Preparation and characterization of di-, tri-, and tetranuclear schiff base complexes derived from diamines and 3,4-dihydroxybenzaldehyde.

A series of new di-, tri-, and tetranuclear Co(II) and Cu(II) complexes of three new diSchiff base ligands were synthesized by two different methods. The first method involved the synthesis of the three ligands from condensation reaction of 3,4-dihydroxybenzaldehyde (L'H2) with ethylenediamine (en), o-phenylenediamine (o-PD), or 4,5-dimethyl-1,2-phenylendiamine (DMPD) in a mole ratio of 2 : 1 followed by the reaction of the resulting Schiff bases ligands with Cu(II) or Co(II) ions in the presence of 2,2'-bipyridyl (L) to form the di- and trinuclear metal complexes. The second method involved the condensation of the copper complex LCu(II)L' (L = 2,2'-bipyridyl, L' = 4-formylbenzene-1,2-bis(olate)) with en, o-PD, or DMPD in a mole ratio of 2 : 1, respectively, followed by reaction with CuCl2 or Cu(ClO4)2 to form di-, tri-, and tetranuclear copper (II) complexes, respectively. The structures of the ligands and metal complexes were characterized by elemental analyses, NMR, and FTIR spectra. The geometries of metal complexes were suggested according to elemental analysis, electronic spectra, thermal analyses, atomic absorption, and magnetic moments and conductivity measurements.

1. Introduction

Multinuclear transition metal complexes have become a central theme of current research because of their potentially useful properties. They are involved in some notable catalytic processes. Their important use for modelling the metal active sites of metalloproteins and their recent applications in the area of nanoscale materials have drawn the focal point of attraction of modern chemists towards the synthesis and characterization of such metal complexes [1]. A number of dinuclear complexes from various types of ligand systems have been prepared and examined in terms of their oxygen uptake or redox processes of oxygen, their catalytic activity, and their antibacterial and antifungal activities [2-4]. One of the synthetic strategies to prepare polynuclear transition metal complexes is the use of simple metal ion complexes which have the appropriate functionality to act as ligands for another metal ion [1]. There is currently a great deal of interest in the synthesis and characterization of polynuclear cobalt, nickel, and copper complexes due to their wide-ranging potential applications such as catalysts, electron transfer mediators in dye-sensitized solar cells, antiviral agents, and molecular nanomagnets [5]. Trinuclear cobalt complexes draw their speciality from their use as catalysts in epoxidation of olefins and in the autoxidation of hydrocarbons [1]. The presence of copper (II) ion in polynuclear complexes has received a wide interest in the fields of (i) the magnetostructural relationship, (ii) the characterization of active sites in multicopper proteins [5], and (iii) biological activity such as antitumor, antiviral, and anti-inflammatory [5]. Schiff bases derived from 3,4-dihydroxybenzaldehyde [3, 4] and diamines like ethylene diamine [5], 2,6-diaminopyridine [2], and 1,4-diaminobenzene [3, 4] represent an important series of chelating agents that have been used to synthesize mono-, di-, or polynuclear transition metal complexes [3-5] in which copper (II) complexes in particular represent models of physical and chemical behavior of biological copper systems that mimic copper metalloproteins such as hemocyanin and tyrosinase [2, 5]. Much attention has been paid to the synthesis and properties of molecules containing the copper (II) complex of 1,10-phenanthroline and 2,2′-bipyridyl units [2, 5–8] which are useful for applications in several fields.

For example, a π-conjugated polymer bearing 2,2′-bipyridyl units was found to serve as a unique electrically conducting polymer complex with transition metals such as Ru, Ni, and Fe [6-8]. Nickel (II) mixed ligand complexes involving an aromatic Schiff base and 1,10-phenanthroline showed higher cytotoxic activity than those of the individual ligands [7]. The copper (II) complex of 1,10-phenanthroline was the first synthetic transition metal complex effectively exhibiting nucleolytic activity [5]. In this work, we study for the first time the synthesis of di- and trinuclear copper (II) and cobalt (II) complexes as synthetic models for multicenter active sites of biological systems by following two methods. The first method (method 1) involves the reaction of metal salts with each of the following three new diSchiff base ligands: N,N′-bis(3,4-dihydroxybenzylidene)ethan-1,2-diamine (EDH ) (Figure 2), N,N′-bis(3,4-dihydroxybenzylidene)benzene-1,2-diamine (PDH ) (Figure 3), and N,N′-bis(3,4-dihydroxybenzylidene)-4,5-dimethyl-1,2-diamine (MPDH ) (Figure 4) prepared from the condensation reaction of 3,4-dihydroxybenzaldehyde with ethylenediamine (en), o-phenylenediamine (o-PD), or 4,5-dimethyl-1,2-phenylendiamine (DMPD), respectively. The second method (method 2) involves the condensation reaction of mononuclear copper (II) mixed ligand complex of 3,4-dihydroxybenzaldehyde and 2,2′-bipyridyl (LCu(II)L′) (Figure 1) (L = 2,2′-bipyridyl, L′ = 4-formylbenzene-1,2-bis(olate)) with (en), o-PD, or DMPD followed by further reaction with the metal salts to form the tri- and tetrahomonuclear metal complexes.

