Synthesis and Temperature-Induced Structural Phase and Spin Transitions in Hexadecylboron-Capped Cobalt(II) Hexachloroclathrochelate and Its Diamagnetic Iron(II)-Encapsulating Analogue

Template condensation of dichloroglyoxime
with n-hexadecylboronic acid on the corresponding
metal ion as a matrix under vigorous reaction conditions afforded n-hexadecylboron-capped iron and cobalt­(II) hexachloroclathrochelates.
The complexes obtained were characterized using elemental analysis,
MALDI-TOF mass spectrometry, IR, UV–vis, 1H and 13C­{1H} NMR, 57Fe Mössbauer spectroscopies,
SQUID magnetometry, electron paramagnetic resonance, and cyclic voltammetry
(CV) and by X-ray crystallography. The multitemperature single-crystal
X-ray diffraction, SQUID magnetometry, and differential scanning calorimetry
experiments were performed to study the temperature-induced spin-crossover
[for the paramagnetic cobalt­(II) complex] and the crystal-to-crystal
phase transitions (for both of these clathrochelates) in the solid
state. Analysis of their crystal packing using the molecular Voronoi
polyhedra and the Hirshfeld surfaces reveals the structural rearrangements
of the apical long-chain alkyl substituents resulting from such phase
transitions being more pronounced for a macrobicyclic cobalt­(II) complex.
Its fine-crystalline sample undergoes the gradual and fully reversible
spin transition centered at approximately 225 K. The density functional
theory calculated parameters for an isolated molecule of this cobalt­(II)
hexachloroclathrochelate in its low- and high-spin states were found
to be in excellent agreement with the experimental data and allowed
to localize the spin density within a macrobicyclic framework. CV
of the cobalt­(II) complex in the cathodic range contains one reversible
wave assigned to the Co2+/+ redox couple with the reduced
anionic cobalt­(I)-containing species stabilized by the electronic
effect of six strong electron-withdrawing chlorine substituents. The
quasireversible character of the Fe2+/+ wave suggests that
the anionic iron­(I)-containing macrobicyclic species undergo substantial
structural changes and side chemical reactions after such metal-centered
reduction.