Anion effects in Cu-crosslinked palindromic artificial tripeptides with pendant Bpy ligands

An artificial peptide with three pendant bipyridine (bpy) ligands on a palindromic backbone was designed and used to make multimetallic assemblies. The tripeptide is synthesized by addition of ligand-substituted aminoethylglycine monomers to both ends of a bpy-substituted diacid. The tripeptide was treated with Cu^II acetate, tetrafluoroborate, and nitrate salts, and the chelation stoichiometry was confirmed in spectrophotometric titrations. NMR spectroscopy, mass spectrometry, and analytical HPLC separations confirm the identity and purity of the product, which is a tripeptide duplex with three Cu^II coordinative crosslinks. UV/Vis absorbance and EPR spectroscopy were used to assess the geometry of the metal complexes and examine the effects of coordinative anions on the coordination geometry about the Cu centers. We find that the distorted square-planar geometry in the Cu duplex shifts to a tetrahedral geometry upon addition of I^- and that counteranions can be used to tune the interaction between metal complexes in multimetallic assemblies.