DFT and experimental studies of perchlorate ion coordination in cis/trans-copper(II) complexes of tetradentate pyridyl ligands

Spectral and structural studies of N,N'-dimethyl-N,N'-bis(pyridin-2-ylmethyl)-1,2-diaminoethane (L-1), N,N'-dimethyl-N,N'-bis(pyridin-2-ylmethyl)-1,3-diaminopropane (L-2), 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-1 and L-2 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 C-13 NMR spectra recorded at different pH values. Furthermore, for their copper complexes, two possible geometrical isomers (cis and trans) are obtained; for example, cis-[CuL1](2+), where both NCH3 groups are oriented in the same plane, is in equilibrium with its trans isomer and, as a result, a small energy difference (0.9 kcalmol(-1)) appears. In contrast, trans-[CuL2](2+) does not equilibrate with the cis isomer, because there is a greater energy difference (Delta E = 3.9 kcalmol(-1)) between the two isomers. Indeed, when [CuL1](2+) with two perchlorate ions was optimized, it was found that although only cis-[CuL1](2+) was able to accommodate a perchlorate ion in the axial position of the square pyramidal geometry by stabilizing the structure with two cis NCH3 groups through H-bonds, the trans isomer was unable to retain the perchlorate ion in the coordination sphere, because the NCH3 groups were in the trans 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 cis-[CuL1(ClO4)](+) is resolved, whereas the trans isomer is not. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).