N. N. Efimov

Polymorphism in a Cobalt-Based Single-Ion Magnet Tuning Its Barrier to Magnetization Relaxation

A large barrier to magnetization reversal, a signature of a good single-molecule magnet (SMM), strongly depends on the structural environment of a paramagnetic metal ion. In a crystalline state, where SMM properties are usually measured, this environment is influenced by crystal packing, which may be different for the same chemical compound, as in polymorphs. Here we show that polymorphism can dramatically change the magnetic behavior of an SMM even with a very rigid coordination geometry. For a cobalt(II) clathrochelate, it results in an increase of the effective barrier from 109 to 180 cm-1, the latter value being the largest one reported to date for cobalt-based SMMs. Our finding thus highlights the importance of identifying possible polymorphic phases in search of new, even more efficient SMMs.

Heterometallic Na6Co3 Phenylsilsesquioxane Exhibiting Slow Dynamic Behavior in its Magnetization

A heterometallic phenylsilsesquioxane [(PhSiO1,5)22(CoO)3(NaO0.5)6]⋅(EtOH)6⋅(H2O) 1 cage architecture of Co(II) ions in a triangular topology exhibits a slow dynamic behavior in its magnetization, induced by the freezing of the spins of individual molecules.

New binuclear ferrocenecarboxylates of rare-earth metals as precursors for ferrites: Syntheses, structures, and solid-phase thermolysis

New ferrocenecarboxylates of rare-earth metals, [Ln2(μ-O,η2-OOCFc)2(μ2-O,O′-OOCFc)2(η2-NO3)2(DMSO)4] (Ln = Gd (I), Tb (II), and Y (III)) and [Gd2(μ-O,η2-OOCFc)2(η2-OOCFc)4(DMSO)2(H2O)2] · 2DMSO · 2CH2Cl2 (IV), are synthesized and characterized by X-ray diffraction analysis. Unlike all earlier known ferrocenecarboxylates of rare-earth metals, in isostructural compounds I–III the Ln atoms are linked by four bridging carboxyl residues, two of which are chelate-bridging (the coordination number of Ln is 9). Binuclear structure IV is formed by two chelate-bridging carboxylate ligands (the coordination number of Gd is 9). Weak antiferromagnetic and weak ferromagnetic interactions between the Gd atoms are observed in complexes I and IV, respectively. The thermal decomposition of the synthesized compounds is studied by differential scanning calorimetry and thermogravimetry. According to the X-ray diffraction data, the final thermolysis products of the complexes in air are garnets Ln3Fe5O12.

anti-syn and anti-anti coordination of the bridging CO32− group in [Cu2(Phen)4(μ-CO3)]B10H10 binuclear complexes: Synthesis, structure, and magnetic properties

A redox reaction that occurs in the [Cu2B10H10]/Phen system in CH3CN/DMSO and CH3CN/DMF in air yields a Cu(II) binuclear complex, [(Phen)2Cu(CO3)Cu(Phen)2]2+. The [Cu2(Phen)4(μ-CO3)]B10H10 · 2.5DMSO · 2H2O (I) and [Cu2(Phen)4(μ-CO3)]B10H10 · 4DMF (II) compounds have been isolated and studied by X-ray crystallography at 150 K and EPR at 295 K. Their magnetic properties have been studied in the range 300–2 K. In the cations of both compounds, the bridging CO32− group is bidentately coordinated to two Cu atoms. The cations in I and II have different spatial orientations of the Cu-O bonds: anti-syn and anti-anti, respectively. Compound I has weak magnetic interactions caused by a short Cu…Cu distance (4.441 Å) in the dimer. No exchange coupling is observed in II.

Synthesis, structure, and ESR spectra of the new heteronuclear complex {Li4(VO)2[(OOC)2C(H)Bu]4(H2O)8}·H2O

The reaction of an aqueous solution of vanadyl sulfate VOSO4·3H2O, butylmalonic acid (H2Bumal = C4H10(COOH)2), and lithium carbonate at pH ∼4–5 gave crystals of the complex Li4(VO)2(Bumal)4(H2O)8]·H2O (1). The structure of complex 1 was established by X-ray diffraction analysis. The molecule of complex 1 consists of two mononuclear bischelate dianionic units {VO(Bumal)2(H2O)} linked by four lithium ions to form the hexanuclear heterometallic {V2Li4} structure. The crystals and an ethanolic solution of compound 1 were studied by ESR spectroscopy.

