSQ) complex. The experimental charge density at 106 K yields detailed information about the bonding between the Y3+ ion and the semiquinonate ligand; the topological charge of the yttrium atom indicates a transfer of about 1.5 electrons from the radical toward the Y3+ ion in the complex, in agreement with DFT calculations. The electron density deformation map reveals well-resolved oxygen lone pairs with one lobe polarized toward the yttrium atom. The determination of the induced spin density at 1.9 K under an applied magnetic field of 9.5 T permits the visualization of the delocalized magnetic orbital of the radical throughout the entire molecule. The spin is mainly distributed on the oxygen atoms [O1 (0.12(1) mu B), O2(0.11(1) mu B)] and the carbon atoms [C21 (0.24(1) mu B), C22(0.20(1) mu B), C24(0.16(1) mu B), C25(0.12(1) mu B)] of the carbonyl ring. A significant spin delocalization on the yttrium site of 0.08(2) mu B is observed, proving that a direct overlap with the radical magnetic orbital can occur at the rare earth site and lead to antiferromagnetic coupling. The DFT calculations are in good quantitative agreement with the experimental charge density results, but they underestimate the spin delocalization of the oxygen toward the yttrium and the carbon atoms of the carbonyl ring.

Journal Article↗
PubMed2004

Finite-size effects in single chain magnets: an experimental and theoretical study.

The problem of finite-size effects in s=1/2 Ising systems showing slow dynamics of the magnetization is investigated introducing diamagnetic impurities in a Co2+-radical chain. The static magnetic properties have been measured and analyzed considering the peculiarities induced by the ferrimagnetic character of the compound. The dynamic susceptibility shows that an Arrhenius law is observed with the same energy barrier for the pure and the doped compounds while the prefactor decreases, as theoretically predicted. Multiple spin reversal has also been investigated.

Journal Article↗
PubMed2002

Ising-type magnetic anisotropy in a cobalt(II) nitronyl nitroxide compound: a key to understanding the formation of molecular magnetic nanowires.

The compound [Co(hfac)2-(NITPhOMe)2] (2) (hfac = hexafluoroacetylacetonate, NITPhOMe = 4'-methoxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) crystallizes in the triclinic P1 space group, a= 10.870(5), b = 11.520(5), c = 19.749(5) A, alpha = 78.05(5), beta = 84.20(5), gamma = 64.51(5) degrees, Z = 2. It can be considered a model system for studying the nature of the magnetic anisotropy of [Co(hfac)2(NITPhOMe)] (1), which was recently reported to behave as a molecular magnetic wire. The magnetic anisotropy of 2 was investigated by EPR spectroscopy and SQUID magnetometry both in the polycrystalline powder and in a single crystal. The experimental magnetic anisotropy was related to the anisotropy of the central ion and to the exchange interaction between the cobalt(II) ion and the radicals.

Journal Article↗
PubMed2001

Hydroxo-bridged Cubane-type tetrairon(II) clusters supported by sterically-hindered carboxylate ligands.

A series of hydroxo-bridged cubane-type tetrairon(II) clusters, [Fe(4)(mu-OH)(4)(mu-O(2)CAr(4)(-)t(BuPh))(2)(mu-OTf)(2)L(4)] (L = C(5)H(5)N (1), 4-(t)BuC(5)H(4)N (2), 3-FC(5)H(4)N (3)), were synthesized by using a sterically hindered carboxylate ligand, 2,6-di(4-tert-butylphenyl)benzoate (Ar(4)(-)t(BuPh)CO(2)(-)). Three different bridging units that mediate weak antiferromagnetic coupling interactions between the metal centers support the unprecedented cubane-type [Fe(4)(mu-OH)(4)](4+) cores in 1-3. The solution structures of 1 and 3 probed by FT-IR and (19)F NMR spectroscopy are consistent with the solid-state geometry determined by X-ray crystallography. Zero-field Mössbauer spectra of 1-3 at 4.2 K are characteristic of high-spin iron(II) centers in nearly identical coordination environments. Compound 1 undergoes two irreversible oxidation processes at ca -10 and +880 mV (vs Fc/Fc(+)), the former approaching quasi-reversible behavior with increased scan rates and a narrow potential sweep range. Comparisons are made with analogous known [Fe(4)X(4)](n)()(+) (X = O, S) units, and the structural integrity of tetrairon fragments upon a change in oxidation state is discussed together with some possible biological implications.