Figure 2

Figure 3

Figure 4

Figure 1

The structures of the prepared compounds were elucidated depending on elmental analyses, Uv-vis, NMR, and FTIR spectra as well as, thermal analyses, atomic absorption, conductivity measurements, and magnetic susceptibility of metal complexes.

2. Experimental

2.1. Materials and Methods

All chemicals were of reagent grade and were used as received except o-phenylenediamine and ethanol which were purified and dried as reported previously [9, 10]. Melting points (uncorrected) were determined on Gallenkamp M.F.B 600-010f melting point apparatus.

The elemental analyses were performed on Eurovector EA 3000A. 1HNMR and 13CNMR were carried out by using Bruker UltraShield 300 MHz NMR spectrophotometer. FTIR spectra were recorded as KBr and CsI discs using Shimadzu FTIR-8400S, Fourier Transform Infrared spectrophotometer. The electronic spectra were recorded in DMF on Shimadzu Uv-visible-160 Spectrophotometer. Thermal analyses (TG & DTG) were carried out under nitrogen atmosphere by using Netzsch Sat 409 PG/PC at a heating rate of 20°C/min under nitrogen atmosphere over a temperature range of 25–1000°C. The metal contents of the complexes were determined by atomic absorption technique using Varian-AA 775 Atomic Absorption spectrophotometer. Electrical conductivity measurements for complexes (10−3 M) in DMF at room temperature were carried out by using Hunts Capacitors Trade Mark British made conductivity meter. Magnetic moment (μ eff B.M) for the prepared complexes was measured at room temperature by using Bruker Magnet B.M-6.

2.2. Preparation Methods

2.2.1. Method 1

Synthesis of Schiff Bases   , , and   : General Procedure. To a solution of diamine (0.0217, 0.0391, and 0.0493 g for en, O-PD, and DMPD, resp., 0.362 mmol) in a minimum amount of absolute ethanol (en, O-PD) or methanol (DMPD) containing 2 drops of piperidine an ethanolic solution of 3,4-dihydroxybenzaldehyde (0.1 g, 0.724 mmol) was added. Precipitation took place immediately giving yellow, brown, and orange-yellow products, respectively. The mixtures were heated under reflux with continuous stirring for 1 h, 1 h, and 1.5 h, respectively, to allow for complete precipitation. The products were filtered, washed with ethanol, methanol, and ether, and vacuum dried.

Synthesis of Binuclear ( , ) and Tetranuclear  ( ) Copper Bis(bipyridyl) Schiff Base Complexes. Dinuclear copper complexes C and C were prepared as follows: to a stirred ethanolic solution of Schiff bases (0.05 g) (0.166 and 0.143 mmol for EDH and PDH , resp.) CuCl2·2H2O (0.0567 and 0.0489 g, 0.332 and 0.286 mmol, resp.), 2,2′-bipyridyl (0.0519 and 0.0448 g, 0.332 and 0.287 mmol, resp.), and triethylamine (NEt3) (0.0673 and 0.0580 g, 0.665 and 0.574 mmol, resp.) were added in a minimum amount of ethanol. Precipitation took place immediately. Reflux was continued for 4 h with continuous stirring. The products were filtered off, washed with ethanol, and vacuum dried. C was prepared by treating an ethanolic solution of MPDH (0.050 g, 0.132 mmol) with a solution mixture of excess CuCl2·2H2O (0.100 g, 0.586 mmol), 2,2′-bipyridyl (0.0414 g, 0.265 mmol), and NEt3 (0.0537 g, 0.531 mmol) in ethanol. The mixture was heated under reflux for 4 h. A brown precipitate was formed. The product was filtered off, washed several times with hot ethanol, and vacuum dried.

