An oxamato bridged trinuclear copper(II) complex: Synthesis, crystal structure, reactivity, DNA binding study and magnetic properties

Inorganica Chimica Acta 376 (2011) 129–135

Contents lists available at ScienceDirect

Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica

An oxamato bridged trinuclear copper(II) complex: Synthesis, crystal structure, reactivity, DNA binding study and magnetic properties S. Dey a, S. Sarkar a, T. Mukherjee a, B. Mondal a, E. Zangrando b, J.P. Sutter c, P. Chattopadhyay a,⇑ a

Department of Chemistry, Burdwan University, Golapbag, Burdwan 713104, India Dipartimento di Scienze Chimiche e Farmaceutiche, Via Licio Giorgieri 1, 34127 Trieste, Italy c Laboratoire de Chimie de Coordination du CNRS, 205, route de Narbonne, 31077 Toulouse, France b

a r t i c l e

i n f o

Article history: Received 6 March 2011 Received in revised form 24 May 2011 Accepted 2 June 2011 Available online 13 June 2011 Keywords: Oxamato bridge Trinuclear copper(II) complex Crystal structures

a b s t r a c t A linear tri-nuclear oxamato bridged copper(II) complex [Cu3(pba)(dpa)2(H2O)(ClO4)](ClO4)H2O (1) (pbaH4 = 1,3-propanediylbis(oxamic acid), dpa = 2,20 -dipyridylamine) was isolated from the reaction mixture of Na2[Cu(pba)]3H2O, copper perchlorate hexahydrate and dipyridylamine in methanol. On reaction with dpa or DMF in basic medium (KOH) at ambient temperature complex 1 changed to dinuclear oxalate bridged copper(II) derivatives, [Cu2(l-C2O4)(dpa)4](ClO4)2 (2) and [Cu2(l-C2O4)(dpa)2 (DMF)2](ClO4)2 (3), respectively. The complexes 1, 2 and 3 have been characterized by physicochemical and spectroscopic tools, and also by the X-ray single crystal analysis. The hydrolysis of 1 in basic medium and thermo-gravimetric analysis has been studied. Absorption and emission spectral studies showed that complex 1 interacts with calf thymus-DNA (CT-DNA) with a binding constant (Kb) of 4.01  104 M1 and linear Stern–Volmer quenching constant (Ksv) of 6.9  104. A strong anti-ferromagnetic interaction with a coupling constant JCuCu of 320.0 ± 0.3 cm1 was observed from the study of magnetic behavior of complex 1 in the temperature range of 2–300 K. Electrochemical equivalency of three copper(II) ions in 1 was identified by getting only one quasi reversible cyclic voltammogram. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction There has been a great deal of interest in the investigation relating to the structure and the chemical properties of polynuclear transition-metal compounds with polyatomic bridging ligands mainly for the rational design of functional materials [1–9]. Moreover, the chemistry of polynuclear Cu(II) complexes has drawn considerable attention due to the presence of multicopper active sites in blue copper oxidases (e.g., laccase, ascorbate oxidase, and ceruloplasmin) [10–14] and in the development of new inorganic materials showing molecular ferromagnetism [15]. Cumulative spectroscopic and bonding studies on Rhus vernicifera laccase have focused on the presence of a trinuclear active site composed of two types of metal centers [13,16]. The crystal structure of ascorbate oxidase from Zucchini revealed at the biosite three copper atoms having an approximate arrangement of isosceles triangle [14]. Depending on the ligands used, trinuclear Cu(II) complexes have shown to possess different arrangements of metal atoms, namely equilateral-, isosceles-, scalene-triangular beside linear clusters [17]. Here, we report an account on the synthesis, spectroscopic and structural characterization, reactivity and magnetic properties of an oxamato-bridged linear trinuclear copper(II) complex, ⇑ Corresponding author. E-mail address: [email protected] (P. Chattopadhyay). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.06.005

[Cu3(pba)(dpa)2(H2O)(ClO4)]ClO4H2O (1), where pbaH4 = 1,3-propylenebis(oxamic acid) and dpa = 2,20 -dipyridylamine. Several homo-trinuclear copper(II) complexes or hetero-bimetallic transition metal complexes have been synthesized using the coordination capability of metallo-ligands [18–25] in search of new magnetic material. But still no attention has been paid on the reactivity of this type of complexes in different chemical environments. In presence of potassium hydroxide at ambient temperature the oxamato bridged complex 1 reacted with dipyridylamine (dpa) or dimethylformamide (DMF) to form new dinuclear oxalato-bridged copper(II) derivatives [Cu2(l-C2O4)(dpa)4](ClO4)2 (2) and [Cu2(lC2O4)(dpa)2(DMF)2](ClO4)2 (3), respectively. In addition, the biological activity of the trinuclear complex 1 towards CT-DNA has been explored and the interaction studied by means of absorption and emission spectra to obtain the binding constant (Kb) and the linear Stern–Volmer quenching constant (Ksv). 2. Experimental 2.1. Materials and physical measurements All chemicals and reagents were obtained from commercial sources and used as received. Solvents were distilled from an appropriate drying agent. Calf thymus-DNA, Tris–HCl buffer solution from Bangalore-Genie and ethidium bromide (EB) from Sigma were used