Algorithms↗
PubMed2001

Pressure- and temperature-induced valence tautomeric interconversion in a o-dioxolene adduct of a cobalt-tetraazamacrocycle complex.

An electronic switch at the molecular level has been realized by using a class of ionic compounds of the formula [Co(L)(diox)]Y (L = tetraazamacrocyclic ligand, Y = mononegative anion). Such compounds undergo temperature- and pressure-induced intramolecular one-electron transfer equilibria. The transition temperature of interconversion varies with the nature of the counterions Y (Y = PF6, BPh4, I). Surprisingly the effect of the anion on the transition temperature is not only governed by its volume but also by its coulombic interaction.

Journal Article↗
PubMed2000

Phonon-assisted tunneling in the quantum regime of Mn12 acetate

Longitudinal or transverse magnetic fields applied on a crystal of Mn12 acetate allows one to observe independent tunnel transitions between m = -S+p and m = S-n-p ( n = 6-10, p = 0-2 in longitudinal field and n = p = 0 in transverse field). We observe a smooth transition (in longitudinal) from coherent ground-state to thermally activated tunneling. Furthermore, two ground-state relaxation regimes show a crossover between quantum spin relaxation far from equilibrium and near equilibrium, when the environment destroys multimolecule correlations. Finally, we stress that the complete Hamiltonian of Mn12 should contain odd spin operators of low order.

Journal Article↗
PubMed2000

Anions derived from squaric acid form interionic pi-stack and layered, hydrogen-bonded superstructures with organometallic sandwich cations: the magnetic behaviour of crystalline

Depending on the stoichiometric ratio, squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione, H2SQA) reacts with [(eta6-C6H6)2Cr] in THF to form the crystalline material [(eta6-C6H6)2Cr][HSQA] (1) and in water to yield [[(eta6-C6H6)2Cr]2][SQA] x 6H2O (3); it also reacts with [(eta5-C5H5)2Co][OH] in water to form [[(eta5-C5H5)2Co]2][SQA] x 6H2O (4). Compound 1 is almost isostructural with the previously reported salt [(eta5-C5H5)2Co][HSQA] (2); its structure is based on pi-pi stacks between the benzene ligands and the hydrogen squarate anionic chains (pi-pi distance 3.375 A). Compounds 3 and 4 are isomorphous and have a structure in which layers of organometallic cations intercalate with layers of water molecules hydrogen bonded to squarate dianions. All crystals contain charge-assisted C-Hdelta+...Odelta- hydrogen bonds between the organometallic and the organic components, while negative O-H(-)...O(-) and O-H...O(2-) interactions are present in the pairs 1/3 and 2/4, respectively. In constrast to most organic salts of [(eta6-C6H6)2Cr]+ and [(eta5-C5H5)2Co]+ which are yellow, crystals of compounds 1-4 are orange. Reflectance spectra measured on the crystalline material 1 show the presence of an intense tail that can be assigned to a charge-transfer transition through the [(eta6-C6H6)2Cr]+/[HSQA]- pi-stacking interactions, while the pi stacking in 2 causes only a broadening of the band. The magnetic behaviour of 1 and 3 has been investigated by SQUID magnetometry. Both compounds are characterised by a weak antiferromagnetic interaction between the S=1/2 Cr centres of the [(eta6-C6H6)2Cr]+ cations, which is significantly stronger in 1 due to the pi-stacking with the HSQA- anions.

