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Synthesis of heterobinuclear Cu(II)Zn(II) complexes derived from lateral macrobicyclic tricompartmental ligands: Spectral, electrochemical and kinetic studies
Polyhedron 25 (2006) 2623–2628 www.elsevier.com/locate/poly
Synthesis of heterobinuclear Cu(II)AZn(II) complexes derived from lateral macrobicyclic tricompartmental ligands: Spectral, electrochemical and kinetic studies M. Thirumavalavan b
a,b
, P. Akilan a, M. Kandaswamy
a,*
a Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai 600 025, Tamil Nadu, India Departamento de Engenharia Quı´mica, Instituto Superior Te´cnico, Complexo I, Av. Rovisco Pais 1, 1049-002 Lisboa, Portugal
Received 10 November 2005; accepted 27 March 2006 Available online 5 April 2006
Abstract Novel macrobicyclic heterobinuclear Cu(II)AZn(II) complexes have been synthesized from the corresponding mononuclear copper(II) complexes using the precursor compound 3,4:9,10-dibenzo-1,12[N,N 0 -bis{(3-formyl-2-hydroxy-5-methyl)benzyl}diaza]-5,8-dioxocyclotetradecane via template method by Shiff’s base condensation. Electrochemical and kinetic studies of the complexes have been carried out on the basis of macrocyclic ring size. Cyclic voltammetry and controlled electrolysis studies indicate that the copper(II) metal ion in the heterobinuclear complexes undergo quasireversible one electron reduction whereas the zinc(II) metal ion does not undergo any reduction in the potential range. The examination of kinetics of catechol oxidation and hydrolysis of 4-nitrophenyl phosphate vindicates that the catalytic activities of the complexes are found to increase with macrocyclic ring size of the complexes. As, the macrocyclic ring size increases, the spectral, electrochemical and catalytic studies of the complexes show remarkable variation due to distortion in the geometry of metal centre. 2006 Elsevier Ltd. All rights reserved. Keywords: Heterobinuclear complexes; Macrobicyclic complexes; Cyclic voltammetry; Macrocyclic ring size; Catechol oxidation; Phosphate hydrolysis
1. Introduction Heteronuclear metal complexes are of current interest for their physiochemical propensities and functions arising from an interplay of dissimilar metal ions in close proximity [1] and this study initiates the research work which involves the synthesis of new lateral macrobicyclic heterobinuclear complexes. The devise of dinucleating ligands that incorporate dissimilar metal ions to form discrete heterobinuclear complexes is of much importance in attempts to mimic the active sites of metalloenzymes [2] and to search appropriate systems for binding and activating simple molecules [3–5] and in investigations concerning the mutual influence of two metal centers on the electronic, *
Corresponding author. Tel./fax: +91 44 2230 0488. E-mail address: [email protected] (M. Kandaswamy).
0277-5387/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.03.018
magnetic and electrochemical properties of such herterobinuclear systems [6]. So far phenol-based macrocyclic ligands having two dissimilar metal binding sites sharing two phenolic oxygen atoms have been used for the study of di-l-phenolato heterobinuclear metal complexes [7–9] and studies on macrobicyclic systems are sparse. Hence, herein we report synthesis and characterization of lateral macrobicyclic tricompartmental heterobinuclear Cu(II)AZn(II) complexes. The macrobicyclic ligands show a site specificity of metal ions in producing CuIIZnII or ZnIICuII complexes depending upon the synthetic strategy. The reported heterobinuclear CuIIZnII complexes have been derived from the corresponding mononuclear CuII precursors by the reaction with zinc(II) perchlorate salt. Earlier reports [10–13] witnessed the catalytic efficiency of several homobinuclear transition metal complexes, but only a few heterobinuclear complexes [14,15] have been
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witnessed as model complexes. Hence, the synthesized heterobinuclear complexes are used for modeling the properties of the active site of the metallobiosites. In our earlier works [16,17] we have reported the spectral, electrochemical and kinetic properties of homobinuclear copper(II) complexes and in this work we rationalize how the heterobinuclear complexes behave differently from homobinuclear complexes. 2. Experimental Elemental analysis was carried out on a Carlo Erba Model 1106 elemental analyzer. 1H NMR spectra were recorded using FX-80-Q Fourier transition NMR spectrometer. Electronic spectral studies were carried out on a Hitachi 320 spectrophotometer in the range 200– 800 nm. IR spectra were recorded on a Hitachi 270-50 spectrophotometer on KBr disks in the range 4000– 250 cm1. The mass spectra were obtained on JEOL DX303 mass spectrometer. Cyclic Voltammograms were obtained on CHI600A (CHI Instruments Co., USA) electrochemical analyzer using DMF. The measurements were carried out under oxygen free condition by purging nitrogen gas, using a three electrode cell in which glassy carbon electrode was working electrode, saturated Ag/AgCl electrode was reference electrode and platinum wire was used as auxiliary electrode. Glassy carbon electrode surface was pretreated by metallographic polishing with alumina on a velvet cloth (abraded with fine grade of emery paper), followed by ultrasonic cleaning in double distilled water and washing with small amount of highly diluted acid and DMF. Ferrocene/ferrocenium (1+) couple was used as standard and E1/2 of the Ferrocene/ferrocenium (Fc/Fc+) couple under the experimental condition is 470 mV in DMF and DEp for Fc/Fc+ is 70 mV. Tetra (n-butyl)ammonium perchlorate (TBAP) was used as supporting electrolyte. The kinetic investigation of catechol oxidation and hydrolysis of 4-nitrophenyl phosphate was carried out spectrophotometrically by choosing the strongest absorbance at appropriate wavelength and monitoring the increase in the absorbance at this wavelength as a function of time. A plot of log (A/A At) versus time was made for each complexes and the rate constant was calculated. All the kinetic studies were carried out in DMF solvent at room temperature (25 C). 2.1. Materials 5-Methylsalicylaldehyde [18], 3-chloromethyl-5-methylsalicylaldehyde [19] and 3,4:9,10-dibenzo-1,12-diaza-5, 8-dioxocyclotetradecane [20,21] were prepared from the literature methods. TBAP used as supporting electrolyte in electrochemical measurement was purchased from Fluka and recrystallised from hot methanol. (Caution! TBAP is potentially explosive; hence care should be taken in handling the compound). DMF and CH3CN were obtained from E. MERCK. All other chemicals and solvents were