130

S. Dey et al. / Inorganica Chimica Acta 376 (2011) 129–135

[Cu2(l-C2O4)(dpa)2(DMF)2](ClO4)2 (3): Anal. Calc. for C28H32Cl2 Cu2N8O14: C, 37.25; H, 3.54; N, 12.41; Cu, 14.07. Found: C, 37.55; H, 3.48; N, 12.71; Cu, 13.96%. IR (cm1): mC@N, 1470; mC@O, 1653s; msClO4 , 1092; masClO4 , 624. Conductivity (Ko, X1 cm2 mol1) in dime thylformamide: 150.

as received. The metallo-ligand, Na2[Cu(pba)]3H2O was prepared following the reported procedure [20] with a little modification. The elemental analyses (C, H and N) were performed on a PerkinElmer model 2400 elemental analyzer. Copper analyses were carried out by Varian atomic absorption spectrophotometer (AAS) model-AA55B, GTA equipped with graphite furnace. IR spectra were obtained on JASCO FT-IR model 460 plus spectrometer with KBr disk, electronic absorption spectra on a JASCO UV–Vis/NIR spectrophotometer model V-570, and the fluorescence spectra on a Fluorometer Hitachi-2000. The magnetic susceptibility measurements were performed at room temperature by using a vibrating sample magnetometer PAR 155 model. Molar conductance (KM) was measured in a Systronics conductivity meter 304 model using solutions 103 mol L1 in appropriate solvents. Magnetic behavior for complex 1 has been recorded on a crushed crystal sample of 21.5 mg dispersed in grease and hold in a medical capsule. Thermo-gravimetric analysis was performed on a Perkin-Elmer Diamond TG/DTA Thermal Analyzer from room temperature to 330 °C at a heating rate of 20 °C min1 using platinum crucibles. Measurements down to 2 K were completed by using a Quantum Design MPMS-5S susceptometer. Electrochemical measurements were recorded on a computer controlled EG&G PAR model 270 VERSTAT electrochemical instruments with TEAP as supporting electrolyte, a Pt-disk as working electrode and a Pt-wire as auxiliary electrode. All the measurements were made at 298 K with acetonitrile as solvent.

Diffraction data for all the complexes were collected at room temperature on a Nonius DIP-1030H system equipped with Mo Ka radiation (k = 0.71073 Å). Cell refinement, indexing and scaling of the data sets were performed by using programs Denzo and Scalepack [26]. The structures were solved by direct methods and subsequent Fourier analyses [27] and refined by the full-matrix least-squares method [27] based on F2 with all reflections. Hydrogen atoms bonded to carbon were included at calculated positions, those of water molecules were located from a DFourier map and refined with distance constraints. Complex 1 shows a disorder of the central propylene CH2 group which was refined over two positions having 0.64(1)/0.36(1) occupancies. Three perchlorate oxygens of 3 are disordered over two positions (occupancy 0.58/ 0.42) about one Cl–O bond. All the calculations were performed using the WinGX System, Ver 1.80.05 [28]. Crystal data and details of refinement for complexes 1 and 3 are summarized in Table 1, data of 2 have been deposited with the CCDC center.

2.2. Synthesis of trinuclear complex 1

2.5. DNA binding experiments

To a solution of 2,20 -dipyridylamine (2.0 mmol, 342 mg) in methanol, copper perchlorate hexahydrate (2.0 mmol, 742 mg) in methanol (25 mL) was added dropwise and the mixture was stirred for 4.0 h at ambient temperature. Then an aqueous solution of metallo-ligand, Na2[Cu(pba)]3H2O (1.0 mmol, 378 mg) was added dropwise to the resulting mixture at stirring condition. The mixture was filtered and the clear filtrate was kept aside. After a few days, deep blue colored crystalline complex was obtained by filtration followed by washing with water, methanol and dried in vacuo. Single crystals suitable for X-ray diffraction were obtained from this solution. Yield: 80–85%. [Cu3(pba)(dpa)2(H2O)(ClO4)]ClO4H2O (1): Anal. Calc. for C27H28Cu3N8O16Cl2: C, 32.99; H, 2.85, N, 11.40; Cu, 19.39. Found: C, 33.27; H, 2.76; N, 11.57; Cu, 19.21%. IR (cm1): mC@N, 1472; mC@O, 1600; msClO4 , 1100; masClO4 , 622. Conductivity (Ko, X1 cm2 mol1) in methanol: 104.

All the experiments involving CT-DNA, which were studied by UV absorbance and fluorescence displacement of ethidium bromide (EB), were performed following our previously reported method [29].

2.3. Synthesis of complexes 2 and 3 To a methanolic solution of complex, 1 (1.0 mmol, 964 mg), KOH (1.0 mmol, 56 mg) in methanol (20 mL) was added and the mixture was stirred for 15 min. Then a methanolic solution of dipyridylamine (1.0 mmol, 0.17 g) was added dropwise continuing to stir further for 1 h. The crystalline corresponding product of 2, [Cu2(l-C2O4)(dpa)4](ClO4)22H2O, was obtained from the resulting solution after a few days by allowing slow evaporation of the solvent at room temperature. Yield: 70–75%. [Cu2(l-C2O4)(dpa)4](ClO4)2 (2) Anal. Calc. for C42H36Cl2Cu2N12O12: C, 45.91; H, 3.30l; N, 15.30; Cu, 11.57. Found: C, 44.72; H, 3.48; N, 14.96; Cu, 11.03%. IR (cm1): mC@N, 1482; mC@O, 1655; msClO4 , 1084; masClO4 , 625. Conductivity (Ko, X1 cm2 mol1) in dimethylformamide: 143. Complex [Cu2(l-C2O4)(dpa)2(DMF)2](ClO4)2 (3) was obtained by stirring for 1 h at ambient temperature a mixture of complex 1 (491 mg) and KOH (50 mg) in 15 mL dimethylformamide (DMF) instead of methanol. Then the resulting solution was kept aside and green colored crystals were got after several days.