Synthesis of a Trinuclear Copper Bis(bipyridyl) Schiff Base Complex ( ). To an ethanol solution of C (0.05 g, 0.06 mmol) CuCl2·2H2O (0.0115 g, 0.06 mmol) dissolved in a minimum amount of ethanol was added with continuous stirring for 1 h during which the color of solution changed to dark brown. The mixture was heated under reflux for 4 h. A brown precipitate was formed. The product was filtered off, washed with ethanol, and vacuum dried.

Synthesis of Trinuclear Copper Tris(bipyridyl) Complexes of   ( ) and ( ). To a hot solution of C (0.05 g, 0.06 mmol) in hot ethanol Cu(ClO4)2·6H2O (0.0222 g, 0.06 mmol) and 2,2-bipyridyl (0.0093 g, 0.06 mmol) in ethanol were added with continuous stirring for 1 h followed by heating under reflux for 4 h to allow for complete precipitation. The resulting product (C ) was filtered off, washed with hot ethanol, and vacuum dried. C was prepared by adding a solution of Cu(ClO4)2·6H2O (0.0678 g), 2,2′-bipyridyl (0.0285 g) (0.183 mmol each), and NEt3 (0.0246 g, 0.244 mmol) in ethanol to MPDH (0.0229 g, 0.061 mmol) dissolved in a minimum amount of ethanol. A dark brown precipitate started to appear. The mixture was heated under reflux for 3-4 h for complete precipitation. The product was filtered, washed with hot ethanol, and vacuum dried.

Synthesis of Tetra- and Trinuclear Cobalt Bis- and Tris(bipyridyl) Complexes of    and   (    and   ).   C was prepared as follows: a solution of 2,2′-bipyridyl (0.0517 g, 0.332 mmol) and NEt3 (0.0672 g, 0.665 mmol) in a minimum amount of ethanol was added to a solution of EDH (0.0499 g, 0.1664 mmol) in warm ethanol with continuous stirring. Then a solution of CoCl2·6H2O (0.1592 g, 0.669 mmol) in ethanol was added. The color of solution was changed from blue to brown. The reaction mixture was then heated under reflux for 4 h. A brown precipitate was formed. The product was filtered, washed with hot ethanol, and vacuum dried. The preparation and purification of C (dark green) was carried out in the same manner, but the quantities of the reactants were MPDH (0.0625 g, 0.1664 mmol), 2,2′-bipyridyl (0,0778 g, 0.4992 mmol), CoCl2·6H2O (0.1187 g, 0.499 mmol), and NEt3 (0.0672 g, 0.665 mmol) and the color of solution after the addition of the cobalt salt was changed from yellow to green.

2.2.2. Method 2

In this method the metal complexes were prepared from condensation reaction of the Cu(II) complex precursor (LCuL ) (L = 2,2′-bipyridyl, L= 4-formylbenzene-1,2-bis(olate)) with the diamines followed by the reaction with the metal salts to form tri- and tetranuclear complexes.

Synthesis of .  This complex was prepared by following a previously published method [11] with modification. A solution of CuCl2·2H2O (0.1234 g, 0.724 mmol) in ethanol was added to an ethanolic mixture of 3,4-dihydroxybenzaldehyde (0.1 g, 0.724 mmol), 2,2′-bipyridyl (0.1130 g, 0.724 mmol), and triethylamine (0.1465 g, 1.448 mmol). The reaction mixture was stirred for 20 min. at room temperature during which a brown precipitate was formed. The mixture was heated under reflux for 2 h and the resulting product was separated by filtration, washed with hot ethanol, and dried under vacuum. The product was characterized by elemental analysis and the FTIR spectral analysis.

Synthesis of Binuclear Copper Bis(bipyridyl) Schiff Base Complexes , , and . An ethanol solution of diamine (en, O-PD, and DMPD, 0.0140, 0.0151, and 0.0191 g, resp., 0.14 mmol) was added to a solution of LCuL′ (0.1 g, 0.28 mmol) in hot ethanol with stirring for 30 min. The mixture was then heated under reflux for 3 h to allow for complete precipitation. The products were filtered off, washed with ethanol and ether, and vacuum dried.

Synthesis of Trinuclear Copper Bis(bipyridyl) Schiff Base Complexes and . To a solution of C and C (0.0441 and 0.0519 g, resp., 0.06 mmol) in hot ethanol CuCl2·2H2O (0.0115 g, 0.06 mmol) was added in a minimum amount of ethanol with stirring for 1 h. The colors of solutions changed to brown and precipitation of products took place. The mixtures were heated under reflux for 4 h and the products were filtered off, washed with ethanol, and vacuum dried.

