Cyclovoltammetric and spectroscopic characterization of optically active cobalt(II) and copper(II) complexes with the Schiff base derived from (N,N¢)-(1R,2R)-(-)-.
Transition Metal Chemistry 27: 501–505, 2002. 2002 Kluwer Academic Publishers. Printed in the Netherlands.
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Cyclovoltammetric and spectroscopic characterization of optically active cobalt(II) and copper(II) complexes with the Schiff base derived from (N,N¢)-(1R,2R)-(-)cyclohexylenediamine and 2-hydroxyacetophenone Edward Szłyk*, Stanisław Biniak, Andrzej Surdykowski, Iwona Łakomska and Magdalena Barwiołek Faculty of Chemistry, Nicholas Copernicus University, Gagarina 7, 87-100 Torun˜, Poland Erik Larsen Department of Chemistry, The Royal Veterinary and Agricultural University, DK-1871 Frederiksberg, Denmark
Received 20 June 2001; accepted 23 July 2001
Abstract The Schiff base prepared by reacting (-)-(1R,2R)-1,2-cyclohexanediamine with 2-hydroxyacetophenone was used as a ligand for CoII and CuII. The coordination compounds were studied by u.v.–vis. absorption and by circular dichroism (c.d.) spectroscopy in solution. The complexes are four-coordinated in a slightly distorted square planar symmetry. The distortion from planarity is a main factor influencing the chiral surroundings of the metal ion. The d–d and c.t. transitions are consistent with the observed distortion, which arises from intramolecular interactions between the methyl groups attached to the Schiff base imine carbon and hydrogen atoms of the cyclohexane ring. The electrochemical properties of the CoII and CuII complexes were observed in MeCN but investigations revealed weaker oxygen activation than of CoII analogue with salicylaldehyde. The CuII complex is reduced in H2O to CuI which disproportionates to CuII and Cu0.
Introduction Chiral Schiff bases obtained from optically active 1,2cyclohexanediamine (-)chxn are interesting ligands, because the p-system in a Schiff base imposes a geometrical constraint and also affects the electronic structure. This situation has given rise to the catalytic properties of transition metal Schiff base complexes in oxidising hydrocarbons [1, 2] or reducing organic halides [3, 4]. In this study we have employed the rigidity of the cyclohexanediamine derivative of 2hydroxyacetophenone which results in a planar cobalt (II) and copper(II) complex with an electronic structure revealed by the absorption and c.d. spectra. Ligand field spectra of tetradentate Schiff base chelates of cobalt(II) and copper(II) were discussed in relation to the conformational changes and the affect of the chromophore moiety of the molecule on the optical activity [5]. Copper(II) complexes of Schiff bases, made by condensing diamines with 2,4-pentanedione or salicylaldehyde, have been investigated by many techniques [6]. The structure of [CuR-pn(acac)2] in solution has been studied by absorption and circular dichroism (c.d.) spectroscopy and its crystal structure was determined by X-ray diffraction. Based on these results one may draw the conclusion, that chiral copper(II) complexes are * Author for correspondence
significantly distorted towards a tetrahedral structure and that the methyl groups on the imine carbon and the phenyl moiety avoid close contact. The structure of analogous coordination compounds can be understood or predicted on this basis. The electronic structure of 2hydroxyacetophenone is less suited for stereochemical studies by c.d. because of the low effective symmetry, so that the internal pfip* transition moments are found to have directions far from the NAO direction or its perpendicular direction in the plane of the molecule. An earlier study on such salicylaldimine copper(II) compounds did not attempt to relate c.d. to electronic or geometric structure. The structure of cobalt(II) Schiff base coordination compounds derived from substituted 1,2-cyclohexanediamines and 2-hydroxyacetophenone have not been studied in detail and at present it is not possible to predict how the ligands will orient themselves around a metal ion. Electrochemical characteristics of tetradentate Schiff base chelates of cobalt(II) and copper(II) will be discussed and related to the structural features in solution and towards oxygen activation. Such reactions are not yet fully understood, even though reduction of dioxygen in aqueous and non-aqueous solutions has received a great deal of attention due to its involvement in biological reactions systems [7–9]. Transition metal ions in chelates of the N4 or N2O2 planar type are known to form adducts in the solid state and in solution
502 with dioxygen via charge-transfer from the central metal ion to oxygen [9, 10]. The reduction of copper(II) complexes may produce unstable copper(I) complexes or copper(0) species [9]. The copper(I) complex is more stable in tetrahedral geometry than copper(II) but can disproportionate when oxygen is bonded in the coordination sphere. In this study we compare the electrochemical behaviour of the metal complex alone and in combination with oxygen.
