Oximato bridged dinuclear copper(II) complexes: Synthesis, crystal structure, magnetic, thermal, electrochemical aspects and BVS analysis

Polyhedron 43 (2012) 89–96

Contents lists available at SciVerse ScienceDirect

Polyhedron journal homepage: www.elsevier.com/locate/poly

Oximato bridged dinuclear copper(II) complexes: Synthesis, crystal structure, magnetic, thermal, electrochemical aspects and BVS analysis Jnan Prakash Naskar a,⇑, Bhargab Guhathakurta a, Liping Lu b, Miaoli Zhu b,⇑ a

Department of Chemistry, Jadavpur University, Calcutta 700 032, India Institute of Molecular Science, Key Laboratory of Chemical Biology and Molecular Engineering of the Education Ministry, Shanxi University, 92 Wucheng Road, Taiyuan, Shanxi 030006, People’s Republic of China b

a r t i c l e

i n f o

Article history: Received 14 February 2012 Accepted 4 June 2012 Available online 15 June 2012 Keywords: Schiff-base ligand Dinuclear copper(II) complexes Crystal structure Redox properties Thermal behavior BVS analysis

a b s t r a c t Reactions of the Schiff-base ligand, 2-hydroxyimino-3-(2-hydrazonopyridyl)-butane (LH) with copper(II) perchlorate hexahydrate and copper(II) nitrate trihydrate in 1:1 molar proportion in methanol give rise respectively to Cu2L2(ClO4)2 (1a) and Cu2L2(NO3)2 (1b) in substantial yields. The complexes have been characterized by C, H and N microanalyses, copper estimation, ESI-MS, FT-IR, UV–Vis spectra, molar electric conductivity and room temperature magnetic susceptibility measurements. The X-ray crystal structures of both 1a and 1b have been determined. The structures reveal that both the complexes are oximato bridged dinuclear Cu(II) assemblies. The electrochemical studies of 1a and 1b show Cu(II) to Cu(III) oxidation in methanol. The thermal behavior of the nitrate analog, 1b, has also been forwarded to delve into the structure. Bond-Valence Sum (BVS) method of analyses was also performed to assign the oxidation state for each copper center both in 1a and 1b. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The electron-transfer properties of binuclear copper(II) complexes are of contemporary interest. The reason is obvious. Binuclear copper(II) assemblies often serve as models for the oxidoreductive functions of copper proteins [1]. Search for synthetic analogs of binuclear copper(II) systems to mimic the type 3 active site of copper proteins has resulted in the syntheses and studies of a large number of binuclear systems [2–5]. From this study two situations are emerged. In one situation, the two copper centers undergo a two electron transfer process involving both the copper centers at identical standard potential [6–8], while in the other case they have two successive and distinct monoelectronic steps [9–12]. Although oxidation of Cu(II) ¢ Cu(III) is common in many mononuclear tri- and tetra-peptide complexes [13,14] with potentials varying from +0.40 to +1.0 V, no examples of synthetic binuclear copper systems with sequential single electron oxidation steps are known except for the first report of single electron oxidation occurring in trinuclear copper(II) systems [15,16]. The first promising example along that line for a binuclear copper(II) complex, however, appears in 1985 [17]. Relevance of binuclear copper(II) complexes in biology is also significant. Hemocyanin and Tyrosinase contain a coupled binuclear copper system. These proteins contain two copper(II) centers. However, this system

⇑ Corresponding authors. Fax: +91 (33) 24146223 (J.P. Naskar). E-mail addresses: [email protected] (J.P. Naskar), [email protected] (M. Zhu). 0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2012.06.001

is EPR inactive due to strong antiferromagnetic spin coupling between the two spin-half copper(II) ions [18,19]. Again, oximato bridged dinuclear copper(II) complexes are of current interest both from structural and magnetic view points [20–24]. This is most likely due to the versatility of the deprotonated oxime moiety. As a diatomic (l-1,2) bridging ligand between copper(II) ions, it can adopt either inplane and out-plane coordinating mode [25]. The former in-plane planar mode stabilizes the singlet state in dinuclear copper(II) assembly favoring strong antiferromagnetic interaction. The other non-planar situation gives rise to weak antiferromagnetic or ferromagnetic behavior [26,27]. Diamagnetic copper(II) dimers are, however, also known [28,29]. Dinuclear copper(II) complexes have been the testing grounds for various theories in magnetochemistry [30–33]. These aspects have kindled our interest in binuclear copper(II) systems. Here in we wish to report the syntheses, structures, magnetic and electrochemical aspects of two l-1,2 oximato bridged copper(II) dimeric complexes generated from a oxime based ligand under deprotonation. The complexes display a rare two-step sequential metal centered one-electron oxidation.

2. Experimental 2.1. Materials and measurements All chemicals were of analytical reagent grade and used without further purification. 2-Hydrazino pyridine was procured from Aldrich, USA.