Synthesis of Trinuclear Copper Tris(bipyridyl) Schiff Base Complexes and . A solution mixture of 2,2′-bipyridyl (0.0093 g, 0.06 mmol) and Cu(ClO4)2·6H2O (0.0222 g, 0.06 mmol) in ethanol was added to a solution of C and C (0.0441 and 0.0536 g, resp., 0.06 mmol) in hot ethanol with stirring for 1 h until the formation of precipitates was observed. The mixtures were then heated under reflux for 4 h and the products were filtered off, washed with hot ethanol, and vacuum dried.

3. Results and Discussion

3.1. Synthesis

The synthesis routes for the metal complexes by the two methods are illustrated in Scheme 1. Method 1 involves the condensation reaction of the diamines with two molecules of 3,4-dihydroxybenzaldehyde (LH2) to form the diSchiff bases as a first step for the formation of metal complexes. In the second method the condensation reaction occurs between the diamine molecule and two molecules of the mixed ligand copper complex precursor LCuL . Although the last method is simpler than the first method to avoid side reactions, the copper complex  precursor is slightly soluble in cold ethanol and therefore reaction with this complex required heating in large amount of solvent.

Scheme 1

The synthesis routes of the studied metal complexes by two different methods.

3.2. Physical Properties

The physical properties and results obtained from elemental analyses of the prepared compounds are described in Table 1. The analytical data are quite agreeable with calculated values with few exceptions which were attributed to incomplete combustion of the compounds. The molecular formula of the prepared compounds has been suggested according to the aforementioned data together with those obtained from spectral and thermal analyses as well as conductivity and magnetic susceptibility measurements of metal complexes. All complexes were noncrystalline which made it difficult to obtain their single crystal structures.

Table 1

The physical properties and analytical data for Schiff bases and their metal complexes.

Symbol	Color	(m.p.) °C	Yield %	CHN % analysis found (Calc.)			M % found (Calc.)	Cl % found (Calc.)
				C %	H %	N %
EDH4	Yellow	210 dec.	67.07	65.04 (64.00)	5.77 (5.33)	9.78 (9.33)	—	—
PDH4·4H2O	Dark brown	240	30.03	57.78 (57.14)	3.84 (4.28)	7.32 (6.66)	—	—
MPDH4·MeOH	Pale yellow	222	56.82	67.50 (67.60)	6.24 (5.88)	7.72 (6.86)	—	—
C1	Brown	>280	68.92	53.29 (53.52)	4.31 (4.46)	10.39 (10.40)	16.05 (15.74)	—
C2	Dark brown	>280	37.55	52.77 (53.65)	4.46 (4.51)	8.92 (9.38)	14.95 (14.20)	—
C3	Brown	>280	62.24	46.00 (45.89)	3.79 (3.09)	8.25 (7.64)	23.79 (23.14)	12.57 (12.93)
C4	Dark brown	>280	67.53	44.67 (44.18)	3.78 (4.09)	7.80 (8.59)	19.92 (19.50)	7.47 (7.26)
C5	Dark brown	>280	76.95	47.77 (47.84)	3.90 (3.12)	10.14 (9.71)	16.00 (16.52)	5.55 (6.15)
C6	Dark brown	>280	63.57	46.23 (46.65)	3.82 (3.88)	9.13 (8.37)	14.98 (14.25)	5.82 (5.31)
C7	Dark brown	>280	26.10	37.78 (37.70)	4.22 (4.04)	6.85 (7.29)	20.85 (20.44)	12.11 (12.32)
C8	Dark green	>280	26.68	55.44 (55.20)	5.32 (4.99)	9.90 (9.99)	13.74 (14.02)	6.03 (5.63)
C9	Dark brown	>280	68.06	59.50 (58.76)	4.55 (3.81)	11.73 (11.42)	17.93 (17.28)	—
C10	Dark brown	>280	86.26	59.38 (59.91)	4.15 (3.74)	9.61 (10.48)	16.54 (16.22)	—
C11	Brown	>280	77.00	58.39 (58.26)	4.12 (4.39)	8.92 (9.71)	15.22 (14.69)	—
C12	Dark brown	>280	52.56	36.24 (37.04)	4.02 (3.43)	7.9 (7.20)	22.42 (21.79)	11.59 (12.17)
C13	Brown	>280	57.05	45.80 (46.25)	4.12 (4.40)	8.33 (7.70)	16.47 (17.49)	7.08 (6.51)
C14	Reddish brown	>280	93.03	51.24 (50.64)	3.90 (4.52)	10.53 (10.20)	15.23 (14.54)	5.14 (5.41)
C15	Brown	>280	65.50	45.18 (45.81)	3.96 (3.66)	7.93 (8.55)	13.91 (14.55)	6.05 (6.07)

