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Synthesis, spectral, electrochemical and magnetic properties of new phenoxo-bridged dicopper(II) complexes derived from unsymmetrical binucleating ligands with imino and amino side arms
www.elsevier.nl/locate/poly Polyhedron 19 (2000) 1769 – 1775
Synthesis, spectral, electrochemical and magnetic properties of new phenoxo-bridged dicopper(II) complexes derived from unsymmetrical binucleating ligands with imino and amino side arms P. Amudha, P. Akilan, M. Kandaswamy * Department of Inorganic Chemistry, School of Chemical Sciences, Uni6ersity of Madras, Guindy Campus, Chennai 25, India Received 8 February 2000; accepted 27 April 2000
Abstract New unsymmetrical binuclear copper(II) complexes of the general formula [Cu2L(X)]ClO4, (X= OH, OAc) have been prepared by in situ complexation of the compound 3-[N-methyl-N-(2-hydroxy-3,5-dichlorobenzyl)aminomethyl]-5-bromosalicylaldehyde (mhdab) with various primary amines, copper(II) perchlorate and appropriate sodium salt. Positive ion FAB mass spectral studies of the complexes show the presence of a dicopper core in the complexes. Electronic spectra of the complexes show a broad band for the d–d transition in the region around 635–685 nm. A cyclic voltammetric study of the complexes shows two quasi-reversible reduction waves in the range E 1pc = −0.52 to −0.86 V and E 2pc = − 1.02 to −1.16 V, and the reduction potentials are sensitive towards the chemical environment around the copper atoms. Cryomagnetic investigation of the complexes shows a moderately strong antiferromagnetic spin exchange interaction between the two copper(II) ions ( − 2J 60 – 170 cm − 1). ESR spectra of the complexes show a broad band at room temperature. The electrochemical and magnetic properties of the complexes are compared on the basis of the rigidity and conjugation of the ligands H2L1, H2L2 and H2L3. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Binuclear Cu(II) complexes; Magnetic properties; Electrochemistry; ESR
Interest in the synthesis of new binuclear copper(II) complexes is mainly due to their applications in bioinorganic chemistry, magnetochemistry and homogeneous catalysis. Copper is an essential trace element for life processes and several copper-containing proteins have been identified. In particular, a dicopper active center is found in many copper proteins like hemocyanin, tyrosinase and also in multinuclear proteins like laccase, ceruloplasmin, catechol oxidases and ascorbate oxidase [1–4]. Although all these proteins contain binuclear copper active sites, their functions, nature of the amino acid coordination and stereochemistry of the active site vary. Understanding the functional and structural properties of these binuclear active site by developing small dicopper complexes as models for these metalloproteins is the main objective of bioinor* Corresponding author. Tel.: +91-44-245-4515; fax: + 91-44-2352494. E-mail address: [email protected] (M. Kandaswamy).
ganic chemistry, which links the chemistry of coordination complexes and life processes. In magnetochemistry, the study of magnetic properties of these dicopper complexes offered considerable information about the basic factors governing the superexchange phenomenon, which is very much important in understanding the physics of magnetic materials, superconductivity and in the construction of molecular ferromagnets [5]. From Robson’s pioneering work on binuclear copper(II) complexes, most of the work is concerned with the studies of symmetrical dicopper complexes and the study of unsymmetrical dicopper complexes is sparse [4,6]. Synthesis of unsymmetrical binucleating ligands and their copper(II) complexes has gained attention in recent years with the establishment of the unsymmetrical nature of the active site of hemocyanin [7] and tyrosinase [8]. The present work deals with the synthesis, spectral, electrochemical and magnetic properties of a few unsymmetrical binuclear copper(II) complexes.
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1. Experimental
1.1. Materials and methods All the chemicals were obtained from commercial sources and used without any further purifications. 2-Aminophenylbenzimidazole [9] and 2-methylaminoethyl-4,6- dichlorophenol [10] were prepared by following the literature procedures. Commercial grade methanol was distilled from quick lime and used. HPLC grade dimethylformamide was purchased from Qualigens’s fine chemicals. Elemental analyses were carried out using a Carlo Erba elemental Analyser Model 1106. 1 H and 13C NMR spectra were recorded in CDCl3 using a JOEL GSX 400 NMR spectrometer. Mass (EI) spectra were recorded on a JMS-DX303 HF Mass spectrometer. Positive ion fast atom bombardment (FAB) mass spectra were recorded on a SX 102/DA-6000 mass spectrometer in which the matrix solvent used was 3-nitrobenzyl alcohol. IR spectra were recorded on a Hitachi 270-50 spectrophotometer in the spectral range 400 – 4000 cm − 1 using KBr discs. Electronic spectra of the complexes were recorded using a Hewlett – Packerd 8452A spectrophotometer. Molar conductivity was measured by using an Elico model SX80 conductivity bridge using freshly prepared solution of the complex in DMF. Cyclic voltammograms were obtained on an apparatus comprising a PAR model 173 potentiostat/ galvanostat, model 175 universal programmer, model 176 current/voltage convertor, 179 coulometer and a Perkin–Elmer Hitachi 057 X-Y recorder. The measurements were carried out under oxygen free condition using a three electrode cell in which the working and Table 1 1 H and 13C NMR spectral assignment for mhdab
auxiliary electrodes were small platinum foils (1 ×1 cm) and the reference electrode was saturated Ag/AgCl. Tetra(n-butyl)ammonium perchlorate was used as supporting electrolyte. The magnetic susceptibility of the complexes was measured in the temperature range 77– 300 K using a PAR model 155 vibrating sample magnetometer, and the instrument was calibrated with the use of metallic nickel. Room temperature magnetic moment of the complexes was measured using Guoy balance, in which Hg[Co(SCN)4] was used as the calibrant. X-band ESR spectra of the complexes were recorded on Varian EPR-E 112 spectrophotometer using DPPH as the reference.
