Closely-Related Zn^II₂Ln^III₂ Complexes (Ln^III = Gd, Yb) with Either Magnetic Refrigerant or Luminescent Single-Molecule Magnet Properties

The reaction of the compartmental ligand <i>N</i>,<i>N</i>′,<i>N</i>″-trimethyl-<i>N</i>,<i>N</i>″-bis­(2-hydroxy-3-methoxy-5-methylbenzyl)­diethylenetriamine (H₂L) with Zn­(NO₃)₂·6H₂O and subsequently with Ln­(NO₃)₃·5H₂O (Ln^III = Gd and Yb) and triethylamine in MeOH using a 1:1:1:1 molar ratio leads to the formation of the tetranuclear complexes {(μ₃-CO₃)₂[Zn­(μ-L)­Gd­(NO₃)]₂}·4CH₃OH (<b>1</b>) and­{(μ₃-CO₃)₂[Zn­(μ-L)­Yb­(H₂O)]₂}­(NO₃)₂·4CH₃OH (<b>2</b>). When the reaction was performed in the absence of triethylamine, the dinuclear compound [Zn­(μ-L)­(μ-NO₃)­Yb­(NO₃)₂] (<b>3</b>) is obtained. The structures of <b>1</b> and <b>2</b> consist of two diphenoxo-bridged Zn^II–Ln^III units connected by two carbonate bridging ligands. Within the dinuclear units, Zn^II and Ln^III ions occupy the N₃O₂ inner and the O₄ outer sites of the compartmental ligand, respectively. The remaining positions on the Ln^III ions are occupied by oxygen atoms belonging to the carbonate bridging groups, by a bidentate nitrate ion in <b>1</b>, and by a coordinated water molecule in <b>2</b>, leading to rather asymmetric GdO₉ and trigonal dodecahedron YbO₈ coordination spheres, respectively. Complex <b>3</b> is made of acetate–diphenoxo triply bridged Zn^IIYb^III dinuclear units, where the Yb^III exhibits a YbO₉ coordination environment. Variable-temperature magnetization measurements and heat capacity data demonstrate that <b>1</b> has a significant magneto–caloric effect, with a maximum value of −Δ<i>S</i>_m = 18.5 J kg^–1 K^–1 at <i>T</i> = 1.9 K and <b>B</b> = 7 T. Complexes <b>2</b> and <b>3</b> show slow relaxation of the magnetization and single-molecule magnet (SMM) behavior under an applied direct-current field of 1000 Oe. The fit of the high-temperature data to the Arrhenius equation affords an effective energy barrier for the reversal of the magnetization of 19.4(7) K with τ_o = 3.1 × 10^–6 s and 27.0(9) K with τ_o = 8.8 × 10^–7 s for <b>2</b> and <b>3</b>, respectively. However, the fit of the full range of temperature data indicates that the relaxation process could take place through a Raman-like process rather than through an activated Orbach process. The chromophoric L^2– ligand is able to act as an “antenna” group, sensitizing the near-infrared (NIR) Yb^III-based luminescence in complexes <b>2</b> and <b>3</b> through an intramolecular energy transfer to the excited states of the accepting Yb^III ion. These complexes show several bands in the 945–1050 nm region, corresponding to ²F_5/2→²F_7/2 transitions arising from the ligand field splitting of both multiplets. The observed luminescence lifetimes τ_obs are 0.515 and 10 μs for <b>2</b> and <b>3</b>, respectively. The shorter lifetime for <b>2</b> is due to the presence of one coordinated water molecule on the Yb^III center (and to a lesser extent noncoordinated water molecules), facilitating vibrational quenching via O–H oscillators. Therefore, complexes <b>2</b> and <b>3</b>, combining field-induced SMM behavior and NIR luminescence, can be considered to be dual magneto–luminescent materials

Magnetism of Dendrimer-Coated Gold Nanoparticles: A Size and Functionalization Study

Highly sensitive magnetometry reveals paramagnetism in dendrimer-coated gold nanoparticles. Different types of such nanoparticles, as a result of (i) functionalizing with two distinct Percec-type dendrons, linked to gold via dodecanethiol groups, and (ii) postsynthesis annealing in a solvent-free environment that further promotes their growth have been prepared. Ultimately, for each of the two functionalization configurations, we obtain highly monodisperse and stable nanoparticles of two different sizes, with spherical shape. These characteristics allow singling out the source of the measured paramagnetic signals as exclusively arising from the undercoordinated gold atoms on the surfaces of the nanoparticles. Bulk gold and the functional groups of the ligands contribute only diamagnetically

