Synthesis, X-ray structures and properties of the first tris-dioximate cobalt clathrochelates with nonequivalent chelate ribbed fragments

Abstract

One-pot macrocyclization and reduction of the kinetically inert nonmacrocyclic cobalt(III) bis-α-benzyldioximate and dimethylglyoximate with BF 3·O(C 2H 5) 2 and metallic silver followed by cycloaddition of the corresponding α-dioxime to the generated in situ macrocyclic cobalt(II) bis-dioximates afforded the cobalt(II) clathrochelates with nonequivalent α-dioximate fragments. The complexes obtained were characterized using elemental analysis, MALDI-TOF mass spectrometry, IR, UV–Vis, 1H, 13C{ 1H} and 19F NMR spectroscopies, magnetochemistry, EPR, and X-ray crystallography. The coordination polyhedra of an encapsulated in a three-dimensional macrobicyclic ligand cavity cobalt(II) ion have a distorted trigonal prismatic geometry. The displacement of a caged metal ion from the centers of these polyhedra is caused mainly by the Jahn–Teller effect. Magnetochemical data for cobalt(II) clathrochelates obtained characterize them as the low-spin complexes in the temperature range of 2–400 K. The cyclic voltammograms of the synthesized clathrochelates contain the one-electron oxidation and reduction waves. The increase of the electron-donating properties of the ribbed substituents causes the negative shift of these waves. A comparative analysis of the reduction and oxidation potentials allowed to assign these processes to the cobalt-centered reduction and oxidation. The “electrochemical gap” values for clathrochelates studied are very small and characteristic of the complexes with the redox processes localized on the molecular orbitals which are close in energy.