90

J.P. Naskar et al. / Polyhedron 43 (2012) 89–96

Diacetyl-monoxime monohydrazone was prepared by a reported procedure [34]. The melting point was determined by an electro-thermal digital melting point apparatus (SUMSIM India) and is uncorrected. Copper was estimated gravimetrically as CuSCN. Microanalyses were performed with a Perkin-Elmer 2400II elemental analyzer. FTIR spectra (KBr disc) were recorded with a Nicolet Magna-IR spectrophotometer (Series II). UV–Vis spectra (in MeOH) were recorded on a Shimadzu UV-160A spectrophotometer, 300 MHz 1H NMR spectrum (in CDCl3: reference, TMS) on a Bruker DPX300 spectrometer and ESI mass spectrum on a Waters QTOF Micro YA263 spectrometer. Solution conductivity measurements were carried out in methanolic solution at room temperature on a Systronics (India) direct reading conductivity meter (model 304). Cyclic voltammetric (CV) experiments were performed under nitrogen in dry and degassed CH3OH on a BAS Epsilon electrochemical workstation at 293 K. The conventional threeelectrode assembly is comprised of a BAS Glassy Carbon (GC) working electrode, a platinum-wire auxiliary electrode and an Ag/AgCl reference electrode. The supporting electrolyte is n-Et4NClO4 (0.1 M). Thermal analyses (TGA/DTA) were performed under nitrogen atmosphere (150 ml/min) on a PerkinElmer instrument (model: Pyris Diamond TG/DTA). Magnetic susceptibility was determined at room temperature with a PAR 155 vibrating sample magnetometer fitted with a walker scientific L75FBAL magnet. The magnetometer was calibrated with Hg[Co(SCN)4] and the susceptibility data were corrected for diamagnetism using Pascal’s constants. Caution! Perchlorate salts of metal complexes can be explosive [35]. Although no detonation tendencies have been observed, care is advised and handling of only small quantities recommended. 2.1.1. Preparation of the ligand 2.1.1.1. 2-Hydroxyimino-3-(2-hydrazonopyridyl)-butane LH. The ligand was prepared following a procedure reported earlier by us [36]. Diacetyl-monoxime (0.1 g, 0.001 mol) was dissolved in 5 ml of dry methanol. A small amount of insoluble material was removed by filtration. To this light yellow solution, 0.120 g (0.001 mol) of 2hydrazino pyridine dissolved in 5 ml of dry methanol was added dropwise with continuous stirring. The color of the resulting solution was orange. This resulting reaction mixture was heated under reflux for 7 h. After refluxing, the yellowish-orange reaction mixture was left in air for slow evaporation. After 2 days, when the volume was reduced to about 3 ml, it was filtered, washed with 10 ml of cold diethyl ether and was dried in vacuum over fused CaCl2. The ligand is reddish-brown. It is soluble in methanol and chloroform but insoluble in acetone. Yield: 130 mg (66%), mp: 205 °C. C9H12N4O (192.096): Anal. Calc. for C9H12N4O: C, 56.22; H, 6.30; N, 29.15. Found: C, 56.16; H, 6.21; N, 29.32%. FTIR (KBr): v 3334 vb(OH), 1609 s(C@N of pyridine), 1575 s(C@N of imine), 1514 s(C@N of oxime) and 930 (N–O) cm1. UV–Vis (MeOH): kmax (e/ l mol1 cm1) 299 nm (21 4285) and 211 nm (79 813). 1H NMR (CDCl3): d 8.14 (1H, d, ring proton of pyridine), d 7.65 (1H, t, another ring proton of pyridine), d 7.35 (1H, d, another ring proton of pyridine), d 6.83 (1H, t, another ring proton of pyridine), d 9.50 (1H, s, oximato proton), d 8.47 (1H, s, –NH– proton), d 2.22 (3H, s, methyl protons), d 2.11 (3H, s, methyl protons) ppm. 2.1.2. Preparation of Cu2L2(ClO4)2 (1a) Thirty milligrams (0.16 mmol) of LH was taken in 15 ml of MeOH. Then it was filtered to remove the trace amount of insoluble materials. The filtrate was yellowish in color. Then 58 mg (0.16 mmol) of Cu(ClO4)26H2O was dissolved in 5 ml of MeOH. The greenish metal solution was added dropwise with constant stirring to the ligand solution. At the end of addition, a dark green solution was obtained. It was left in air for slow evaporation. After

