Abstract
Salen type complexes, CuL, the corresponding tetrahydrosalen type complexes, Cu[H
4]L, and
N,
N′-dimethylated tetrahydrosalen type complexes, Cu[H
2Me
2]L, were investigated using cyclic voltammetry, and electronic and ESR spectroscopy. In addition, the analogous copper(II) complexes with a derivative of the tetradentate ligand ‘salphen’ [salphen=H
2salphen=
N,
N′-disalicylidene-1,2-diaminobenzene] were studied. Solutions of CuL, Cu[H
4]L and Cu[H
2Me
2]L are air-stable at ambient temperature, except for the complex Cu(
t
Bu, Me)[H
4]salphen [H
2(
t
Bu, Me)[H
4]salphen=
N,
N′-bis(2-hydroxy-3-
tert-butyl-5-methylbenzyl)-1,2-diaminobenzene]. Cu(
t
Bu, Me)[H
4]salphen interacts with dioxygen and the ligand is oxidatively dehydrogenated (–CH
2–NH–→–CN–) to form Cu(
t
Bu, Me)[H
2]salphen and finally, in the presence of base, Cu(
t
Bu, Me)salphen. X-ray structure analysis of Cu(
t
Bu, Me)[H
2Me
2]salen confirms a slightly tetrahedrally distorted planar geometry of the CuN
2O
2 coordination core. The complexes were subjected to spectrophotometric titration with pyridine, to determine the equilibrium constants for adduct formation. It was found that the metal center in the complexes studied is only of weak Lewis acidity. In dichlormethane, the oxidation Cu(II)/Cu(III) is quasireversible for the CuL type complexes, but irreversible for the Cu[H
4]L and Cu[H
2Me
2]L type. A poorly defined wave was observed for the irreversible reduction Cu(II)/Cu(I) at potentials less than −1.0 V. The ESR spectra of CuL at both 77 K and room temperature reveal that very well resolved lines can be attributed to the interaction of an unpaired electron spin with the copper nuclear spin,
14N donor nuclei and to a distant interaction with two equivalent protons [∣
A
Cu(iso)∣≈253 MHz, ∣
A
N(iso)∣≈43 MHz, ∣
A
N(iso)∣≈20 MHz]. These protons are attached to the carbon atoms adjacent to the
14N nuclei. In contrast to CuL, the number of lines in the spectra of the complexes Cu[H
4]L and Cu[H
2Me
2]L is greatly reduced. At room temperature, only a quintet with a considerably smaller nitrogen shf splitting constant [∣
A
N(iso)∣≈27 MHz] is observed. Both factors, planarity and conjugation, are thus essential for the observation of distant hydrogen shf splitting in CuL. Due to the CN bond hydrogenation, the coordination polyhedra of the complexes Cu[H
4]L and Cu[H
2Me
2]L is more flexible and more sensitive to ligand modification than that of CuL. The electron-withdrawing effect of the phenyl ring of the phenylenediamine bridge is reflected in a reduction of the copper hyperfine coupling constants in Cu(
t
Bu, Me)[H
4]salphen and Cu(
t
Bu, Me)[H
2Me
2]salphen complexes [∣
A
Cu(iso)∣≈215 MHz].