Pathways of directed synthesis of iron(II) clathrochelates and polyclathrochelates with non-equivalent capping groups starting from antimony- and germanium-containing precursors

Alternative methods for directed syntheses of clathrochelates and polyclathrochelates with two non-equivalent capping fragments using semiclathrochelate precursors and capping group reactions are described. The lability of antimony-capped iron(II) clathrochelates in remetallation (a capping group exchange) reactions allowed to obtain of mono- and bis-clathrochelates of a general formula FeNx3X1X2 and (FeNx3X1)2X2 (where Nx2− is cyclohexanedione-1,2-dioxime dianion; X1, X2 are different capping groups). The first and unique dioximate [Fe(HNx)3(Sb(C6H5)3)](ClO4) semiclathrochelate was isolated and characterized. The reaction of this complex with differing mono- and bifunctional cross-linking agents (Lewis acids) led to the formation of clathrochelates molecules with non-equivalent capping groups. The clathrochelates with a labile triethylantimony capping group underwent remetallation in the presence of silicon dioxide as a catalyst. The first stage led to the formation of a surface-immobilized mixed SbSi-capped clathrochelate, which under the action of another capping agent desorbed from the surface gives a mono- or bis-clathrochelate depending on the nature of this agent. An alternative pathway using the bis-capping agents has been implemented for synthesis of bis-clathrochelates when reactive fragments of an initial azomethine ligand demonstrate essentially different chemical properties. The oximehydrazonate germanium-capped iron(II) bis-clathrochelate was synthesized starting from a bis-semiclathrochelate, which was initially isolated with bis-capping germanium(IV) tetraethoxide. A further H+-catalyzed macrocyclization with triethyl orthoformate produced germanium-capped bis-clathrochelate. The obtained mono- and bis-clathrochelates have been characterized using elemental analysis, PD mass, IR, UV–Vis and 1H and 13C NMR spectra, and X-ray crystallography (for FeNx3(Sb(C2H5)3)2 complex), as well as by cyclic voltammograms. The distortion angle ϕ values and the main distances in the clathrochelate frameworks have been deduced using 57Fe Mössbauer parameters, and EXAFS data and molecular mechanics calculations, respectively.