Anna V. Vologzhanina

Ligand Aspect Ratio as a Decisive Factor for the Self-Assembly of Coordination Cages

It is possible to control the geometry and the composition of metallasupramolecular assemblies via the aspect ratio of their ligands. This point is demonstrated for a series of iron- and palladium-based coordination cages. Functionalized clathrochelate complexes with variable aspect ratios were used as rod-like metalloligands. A cubic Fe(II)8L12 cage was obtained from a metalloligand with an intermediate aspect ratio. By increasing the length or by decreasing the width of the ligand, the self-assembly process resulted in the clean formation of tetrahedral Fe(II)4L6 cages instead of cubic cages. In a related fashion, it was possible to control the geometry of Pd(II)-based coordination cages. A metalloligand with a large aspect ratio gave an entropically favored tetrahedral Pd(II)4L8 assembly, whereas an octahedral Pd(II)6L12 cage was formed with a ligand of the same length but with an increased width. The aspect ratio can also be used to control the composition of dynamic mixtures of Pd(II) cages. Out of two metalloligands with only marginally different aspect ratios, one gave rise to a self-sorted collection of Pd(II)4L8 and Pd(II)6L12 cages, whereas the other did not.

Voronoi-Dirichlet tesselation as a tool for investigation of polymorphism in molecular crystals with CwHxNyOz composition and photochromic properties.

The non-bonded interactions in five sets of polymorph substances with photochromic properties have been investigated within the Voronoi-Dirichlet approach. Twenty compounds with the general formula C(w)H(x)N(y)O(z) were analyzed. Among ten possible types of non-bonded interactions at least five types are observed in the crystal structures of compounds under discussion. For all the structures the majority of interactions involve H atoms, namely London forces (H...H and H...C) and hydrogen bonds (H...O and H...N). A conformational polymorph was stated to be characterized by a unique set of inter- and intramolecular non-bonded interactions. It was quantitatively demonstrated that molecules in the same conformation can pack in a different way, and, vice versa, the change in conformation of a molecule does not prevent a substance from realising the same set of intermolecular contacts. In accordance with the data obtained for 2,4-dinitrobenzylpyridine derivatives, only conformational polymorphs with an intramolecular N...N interaction between a nitro group and a pyridine are photochromic.

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.

A Heterometallic (Fe6Na8) Cage-like Silsesquioxane: Synthesis, Structure, Spin Glass Behavior and High Catalytic Activity

The exotic “Asian Lantern” heterometallic cage silsesquioxane [(PhSiO1.5)20(FeO1.5)6(NaO0.5)8(n-BuOH)9.6(C7H8)] (I) was obtained and characterized by X-ray diffraction, EXAFS, topological analyses and DFT calculation. The magnetic property investigations revealed that it shows an unusual spin glass-like behavior induced by a particular triangular arrangement of Fe(III) ions. Cyclohexane and other alkanes as well as benzene can be oxidized to the corresponding alkyl hydroperoxides and phenol, respectively, by hydrogen peroxide in air in the presence of catalytic amounts of complex I and nitric acid. The I-catalyzed reaction of cyclohexane, c-C6H12, with H216O2 in an atmosphere of 18O2 gave a mixture of labeled and non-labeled cyclohexyl hydroperoxides, c-C6H11–16O–16OH and c-C6H11–18O–18OH, respectively, with an 18O incorporation level of ca. 12%. Compound I also revealed high efficiency in the oxidative amidation of alcohols into amides: in the presence of complex I, only 500 ppm of iron was allowed to reach TON and TOF values of 1660 and 92 h−1.

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.

Apically linked iron(II) α-dioximate and α-oximehydrazonate bis-clathrochelates: synthesis, structure and electrocatalytic properties.

Iron(II) α-oximehydrazonate and α-dioximate bis-clathrochelates with apical hydrocarbon linkers were obtained by template condensation on an iron(II) ion followed by H(+)-catalyzed macrobicyclization of the bis-semiclathrochelate precursor with formaldehyde and triethyl orthoformate, and by transmetallation of the triethylantimony-containing clathrochelate precursor with diboron-containing bifunctional Lewis acids, respectively. The geometry of the para-phenylenediboron-capped iron(II) bis-clathrochelate studied by single-crystal X-ray diffraction is intermediate between a trigonal prism and a trigonal antiprism with a distortion angle of 20.4°; the rigidity of its C6H4 linker results in the presence of the expected three-fold pseudo-rotational B···Fe···B···B···Fe···B axis and a staggered conformation of the cyclohexane-containing chelate moieties. The cyclic voltammograms (CVs) for the oximehydrazonate bis-clathrochelates contain single one-electron (for each metallocentre, and therefore, two electrons per molecule) quasi-reversible reduction waves assigned to the redox-processes of Fe(2+/+), and no interaction is observed between the two encapsulated iron(I)-containing metallocenters; six strong electron-withdrawing ethoxy substituents in the 1,3,5-triazacyclohexane capping fragments substantially affect the potential of this reduction. The corresponding waves for the dioximate complexes are irreversible: due to the structural rigidity of the caging tris-dioximate ligands, their reduced dianionic forms are unstable on the CV time scale. The CV for the hexaethoxy bis-clathrochelate complex contains one two-electron reversible oxidation wave assigned to the metal-centered oxidation of Fe(2+/3+), whereas those for its dioximate analogs are quasi-reversible. The relative lability of the ligand cavity in binuclear oximehydrazonates causes a stabilization of both the oxidized and the reduced forms; the reduced iron(I)-containing species are highly electrocatalytically active in the hydrogen-producing 2H(+)/H2 reaction. Their higher activity as compared with that for dioximate bis-clathrochelates was explained by the higher availability of the catalytically active metallocentres for H(+) ions.