1.2. Preparation of the precursor ligands 1.2.1. 3 -[N-Methyl-N-(2 -hydroxy-3,5 -dichlorobenzyl)aminomethyl] -5 -bromosalicylaldehyde (mhdab) 2-Methylaminomethyl-4,6-dichlorophenol (2.05 g, 0.01 mol) and paraformaldehyde (0.33 g, 0.011 mol) were stirred in 75 cm3 glacial acetic acid for 24 h. To this mixture 5-bromosalicylaldehyde (2.0 g, 0.02 mol) was added and stirred for 48 h. The mixture was heated on a water bath for 7 h and neutralized with solid Na2CO3. The residue was extracted with CH2Cl2. The solvent was evaporated. Column chromatographic separation on silica gel (4:1 CH2Cl2 –MeOH) afforded the product. M.p. 97°C, Yield 1 g, (25%), IR KBr disc 1680 cm − 1 for CHO, Mass (EI) m/z= 416 [M −2]. Anal. Calc. for C16H14BrCl2NO3: C, 45.83; H, 3.34; N, 3.34. Found: C, 45.95; H, 3.43; N, 3.41%. Table 1 accounts the 1H and 13 C NMR peak assignment of the compound.
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1100 and 620 cm − 1. Conductance (Lm/S cm2 mol − 1) in DMF 60. lmax (nm) (o/M − 1 cm − 1) in DMF 685(276), 410(s), 275(15110).
1.3.3. [Cu2L 2(OH)]ClO4 ·2H2O (3) Dark brown solid. Anal. Calc. for C29H26BrCl3Cu2N4O9: C, 39.21; H, 2.93; N, 6.31. Found C, 39.63; H, 2.95; N, 6.42%. Selected IR (KBr): 3450, 1620, 1100 and 620 cm − 1. Conductance (Lm/S cm2 mol − 1) in DMF 55. lmax (nm) (o/M − 1 cm − 1) in DMF 623(216), 386(2709), 290(12123). 1.3.4. [Cu2L 2(OAc)]ClO4 ·H2O (4) Dark green solid. Anal. Calc. for C31H26BrCl3Cu2N4O9: C, 40.83; H, 2.85; N, 6.15. Found C, 40.54; H, 2.88; N, 6.26%. Selected IR (KBr): 3450, 1610, 1540, 1100 and 620 cm − 1. Conductance (Lm/S cm2 mol − 1) in DMF 65. lmax (nm) (o/M − 1 cm − 1) in DMF 641(213), 391(s), 286(15110). Fig. 1. Schematic diagram for the synthesis of the intermediate mhdab and ligands.
1.3. Preparation of the complexes The m-phenoxo-m-(X)-dicopper(II) complexes [Cu2L1 – 3(X)]ClO4 (1 – 6) were prepared by following the general procedure [11]. To the precursor compound mhdab (0.42 g, 1 mmol), dissolved in 75 cm3 of methanol, primary amine (1 mmol) (2-aminomethylpyridine for H2L1 or 2-aminophenylbenzimidazole for H2L2 or N,N-dimethylpropanediamine for H2L3) was added and refluxed for 2 h. The resulting solution was added to a solution of copper(II) perchlorate hexahydrate (0.75 g, 2 mmol) dissolved in 50 ml of methanol and refluxed for a further hour. The mixture was then reacted with one equivalent of NaX (X = OH, OAc), dissolved in 5 cm3 of 1:4 water – methanol and refluxed for a further hour. The solid complex that deposited on slow evaporation of the solvent was filtered, washed with diethyl ether and dried.
1.3.1. [Cu2L 1(OH)]ClO4 ·H2O·MeOH (1) Dark green plates were deposited, which were filtered and dried, whereupon the crystals turned to green powder. Anal. Calc. for C23H25BrCl3Cu2N3O9: C, 34.45; H, 3.12; N, 5.24. Found: C, 34.55; H, 3.23; N, 5.42%. Selected IR (KBr): 3450, 1630, 1100 and 620 cm − 1. Conductance (Lm/S cm2 mol − 1) in DMF 65. lmax (nm) (o/M − 1 cm − 1) in DMF 658(219), 384(1015), 274(15876). 1.3.2. [Cu2L 1(OAc)]ClO4 ·2H2O (2) Dark green solid. Anal. Calc. for C24H25BrCl3Cu2N3O10: C, 34.76; H, 3.02; N, 5.07. Found: C, 34.63; H, 3.15; N, 5.12%. Selected IR (KBr): 3450, 1620, 1540,
1.3.5. [Cu2L 3(OH)]ClO4 ·2H2O (5) Green solid. Anal. Calc. for C21H29BrCl3Cu2N3O9: C, 32.31; H, 3.72; N, 5.39. Found C, 32.52; H, 3.81; N, 5.45%. Selected IR (KBr): 3450, 1610, 1100 and 620 cm − 1. Conductance (Lm/S cm2 mol − 1) in DMF 65. lmax (nm) (o/M − 1 cm − 1) in DMF 664(275), 410(s), 276(16032). 1.3.6. [Cu2L 3(OAc)]ClO4 ·MeOH (6) Dark green solid. Anal. Calc. for C24H31BrCl3Cu2N3O9: C, 35.20; H, 3.79; N, 5.13. Found: C, 35.26; H, 3.95; N, 5.21%. Selected IR (KBr): 1630, 1540, 1100 and 620 cm − 1. Conductance (Lm/S cm2 mol − 1) in DMF 55. lmax (nm) (o/M − 1 cm − 1) in DMF 681(168), 398(s), 298(18139).
2. Results and discussion
2.1. Syntheses Reaction of 5-bromosalicylaldehyde, 2-methylaminomethyl-4,6-dichlorophenol and paraformaldehyde in acetic acid medium afforded the desired precursor compound mhdab. The IR spectra of the compound showed the characteristic peak at 1680 cm − 1 for the aldehyde group. Condensation of the aldehyde mhdab with 2aminomethylpyridine, 2-aminophenyl-benzimidazole and N,N-dimethylpropanediamine gave the ligands H2L1, H2L2 and H2L3, respectively (Fig. 1), which were complexed in situ with copper(II) perchlorate hexahydrate and appropriate sodium salt in methanol medium. In all the complexes the characteristic IR peak for the CHO group at 1680 cm − 1 is absent and the characteristic peak for the CN around 1610–1630 cm − 1 is
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observed, which indicates the effective condensation of the aldehyde group with the primary amines.