Experimental Materials Co(OAc)2 Æ 4H2O and Cu(OAc)2 Æ H2O (analytical grade) were purchased from POCh (Gliwice, Poland). 2Hydroxyacetophenone (98%) (hacphenH), was supplied by Aldrich. Trans-(1R, 2R)-1,2-cyclohexanediammonium (+)D-tartrate was prepared according to the method of Galsbol et al. [11] and used for isolation of (-)-(1R,2R)-trans-cyclohexanediamine (-)chxn. MeCN and CH2Cl2, both 99.9% HPLC grade, (Sigma-Aldrich) as well as n-Bu4NBF4 (Fluka) was used as received. Na2SO4 (p.a., Merck) solutions were prepared using doubly distilled H2O.
fitting was performed using the non-linear least-squares algorithms and assuming a Gaussian/Lorenzian mix of the variable proportion peak shape. The peak fitting was repeated until an acceptable adjustment was obtained (error 5%). The positions of resolved peaks were determined in accordance with the literature data and empirically obtained values. Synthesis An EtOH solution of (-)chxn (1.82 g, 16 mmol) was mixed with 2-hydroxyacetophenone (4.35 g, 32 mmol) in EtOH solution and stirred under reflux for 3 h. The resulting yellow solution of the Schiff base was concentrated by evaporation on a vacuum line to dryness and the solid was recrystallized from EtOH. The Schiff base (3.5 g, 10 mmol) was dissolved in n-PrOH and Co(OAc)2 Æ 4H2O (2.49 g, 10 mmol) or Cu(OAc)2 Æ H2O (1.99 g, 10 mmol) in hot MeOH(40 cm3) was added under Ar. The complex precipitated immediately, but the reaction mixture was stirred and heated under reflux for 30 min, filtered, washed with EtOH and dried on a vacuum line. Elemental analyses: [Co(-)chxn(hacphen)2] (Found: C, 64.9; H, 5.9; N, 6.9; Co 14.5. C22H24N2O2Co calcd.: C, 64.55; H, 5,7; N, 6,6; Co; 14.3%). [Cu(-)chxn(hacphen)2] (Found: C, 64.15; H, 5.8; N, 6.8; Co 15.4. C22H24N2O2Cu calcd.: C, 64.4; H, 5,65; N, 6,5; Cu; 15.2%).
