Synthesis, X-ray characterization, NMR and ab initio molecular-orbital studies of some cadmium(II) macrocyclic Schiff-base complexes with two 2-aminoethyl pendant arms

Abstract

Three new pendant arm Schiff-base macrocyclic complexes, [CdL n ] 2+ (n = 5, 6, 7), have been prepared via cyclocondensation of 2,6-diacetylpyridine with three different branched hexaamines in the presence of Cd(II). The ligands are 15-, 16-and 17-membered pentaaza macrocycles having two 2-aminoethyl pendant arms [L 5 = 2,13-dimethyl-6,9-bis(aminoethyl)--pentaene]. All complexes were investigated by IR, 1 H and 13 C NMR, COSY(H,H) and HETCOR(H,C) spectroscopy and X-ray diffraction. In the solid state structure of each complex the Cd(II) ion is situated centrally within an approximately planar pentaaza macrocyclic ring, binding to the five nitrogen atoms, and also to the two pendant amines which are located on opposite sides of the macrocyclic plane. ab initio HF-MO calculations using a standard 3-21G* basis set have been used to verify that these similar basic structures correspond to energy minima in the gas phase.

Synthesis and characterization of Cd(II) macrocyclic schiff base complex with two 2-Aminoethyl pendant arms

“…The 1 H NMR spectrum of [CdL] 2+ in DMSO shows a peak at 8.86 ppm, corresponding to the imine protons and a doublet of doublet (8.57 ppm) and a doublet (8.48 ppm) assigned to the para-and meta-pyridyl protons, respectively [23]. In the region characteristics of the benzene protons (7.82-7.01 ppm) multi-signals were found.…”

“…The spectrum exhibited a strong band at 1596 cm -1 , as expected, for the high energy ring vibrations of the pyridine. The presence of unreacted pendant primary amine groups in the complex was evident from their IR spectra, indicated by the appearance of two strong peaks at 3241 and 3296 cm -1 assigned to the symmetric and asymmetric NH 2 stretching modes [23]. The strong absorption bands at 1384, 1290, 836 and 748 cm -1 support the presence of nitrate groups as the counter-ions in the complex [31].…”

“…As we were unable to prepare crystals suitable for X-ray [23] would hold, and thus undertook a full geometry optimization at the HF/3-21G* level of theory, using LanL2DZ basis set for Cd. We also made use of the same level of ab initio calculations for [CdL] 2+ to examine the comparability of the predicted gas phase structure with that determined in the solid state (Scheme 1).…”

“…The latter reactions produce tetraaza macrocycles carrying a single 2-aminoethyl pendant arm [19]. We have also produced Cd(II) pentaaza macrocyclic complexes with two 2-aminoethyl pendant arms, using branched hexaamines and 2,6-diacetylpyridine (Scheme 1) [23].…”

Synthesis and characterization of three heptaaza manganese(II) macrocyclic Schiff base complexes containing two 2-pyridylmethyl pendant arms

“…In the last decade, we have used 2,6-diacetylpyridine, 2,6-diformylpyridine, and 2,9-dicarboxaldehyde-1,10-phenanthroline as precursors to achieve several kinds of cyclocondensation reactions with a series of multi-amine derivatives, and a number of pendant-armed macrocycles have been prepared in this way [11][12][13]. Recently, three branched hexadentate amines (1)(2)(3) and related Mn(II) Schiff base complexes with two 2-pyridylmethyl pendant arms by the metal ion-templated [1 ?…”