Supramolecular Assemblies with Calix[6]arenes and Copper Ions: from Dinuclear to Trinuclear Linear Arrangements of Hydroxo−Cu(II) Complexes

Complexation of copper(II) by calix[6]arene-based ligands bearing either two or three N-benzylimidazole coordinating
arms under basic conditions has been studied. Whereas the tris(imidazole) derivative stabilizes dicationic 5-coordinate
aqua complexes in a mononuclear state with an intracavity bound guest, in the presence of hydroxide ions, the
latter undergo dimerization. An X-ray structure revealed decoordination of one imidazole arm and formation of a
bis(hydroxo) bridged Cu(II) core with a square-planar geometry for both metal centers sandwiched by two empty
calixarene cavities. Upon methanolysis, the dinuclear complex underwent an unexpected rearrangement leading to
the clean formation of a trinuclear complex. X-ray diffraction analyses of this novel species revealed a trinuclear
core constructed around a central Cu(II) ion that is doubly bridged through either methoxide or hydroxide anions
to two Cu(II) ions hold by two calixarene units. The same complex could be directly synthesized by reacting the
ligand with copper(II) perchlorate in a 2:3 ratio in the presence of base. In solution, the tetrahydroxo Cu3 complex
was characterized by UV−vis and 1H NMR spectroscopies and displayed an electron paramagnetic resonance
(EPR) signal only below 100 K that accounts for a S = 1/2 fundamental state. Formation of the same di- and
trinuclear species was observed with a calix[6]arene-based bis(imidazole) ligand, which demonstrates the generality
of the reaction schemes. All these results emphasize the versatility of the calix[6]arene scaffold for the stabilization
of metal complexes with various nuclearities.