Alexander S. Belov

Interaction of dichloride iron(II) clathrochelate with dimercaptomaleodinitrile: synthesis of the precursor monoribbed-functionalized phthalocyaninoclathrochelates and the unexpected formation of a new thiophene-containing heterocyclic system in the ribbed chelate fragment of the clathrochelate framework.

Clathrochelate iron(II) FeBd 2(S2C2(CN)2Gm)(BF)2 tris-dioximate with a ribbed vic-dinitrile fragment was synthesized as a precursor of the monoribbed-functionalized hybrid phthalocyaninoclathrochelates by nucleophilic substitution of the vic-dichloride FeBd2(Cl2Gm)(BF)2 clathrochelate (1) with the potassium salt of dimercaptomaleodinitrile. Reaction of nitromethane with this salt was followed by the condensation of the reaction products with 1 to yield the clathrochelate with an annulated previously unknown thiazinothiophene heterocyclic system in the ribbed fragment. Both complexes were characterized on the basis of elemental analysis; MALDI-TOF mass spectrometry; IR, UV-vis, (57)Fe Mössbauer, and NMR spectroscopies; and X-ray crystallography.

Transition Ion Strikes Back: Large Magnetic Susceptibility Anisotropy in Cobalt(II) Clathrochelates

Transition-metal complexes are rarely considered as paramagnetic tags for NMR spectroscopy due to them generally having relatively low magnetic anisotropy. Here we report cobalt(II) cage complexes with the largest (among the transition-metal complexes) axial anisotropy of magnetic susceptibility, reaching as high as 12.6 × 10(-32) m(3) at room temperature. This remarkable anisotropy, which results from an unusual trigonal prismatic geometry of the complexes and translates into large negative value of the zero-field splitting energy, is high enough to promote reliable paramagnetic pseudocontact shifts at the distance beyond 2 nm. Our finding paves the way toward the applications of cobalt(II) clathrochelates as future paramagnetic tags. Given the incredible stability and functionalization versatility of clathrochelates, the fine-tuning of the caging ligand may lead to new chemically stable mononuclear single-molecule magnets, for which magnetic anisotropy is of importance.

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.

Synthesis and Temperature-Induced Structural Phase and Spin Transitions in Hexadecylboron-Capped Cobalt(II) Hexachloroclathrochelate and Its Diamagnetic Iron(II)-Encapsulating Analogue.

Template condensation of dichloroglyoxime with n-hexadecylboronic acid on the corresponding metal ion as a matrix under vigorous reaction conditions afforded n-hexadecylboron-capped iron and cobalt(II) hexachloroclathrochelates. The complexes obtained were characterized using elemental analysis, MALDI-TOF mass spectrometry, IR, UV-vis, (1)H and (13)C{(1)H} NMR, (57)Fe Mössbauer spectroscopies, SQUID magnetometry, electron paramagnetic resonance, and cyclic voltammetry (CV) and by X-ray crystallography. The multitemperature single-crystal X-ray diffraction, SQUID magnetometry, and differential scanning calorimetry experiments were performed to study the temperature-induced spin-crossover [for the paramagnetic cobalt(II) complex] and the crystal-to-crystal phase transitions (for both of these clathrochelates) in the solid state. Analysis of their crystal packing using the molecular Voronoi polyhedra and the Hirshfeld surfaces reveals the structural rearrangements of the apical long-chain alkyl substituents resulting from such phase transitions being more pronounced for a macrobicyclic cobalt(II) complex. Its fine-crystalline sample undergoes the gradual and fully reversible spin transition centered at approximately 225 K. The density functional theory calculated parameters for an isolated molecule of this cobalt(II) hexachloroclathrochelate in its low- and high-spin states were found to be in excellent agreement with the experimental data and allowed to localize the spin density within a macrobicyclic framework. CV of the cobalt(II) complex in the cathodic range contains one reversible wave assigned to the Co(2+/+) redox couple with the reduced anionic cobalt(I)-containing species stabilized by the electronic effect of six strong electron-withdrawing chlorine substituents. The quasireversible character of the Fe(2+/+) wave suggests that the anionic iron(I)-containing macrobicyclic species undergo substantial structural changes and side chemical reactions after such metal-centered reduction.

Template synthesis and structures of apically functionalized iron(ii) clathrochelates

Template condensation of three nioxime or dichloroglyoxime molecules with monosubstituted boronic acids on an iron(ii) ion matrix afforded apically functionalized tris-nioximate clathrochelates and hexachloride precursors of apically-ribbed functionalized clathratochelates, respectively. The macrobicyclic tris-nioximates containing apical o-dimethoxyphenyl, p-bromophenyl, phenylethynyl, biphenylyl, 6-pyrimidinyl, or tert-butyl substituents and the dibenzothienyl-containing hexachloroglyoximate clathrochelate thus obtained were characterized by elemental analysis and IR, UV-Vis, 1H and 13C NMR, 57Fe Mössbauer and PD mass spectroscopies. The structures of o-dimethoxyphenyl- and phenylethynyl-containing iron(ii) tris-nioximates and the dibenzothienyl precursor were established by X-ray diffraction analysis.