2 days, the dark green compound that appeared was filtered and washed with 10 ml of ice-cold diethyl ether. Then it was dried in vacuum over fused CaCl2. Yield: 36 mg (65%). Anal. Calc. for C18H22N8Cu2O10Cl2: C, 30.50; H, 3.13; N, 15.81; Cu, 17.94. Found C, 30.39; H, 3.21; N, 15.72; Cu, 17.82%. The compound is soluble in methanol but insoluble in benzene, petroleum ether (40–60) and diethyl ether. FT-IR (KBr): v 3492 br(OH), 1620 s(C@N of imine), 1501 s(C@N of oxime), 1162 s(N–O), 1117 vs(ClO4) and 622 s(ClO4) cm1. UV–Vis (MeOH): kmax (e/l mol1 cm1) 599 nm (538) [d–d band of Cu(II)], 245 nm (22 969), 297 nm (17 843), 382 nm (11 565), 476 nm (7169). ESI-MS (positive ion mode) (m/ z): 254.0500 for [Cu632L2]2+ (22%), 507.0621 for [Cu632L2H+]1+ (100%) and 509.0503 for [Cu652L2H+]1+ (91%) KM (MeOH): 163.09 cm2 ohm1 mol1 (1:2 electrolyte). leff/lB: 1.85 (at 298 K) per Cu atom. Shining dark green needle-shaped single crystals of 1a suitable for X-ray crystallography were grown from methanol-diethyl ether direct diffusion technique. 2.1.3. Preparation of Cu2L2(NO3)2 (1b) Twenty milligrams (0.10 mmol) of LH was taken in 10 ml of MeOH. It was filtered to remove the trace amount of insoluble materials. The color of the solution was yellow. Twenty-six milligrams (0.10 mmol) of Cu(NO3)23H2O was dissolved in 5 ml of MeOH. The blue metal solution was added dropwise with constant stirring to the ligand solution. At the end of addition, a dark green solution was obtained. It was left in air for slow evaporation. After 2 days, shining and crystalline dark green compound was obtained. It was filtered and washed with ice-cold diethyl ether. Then it was dried in vacuum over fused CaCl2. Yield: 21 mg (63%). Anal. Calc. for C18H22N10Cu2O8: C, 34.11; H, 3.50; N, 22.11; Cu, 20.07. Found C, 33.89; H, 3.63; N, 21.97; Cu, 20.23%. The compound is soluble in MeOH but insoluble in benzene, petroleum ether (40–60) and diethyl ether. FT-IR (KBr): v 3436 br(OH), 1619 s(C@N of imine), 1478 s(C@N of oxime), 1178 s(N–O) and 1384 s(NO3) cm1. UV–Vis (MeOH): kmax (e/l mol1 cm1) 599 nm (598) [d–d band of Cu(II)], 246 nm (28 264), 298 nm (20 826), 384 nm (10 379), 476 nm (7658). ESI-MS (positive ion mode) (m/z): 254.0843 for [Cu632L2]2+ (100%), 256.0859 for [Cu652L2]2+ (37%), 507.1348 for [Cu632L2H+]1+ (42%) and 509.1232 for [Cu652L2H+]1+ (38%). KM (MeOH): 189.87 cm2 ohm1 mol1 (1:2 electrolyte). leff/lB: 1.88 (at 298 K) per Cu atom. We got shining needle-shaped single crystals fit for X-ray crystallography of 1b from recrystallization of it in MeOH. 2.2. Crystal structures determination Single crystals suitable for X-ray crystallographic analysis were selected following examination under a microscope. Intensity data were collected at 298(2) K on a Bruker Smart Apex II diffractometer equipped with 1 K charge-coupled device (CCD) area detector by using a graphite monochromator utilizing Mo Ka radiation (k = 0.71073 Å). Cell parameters were determined using SMART software [37]. Data reduction and corrections were performed using SAINT Plus [37]. Absorption corrections were made via SADABAS [38]. The structures were solved by direct methods with the program SHELXS-97 and refined by full-matrix least-squares methods on all F2 data with SHELXL-97 [39]. The non-H atoms were refined anisotropically. Hydrogen atoms attached with C atoms were added theoretically and treated as riding on the concerned atoms. The final cycle of full-matrix least-squares refinement was based on observed reflections and variable parameters. A summary of data collection, structure refinement for complexes 1a and 1b are given in Table 1. Selected bond lengths, bond angles and hydrogen-bond geometry are given in Tables 2 and 3, respectively.

91

J.P. Naskar et al. / Polyhedron 43 (2012) 89–96 Table 1 Crystal data and structure refinement for complexes 1a and 1b.

3. Results and discussion 3.1. Synthesis and formulation

Complex

1a

1b

Empirical formula Formula weight T (K) Wavelength (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) b (°) V (Å3) Absorption coefficient (mm1) h range for data collection (°) Absorption correction Refinement method

C18H22Cu2N8O10Cl2 708.42 298(2) 0.71073 monoclinic P21/c

C18H22Cu2N10O8 633.54 298(2) 0.71073 monoclinic P21/c

6.672(4) 10.918(7) 18.388(12) 98.255(7) 1325.6(15) 1.874

6.7647(3) 10.0553(3) 18.3223(8) 101.314(2) 1222.08(8) 1.806

2.18–25.00 multi-scan full-matrix leastsquares on F2 2 1.775 716 0.20  0.03  0.02 9105 187 0.0492, 0.1369 0.0724, 0.1543 1.052 0.872 and 0.426

2.27–25.05 multi-scan full-matrix leastsquares on F2 2 1.722 644 0.10  0.04  0.04 16 665 174 0.0442, 0.0953 0.0720, 0.1060 1.047 0.382 and 0.3334

Z

qcalc (g cm3) F(0 0 0) Crystal size (mm) Reflections collected Parameters R1, all data, R1 [I > 2r(I)] wR2, all data, wR2 [I > 2r(I)] S on F2 Largest difference in peak and hole (e Å3)

H3C HO

Table 2 Selected bond distances (Å) and angles (°) for complexes 1a and 1b. Complex 1a Cu1–O1 Cu1–N3 Cu1–N4 Cu1–N1 Cu1–O2 N1–C1 N1–C2 N2–N3 N2–C1 N3–C6 N4–C7 O1–N4#1 O1–Cu1–N3 O1–Cu1–N4 N3–Cu1–N4 O1–Cu1–N1 N3–Cu1–N1 N4–Cu1–N1 O1–Cu1–O2 N4–Cu1–O2 N1–Cu1–O2 N3–Cu1–O2 C1–N1–Cu1 O3–Cl1–O4 #1 1  x, 2  y, 1  z Complex 1b Cu1–O1 Cu1–N4#1 Cu1–O2 N2–N3 O1–Cu1–N3#1 N3#1–Cu1–N4#1 N3#1–Cu1–N1#1 O1–Cu1–O2 N4#1–Cu1–O2 #1 1  x, 1  y, 2  z.

The ligand employed for the present work is LH. LH was prepared out of the Schiff-base condensation of diacetylmonoxime monohydrazone and 2-hydrazino pyridine.