2.4. X-ray crystal structure analysis

3. Results and discussion 3.1. Synthesis and characterization In this work the copper(II) complex of 1,3-propylenebis(oxamide) of formulation Na2[Cu(pba)]3H2O was prepared according to Table 1 Crystal data and details of refinement data for complexes 1 and 3. Complex

1

3

Formula Formula weight Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z qcalc (g cm3) F (0 0 0) h (°) Reflections collected Independent reflections (Rint) Data [I > 2r(I)] Parameters Goodness-of-fit (GOF) on F2 Final R indices [I > 2r(I)]

C27H28Cu3N8O16Cl2 982.09 monoclinic P21/c

C28H32Cl2Cu2N8O14 902.60 triclinic  P1

11.347(3) 19.885(4) 16.189(4) 90.0 102.82(3) 90.0 3561.7(15) 4 1.831 1980 2.25–27.10 41 589 7521 (0.0487) 4322 522 0.876 R1 = 0.0473 wR2 = 0.1231 R1 = 0.0813 wR2 = 0.1361 0.618 and 0.518

8.515(2) 9.121(2) 12.729(3) 70.22(2) 74.51(3) 77.59(2) 888.0(4) 1 1.688 460 2.51–27.88 10 455 4081 (0.0294) 3098 273 1.031 R1 = 0.0449 wR2 = 0.1294 R1 = 0.0528 wR2 = 0.1354 0.509 and 0.374

R indices (all data)

Dq (e Å3)

S. Dey et al. / Inorganica Chimica Acta 376 (2011) 129–135

131

the literature method [20] and was used as starting material to react with copper(II) perchlorate hexahydrate and dpa for the synthesis of the trinuclear copper(II) complex, 1 in neutral methanol medium at ambient temperature. The novel complex 1 was characterized by physico-chemical and spectroscopic tools and revealed to be stable in air and soluble in methanol, DMF and DMSO and insoluble in hexane, ether and dichloromethane. The conductivity measurement in methanol showed a conductance of 104 Ko mol1 cm1 at 300 K suggesting that in solution complex 1 exists as 1:1 electrolyte and it has the same structure both in solution and solid state.

paired about a center of symmetry leading the central copper ions to be at 2.769(3) Å from the oxygen O(4) of the symmetry related trinuclear complex, and the atom Cu(2) fulfills the square pyramidal coordination as cited above (Fig. 2). The disorder found in the propylene bridge (see Section 2) may be due to close steric contacts with the symmetry related complex. A search in CCDC indicates that this type of arrangement (Fig. 2) is observed for the first time in these trinuclear complexes.

3.2. Structural description of complex 1

The IR spectrum of the complex 1 showed a broad intense band at 1100 cm1 and another at 622 cm1 assigned to mClO4 symmetric and asymmetric stretching, in addition to a characteristic band at 1472 cm1 assignable to the mC@N stretching frequency. A characteristic band at 1600 cm1 indicates the presence of C@O group in complex 1. The electronic spectrum of the copper(II) complex was recorded in the UV–Vis region using methanol as solvent. Three absorption bands with varied intensity were observed. An intense bands at 224 nm (s, e = 19 512 dm3 mol1 cm1) and at 313 nm (s, e = 10 0 12 dm3 mol1 cm1) may be attributable to the spin exchange interaction between the copper(II) ions through the p-path orbital set up by the oxamato bridge [33]. A broad band observed at the 627 nm (b, e = 112 dm3 mol1 cm1) is in agreement with the d–d transition for the two terminal copper(II) ions in the square pyramidal geometry.

The ORTEP view of the tri-nuclear copper(II) complex 1 with the atom numbering scheme is illustrated in Fig. 1, and a selection of bond distances and angles are listed in Table 2. In the complex the oxamato groups bridge the neighboring copper atoms and the coordination sphere around each metal ion can be described as a distorted square pyramidal geometry. In fact the terminal metals Cu(1) and Cu(3) are chelated by the N donors of the dipyridylamine ligand and by two O atoms from the oxamate moiety of the central Cu(pba) metallo ligand, fulfilling the coordination at the apical site through a perchlorate ion and a water molecule, respectively. The central metal ion Cu(2) is surrounded by two nitrogen and two oxygen atoms from the oxamato bridge and completes the squared pyramidal geometry with a pba oxygen of a symmetry related trinuclear complex (see below). The Cu–Cu separations within the trinuclear unit are Cu(1)– Cu(2) = 5.194(1) Å and Cu(2)–Cu(3) = 5.181(1) Å with an intermetallic angle of 175.36(2)°. All these values compare with those measured in similar reported complexes [30–32]. All the copper atoms do not show any significant displacement from the mean base plane, and only for Cu(1) the donors in the basal square plane present a slight tetrahedral distortion of ±0.131(2) Å. The basal mean planes are not coplanar and form dihedral angles of 5.4(2)° (Cu(1)/Cu(2)) and of 9.0(2)° (Cu(2)/Cu(3)). The terminal Cu–N(py) bond distances fall in a range 1.961(3)– 1.992(3) Å, and appear slightly longer then the Cu(2)–N(pba) ones that average to 1.927(3) Å. On the other hand the Cu(1)–O(1) and Cu(3)–O(6) distances of the oxamato bridge on the side of nitrogen atoms (of 1.939(3) and 1.947(3) Å, respectively) are shorter than all the other Cu–O distances that fall in a range 1.996(3)–2.010(3) Å. The Cu–O bond lengths relative to the apical O donor are, as expected, significantly longer (Cu(1)–O(11) = 2.446(5), Cu(3)–O (1w) = 2.355(4) Å). Moreover in the crystal the complexes are

Fig. 1. ORTEP drawing of the cation of [Cu3(pba)(dpa)2(H2O)(ClO4)]ClO4H2O (1).