Structural peculiarities of a homologous series of iron(II) cage complexes with ribbed glyoximate, methylglyoximate, and dimethylglyoximate chelate fragments

The crystal and molecular structures of four representatives of the homologous series of iron(ii) clathrochelates FeBd2Gm(BF)2·1.5CHCl3, FeBd2Mm(BF)2·CH2Cl2, FeBd2Dm(BF)2·CH2Cl2, and FeBd2Dm(BF)2·0.5C6H6·0.5iso-C8H18 whose frameworks contain two α-benzyldioximate (Bd) chelate rings and one acyclic α-dioximate fragment (Gm2−, Mm2−, and Dm2− are glyoxime, methylglyoxime, and dimethylglyoxime dianions, respectively) were studied by X-ray diffraction. Two types, A and B, of independent macrobicycles were observed in the unit cell of the crystal FeBd2Gm(BF)2·1.5CHCl3. The geometry of the FeN6 coordination polyhedra of encapsulated iron(ii) ions is intermediate between a trigonal prism (TP) and a trigonal antiprism (TAP). The encapsulated iron(ii) ions occupy centers of the TP-TAP polyhedra. For the macrobicyclic derivatives of dimethylglyoxime, the distortion angles φ(average values 24.0 and 24.7°) are larger than for the glyoximate and methylglyoximate analogs (average values 20.7, 22.6, and 23.2°). The larger trigonal-prismatic distortion of the coordination polyhedron in type-B FeBd2Gm(BF)2 molecules as compared to type-A FeBd2Gm(BF)2 can be explained by a difference in their intermolecular interactions. The carbon-carbon distances in the glyoximate and methylglyoximate ribbed fragments of the clathrochelates FeBd2Gm(BF)2 and FeBd2Mm(BF)2 (average values 1.414 shorter than in the α-benzyldioximate chelate rings (about 1.45 FeBd2Gm(BF)2·1.5CHCl3 and FeBd2Dm(BF)2·CH2Cl2 have laminated structures in which chains of the clathrochelate molecules are differently oriented relative to the axes of the unit cells. The crystals FeBd2Mm(BF)2·CH2Cl2 and FeBd2Dm(BF)2·0.5C6H6·0.5 iso-C8H18 have framework structures and their solvate molecules occupy voids between the dimeric associates and the macrobicyclic molecules, respectively.

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.

Supramolecular organization of the crystals of allylsulfide clathrochelate: influence of the nature of solvate molecules

The influence of the nature of solvate molecules on the supramolecular organization of the crystals of α-benzyldioximate FeBd2((AllylS)2Gm)(BF)2 clathrochelate containing the allylsulfide substituents was studied by X-ray diffraction analysis.

Polarized beam for the IUCF cooler injector synchrotron

The Indiana University Cyclotron Facility (IUCF) is in the process of commissioning a new synchrotron that is designed to rapidly accumulate, accelerate and inject beam into the IUCF Cooler ring. The Cooler Injector Synchrotron (CIS) is filled by the strip injection of 7 MeV negative ions that were pre-accelerated by an RFQ-DTL linear accelerator. This pre-accelerator has an acceptance of 1.0 π mm mrad normalized with an injection energy of 25 keV. The injected beam should be pulsed with a maximum repetition rate of 5 Hz, have a maximum pulse width of 300 μs and a peak beam current that exceeds 300 μA. Beam commissioning will be complete by the fall of 1998 at which time CIS will be used exclusively to fill the Cooler ring with both unpolarized and polarized beams. A pulsed source of polarized negative hydrogen and deuterium ions is under construction at IUCF. The source will consist of a pulsed atomic beam which is ionized by the resonant charge exchange method. Progress in the design and construction of...

Synthesis, X-ray structure and electrochemical properties of hybrid binuclear metallophthalocyaninate-capped tris-pyridineoximates

New antimony-capped iron and nickel(II) tris-pyridineoximates with a labile triethylantimony cross-linking group were obtained by template condensation of 2-acetylpyridinoxime with triethylantimony(V) dibromide on the corresponding metal ion as a matrix. They easily undergo transmetallation (capping group exchange) with Lewis-acidic zirconium and hafnium(IV) phthalocyaninates (Pc) to give binuclear MPc-capped tris-pyridineoximates. The obtained hybrid complexes and their precursors were thoroughly characterized (among others, by single-crystal X-ray diffraction), and their redox properties were studied by cyclic and differential pulse voltammetry.