1.876(4) 1.935(4) 1.987(4) 2.008(5) 2.476(4) 1.349(7) 1.367(7) 1.356(6) 1.374(7) 1.285(7) 1.299(6) 1.348(6) 169.35(18) 108.28 (16) 79.67(18) 92.14(17) 79.56 (19) 159.18 (17) 95.21(16) 84.04(18) 98.47(19) 92.63(16) 113.8(4) 107.5(4)

C1–C5 C2–C3 C3–C4 C4–C5 C6–C7 C6–C8 Cl1–O3 Cl1–O5 Cl1–O4 Cl1–O2

1.382(8) 1.347(10) 1.407(10) 1.375(9) 1.484(8) 1.496(6) 1.384(5) 1.386(7) 1.394(5) 1.408(4)

N1–C1–C2 C3–C1–C2 N2–C3–C1 N2–C3–C4 C1–C3–C4 N3–C5–C6 C8–C6–N4 C8–C6–C5 N4–C6–C5 N4–C7–N5 N5–C8–C6

122.2(4) 123.6(4) 115.8(3) 123.8(4) 120.4(4) 131.6(4) 109.5(4) 125.4(4) 125.1(4) 111.1(4) 105.5(3

1.880(3) 1.993(3) 2.336(3) 1.356(4) 166.73(13) 79.58(14) 79.81(14) 94.14(12) 88.04(14)

Cu1–N3#1 Cu1–N1#1 N1–C1 N4–O1 O1–Cu1–N4#1 O1–Cu1–N1#1 N4#1–Cu1–N1#1 N3#1–Cu1–O2 N1#1–Cu1–O2

1.945(3) 2.011(3) 1.339(5) 1.335(4) 107.72(12) 91.76(13) 158.98(14) 97.23(13) 98.59(14)

CH3

N

N

N

NH

LH

The yield was almost quantitative. Reactions of the 1:1 stoichiometric proportions of LH with the copper(II) salts, copper(II)perchlorate hexahydrate and copper(II)nitrate trihydrate, in methanol at room temperature afforded the green dimeric oximato bridged copper(II) compounds, Cu2L2(ClO4)2 (1a) and Cu2L2(NO3)2 (1b) in considerable yields. The elemental analyses data are consistent with the proposed empirical formulae. The complexes are neutral and soluble in methanol. The room temperature (298 K) effective magnetic moments of the complexes, 1a and 1b, are 1.85 and 1.88 lB, respectively. These values are consistent with an S = ½ spin state as expected for d9 (t26e3) systems. A value of 0.424 cm3 mol1 K for vMT was found for 1a (0.438 cm3 mol1 K for 1b) at 298 K, much smaller than the spin-only value of 0.75 cm3 mol1 K expected for a non-interacting S = ½ spin system, providing evidence of strong antiferromagnetic interactions both in 1a and 1b [4].

3.2. Molecular structures 3.2.1. Cu2L2(ClO4)2 (1a) The X-ray crystal structure (Fig. 1) of 1a reveals that it is a dimeric Cu(II) compound with double oximato-bridges (l1,2-N,O). It has a crystallographic center of symmetry with a Cu  Cu distance of 3.609(2) Å. This value is almost close to other symmetric l1,2-N,O double oximato bridged dinuclear copper(II) complexes (Cu  Cu distance of 3.763 Å) [4]. Each copper atom in 1a is five coordinate with ‘N3O2’ coordination chromophore. The tridentate uninegative deprotonated ligand, L1 offers three nitrogens as donors to the Cu1 center – one from the oxime moiety (N4), another from the imine nitrogen, (N3), directly attached to the diacetylmonoxime and the last nitrogen (N1) comes from the appended 2-hydrazino pyridyl moiety. The neighboring ligand in its turn shares its lone oximato oxygen (O1) to Cu1. The coordination sphere of each copper ion is an approximate square pyramid with the perchlorato oxygen atom (O2) in an axial position at a distance of 2.476(4) Å from the copper center. We can compare this structure with another symmetrical (l-1,2) bridging oxime containing copper(II) dinuclear complex having ‘N3O2’ chromophore for the copper center [40]. Here each copper is in square pyramidal geometry having weak apical coordination from a water molecule. However, due to twisted boat conformation of the dinuclear core, the two Cu–O(H2O) distances are different, 2.346(6) and 2.294(8) Å. In our case this is the same. In complex 1a, the atoms in the equatorial plane form shorter bonds [Cu1–N1 = 2.008(5) Å, Cu1–N4 = 1.987(4) Å, Cu1–N3 = 1.935(4) Å, Cu1–O1 = 1.876(4) Å]. The tridenticity of the deprotonated form of the oxime based ligand, LH, to copper(II) binding is, however, also known [41]. In

92

J.P. Naskar et al. / Polyhedron 43 (2012) 89–96

Table 3 Hydrogen bonds (Å and °) for complexes 1a and 1b. D–HA

d(D–H)

d(H  A)

d(D  A)

\(D–H–A)

Symmetry code

Complex 1a N2–H2A. . .O2#2

0.81(5)

2.16(5)

2.946(7)

166(5)

x  1, y, z

Complex 1b N2–H2. . .O3#2 N2–H2. . .O2#2

0.86 0.86

2.36 2.24

3.156(6) 2.995(5)

Fig. 1.

