A new tetraiminodiphenol macrocyclic ligand and its two dicopper(II) complexes: Syntheses, crystal structures, electrochemistry and magnetochemistry

Journal of Molecular Structure 1020 (2012) 127–133

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A new tetraiminodiphenol macrocyclic ligand and its two dicopper(II) complexes: Syntheses, crystal structures, electrochemistry and magnetochemistry Samit Majumder a, Michel Fleck b, C. Robert Lucas c,⇑, Sasankasekhar Mohanta a,⇑ a

Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India Institute for Mineralogy and Crystallography, University of Vienna, Althanstr, 14, A-1090 Vienna, Austria c Department of Chemistry, Memorial University of Newfoundland, St. John’s, NL A1B 3X7, Canada b

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 27 January 2012 Received in revised form 2 April 2012 Accepted 2 April 2012 Available online 10 April 2012

This paper deals with the diprotonated salt, [H4L](ClO4)2 (1), of the tetraiminodiphenolate macrocyclic ligand, H2L which is the [2 + 2] condensation product of 4-ethyl-2,6-diformylphenol and 2,20 -dimethyl1,3-diaminopropane, and two diphenoxo-bridged dicopper(II) complexes [Cu2L(ClO4)2] (2) and [Cu2L (ClO4)](ClO4) (3). The syntheses, characterization and crystal structures of 1–3 and the magnetic and electrochemical properties of the metal complexes 2 and 3 are presented. The cationic macrocycle [H4L]2+ is stabilized by four intramolecular and symmetric NAH. . .O. . .HAN hydrogen bonds involving phenoxo oxygen and imino nitrogen atoms. One metal center in 3 is tetracoordinated and in a square planar geometry. The second metal ion of 3 and both the metal ions in 2 are pentacoordinated and in a square pyramidal geometry; one perchlorate oxygen atom occupies the apical position in each case. Both 2 and 3 exhibit quasireversible two-step one-electron couples in the reduction window. The E½ (DEp) values (in mV) for the CuIICuII/CuIICuI couple is 0.466 V (0.067 V) for 2 and 0.490 V (0.076 V) for 3, while the E½ (DEp) values (in mV) for the CuIICuI/CuICuI couple is 0.953 V (0.060 V) for 2 and 0.985 V (0.069 V) for 3. Variable-temperature (2–300 K) magnetic susceptibility measurements of the two compounds reveal that the metal centers in both of the complexes are coupled by strong antiferromagnetic interactions with 2J values (H = JS1S2) of 780 and 820 cm1 for 2 and 3, respectively. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: Macrocyclic ligand Dicopper(II) Magnetochemistry Electrochemistry Crystal structures

1. Introduction Interest in the investigation of new dinuclear complexes is still continuing, although a number of such complexes were reported earlier due to their relevance in bioinorganic chemistry [1–3], magnetochemistry [4–17], electrochemistry [13–19] and homogeneous catalysis [20–23]. Several works on dicopper(II) complexes have been reported due to their relevance to copper-containing enzymes such as hemocyanin, tyrosinase and catechol oxidases [24–26]. Again, being the simplest exchange-coupled systems, magnetic properties of dicopper(II) complexes have been studied [5–11,13–17] and a number of magneto-structural correlations have been determined [5]. However, new dicopper(II) systems derived from new preorganized ligands deserve study for the enlightenment they can shed on the structure–property correlations. In 1970, Robson introduced 4-methyl-2,6-diformylphenol as the precursor for the synthesis of preorganized bi-/oligonucleating ligands and since then there has been a continuous growth of this area; several macrocyclic [27–36] and acyclic ligands [24–26] and ⇑ Corresponding authors. E-mail addresses: (S. Mohanta).

[email protected]

(C.R.

Lucas),

[email protected]

0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.04.003

di-/oligonuclear metal complexes derived therefrom have been reported. By changing the 4-substituent from methyl [5–9,14, 17–19,32–36] to chloro [12,29], Fluoro [15,30], bromo [11,31], butyl [14], tertiarybutyl [10], trifluromethyl [16], etc., several ligands and their metal complexes have also become known. These complexes have occupied a dominating position in coordination chemistry research that explores the electrochemical, magnetic, biomimetic and catalytic phenomena. In these and other types of complexes, small changes in the ligand environment can result in tremendous change of the metal complexes with respect to topology/composition, structural parameters and magnetic or electrochemical properties. For example, change of magnetic and electrochemical properties in dicopper(II) complexes of tetraiminodiphenolate macrocyclic ligands are known on changing the 4substituent of the 4-substituent-2,6-diformylphenol fragment of the ligand from methyl to butyl, t-butyl or trifluromethyl. Similarly, the number of components, self-assembly and topology of the complexes of 3-ethoxysalicylaldehyde-diamine ligands [37,38] are dramatically different from the complexes of 3-methoxysalicylaldehyde-diamine ligands [39,40]. With the expectation of variation in their structure and properties, we have begun studies of systems derived from ligands having 4-ethyl-2,6-diformylphenol [41] as a precursor. In comparison to the 4-methyl