of analytical grade and were used as received without any further purification. 2.2. Synthesis of precursor compound 2.2.1. 3,4:9,10-Dibenzo-1,12[N,N 0 -bis{(3-formyl-2hydroxy-5-methyl)benzyl}diaza]-5,8-dioxocyclotetradecane [16] (precursor compound) A mixture of 3,4:9,10-dibenzo-1,12-diaza-5,8-dioxocyclotetradecane (0.95 g, 3.2 mmol) and triethylamine (1.32 g, 6.4 mmol) in 30 ml of THF was added slowly to a stirred solution of 3-chloromethyl-5-methylsalicylaldehyde (1.16 g, 6.4 mmol) in 30 ml of THF. After the addition was over the stirring was continued further for one more hour. The whole solution was then refluxed under water bath for 3 h and was allowed to cool on standing at room temperature. Copious water was added to this solution to dissolve any salt obtained. Then the required compound was extracted in organic medium using chloroform. The extraction was repeated for two to three times. A pale yellow compound was obtained on evaporation of solvent at room temperature (25 C). Light yellow microcrystals were obtained on recrystallization from chloroform. Yield: 1.82 g (80%), m.p.: 100 C. Anal. Calc. for C36H38O6N2: C, 60.50; H, 5.32; N, 3.92. Found: C, 60.72; H, 5.36; N, 3.98%. IR data (m cm1): 1678 (mC@O, s), 3448 (mOH, br). 1H NMR (d ppm in CDCl3): 10.0 (s, 2H, CHO protons), 7.0 (m, 12H, aromatic protons), 4.7 (m, 4H, methylene protons attached to oxygen atom), 3.8 (m, 8H, benzylic protons), 3.5 (s, 4H, methylene protons attached to nitrogen atom), 2.3 (s, 6H, CH3 protons attached to aromatic ring). Mass (EI) m/z: 594 (m+). 2.3. Synthesis of macrobicyclic heterobinuclear complexes 2.3.1. [CuZnL1](ClO4)2 A methanolic solution (20 ml) of zinc(II)perchlorate hexahydrate (0.707 g, 1.9 mmol) [Caution! zinc(II) perchlorate salt is potentially explosive; hence care should be taken in handling the compound] was added to the hot solution of mononuclear copper(II) complex [16] [CuL1](ClO4) (1.562 g, 1.9 mmol) in 25 ml of methanol followed by the addition triethylamine (0.192 g, 1.9 mmol) in 5 ml of methanol. The solution was refluxed on water bath for 24 h. After the reaction was over, the solution was filtered at hot condition and allowed to stand at room temperature. After slow evaporation of the solvent at 25 C, the dark green compound obtained was washed with methanol and dried in vacuum. Yield: 1.48 g (80%). Anal. Calc. C42H46O12N6Cl2CuZn: C, 49.17; H, 4.48; N, 8.19. Found: C, 49.28; H, 4.62; N, 8.20%. Selected IR (KBr): 1628 (s), 1100 (s), 622 (s) cm1. kmax (nm) (e/M1 cm1) in CH3CN: 565 (121), 372 (16 870), 272 (37 200). 2.3.2. [CuZnL2](ClO4)2 [CuZnL2](ClO4)2 was synthesized by following the previously described procedure for [CuZnL1](ClO4)2, using the
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mononuclear complex [CuL2](ClO4) instead [CuL1](ClO4). The compound obtained was dark green. Yield: 1.44 g (77%). Anal. Calc. for C43H48O12N6Cl2CuZn: C, 49.66; H, 4.61; N, 8.08. Found: C, 49.79; H, 4.73; N, 8.20%. Selected IR (KBr): 1620 (s), 1100 (s), 626 (s) cm1. kmax (nm) (e/ M1 cm1) in CH3CN: 595 (131), 352 (14 590), 271 (37 160). 2.3.3. [CuZnL3](ClO4)2 [CuZnL3](ClO4)2 was synthesized by following the previously described procedure for [CuZnL1](ClO4)2, using the mononuclear complex [CuL3](ClO4) instead [CuL1](ClO4). The compound obtained was dark green. Yield: 1.49 g (79%). Anal. Calc. for C44H50O12N6Cl2CuZn: C, 50.14; H, 4.74; N, 7.97. Found: C, 50.22; H, 4.84; N, 8.11%. Selected IR (KBr): 1622 (s), 1101 (s), 627 (s) cm1. kmax (nm) (e/M1 cm1) in CH3CN: 625 (125), 355 (15 440), 273 (32 400). 2.3.4. [CuZnL4](ClO4)2 [CuZnL4](ClO4)2 was synthesized by following the previously described procedure for [CuZnL1](ClO4)2, using the mononuclear complex [CuL4](ClO4) instead [CuL1](ClO4). The compound obtained was blackish green. Yield: 1.51 g (78%). Anal. Calc. for C46H46O12N6Cl2CuZn: C, 51.44; H, 4.28; N, 7.82. Found: C, 51.62; H, 4.41; N, 7.93%. Selected IR (KBr): 1628 (s), 1100 (s), 625 (s) cm1. kmax (nm) (e/M1 cm1) in CH3CN: 605 (110), 412 (13 400), 375 (12 300), 275 (33 500).
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2.3.5. [CuZnL5](ClO4)2 [CuZnL5](ClO4)2 was synthesized by following the previously described procedure for [CuZnL1](ClO4)2, using the mononuclear complex [CuL5](ClO4) instead [CuL1](ClO4). The compound obtained was blackish green. Yield: 1.62 g (80%). Anal. Calc. for C50H48O12N6Cl2CuZn: C, 53.52; H, 4.28; N, 7.49, Found: C, 53.59; H, 4.37; N, 7.59%. Selected IR (KBr): 1624 (s), 1101 (s), 625 (s) cm1. kmax (nm) (e/M1 cm1) in CH3CN: 635 (140), 401 (15 300), 368 (18 300), 268 (34 000). In all cases we could obtain only microcrystals and attempts to obtain the crystals suitable for X-ray analysis were unsuccessful. 3. Results and discussion Schiff’s base condensation of the precursor compounds with diamines in the presence of metal ion by template method results into various macrobicyclic hetero binuclear Cu(II)AZn(II) complexes. Scheme 1 represents the synthetic pathway of hetero binuclear Cu(II)AZn(II) complexes. In our previous study [16] the crystal structures of mono and binuclear copper(II) complexes have been rationalized. Hence, in accordance with this, it is proposed that the geometry around copper(II) and zinc(II) ion is distorted square planar. Extensive spectral, electrochemical and kinetic studies of the heterobinuclear complexes were carried out and discussed.
