Synthesis of Some Transition Metal Complexes of a Novel Schiff Base Ligand Derived from 2,2'-bis(p-Methoxyphenylamine) and Salicylicaldehyde

A novel Schiff base ligand derived from 2,2'-bis(p-methoxyphenylamine) and salicylicaldehyde and its transition metal complexes with Cu (Ⅱ), Co (Ⅱ) and Mn (Ⅱ) have been synthesized. Their spectral properties and electrochemical behavior were investigated.


Introduction
During the past two decades, considerable attention has been paid to the chemistry of the metal complexes of Schiff bases containing nitrogen and other donors [1][2][3][4]. This may be attributed to their stability, biological activity [5] and potential applications in many fields such as oxidation catalysis [6], electrochemistry [7], etc. Herein we report the synthesis of a novel Schiff base ligand (H 2 L) and its Cu (II), Co (II) and Mn (II) complexes. Their spectral properties and electrochemical behavior were investigated.

Ligand synthesis
The ligand (H 2 L) was prepared as outlined in Figure 1.

Complexes
Elemental analyses indicate that the complexes of H 2 L with Cu (II), Co (II) and Mn (II) can be formulated as M·L. The disappearance of the OH band of the free ligand in the IR of the metal complexes indicates that the OH group is deprotonated and coordinated to the metal ion as -O -. On the other hand, the C=N stretching mode is shifted to a lower frequency by about 29 cm -1 , compared to the free ligand. These IR results indicate that the ligand is coordinated to Cu (II), Co (II), Mn (II) via both N and O. The new IR bands appearing at 420 ~ 430 cm -1 and 541 ~ 560 cm -1 are assigned to ν (M-O) and ν (M-N) vibrations, respectively. In addition, no new bands at 1610 ~ 1550 cm -1 and 1420 ~ 1300 cm -1 are observed, indicating that the complexes do not contain CH 3 COOanions, which is in accordance with the elemental analysis results for the complexes. According to the aforementioned data, we propose for the complexes prepared the structure shown in Figure 2. It is suggested that the complexes are square planar or nearly square planar, coordinated according to the common stereochemistry of this kind of compounds.

Electrochemistry
The electrochemical behaviors of the H 2 L ligand and the MnL, CuL and CoL complexes were examined by means of cyclic voltammetry in CH 2 Cl 2 . A typical cyclic voltammogram of H 2 L and its complexes is shown in Figure 3 and the results are summarized in Table 1. The MnL complex shows two oxidation processes at E p,a = 0.58 and 0.895V. The first wave is nearly reversible with ΔEp=110mV. This process is consistent with a one-electron oxidation to form the mixed valence Mn(II, III) species. The second wave is irreversible at 0.895 V. On the other hand, the complexes of CuL and CoL only show one oxidation process at 0.83 and 0.782V, respectively, which are irreversible. This process is attributed to the oxidation of ligand. The observed reaction voltages of the complexes of CuL and CoL are lower than that of the ligand, while the MnL one is higher.

Acknowledgments
We thank National Natural Science Foundation of China (No. 29871014) and the Doctoral Foundation of Lanzhou University for financial support.

General
The C, H, N data were determined using a Varian EL elemental analyzer. IR spectra was recorded on a Nicolet 170SX FT IR spectrophotometer using KBr discs in the range ν = 400-4000 cm -1 . The cyclic voltammetry experiments were carried out using a CHI660A electrochemical workstation (Covarda, USA). All chemicals were analytical grade and used without further purification.

Preparation of the Ligand
Salicylicaldehyde (20 mmol) was added to a solution of 2,2'-bi(p-methoxyphenylamine) (10 mmol) in ethanol (30 mL). The mixture was continuously stirred for 3 h at room temperature, and the resulting yellow product was collected by filtration to give pale white crystals in 94% yield.