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2.2. Syntheses of the complexes

N

OH

N

N

OH

N

Chart 1. Chemical structure of H2L.

analogue, new and interesting structures/topologies/properties have already been observed in the complexes derived from an acyclic ligand having 4-ethyl-2,6-diformylphenol as a fragment. To explore further the systems having 4-ethyl-2,6-diformylphenol as a fragment, we have synthesized the diprotonated salt, [H4L](ClO4)2 (1) of the tetraiminodiphenolate macrocyclic ligand, H2L (Chart 1) which is the [2 + 2] condensation product of 4-ethyl-2,6-diformylphenol and 2,20 -dimethyl-1,3-diaminopropane, and two diphenoxo-bridged dicopper(II) complexes [Cu2L(ClO4)2] (2) and [Cu2L(ClO4)](ClO4) (3). Herein, we report the syntheses, characterization and crystal structures of 1–3 and the magnetic and electrochemical properties of the metal complexes 2 and 3.

2.2.1. [H4L](ClO4)2 (1) The diprotonated perchlorate salt, [H4L](ClO4)2, of the macrocyclic Schiff base ligand H2L was synthesized by condensation of 4ethyl-2,6-diformylphenol with 2,20 -dimethyl-1,3-diaminopropane following a known procedure to prepare the related other macrocyclic ligands [27,28]. To a boiling methanol solution (30 mL) containing 2,6-diformyl-4-ethylphenol (0.356 g, 2 mmol), NaClO4 (1 g, 8 mmol) and acetic acid (0.25 ml, 4 mmol) was added a methanol solution (5 mL) of 2,20 -dimethyl-1,3-propanediamine (0.208 g, 2 mmol). The solution was removed from the source of heating and kept at room temperature overnight. The product separated as orange crystals that were collected by filtration and washed with methanol and diethyl ether. (Yield: 0.63 g, 92%) Anal. Calcd. for C30H42N4Cl2O10 (689.59): C, 52.25; H, 6.14; N, 8.12%. Found C, 52.40; H, 6.26; N, 8.24%. IR data, m(cm1): 1649 vs (C@N), 1088 vs, 625 w (ClO4). 2.2.2. [Cu2L(ClO4)2] (2) To a stirred acetonitrile solution (10 mL) of H4L(ClO4)2 (0.689 g, 1 mmol), an acetonitrile solution (5 mL) of Cu(ClO4)26H2O (0.742 g, 2 mmol) was added. To the resulting reddish solution, was added dropwise an acetonitrile solution (5 mL) of triethylamine (0.404 g, 4 mmol) to produce a green solution. After 2 h stirring, the green solution was filtered to remove out any suspended particles. The filtrate was kept at room temperature for slow evaporation. After a few days, diffraction quality green crystals deposited were collected by filtration and air dried. (Yield: 0.568 g, 70%) Anal. Calcd. for C30H38N4O10Cl2Cu2 (812.65): C, 44.34; H, 4.71; N, 6.89%. Found C, 44.52; H, 4.84; N, 6.78%. IR data, m(cm1): 1641 vs (C@N), 1105 vs, 624 w (ClO4). UV–vis (dmf, kmax/nm (e/M1 cm1)): 357(12,800), 604 (136).

