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

Abstract

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.

Preparation and Electrochemistry of Iron, Ruthenium, and Cobalt(II) Hexaphenanthrene Clathrochelates Designed for Efficient Electrocatalytic Hydrogen Production and Their Physisorption on Carbon Materials

“…This is consistent with observations made on other iron(II) cage complexes. 1,[8][9][10][11][12]14 Regarding the second anodic wave, the situation is different (Fig. 3b).…”

“…This is a characteristic of the tris-dioximate iron(II) clathrochelates, as reported elsewhere. 1,[8][9][10][11][12]14 On the Pt working electrode, the situation that prevails in the same potential range is more complex (Fig. 2b).…”

“…Published August 20, 2019. The electrochemical activity of 3d-metal cage complexes, such as clathrochelates, 1,2 with regard to the hydrogen evolution reaction (HER) in various acidic media, is a topic of great practical interest, which has been scrutinized by different research teams. [3][4][5][6][7][8][9][10] It is recognized that these compounds could be used as the (pre)catalysts, instead of metallic platinum, at the cathodes of proton exchange membrane (PEM) water electrolysis cells for the production of molecular hydrogen at industrial scale. 3,11 Their surface immobilization onto appropriate electrode materials of large surface area to form monolayer thick molecular mats, remains a target that would substantially reduce the catalyst loadings (down to the μg.cm -2 scale, compared to the current mg.cm -2 scale) and would optimize the HER efficiency.…”

Precise Fingerprint Determination of Vibrational Infrared Spectra in a Series of Co(II) Clathrochelates through Experimental and Theoretical Analyses

“…Cage complexes with encapsulated transition d -metal ion containing the derivatives of Fe, Ni or Co (e.g., clathrochelates , ) were synthesized, − among which, the macrobicyclic cobalt­(II) tris-α-dioximates (Scheme ) allow the fabrication of a broad and interesting family of metal complexes. Such complexes have been highlighted , due to their efficiency as both homogeneous − and immobilized − electrocatalysts for hydrogen production. They are also reported to be good precatalysts for the deposition of highly catalytically active metal nanoparticles. − Their three-dimensional macropolycyclic ligands with suitable electron-withdrawing substituents (especially halogen atoms) in their ribbed chelating ligand are able to stabilize the low (probably, catalytically active) oxidation states of their encapsulated transition metal ions (in particular, iron­(I) and cobalt­(I) oxidation states ,− ), which makes them unique candidates to be applied as molecular electrocatalysts for the HER. , …”