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.