DFT and Experimental Studies of Perchlorate Ion Coordination incis/trans‐Copper(II) Complexes of Tetradentate Pyridyl Ligands

Abstract

AbstractSpectral and structural studies ofN,N′‐dimethyl‐N,N′‐bis(pyridin‐2‐ylmethyl)‐1,2‐diaminoethane (L1),N,N′‐dimethyl‐N,N′‐bis(pyridin‐2‐ylmethyl)‐1,3‐diaminopropane (L2), 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 L1and L2influence 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 with13C NMR spectra recorded at different pH values. Furthermore, for their copper complexes, two possible geometrical isomers (cisandtrans) are obtained; for example,cis‐[CuL1]2+, where both NCH3groups are oriented in the same plane, is in equilibrium with itstransisomer and, as a result, a small energy difference (0.9 kcal mol–1) appears. In contrast,trans‐[CuL2]2+does not equilibrate with thecisisomer, because there is a greater energy difference (ΔE= 3.9 kcal mol–1) between the two isomers. Indeed, when [CuL1]2+with two perchlorate ions was optimized, it was found that although onlycis‐[CuL1]2+was able to accommodate a perchlorate ion in the axial position of the square pyramidal geometry by stabilizing the structure with twocisNCH3groups through H‐bonds, thetransisomer was unable to retain the perchlorate ion in the coordination sphere, because the NCH3groups were in thetransposition; 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 whichcis‐[CuL1(ClO4)]+is resolved, whereas thetransisomer is not. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)