Irina Belaya

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.

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.

Encapsulation by Covalent Capsules

Overwhelming majority of covalent capsules are neutral compounds that have been prepared using classical organic reactions and synthetic approaches from classical [1–9] and modern [10–12] macrocyclic chemistry. Those are based on “bottom-to-up” principle, such as imine condensations (in many cases, followed by reduction of the cage Schiff bases formed to their amine analogs) and condensations in high-dilution conditions of diamine- or dihydroxyl-containing components (mostly the reactive macrocyclic precursors) with terminal dihalogenoalkanes or chloroanhydrides of suitable dicarboxylic acids. In contrast to supramolecular and coordination capsules (Chaps. 3 and 4), the template effect of encapsulated species is less pronounced, but in several cases, templation of these reactions by alkali or alkali-earth metal cations, neutral guests, and caged anions is reported.

Synthesis and Reactivity of Cage Metal Complexes

Synthesis of cage metal complexes of different symmetry and functionality is one of the most important tasks in the chemistry of these compounds with unique or uncommon chemical, physicochemical, and physical properties. Until publication of our first book on clathrochelate chemistry in 2002 [1], main synthetic approaches for preparation of various types of clathrochelates have included: 1. Template self-assembly on a suitable metal ion as a matrix (for all the classes of such cage complexes) 2. Coordination of an initially prepared encapsulating ligand toward an appropriate metal ion (for the conformationally labile polyamine and polyimine ligands) 3. Nucleophilic substitution with highly active nucleophiles (for the reactive halogenoclathrochelates) 4. Transmetallation (a capping group exchange) reaction [for the cage complexes with labile cross-linking group(s)] 5. Chemical transformation of apical substituent(s) at a cage framework [for the clathrochelates with reactive terminal group(s)] 6. Condensation of an appropriate macrocyclic precursor with a suitable ligand synthon 7. Macrobicyclization (cross-linking) of a reactive semiclathrochelate precursor (first of all, for the hydrazonate, oximehydrazonate, and azineoximate cage complexes) 8. Imine condensation (for the Schiff-base clathrochelates)

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

Spacial and Electronic Structure of Cage Metal Complexes

Molecular structures of cage metal complexes in the solid state have been mainly elucidated by single crystal X-ray diffraction experiments and, in several cases, by EXAFS method; those in solutions have been obtained mainly using various NMR techniques.

Practical Applications of Cage Metal Complexes

Main fields of potent practical applications of the metal clathrochelates and the corresponding clathrochelate-containing derivatives and nanosystems summarized in this chapter are based on their unprecedented chemical robustness, unusual redox and magnetic properties of an encapsulation metal ion, and three-dimensional shape of these cage molecules as well as on special selectivity of their self-assembly reactions.

Magnetic Properties, EPR, and Paramagnetic NMR Spectra of Cage Metal Complexes

Magnetometry, EPR, and NMR methods have been widely used to study electronic structure of paramagnetic transition d- and f-metal cage complexes, their magnetic characteristics, and SCO and SMM behavior of the (pseudo)clathrochelates as well as to deduce the geometry of coordination polyhedra of encapsulated metal ion(s).

Reactivity of Encapsulated Species

Unusual chemical and photochemical reactivity (or, in contrast, incredible stability) of encapsulated species and the unexpected products of such reactions within the confined cavities of molecular capsules have been observed in many cases. This can be explained by steric restrictions due to relative rigidity of covalent and coordination capsules, by isolation of these guests from external factors such as solvent effects, and by relatively strong supramolecular host–guest interactions.