2.2. Spectroscopy The positive ion FAB mass spectral study was carried out for selected complexes 1, 4 and 6 to confirm the binuclearity of the complexes. All three complexes show the molecular ion peak assignable to the [Cu2L(X)]+ fragment. The molecular ion peak for complex 1 is observed at m/z= 668 for the [Cu2L1(OH)H2O]+ fragment, the molecular ion peak for complex 4 is observed at m/z = 792 for the fragment [Cu2L2(OAc)]+ and complex 6 shows the molecular ion peak for the fragment [Cu2L3(OAc)MeOH]+ at m/z =719. The FAB mass spectrum of complex 4 is given in Fig. 2. IR spectra of the complexes show the characteristic peak for the CN group around 1610 – 1630 cm − 1, along with a broad band centered around 3400 cm − 1, indicating the presence of coordinated or lattice water and a strong symmetrical peak around 1100 cm − 1, indicating the presence of an uncoordinated ClO4 ion in the complexes. A characteristic peak for the acetate of the complexes 2, 4 and 6 was observed in the region 1540 cm − 1. The electronic spectra of the complexes were recorded in DMF, which show a weak band in the region around 635 – 685 nm for the d – d transition along with shoulders around 375 – 430 nm, which are
most probably due to the charge transfer transitions originated from the phenolate to copper [12] or benzimidazole to copper [13]. The intense peak observed below 300 nm is assigned to the ligand–ligand charge transfer transition. Appearance of a single d–d band in the region around 650 nm indicates that the coordination geometry around the copper atoms is basically four-coordinated planar with weak axial interactions and the red shift in the lmax value for complexes 1, 2 and 5, 6 with respect to the complexes 3, 4, indicates that the complexes of the ligands H2L1 and H2L3 have relatively more distorted geometry than the complexes of the ligand H2L2 [14].
2.3. Electrochemical studies All the complexes show a molar conductance value in the range 45–60 Lm/S cm2 mol − 1 in DMF, which indicates that the complexes are 1:1 electrolyte in nature [15]. The cyclic voltammograms of the complexes show two redox waves in the cathodic potential region and the data are given in Table 2. Constant potential electrolysis of complexes 2 and 5 at 100 mV more negative to their first reduction potential consumed one electron per molecule and consumed two electrons per molecule at the potential − 1.25 V, which indicates the involvement of two single electron transfers in the reduction processes. Based on the coulometric results,
Fig. 2. FAB mass spectra for complex 4. Table 2 Electrochemical data for the complexes Complex
E 1pc (V)
E 1pa (V)
E 11/2 (V)
DE (mV)
E 2pc (V)
E 2pa (V)
E 21/2 (V)
DE (mV)
1 2 3 4 5 6
−0.64 −0.58 −0.86 −0.81 −0.62 −0.52
−0.50 −0.46 −0.62 −0.55 −0.34 −0.32
−0.57 −0.52 −0.74 −0.68 −0.48 −0.40
140 120 240 270 280 200
−1.15 −1.05 −1.11 −1.10 −1.12 −1.02
−1.00 −0.79 −1.02 −1.00 −0.96 −0.68
−1.07 −0.92 −1.06 −1.05 −1.02 −0.85
150 260 90 100 280 340
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Fig. 3. Cyclic voltammogram for complex 3.
the two reduction peaks observed in the cyclic voltammograms are attributed to the two stepwise one electron transfers as given below [Cu(II)Cu(II)] v [Cu(II)Cu(I)] v [Cu(I)Cu(I)] The cyclic voltammetric response of the complexes follow the typical characteristics of a quasi-reversible redox process. The cyclic voltammogram for complex 3 was given in Fig. 3. A closer observation of the electrochemical data of the complexes of the ligands H2L1, H2L2 and H2L3 will be interesting to identify which end of the present unsymmetrical complexes is reduced first in one electron reductive processes. Although all the complexes undergo reduction at negative potentials, it is interesting to note that for all the complexes, irrespective of their exogenous bridging donor, there is a notable shift in the first reduction potential and there is only a minor shift in the second reduction potential. Since nitrogen is a soft donor when compared with the phenoxide oxy-gen [16], it is reasonable to say that the first reduction is associated with the copper atom in the ONN compartment in these ligands. The pronounced shift in the reduction potential is most probably due to the chemi-
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cally different environments of this compartment. The second reduction peak observed at almost same potential seems to be associated with the copper atom in the ONO compartment, where the chemical environment is identical for all the three ligands. From the table it is observed that the order of first reduction potential for the complexes of the ligand is H2L3 B H2L1 B H2L2. The NNO compartment comprises a N,N-dimethylpropanediamine side arm in the ligand H2L3, 2-methylpyridine in ligand H2L1 and 2%phenylbenzimidazole in the ligand H2L2. The reduction observed at high negative potential for the complexes of the ligand H2L2 (3–4) may be the result of the greater planarity of the complexes induced by the rigidity [17] due to direct attachment of the benzimidazole group [18] to the phenyl ring and conjugation [19] of the phenylbenzimidazole side arm. Further, the complexes of the ligand H2L3 undergo reduction at less negative potential when compared with the complexes of the H2L1 ligand. The easy reduction of the complexes of the ligand H2L3 may be due to the more flexible [20] nature of the N,N-dimethylpropanediamine side arm when compared with the 2-methylpyridine side arm. A comparison of the reduction potentials of the complexes of a particular ligand indicates that the reduction potential is exogenous, bridging donor dependent and the acetato bridged complexes undergo reduction at less negative potential when compared with the hydroxo bridged complexes. This behaviour seems to be associated with the increased number of atoms (OCO) involved in the acetate bridge, which may provide more flexibility for the acetato bridged complexes as against the hydroxo bridged complexes.