New Dioximes as Bridging Ligands in 3d/4f-Metal Cluster Chemistry: One-Dimensional Chains of Ferromagnetically Coupled {Cu₆Ln₂} Clusters Bearing Acenaphthenequinone Dioxime and Exhibiting Magnetocaloric Properties

The employment of the tetradentate ligand acenaphthenequinone dioxime (acndH₂) for a first time in heterometallic Cu^II/Ln^III (Ln = Gd and Dy) chemistry has afforded the one-dimensional coordination polymers [Cu₆Gd₂(acnd)₆­(acndH)₆­(MeOH)₆]_<i>n</i> (<b>1</b>) and [Cu₆Dy₂(acnd)₆­(acndH)₆­(MeOH)₂]_<i>n</i> (<b>2</b>), which consist of repeating {Cu₆Ln₂} clusters that are intermolecularly linked to each other through the oximate groups of two η²:η¹:η¹:μ₃ acnd^2– ligands. The [Cu₆Ln₂­(μ₃-NO)₆­(μ-NO)₈]⁴⁺ core is unprecedented in heterometallic cluster chemistry and comprises two symmetry-related {Cu₃Ln} subunits, each with a distorted trigonal pyramidal topology. Magnetic susceptibility studies revealed the presence of predominant ferromagnetic exchange interactions within the {Cu₃Ln} subunits and weak antiferromagnetic interactions between them. As a result, the magnetic and magnetocaloric properties of the {Cu₆Gd₂}_<i>n</i> compound could be rationalized in terms of two weakly coupled <i>S</i> = 5 spins that yield a magnetic entropy change of −Δ<i>S</i>_m = 11.8 J kg^–1 K^–1 at <i>T</i> = 1.6 K for μ₀Δ<i>H</i> = 7 T

Thiocyanate Complexes of Uranium in Multiple Oxidation States: A Combined Structural, Magnetic, Spectroscopic, Spectroelectrochemical, and Theoretical Study

A comprehensive study of the complexes A₄[U­(NCS)₈] (A = Cs, Et₄N, ⁿBu₄N) and A₃[UO₂(NCS)₅] (A = Cs, Et₄N) is described, with the crystal structures of [ⁿBu₄N]₄[U­(NCS)₈]·2MeCN and Cs₃[UO₂(NCS)₅]·O_0.5 reported. The magnetic properties of square antiprismatic Cs₄[U­(NCS)₈] and cubic [Et₄N]₄[U­(NCS)₈] have been probed by SQUID magnetometry. The geometry has an important impact on the low-temperature magnetic moments: at 2 K, μ_eff = 1.21 μ_B and 0.53 μ_B, respectively. Electronic absorption and photoluminescence spectra of the uranium­(IV) compounds have been measured. The redox chemistry of [Et₄N]₄[U­(NCS)₈] has been explored using IR and UV–vis spectroelectrochemical methods. Reversible 1-electron oxidation of one of the coordinated thiocyanate ligands occurs at +0.22 V vs Fc/Fc⁺, followed by an irreversible oxidation to form dithiocyanogen (NCS)₂ which upon back reduction regenerates thiocyanate anions coordinating to UO₂²⁺. NBO calculations agree with the experimental spectra, suggesting that the initial electron loss of [U­(NCS)₈]^4– is delocalized over all NCS^– ligands. Reduction of the uranyl­(VI) complex [Et₄N]₃[UO₂(NCS)₅] to uranyl­(V) is accompanied by immediate disproportionation and has only been studied by DFT methods. The bonding in [An­(NCS)₈]^4– (An = Th, U) and [UO₂(NCS)₅]^3– has been explored by a combination of DFT and QTAIM analysis, and the U–N bonds are predominantly ionic, with the uranyl­(V) species more ionic that the uranyl­(VI) ion. Additionally, the U­(IV)–NCS ion is more ionic than what was found for U­(IV)–Cl complexes

Molecular Nanoscale Magnetic Refrigerants: A Ferrimagnetic {Cu^II₁₅Gd^III₇} Cagelike Cluster from the Use of Pyridine-2,6-dimethanol