2. Experimental Caution! Perchlorate complexes of metal ions are potentially explosive. Only a small amount of material should be prepared, and it should be handled with caution. 2.1. Materials and physical methods All reagents and solvents were purchased from commercial sources and used as received. 4-ethyl-2,6-diformylphenol was synthesized by a known procedure [42]. Elemental (C, H and N) analyses were performed on a Perkin-Elmer 2400 II analyzer. IR spectra were recorded in the region 400–4000 cm1 on a Bruker-Optics Alpha–T spectrophotometer with samples as KBr disks. Electronic spectra were obtained by using a Hitachi U-3501 spectrophotometer. The electrospray ionization mass spectra were recorded on a Micromass Qtof YA 263 mass spectrometer. Variable-temperature (2–300 K) magnetic susceptibility measurements under a fixed field strength of 1 T were carried out with a Quantum Design MPMS SQUID magnetometer. Diamagnetic corrections were estimated from Pascal constants. Cyclic voltammetric (CV) and square wave voltammetric (SWV) measurements were done using a Bioanalytical System EPSILON electrochemical analyzer. The concentration of the supporting electrolyte, tetrabutylammonium perchlorate (TBAP) was 0.1 M, while that of the complexes was 1 mM. Cyclic voltammetric measurements for 2 and 3 were carried out in dimethylsulphoxide solution, with a three-electrode assembly comprising a glassy carbon disk working electrode, a platinum auxiliary electrode and an aqueous Ag/AgCl reference electrode, the later of which was connected with the solution through a salt bridge (tetrabutylammonium perchlorate in acetonitrile). Under identical conditions, the E½ value of the ferrocene/ferrocenium couple was 0.380 V (DE = 0.062 V) in dimethylsulphoxide.

2.2.3. [Cu2L(ClO4)](ClO4) (3) To a boiling methanol solution (30 mL) of 4-ethyl-2,6-diformylphenol (0.356 g, 2 mmol), was added a methanol solution (10 mL) containing Cu(ClO4)26H2O (0.371 g, 1 mmol) and Cu(OAc)22H2O (0.199 g, 1 mmol). A methanol solution (20 mL) of 2,20 -dimethyl1,3-propanediamine (0.202 g, 2 mmol) was then added dropwise. The resulting green solution was refluxed for 1 h. On cooling this solution, green solid deposited which was collected by filtration and washed well with methanol. Recrystallization from acetonitrile/toluene mixture produced green crystals suitable for X-ray diffraction. (Yield: 0.650 g, 80%) Anal. Calcd. for C30H38N4O10Cl2Cu2(812.65): C, 44.34; H, 4.71; N, 6.89%. Found C, 44.50; H, 4.60; N, 6.98%. IR data, m(cm1): 1640 vs (C@N), 1105 vs, 624 w (ClO4). UV–vis (dmf, kmax/nm (e/M1 cm1)): 357(12,800), 604 (136). 2.3. Crystal structure determination of [H4L](ClO4)2 (1), [Cu2L(ClO4)2] (2) and [Cu2L(ClO4)](ClO4) (3) The crystallographic data of these three compounds 1–3 are summarized in Table 1. Diffraction data for 1 were collected on a Nonius Kappa diffractometer at 120 K whereas the data for 2 and 3 were collected on a Nonius APEX-II diffractometer with CCD-area detector at 293 K using graphite-monochromated Mo Ka radiation (k = 0.71073 Å) with data collection and reduction performed using the NONIUS program suite DENZO-SMN package [43,44]. All data for the compound 1–3 were corrected for Lorentz, polarization, background and absorption effects. For all the compounds, crystal structures were determined by direct methods and subsequent Fourier and difference Fourier syntheses, followed by full-matrix least-squares refinements on F2 using SHELXL-97 [45]. The perchlorate oxygen atoms of 1 (O2, O3, O4 and O5) were disordered over two sites with 0.5 occupancy for each. The following hydrogen

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S. Majumder et al. / Journal of Molecular Structure 1020 (2012) 127–133 Table 1 Crystallographic Data for 1–3. Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z D (calculated, g cm3) k (Mo Ka), Å l (mm1) T (K) F(000) 2h range for data collection (°) Index ranges

No. measured reflections No. independent reflections Rint No. refined parameters No. observed reflections (I P 2r (I)) Goodness-of-fit on F2 (S) R1a, wR2b (I P 2r (I)) R1a, wR2b (all data) Max., min. electron density (eÅ3) a b

C30H42N4Cl2O10 689.59 Orthorhombic Fdd2 18.917(4) 40.782(8) 8.6499(17) 90.00 90.00 90.00 6673(2) 8 1.373 0.71073 0.255 293(2) 2912 8.54–59.60 26 6 h 6 26 55 6 k 6 56 11 6 l 6 12 4644 4644 0.0000 320 3805 1.014 0.0379, 0.0816 0.0551, 0.0899 0.194, 0.316