2.4. Magnetic properties The magnetic moments of the complexes are subnormal at room temperature, suggesting the presence of an antiferromagnetic interaction within the complex. A variable temperature magnetic moment measurement was carried out for complexes 1–3, 5 and 6 in the temperature range 77–300 K to evaluate the temperature dependence of magnetic properties of the complexes. Analyses were carried out by fitting the experimental
Table 3 Temperature dependence magnetic parameters and ESR spectrum data for the complexes Complex
1 2 3 4 5 6
meff (BM per molecule)
−2J (cm−1)
P (%)
298 K
77 K
2.19 2.35 1.86 1.92 2.15 2.64
1.17 1.06 1.10
82 59 167
0.25 0.18 0.51
1.08 1.56
97 56
0.19 0.45
ESR data (g) g
gÞ
2.19 2.18 2.05 2.20 2.08 2.20
2.06 2.02
2.02 2.01
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Fig. 4. Temperature dependence of magnetic properties for 1.
susceptibility values to the modified Bleaney – Bowers equation. xm =(Ng 2b 2/3kT) [3+ exp(−2J/kT)] − 1 (1 − P) +(0.45P/T)+ Na where xm is the molar magnetic susceptibility of the complex, P is the percentage of monomeric impurities and other symbols have their usual meanings. Na and g have been fixed as 120×10−6 cm3 mol−1 and 2.20, respectively for all magnetic simulations. The best fit parameters are tabulated in Table 3. Temperature dependence of the magnetic properties for complexes 1 is given in Fig. 4. The exchange integral values (−2J) observed for the complexes is in the range 60 – 170 cm−1, which indicates the presence of antiferromagnetic coupling between the copper atoms. A comparison of the observed −2J value
of the complexes 1, 3 and 5 shows that for the phenylbenzimidazole substituted complex 3, the − 2J value is 167 cm − 1, indicating the strong antiferromagnetic coupling between the copper atoms when compared with complexes 1 and 5 (− 2J=82 and 97 cm − 1, respectively). In the complexes of all the three ligands reported in the present work, the geometry around the phenolic side arm (ONO) would be the same, but the geometry around the other copper atom (ONN) will be different due to the variations in chemical environments. As observed in the electrochemical properties of the complexes, the most probable reason for the strong exchange interaction may be the planarity of the phenylbenzimidazole compartment due to rigidity and conjugation of the phenylbenzimidazole side arm, which seems to reduce the dihedral angle between the two copper planes, thereby causing effective antiferromagnetic coupling in the complex. Relatively less exchange interaction observed in complexes 1 and 5 may be due to the flexibility of the N,N%-dimethylpropanediamine side arm of the ligand H2L3 and five membered ring formation in complex 1 of the ligand H2L1, which may increase the dihedral angle between the copper planes, resulting in a weak exchange interaction in these complexes. The variation in the − 2J values of complexes 1, 2 or 5, 6 is due to the effect of the exogenous bridging donor. The higher − 2J value observed for the acetato-bridged complexes when compared with their corresponding hydroxo bridged complexes is possibly due to the counter-complementary nature of the overlap of the magnetic orbitals involving the monoatomic and three atom bridging ligands [21–23].
2.5. EPR spectroscopy Room temperature ESR spectra of the complexes were studied. The ESR spectra of complexes 1, 3 and 5
Fig. 5. ESR spectra for complex (a) 1; (b) 3 and (c) 5. The half-field signal for complex 3 is inserted.
P. Amudha et al. / Polyhedron 19 (2000) 1769–1775
are given in Fig. 5. The g value of the complexes was evaluated using the relationship hy =gbH and given in Table 3. The ESR spectra of the complexes of the ligands H2L1 and H2L3 show asymmetric signal with poorly resolved hyperfine splitting signals; however’ the complexes of the ligand H2L2 show only a broad signal. In addition, all the complexes show a signal for DMs= 2 transition at the low field region around 1600 G, with g:4. g \gÞ values observed for the complexes suggest a planar-based geometry with dx 2 − y 2 ground state [24]. Appearance of a hyperfine splitting component for the complexes of the ligands H2L1 and H2L3 indicates that the antiferromagnetic coupling in the complexes is weak when compared with the complexes of the ligand H2L2.
References [1] H. Holden Thorp, in: H. Holden Thorp, V.L. Pecoraro (Eds.), Mechanistic Bioinorganic Chemistry, The American Chemical Society, Washington, DC, 1995, p. 1. [2] E.I. Solomon, V.M. Sundaram, T.E. Machonkin, Chem. Rev. 96 (1996) 2563. [3] E.I. Solomon, M.J. Baldwin, M.D. Lowery, Coord. Chem. Rev. 92 (1992) 521. [4] P.A. Vigato, S. Tamburini, Coord. Chem. Rev. 106 (1990) 25. [5] (a) D. Willett, G. Gatteschi, O. Kahn (Eds.), Magnetostructural Correlations in Exchange Coupled Systems, NATO-ASI Series, Reidal, Dordrecht, The Netherlands, 1985. (b) O. Kahn, Molecular Magnetism, VCH, New York, 1993. [6] E. Spodine, J. Manzur, Coord. Chem. Rev. 119 (1992) 171.
