Pseudoclathrochelate n-hexadecylboron-capped metal(II) tris-pyrazoloximates: synthesis, X-ray structure, spectral and magnetic characteristics

Self-assembly of monohexadecylboron-capped zinc, cobalt, iron and manganese(II) tris-pyrazoloximate pseudoclathrochelates was performed using one-pot template condensation of 2-acetylpyrazole oxime with n-hexadecylboronic acid as a capping Lewis-acidic agent on the corresponding metal ion as a matrix. Hydrogen atoms of three terminal pyrazolyl NH groups of three pyrazoloxime synthons form a pseudomacrobicyclic ligand through their hydrogen bonding with a chloride counterion Cl− … (HN)3, thus allowing an encapsulation of the corresponding metal ion. The complexes obtained were characterized using elemental analysis, MALDI-TOF mass spectrometry, IR, UV-Vis-NIR, 57Fe Mossbauer (for the iron complex), 1H and 13C NMR spectroscopy, as well as by SQUID magnetometry. The structure of the complexes synthesized was also confirmed by single crystal X-ray diffraction analysis of the zinc(II) pseudoclathrochelate. Its pseudocaged zinc(II) ion is located almost in the center of ZnN6-coordination polyhedron, which adopts a trigonal prismatic geometry with a very low distortion angle. As follows from variable-temperature magnetometry data, the χT values for manganese and iron(II) pseudoclathrochelates in the temperature range 50–300 К are close to theoretical pure high-spin values without of orbital contribution. 1H and 13C NMR chemical shifts of their pyrazoloximate nuclei reach to some hundreds of ppm, while those chemical shifts of the normal hexadecyl chain's nuclei, on the distance of more than five chemical bonds from the pseudoencapsulated paramagnetic metal ion, are close to those for the diamagnetic zinc(II) pseudoclathrochelate. This is indicative of the contact nature of thus induced paramagnetic shifts, as well as a weak pseudocontact contribution.