3.3. Infrared Spectra

The important vibrational modes of IR spectra for the free Schiff bases and their metal complexes are described in Table 2. The spectra of the free ligands displayed strong to moderate absorption band in the wavenumber region 3251–3263 cm−1 which were assigned to intramolecular hydrogen bonding of the two adjacent OH groups [12]. These bands were absent in the spectra of all metal complexes which indicates that the phenolic oxygen atoms were bonded to the metal ions [3–5, 11]. The spectrum of the mixed ligand copper (II) complex precursor CuLL displayed strong absorption bands at 1660 and 1540 cm−1 assigned to the stretching vibrations of the C=O and C=N groups of L and L moieties, respectively [11, 13]. The low intensity bands observed at lower wavenumber region at 440 and 345 cm−1 were assignable to stretching vibrations of Cu–O and Cu–N bonds, respectively [3, 5].

Table 2

Significant bands in the FTIR spectra (cm−1) for Schiff bases and their metal complexes.

Symbol	νOH	νC−H	νC=N imine	νC=N bipy.	νClO4 ionic (Coord.)	ν H2O lattice (Coord.)	νM−O	νM−N	νM−Cl
EDH4	3263	2839	1651	—	—	—	—	—	—
			1608
PDH4·4H2O	3253	2750	1631	—	—	3448–3417	—	—	—
			1604
MPDH4·MeOH	3251	2985	1670	—	—	—	—	—	—
C1	—	2860	1654	1519	—	(3150, 767, 651)	420	370	—
		2760	1608
C2	—	2950	1640	1500	—	3400 (3250, 775, 660)	460	350	—
			1610
C3	—	2900	1658	1570	—	3444	474	385	297a
			1604						254b
C4	—	2800	1643	1543	—	(3356, 771, 729)	470	385	340a
		2750	1604
C5	—	2819	1651	1570	1111, 1083, 1037	—	470	333	—
		2746	1608
C6	—	2950	1660	1570	(1093, 1040)	3580 (3240, 750, 675)	550	341	—
		2800	1610
C7	—	2750	1653	1580	—	3750 (3300, 770, 650)	490	405	312a
			1610						241b
C8	—	2980	1640	1560	—	(3240, 775, 655)	560	395	325a
		2870	1610						270b
C9	—	2951	1655	1573	—	—	478	358	—
		2839
C10	—	2980	1630	1590	—	3550	490	400	—
C11	—	2823	1651	1570	—	3456	489	389	—
		2754	1608
C12	—	2950	1645	1550	—	3700 (3250, 690, 640)	450	322	304a
		2800	1600						250b
C13	—	2960	1660	1550	—	(3300, 770, 640)	560	343
		2850	1610
C14	—	2750	1639	1570	1103, 1050	—	459	393
			1604
C15	—	2800	1620	1550	(1091, 1050)	3600 (3380, 771, 740)	510	400

aTerminal; bbridged.

The strong bands observed at 1631–1670 cm−1 and 1604, 1608 cm−1 in the spectra of the free Schiff bases were assigned to the asymmetric symmetric stretching vibration of the azomethine group (νC=N) [2-5]. These bands were shifted to lower frequency in all complexes (except C , C , C , C , and C ) indicating the coordination of the Schiff bases with the metal ions through the azomethine nitrogens [5]. All complexes exhibited strong bands at wavenumber range 1500–1590 cm−1 attributed to ν C=N of coordinated bipyridyl ligand [14]. The spectra of the complexes C , C , C , and C exhibited strong bands assigned to the stretching vibrational modes of ClO4 anion (ν ClO) which behaved as monodentate ligand in C and C complexes [15] and as a free ion in C and C complexes [15]. The bands appeared at 3400–3750 cm−1 in the spectra of PDH , C , C , C , C , C , C , C , and C were attributed to vibrational modes of lattice H2O [15], while the bands which appeared at 3100–3380 cm−1 and 621–775 cm−1 in the spectra of C , C , C , C , C , C , C , C , and C were due to coordinated HO [15]. The spectrum of MPDH exhibited a strong band in the range 3552–3421 cm−1 and another band at 1195–1161 cm−1 attributed to OH and C–O stretching vibrations of methanol embedded in the crystal lattice of the ligand [13, 15]. Further bands which appeared at lower frequencies in the spectra of metal complexes were assigned to M–O, M–N and M–Cl stretching vibrations (Table 2). Figure 5 shows representative FTIR spectra of C and C prepared by method 1 from MPDH and EDH , respectively.