C30H38N4O10Cl2Cu2 812.65 Monoclinic P 2(1)/c 8.6461(4) 10.8379(5) 18.3254(8) 90.00 102.1300(10) 90.00 1678.85(13) 2 1.608 0.71073 1.488 293(2) 836 5.90–58.06 11 6 h 6 11 14 6 k 6 14 25 6 l 6 25 17,148 4175 0.0182 266 3604 1.052 0.0276, 0.0682 0.0356, 0.0728 0.397, 0.294

C30H38N4O10Cl2Cu2 812.65 Monoclinic P 2(1)/n 13.2937(5) 14.2150(6) 18.5904(7) 90.00 108.391(2) 90.00 3333.6(2) 4 1.619 0.71073 1.498 293(2) 1672 4.38–67.02 19 6 h 6 20 21 6 k 6 21 28 6 l 6 24 57,437 12,441 0.0397 569 7584 1.011 0.0472, 0.1072 0.1007, 0.1293 1.395, 0.667

R1 = [R||Fo|  |Fc||/R|Fo|]. wR2 = [Rw(F 2o  F 2c )2/Rw(F 2o )2]1/2.

atoms were placed at geometrically fixed positions: H8A, H8B, H8C linked with C8 in 1; H8A, H8B and H8C linked with C8, H12A, H12B and H12C linked with C12, H13A, H13B and H13C linked with C13 and H15A and H15B linked with C15 in 2; H8A, H8B and H8C linked with C8 and H23A, H23B and H23C linked with C23 in 3. All other hydrogen atoms in 1–3 were located and refined freely. The hydrogen atoms were refined isotropically, while the nonhydrogen atoms were refined anisotropically. The final refinements converged at the R1 values (I > 2r (I)) 0.0379, 0.0276 and 0.0472 for 1, 2 and 3, respectively.

3. Results and discussion 3.1. Syntheses and characterization The diprotonated perchlorate salt, [H4L](ClO4)2, of the macrocyclic Schiff base ligand H2L and the dinuclear copper(II) complexes described herein are facilely synthesized in high yields under mild conditions. The complex 2 is readily obtained in high yield from the reaction of the perchlorate salt of the macrocyclic ligand, copper(II) perchlorate and triethylamine as base in 1:2:4 ratio. On the other hand, metal-templated condensation of 2,6-diformyl-4-ethyphenol and 2,20 -dimethyl-1,3-diaminopropane in presence of copper(II) acetate and copper(II) perchlorate results in the formation of the dicopper(II) complex 3. Thus, variation in composition takes place on changing the reaction route. The IR spectrum of the diprotonated perchlorate salt (1) of the macrocyclic ligand H2L exhibits characteristic bands at 1649 cm1 due to mC@N and at 1088 and 625 cm1 due to ionic perchlorate. In both the complexes 2 and 3, the mC@N absorption appears at slightly lower energy (1641 cm1 for 2 and 1640 cm1 for 3). The two binuclear complexes show two peaks at ca. 1105 cm1 and a peak ca. 624 cm1 due to perchlorate ion. The

splitting of the peak around 1105 cm1 indicates the presence of coordinated perchlorate. Composition of the complexes was further verified by electrospray ionization mass spectra (ES–MS positive). The spectra of both the complexes were recorded in acetonitrile. Both (Fig. 1 for complex 2) exhibit two abundant peaks at m/z 713.20 (17%; line to line separation 1.0) and 306.87 (100%; line to line separation 0.5). The peaks at m/z 306.87 and 713.20 are assignable to a dicationic [Cu2(L)]2+ (C30H38N4O2Cu2) and a monocationic [Cu2(L)(ClO4)]+ (C30H38N4O6ClCu2) species. As shown in Fig. 1, the isotopic distribution of the observed and simulated spectral patterns are in excellent agreement with each other, indicating right assignment. The electronic spectra of the copper(II) complexes were recorded in DMF medium. The electronic spectra of the complexes exhibit band at 604 nm (e = 136 L mol1 cm1) and 357 nm (e = 12,800 L mol1 cm1). The lower energy band is assigned to a 2 dz ! dx2 y2 transition (the unpaired electron reside in the dx2 y2

306.87 100 306.87

%

713.20 Simulated

50

713.20

306.87 Observed 713.20 0 300

700

1100

1400

m/z

Fig. 1. Electrospray mass spectrum (ESI-MS positive) of [Cu2L(ClO4)2] (2) in acetonitrile showing observed and simulated isotopic distribution pattern.

