Genrikh E. Zelinskii

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.

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.

Cytotoxicity of electrophilic iron(II)–clathrochelates in human promyelocytic leukemia cell line

We observed that electrophilic iron(II)-clathrochelates exhibit significant cytotoxicity in human promyelocytic leukemia cells (IC50=6.5±4.6μM), which correlates with the enhancement of intracellular oxidative stress (17-fold increase with respect to the cells treated with the solvent only). Based on in vitro studies we suggested that this effect is caused by alkylation of glutathione leading to inhibition of the cellular antioxidative system and by catalytic generation of reactive oxygen species by products of the alkylation reaction.

The molecular design of cage metal complexes for biological applications: pathways of the synthesis, and X-ray structures of a series of new N2-, S2- and O2-alicyclic iron(ii) di- and tetrachloroclathrochelates

The synthesis of new metal(II) di- and tetrahalogenoclathrochelates with apical functionalizing substituents as reactive macrobicyclic precursors is a key stage of the molecular design of cage metal complexes – prospective biological effectors. We found that the most convenient multistep synthetic pathway for their preparation includes (i) direct template condensation of a dihalogeno-α-dioxime with an appropriately functionalized boronic acid on the corresponding metal ion as a matrix, giving an apically functionalized metal hexahalogenoclathrochelate in a high yield; and (ii) its stepwise nucleophilic substitution with S2-, N2- or O2-bis-nucleophiles, forming stable six-membered alicyclic ribbed fragments, thus allowing obtaining the corresponding apically functionalized di- and tetrahalogenoclathrochelates. The latter reaction of an iron(II) hexachloroclathrochelate with different N2-, S2- and O2-bis-nucleophilic agents afforded chloroclathrochelate complexes with equivalent and non-equivalent alicyclic ribbed substituents, such as N2-, S2 or O2-containing six-membered cycles. In the case of anionic forms of pyrocatechol and 1,2-ethanedithiol as O2- and S2-bis-nucleophiles, generated in situ in the presence of triethylamine, such substitution proceeds easily and in a high yield. In the case of anionic derivatives of ethylenediamine as N2-bis-nucleophiles, only a mono-N2-alicyclic iron(II) tetrachloroclathrochelate was obtained in a moderate yield. The S2-alicyclic iron(II) tetrachloroclathrochelate underwent a further nucleophilic substitution of one of the two dichloroglyoximate fragments, giving its N2, S2-alicyclic dichloroclathrochelate derivative with three non-equivalent ribbed chelate fragments. The complexes obtained were characterized using elemental analysis, MALDI-TOF mass spectrometry, and IR, UV-vis, 1H and 13C{1H} NMR spectroscopies, and by single crystal X-ray diffraction (XRD). As follows from XRD data for four O2-, S2- and N2-ribbed-functionalized iron(II) clathrochelates, the geometry of their FeN6-coordination polyhedra is intermediate between a trigonal prism and a trigonal antiprism. UV-vis spectra of these cage complexes are indicative of a dramatic redistribution of the electron density in a quasiaromatic clathrochelate framework caused by its ribbed functionalization with six-membered O2-, S2- and/or N2-alicyclic substituent(s).

A New Series of Cobalt and Iron Clathrochelates with Perfluorinated Ribbed Substituents

The study tackles one of the challenges in developing platinum-free molecular electrocatalysts for hydrogen evolution, which is to seek for new possibilities to ensure large turnover numbers by stabilizing electrocatalytic intermediates. These species are often much more reactive than the initial electrocatalysts, and if not properly stabilized by a suitable choice of functionalizing substituents, they have a limited long-time activity. Here, we describe new iron and cobalt(II) cage complexes (clathrochelates) that in contrast to many previously reported complexes of this type do not act as electrocatalysts for hydrogen evolution. We argue that the most probable reason for this behavior is an excessive stabilization of the metal(I) species by perfluoroaryl ribbed groups, resulting in an unprecedented long-term stability of the metal(I) complexes even in acidic solutions.

CCDC 1433324: Experimental Crystal Structure Determination

Related Article: Genrikh E. Zelinskii, Alexander S. Belov, Irina G. Belaya, Anna V. Vologzhanina, Valentin V. Novikov, Oleg A. Varzatskii, Yan Z. Voloshin||New J.Chem.|||doi:10.1039/C7NJ03051G

Clathrochelate iron(II) tris-nioximates with non-equivalent capping groups and their precursors: synthetic strategies, X-ray structure, and reactivity

Abstract Template cross-linking of nioxime using equimolar amounts of boric and 4-vinylphenylboronic acids on an iron(II) ion as a matrix gave a mixture of mono- and divinyl-terminated clathrochelate products, which were chromatographically isolated in moderate yields and characterized by X-ray diffraction. The target complex FeNx3(BOCH3)(4-BC6H4CH=CH2) with non-equivalent capping groups was also obtained in a low yield using a transmetallation (re-boronation) of its dimethoxyboron-capped clathrochelate precursor. Re-boronation of the monomethoxyboron-capped cage complex with benzene-1,4-diboronic acid as a bifunctional Lewis-acidic agent afforded mainly the clathrochelate product of its 1:1 re-boronation having a terminal B(OH)2 group. The iron(II) clathrochelate with labile triethylantimony capping groups underwent a transmetallation on the surface of silica gel giving an immobilized Sb, Si-capped macrobicyclic intermediate. Its desorption with 4-vinylphenylboronic acid unexpectedly gave the monovinyl-terminated iron(II) semiclathrochelate as the major product, isolated in a high yield; it was X-ray structurally characterized. The geometry of FeN6-coordination polyhedra of the above semi- and clathrochelates is intermediate between a trigonal prism and a trigonal antiprism; that of the monocapped iron(II) semiclathrochelate is more TAP-distorted and its pseudoencapsulated iron(II) ion is shifted from the center of this polyhedron by 0.02 Å in the direction of the capping boron atom.