ORTEP

153.2 146.7

x, 1  y, 2  z x, 1  y, 2  z

plots of 1a. Symmetry code (i) 1  x, 2  y, 1  z.

order to delve into the geometry around copper, we have taken recourse to the calculation of trigonality index (s) [42–44]. By definition, s = (a  b)/60; where a is the largest angle and b, the second largest angle, around the central metal atom with the surrounded donors. A value of 0 for s is a signature for a perfectly square pyramidal geometry while it is of unity for a regular trigonal bipyramidal core. Taking, \O1–Cu1–N3 = 169.35(18)° (a) and \N4–Cu1–N1 = 159.18(17)° as b, s value comes out of 0.1695 for 1a. Thus the geometry of our compound (1a) can best be described as square pyramidal. It is quite distorted. Crystal packing diagram illustrating the hydrogen bonding in 1a is shown in (Fig. 2). 3.2.2. Cu2L2(NO3)2 (1b) The molecular structure (Fig. 3) of 1b, similar to 1a, is a dimer of two symmetric oximato bridged Cu(II) centers. However, alike 1a, the two copper centers in the dimeric unit of 1b are also coordinatively similar. Both copper centers are five coordinate having ‘N3O2’ chromophore. The apical donor oxygen atom in Cu1 comes from the coordinated nitrate oxygen, O2. The Cu1–O2 distance is of 2.335(3) Å. In 1b, the Cu  Cu distance is 3.608(1) Å. Crystal packing diagram illustrating the different hydrogen bonding patterns are shown in (Fig. 4). The hydrogen bond distances (N2– H2. . .O2i and N2–H2. . .O3i, i x, 1  y, 2  z) are 2.995(5) and 3.156(6) Å, respectively, which are weaker than normal H-bond. The length of the O  H  O bond is only 2.428 Å which is one of the shortest H-bonds known [45,46]. The s value for Cu1 and Cu2 in 1b is of similar magnitude as expected. This is of 0.1291

Fig. 2. A view of part of the crystal structure of 1a, showing the formation of a hydrogen-bonded (dotting lines) chain, O red, C gray, N blue and Cu cyan. For the sake of clarity, H atoms not involved in the motifs shown have been omitted. (Colour online.)

J.P. Naskar et al. / Polyhedron 43 (2012) 89–96

Fig. 3.

ORTEP

93

plots of 1b. Symmetry code: (i) 1  x, 1  y, 2  z.

Fig. 4. A view of part of the crystal structure of 1b, showing the formation of a hydrogen-bonded (dotting lines) chain, O red, C gray, N blue and Cu cyan. For the sake of clarity, H atoms not involved in the motifs shown have been omitted. (Colour online.)

for Cu1 [\O1–Cu1–N3 = 166.73(13)° (a) and \N4–Cu1–N1 = 158.98(14)° as b]. Thus the geometries of both the copper centers in 1b are distorted square pyramidal. However, compared to 1a, this distortion seems to be less. Most likely this is due to the coordination of the relatively more bulky perchlorate ion in 1a rather than the nitrate in 1b. The molecules of both of the complexes are linked by N–H  O (ClO4 or NO3) hydrogen bonds to form hydrogen-bonded chains (Figs. 2 and 4) containing the R22 ð16Þ motif. 3.3. Electrochemistry The redox properties of the copper(II) dimeric compounds (1a and 1b) have been examined in methanol at GC electrodes under

a N2 atmosphere. The dimeric copper(II) complexes, 1a and 1b, exhibit two consecutive responses OXI and OXII on the positive side of the Ag/AgCl in their cyclic-voltammograms. CV of 1a (Fig. 5) shows a quasi-reversible oxidation (OXI) at a potential of 0.588 V versus Ag/AgCl. The corresponding peak current, ipaI is 2.758 lA. The second consecutive oxidation can be discernable at 0.811 V versus Ag/AgCl with a peak current, ipaII of 10.61 lA. The ligand, LH, is electrochemically inert in the potential range of interest here. Thus this oxidation can safely be assigned as metal centered. Comparison of the voltammetric peak current with those of the ferrocene–ferrocenium couple under the same experimental condition establishes that the oxidative responses in 1a involves one electron in each step. This process is one electron two step pathway. Complex 1b shows similar electrochemical responses

94

J.P. Naskar et al. / Polyhedron 43 (2012) 89–96

K con ¼

Fig. 5. CV of 1a in methanol at a scan rate of 50 mV s1. Analyte concentration was 0.30  103 (M).

Fig. 6. CV of 1b in methanol at a scan rate of 100 mV s1. Analyte concentration was 0.40  103 (M).

within the same potential window (Fig. 6). 1b shows first metal centered oxidative response at 0.232 V versus Ag/AgCl with a peak current, ipaI of 13.06 lA. The second consecutive oxidation can be observed at 0.899 V versus Ag/AgCl with ipaII of 27.0 lA. The electrochemical data for 1a and 1b are tabulated in Table 4. In general, the peak current, ip, increases with square root of scan rate (m1/2) but not in proportionality. Again, the cathodic peak potential, Epc, shifts more negatively with the increase in m. Thus the metal centered oxidations for 1a and 1b are quasi-reversible in nature [47]. The electrochemical responses of 1a and 1b can be assigned as following:

CuII ; CuII  e $ CuIII ; CuII CuIII ; CuII  e $ CuIII ; CuIII The electrochemical processes in 1a and 1b are two step sequential one electron oxidation and quasi-reversible in nature. These redox assignments for both in 1a and 1b can be substantiated from a stability consideration of the mixed-valence species, Cu(III), Cu(II). This stability can be expressed in terms of the conproportionation constant, Kcon, where DE = [E1/2 (OxII)  E1/2 (OxI)] [48].