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Fig. 2. Crystal structure of cationic part of [H4L](ClO4) (1). Hydrogen atoms are omitted for clarity. A, x, 2y, z.

Table 2 Selected bond lengths (Å) and angles (°) for [H4L](ClO4)2 (1). N(1)AC(9) N(2)AC(14) N(1)AC(10) N(2)AC(15) O(1)AC(1) N(1)  O(1) N(1)AH(1) O(1)  H(1) N(2)  O(1) N(2)AH(2) O(1)  H(2)

1.298(2) 1.302(2) 1.459(2) 1.467(2) 1.285(2) 2.594(2) 0.85(2) 1.85(2) 2.570(2) 0.88(2) 2.570(2)

N(1)AC(9)AC(2) N(2)AC(14)AC(6) C(9)AN(1)AC(10) C(14)AN(2)AC(15) N(1)AC(10)AC(11) N(2)AC(15)AC(11A) N(1)AH(1)  O(1) N(2)AH(2)  O(1)

122.1(2) 121.7(2) 124.2(2) 125.0(2) 112.9(1) 114.9(1) 144.3(2) 141(2)

orbital as evident from the coordination environment adopted by the metal centre), while the intense peak at 357 nm arises due to ligand-to-metal charge transfer transition.

Fig. 3. Crystal structure of [Cu2L(ClO4)2] (2). Hydrogen atoms are omitted for clarity. A, 2x, 2y, z.

standard C(sp2)AOH and C(sp2)AO bond length [46]. The values of these structural parameters are in the range as observed in previously reported related macrocyclic systems [27,28]. The macrocyle is stabilized by four intramolecular hydrogen bonds involving phenoxo oxygen and imino nitrogen atoms. Considering the stabilization factor, the crystal structure can be described as a superposition of two form: (i) a molecular species in which a H atom is attached to the two alternative N atoms and two phenoxo oxygen atoms and (ii) a zwitterionic form in which four nitrogen atoms are protonated and negative charges are located on two oxygen atoms. The hydrogen bonds may be either asymmetric NAH. . .OAH. . .N or symmetric NAH. . .O. . .HAN with the later being the reasonable choice because the range of NAH distances (0.85(2) and 0.88(2) Å) is much smaller than O. . .H distances (1.85(2) and 1.83(2) Å). The donor–acceptor contacts of the four NAH. . .O interactions lie in the range 2.570(2)– 2.594(2) Å, while the NAH. . .O angles vary between 141(2) and 144.3(2)°, clearly indicating that the intramolecular hydrogen bonds are quite strong. In previously reported related macrocylic systems, similar types of hydrogen bonding have also been observed. The dihedral angle between the two symmetry related phenyl rings of 1 is 22.8° which is slightly larger than the dihedral angle (20°) observed in previously related deprotonated macrocyclic systems containing at the 4-position a methyl group instead of an ethyl group as in 1.

3.2. Description of the structure of [H4L](ClO4)2 (1)

3.3. Description of the structure of [Cu2L(ClO4)2] (2) and [Cu2L(ClO4)](ClO4) (3)

The crystal structure of [H4L](ClO4)2 (1) is shown in Fig. 2, while selected bond lengths and angles including the geometries of the hydrogen bonds are listed in Table 2. The compound has a centrosymmetric structure with the center of inversion at the geometric center of the cation. The crystal structure of 1 shows that the diprotonated Schiff base macrocycle adopts a folded conformation, reminiscent of calixarenes and related Schiff base macrocycles, with the phenyl rings folded downwards to leave the N and O donor atoms on an exposed face. The azomethane linkage is evident from the N1AC9 and N2AC14 bond lengths (1.298(2) and 1.302(2) Å, respectively), N1AC9AC2 and N2AC14AC6 angles (122.1(2) and 121.7(2)°, respectively) and C10AN1AC9AC2 and C15AN2AC14AC6 torsion angles (173.42 and 176.23°, respectively). The bond length involving the phenyl carbon and phenoxo oxygen atoms, C1AO1, is 1.285(2) Å and this value is close to the

