Evolution of the Coordination‐Sphere Symmetry in Copper(II), Nickel(II), and Zinc(II) Complexes with N,N′‐Double‐Armed Diaza‐Crown Ethers: Experimental and Theoretical Approaches

Abstract

AbstractN,N′‐Bis(triazolyl)diaza[18]crown‐6 and corresponding diaza[15]crown‐5 ethers were synthesized by means of click chemistry. The interaction of these ligands with NiII, CuII, and ZnII cations were studied by UV/Vis and 1H and 13C NMR spectroscopy. The solid‐state structure of NiII and ZnII complexes formed by the diaza[18]crown‐6 ligand were determined by means of X‐ray crystallography. The NiII complex is centrosymmetric; the geometry around the metal ion is slightly distorted octahedral whereby the equatorial sites were occupied by four N atoms, and the axial positions by two O atoms that come from the crown moiety. For the ZnII–diaza[18]crown‐6 complex, an irregular octahedral coordination was observed whereby the metal ion is asymmetrically placed in the macrocyclic cavity. The equatorial plane is occupied by two N and two O atoms of the crown moiety, and two N atoms of the triazolyl motifs on the pendant arms occupy the axial positions. The diamagnetic character of the ZnII ion allows its structural study in solution by NMR spectroscopy. A dynamic behavior was observed at room temperature, which corresponds to the displacement of the ZnII ion between the bond end and the nonbond end of the macrocycle. This movement results in an S4‐symmetrical structure in solution. Quantum chemical calculations at the DFT level have allowed us to interpret the experimental results observed in the solid state for the symmetry of the complexes in terms of covalent and noncovalent interactions, which favor the centrosymmetric and irregular octahedral coordination modes, respectively. Only the structure of the CuII complex with N,N′‐bis(triazolyl)diaza[15]crown‐5 ligand has been investigated in the solid state, for which a pentagonal bipyramidal coordination sphere was observed. This coordination geometry was confirmed in solution in MeCN by UV/Vis spectroscopy, and also for the ZnII complex by NMR spectroscopy. In the case of the NiII complex, a structural modification was suggested in solution in MeCN based on the UV/Vis spectrum. The rearrangement of heptadentate coordination to hexadentate is proposed.