Instrumentation U.v.–vis. absorption spectra were recorded on Milton Roy Spectronic 1201 and Specord M-40 spectrophotometers. The sample concentration was 6.2 · 10)5 M in MeCN. Free ligand spectra were measured in EtOH (1.25 · 10)4 M ) and MeCN (1.1 · 10)4 M ). C.d. spectra were measured with a JASCO 710 spectropolarimeter using the same sample solution as in u.v.–vis. Voltammetric measurements were made on a modular electrochemical system Autolab (Eco Chemie), equipped with a potentiostat PSTAT 10 and driven by a GPES3 software (Eco Chemie). A three-electrode cell was used and the solution was kept under Ar, or in air, at room temperature. Pt mini-disc working electrodes (1 mm diam) (Cypress System) were used. The Pt wire as a counter electrode and Ag/AgCl/KCl(sat.) as the reference electrode was employed. All potentials are quoted with respect to this reference. The X-ray photoelectron spectra (XPS) were carried out with an EscaLab 210 (V.G. Scientific Ltd.) spectrometer using non-monochromatized AlKa radiation (1486.6 eV), the source being operated at 15 kV and 34 mA. Samples for XPS measurements were prepared as follows: a Pt disc (8 mm diam.) was used as a working electrode and subjected to cyclic voltammetry, for 10–15 cycles when voltammograms become constant. The Pt disc was then removed from the electrolyte solution, washed with EtOH, and examined with XPS. The high resolution scans were performed over the 928–958 eV range (Cu 2p3/2 spectrum) and 776–792 eV range (Co 2p3/2 spectrum). After substraction of the base line (Shirley-type), the curve
Results and discussion Electronic spectra and circular dichroism The free ligand absorption spectrum and c.d. of (1R,2R)-(-)-cyclohexylenebis-(2-hydroxyacetophenoimine) (-)chxn(hacphenH)2 in acetonitrile, revealed a minimum at 325 nm, assigned to essentially nfip* transitions involving promotion of one of the lone nitrogen atom pairs to the antibonding p*-orbital associated with the azomethene group. The Cotton effects with a minimum c.d. at 264 nm and a maximum c.d. at 242 nm were assigned to exciton coupled pfip* transitions. The splitting between the bands is much larger than the energy splitting because of near cancellation of two c.d. bands of opposite sign having the observed residuals. The wing shaped c.d. components are typical for exciton-coupled transitions [12]. [Cu(-)chxn(hacphenH)2] exhibits an absorption band at 702 nm (14200 cm)1) (0.12) and a second band, which is split in c.d. with a maximum at 590 nm (16900 cm)1) ()1.30) and 488 nm (20500 cm)1) (0.37). These are considered to be d–d transitions and they can be reasonably represented by er and ep parameters, giving the D4h holohedrized one electron diagram [13]. Spectral observations and the programme for personal computers, written by Bendix [14], were used to calculate the ligand field parameters, with er ¼ 8700 cm)1, ep ¼ 2900 cm)1 giving the following assignment for [Cu(-)chxn(hacphen)2]:
503
xz ! x2 y 2 2
2
2
z !x y xz; yz ! x2 y 2
m obs:ðcm1 Þ
m calcd:ðcm1 Þ
14; 200
14; 500
16; 900 20; 500
17; 400 20; 300
The higher energy c.d. components at 397 nm ()5.92), 387 nm ()15.68), 348 nm (8.47), 271 nm ()17.11) are all considered to be charge-transfer bands in which electronic transitions may take place from the internal ligand p orbitals and non-bonding oxygen orbitals to the strongly antibonding 3d orbital, when O, N, N, O are placed at the x- and y-axes. The dominant sign for these c.d. bands is negative. They are very different from exciton-coupled transitions, which integrate to zero. The pfid transitions obtain an electric transition moment, P, which is defined from the atomic arrangement in the mean coordination plane. The magnetic transition moment, M, with the same transition, is mainly derived from tetrahedral out-of-plane distortion as a charge rotation during the transition. To produce a negative rotational strength M ¼ PM the distortion has to take the same shape as presented in Figure 1 and the similar donor atom arrangements were observed for [Ni(-)chxn(salal)2], and [Ni(-)chxn(hacphen)2] [15]. [Co(-)chxn(hacphen)2] has the strong positive c.d. at 551 nm (De ¼ 0.39) and the second one, which is split slightly negative at 460 nm (De ¼ )0.06) and positive 408 nm (De ¼ 0.21) (Figure 2). These components can be considered as d–d transitions. Two c.d.’s at 357 nm (0.08) and 304 nm ()0.35) are assigned to the metal ligand charge-transfer transitions. This couplet also gives the non-zero values to the dipole strength. The dominant sign of these bands is positive, hence the distortion from planarity can be assumed to be a mirror image in relation to [Co(-)chxn(salal)2] [16]. C.d. spectra favour a k conformation for the cyclohexane ring in the metallocycle, i.e. identical to the nickel(II) complexes [15–17]. The above argument can be rationalised in terms of the different conformations in both complexes, one being slightly distorted as found in the crystal structure of [Ni(-)chxn(hacphen)2] [19], where the phenyl rings reveal an open umbrella shape and the same
Fig. 2. C.d. spectrum of [Co(-)chxn(hacphen)2] in MeCN solution.