Scheme 1. Synthesis of macrobicyclic heterobinuclear Cu(II)AZn(II) complexes.
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3.1. Spectral studies The IR spectra of the complexes show m(C@N) peak [22] at 1620–1630 cm1. The peaks appeared near 1100 and 620 cm1 are characteristics of the uncoordinated perchlorate ion. The electronic spectra of the complexes show three main transitions. A weak band observed in the range 565–635 nm is due to d–d transition of the Cu(II) ion presents in the heterobinuclear complexes. On moving from the complexes of ligand L1 to L3 and L4 to L5, an increase in kmax (red shift) [23] of the d–d transition of Cu(II) ion in the complexes was observed. A moderate intensive band observed in the range 352–375 nm is due to ligand-to-metal charge transfer transition and the strong band observed in the range 268–275 nm is due to intraligand charge transfer transition [24]. Apart from these, additional peaks are observed for the complexes of ligands L4 to L5 due to the presence of aromatic ring in the imine nitrogen compartment. The solid state ESR spectra of the heterobinuclear Cu(II)AZn(II) complexes show four line hyperfine splittings due to Cu(II) ion with a nuclear hyperfine spin of 3/2. Fig. 1 represents the ESR spectrum of the complex [CuZnL1](ClO4)2 with gi value 2.14 and g^ value 2.02. 3.2. Electrochemistry The electrochemical properties of the complexes reported in the present work were studied by cyclic voltammetry in the potential range 0 to 1.4 V in dimethylformamide containing 101 M tetra(n-butyl)ammonium perchlorate and the data are represented in Table 1. All
Fig. 1. ESR spectrum of complex [CuZnL1](ClO4)2.
Table 1 Electrochemical dataa of heterobinuclear Cu(II)AZn(II) complexes in DMF medium (reduction) Complexes 1
[CuZnL ](ClO4)2 [CuZnL2](ClO4)2 [CuZnL3](ClO4)2 [CuZnL4](ClO4)2 [CuZnL5](ClO4)2
E1pc ðVÞ
E1pa ðVÞ
E11=2 ðVÞ
DE (mV)
0.98 0.82 0.78 1.08 0.90
0.88 0.75 0.69 0.95 0.79
0.93 0.79 0.74 1.04 0.84
100 130 90 130 110
a Measured by CV at 25 mV/s. E vs. Ag/AgCl conditions: GC working and Ag/AgCl reference electrodes; supporting electrolyte TBAP; concentration of complex 1 · 103 M, concentration of TBAP 1 · 101 M.
the heterobinuclear Cu(II)AZn(II) complexes undergo one quasi reversible reduction at negative potential. Fig. 2 depicts the cyclic voltammograms of the heterobinuclear Cu(II)AZn(II) complexes and the redox processes are assigned as follows: CuII ZnII CuI ZnII The interesting feature connoted for heterobinuclear complexes is shifting of reduction potential towards anodic from 0.98 to 0.78 as we move from complexes of ligand L1 to L3. This is because of the reason that the increase in chain length in imine nitrogen compartment increases the macrocyclic ring size and hence causes more flexibility [25–27] which, in turn makes the reduction of the complexes easier. In our earlier work [16] we have reported the reduction potentials of the corresponding mono and
Fig. 2. Cyclic voltammograms of heterobinuclear complexes (1 · 103M) (a) [CuZnL1](ClO4)2 (b) [CuZnL2](ClO4)2 (c) [CuZnL3](ClO4)2.
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binuclear copper(II) complexes and in the case of heterobinuclear Cu(II)AZn(II) complexes the observed reduction potentials fall in the vicinity of the potentials of the corresponding mononuclear copper(II) complexes. Hence, it is worthwhile to assign this potential to the corresponding Cu(II) ion presents in the imine nitrogen compartment. The Zn(II) ion presents in the tertiary nitrogen compartment does not undergo reduction in this potential range. Thus, cyclic voltammetric technique is a useful tool to assign the site specificity of metal ion in the heterobinuclear complexes. An inspection on the reduction potential of the heterobinuclear Cu(II)AZn(II) complexes reveals that the observed reduction potentials are comparatively lesser than the corresponding homobinuclear [16] Cu(II) complexes. This clearly indicates that the interaction between two metal ions in homobinuclear Cu(II) complexes is effective when compared to the heterobinuclear Cu(II)AZn(II) complexes. Because of this, heterobinuclear complexes have more or less the same reduction potential as mononuclear Cu(II) complexes.
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Fig. 3. Catecholase activity of heterobinuclear complexes (a) [CuZnL1](ClO4)2 (b) [CuZnL2](ClO4)2.
3.3. Kinetic studies All the heterobinucler Cu(II)AZn(II) complexes were subjected for both catecholase activity and phosphate hydrolysis. The investigation of the kinetic studies of the heterobinuclear Cu(II)AZn(II) complexes implies that the complexes act as model for catecholase and phosphate hydrolase. The kinetics of the catecholase activity was carried out using 103 M of complex and 101 M of 3,5-ditert-butylcatechol in DMF. The product o-quinone is fairly stable and has a strong absorbance at 390 nm. The kinetics of the hydrolysis of 4-nitrophenyl phosphate was carried out using an equal concentration of the complex catalyst and tetramethylammonium hydroxide (103 M) and 4-nitrophenyl phosphate (101 M) in DMF. The chromophore leaving group 4-nitrophenolate, which is the hydrolysis product of the substrate was monitored at 420 nm. The course of the reaction in both cases was followed nearly 45 min at regular time interval. The slope was determined by monitoring the growth of the band of the corresponding product. Plots of log (A/A At) versus time for both catecholase activity and phosphate hydrolysis are obtained and shown in Figs. 3 and 4, respectively. The rate constant values are also reported in Table 2. The first apparent result was that the reactivities of the complexes differ significantly, as the size of macrocycle increases. In both kinetics, the catalytic activities of heterobinuclear Cu(II)AZn(II) complexes are found to increase as the macrocyclic ring size increases due to the intrinsic flexibility. The catalytic activity of the complexes containing aromatic diimines is found to be less than that of the complexes containing aliphatic diimines. This can be explained that the planarity, which is associated with aromatic ring, imparts less catalytic efficiency due to the rigidity of the systems as observed in the
Fig. 4. Catalytic activity of heterobinuclear complexes for hydrolysis of 4-nitrophenylphosphate (a) [CuZnL1](ClO4)2 (b) [CuZnL2](ClO4)2.