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[7] W.P.J. Gaykema, A. Volbeda, W.G.H. Hol, J. Mol. Biol. 187 (1985) 2255. [8] K.J. Lerch, M. Huber, H.-J. Schneider, R. Drexel, B. Linzen, J. Inorg. Biochem. 26 (1986) 213. [9] D.W. Hein, R.J. Alhem, J.J. Leavitt, J. Am. Chem. Soc. 79 (1957) 427. [10] W.J. Burke, E.L.M. Glennie, C. Weatherbee, J. Org. Chem. 29 (1962) 909. [11] V. McKee, M. Zvagulis, J.V. Dogdigian, M.G. Patch, C.A. Reed, J. Am. Chem. Soc. 106 (1984) 4765. [12] A.B.P. Lever, Electronic Spectra of Inorganic and Coordination Compounds, 2nd Ed., Elsevier, Amsterdam, 1984, 1986. [13] L. Casella, O. Carugo, M. Gullotti, S. Doldi, M. Frassoni, Inorg. Chem. 35 (1995) 1101. [14] B.J. Hathway, in: G. Wilkinson (Ed.), Comprehensive Coronation Chemistry, vol. 5, Pregamon, New York, 1987, p. 353. [15] W.J. Geary, Coord. Chem. Rev. 7 (1971) 85. [16] A. Benzekeri, J. Dudourdeaux, J.M. Latour, P. Rey, J. Laugier, J. Chem. Soc., Dalton Trans. (1991) 3359. [17] (a) R.L. Lintvedt, L.S. Kramer, Inorg. Chem. 22 (1983) 796. (b) R.R. Gagne, C.L. Spiro, T.J. Smith, C.A. Hamann, A.K. Shiemke, J. Am. Chem. Soc. 103 (1981) 4073. [18] A.K. Mishra, S.K. Dogra, Bull. Chem. Soc. Jpn. 58 (1985) 3587. [19] N.A. Bailey, D.E. Fenton, J. Lay, P.B. Roberts, J.M. Latour, D. Limosin, J. Chem. Soc., Dalton Trans. (1986) 2681. [20] (a) G.C. Patterson, R.H. Holm, Bioinorg. Chem. 4 (1975) 257. (b) D.E. Fenton, R.L. Lintvedt, J. Am. Chem. Soc. 100 (1978) 6368. [21] Y. Nishida, S. Kida, J. Chem. Soc., Dalton Trans. (1986) 2633. [22] K. Geetha, M. Netahji, A.R. Chakravarthy, N.Y. Vasanthacharya, Inorg. Chem. 35 (1996) 7666. [23] T.N. Sorrell, C.J. O’Connor, D.P. Anderson, J.H. Reibenspies, J. Am. Chem. Soc. 107 (1985) 4199. [24] R.C. Agarwal, N.K. Singh, R.P. Singh, Inorg. Chem. 20 (1981) 2794.
Synthesis, spectral, electrochemical and magnetic properties of new phenoxo-bridged dicopper(II) complexes derived from unsymmetrical binucleating ligands with imino and amino side arms P. Amudha, P. Akilan, M. Kandaswamy * Department of Inorganic Chemistry, School of Chemical Sciences, Uni6ersity of Madras, Guindy Campus, Chennai 25, India Received 8 February 2000; accepted 27 April 2000
Abstract New unsymmetrical binuclear copper(II) complexes of the general formula [Cu2L(X)]ClO4, (X= OH, OAc) have been prepared by in situ complexation of the compound 3-[N-methyl-N-(2-hydroxy-3,5-dichlorobenzyl)aminomethyl]-5-bromosalicylaldehyde (mhdab) with various primary amines, copper(II) perchlorate and appropriate sodium salt. Positive ion FAB mass spectral studies of the complexes show the presence of a dicopper core in the complexes. Electronic spectra of the complexes show a broad band for the d–d transition in the region around 635–685 nm. A cyclic voltammetric study of the complexes shows two quasi-reversible reduction waves in the range E 1pc = −0.52 to −0.86 V and E 2pc = − 1.02 to −1.16 V, and the reduction potentials are sensitive towards the chemical environment around the copper atoms. Cryomagnetic investigation of the complexes shows a moderately strong antiferromagnetic spin exchange interaction between the two copper(II) ions ( − 2J 60 – 170 cm − 1). ESR spectra of the complexes show a broad band at room temperature. The electrochemical and magnetic properties of the complexes are compared on the basis of the rigidity and conjugation of the ligands H2L1, H2L2 and H2L3. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Binuclear Cu(II) complexes; Magnetic properties; Electrochemistry; ESR
Interest in the synthesis of new binuclear copper(II) complexes is mainly due to their applications in bioinorganic chemistry, magnetochemistry and homogeneous catalysis. Copper is an essential trace element for life processes and several copper-containing proteins have been identified. In particular, a dicopper active center is found in many copper proteins like hemocyanin, tyrosinase and also in multinuclear proteins like laccase, ceruloplasmin, catechol oxidases and ascorbate oxidase [1–4]. Although all these proteins contain binuclear copper active sites, their functions, nature of the amino acid coordination and stereochemistry of the active site vary. Understanding the functional and structural properties of these binuclear active site by developing small dicopper complexes as models for these metalloproteins is the main objective of bioinor* Corresponding author. Tel.: +91-44-245-4515; fax: + 91-44-2352494. E-mail address: [email protected] (M. Kandaswamy).
ganic chemistry, which links the chemistry of coordination complexes and life processes. In magnetochemistry, the study of magnetic properties of these dicopper complexes offered considerable information about the basic factors governing the superexchange phenomenon, which is very much important in understanding the physics of magnetic materials, superconductivity and in the construction of molecular ferromagnets [5]. From Robson’s pioneering work on binuclear copper(II) complexes, most of the work is concerned with the studies of symmetrical dicopper complexes and the study of unsymmetrical dicopper complexes is sparse [4,6]. Synthesis of unsymmetrical binucleating ligands and their copper(II) complexes has gained attention in recent years with the establishment of the unsymmetrical nature of the active site of hemocyanin [7] and tyrosinase [8]. The present work deals with the synthesis, spectral, electrochemical and magnetic properties of a few unsymmetrical binuclear copper(II) complexes.
