Exploring the Limits of Ligand Rigidification in Transition Metal Complexes with Mono- N -Functionalized Pyclen Derivatives.

We report the synthesis of a new family of side-bridged pyclen ligands. The incorporation of an ethylene bridge between two adjacent nitrogen atoms was reached from the pyclen-oxalate precursor described previously. Three new side-bridged pyclen macrocycles, H sb-3-pc1a , sb-3-pc1py , and H sb-3-pc1pa , were obtained with the aim to assess their coordination properties toward Cu 2+ and Zn 2+ ions. We also prepared their nonreinforced analogues H 3-pc1a , 3-pc1py , and H 3-pc1pa as comparative benchmarks. The two series of ligands were characterized and their coordination properties were investigated in detail. The Zn 2+ and Cu 2+ complexes with the nonside-bridged series H 3-pc1a , 3-pc1py , and H 3-pc1pa were successfully isolated and their structures were assessed by X-ray diffraction studies. In the case of the side-bridged family, the synthesis of the complexes was far more difficult and, in some cases, unsuccessful. The results of our studies demonstrate that this difficulty is related to the extreme stiffening and basicity of such side-bridged pyclens.