Table 2 Catalytic activitya of heterobinuclear Cu(II)AZn(II) complexes for catecholase activity and hydrolysis of 4-nitrophenyl phosphate Complexes
Rate constant (min1) Catecholase activity
1
[CuZnL ](ClO4)2 [CuZnL2](ClO4)2 [CuZnL3](ClO4)2 [CuZnL4](ClO4)2 [CuZnL5](ClO4)2 a
3
2.91 · 10 3.05 · 103 3.39 · 103 2.81 · 103 2.89 · 103
Hydrolysis activity 1.75 · 102 2.27 · 102 3.43 · 102 1.68 · 102 2.12 · 102
Measured spectrophotometrically in DMF medium.
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case of electrochemical reduction of the complexes. It seems interesting to observe that the rate constant values of heterobinuclear Cu(II)AZn(II) for catecholase activities are less when compared to the rate constant values of corresponding mono and homobinuclear [16] Cu(II) complexes. 4. Conclusion This work involves the synthesis of lateral macrobicyclic compartmental heterobinuclear Cu(II)AZn(II) complexes with phenolate bridging atoms. Their extensive spectral, electrochemical and catalytic studies were carried out with slight modification in the ligand framework and compared with their corresponding mono and homobinuclear Cu(II) complexes. The increase in macrocyclic ring size in the ligand framework influences the spectral, electrochemical and catalytic properties of the complexes. Electronic spectral studies indicate that the d–d transition of Cu(II) ion is responsible for the appearance of band in the visible region. Electrochemical studies imply that the Zn(II) ion does not undergo reduction in the given potential range. Acknowledgements The author M.T. gratefully appreciates the financial support received from CSIR and UGC major project sanctioned to M.K., New Delhi. References [1] M. Yonemura, H. Okawa, M. Ohba, D.E. Fenton, L.K. Thompson, Chem. Commun. (2000) 817. [2] G.T. Babcock, L.E. Vickery, G. Palmer, J. Biol. Chem. 253 (1978) 2400. [3] P.A. Vigato, S. Tamburini, D.E. Fenton, Coord. Chem. Rev. 106 (1990) 25.
[4] F. Arena, C. Floriani, A.C. Villa, C. Guastini, Inorg. Chem. 25 (1986) 4589. [5] Y. Aratake, H. Okawa, E. Asato, H. Sakiyama, M. Kodera, S. Kida, M. Sakamoto, J. Chem. Soc., Dalton Trans. (1990) 2941. [6] H. Okawa, J. Nishio, M. Ohba, M. Tadokoro, N. Matsumoto, M. Koikawa, S. Kida, D.E. Fenton, Inorg. Chem. 32 (1993) 2949. [7] M. Yonemura, K. Arimura, K. Inoue, N. Usuki, M. Ohba, H. Okawa, Inorg. Chem. 41 (2002) 582. [8] H. Okawa, H. Furutachi, D.E. Fenton, Coord. Chem. Rev. 174 (1998) 51. [9] M. Yonemura, N. Usuki, Y. Nakamura, H. Okawa, J. Chem. Soc., Dalton Trans. (2000) 3624. [10] J.R. Morrow, W.C. Trogler, Inorg. Chem. 27 (1998) 3387. [11] N.P. Sadler, C.C. Chuang, R.M. Milburn, Inorg. Chem. 34 (1995) 402. [12] I.M. Atkinson, L.F. Lindoy, Coord. Chem. Rev. (2000) 200. [13] P. Hendry, A.M. Sargeson, Inorg. Chem. 29 (1990) 92. [14] N. Strater, T. Klaubunde, P. Tucker, H. Witzel, B. Krebs, Science 268 (1995) 1489. [15] M.P. Engloff, P.T.W. Coher, P. Reimer, D. Barfard, J. Mol. Biol. 254 (1995) 942. [16] M. Thirumavalavan, P. Akilan, M. Kandaswamy, K. Chinnakali, G. Senthil Kumar, H.K. Fun, Inorg. Chem. 42 (2003) 3308. [17] M. Thirumavalavan, P. Akilan, M. Kandaswamy, Inorg. Chem. Commun. 5 (2002) 422. [18] J.C. Duff, J. Chem. Soc. (1941) 547. [19] J.D. Crane, D.E. Fenton, J.M. Latour, A.J. Smith, J. Chem. Soc., Dalton Trans. (1991) 2279. [20] L.G. Armstrong, L.F. Lindoy, Inorg. Chem. 14 (1975) 1322. [21] P.G. Grimsley, L.F. Lindoy, H.C. Lip, R.J. Smith, J.T. Baker, Aust. J. Chem. 30 (1977) 2095. [22] G. Das, R. Shukala, S. Mandal, R. Singh, P.K. Bharadwaj, J.V. Singh, K.U. Whitmire, Inorg. Chem. 36 (1997) 323. [23] R. Bhalla, M. Helliwell, C.D. Garner, Inorg. Chem. 36 (1997) 2944. [24] P. Amudha, M. Kandaswamy, L. Govindaswamy, D. Velmurugan, Inorg. Chem. 37 (1998) 4490. [25] E. Gao, W. Bu, G. Yang, D. Liao, Z. Jiang, S. Yan, G. Way, J. Chem. Soc. Dalton Trans. (2000) 1431. [26] P.A. Connick, K.A. Macor, Inorg. Chem. 30 (1991) 4654. [27] H. Okawa, M. Tadokora, T.Y. Aratake, M. Ohba, K. Shindom, M. Mitsumi, M. Koikawa, M. Tomono, D.E. Fenton, J. Chem. Soc., Dalton Trans. (1993) 253.