Figure 5

FTIR spectra of (a) C and (b) C .

3.4. 1H NMR and 13C NMR Spectra

The 1H NMR spectra of diSchiff bases and the binuclear bis(bipyridyl) copper complex of MPDH (C ) were recorded in DMSO and the chemical shifts and peak assignments are given in Table 3. The spectra of the Schiff base ligands showed a broad peak in the range δ = 8.5–9.9 ppm attributed to phenolic hydroxyl protons [3, 4, 16] as is demonstrated by the spectra of EDH and MPDH shown in Figure 6.

Table 3

1H NMR data of the three Schiff base ligands and the Cu(II) complex C in DMSO.

EDH4		PDH4
Chemical shift δ (ppm)	Assignments	Chemical shifts δ (ppm)	Assignments
(8.8–9.9, 4H, b)	Protons of OH	(9.60–9.75, 4H, b)	Protons of OH
(8.1, 2H, s)	Protons of azomethine	(8.7–7.9, 2H, b)	Protons of azomethine
(6.7–7.4, 6H, m)	Aromatic protons	(6.5–7.9, 10H, b)	Aromatic protons
(3.9, 4H, s)	Protons of NCH2	(3–3.5, 8H, b)	Protons of H2O
(3–3.5, 2H, b)	Protons of H2O (in DMSO)	(2.5, 6H, s)	Protons of DMSO
(2.5, 6H, s)	Protons of DMSO
MPDH4		C11 complex
Chemical shifts δ (ppm)	Assignments	Chemical shifts δ (ppm)	Assignments
(8.5–8.68, 4H, b)	Protons of OH	(10.1–10.98, 2H, b)	Protons of azomethine
(8.0–8.28, 2H, b)	Protons of azomethine	(6.7–7.5, 24H, b)	Aromatic protons of benzene rings and bipyridyl
(6.9–7.6, 10, m)	Aromatic protons	(3.2, 6H, m)	Protons of H2O
(3.0–3.6, 2H, m)	Protons of H2O	(2.4–2.8, 6H, m)	Protons of DMSO
(2.5, 6H, s)	Protons of DMSO	(0.97–1.85, 6H, b)	Protons of CH3
(1.5–1.7, 6H, m)	Protons of CH3	—	—

Figure 6

1H NMR spectra of diSchiff base ligands EDH and MPDH .

The absence of this peak in the spectrum of complex C (Table 3) confirms the involvement of deprotonated hydroxyls in chelation to the metal ion [16, 17]. The peaks displayed by 1H NMR spectra of Schiff bases in the range δ = 7.9–8.7 ppm were attributed to chemical shifts of the azomethine protons (HC=N) [3, 4, 16, 17]. The spectrum of C exhibited the absence of the signals related to OH protons and the appearance of the azomethine proton signals downfield which confirms the formation of the metal complex [16, 17].

Signals of aromatic and aliphatic protons were observed in the chemical shift ranges 6.5–7.9 and 1.5–3.9 ppm, respectively [13]. Chemical shifts for 13C NMR of EDH and MPDH in DMSO are described in Table 4. The signals assigned to the chemical shifts of methylene and methyl groups for the two ligands, respectively, were observed at 60.99 (CH2) and at 19.8 and 20.1 (CH3) ppm [18], while the signals of aromatic carbons were located at 113.6–161.2 and 110.8–146.8 ppm, respectively [18, 19]. The signals observed at 167.2 and 150.6–152.7 ppm, respectively, were attributed to the chemical shifts of azomethine carbons which confirms the formation of the Schiff bases [20-22].

Table 4

Chemical shifts (ppm) for 13C NMR of Schiff bases EDH and MPDH in DMSO.