conformation can be proposed for the complexes studied. Due to the low effective symmetry in these compounds and cobalt(II) analogs only detailed calculations can explain all c.d. bands. Copper(II) complex cyclovoltammetry in aqueous solution [Cu(-)chxn(hacphen)2] is sufficiently soluble in water (0.1 mM), hence cyclo voltammetric (c.v.) curves in oxygenated and deoxygenated solutions were recorded. The cathodic peak observed between 0.02 and )0.06 V can be assigned to the copper(II)/copper(I) redox couple (E0 ¼ )0.04 V). The potential sweeping towards the negative values resulted in currents, which can be related to the disproportionation of the copper(I) species. With the increase of the scan numbers the anodic peak of copper(0) oxidation (Ep.a ¼ 0.15 V) has reduced gradually. When the system has been stabilised, the anodic peak was twofold higher than the cathodic one. In oxygenated solutions the c.v. shape of the cathodic part has been changed, but the oxygen reduction peak was not observed between )0.1 and )0.7 V. Simultaneously the anodic peak (Ep.a. ¼ 0.15 V) has been reduced to one-tenth of the starting value in comparison to the deoxygenated solution. These discrepancies can be explained by the partial stabilisation of the copper(I) complex due to oxygen adduct formation (via a chargetransfer process). The following reactions can be proposed for the system studied: (a) in deoxygenated solution ½CuII ð-ÞchxnðhacphenÞ2 þ e ! ½CuI ð-ÞchxnðhacphenÞ2
ðEp:c: ¼ 0:04 VÞ
ð1Þ
2½CuI ð-ÞchxnðhacphenÞ2 ! ½CuII ð-ÞchxnðhacphenÞ2 þ Cu0 þ ð-ÞchxnðhacphenÞ2 ð2Þ Fig. 1. A schematic structure of [M(-)chxn(hacphen)2] complex and tetrahedral distortion of the coordination plane.
Cu0 2e ! CuII ðEp:a: ¼ 0:15 VÞ
ð3Þ
504 ½CuII ð-ÞchxnðhacphenÞ2 þ e ! ½CuI ð-ÞchxnðhacphenÞ2 ðEp:c: ¼ 0:04VÞ
ð4Þ
½CuI ð-ÞchxnðhacphenÞ2 þ O2 ! ½CuII ð-ÞchxnðhacphenÞ2 O2 ðintramolecular c.t.Þ ð5Þ The observed reduction of peak intensity can be related to the adsorption of the copper(I) complexes on the electrode surface and the oxidation process was detected. The interaction between the electrode and complexes can additionally stabilise the tetracoordinated monomeric copper(I) complex. Also the lack of wave at negative potentials (below 0.45 V) allows an exclusion of the dimerization in the presence of dioxygen [15]. Copper(II) complex cyclovoltammetry in non-aqueous solutions The c.v. curves recorded for [CuII(-)chxn(hacphen)2] in the deoxygenated MeCN solution reveals the reduction wave at E1/2 ¼ )0.375 V and the anodic peak at Ep.a.1 ¼ )0.165 V. In the anodic part of the c.v.’s, two waves were observed (E1/2 ¼ 0.75 and 1.00 V) out of which the one at 1.0 V ceases during cyclisation. In oxygenated solutions the first anodic peak Ep.a ¼ )0.180 V is of much smaller magnitude and vanishes after the third scan, whereas the large wave of the oxygen reduction (onset at )0.25 V) gradually shifts to the more negative values. It was noted that at the end of the process the electrode was covered with a precipitate, as was similarly found for N,N0 -(1R,2R) (-)-1,2-cyclohexylenebis(salicylideneiminato)cobalt(II) [CoII(-)chxn(salal)2] and other transition metal Schiff base complexes [10, 18]. In M(salen)2 (M ¼ Co, Cu, Ni and Mn), the redox conductive polymer films were formed during cyclisation [18]. Therefore the precipitate obtained was examined by an XPS method and spectra