0277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0277-5387(00)00418-6
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P. Amudha et al. / Polyhedron 19 (2000) 1769–1775
1. Experimental
1.1. Materials and methods All the chemicals were obtained from commercial sources and used without any further purifications. 2-Aminophenylbenzimidazole [9] and 2-methylaminoethyl-4,6- dichlorophenol [10] were prepared by following the literature procedures. Commercial grade methanol was distilled from quick lime and used. HPLC grade dimethylformamide was purchased from Qualigens’s fine chemicals. Elemental analyses were carried out using a Carlo Erba elemental Analyser Model 1106. 1 H and 13C NMR spectra were recorded in CDCl3 using a JOEL GSX 400 NMR spectrometer. Mass (EI) spectra were recorded on a JMS-DX303 HF Mass spectrometer. Positive ion fast atom bombardment (FAB) mass spectra were recorded on a SX 102/DA-6000 mass spectrometer in which the matrix solvent used was 3-nitrobenzyl alcohol. IR spectra were recorded on a Hitachi 270-50 spectrophotometer in the spectral range 400 – 4000 cm − 1 using KBr discs. Electronic spectra of the complexes were recorded using a Hewlett – Packerd 8452A spectrophotometer. Molar conductivity was measured by using an Elico model SX80 conductivity bridge using freshly prepared solution of the complex in DMF. Cyclic voltammograms were obtained on an apparatus comprising a PAR model 173 potentiostat/ galvanostat, model 175 universal programmer, model 176 current/voltage convertor, 179 coulometer and a Perkin–Elmer Hitachi 057 X-Y recorder. The measurements were carried out under oxygen free condition using a three electrode cell in which the working and Table 1 1 H and 13C NMR spectral assignment for mhdab
auxiliary electrodes were small platinum foils (1 ×1 cm) and the reference electrode was saturated Ag/AgCl. Tetra(n-butyl)ammonium perchlorate was used as supporting electrolyte. The magnetic susceptibility of the complexes was measured in the temperature range 77– 300 K using a PAR model 155 vibrating sample magnetometer, and the instrument was calibrated with the use of metallic nickel. Room temperature magnetic moment of the complexes was measured using Guoy balance, in which Hg[Co(SCN)4] was used as the calibrant. X-band ESR spectra of the complexes were recorded on Varian EPR-E 112 spectrophotometer using DPPH as the reference.
1.2. Preparation of the precursor ligands 1.2.1. 3 -[N-Methyl-N-(2 -hydroxy-3,5 -dichlorobenzyl)aminomethyl] -5 -bromosalicylaldehyde (mhdab) 2-Methylaminomethyl-4,6-dichlorophenol (2.05 g, 0.01 mol) and paraformaldehyde (0.33 g, 0.011 mol) were stirred in 75 cm3 glacial acetic acid for 24 h. To this mixture 5-bromosalicylaldehyde (2.0 g, 0.02 mol) was added and stirred for 48 h. The mixture was heated on a water bath for 7 h and neutralized with solid Na2CO3. The residue was extracted with CH2Cl2. The solvent was evaporated. Column chromatographic separation on silica gel (4:1 CH2Cl2 –MeOH) afforded the product. M.p. 97°C, Yield 1 g, (25%), IR KBr disc 1680 cm − 1 for CHO, Mass (EI) m/z= 416 [M −2]. Anal. Calc. for C16H14BrCl2NO3: C, 45.83; H, 3.34; N, 3.34. Found: C, 45.95; H, 3.43; N, 3.41%. Table 1 accounts the 1H and 13 C NMR peak assignment of the compound.
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1100 and 620 cm − 1. Conductance (Lm/S cm2 mol − 1) in DMF 60. lmax (nm) (o/M − 1 cm − 1) in DMF 685(276), 410(s), 275(15110).
1.3.3. [Cu2L 2(OH)]ClO4 ·2H2O (3) Dark brown solid. Anal. Calc. for C29H26BrCl3Cu2N4O9: C, 39.21; H, 2.93; N, 6.31. Found C, 39.63; H, 2.95; N, 6.42%. Selected IR (KBr): 3450, 1620, 1100 and 620 cm − 1. Conductance (Lm/S cm2 mol − 1) in DMF 55. lmax (nm) (o/M − 1 cm − 1) in DMF 623(216), 386(2709), 290(12123). 1.3.4. [Cu2L 2(OAc)]ClO4 ·H2O (4) Dark green solid. Anal. Calc. for C31H26BrCl3Cu2N4O9: C, 40.83; H, 2.85; N, 6.15. Found C, 40.54; H, 2.88; N, 6.26%. Selected IR (KBr): 3450, 1610, 1540, 1100 and 620 cm − 1. Conductance (Lm/S cm2 mol − 1) in DMF 65. lmax (nm) (o/M − 1 cm − 1) in DMF 641(213), 391(s), 286(15110). Fig. 1. Schematic diagram for the synthesis of the intermediate mhdab and ligands.
1.3. Preparation of the complexes The m-phenoxo-m-(X)-dicopper(II) complexes [Cu2L1 – 3(X)]ClO4 (1 – 6) were prepared by following the general procedure [11]. To the precursor compound mhdab (0.42 g, 1 mmol), dissolved in 75 cm3 of methanol, primary amine (1 mmol) (2-aminomethylpyridine for H2L1 or 2-aminophenylbenzimidazole for H2L2 or N,N-dimethylpropanediamine for H2L3) was added and refluxed for 2 h. The resulting solution was added to a solution of copper(II) perchlorate hexahydrate (0.75 g, 2 mmol) dissolved in 50 ml of methanol and refluxed for a further hour. The mixture was then reacted with one equivalent of NaX (X = OH, OAc), dissolved in 5 cm3 of 1:4 water – methanol and refluxed for a further hour. The solid complex that deposited on slow evaporation of the solvent was filtered, washed with diethyl ether and dried.