The structures of compounds 2 and 3 are shown in Figs. 3 and 4, while selected bond lengths and angles are listed in Tables 3 and 4, respectively. The structures show that both are diphenoxo-bridged dicopper(II) compounds. Each of the two N(imine)2O(phenoxo)2 compartments provide four coordination positions to each of the two metal centers in 2 and 3. While the two symmetry related metal ions (Cu1 and Cu1A) and one (Cu2) of the two metal ions in 3 are pentacoordinated, the second metal ion (Cu1) in 3 is tetracoordinated. The fifth coordination position of Cu1/Cu1A in 2 as well as of Cu2 in 3 is occupied by a perchlorate oxygen atom, O2/O2A in 2 and O3 in 3. The geometry of the tetracoordinated metal ion, Cu1 in 3, is slightly distorted square planar; with the deviation of the donor atoms from the least-squares N2O2 plane is 0.005 Å and displacement of the metal center from the least-squares N2O2 plane is ca. 0.03 Å. On the other hand, the coordination environment of

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S. Majumder et al. / Journal of Molecular Structure 1020 (2012) 127–133 Table 4 Selected bond lengths (Å) and angles (°) for [Cu2L(ClO4)](ClO4) (3). Cu(1)AO(1) Cu(1)AO(2) Cu(1)AN(1) Cu(1)AN(4)

1.950(2) 1.957(2) 1.956(2) 1.959(2)

Cu(1)  Cu(2)

3.0369(4)

O(1)ACu(1)AN(4) O(2)ACu(1)AN(1) O(1)ACu(1)AO(2) O(1)ACu(1)AN(1) O(2)ACu(1)AN(4) N(1)ACu(1)AN(4) Cu(1)AO(1)ACu(2) Cu(1)AO(2)ACu(2)

169.55(8) 170.81(7) 78.17(6) 92.80(7) 91.43(8) 97.55(8) 102.18(7) 101.51(7)

Cu(2)AO(1) Cu(2)AO(2) Cu(2)AN(2) Cu(2)AN(3) Cu(2)AO(3)

1.953(2) 1.965(2) 1.957(2) 1.948(2) 2.422(2)

O(1)ACu(2)AN(3) O(2)ACu(2)AN(2) O(3)ACu(2)AO(1) O(3)ACu(2)AO(2) O(3)ACu(2)AN(2) O(3)ACu(2)AN(3) O(1)ACu(2)AO(2) O(1)ACu(2)AN(2) O(2)ACu(2)AN(3) N(2)ACu(2)AN(3)

168.49(7) 170.29(8) 90.55(8) 84.82(8) 98.43(9) 93.94(9) 77.91(6) 92.86(7) 91.93(7) 96.95(8)

40

Fig. 4. Perspective view of cationic part [Cu2L(ClO4)](ClO4) (3). Hydrogen atoms are omitted for clarity.

Table 3 Selected bond lengths (Å) and angles (°) for [Cu2L(ClO4)2] (2). Cu(1)AO(1) Cu(1)AO(1A) Cu(1)AN(1) Cu(1)AN(2A) Cu(1)AO(2)

1.990(1) 1.952(1) 1.954(2) 1.997(1) 2.411(2)

Cu(1)  Cu(1A)

3.0399(4)

O(1)ACu(1)AN(2A) O(1A)ACu(1)AN(1) O(2)ACu(1)AO(1) O(2)ACu(1)AO(1A) O(2)ACu(1)AN(1) O(2)ACu(1)AN(2A) O(1)ACu(1)AO(1A) O(1)ACu(1)AN(1) O(1A)ACu(1)AN(2A) N(1)ACu(1)AN(2A)

170.48(5) 169.68(6) 89.47(5) 84.81(6) 96.46(7) 91.86(6) 79.10(5) 90.66(6) 91.63(5) 98.55(6)

Cu(1)AO(1)ACu(1A)

100.90(5)

the three pentacoordinated metal ions in 2/3 is slightly distorted square pyramidal as evidenced by the discrimination parameter (s) value (0.01 for Cu1/Cu1A in 2; 0.03 for Cu2 in 3); both the deviation (0.007 Å for Cu1/Cu1A in 2; 0.018 Å for Cu2 in 3) of the donor atoms and displacement (0.03 Å for Cu1/Cu1A in 2; 0.07 Å for Cu2 in 3) of the metal center from the least-squares N2O2 plane are very small. The CuAN(imine) and CuAO(phenoxo) bond distances for the three crystallographically different copper(II) centers (Cu1 for 2 and Cu1 and Cu2 for 3) are not very different and lie in the range 1.948(2)–1.997(1) Å and 1.950(2)–1.990(1) Å, respectively. In contrast to the bond distances involving four ligand centers in the basal plane, the CuAO(apical) bond distances are significantly longer (Cu1AO2 = 2.411(2) Å for 2 and Cu2AO3 = 2.422(2) Å for 3) due to Jahn–Teller distortion. The ranges of the cisoid angles (79.10(5)– 98.55(6)° in 2; 78.17(6)–97.55(8)° for Cu1 and 77.91(6)– 98.43(9)° for Cu2 in 3) and the transoid angles (169.68(6) and 170.48(5)° for Cu1 in 2; 169.55(8) and 170.81(7)° for Cu1 and

