N,N,N′,N′-Tetrakis((1H-Pyrazol-3-Yl)methyl)-1,2-Ethanediamine Ligands as Versatile Scaffolds for Second Coordination Sphere Design

Abstract

The polypyrazole-containing ligands, N,N,N′,N′-tetrakis((5-tert-Butyl-1H-pyrazol-3-yl)methyl)-1,2-ethanediamine (L1) and N,N,N′,N′-tetrakis((5-Phenyl-1H-pyrazol-3-yl)methyl)-1,2-ethanediamine (L2) were synthesized and shown to form hexadentate monometallic coordination compounds with Co^II, Ni^II, Cu^II and Zn^II metal ions. Exchange studies with the metal ions show binding preference in the order of Ni^II>Co^II/Cu^II>Zn^II. The compounds were confirmed by NMR, UV-Vis studies and the X-ray crystals structures of [Co(L1)](PF₆)₂, [Ni(L1)](PF₆)₂, [Cu(L1)]ClPF₆, [Zn(L1)](PF₆)₂, and [Ni(L2)]Cl_2. In the solid state, the arrangement of the bulky pyrazole units leads to the formation of two distinct cationic pockets containing the N−H groups of the pyrazoles. These cationic pockets are found to bind Cl⁻ or PF₆⁻ counterions through hydrogen bonding and anions titrations revealed good affinity for Cl⁻ and Br⁻. The compounds can be dehydronated up to two times using mild organic bases, highlighting the potential of these ligands to tune the electronic properties of the metal center.