½CuðIIIÞCuðIIÞ2 ¼ exp½nFðDEÞ=RT ½CuðIIIÞCuðIIIÞ½CuðIIÞCuðIIÞ

The larger the separation of the potentials of the couple (DE), the greater is the stability of the mixed-valence species with respect to conproportionation. Taking DE for 1a of 0.349 V versus SCE (after reference conversion to SCE) the magnitude of the constant, Kcon comes out as 3.04  104. Similarly a value for DE in 1b of 0.611 V versus SCE, the magnitude of Kcon is found to be 9.55  105. These values of Kcon for 1a and 1b are comparable with those in the literature [48,49]. It is pertinent to note that the unstability of a mixed-valence species under the electrochemical time domain would give rise to a value of 0 for the conproportionation constant, Kcon [8]. Kcon value for 1a is of 104, while this is of 105 for 1b. This difference in electrochemical behavior between 1a and 1b is most likely due to the change in coordination environment. We could not isolate the oxidized species in pure form due to their instability at higher potentials. Stabilization of the trivalent oxidation state of copper by tridentate imine–oxime ligands is, however, known [50,51]. In our case, penta-coordination renders the electrogenerated dinuclear Cu(III) species unstable. Earlier such problems were also encountered for the electrochemical stabilization of +III state of copper in its penta-coordinational environment [52]. It may be noted that while examples of five-coordinate species for d8 systems are not rare [53,54], surprisingly for Cu(III), to date, there are only few examples [55]. It can be seen from Table 4, that the separation between the E1/2 values of the two couples in 1a is 305 mV versus Ag/AgCl, while this value is of 612 mV versus Ag/ AgCl for 1b. It is known that non-interacting metal centers of similar environments do not have a separation larger than 50 mV [56,57]. The strong copper–copper interactions in our present binuclear complexes, 1a and 1b, can induce CV wave splitting by a magnitude of a few hundred millivolts. Similar separation in cyclic-voltammetric waves can also be noticed in other magneticallycoupled binuclear copper(II) complexes [17]. 3.4. Thermal behavior 1.566 mg of the compound (1b) was heated from 40 to 500 °C in a platinum crucible at a heating rate of 10 °C per min. Powdered a-Al2O3 was used as a reference. The TGA plot (Fig. 7) shows a continuous mass loss from the very beginning. However, above 215 °C, a sudden but steady and sharp loss is observed. This is virtually completed only within 380 °C. The corresponding DTA plot indicates that this process is highly exothermic. This corresponds to the loss of ligand framework as well as the coordinated nitrate moieties. The ultimate burnt residue in the crucible was black in hue. This is most likely CuO. Coordinated nitrates in copper(II) compound is known to be decomposed in the temperature range of 250–340 °C [58]. 3.5. BVS analysis In order to assign the oxidation state of each copper center in 1a and 1b, we have taken recourse to the calculation based on BondValence Sum (BVS) method [59–63]. In this method, the valence s of a bond between two atoms i and j is related by an empirical

Table 4 Cyclic voltammetric data for 1a and 1b. Compound

EpaI(ipaI)

EpcI(ipcI)

E1/2(I)

Ipa/Ipc(I)

EpaII(ipaII)

EpcII(ipcII)

E1/2(II)

Ipa/Ipc(II)

1a 1b

0.588(2.76) 0.232(13.06)

0.295(2.44) 0.147(12.32)

0.441 0.189

1.13 1.06

0.811(10.61) 0.899(27.00)

0.681(0.623) 0.702(0.907)

0.746 0.801

17.03 29.77

EpcI, EpcII = cathodic peak potential, V; EpaI, EpaII = anodic peak potential, V; ipcI, ipcII = cathodic peak current, lA; ipaI, ipaII = anodic peak current, lA; E1/2 = 0.5(Epc + Epa) V.

95

J.P. Naskar et al. / Polyhedron 43 (2012) 89–96

Fig. 7. TGA/DTA plot of Cu2L2(NO3)2.

expression (1) where rij is the length of the bond (expressed in Å), and r0 a parameter

sij ¼ exp½ðr0  rij Þ=0:37

ð1Þ

characteristic of the bond. This r0, known as bond valence parameter, is however geometry and coordination number specific. The oxidation number Ni of the atom i is simply the algebraic sum of these s values of all the bonds (n) around the atom, i (2).

Ni ¼

n X sij

ð2Þ

i¼1

This Ni is known as the BVS of the ith atom. Thus if r0 is known for a particular bond type, the BVS can be calculated from the crystallographically determined rij values. To find out r0, earlier Datta and co-workers [64] in 1995 had solved Eq. (3) for r00 akin to (2). n n X X Ni ¼ sij ¼ exp½ðr 00  r ij Þ=0:37 i¼1

Table 5 Bond valence values for individual copper centers in (1a) and (1b). Compound

Bond type

Bond distance (Å)

Bond valence

Bond valance sum for individual copper

1a

Cu(1)–N(1) Cu(1)–N(3) Cu(1)–N(4) Cu(1)–O(1) Cu(1)–O(2)

2.008 1.935 1.987 1.876 2.476

0.116 0.558 0.485 0.587 0.116

2.204 for Cu1

1b

Cu(1)–N(1) Cu(1)–N(3) Cu(1)–N(4) Cu(1)–O(1) Cu(1)–O(2)