Current (A)X10

6

30 20 10 0 -10 -20 -30 -0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

Potential (V) Fig. 5. Cyclic voltammogram of [Cu2L(ClO4)](ClO4) (3).

168.49(7) and 170.29(8)° for Cu2 in 3) indicate that the coordination environment around the metal center is distorted. In compound 2, the flatness of the macrocyclic ligand is evident from the dihedral angle of 0° between the two phenyl rings. Unlike in 2, the macrocyclic ligand in 3 adopts a puckered configuration in which the interplanar angle between the planes of the two aromatic rings is 45.6°. The metal. . .metal separation (3.0399(4) in 2; 3.0369(4) Å in 3) and the CuAO(phenoxo)ACu bridge angles 100.90(5)° in 2; 102.18(7) and 101.51(7)° in 3) are comparable with each other and also with previously reported similar macrocyclic dicopper(II) systems [5–19]. The dihedral angles in 2 and 3 between the basal planes of the two metal ions are 0 and 2.7°, respectively, indicating the bridging moiety is perfectly planar in 2 and almost planar in 3.

3.4. Electrochemical measurements The electrochemical properties of the complexes were studied by cyclic voltammetry (CV) and square wave voltammetry (SWV) in DMSO at 25 °C under a nitrogen atmosphere using a glassy carbon working electrode. A potential window ranging from 0.2 to 1.4 V in which the ligand is electrochemically silent and a scan rate of 200 mV/s was employed. For both the complexes, two cathodic responses and, in the return sweep, two anodic responses are observed. The electrochemical responses of the complex 3 is shown in Fig. 5. Two cathodic responses are observed at 0.500

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Table 5 Electrochemical measurementsa of 2 and 3 in DMSO. CuIICuII ? CuIICuI

Complex

E1½ 2 3 a

(V)

0.466 0.490

(V)

0.067 0.076

Ipa/Ipc

SWV(V)

E1½ (V)

DEp (V)

Ipa/Ipc

SWV(V)

0.90 0.98

0.471 0.492

0.953 0.985

0.060 0.069

0.84 0.84

0.969 0.985

All the potentials are against Ag/AgCl reference electrode and scan rate in all cases is 200 mV s1 for cyclic voltammogram. For cyclic voltammogram.

60

0.10

50

0.08 -1

40

χMT / cm K mol

30

0.06

3

Current (A)X 10

6

b

CuIICuI ? CuICuI

DEpb

20 10

0.04

0.02

0 -0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

0.00

Potential (V) Fig. 6. Square wave voltammogram of [Cu2L(ClO4)](ClO4) (3).

50

100

150

200

250

300

T /K Fig. 8. vM T versus T for [Cu2L(ClO4)](ClO4) (3) between 2 and 300 K. (N experimental; — calculated).

0.10

-1

0.08

0.06

3

χMT / cm K mol

0

0.04

0.02

0.00 0

50

100

150

200

250

300

T /K Fig. 7. vM T versus T for [Cu2L(ClO4)2] (2) between 2 and 300 K. (N experimental; — calculated).

and 0.987 V for 2 and at 0.528 and 1.021 V for 3. The anodic peaks appear at 0.921 and 0.433 V for 2 and 0.952 and 0.452 V for 3. The pairs of cathodic peaks are due to the stepwise one-electron reductions CuIICuII ? CuIICuI and CuIICuI ? CuICuI, while the pairs of anodic peaks arise due to the stepwise oxidations CuICuI ? CuIICuI and CuIICuI ? CuIICuII. The E½ values of the CuIICuII ? CuIICuI and CuIICuI ? CuICuI peaks are 0.466 and 0.953 V for complex 2 and 0.490 and 0.985 V for complex 3, while the DEp values of these two peaks were 0.067 and 0.060 V for 2 and 0.076 and 0.069 V for 3. This values clearly indicate that both the two couples for both the complexes are quasireversible. The ratio of redox peak current ipa/ipc is in the range 0.84–0.98 (Table 5) also suggesting the quasireversible nature of the redox process. The peak positions (0.471 and 0.969 V for 2 and 0.492 and 0.985 V for 3; shown in Fig. 6) observed in the SWV are close to the E½ values observed in CV. It is relevant at this point to men-