Journal Article↗
PubMed2000

Effects of nuclear spins on the quantum relaxation of the magnetization for the molecular nanomagnet Fe8

The strong influence of nuclear spins on resonant quantum tunneling in the molecular cluster Fe8 is demonstrated for the first time by comparing the relaxation rate of the standard Fe8 sample with two isotopic modified samples: (i) 56Fe is replaced by 57Fe, and (ii) a fraction of 1H is replaced by 2H. By using a recently developed "hole digging" method, we measured an intrinsic broadening which is driven by the hyperfine fields. Our measurements are in good agreement with numerical hyperfine calculations. For T>1.5 K, the influence of nuclear spins on the relaxation rate is less important, suggesting that spin-phonon coupling dominates the relaxation rate.

Journal Article↗
PubMed2000

Antiferromagnetic Coupling in a Gadolinium(III) Semiquinonato Complex.

The strongest antiferromagnetic coupling to Gd(III) so far reported was found for the complex [Gd(Hbpz(3))(2)(dtbsq)] small middle dot2 CHCl(3) (1; Hbpz(3)=hydrotris(pyrazolyl)borate; dtbsq=3,5-di-tert-butylsemiquinonato; see structure). At 245 K the magnetic susceptibility of 1 is lower than expected for two uncorrelated spins of 7/2 and 1/2, and the lowering of chiT with increasing temperature suggests that this is due to antiferromagnetic interaction between Gd(III) and the radical.

Journal Article↗
PubMed1991

[Hypersensitivity of the carotid sinus in syncope and its diagnostic role: therapeutic implications and follow-up].

Our aim was to assess the incidence and clinical characteristics of carotid sinus hypersensitivity in a group of subjects suffering from syncope. The total number of patients was 118. Twenty-four of them (20%) were diagnosed as epileptics (first attack) and 38 (32%) as vasodepressors. 56 patients with unexplained syncope underwent carotid sinus massage during electrocardiographic registration and pressure monitoring. 41 patients were found to have carotid sinus hypersensitivity. 34 of these 41 patients received implantation of a pacemaker. Abnormal sinus node function was noted in 30, abnormal atrioventricular node function in 3 and combined abnormal sinus node and AV node function in 1. Furthermore three other patients who were negative to massage were implanted with a pacemaker because they had an organic heart disease and recurrent syncope. Different tests as 24-hour monitoring ecg and electrophysiologic study showed no better results in the diagnosis of syncope. Our data demonstrates the diagnostic importance of carotid sinus massage in the diagnosis of syncope. The validity of this method is confirmed by the 24 month follow-up.

Adolescent↗
PubMed1990

[Incidence of exertion hypertension in normotensive individuals].

Exercise hypertension is the blood pressure response to dynamic exercise stress which is considered excessive as compared to what is normally observed in a healthy population. It is a useful indicator to assist in detecting those persons who may develop sustained hypertension. Research has been conducted on a large number of patients, carefully selected using as criteria: arterial pressure, presence of other diseases and age. The examination has been conducted by observing the stressor test on the cycloergometer and annotating the Pa with the traditional method. The data was then analyzed using statistical methods. We conclude that at the moment of maximum effort the patients can be divided into two groups based upon PaD values superior or inferior to 20% of base values or to 95 mmHg. Those who exceed these values (43% of all patients studied) are considered as effort hypertensive subjects. The work's originality derives from the statistically proven consideration that the evaluation of PaD at the 8th minute of the test with cardiac rate between 70 and 90% of the maximum theoretical cardiac rate is predictive of effort hypertension. Hence continuation of the test can be avoided with advantages in terms of time and reduction of the generic risks inherent in carrying out a maximal test. Follow up on the patients is in progress.

Adolescent↗
PubMedDate not supplied

[Physical exercise and increase in arterial pressure and temperature in a female heterozygote for anhidrotic ectodermal dysplasia].

A 51-year-old woman suffered from an increase in body temperature from 37 degrees to 38.4 degrees C after physical exercise. She did not sweat. The patient also had labile hypertension with maximum values reaching 210/130 mmHg. Tests were carried out to explore the possibility of a link between the increase in body temperature and her hypertension. Evaluation of the patient's blood pressure and temperature changes after exercise and after environmental modification suggests a pathogenetic link between hyperthermia and hypertension.

Blood Pressure↗