3.3. Spectral characterization

3.4. Reactivity of complex 1 On reaction of complex 1 with dipyridylamine (dpa) and dimethylformamide (DMF) in alkaline medium, two dinuclear oxalato-bridged copper(II) complexes, 2 and 3, respectively, were obtained (Scheme 1). The conductivity measurement of 2 and 3 in dimethylformamide showed conductances in the range of 143–152 Ko mol1 cm1 at 300 K. These values suggest that each complex exists in solution as 1:2 electrolyte [34]. The IR spectrum of 2 displayed characteristic bands of the oxalate bridging ligand [5,35]: masym(C–O) at 1655s and msym(C–O) at 1356m. A weak signal of the d(O–C–O) bending is at 850 cm1, while the absorption frequencies at 1084 cm1 and 625 suggest the presence of perchlorate ion [34]. Complex 3 presents the characteristic bands of the oxalate bridging ligand: masym(C–O) at 1653s, msym(C–O) at 1356m and d(O–C–O) at 823 cm1 and the absorption frequencies at 1092 and 624 cm1 for perchlorate ion. 3.4.1. Structural description of complexes 2 and 3 The crystal and molecular structure of 2, as dihydrated complex [Cu2(l-C2O4)(dpa)4](ClO4)22H2O was also determined by X-ray diffraction analysis. Since this has been already reported [36,37] a picture of the complex cation is reported as Supporting Material (Fig. S1), and it will be not described in detail. Here, differently from the other copper complexes reported, the metals present a distorted octahedral geometry, being chelated by two dpa ligands and by the oxalate anion. The ORTEP view of the dinuclear copper(II) complex [Cu2(lC2O4)(dpa)2(DMF)2](ClO4)2 (3) with the atom numbering scheme is illustrated in Fig. 3, and a selection of bond distances and angles are listed in Table 3. The complex is located on a crystallographic center of symmetry and the independent metal exhibits a square pyramidal coordination geometry being chelated by the dpa ligand and oxalate anion in the basal plane, while the apical position is occupied by a DMF coordinated via carbonyl group. The oxalate moiety separates the metals at 5.226(2) Å, a distance significantly shorter than that measured in 2 (5.685(2) Å) dictated in the latter

132

S. Dey et al. / Inorganica Chimica Acta 376 (2011) 129–135

Table 2 Selected bond lengths (Å) and angles (°) for complex 1. Cu(1)–O(1) Cu(1)–O(2) Cu(1)–N(1) Cu(1)–N(2) Cu(1)–O(11)

1.939(3) 2.010(3) 1.961(3) 1.987(4) 2.446(5)

Bond angles (°) O(1)–Cu(1)–O(2) O(1)–Cu(1)–N(1) O(1)–Cu(1)–N(2) N(1)–Cu(1)–O(2) N(2)–Cu(1)–O(2) N(1)–Cu(1)–N(2) O(1)–Cu(1)–O(11) O(2)–Cu(1)–O(11) N(1)–Cu(1)–O(11) N(2)–Cu(1)–O(11)

Cu(2)–O(3) Cu(2)–O(4) Cu(2)–N(7) Cu(2)–N(8) Cu(2)–O(40 )

83.55(11) 174.81(13) 92.26(13) 91.28(13) 162.72(14) 92.61(15) 88.40(15) 94.19(14) 92.30(16) 102.47(15)

2.000(3) 1.999(3) 1.922(3) 1.931(3) 2.769(3)

O(4)–Cu(2)–O(3) N(7)–Cu(2)–O(3) N(8)–Cu(2)–O(3) N(7)–Cu(2)–O(4) N(8)–Cu(2)–O(4) N(7)–Cu(2)–N(8) O(3)–Cu(2)–O(40 ) O(4)–Cu(2)–O(40 ) N(7)–Cu(2)–O(40 ) N(8)–Cu(2)–O(40 )

95.10(10) 84.65(12) 179.35(15) 179.14(14) 84.76(12) 95.50(13) 83.81(10) 82.52(11) 98.28(13) 95.54(13)

Cu(3)–O(5) Cu(3)–O(6) Cu(3)–N(5) Cu(3)–N(6) Cu(3)–O(1w) O(6)–Cu(3)–O(5) N(5)–Cu(3)–O(5) N(6)–Cu(3)–O(5) O(6)–Cu(3)–N(5) O(6)–Cu(3)–N(6) N(5)–Cu(3)–N(6) O(6)–Cu(3)–O(1w) N(5)–Cu(3)–O(1w) N(6)–Cu(3)–O(1w) O(5)–Cu(3)–O(1w)

1.996(3) 1.947(3) 1.982(3) 1.992(3) 2.355(4) 83.69(11) 170.74(13) 92.52(12) 91.34(12) 176.19(12) 92.38(13) 93.11(14) 94.46(15) 87.43(14) 93.61(13)

Primed atom at x + 1, y, z + 1.