Synthesis of heterobinuclear Cu(II)AZn(II) complexes derived from lateral macrobicyclic tricompartmental ligands: Spectral, electrochemical and kinetic studies M. Thirumavalavan b
a,b
, P. Akilan a, M. Kandaswamy
a,*
a Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai 600 025, Tamil Nadu, India Departamento de Engenharia Quı´mica, Instituto Superior Te´cnico, Complexo I, Av. Rovisco Pais 1, 1049-002 Lisboa, Portugal
Received 10 November 2005; accepted 27 March 2006 Available online 5 April 2006
Abstract Novel macrobicyclic heterobinuclear Cu(II)AZn(II) complexes have been synthesized from the corresponding mononuclear copper(II) complexes using the precursor compound 3,4:9,10-dibenzo-1,12[N,N 0 -bis{(3-formyl-2-hydroxy-5-methyl)benzyl}diaza]-5,8-dioxocyclotetradecane via template method by Shiff’s base condensation. Electrochemical and kinetic studies of the complexes have been carried out on the basis of macrocyclic ring size. Cyclic voltammetry and controlled electrolysis studies indicate that the copper(II) metal ion in the heterobinuclear complexes undergo quasireversible one electron reduction whereas the zinc(II) metal ion does not undergo any reduction in the potential range. The examination of kinetics of catechol oxidation and hydrolysis of 4-nitrophenyl phosphate vindicates that the catalytic activities of the complexes are found to increase with macrocyclic ring size of the complexes. As, the macrocyclic ring size increases, the spectral, electrochemical and catalytic studies of the complexes show remarkable variation due to distortion in the geometry of metal centre. 2006 Elsevier Ltd. All rights reserved. Keywords: Heterobinuclear complexes; Macrobicyclic complexes; Cyclic voltammetry; Macrocyclic ring size; Catechol oxidation; Phosphate hydrolysis
1. Introduction Heteronuclear metal complexes are of current interest for their physiochemical propensities and functions arising from an interplay of dissimilar metal ions in close proximity [1] and this study initiates the research work which involves the synthesis of new lateral macrobicyclic heterobinuclear complexes. The devise of dinucleating ligands that incorporate dissimilar metal ions to form discrete heterobinuclear complexes is of much importance in attempts to mimic the active sites of metalloenzymes [2] and to search appropriate systems for binding and activating simple molecules [3–5] and in investigations concerning the mutual influence of two metal centers on the electronic, *
Corresponding author. Tel./fax: +91 44 2230 0488. E-mail address: [email protected] (M. Kandaswamy).
0277-5387/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.03.018
magnetic and electrochemical properties of such herterobinuclear systems [6]. So far phenol-based macrocyclic ligands having two dissimilar metal binding sites sharing two phenolic oxygen atoms have been used for the study of di-l-phenolato heterobinuclear metal complexes [7–9] and studies on macrobicyclic systems are sparse. Hence, herein we report synthesis and characterization of lateral macrobicyclic tricompartmental heterobinuclear Cu(II)AZn(II) complexes. The macrobicyclic ligands show a site specificity of metal ions in producing CuIIZnII or ZnIICuII complexes depending upon the synthetic strategy. The reported heterobinuclear CuIIZnII complexes have been derived from the corresponding mononuclear CuII precursors by the reaction with zinc(II) perchlorate salt. Earlier reports [10–13] witnessed the catalytic efficiency of several homobinuclear transition metal complexes, but only a few heterobinuclear complexes [14,15] have been
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witnessed as model complexes. Hence, the synthesized heterobinuclear complexes are used for modeling the properties of the active site of the metallobiosites. In our earlier works [16,17] we have reported the spectral, electrochemical and kinetic properties of homobinuclear copper(II) complexes and in this work we rationalize how the heterobinuclear complexes behave differently from homobinuclear complexes. 2. Experimental Elemental analysis was carried out on a Carlo Erba Model 1106 elemental analyzer. 1H NMR spectra were recorded using FX-80-Q Fourier transition NMR spectrometer. Electronic spectral studies were carried out on a Hitachi 320 spectrophotometer in the range 200– 800 nm. IR spectra were recorded on a Hitachi 270-50 spectrophotometer on KBr disks in the range 4000– 250 cm1. The mass spectra were obtained on JEOL DX303 mass spectrometer. Cyclic Voltammograms were obtained on CHI600A (CHI Instruments Co., USA) electrochemical analyzer using DMF. The measurements were carried out under oxygen free condition by purging nitrogen gas, using a three electrode cell in which glassy carbon electrode was working electrode, saturated Ag/AgCl electrode was reference electrode and platinum wire was used as auxiliary electrode. Glassy carbon electrode surface was pretreated by metallographic polishing with alumina on a velvet cloth (abraded with fine grade of emery paper), followed by ultrasonic cleaning in double distilled water and washing with small amount of highly diluted acid and DMF. Ferrocene/ferrocenium (1+) couple was used as standard and E1/2 of the Ferrocene/ferrocenium (Fc/Fc+) couple under the experimental condition is 470 mV in DMF and DEp for Fc/Fc+ is 70 mV. Tetra (n-butyl)ammonium perchlorate (TBAP) was used as supporting electrolyte. The kinetic investigation of catechol oxidation and hydrolysis of 4-nitrophenyl phosphate was carried out spectrophotometrically by choosing the strongest absorbance at appropriate wavelength and monitoring the increase in the absorbance at this wavelength as a function of time. A plot of log (A/A At) versus time was made for each complexes and the rate constant was calculated. All the kinetic studies were carried out in DMF solvent at room temperature (25 C). 2.1. Materials 5-Methylsalicylaldehyde [18], 3-chloromethyl-5-methylsalicylaldehyde [19] and 3,4:9,10-dibenzo-1,12-diaza-5, 8-dioxocyclotetradecane [20,21] were prepared from the literature methods. TBAP used as supporting electrolyte in electrochemical measurement was purchased from Fluka and recrystallised from hot methanol. (Caution! TBAP is potentially explosive; hence care should be taken in handling the compound). DMF and CH3CN were obtained from E. MERCK. All other chemicals and solvents were
