Copper Complexes Bearing Pendant, Protonatable Noncoordinating Groups: Synthesis, Characterization, and Protonation Chemistry

Abstract

We have derivatized the pendant amine of (5-amino-5-methyl-3,7-diaza-2,9-nonanedioato)chlorocopper(II) with 3-pyridinecarboxaldehyde and 4-pyridinecarboxaldehyde. These new complexes, along with the starting material, comprise a set of copper complexes with chemically identical inner coordination spheres that provide protonatable nitrogen attached to the ligand framework. The 3-pyridinecarboxaldehyde adduct has been characterized by X-ray crystallography. This complex crystallized in the space group P21/n with a = 12.025(8) Å, b = 12.304(5) Å, c = 13.311(8) Å, β = 94.76(5)°, V = 1962(1) Å3, and Z = 4 (R, Rw = 0.049; 0.045). pH-dependent changes in the electronic spectroscopy and electrochemistry of these complexes have been observed. The parent complex undergoes a ∼300 cm-1 shift in λmax and a +140 mV shift in midpoint potential on going from high pH to low pH, while the pyridinecarboxaldehyde Schiff-base derivatives undergo smaller changes (78 and 110 cm-1 and 104 and 100 mV, respectively). The origins of the shift in λmax are investigated spectroscopically. A number of explanations for these shifts can be ruled out, including metal complex dimerization and axial coordination to the metal complexes. The data tend to support the suggestion that the spectroscopic shifts are largely due to the electrostatic influence of the pendant charge on the transition metal chromophore.