of the deposited material as well as solid [Cu(-)chxn(hacphen)2] are presented at Figure 3. Bond energies (BE ¼ 936.2 and 934.3 eV) found for the pure complex (Figure 3a) are characteristic for copper(II) ions. However the spectrum of the electrochemically deposited complex on the Pt electrode revealed Cu 2p3/2 (BE ¼ 933.7 eV) and lack of satellites in the 940–948 eV range, that can be related to the copper(I) oxidation state. Besides, the Cu 2p1/2 component is located at BE ¼ 953.8 eV, similar as reported for copper(I) complexes with N macrocycles [20]. Because the observed potentials of the cathodic wave in oxygenated solutions have similar values as recorded for the Pt electrode in the absence of the complex in solution, one can propose that [CuII(-)chxn(hacphen)2] has the minor ability to oxygen activation. This feature can be related to deformation of the coordination sphere from planarity, which is caused by interactions between the methyl
Fig. 3. Cu 2p3/2 XPS spectra of Cu species in the solid CuII (-)chxn(hacphen)2 and electrochemically deposited at Pt electrode: (a) original complex, (b) deposited complex in oxygenated solution.
groups on the imine carbon and the methine carbon in the cyclohexane ring, as is also evident from the c.d. spectra and 1H-NOE experiment for [Ni(-)chxn(hacphen)2] [20]. Cobalt(II) complex: cyclovoltammetry in non-aqueous solutions In oxygenated and deoxygenated solutions the anodic peak at Ep.a. ¼ 0.97 V without a cathodic response was observed. Similarly as for the copper(II) complex this can be related to an electrochemically non-active product formation. The precipitate observed on the electrode surface was examined by an XPS method and compared to the spectrum of the solid [Co(-)chxn(hacphen)2] (Figure 4). Bond energies for Co 2p3/2 (BE ¼ 782.6 and 787.8 eV) found for the complex (Figure 4a) are characteristic for cobalt(II) and cobalt(II) satellite ions, whereas cobalt(III) can be excluded. The spectrum of the complex, electrochemically deposited on the electrode surface, revealed the same cobalt 2p3/2 signals at BE ¼ 781.8 eV and satellite at 787.0 eV, which can be assigned to formation on the Pt electrode electrochemically inactive products. In the oxygenated system the stable cathodic peak due to oxygen reduction was detected at Ep.c. ¼ )0.41 V, suggesting the ability of the complex to catalyse the process with formation of stable oxygen adducts analogous to other cobalt(II) complexes [8, 10]. C.v. curves of [Co(II)(-)chxn(hacphen)2] recorded in the oxygen-free DCM solutions do not reveal the
505 were found, suggesting the catalytic activity of this compound in electroreduction of oxygen. However the electrocatalytic activity is lower than for [CoII(-)chxn(salal)2], and can be related to stronger deformation of the coordination sphere [20], caused by methyl substitution the imine carbon. The stronger tetrahedral deformation of cobalt(II) than copper(II) complexes seems to be the second parameter, which is important in oxygen activation. The chirality of the Schiff base reduces the deformation, which is affected mainly by the substituent at the imine carbon. Further studies will be focused on characterisation of oxygen adducts, postulated in this work.
Acknowledgement Financial support from the Polish State Committee for Scientific Research grant 3T09A 076 10 is gratefully acknowledged.