of analytical grade and were used as received without any further purification. 2.2. Synthesis of precursor compound 2.2.1. 3,4:9,10-Dibenzo-1,12[N,N 0 -bis{(3-formyl-2hydroxy-5-methyl)benzyl}diaza]-5,8-dioxocyclotetradecane [16] (precursor compound) A mixture of 3,4:9,10-dibenzo-1,12-diaza-5,8-dioxocyclotetradecane (0.95 g, 3.2 mmol) and triethylamine (1.32 g, 6.4 mmol) in 30 ml of THF was added slowly to a stirred solution of 3-chloromethyl-5-methylsalicylaldehyde (1.16 g, 6.4 mmol) in 30 ml of THF. After the addition was over the stirring was continued further for one more hour. The whole solution was then refluxed under water bath for 3 h and was allowed to cool on standing at room temperature. Copious water was added to this solution to dissolve any salt obtained. Then the required compound was extracted in organic medium using chloroform. The extraction was repeated for two to three times. A pale yellow compound was obtained on evaporation of solvent at room temperature (25 C). Light yellow microcrystals were obtained on recrystallization from chloroform. Yield: 1.82 g (80%), m.p.: 100 C. Anal. Calc. for C36H38O6N2: C, 60.50; H, 5.32; N, 3.92. Found: C, 60.72; H, 5.36; N, 3.98%. IR data (m cm1): 1678 (mC@O, s), 3448 (mOH, br). 1H NMR (d ppm in CDCl3): 10.0 (s, 2H, CHO protons), 7.0 (m, 12H, aromatic protons), 4.7 (m, 4H, methylene protons attached to oxygen atom), 3.8 (m, 8H, benzylic protons), 3.5 (s, 4H, methylene protons attached to nitrogen atom), 2.3 (s, 6H, CH3 protons attached to aromatic ring). Mass (EI) m/z: 594 (m+). 2.3. Synthesis of macrobicyclic heterobinuclear complexes 2.3.1. [CuZnL1](ClO4)2 A methanolic solution (20 ml) of zinc(II)perchlorate hexahydrate (0.707 g, 1.9 mmol) [Caution! zinc(II) perchlorate salt is potentially explosive; hence care should be taken in handling the compound] was added to the hot solution of mononuclear copper(II) complex [16] [CuL1](ClO4) (1.562 g, 1.9 mmol) in 25 ml of methanol followed by the addition triethylamine (0.192 g, 1.9 mmol) in 5 ml of methanol. The solution was refluxed on water bath for 24 h. After the reaction was over, the solution was filtered at hot condition and allowed to stand at room temperature. After slow evaporation of the solvent at 25 C, the dark green compound obtained was washed with methanol and dried in vacuum. Yield: 1.48 g (80%). Anal. Calc. C42H46O12N6Cl2CuZn: C, 49.17; H, 4.48; N, 8.19. Found: C, 49.28; H, 4.62; N, 8.20%. Selected IR (KBr): 1628 (s), 1100 (s), 622 (s) cm1. kmax (nm) (e/M1 cm1) in CH3CN: 565 (121), 372 (16 870), 272 (37 200). 2.3.2. [CuZnL2](ClO4)2 [CuZnL2](ClO4)2 was synthesized by following the previously described procedure for [CuZnL1](ClO4)2, using the
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mononuclear complex [CuL2](ClO4) instead [CuL1](ClO4). The compound obtained was dark green. Yield: 1.44 g (77%). Anal. Calc. for C43H48O12N6Cl2CuZn: C, 49.66; H, 4.61; N, 8.08. Found: C, 49.79; H, 4.73; N, 8.20%. Selected IR (KBr): 1620 (s), 1100 (s), 626 (s) cm1. kmax (nm) (e/ M1 cm1) in CH3CN: 595 (131), 352 (14 590), 271 (37 160). 2.3.3. [CuZnL3](ClO4)2 [CuZnL3](ClO4)2 was synthesized by following the previously described procedure for [CuZnL1](ClO4)2, using the mononuclear complex [CuL3](ClO4) instead [CuL1](ClO4). The compound obtained was dark green. Yield: 1.49 g (79%). Anal. Calc. for C44H50O12N6Cl2CuZn: C, 50.14; H, 4.74; N, 7.97. Found: C, 50.22; H, 4.84; N, 8.11%. Selected IR (KBr): 1622 (s), 1101 (s), 627 (s) cm1. kmax (nm) (e/M1 cm1) in CH3CN: 625 (125), 355 (15 440), 273 (32 400). 2.3.4. [CuZnL4](ClO4)2 [CuZnL4](ClO4)2 was synthesized by following the previously described procedure for [CuZnL1](ClO4)2, using the mononuclear complex [CuL4](ClO4) instead [CuL1](ClO4). The compound obtained was blackish green. Yield: 1.51 g (78%). Anal. Calc. for C46H46O12N6Cl2CuZn: C, 51.44; H, 4.28; N, 7.82. Found: C, 51.62; H, 4.41; N, 7.93%. Selected IR (KBr): 1628 (s), 1100 (s), 625 (s) cm1. kmax (nm) (e/M1 cm1) in CH3CN: 605 (110), 412 (13 400), 375 (12 300), 275 (33 500).
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2.3.5. [CuZnL5](ClO4)2 [CuZnL5](ClO4)2 was synthesized by following the previously described procedure for [CuZnL1](ClO4)2, using the mononuclear complex [CuL5](ClO4) instead [CuL1](ClO4). The compound obtained was blackish green. Yield: 1.62 g (80%). Anal. Calc. for C50H48O12N6Cl2CuZn: C, 53.52; H, 4.28; N, 7.49, Found: C, 53.59; H, 4.37; N, 7.59%. Selected IR (KBr): 1624 (s), 1101 (s), 625 (s) cm1. kmax (nm) (e/M1 cm1) in CH3CN: 635 (140), 401 (15 300), 368 (18 300), 268 (34 000). In all cases we could obtain only microcrystals and attempts to obtain the crystals suitable for X-ray analysis were unsuccessful. 3. Results and discussion Schiff’s base condensation of the precursor compounds with diamines in the presence of metal ion by template method results into various macrobicyclic hetero binuclear Cu(II)AZn(II) complexes. Scheme 1 represents the synthetic pathway of hetero binuclear Cu(II)AZn(II) complexes. In our previous study [16] the crystal structures of mono and binuclear copper(II) complexes have been rationalized. Hence, in accordance with this, it is proposed that the geometry around copper(II) and zinc(II) ion is distorted square planar. Extensive spectral, electrochemical and kinetic studies of the heterobinuclear complexes were carried out and discussed.
