Methylresorcinarene: a reaction vessel to control the coordination geometry of copper(II) in pyridine N-oxide copper(II) complexes

Pyridine and 2-picolinic acid N-oxides form 2 : 2 and 2 : 1 ligand : metal (L : M) discrete L2M2 and polymeric complexes with CuCl2 and Cu(NO3)2, respectively, with copper(II) salts. The N-oxides also form 1 : 1 host–guest complexes with methylresorcinarene. In combination, the three components form a unique 2 : 2 : 1 host–ligand–metal complex. The methylresorcinarene acts as a reaction vessel/protecting group to control the coordination of copper(II) from cis-see-saw to trans-square planar, and from octahedral to square planar coordination geometry. These processes were studied in solution and in the solid state via1H NMR spectroscopy and single crystal X-ray diffraction.