The employment of pyridine-2,6-dimethanol in 3d/4f metal cluster chemistry has afforded a new {CuII15GdIII7} cagelike molecule with a beautiful structure built by fused triangular subunits; the compound exhibits an overall ferrimagnetic behavior with an appreciable ground-state spin value and shows promise as a low-temperature magnetic refrigerant

Molecular Nanoscale Magnetic Refrigerants: A Ferrimagnetic {Cu^II₁₅Gd^III₇} Cagelike Cluster from the Use of Pyridine-2,6-dimethanol

The employment of pyridine-2,6-dimethanol in 3d/4f metal cluster chemistry has afforded a new {Cu^II₁₅Gd^III₇} cagelike molecule with a beautiful structure built by fused triangular subunits; the compound exhibits an overall ferrimagnetic behavior with an appreciable ground-state spin value and shows promise as a low-temperature magnetic refrigerant

Asymmetric [2+2+1] cyclopentannulation of olefins. Ring expansion of 2-N-methyl-N-tosyl-cyclobutanone

alpha-N-Methyl-N-tosyl cyclobutanones 2 which had been previously prepared in good yields and high enantiomeric excesses from olefins and chiral keteniminium salts have been converted into the corresponding oxiranes 3 by reaction with dimethylsulfonium methylid. The stereochemistry of this reaction was found to be dependent on several factors which have been analyzed. Treatment of these oxiranes with a stoichiometric amount of lithium iodide in refluxing tetrahydrofuran gave excellent yields of monocyclic or fused cyclopentenones 4 resulting from a P-elimination of N-methyl-N-tosylamide from a primarily formed cyclopentanone. The ring-expansion was totally selective but for oxiranes attached to a bicyclo[4.2.0]octanone system. In all cases, the enantiomeric purities of the starting cyclobutanones were preserved throughout the sequence which thus represents a useful [2+2+1] strategy for the cyclopentannulation of olefins. (C) 2002 Elsevier Science Ltd. All rights reserved

Heptanuclear disk-like MII3LnIII4 (M=Ni, Co) coordination clusters: synthesis, structures and magnetic properties

The synthesis, characterization, crystal structures and magnetic properties of isoskeletal heptanuclear disk-like MII 3LnIII 4 coordination clusters with the general formula [CoII 3LnIII 4(µ3- OH)6L6(CF3SO3)](CF3SO3)5] where Ln= Gd (2), Y (3) and [NiII 3LnIII 4(µ3- OH)6L6(CF3SO3)](CF3SO3)5] where Ln= Dy (4), Gd (5), Y (6) are presented. All the compounds are stable in solution as confirmed by ESI-MS. Magnetic studies were performed for compounds 2, 4, 5 and 6 and indicate ferromagnetic coupling while the magnetocaloric properties of 5 are characterized by ?Sm = -15.4 Jkg-1K -1 at T = 5.0 K and ?Tad = 5.9 K at T = 2.3 K, for µ0?H = 7 T

A Detailed Study of the Magnetism of Chiral {Cr₇M} Rings: An Investigation into Parametrization and Transferability of Parameters

Compounds of general formula [Cr₇MF₃(Etglu)­(O₂C^tBu)₁₅(Phpy)] [H₅Etglu = <i>N</i>-ethyl-d-glucamine; Phpy = 4-phenylpyridine; M = Zn (<b>1</b>), Mn (<b>2</b>), Ni (<b>3</b>)] have been prepared. The structures contain an irregular octagon of metal sites formed around the penta-deprotonated Etglu^5– ligand; the chirality of <i>N</i>-ethyl-d-glucamine is retained in the final product. The seven Cr^III sites have a range of coordination environments, and the divalent metal site is crystallographically identified and has a Phpy ligand attached to it. By using complementary experimental techniques, including magnetization and specific heat measurements, inelastic neutron scattering, and electron paramagnetic resonance spectroscopy, we have investigated the magnetic features of this family of {Cr₇M} rings. Microscopic parameters of the spin Hamiltonian have been determined as a result of best fits of the different experimental data, allowing a direct comparison with corresponding parameters found in the parent compounds. We examine whether these parameters can be transferred between compounds and compare them with those of an earlier family of heterometallic rings