Scheme 1. Synthesis of macrobicyclic heterobinuclear Cu(II)AZn(II) complexes.
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3.1. Spectral studies The IR spectra of the complexes show m(C@N) peak [22] at 1620–1630 cm1. The peaks appeared near 1100 and 620 cm1 are characteristics of the uncoordinated perchlorate ion. The electronic spectra of the complexes show three main transitions. A weak band observed in the range 565–635 nm is due to d–d transition of the Cu(II) ion presents in the heterobinuclear complexes. On moving from the complexes of ligand L1 to L3 and L4 to L5, an increase in kmax (red shift) [23] of the d–d transition of Cu(II) ion in the complexes was observed. A moderate intensive band observed in the range 352–375 nm is due to ligand-to-metal charge transfer transition and the strong band observed in the range 268–275 nm is due to intraligand charge transfer transition [24]. Apart from these, additional peaks are observed for the complexes of ligands L4 to L5 due to the presence of aromatic ring in the imine nitrogen compartment. The solid state ESR spectra of the heterobinuclear Cu(II)AZn(II) complexes show four line hyperfine splittings due to Cu(II) ion with a nuclear hyperfine spin of 3/2. Fig. 1 represents the ESR spectrum of the complex [CuZnL1](ClO4)2 with gi value 2.14 and g^ value 2.02. 3.2. Electrochemistry The electrochemical properties of the complexes reported in the present work were studied by cyclic voltammetry in the potential range 0 to 1.4 V in dimethylformamide containing 101 M tetra(n-butyl)ammonium perchlorate and the data are represented in Table 1. All
Fig. 1. ESR spectrum of complex [CuZnL1](ClO4)2.
Table 1 Electrochemical dataa of heterobinuclear Cu(II)AZn(II) complexes in DMF medium (reduction) Complexes 1
[CuZnL ](ClO4)2 [CuZnL2](ClO4)2 [CuZnL3](ClO4)2 [CuZnL4](ClO4)2 [CuZnL5](ClO4)2
E1pc ðVÞ
E1pa ðVÞ
E11=2 ðVÞ
DE (mV)
0.98 0.82 0.78 1.08 0.90
0.88 0.75 0.69 0.95 0.79
0.93 0.79 0.74 1.04 0.84
100 130 90 130 110
a Measured by CV at 25 mV/s. E vs. Ag/AgCl conditions: GC working and Ag/AgCl reference electrodes; supporting electrolyte TBAP; concentration of complex 1 · 103 M, concentration of TBAP 1 · 101 M.
the heterobinuclear Cu(II)AZn(II) complexes undergo one quasi reversible reduction at negative potential. Fig. 2 depicts the cyclic voltammograms of the heterobinuclear Cu(II)AZn(II) complexes and the redox processes are assigned as follows: CuII ZnII CuI ZnII The interesting feature connoted for heterobinuclear complexes is shifting of reduction potential towards anodic from 0.98 to 0.78 as we move from complexes of ligand L1 to L3. This is because of the reason that the increase in chain length in imine nitrogen compartment increases the macrocyclic ring size and hence causes more flexibility [25–27] which, in turn makes the reduction of the complexes easier. In our earlier work [16] we have reported the reduction potentials of the corresponding mono and
Fig. 2. Cyclic voltammograms of heterobinuclear complexes (1 · 103M) (a) [CuZnL1](ClO4)2 (b) [CuZnL2](ClO4)2 (c) [CuZnL3](ClO4)2.
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binuclear copper(II) complexes and in the case of heterobinuclear Cu(II)AZn(II) complexes the observed reduction potentials fall in the vicinity of the potentials of the corresponding mononuclear copper(II) complexes. Hence, it is worthwhile to assign this potential to the corresponding Cu(II) ion presents in the imine nitrogen compartment. The Zn(II) ion presents in the tertiary nitrogen compartment does not undergo reduction in this potential range. Thus, cyclic voltammetric technique is a useful tool to assign the site specificity of metal ion in the heterobinuclear complexes. An inspection on the reduction potential of the heterobinuclear Cu(II)AZn(II) complexes reveals that the observed reduction potentials are comparatively lesser than the corresponding homobinuclear [16] Cu(II) complexes. This clearly indicates that the interaction between two metal ions in homobinuclear Cu(II) complexes is effective when compared to the heterobinuclear Cu(II)AZn(II) complexes. Because of this, heterobinuclear complexes have more or less the same reduction potential as mononuclear Cu(II) complexes.
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Fig. 3. Catecholase activity of heterobinuclear complexes (a) [CuZnL1](ClO4)2 (b) [CuZnL2](ClO4)2.
3.3. Kinetic studies All the heterobinucler Cu(II)AZn(II) complexes were subjected for both catecholase activity and phosphate hydrolysis. The investigation of the kinetic studies of the heterobinuclear Cu(II)AZn(II) complexes implies that the complexes act as model for catecholase and phosphate hydrolase. The kinetics of the catecholase activity was carried out using 103 M of complex and 101 M of 3,5-ditert-butylcatechol in DMF. The product o-quinone is fairly stable and has a strong absorbance at 390 nm. The kinetics of the hydrolysis of 4-nitrophenyl phosphate was carried out using an equal concentration of the complex catalyst and tetramethylammonium hydroxide (103 M) and 4-nitrophenyl phosphate (101 M) in DMF. The chromophore leaving group 4-nitrophenolate, which is the hydrolysis product of the substrate was monitored at 420 nm. The course of the reaction in both cases was followed nearly 45 min at regular time interval. The slope was determined by monitoring the growth of the band of the corresponding product. Plots of log (A/A At) versus time for both catecholase activity and phosphate hydrolysis are obtained and shown in Figs. 3 and 4, respectively. The rate constant values are also reported in Table 2. The first apparent result was that the reactivities of the complexes differ significantly, as the size of macrocycle increases. In both kinetics, the catalytic activities of heterobinuclear Cu(II)AZn(II) complexes are found to increase as the macrocyclic ring size increases due to the intrinsic flexibility. The catalytic activity of the complexes containing aromatic diimines is found to be less than that of the complexes containing aliphatic diimines. This can be explained that the planarity, which is associated with aromatic ring, imparts less catalytic efficiency due to the rigidity of the systems as observed in the
Fig. 4. Catalytic activity of heterobinuclear complexes for hydrolysis of 4-nitrophenylphosphate (a) [CuZnL1](ClO4)2 (b) [CuZnL2](ClO4)2.