Compound	Chemical shifts δ (ppm)	Assignments
EDH4	60.99	Carbon of CH2
	113.6–161.2	Aromatic carbons
	167.2	Carbon of HC=N
MPDH4	19.8, 20.1	Carbon of methyl group
	110.8–146.8	Aromatic carbons
	150.6, 152.7	Carbon of HC=N

3.5. Electronic Spectra Conductivity and Magnetic Susceptibility Measurements

The results of electronic spectra of the ligands and their metal complexes in DMF are described in Table 5. The three ligands exhibited high intensity bands which appeared at wavenumber region 33333–24390 cm−1 and low intensity bands at 27777–20833 cm−1 which were assigned to π → π* and n → π* transitions, respectively [13]. The spectra of metal complexes exhibited hypsochromic shifts of the ligand π → π* band which refers to complex formation with the metal ions [18]. The spectra of complexes exhibited additional medium intensity bands in the near Uv to visible region at 28248–23255 cm−1 which were attributed to charge transfer transitions [23]. The copper complexes (C , C , C , C , C , and C ) displayed bands in the regions 12970–10298 cm−1, 18833–15313 cm−1, and 21739–19920 cm−1 assigned to B → A, B → B, and B → E transitions, respectively, of square planar Cu(II) complexes [23-27] while the spectra of the copper complexes (C , C , C , C , C , C , and C ) displayed bands in the regions 13513–10460, 18868–15431, and 23148–22472 which were attributed to the transitions of tetragonally distorted octahedral Cu(II) complexes [23-25]. The two cobalt complexes (C and C ) exhibited two bands observed at 15731 and 15983 cm−1, respectively, which were assigned to T → T (P) (ν 3) and at 10504 and 9900 cm−1, respectively, corresponding to the transition T → A (ν 2) of octahedral Co(II) complexes [23]. The energies of ν 1 (T → T) as well as the values of the spectral parameters Dq/B, B, 10Dq, and nephelauxetic ratio β for the Co(II) complexes C (7083 cm−1, 0.9, 787 cm−1, 7080 cm−1, and 0.811, resp.), and C (6456 cm−1, 0.7, 807 cm−1, 5740 cm−1, and 0.831, resp.), were calculated by applying the band ratio ν 3/ν 2 on Tanabe-Sugano diagram of d7 complexes. The values of β indicate a covalent bonding character of both complexes [23]. Conductivity measurements in DMF showed nonelectrolytic nature for all compounds (Table 3) except C , C , and C which were electrolytes with ionic ratio (1 : 2) [28]. Magnetic susceptibility measurements at room temperature showed that the magnetic moment (μ eff) of the Cu(II) complex precursor CuLL (1.69 B.M) agrees with square planar geometry of the complex [11, 23]. The values of μ eff of the other complexes were less than those expected for copper and cobalt ions which imply that the di- and trinuclear copper (II) and cobalt (II) complexes possess antiferromagnetic properties by a strong intramolecular antiferromagnetic spin exchange interaction [5]. According to the aforementioned results in addition to elemental analysis and FTIR and NMR spectra the stereochemical structures of the studied complexes were suggested as is illustrated in Scheme 2.

Table 5

Electronic spectra, magnetic moments, and molar conductivity data of Schiff bases and their metal complexes.

Symbol	Band positions (cm−1)	Assignment	μ eff (B.M)	Molar conductivity S·mol−1·cm2 in DMF
EDH4	33222, 24691	π → π*	—	0.0018
PDH4	33222, 27777	π → π*	—	0.011
	20833	n → π*
MPDH4	33333	π → π*	—	0.0007
	27777	n → π*
C1	32362, 29154	Intraligand π → π*	0.386 oh	0.022
	23255	C.T
	15431	2B1g → 2B2g
	13513	2B1g → 2A1g
C2	33444	Intraligand π → π*	0.514 oh	0.251
	26315	C.T
	18868	2B1g → 2B2g
C3	33333	Intraligand π → π*	1.260 Sq.	0.093
	25641	C.T
	15625	2B1g → 2B2g
	12658	2B1g → 2A1g
C4	37735, 34843	Intraligand π → π*	1.023 oh	0.046
	27247	C.T
	23148	2B1g → 2Eg
	10989	2B1g → 2A1g
C5	34013, 27247	Intraligand π → π*	1.061 Sq.	149
	23313	C.T
	16025	2B1g → 2B2g
C6	34129, 28571	Intraligand π → π*	0.810 oh	0.114
	25974	C.T
	22883	2B1g → 2Eg
	10460	2B1g → 2A1g
C7	36363, 33333	Intraligand π → π*	2.440 oh	0.043
	31250
	26178	C.T
	15731	4T1g → 4T1g (P)
	10504	4T1g → 4A2g
	7083 cal.	4T1g → 4T2g
C8	34843, 33003	Intraligand π → π*	0.200 oh	0.273
	28248	C.T
	15983.	4T1g → 4T1g (P)
	9900	4T1g → 4A2g
	6456 cal.	4T1g → 4T2g
C9	36363, 27247	Intraligand π → π*	1.092 Sq.	0.0002
	23310	C.T
	19920	2B1g → 2Eg
	15313	2B1g → 2B2g
	10298	2B1g → 2A1g
C11	29585	Intraligand π → π*	2.017 Sq.	0.005
	26041	C.T
	21739	2B1g → 2Eg
	15873	2B1g → 2B2g
C12	36496, 32894	Intraligand π → π*	1.023 oh	0.013
	27624	C.T
	22935	2B1g → 2Eg
	10482	2B1g → 2A1g
C13	33333, 29239	Intraligand π → π*	0.440	163
	27027	C.T
	22472	2B1g → 2Eg
C14	34013, 29154	Intraligand π → π*	1.89 Sq.	158
	27027	C.T
	18833.	2B1g → 2B2g
C15	29325	Intraligand π → π*	1.783 oh	0.023
	27397	C.T
	22727	2B1g → 2Eg

