Oligonuclear Copper Complexes of a Bioinspired Pyrazolate-Bridging Ligand: Synthesis, Structures, and Equilibria in Solution

The synthesis of a new bioinspired dinucleating ligand scaffold based on a bridging pyrazolate with appended
bis[2-(1-methylimidazolyl)methyl]aminomethyl chelate arms is reported. This ligand forms very stable copper
complexes, and a series of different species is present in solution depending on the pH. Interconversions between
these solution species are tracked and characterized spectroscopically, and X-ray crystallographic structures of
three distinct complexes that correspond to the species present in solution from acidic to basic pH have been
determined. Overall, this provides a comprehensive picture of the copper coordination chemistry of the new ligand
system. Alterations in the protonation state are accompanied by changes in nuclearity and pyrazolate binding,
which cause pronounced changes in color and magnetic properties. Antiferromagnetic coupling between the copper(II) ions is switched on or off depending on the pyrazole binding mode.