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-a-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-ribbedfunctionalized
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).