Novel Cage-Like Hexanuclear Nickel(II) Silsesquioxane. Synthesis, Structure, and Catalytic Activity in Oxidations with Peroxides

New hexanuclear nickel(II) silsesquioxane [(PhSiO1.5)12(NiO)6(NaCl)] (1) was synthesized as its dioxane-benzonitrile-water complex (PhSiO1,5)12(NiO)6(NaCl)(C4H8O2)13(PhCN)2(H2O)2 and studied by X-ray and topological analysis. The compound exhibits cylinder-like type of molecular architecture and represents very rare case of polyhedral complexation of metallasilsesquioxane with benzonitrile. Complex 1 exhibited catalytic activity in activation of such small molecules as light alkanes and alcohols. Namely, oxidation of alcohols with tert-butylhydroperoxide and alkanes with meta-chloroperoxybenzoic acid. The oxidation of methylcyclohexane gave rise to the isomeric ketones and unusual distribution of alcohol isomers.

Unusual Tri-, Hexa-, and Nonanuclear Cu(II) Cage Methylsilsesquioxanes: Synthesis, Structures, and Catalytic Activity in Oxidations with Peroxides

Three types of unusual cagelike copper(II) methylsilsesquioxanes, namely, nona- [(MeSiO1.5)18(CuO)9] 1, hexa- [(MeSiO1.5)10(HO0.5)2(CuO)6(C12H8N2)2(MeSiO1.5)10(HO0.5)1.33(CH3COO0.5)0.67(CuO)6(C12H8N2)2] 2, [(MeSiO1.5)10(CuO)6(MeO0.5)2(C10H8N2)2] 3, and trinuclear [(MeSiO1.5)8(CuO)3(C10H8N2)2] 4, were obtained in 44%, 27%, 20%, and 16% yields, respectively. Nuclearity and structural fashion of products was controlled by the choice of solvent system and ligand, specifically assisting the assembling of cage. Structures of 1-4 were determined by single-crystal X-ray diffraction analysis. Compounds 1 and 4 are the first cage metallasilsesquioxanes, containing nine and three Cu ions, respectively. Product 1 is the first observation of nonanuclear metallasilsesquioxane ever. Unique architecture of 4 represents early unknown type of molecular geometry, based on two condensed pentamembered siloxane cycles. Topological analysis of metal clusters in products 1-4 is provided. Complex 1 efficiently catalyzes oxidation of alcohols with tert-butylhydroperoxide TBHP to ketones or alkanes with H2O2 to alkyl hydroperoxides in acetonitrile.

Chloride ion-aided self-assembly of pseudoclathrochelate metal tris-pyrazoloximates.

Chloride ion-aided one-pot template self-assembly of a mixed pyrazoloxime ligand with phenylboronic acid on a corresponding metal(II) ion as a matrix afforded the first boron-capped zinc, cobalt, iron, and manganese pseudoclathrochelate tris-pyrazoloximates. The presence of a pseudocross-linking hydrogen-bonded chloride ion is critical for their formation, as the same chloride-capped complexes were isolated even in the presence of large excesses of bromide and iodide ions. As revealed by X-ray diffraction, all complexes are capped with a chloride ion via three N-H···Cl hydrogen bonds that stabilize their pseudomacrobicyclic frameworks. The MN6 coordination polyhedra possess a distorted trigonal prismatic geometry, with the distortion angles φ between their nonequivalent N3 bases of approximately 0°. Temperature dependences of the effective magnetic moment for the paramagnetic complexes showed the encapsulated metal(II) ions to be in a high-spin state in the temperature range of 2-300 K. In the case of the iron(II) pseudoclathrochelate, density functional theory (DFT) and time-dependent DFT calculations were used to assess its spin state as well as the (57)Fe Mössbauer and UV-vis-NIR parameters. Cyclic voltammetry studies performed for these pseudomacrobicyclic complexes showed them to undergo irreversible or quasi-reversible metal-localized oxidations and reductions. As no changes are observed in the presence of a substantial excess of bromide ion, no anion-exchange reaction occurs, and thus the pseudoclathrochelates have a high affinity toward chloride anions in solution.