Molecular design of cage iron(ii) and cobalt(ii,iii) complexes with a second fluorine-enriched superhydrophobic shell

Abstract

Pentafluorophenylboron-capped iron and cobalt(II) hexachloroclathrochelate precursors were obtained by the one-pot template condensation of dichloroglyoxime with pentafluorophenylboronic acid on iron and cobalt(II) ions under vigorous reaction conditions in trifluoroacetic acid media. These reactive precursors easily undergo nucleophilic substitution with (per)fluoroarylthiolate anions, giving (per)fluoroarylsulfide macrobicyclic complexes with encapsulated iron and cobalt(II) ions; nucleophilic substitution of the cobalt(II) hexachloroclathrochelate precursor with a pentafluorophenylsulfide anion gave the target hexasulfide monoclathrochelate and the mixed-valence Co^IIICo^IICo^III bis-clathrochelate as a side product. The complexes obtained were characterized using elemental analysis, MALDI-TOF mass spectrometry, IR, UV-Vis, ⁵⁷Fe Mössbauer (for the X-rayed iron complexes), ¹H, ¹¹B, ¹³C and ¹⁹F NMR spectroscopies and by X-ray diffraction; their redox and electrocatalytic behaviors were studied using cyclic voltammetry and gas chromatography. As can be seen from the single-crystal X-ray diffraction data, the second superhydrophobic shell of such caged metal ions is formed by fluorine atoms of both the apical and ribbed (per)fluoroaryl peripheral groups. The main bond distances and chelate NC–CN angles in their molecules are similar, but rotational elongation (contraction) along the molecular C₃-pseudoaxes, accompanied by changes in the geometry of the corresponding MN₆-coordination polyhedra from a trigonal prism to a trigonal antiprism, allowed encapsulating Fe²⁺, Co²⁺ and Co³⁺ ions. The nature of an encapsulated metal ion and its oxidation state affect the M–N bond lengths, and, for cobalt(II) clathrochelate with an electronic configuration d⁷ the Jahn–Teller structural effect is observed as an alternation of the Co–N distances. Pentafluorophenylboron-capped hexachloroclathrochelate precursors, giving stable catalytically active metal(I)-containing intermediates due to the electron-withdrawing effect of their six ribbed chlorine substituents, were found to show moderate electrocatalytic activity in a 2H⁺/H₂ hydrogen-forming reaction. In the case of their ribbed-functionalized sulfide derivatives, the strong electron-withdrawing (per)fluoroaryl groups do not stabilize the reduced electrocatalytically active metal(I)-containing species as their mesomeric effect is absent or substantially decreased by steric hindrances between them.