Fig. 2. Centrosymmetric hexanuclear entity of compound 1 showing the pair of Cu(2)–O(40 ) interactions.

Fig. 3. ORTEP diagram of [Cu2(l-C2O4)(dpa)2(DMF)2](ClO4)2 (3). Of the disordered ClO4  anion only the species at higher occupancy is shown.

[Cu(H2O)6]ClO4 + dpa Stir/4h

MeOH

N

O

O

OH[Cu2(μ-C2O4)(dpa)4](ClO4)2.2H2O (2)

OH-

O

O

O

.3H2O

Na2[Cu(pba)].3H2O

MeOH/Stir 6 h

H N

[Cu3(pba)(dpa)(H2O)(ClO4)]ClO4.H2O (1) dpa

N Cu

Na2

Mixture Na2[Cu(pba)].3H2O

O

N

N

dpa

DMF

[Cu2(μ-C2O4)(dpa)2(DMF)2](ClO4)2 (3)

Scheme 1. Synthetic procedure of formation the copper(II) complexes (1–3).

by the steric crowding of two dpa ligands in the coordination sphere. The Cu–N distances, of 1.970(2) and 1.981(2) Å, are slightly shorter than the Cu–O(oxalate) of 1.995(2) and 2.015(2) Å, while the apical bond length is, as expected, slightly longer (2.202(2) Å). As evident from the picture the O3–Cu–X angles in a range 91.17(10)–99.42(9)° are indicative of the deviation from ideal values leading the copper ion to be displaced from the O2N2 basal plane by 0.195(1) Å. The pyridine mean plane form a dihedral angle of 22.5(2)°. The amino group of dpa forms a hydrogen bond with an oxygen of the perchlorate anion (N. . .O distance of 2.93 Å), which in turn is located above the copper ions indicating a possible Cu–OClO3 interaction in solution. In the crystal the Cu–O13 is 2.97(1) Å. It

Table 3 Selected bond lengths (Å) and angles (°) for complex 3. Cu–O(1) Cu–O(2) Cu–O(3) Bond angles (°) O(1)–Cu–O(2) O(1)–Cu–O(3) N(1)–Cu–O(1) N(2)–Cu–O(1) N(1)–Cu–O(2)

1.995(2) 2.015(2) 2.202(2) 82.77(8) 99.42(9) 168.29(9) 92.58(9) 91.01(9)

Cu–N(1) Cu–N(2)

N(2)–Cu–O(2) O(2)–Cu–O(3) N(1)–Cu–O(3) N(2)–Cu–O(3) N(1)–Cu–N(2)

1.970(2) 1.981(2)

167.53(9) 97.24(10) 91.17(10) 94.93(10) 91.45(10)

is to note that the nitrate derivative of this complex has been reported but with a high disorder situation [38]. 3.4.2. Study of kinetics of the hydrolysis of 1 by UV–Vis spectroscopy It was recorded a gradual shift in the spectra of complex 1 when it was treated with KOH. The spectra bands shift from long to short wavelength region along with an increase of intensity. This feature may be due to the lowering of the P ? P⁄ transition energy caused by the base hydrolysis of metal ligand bond (Fig. 4). The kinetics for this process was determined by the initial rates method by recording the increase of the hydrolysis species (at 627 nm). The concentration of the base was kept 100 times more than that of the Cu(II) complex. The experiments were conducted at constant temperature of 25 °C.

133

S. Dey et al. / Inorganica Chimica Acta 376 (2011) 129–135

0.35 0.30 0.25 0.20 0.15 0.10 0.05 1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

[C] Fig. 5. Plot of DC/Dt vs. [C] (concentration of hydrolyzed species).

The thermal decomposition of the complexes was studied using thermo-gravimetric analysis. In compound 1, an initial mass loss of 3.9% was observed in the range from room temperature to 109 °C. It corresponds to the release of the lattice and the coordinated water molecules (Calc. 4.0%). After this temperature, the observed plateau suggested the thermal stability of the anhydrous species up to 270 °C. The second weight loss of 10.5% in the range 270–311 °C, corresponds to the removal of perchlorate ion. From the data relative to the thermal decomposition of complex 1 in the temperature range from 27 to 330 °C, a straight line has been obtained by plotting the logarithm of the mole fraction of complex remaining versus time (Fig. 6). It demonstrates that the thermal decomposition process follows a first-order kinetics with the specific reaction-rate constant (k) of 6.2  103 min1. In case of complex 3, the release of the two coordinated DMF molecules occurs in the interval from 154 to 224 °C, and a stable molecular species was formed in the temperature range of 224–290 °C with the weight of the complex hardly loss. Then a complicated degradation occurs with a total weight loss.

