Template synthesis, X-ray structure, spectral and redox properties of the paramagnetic alkylboron-capped cobalt(II) clathrochelates and their diamagnetic iron(II)-containing analogs

Abstract

The n-butyl- and n-hexadecylboron-capped macrobicyclic iron and cobalt(II) tris-dioximates were synthesized by the direct template condensation of the aliphatic and aromatic α-dioximes with alkylboronic acids on either a Fe2+ or a Со2+ cation as a matrix. The clathrochelates obtained were characterized using elemental analysis, MALDI-TOF mass spectrometry, IR, UV–Vis, 1H and 13C{1H} NMR, 57Fe Mössbauer and EPR spectroscopies, magnetochemistry, and by X-ray crystallography; their redox properties were studied by cyclic voltammetry (CV). These low-spin complexes undergo no spin transitions in the temperature range from 5 to 300 K. Their trigonal prismatic (TP)–trigonal antiprismatic (TAP) molecular geometry is mainly governed by the nature of an encapsulated metal ion and that of their ribbed substituents. The encapsulated cobalt(II) ions are shifted from the centers of their CoN6 coordination polyhedra due to Jahn–Teller distortion, and their anisotropic displacement parameters proved the energetical preference for only one distorted structure. The hexadecylboron-capped clathrochelates undergo the temperature-driven phase transitions. The EPR and NMR data suggest that the length of the apical alkyl groups has no impact on the magnetic properties of the cobalt complexes, whereas that of the ribbed substituents affects a distribution of the spin density. The CV data showed that these electron-donor groups stabilize the oxidized and destabilize the reduced clathrochelate species.