Table 2 Catalytic activitya of heterobinuclear Cu(II)AZn(II) complexes for catecholase activity and hydrolysis of 4-nitrophenyl phosphate Complexes
Rate constant (min1) Catecholase activity
1
[CuZnL ](ClO4)2 [CuZnL2](ClO4)2 [CuZnL3](ClO4)2 [CuZnL4](ClO4)2 [CuZnL5](ClO4)2 a
3
2.91 · 10 3.05 · 103 3.39 · 103 2.81 · 103 2.89 · 103
Hydrolysis activity 1.75 · 102 2.27 · 102 3.43 · 102 1.68 · 102 2.12 · 102
Measured spectrophotometrically in DMF medium.
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case of electrochemical reduction of the complexes. It seems interesting to observe that the rate constant values of heterobinuclear Cu(II)AZn(II) for catecholase activities are less when compared to the rate constant values of corresponding mono and homobinuclear [16] Cu(II) complexes. 4. Conclusion This work involves the synthesis of lateral macrobicyclic compartmental heterobinuclear Cu(II)AZn(II) complexes with phenolate bridging atoms. Their extensive spectral, electrochemical and catalytic studies were carried out with slight modification in the ligand framework and compared with their corresponding mono and homobinuclear Cu(II) complexes. The increase in macrocyclic ring size in the ligand framework influences the spectral, electrochemical and catalytic properties of the complexes. Electronic spectral studies indicate that the d–d transition of Cu(II) ion is responsible for the appearance of band in the visible region. Electrochemical studies imply that the Zn(II) ion does not undergo reduction in the given potential range. Acknowledgements The author M.T. gratefully appreciates the financial support received from CSIR and UGC major project sanctioned to M.K., New Delhi. References [1] M. Yonemura, H. Okawa, M. Ohba, D.E. Fenton, L.K. Thompson, Chem. Commun. (2000) 817. [2] G.T. Babcock, L.E. Vickery, G. Palmer, J. Biol. Chem. 253 (1978) 2400. [3] P.A. Vigato, S. Tamburini, D.E. Fenton, Coord. Chem. Rev. 106 (1990) 25.
[4] F. Arena, C. Floriani, A.C. Villa, C. Guastini, Inorg. Chem. 25 (1986) 4589. [5] Y. Aratake, H. Okawa, E. Asato, H. Sakiyama, M. Kodera, S. Kida, M. Sakamoto, J. Chem. Soc., Dalton Trans. (1990) 2941. [6] H. Okawa, J. Nishio, M. Ohba, M. Tadokoro, N. Matsumoto, M. Koikawa, S. Kida, D.E. Fenton, Inorg. Chem. 32 (1993) 2949. [7] M. Yonemura, K. Arimura, K. Inoue, N. Usuki, M. Ohba, H. Okawa, Inorg. Chem. 41 (2002) 582. [8] H. Okawa, H. Furutachi, D.E. Fenton, Coord. Chem. Rev. 174 (1998) 51. [9] M. Yonemura, N. Usuki, Y. Nakamura, H. Okawa, J. Chem. Soc., Dalton Trans. (2000) 3624. [10] J.R. Morrow, W.C. Trogler, Inorg. Chem. 27 (1998) 3387. [11] N.P. Sadler, C.C. Chuang, R.M. Milburn, Inorg. Chem. 34 (1995) 402. [12] I.M. Atkinson, L.F. Lindoy, Coord. Chem. Rev. (2000) 200. [13] P. Hendry, A.M. Sargeson, Inorg. Chem. 29 (1990) 92. [14] N. Strater, T. Klaubunde, P. Tucker, H. Witzel, B. Krebs, Science 268 (1995) 1489. [15] M.P. Engloff, P.T.W. Coher, P. Reimer, D. Barfard, J. Mol. Biol. 254 (1995) 942. [16] M. Thirumavalavan, P. Akilan, M. Kandaswamy, K. Chinnakali, G. Senthil Kumar, H.K. Fun, Inorg. Chem. 42 (2003) 3308. [17] M. Thirumavalavan, P. Akilan, M. Kandaswamy, Inorg. Chem. Commun. 5 (2002) 422. [18] J.C. Duff, J. Chem. Soc. (1941) 547. [19] J.D. Crane, D.E. Fenton, J.M. Latour, A.J. Smith, J. Chem. Soc., Dalton Trans. (1991) 2279. [20] L.G. Armstrong, L.F. Lindoy, Inorg. Chem. 14 (1975) 1322. [21] P.G. Grimsley, L.F. Lindoy, H.C. Lip, R.J. Smith, J.T. Baker, Aust. J. Chem. 30 (1977) 2095. [22] G. Das, R. Shukala, S. Mandal, R. Singh, P.K. Bharadwaj, J.V. Singh, K.U. Whitmire, Inorg. Chem. 36 (1997) 323. [23] R. Bhalla, M. Helliwell, C.D. Garner, Inorg. Chem. 36 (1997) 2944. [24] P. Amudha, M. Kandaswamy, L. Govindaswamy, D. Velmurugan, Inorg. Chem. 37 (1998) 4490. [25] E. Gao, W. Bu, G. Yang, D. Liao, Z. Jiang, S. Yan, G. Way, J. Chem. Soc. Dalton Trans. (2000) 1431. [26] P.A. Connick, K.A. Macor, Inorg. Chem. 30 (1991) 4654. [27] H. Okawa, M. Tadokora, T.Y. Aratake, M. Ohba, K. Shindom, M. Mitsumi, M. Koikawa, M. Tomono, D.E. Fenton, J. Chem. Soc., Dalton Trans. (1993) 253.