1.3.1. [Cu2L 1(OH)]ClO4 ·H2O·MeOH (1) Dark green plates were deposited, which were filtered and dried, whereupon the crystals turned to green powder. Anal. Calc. for C23H25BrCl3Cu2N3O9: C, 34.45; H, 3.12; N, 5.24. Found: C, 34.55; H, 3.23; N, 5.42%. Selected IR (KBr): 3450, 1630, 1100 and 620 cm − 1. Conductance (Lm/S cm2 mol − 1) in DMF 65. lmax (nm) (o/M − 1 cm − 1) in DMF 658(219), 384(1015), 274(15876). 1.3.2. [Cu2L 1(OAc)]ClO4 ·2H2O (2) Dark green solid. Anal. Calc. for C24H25BrCl3Cu2N3O10: C, 34.76; H, 3.02; N, 5.07. Found: C, 34.63; H, 3.15; N, 5.12%. Selected IR (KBr): 3450, 1620, 1540,
1.3.5. [Cu2L 3(OH)]ClO4 ·2H2O (5) Green solid. Anal. Calc. for C21H29BrCl3Cu2N3O9: C, 32.31; H, 3.72; N, 5.39. Found C, 32.52; H, 3.81; N, 5.45%. Selected IR (KBr): 3450, 1610, 1100 and 620 cm − 1. Conductance (Lm/S cm2 mol − 1) in DMF 65. lmax (nm) (o/M − 1 cm − 1) in DMF 664(275), 410(s), 276(16032). 1.3.6. [Cu2L 3(OAc)]ClO4 ·MeOH (6) Dark green solid. Anal. Calc. for C24H31BrCl3Cu2N3O9: C, 35.20; H, 3.79; N, 5.13. Found: C, 35.26; H, 3.95; N, 5.21%. Selected IR (KBr): 1630, 1540, 1100 and 620 cm − 1. Conductance (Lm/S cm2 mol − 1) in DMF 55. lmax (nm) (o/M − 1 cm − 1) in DMF 681(168), 398(s), 298(18139).
2. Results and discussion
2.1. Syntheses Reaction of 5-bromosalicylaldehyde, 2-methylaminomethyl-4,6-dichlorophenol and paraformaldehyde in acetic acid medium afforded the desired precursor compound mhdab. The IR spectra of the compound showed the characteristic peak at 1680 cm − 1 for the aldehyde group. Condensation of the aldehyde mhdab with 2aminomethylpyridine, 2-aminophenyl-benzimidazole and N,N-dimethylpropanediamine gave the ligands H2L1, H2L2 and H2L3, respectively (Fig. 1), which were complexed in situ with copper(II) perchlorate hexahydrate and appropriate sodium salt in methanol medium. In all the complexes the characteristic IR peak for the CHO group at 1680 cm − 1 is absent and the characteristic peak for the CN around 1610–1630 cm − 1 is
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observed, which indicates the effective condensation of the aldehyde group with the primary amines.
2.2. Spectroscopy The positive ion FAB mass spectral study was carried out for selected complexes 1, 4 and 6 to confirm the binuclearity of the complexes. All three complexes show the molecular ion peak assignable to the [Cu2L(X)]+ fragment. The molecular ion peak for complex 1 is observed at m/z= 668 for the [Cu2L1(OH)H2O]+ fragment, the molecular ion peak for complex 4 is observed at m/z = 792 for the fragment [Cu2L2(OAc)]+ and complex 6 shows the molecular ion peak for the fragment [Cu2L3(OAc)MeOH]+ at m/z =719. The FAB mass spectrum of complex 4 is given in Fig. 2. IR spectra of the complexes show the characteristic peak for the CN group around 1610 – 1630 cm − 1, along with a broad band centered around 3400 cm − 1, indicating the presence of coordinated or lattice water and a strong symmetrical peak around 1100 cm − 1, indicating the presence of an uncoordinated ClO4 ion in the complexes. A characteristic peak for the acetate of the complexes 2, 4 and 6 was observed in the region 1540 cm − 1. The electronic spectra of the complexes were recorded in DMF, which show a weak band in the region around 635 – 685 nm for the d – d transition along with shoulders around 375 – 430 nm, which are
most probably due to the charge transfer transitions originated from the phenolate to copper [12] or benzimidazole to copper [13]. The intense peak observed below 300 nm is assigned to the ligand–ligand charge transfer transition. Appearance of a single d–d band in the region around 650 nm indicates that the coordination geometry around the copper atoms is basically four-coordinated planar with weak axial interactions and the red shift in the lmax value for complexes 1, 2 and 5, 6 with respect to the complexes 3, 4, indicates that the complexes of the ligands H2L1 and H2L3 have relatively more distorted geometry than the complexes of the ligand H2L2 [14].
2.3. Electrochemical studies All the complexes show a molar conductance value in the range 45–60 Lm/S cm2 mol − 1 in DMF, which indicates that the complexes are 1:1 electrolyte in nature [15]. The cyclic voltammograms of the complexes show two redox waves in the cathodic potential region and the data are given in Table 2. Constant potential electrolysis of complexes 2 and 5 at 100 mV more negative to their first reduction potential consumed one electron per molecule and consumed two electrons per molecule at the potential − 1.25 V, which indicates the involvement of two single electron transfers in the reduction processes. Based on the coulometric results,
Fig. 2. FAB mass spectra for complex 4. Table 2 Electrochemical data for the complexes Complex
E 1pc (V)
E 1pa (V)
E 11/2 (V)
DE (mV)
E 2pc (V)
E 2pa (V)
E 21/2 (V)
DE (mV)
1 2 3 4 5 6
−0.64 −0.58 −0.86 −0.81 −0.62 −0.52
−0.50 −0.46 −0.62 −0.55 −0.34 −0.32
−0.57 −0.52 −0.74 −0.68 −0.48 −0.40
140 120 240 270 280 200
−1.15 −1.05 −1.11 −1.10 −1.12 −1.02
−1.00 −0.79 −1.02 −1.00 −0.96 −0.68
−1.07 −0.92 −1.06 −1.05 −1.02 −0.85
150 260 90 100 280 340
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Fig. 3. Cyclic voltammogram for complex 3.