0.00 -0.02 -0.04

lnC

3.5. Thermal analysis

-0.06 -0.08 -0.10 -0.12 -0.14 0

2

4

6

8

10 12 14 16 18 20 22

Time (min) Fig. 6. Plot of log [mole fraction of complex remaining] vs. time in thermal analysis of 1.

f

1.30

The binding mode of complex 1 with calf thymus DNA was examined by electronic absorption titration (Fig. 7) and ethidium bromide (EB) fluorescence displacement experiments (Fig. 8). Complex 1 showed a significant hyperchromism effect centered at the 313 nm absorption maximum. The intrinsic binding constant Kb was calculated to be 4.01  104 M1, from the slope to intercept ratio of the graph obtained by plotting the [DNA]/(ea  ef) versus [DNA] (Fig. 9, R = 0.89527, 5 points), by using the following equation [39]

Absorbance

1.25

3.6. DNA-binding studies

a

1.20 1.15 1.10 1.05 1.00 270

285

300

315

330

345

Wavelength(nm) Fig. 7. Electronic spectral change of 1 during titration with CT-DNA in Tris–HCl buffer; [Complex] = 1.56  104 mol L1; [DNA]: (a) 0.0, (b) 1.00  106, (c) 2.00  106, (d) 3.40  106, (e) 7.76  106, (f) 9.7  106 mol L1. The increase of DNA concentration is indicated by an arrow.

½DNA=ðea  ef Þ ¼ ½DNA=ðeb  ef Þ þ 1=½K b ðeb  ef Þ

1.0

where [DNA] is the DNA concentration, ea, eb and ef symbolize the extinction coefficient for each addition of DNA to the complex, for the copper complex in the fully bound form and for the free copper complex, respectively. The linear Stern–Volmer quenching constant Ksv has been calculated from the linear Stern–Volmer equation, [40] by considering the quenching of EB bound to DNA due to the presence of the complex:

0.5

I0 =I ¼ 1 þ K sv ½Q 

2.5 2.0

Absorbance

0.40

ΔC/Δt

Initially, a 2.0 mL methanolic solution of Cu(II) complex having concentration of 1  103 mol dm3 as a substrate solution was poured in a 1 cm quartz cell kept in the spectro-photometer to equilibrate the temperature at 25 °C. Then 0.04 mL 5  103 mol dm3 solution of potassium hydroxide (KOH) in methanol was quickly added and mixed thoroughly. The absorbance was continually monitored at 627 nm at an interval of 2.0 min. The plot of DC/Dt versus [C] gave a straight line and it reveals that the hydrolysis followed a first order kinetic. The rate constant was determined to be 4.9  101 min1 from the slope of the tangent to the DC/Dt versus [C] (concentration of newly formed complex) curve at 25 °C (Fig. 5) where DC = (Ct  C0), Ct is the concentration of the hydrolyzed product at tth time and C0 is the initial concentration.

1.5

0.0 250

300

350

400

Wave length(nm) Fig. 4. UV–Vis spectral change of 1 due to hydrolysis.

where I0 and I represent the fluorescence intensities in the absence and presence of quencher, respectively, and Q is the concentration of quencher. Here, the value of Ksv (6.9  104, R = 0.97556 for five points) (viz. Fig. 10) is comparable to that of the well-established groove binding rather than classical intercalator agent [41].

134

S. Dey et al. / Inorganica Chimica Acta 376 (2011) 129–135

0.65

2400 a

Intensity

2000

0.6

1600

e

0.55

1200 0.5

800 0.45

400 0.4

560

600

640

680

720

760

800

Wavelength(nm)

0.35

Fig. 8. Emission spectra of the CT-DNA-EB system in Tris–HCl buffer based on the titration of complex 1. kex = 522 nm; [EB] = 0.96  106 mol L1; [DNA] = 0.94  105; [Complex]: (a) 9.4  107, (b) 1.88  106, (c) 2.82  106, (d) 3.76  106, (e) 4.70  106 mol L1. The arrow indicates the increase of the complex concentration.

2.8x10

-10

2.6x10

-10

2.4x10

-10

2.2x10

-10

2.0x10

-10

1.8x10

-10

1.6x10

-10

0

50

100

150

200

250

300

Fig. 11. Plot of vMT vs. T for compound 1. Solid line shows the best fit with JCuCu = 320.0 ± 0.3 cm1, zJ0 = 0.22 ± 0.2 cm1, and g = 2.05.

T=2K

0.8

M (µ B)

[DNA]/(εa-εf )

1

0.6

0.4

0.2 0.00

2.50x10

-6

5.00x10

-6

7.50x10

-6

1.00x10

-5

[DNA]

0

Fig. 9. Plot of [DNA]/(ea  ef) vs. [DNA] for the titration of CT-DNA with 1 in Tris– HCl buffer; binding constant Kb = 4.01  104 M1 (R = 0.89527, n = 5).

3.7. Temperature dependence of the molar magnetic susceptibility of 1 While complexes 2 and 3 are not really novel, the newly designed trinuclear copper(II) complex 1, having odd number of paramagnetic copper(II) ions, deserves a more detailed study of its magnetic behavior, which was investigated in the temperature range 2–300 K. The temperature dependency of the molar magnetic susceptibility (vM) is depicted in Fig. 11. As temperature is lowered, vMT exhibits a rapid decrease reaching a plateau value of 0.38 cm3 mol1 K below 120 K. Such behavior is characteristic for strong anti-ferromagnetic interactions between the magnetic

1.35 1.30 1.25

I/I0

1.20 1.15 1.10 1.05

0

1 10

4

2 10

4

3 10

4

4 10

4

5 10

4

H (Oe) Fig. 12. Experimental (points) and calculated (line) field dependence of the magnetization at 2 K.