2.011 1.945 1.993 1.880 2.336

0.454 0.543 0.477 0.581 0.170

2.225 for Cu1

4. Conclusion

ð3Þ

i¼1

To find out r00 , a good number of crystallographically determined structures were considered for an atom environment where the chemically equivalent ligands are attached to the target atom. In doing so, steric strain arising due to bulky groups around the donor atoms were carefully excluded. The r00 values thus obtained were then averaged to get the best fit value of r0. It is pertinent to note that a reliable r0 value is the outcome of the large number of such judiciously selected structures employed for averaging r00 . In this endeavor, a value of 1.679 Å for r0 comes out for a Cu2+–O bond, while r0 value for a Cu2+–N bond was so determined as 1.719 Å [64]. We have taken these values for r00 s to find out the BVS values of all the copper centers in 1a and 1b employing equation (1) in our present calculation. Taking the crystallographically determined bond lengths in Å of three Cu–N bonds and two Cu–O bonds in 1a for copper center, the BVS comes out as 2.204 valence units. Similar exercise gives rise to the BVS value of 2.225 valence units for copper in 1b (Table 5). This result fairly corroborates the stipulated error limit of ±0.25 as proposed earlier by Thorp [65,66]. Thus an oxidation number of +2 can safely be assigned to each copper center in 1a and 1b computationally.

Here we have synthesized and characterized two copper(II) complexes from oxime based Schiff-base ligand, 2-hydroxyimino3-(2-hydrazonopyridyl)-butane (LH). The compounds are dinuclear symmetrical (l-1,2) oxime bridged. The copper(II) centers are antiferromagnetically spin coupled. Electrochemical studies reveal that the Cu(II) centers are quasi-reversibly oxidized to Cu(III). Thermal analyses data also corroborate our experimental findings. Electrospray Ionization Mass Spectra (ESI-MS) in the positive ionization mode of the methanolic solution of the compounds indicates that the dinuclear cores retain their integrity in solution also. BVS method of calculations were also undertaken to assign the oxidation state of each copper center of the Cu(II) dimers.

Acknowledgments J.P.N. gratefully acknowledges the financial support received from the Department of Science & Technology, Government of India, New Delhi. L.P.L. and M.L.Z. appreciated the National Natural Science Foundation of China (Grant No. 21171109) and the Natural Science Foundation of Shanxi Province of China (Grant Nos. 2010011011-2 and 2011011009-1).

96

J.P. Naskar et al. / Polyhedron 43 (2012) 89–96

Appendix A. Supplementary data CCDC 801663 and 801664 contain the supplementary crystallographic data for 1a and 1b, respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/ retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336033; or e-mail: [email protected]. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

J.A. Fee, Struct. Bond. 23 (1975) 1. D.J. Hodgson, Prog. Inorg. Chem. 19 (1975) 173. R.J. Doedens, Prog. Inorg. Chem. 21 (1976) 209. J.P. Naskar, C. Biswas, B. Guhathakurta, N. Aliaga-Alcalde, L. Lu, M. Zhu, Polyhedron 30 (2011) 2310. C.J. Milios, T.C. Stamatatos, S.P. Perlepes, Polyhedron 25 (2006) 134. D.E. Fenton, R.R. Schroeder, R.L. Lintvedt, J. Am. Chem. Soc. 99 (1977) 8367. J.P. Giselbrecht, M. Gross, A.H. Alberts, J.M. Lehn, Inorg. Chem. 19 (1980) 1386. S.K. Mandal, K. Nag, Inorg. Chem. 22 (1983) 2567. A.W. Addison, Inorg. Nucl. Chem. Lett. 12 (1976) 899. E.F. Hasty, L.J. Wilson, D.N. Hendrickson, Inorg. Chem. 17 (1978) 1834. R.R. Gagne, C.A. Koval, T.J. Smith, M.C. Cimoline, J. Am. Chem. Soc. 101 (1979) 4571. D. Datta, A. Chakravorty, Proc. Indian Acad. Sci. 90 (1981) 1. F.P. Bossu, K.L. Chellappa, D.W. Margerum, J. Am. Chem. Soc. 99 (1977) 2195. T.A. Newbecker, S.T. Krisksey Jr., K.L. Chellappa, D.W. Margerum, Inorg. Chem. 18 (1979) 444. D. Datta, A. Chakravorty, Inorg. Chem. 22 (1983) 1611. A. Ramachandraiah, P.S. Zacharias, Inorg. Nucl. Chem. Lett. 16 (1980) 433. P.S. Zacharias, A. Ramachandraiah, Polyhedron 4 (1985) 1013. I. Bertini, H.B. Gray, S.J. Lippard, J.S. Valentine (Eds.), Bioinorganic Chemistry, University Science Books, California, 1994. E.I. Solomon, U.M. Sundaram, T.E. Machonkin, Chem. Rev. 96 (1996) 2563. P. Chaudhuri, Coord. Chem. Rev. 243 (2003) 143. A. Chakravorty, Coord. Chem. Rev. 13 (1974) 1. P.K. Nanda, M. Bera, A.M. da Costa Ferreira, A. Paduan-Filho, D. Ray, Polyhedron 28 (2009) 4065. N.F. Curtis, O.P. Gladkikh, S.L. Heath, K.R. Morgan, Aust. J. Chem. 53 (2000) 577. R. Ruiz, J. Sanz, F. Lloret, M. Julve, J. Faus, C. Bois, M.C. Munoz, J. Chem. Soc., Dalton Trans. 20 (1993) 3035. T.C. Stamatatos, J.C. Vlahopoulou, Y. Sanakis, C.P. Raptopoulou, V. Psycharis, A.K. Boudalis, S.P. Perlepes, Inorg. Chem. Commun. 9 (2006) 814. B. Cervera, R. Ruiz, F. Lloret, M. Julve, J. Cano, J. Faus, C. Bois, J. Mrozinski, J. Chem. Soc., Dalton Trans. 24 (1997) 395. O. Das, N.N. Adarsh, A. Paul, T.K. Paine, Inorg. Chem. 49 (2010) 541. J.A. Bertrand, J.H. Smith, P.G. Eller, Inorg. Chem. 13 (1974) 1649. W.E. Hatfield, Inorg. Chem. 11 (1972) 216.

