DFT and Experimental Studies of Perchlorate Ion Coordination in<i>cis</i>/<i>trans</i>‐Copper(II) Complexes of Tetradentate Pyridyl Ligands

<jats:title>Abstract</jats:title><jats:p>Spectral and structural studies of<jats:italic>N</jats:italic>,<jats:italic>N′</jats:italic>‐dimethyl‐<jats:italic>N</jats:italic>,<jats:italic>N′</jats:italic>‐bis(pyridin‐2‐ylmethyl)‐1,2‐diaminoethane (L<jats:sup>1</jats:sup>),<jats:italic>N</jats:italic>,<jats:italic>N′</jats:italic>‐dimethyl‐<jats:italic>N</jats:italic>,<jats:italic>N′</jats:italic>‐bis(pyridin‐2‐ylmethyl)‐1,3‐diaminopropane (L<jats:sup>2</jats:sup>), and their copper(II) complexes were carried out by spectroscopic and DFT methods. The DFT results show that conventional and nonconventional H‐bonds present in L<jats:sup>1</jats:sup>and L<jats:sup>2</jats:sup>influence the NMR chemical shifts at different pH values, which thus suggests that the protonation sites at the pyridyl and tertiary nitrogen atoms of the ligands change the chemical environment of the adjacent carbon atoms; this observation is consistent with<jats:sup>13</jats:sup>C NMR spectra recorded at different pH values. Furthermore, for their copper complexes, two possible geometrical isomers (<jats:italic>cis</jats:italic>and<jats:italic>trans</jats:italic>) are obtained; for example,<jats:italic>cis</jats:italic>‐[CuL<jats:sup>1</jats:sup>]<jats:sup>2+</jats:sup>, where both NCH<jats:sub>3</jats:sub>groups are oriented in the same plane, is in equilibrium with its<jats:italic>trans</jats:italic>isomer and, as a result, a small energy difference (0.9 kcal mol<jats:sup>–1</jats:sup>) appears. In contrast,<jats:italic>trans</jats:italic>‐[CuL<jats:sup>2</jats:sup>]<jats:sup>2+</jats:sup>does not equilibrate with the<jats:italic>cis</jats:italic>isomer, because there is a greater energy difference (Δ<jats:italic>E</jats:italic>= 3.9 kcal mol<jats:sup>–1</jats:sup>) between the two isomers. Indeed, when [CuL<jats:sup>1</jats:sup>]<jats:sup>2+</jats:sup>with two perchlorate ions was optimized, it was found that although only<jats:italic>cis</jats:italic>‐[CuL<jats:sup>1</jats:sup>]<jats:sup>2+</jats:sup>was able to accommodate a perchlorate ion in the axial position of the square pyramidal geometry by stabilizing the structure with two<jats:italic>cis</jats:italic>NCH<jats:sub>3</jats:sub>groups through H‐bonds, the<jats:italic>trans</jats:italic>isomer was unable to retain the perchlorate ion in the coordination sphere, because the NCH<jats:sub>3</jats:sub>groups were in the<jats:italic>trans</jats:italic>position; this disfavors the presence of the perchlorate ion in the sphere, because of the absence of H‐bonds. This is consistent with the X‐ray structure, in which<jats:italic>cis</jats:italic>‐[CuL<jats:sup>1</jats:sup>(ClO<jats:sub>4</jats:sub>)]<jats:sup>+</jats:sup>is resolved, whereas the<jats:italic>trans</jats:italic>isomer is not. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)</jats:p>