centers. At 300 K vMT is 0.62 cm3 mol1 K, already much below the value of 1.125 cm3 mol1 K anticipated for three S = ½ spin carriers with no exchange coupling. Then vMT rapidly decreases to reach the plateau, and remains constant between 120 and 10 K, and below this temperature a further slight decrease is observed. The plateau corresponds to the S = ½ spin state resulting from the antiferromagnetic interaction in the linear three spin system. The field dependence of the magnetization recorded at 2 K is given in Fig. 12. The behavior of M versus H is consistent with a S = ½, and the best fit with the Brillouin expression yielded g = 2.0 with S = ½. The structure for complex 1 consists of a linear arrangement of three CuII ions bridged by oxamato ligands and two of these units are paired by linkage of the central Cu ions via an inner oxamatooxygen atom. Based on this arrangement the magnetic behavior has been analyzed by a linear three spin model derived from the spin Hamiltonian H ¼ Jðb S1 b S2 b S2 þ b S 3 Þ [42]. The possible interactions in the dinuclear arrangement are anticipated to be very weak due to the long bond, hence these have been considered within the meanfield approximation. The best fit to the experimental data (Fig. 11) yielded JCuCu = 320.0 ± 0.3 cm1, zJ0 = –0.22 ± 0.02 cm1, and g = 2.05. The exchange interaction between the oxamato-bridged Cu-centers in 1 compares well with that found in related compounds [43].

1.00 1.0x10

-6

2.0x10

-6

3.0x10

-6

4.0x10

-6

5.0x10

-

[Complex ] Fig. 10. Plot of I0/I vs. [Complex] for the titration of CT-DNA-EB system with 1. Linear Stern–Volmer quenching constant Ksv = 6.9  104 (R = 0.99596, n = 5).

3.8. Electrochemistry Redox properties of complex 1 was examined by cyclic voltammetry using a Pt-disk working electrode and a Pt-wire auxiliary

S. Dey et al. / Inorganica Chimica Acta 376 (2011) 129–135

electrode in dry MeCN and in presence of [Et4N]ClO4 as supporting electrolyte. The potentials are expressed with reference to saturated KCl electrode. In solution the compound displayed a quasireversible voltammogram having ipc/ipa  1, Epc at 1.31 V and Epa 1.05 V with DE = 260 mV. Though the electrochemical analysis of complex 1, which has three copper(II) ions in different coordination environment and geometries (as observed in solid state), showed only one reduction point, indicating that the three metal centers are electrochemically equivalent in solution. 4. Conclusion A new trinuclear copper(II) complex, [Cu3(pba)(dpa)2(H2O)(ClO4)]ClO4H2O (1) having an almost linear arrangement of three electrochemically equivalent CuII ions bridged by oxamato ligand has been synthesized and structurally characterized. This compound is characterized by very strong antiferromagnetic Cu–Cu interactions with JCuCu = 320 cm1. The oxamato bridge is susceptible to be hydrolyzed in alkaline medium, and complex 1 reacts with other ligands in presence of potassium hydroxide to produce oxalate-bridged dinuclear copper(II) complexes [Cu2(l-C2O4)(dpa)4](ClO4)2 (2) and [Cu2(l-C2O4)(dpa)2(DMF)2](ClO4)2 (3). Initially complex 1 was hydrolyzed by OH ion, then due to the less structural rigidity, an additional dpa ligand coordinates to the metal center to form complex 2. However, in presence of DMF and potassium hydroxide, complex 1 was converted to 3 likely due to the stronger coordination of DMF to copper(II) ion. In fact the Cu– O(DMF) bond length of 2.202(2) Å is comparatively shorter than the Cu–OH2 bond (2.355(4) Å) measured in 1, thus maintaining the square pyramidal coordination sphere of metal ions. Complex 1 interacts with CT-DNA and the investigation indicates a tendency for this complex to be bound within the groove of DNA. Acknowledgements Financial support from Department of Science and Technology (DST), New Delhi, India is gratefully acknowledged. E. Zangrando thanks MIUR-Rome, PRIN No. 2007HMTJWP_002 for financial support. S. Sarkar is thankful to CSIR, New Delhi for Senior Research Fellowship. Appendix A. Supplementary material CCDC 786341, 786342 and 786343 contain the supplementary crystallographic data for the complexes 1, 2 and 3, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2011.06.005. References [1] Z. Smekal, J. Kamenıcek, P. Klasova, G. Wrzeszcz, Z. Sindelar, P. Kopel, Z. Zak, Polyhedron 21 (2002) 1203. [2] M. Du, Y.-M. Guo, X.-H. Bu, Inorg. Chim. Acta 335 (2002) 136.