tion that the potentials required for the CuIICuII ? CuIICuI and CuIICuI ? CuICuI couples in the two title compounds lie in the usual ranges (from +0.090 to 1.240 V and from 0.350 to 1.720 V, respectively) of these two electrochemical conversions observed in the dicopper(II) compounds derived from the related tetraiminodiphenolate macrocyclic ligands [13–19]. It is known that in the negative potential range the electrochemical behavior of such dicopper(II) macrocyclic systems in the negative potential range is found to be sensitive to the electron-inductive nature of the substituent (methyl, butyl, trifluoromethyl, etc.) [14,16] at the para-position (with respect to the phenolate moiety) of the aromatic ring. For example, when other factors are almost identical, both the first and second reduction waves of the electron-withdrawing trifluoromethyl analogue are shifted to less negative potentials in comparison to the methyl analogue, while the first reduction potential of the electron-releasing butyl analogue is shifted to more negative potential in comparison to the methyl analogue [14]. In this context, the electrochemical response of [Cu2L(ClO4)](ClO4) (3) may be compared with that of a methyl analogue [CuII2L1(H2O)(ClO4)]ClO42H2O (4; L1 is the methyl analogue of L) [13]. Interestingly, in spite of the difference by only one CH2 moiety, the electronic inductive effect is reflected in the potential values; both the first and second reductions are shifted to more negative values for 3 (0.490 and 0.985 V) in comparison to 4 (0.403 and 0.927 V).

3.5. Magnetochemistry Variable temperature magnetic susceptibility measurements were performed on powdered samples of the perchlorate salts of complexes 2 (Fig. 7) and 3 (Fig. 8) in the temperature range 2– 300 K. The vM T values at 300 K for the complexes (0.092 and 0.085 cm3 K mol1 for 2 and 3, respectively) are much smaller than the expected value of 0.75 cm3 K mol1 for two isolated copper(II)

S. Majumder et al. / Journal of Molecular Structure 1020 (2012) 127–133

ions with g = 2.0 and S = ½, suggesting the existence of strong antiferromagnetic coupling between the CuII ions. The variable-temperature susceptibility data were fitted to the Bleaney–Bowers equation, the relevant isotropic Heisenberg exchange Hamiltonian ˆ = 2JS1S2. The best fit parameters are 2J = 780 cm1, being H g = 2.13, q = 0.2% and TIP = 50  106 for 2 and 2J = 820 cm1, g = 2.16, q = 0.5% for 3. Previous magneto-structural studies on this family of compounds have revealed a correlation between the strength of the exchange coupling and the CuAO(phenoxo)ACu angle values (h) [5]. In general, 2J values vary in the range 600– 1084 cm1 [5–17] when h angles are between 98.8 and 104.7°. In the present work, the h values are 100.90(5) for 2 and 101.51(7) and 102.18(7) for 3, matching with the experimental results for 2J where the exchange coupling in compound 3 is stronger than in compound 2 consistent with the larger value of the phenoxo bridge angle in 3. 4. Conclusions A new tetraiminodiphenolate macrocyclic ligand has been utilized in the present investigation to derive two diphenoxo-bridged dicopper(II) compounds 2 and 3. These two are new additions in the family of macrocyclic complexes. The structural, magnetic and electrochemical properties of the two compounds have been investigated in details. Both compounds exhibit strong antiferromagnetic interaction and quasireversible two-step reduction–oxidation processes. In addition the deprotonated perchlorate salt of the ligand has been isolated and characterized by single crystal X-ray diffraction. Acknowledgments Financial support from Government of India through DST (SR/ S1/IC-42/2011) and CSIR (Fellowship to S. Majumder) is gratefully acknowledged. Appendix A. Supplementary data CCDC the supplementary crystallographic data. CCDC 864537– 864539 contain the supplementary crystallographic datas in cif format for 1–3, 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) 1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.molstruc.2012.04.003. References [1] M.A. Halcrow, G. Christou, Chem. Rev. 94 (1994) 2421. [2] J.D. Crane, D.E. Fenton, J.M. Lartour, A. Smith, J. Chem. Soc., Dalton Trans. (1991) 2979.

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