New sterically hindered disubstituted imine derivatives of (thia)calix[4]arenes bearing bulky tert-butyl groups at the lower rim: synthesis, structures, and complexation ability toward Co^II and Ni^II cations in solution

New macrocyclic Schiff bases, lower rim disubstituted imine derivatives of thia- and calix[4]arenes 7 and 8 adopted in a cone stereoisomeric conformation, bearing two tert-butyl substituents grafted to iminophenol coordinating sites, and containing a spacer composed of two methylene bridges were synthesized. The prepared compounds were characterized by a complex of physicochemical methods in solution (¹H/¹³C NMR spectroscopy, MALDI TOF mass spectrometry) and in the crystalline phase (IR spectroscopy, single-crystal X-ray diffraction (XRD)). In crystals calix[4]arene 7 forms a solvate in which acetonitrile or methanol molecules are included into the macrocycle cavity, whereas the crystals of thiacalix[4]arene 8 contain no solvent molecules. The difference in the conformational behavior of the macrocyclic platform was evidenced when comparing the crystal structures of calix[4]arene 7 and thiacalix[4]arene 8. The stoichiometry and the logarithm and stability constant values of the corresponding complexes of the synthesized macrocyclic Schiff bases with 3d-metal cations (Co^II, Ni^II) in solution were determined using spectrophotometry titration. When interacting with Co^II cations, compound 7 forms complexes with stoichiometry Lig: M = 1: 1 and 1: 2 (Lig is ligand, and M is metal). In the case of compound 8, complexes with the stoichiometry Lig: M = 1: 4, as well as 1: 2, are observed, which presumably indicates the involvement of “soft” sulfur atoms in the interaction with the metal cations. The replacement of Co^II cations by Ni^II resulted in the formation in the solution of complexes with the stoichiometry Lig: M = 2: 1, 1: 1 and Lig: M = 1: 1, 1: 2 for compounds 7 and 8, respectively.