the two reduction peaks observed in the cyclic voltammograms are attributed to the two stepwise one electron transfers as given below [Cu(II)Cu(II)] v [Cu(II)Cu(I)] v [Cu(I)Cu(I)] The cyclic voltammetric response of the complexes follow the typical characteristics of a quasi-reversible redox process. The cyclic voltammogram for complex 3 was given in Fig. 3. A closer observation of the electrochemical data of the complexes of the ligands H2L1, H2L2 and H2L3 will be interesting to identify which end of the present unsymmetrical complexes is reduced first in one electron reductive processes. Although all the complexes undergo reduction at negative potentials, it is interesting to note that for all the complexes, irrespective of their exogenous bridging donor, there is a notable shift in the first reduction potential and there is only a minor shift in the second reduction potential. Since nitrogen is a soft donor when compared with the phenoxide oxy-gen [16], it is reasonable to say that the first reduction is associated with the copper atom in the ONN compartment in these ligands. The pronounced shift in the reduction potential is most probably due to the chemi-
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cally different environments of this compartment. The second reduction peak observed at almost same potential seems to be associated with the copper atom in the ONO compartment, where the chemical environment is identical for all the three ligands. From the table it is observed that the order of first reduction potential for the complexes of the ligand is H2L3 B H2L1 B H2L2. The NNO compartment comprises a N,N-dimethylpropanediamine side arm in the ligand H2L3, 2-methylpyridine in ligand H2L1 and 2%phenylbenzimidazole in the ligand H2L2. The reduction observed at high negative potential for the complexes of the ligand H2L2 (3–4) may be the result of the greater planarity of the complexes induced by the rigidity [17] due to direct attachment of the benzimidazole group [18] to the phenyl ring and conjugation [19] of the phenylbenzimidazole side arm. Further, the complexes of the ligand H2L3 undergo reduction at less negative potential when compared with the complexes of the H2L1 ligand. The easy reduction of the complexes of the ligand H2L3 may be due to the more flexible [20] nature of the N,N-dimethylpropanediamine side arm when compared with the 2-methylpyridine side arm. A comparison of the reduction potentials of the complexes of a particular ligand indicates that the reduction potential is exogenous, bridging donor dependent and the acetato bridged complexes undergo reduction at less negative potential when compared with the hydroxo bridged complexes. This behaviour seems to be associated with the increased number of atoms (OCO) involved in the acetate bridge, which may provide more flexibility for the acetato bridged complexes as against the hydroxo bridged complexes.
2.4. Magnetic properties The magnetic moments of the complexes are subnormal at room temperature, suggesting the presence of an antiferromagnetic interaction within the complex. A variable temperature magnetic moment measurement was carried out for complexes 1–3, 5 and 6 in the temperature range 77–300 K to evaluate the temperature dependence of magnetic properties of the complexes. Analyses were carried out by fitting the experimental
Table 3 Temperature dependence magnetic parameters and ESR spectrum data for the complexes Complex
1 2 3 4 5 6
meff (BM per molecule)
−2J (cm−1)
P (%)
298 K
77 K
2.19 2.35 1.86 1.92 2.15 2.64
1.17 1.06 1.10
82 59 167
0.25 0.18 0.51
1.08 1.56
97 56
0.19 0.45
ESR data (g) g
gÞ
2.19 2.18 2.05 2.20 2.08 2.20
2.06 2.02
2.02 2.01
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Fig. 4. Temperature dependence of magnetic properties for 1.
susceptibility values to the modified Bleaney – Bowers equation. xm =(Ng 2b 2/3kT) [3+ exp(−2J/kT)] − 1 (1 − P) +(0.45P/T)+ Na where xm is the molar magnetic susceptibility of the complex, P is the percentage of monomeric impurities and other symbols have their usual meanings. Na and g have been fixed as 120×10−6 cm3 mol−1 and 2.20, respectively for all magnetic simulations. The best fit parameters are tabulated in Table 3. Temperature dependence of the magnetic properties for complexes 1 is given in Fig. 4. The exchange integral values (−2J) observed for the complexes is in the range 60 – 170 cm−1, which indicates the presence of antiferromagnetic coupling between the copper atoms. A comparison of the observed −2J value
of the complexes 1, 3 and 5 shows that for the phenylbenzimidazole substituted complex 3, the − 2J value is 167 cm − 1, indicating the strong antiferromagnetic coupling between the copper atoms when compared with complexes 1 and 5 (− 2J=82 and 97 cm − 1, respectively). In the complexes of all the three ligands reported in the present work, the geometry around the phenolic side arm (ONO) would be the same, but the geometry around the other copper atom (ONN) will be different due to the variations in chemical environments. As observed in the electrochemical properties of the complexes, the most probable reason for the strong exchange interaction may be the planarity of the phenylbenzimidazole compartment due to rigidity and conjugation of the phenylbenzimidazole side arm, which seems to reduce the dihedral angle between the two copper planes, thereby causing effective antiferromagnetic coupling in the complex. Relatively less exchange interaction observed in complexes 1 and 5 may be due to the flexibility of the N,N%-dimethylpropanediamine side arm of the ligand H2L3 and five membered ring formation in complex 1 of the ligand H2L1, which may increase the dihedral angle between the copper planes, resulting in a weak exchange interaction in these complexes. The variation in the − 2J values of complexes 1, 2 or 5, 6 is due to the effect of the exogenous bridging donor. The higher − 2J value observed for the acetato-bridged complexes when compared with their corresponding hydroxo bridged complexes is possibly due to the counter-complementary nature of the overlap of the magnetic orbitals involving the monoatomic and three atom bridging ligands [21–23].
2.5. EPR spectroscopy Room temperature ESR spectra of the complexes were studied. The ESR spectra of complexes 1, 3 and 5
Fig. 5. ESR spectra for complex (a) 1; (b) 3 and (c) 5. The half-field signal for complex 3 is inserted.
P. Amudha et al. / Polyhedron 19 (2000) 1769–1775
are given in Fig. 5. The g value of the complexes was evaluated using the relationship hy =gbH and given in Table 3. The ESR spectra of the complexes of the ligands H2L1 and H2L3 show asymmetric signal with poorly resolved hyperfine splitting signals; however’ the complexes of the ligand H2L2 show only a broad signal. In addition, all the complexes show a signal for DMs= 2 transition at the low field region around 1600 G, with g:4. g \gÞ values observed for the complexes suggest a planar-based geometry with dx 2 − y 2 ground state [24]. Appearance of a hyperfine splitting component for the complexes of the ligands H2L1 and H2L3 indicates that the antiferromagnetic coupling in the complexes is weak when compared with the complexes of the ligand H2L2.
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