[30] O. Kahn, Molecular Magnetism, VCH, New York, 1993. [31] C.J. ÓConnor (Ed.), Research Frontiers in Magnetochemistry, World Scientific, Singapore, 1993. [32] J.S. Miller, M. Brillon (Eds.), Magnetism: Molecules to Materials, Wiley-VCH, Weinheim, Germany, 2002. [33] D. Gatteschi, R. Sessoli, Angew. Chem., Int. Ed. 42 (2003) 268. [34] A. Darapsky, H. Spannagel, J. Prakt. Chem. 2 92 (1915) 272. [35] W.C. Wolsey, J. Chem. Educ. 50 (1973) A335. [36] J.P. Naskar, C. Biswas, L. Lu, M. Zhu, J. Chem. Crystallogr. 41 (2011) 502. [37] Bruker, SMART (Version 5.0) and SAINT (Version 6.02). Bruker AXS Inc., Madison, Wisconsin, USA, 2000. [38] G.M. Sheldrick, SADABS, University of Gottingen, Germany, 2000. [39] G.M. Sheldrick, SHELXS97 and SHELXL97, University of Gottingen, Germany, 1997. [40] M. Maekawa, S. Kitagawa, Y. Nakao, S. Sakamoto, A. Yatani, W. Mori, S. Kashino, M. Munakata, Inorg. Chim. Acta 293 (1999) 20. [41] C. Basu, S. Biswas, A.P. Chattopadhyay, H. Stoeckli-Evans, S. Mukherjee, Eur. J. Inorg. Chem. (2008) 4927. [42] A.W. Addison, T.N. Rao, J. Reedijk, J. van Rijn, G.C. Verschoor, J. Chem. Soc., Dalton Trans. 7 (1984) 1349. [43] K.P. Maresca, G.H. Bonavia, J.W. Babich, J. Zubieta, Inorg. Chim. Acta 284 (1999) 252. [44] M. Li, A. Ellern, J.H. Espenson, Inorg. Chem. 44 (2005) 3690. [45] T. Steiner, Angew. Chem., Int. Ed. 41 (2002) 48. [46] J.E. Huhee, Inorganic Chemistry, third ed., Harper & Row, New York, 1983. pp. 268–272. [47] R. Greef, R. Peat, L.M. Peter, D. Pletcher, J. Robinson, Instrumental Methods in Electrochemistry, Ellis Horwood Limited, England, 1985. p. 188. [48] S.K. Mondal, L.K. Thompson, K. Nag, J.P. Charland, E.J. Gabe, Inorg. Chem. 26 (1987) 1391. [49] W. Zhang, S. Liu, C. Ma, D. Jiang, Polyhedron 17 (1998) 3835. [50] M.A.-Z. Eltayeb, Y. Sulfab, Polyhedron 26 (2007) 1. [51] M. Taki, S. Itoh, S. Fukuzumi, J. Am. Chem. Soc. 124 (2002) 998. [52] M.G.B. Drew, J.P. Naskar, S. Chowdhury, D. Datta, Indian J. Chem. 42A (2003) 2531. [53] A.R. Rossi, R. Hoffman, Inorg. Chem. 14 (1975) 365. [54] J.E. Huheey, E.A. Keiter, R.L. Keiter, Inorganic Chemistry, fourth ed., Harper Collins College Publishers, New York, 1993. p. 479. [55] W.E. Keyes, J.B.R. Dunn, T.M. Loehr, J. Am. Chem. Soc. 99 (1977) 4527. [56] F. Ammar, J.M. Saveant, J. Electroanal. Chem. 47 (1973) 115. [57] J.B. Flamagan, S. Margel, A.J. Bard, F.C. Anson, J. Am. Chem. Soc. 100 (1978) 4248. [58] M.T. Wheeler, F. Walmsley, Thermochim. Acta 108 (1986) 325. [59] I.D. Brown, Chem. Soc. Rev. 7 (1978) 359. [60] D. Altermatt, I.D. Brown, Acta Crystallogr., Sect. B 41 (1985) 240. [61] I.D. Brown, Acta Crystallogr., Sect. B 48 (1992) 553. [62] I.D. Brown, D. Altermatt, Acta Crystallogr., Sect. B 41 (1985) 244. [63] J.P. Naskar, S. Hati, D. Datta, Acta Crystallogr., Sect. B 53 (1997) 885. [64] S. Hati, D. Datta, J. Chem. Soc., Dalton Trans. (1995) 1177. [65] H.H. Thorp, Inorg. Chem. 31 (1992) 1585. [66] W. Liu, H.H. Thorp, Inorg. Chem. 32 (1993) 4102.