Scheme 2

Suggested stereochemical structures of the synthesized diSchiff base complexes.

3.6. Thermal Analysis

Thermogravimetric (TG) and differential thermogravimetric (DTG) analyses for the two complexes C and C are shown in Figure 7. The decomposition temperature and the weight losses are described in Table 6. The loss of solvent molecules embedded in the crystal lattice of the complexes as well as uncoordinated ligand groups took place at the first stage at temperature range 78–180°C with peak temperatures at 100 and 98°C, respectively, as is indicated by the DTG curves of the two complexes (Figure 7). The successive loss of coordinated water molecules occurred in the second and third stages at peak temperatures 195, 300 and 210, 325°C, respectively. The loss of bipyridyl and chloride ligands took place at temperature range 400–900°C. The DTG curve of C showed three peaks at 412, 620, and 822°C for this stage. The high percentage of the remaining residues at 1000°C indicates that the two complexes are very stable and require a higher temperature range for complete decomposition which is quite common for polynuclear metal complexes [5].

Figure 7

TG and DTG thermographs of C and C .

Table 6

Thermal decomposition of the copper complexes (C and C).

C8 stable phase [MPD(Co3(bipy)3(H2O)4Cl2] Et3N M.wt = 1260.7	Temp. range of decomp. °C	Weight % loss found (calc.)
↓ 0.45Et3N + Cl + 2H2O	78–211	9.00 (9.28)
↓ 0.55Et3N + 2H2O	212–423	6.80 (7.26)
↓ 2CH3 + 2bipy + C6H2	424–661	32.4 (33.43)
↓ Cl + bipy + HCN	662–998	17.23 (17.33)
C7H4NO2Co2 + C6H4O2Co (residue)	—	34.57 (33.13)
C12 stable phase [ED(Cu4(bipy)2(H2O)6Cl4] 3H2O M.wt = 1166.16	Temp. range of decomp. °C	Weight % loss found (calc.)
↓ 4H2O	73–261	5.80 (6.17)
↓ 5H2O + C2H4	262–365	10.20 (10.11)
↓ C5H4N + bipy	366–542	20.00 (19.46)
↓ 4Cl	543–761	11.8 (12.13)
↓ C5H4N	762–998	6.4 (6.68)
C14H8N2O4Cu4 (residue)	—	45.79 (44.76)

4. Conclusions

The bi-, tri-, and tetranuclear bis- and tris(bipyridyl) copper (II) and cobalt (II) mixed ligand complexes of three diSchiff base ligands derived from 3,4-dihydroxybenzaldehyde and three diamines in a stoichiometric ratio of 2 : 1 were successfully synthesized by two different methods. The structures of the ligands were confirmed by elemental and spectral analysis. Coordination of the metal ions to form trinuclear and tetranuclear complexes took place through the two imino nitrogens and phenolic dianionic oxygen atoms of each ligand molecule as was indicated by FTIR spectra. The formation of binuclear diSchiff base copper bis(bipyridyl) complexes was achieved by reacting the mononuclear copper (II) mixed ligand complex CuLL with the diamines in a 2 : 1 ratio, respectively, as was confirmed by the NMR spectrum of C while trinuclear bis- and tris(bipyridyl) and tetranuclear complexes of the three ligands were achieved by reacting the synthesized Schiff bases with the copper salts in the presence of 2,2′-bipyridyl. The complexes exhibited low values of magnetic moments which made them a good synthetic model for intramolecular antiferromagnetic spin exchange interaction of biological systems. In the future work the enzyme like and metalloprotein activities of these complexes and their biological activities will be studied in detail.