Synthesis and spectroscopic investigation of new nickel(II) complexes with the open chain asymmetric tetraamine 1,4,7,11-tetraazaundecane: Crystal structure of the square-planar complex

The synthesis of the X-ray structurally characterized square-planar [Ni(2,2,3-tet)](ClO4)2 (2) from the [Ni(2,2,3-tet)(NH3)(H2O)]Cl2 · 1/2EtOH · H2O (1), was based on their inter-conversion. The two sets of trans Ni–N bonds display similar to each other lengths (1.900 & 1.910 and 1.996 & 1.966 Å). The relaxation times (T1,2) of water protons and NMR experiments indicate relatively slow octahedral–sq-planar equilibrium, while in the aqueous solutions of 1, the sq-planar species appear to display a structure analogous to 2 in solution. The square planar geometry, the hydrophilicity of the complex and the asymmetry of the ligand generate the conditions for catalytic site formation.