Polymeric heterometallic CuII dimethylmalonate complexes with potassium and cadmium ions

A reaction of (HPiv)2Cu2(Piv)4 with dimethylmalonic acid dipotassium salt (K2Me2mal) leads to the formation of a cage coordination polymer {(μ-H2O)6K8[(μ-H2O)Cu-(μ3,κ2-Me2mal)(μ6,κ2-Me2mal)]2[Cu2(μ5,κ2-Me2mal)2(μ5,κ2-Me2mal)2]}n (1). It was found that when 1 reacted with CdSO4·8H2O in a mixture of EtOH-H2O (3: 1), the potassium ions in 1 were displaced with cadmium(ii) ions with the formation of a heterometallic 1D-polymer [(κ1-H2O)4CdCu(μ,κ2-Me2mal)2]n (2). Compounds 1 and 2 were characterized by X-ray crystallography and ESR spectroscopy.

Unusual structure of new dimethylmalonate coordination polymer with strontium atoms and VO2+ fragments

A reaction of vanadyl sulfate VOSO4·3H2O with dimethylmalonic acid strontium salt led to the coordination polymer {Sr(H2O)8[VO(Me2mal)2]}n (1, Me2mal is the dimethylmalonic acid dianion), in which the carboxylate dianions did not form chelate metallocycles with the vanadyl ions. The complex was studied by ESR spectroscopy. The compound 1 exhibited weak spin-spin exchange interactions of ferromagnetic type between the VIV atoms.

Trigonal Prismatic Tris-pyridineoximate Transition Metal Complexes: A Cobalt(II) Compound with High Magnetic Anisotropy

High magnetic anisotropy is a key property of paramagnetic shift tags, which are mostly studied by NMR spectroscopy, and of single molecule magnets, for which magnetometry is usually used. We successfully employed both these methods in analyzing magnetic properties of a series of transition metal complexes, the so-called clathrochelates. A cobalt complex was found to be both a promising paramagnetic shift tag and a single molecule magnet because of it having large axial magnetic susceptibility tensor anisotropy at room temperature (22.5 × 10-32 m3 mol-1) and a high effective barrier to magnetization reversal (up to 70.5 cm-1). The origin of this large magnetic anisotropy is a negative value of zero-field splitting energy that reaches -86 cm-1 according to magnetometry and NMR measurements.

Synthesis and characterization of Mn(II) coordination compounds with 2-(7-bromo-2-oxo-5-phenyl-3H-1,4-benzdiazepin-1-yl)acetohydrazide and its condensation product with pyruvic acid

Complexes of MnCl2 with 2-(7-bromo-2-oxo-5-phenyl-3H-1,4-benzdiazepin-1-yl)acetohydrazide (Hydr) and its condensation product with pyruvic acid (HPv) of composition [Mn(Hydr)2Cl2] (I) and [Mn(HydrHPv)2Cl2] (II) were prepared. The complexes were characterized by conductivity and magnetic susceptibility measurements, IR and ESR spectroscopy.

Binuclear and Polynuclear Cymantrenecarboxylate Complexes of Heavy Lanthanides

New binuclear cymantrenecarboxylate complexes of rare-earth metals, [Ln2(μ-O,η2-O2CCym)2(μ2-O,O′-O2CCym)2(η2-O2CCym)2(DMSO)4] (Ln = Tb (I), Dy (II); Cym = (η5-C5H4)Mn(CO)3) and [Ln2(μ2-O,O′-O2CCym)4(η2-NO3)2(DMSO)4] (Ln = Tb (III), Dy (IV)), are synthesized and characterized by X-ray diffraction analysis. The carboxylate clusters containing the Mn2+ ion, which is formed due to the destruction of the cymantrenenecarboxylate anion, [Tb4(μ3-OH)4(μ2-O,O′-O2CCym)6(H2O)3(THF)4][MnCl4] · 4CH2Cl2 · 6THF (V) with the cubane-like structure, and [Er2Mn(μ2-O2CCym)6(η2-O2CCym)2((MeO)3PO)4] · 2MePh (VI) with linear structure, are also obtained. The magnetism of complexes I, II, V, and VI is studied in a direct magnetic field. The magnetic properties of complexes II and VI are studied in direct and alternating magnetic fields. Complex II exhibits the properties of a single-molecule magnet. The thermal decomposition of complexes I–IV is studied by differential scanning calorimetry and thermogravimetric analysis. According to the X-ray diffraction analysis data, the final thermolysis products of complexes III and IV in air are multiferroics LnMn2O5.