135

[3] O. Castillo, A. Luque, S. Iglesias, C. Guzman-Miralles, P. Roman, Inorg. Chem. Commun. 4 (2001) 640. [4] H. Nunez, J.J. Timor, J. Server-Carrio, L. Soto, E. Escriva, Inorg. Chim. Acta 318 (2001) 8. [5] J. Tang, E. Gao, W. Bu, D. Liao, S. Yan, Z. Jiang, G. Wang, J. Mol. Struct. 525 (2000) 271. [6] O. Castillo, I. Muga, A. Luque, J.M. Gutierrez-Zorrilla, J. Sertucha, P. Vitoria, P. Roman, Polyhedron 18 (1999) 1235. [7] H.Z. Kou, D.Z. Liao, G.M. Yang, P. Cheng, Z.H. Jiang, S.P. Yan, G.L. Wang, X.K. Yao, H.G. Wang, Polyhedron 17 (1998) 3193. [8] S. Kitagawa, T. Okubo, S. Kawata, M. Kondo, M. Katada, H. Kobayaski, Inorg. Chem. 34 (1995) 4790. [9] L.S. Tuero, J. Garcia-Lozano, E.E. Monto, M.B. Borja, F. Dahan, J.P. Tuchagues, J.P. Legros, J. Chem. Soc., Dalton Trans. (1991) 2619. [10] R. Huber, Angew. Chem., Int. Ed. Engl. 28 (1989) 848. [11] P. Kyritsis, A. Messerschmidt, R. Huber, G.A. Salmon, A.G. Sykes, J. Chem. Soc., Dalton Trans. (1993) 731. [12] W. Kaim, J. Rall, Angew. Chem., Int. Ed. Engl. 35 (1996) 43. and references therein. [13] U.M. Sundaram, H.H. Zhang, B. Hedman, K.O. Hodgson, E.I. Solomon, J. Am. Chem. Soc. 119 (1997) 12525. and references therein. [14] A. Messerschmidt, R. Ladenstein, R. Huber, M. Bolognesi, L. Avigliano, R. Petruzzelli, A. Rossi, A. Finazzi-Agro, J. Mol. Biol. 224 (1992) 179. [15] O. Kahn, Angew. Chem., Int. Ed. Engl. 24 (1985) 834. [16] J.L. Cole, L. Avigliano, L. Morpugno, E.I. Solomon, J. Am. Chem. Soc. 113 (1991) 9080. [17] S. Bandyopadhyay, J.M. Lo, H.H. Yao, F.L. Liao, P. Chattopadhyay, J. Coord. Chem. 59 (2006) 2015. and references therein. [18] T. Ruffer, B. Brauer, A.K. Powell, I. Hewitt, G. Salvan, Inorg. Chim. Acta 360 (2007) 3475. [19] J. Tercero, C. Diaz, M.S.E. Fallah, J. Ribas, X. Solans, M.A. Maestro, J. Mahia, Inorg. Chem. 40 (2001) 3077. [20] S. Turner, O. Kahn, L. Rabardel, J. Am. Chem. Soc. 118 (1996) 6428. [21] V. Baron, B. Gillon, J. Sletten, C. Mathoniere, E. Codjovi, O. Kahn, Inorg. Chim. Acta 235 (1995) 69. [22] K. Nakatani, P. Bergerat, E. Codjovi, Inorg. Chem. 30 (1991) 3977. [23] O. Kahn, Y. Pei, M. Verdaguer, J.P. Renard, J. Sletten, J. Am. Chem. Soc. 110 (1988) 782. [24] O. Kahn, Acc. Chem. Res. 33 (2000) 647. [25] C. Mathonière, J.-P. Sutter, J.V. Yakhmi, in: J.S. Miller, M. Drillon (Eds.), Magnetism: Molecules to Materials, vol. 4th, Wiley-VCH, 2002, pp. 1–40. [26] Z. Otwinowski, W. Minor, in: C.W. Carter Jr., R.M. Sweet (Eds.), Methods in Enzymology, vol. 276, Academic Press, New York, 1997, pp. 307–326. [27] G.M. Sheldrick, Acta Crystallogr., Sect. A A64 (2008) 112. [28] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837. [29] S. Dey, S. Sarkar, H. Paul, E. Zangrando, P. Chattopadhyay, Polyhedron 29 (2010) 1583. [30] J. Tercero, C. Diaz, J. Ribas, J. Mahia, M. Maestro, X. Solans, J. Chem. Soc., Dalton Trans. (2002) 2040. [31] R. Costa, A. Garcia, J. Ribas, T. Mallah, Y. Journaux, J. Sletten, X. Solans, V. Rodriguez, Inorg. Chem. 32 (1993) 3733. [32] R. Costa, A. Garcia, R. Sanchez, J. Ribas, X. Solans, V. Rodriguez, Polyhedron 12 (1993) 2697. [33] R. Vicente, A. Escuer, J. Ferretjans, H. Stoeckli-Evans, X. Solans, M.F. Bardıa, J. Chem. Soc., Dalton Trans. (1997) 167. [34] S. Sarkar, S. Dey, T. Mukherjee, E. Zangrando, M.G.B. Drew, P. Chattopadhyay, J. Mol. Struct. 980 (2010) 94. [35] L. Zhang, W.-M. Bu, S.-P. Yan, Z.-H. Jiang, D.-Z. Liao, G.-L. Wang, Polyhedron 19 (2000) 1105. [36] S. Youngme, G.A. van Albada, N. Chaichit, P. Gunnasoot, P. Kongsaeree, I. Mutikainen, O. Roubeau, J. Reedijk, U. Turpeinen, Inorg. Chim. Acta 353 (2003) 119. [37] R.E. Marsh, Acta Crystallogr., Sect. B61 (2005) 359. [38] S. Youngme, N. Chaichit, C. Pakawatchai, Transition Met. Chem. 29 (2004) 840. [39] A. Wolfe, G.H. Shimer, T. Meehan, Biochemistry 26 (1987) 6392. [40] O. Stern, M. Volmer, Z. Phys. 20 (1919) 183. [41] L. Strekowski, D.B. Harden, R.L. Wydra, K.D. Stewart, W.D. Wilson, J. Mol. Recognit. 2 (1989) 158. [42] O. Kahn, Molecular Magnetism, VCH Weinheim, 1993. [43] Y. Journaux, J. Sletten, O. Kahn, Inorg. Chem. 25 (1986) 439.