References Fig. 4. Co 2p3/2 XPS spectra of original CoII(-)chxn(hacphen)2 (a) and electrochemically deposited species at Pt electrodes (b).
cobalt(II)/cobalt(III) couple due to the lack of peaks in the 0.0 V–0.90 V potential range. The anodic (Ep.a.¼ 1.03 V) and cathodic peaks (Ep.c. ¼ )0.60 V) can be viewed as the outcome of an electrochemically irreversible oxidation process and reduction of the complex. During cyclisation in oxygenated solution the first reduction peak (Ep.c.1 ¼ )0.13 V), which can be assigned to the active oxygen, slowly vanished, whereas the second one (Ep.c.2 ¼ )1.00 V) was shifted towards negative potentials. Such effects suggest the existence of different active forms of oxygen and their rather poor stability in the system studied. A similar measurement for [CoII(-)chxn(salal)2] (in MeCN) was in favour of the active cobalt(II)/cobalt(III) species with the superoxo/ peroxo coordinated oxygen which exhibited catalytic activity [10]. However the complex discussed has weaker properties in the oxygen activation process. The results suggest, that the stereochemistry of the central ion has a main impact on electrochemical processes in oxygenated and deoxygenated solutions. The water-soluble complex [CuII(-)chxn(hacphen)2] during the course of reaction exhibits a tendency to deposite on the electrode surface, but there is no oxygen activation in this solution. In MeCN, electropolymerisation of the copper complex with simultaneous formation of the non-conductive layer was observed. In [CoII(-)chxn(hacphen)2], possible oxygen adducts
1. C.P. Horowitz, S.E. Creager and R.W. Murray, Inorg. Chem., 29, 1006 (1990). 2. M. Gomes de F.T. and O.A.C. Anutunes, Catal. Lett., 38, 133 (1996). 3. J.M. Duprilot, F. Bediouni, J. Devynck, J.C. Folest and C. BiedCharreton, J. Organometal. Chem., 286, 77 (1985). 4. C.E. Dahm, D.G. Peters, Anal. Chem., 66, 3117 (1994). 5. R.S. Downing and F.L. Urbach, J. Am. Chem. Soc., 91, 5977 (1969). 6. J. Llewellyn and T.N. Waters, J. Chem. Soc. Dalton Trans., 2639 (1960). 7. P. Gourec and M. Savy, Electrochim. Acta., 44, 2653 (1990). 8. S.K. Dhar, Inorg. Chim. Acta, 240, 609 (1995). 9. C.X. Cai, K.H. Xue, X.Y. Xu and Q.H. Luo, J. Appl. Electrochem., 27, 793 (1997). 10. E. Szłyk, S. Biniak and E. Larsen, J. Solid State Electrochem., 5, 211 (2001) 11. F. Galsbol, P. Steenbol and B.S. Sorensen, Acta Chem. Scand., 26, 962 (1972). 12. B. Bosnich, J. Am. Chem. Soc., 90, 627 (1968). 13. C.E. Scha¨ffer, Pure Appl. Chem., 24, 361 (1970). 14. Bendix Ligfield V.078 Manual ed. University of Copenhagen. 15. A. Wojtczak, E. Szłyk, M. Jasko´lski and E. Larsen, Acta Chem. Scand., 51, 274 (1997). 16. E. Szłyk, A. Surdykowski, M. Barwiołek and E. Larsen, Transition Met. Chem., 25, 670 (2000). 17. E. Szłyk, A. Wojtczak, E. Larsen, A. Surdykowski and J. Neumann, Inorg. Chim. Acta., 93, 239 (1999). 18. F. Beudiouni, E. Labbe, S. Gutierrez-Granados and J. Devynck, J. Electroanal. Chem., 301, 267 (1991). 19. R.R. Gagne, J.L. Allison, C.A. Kowal, W.S. Mialki, T.J. Smith and R.A. Walton, J. Am. Chem. Soc., 102, 1905 (1980). 20. A. Cinquanti, R. Cini, R. Seerbere and P. Zanello, J. Electroanal. Chem., 121, 301 (1981).
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