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

Abstract

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 (H₂L) and its Cu (II), Co (II) and Mn (II) complexes. Their spectral properties and electrochemical behavior were investigated.

Results and discussion

Ligand synthesis

The ligand (H₂L) was prepared as outlined in Figure 1.

Figure 1.

Synthesis of the ligand

Complexes

Elemental analyses indicate that the complexes of H₂L 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 CH₃COO^- anions, 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.

Figure 2.

The proposed chemical structure for the transition metal complexes

Electrochemistry

The electrochemical behaviors of the H₂L ligand and the MnL, CuL and CoL complexes were examined by means of cyclic $\mathrm{voltammetry}$ in CH₂Cl₂. A typical cyclic$\mathrm{ voltammogram }$of H₂L and its complexes is shown in Figure 3 and the results are summarized in Table 1.

Figure 3.

Cyclic Voltamogram of the Complexes in CH₂Cl_2; Scan Rate 100mV/s

Table 1.

Cyclic$\mathbf{ voltammetric }$data for ligand and complexes in CH₂Cl₂ 

$\mathrm{Ligand and Complexes}$	Ep ,a1/ V	E1/2/ mV	ΔEp1/ mV	Ep ,a2 / V	${\text{E}}_{1/2}^{\prime }$/ mV
L	0.8579	782.4	75.5	—	—
MnL	0.580	495.0	110.0	0.895	800.0
CuL	0.830	745.0	—	—	—
CoL	0.782	700.0	—	—	—

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.

Experimental

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. Calculated for C₂₈H₂₄N₂O₄ : 74.3 % C, 5.4 % H, 6.2 % N. Found: 73.9 % C, 5.6 % H, 6.1 % N. IR (in KBr pellets) cm^-1: 1657 (-C=N-), 3450 (-OH) cm^-1.

Preparation of the Complexes

$\mathrm{ A mixture of }{\mathrm{M(OAc)}}_2·{\mathrm{nH}}_2\mathrm{ O (2\; mmol)\; [M\; =\; Mn\; (Ⅱ),\; Co\; (Ⅱ),\; Cu\; (Ⅱ)],\; LiOH\; and\; ligand\; in\; a}$

$\mathrm{1:2:1 molar ratio was stirred in ethanol (20 mL) at 50℃ for 4h. The crude complexes were filtered off}$

$\mathrm{and then recrystallized from 1:1 water-ethanol. The products were dried in vacuo for 48 h.}$

CuL: brown solid, yield, 85 %; Calculated for C₂₈H₂₂N₂O₄Cu: 65.4 % C, 4.3 % H, 5.5 % N. Found: 65.2 % C, 4.6 % H, 5.7 % N. IR cm^-1: 1628 (-C=N).

CoL: orange solid, yield, 88 %; Calculated for C₂₈H₂₂N₂O₄C: 66.0 % C, 4.4 % H, 5.5 % N. Found: 65.8 % C, 4.3 % H, 5.7 % N. IR cm^-1: 1629 (-C=N).

MnL: dark-brown solid, yield, 88 %; Calculated for C₂₈H₂₂N₂O₄Mn: 66.5 % C, 4.4 % H, 5.5 % N. Found: 66.8 % C, 4.1 % H, 5.2 % N. IR cm^-1: 1628 (-C=N)

Electrochemical Measurements

The cyclic voltammetry experiments were carried out with a three electrode apparatus using a CHI660A electrochemical workstation (Covarda, USA), the working electrode was a glassy carbon disc, polished with an Al₂O₃ suspension prior to every experiment. Ag/AgCl and Pt foil were used as reference and counter electrodes, respectively. The H₂L and complexes solutions (1.0×10^-3 mol·L^-1) in CH₂Cl₂, with tetraethylammonium perchlorate (0.1 mol·L^-1) as supporting electrolyte were purged of oxygen by bubbling nitrogen for 15 min. and then blanketed with the same gas during the experiments. All compounds were investigated at 25℃.The voltammograms were recorded with a potential scan of 100mV·s^-1.