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cating that ligand L3H2 has cis–trans isomerism [12]. The isomer ratio was found to be 64% cis-isomer and. 36% trans-isomer from the 1H-n.m.r and 13C-n.m.r..
 Springer 2005

Transition Metal Chemistry (2005) 30:758–764 DOI 10.1007/s11243-005-6226-1

Synthesis, characterization and redox properties of three new vic-dioximes and their nickel(II) metal complexes Esref Tas*, Mehmet Aslanoglu, Ahmet Kilic and Zeliha Kara Department of Chemistry, Harran University, 63510, Sanliurfa, Turkey Received 18 March 2005; accepted 11 May 2005

Abstract Three novel vic-dioximes: cyclohexylamine-p-tolylglyoxime (L1H2), t-butylamine-p-tolylglyoxime (L2H2) and sec-butylamine-p-tolylglyoxime (L3H2) were prepared by the reaction of anti-p-tolylchloroglyoxime with cyclohexylamine, t-butylamine and sec-butylamine in absolute THF. The detection of H-bonding in all of the Ni(II) complexes by i.r. revealed the square-planar MN4 coordination of mononuclear complexes. MN4 coordination of the [(L1H)2Ni] complex was also determined by 1H and 13C-n.m.r spectroscopy. Mononuclear complexes with a 1:2 metal-ligand ratio were prepared using Ni(II) salts. All Ni(II) complexes are insoluble in common solvents. The ligands and complexes were characterized by elemental analyses, FT-i.r., u.v.–vis., 1H and 13C-n.m.r. spectra, magnetic susceptibility measurements, thermogravimetric analyses (t.g.a.) and cyclic voltammetry.

Introduction Numerous vic-dioximes and their transition metal complexes have been investigated [1]. Due to the presence of mildly acidic hydroxyl groups and slightly basic azomethine groups, vic-dioximes are amphoteric ligands that form corrin-type square-planar, squarepyramidal and octahedral complexes with transition metal ions such as nickel(II), copper(II), palladium(II), cobalt(II) and cobalt(III) as the central atoms [2]. The exceptional stability and unique electronic properties of these complexes can be attributed to their planar structure which is stabilized by hydrogen bonding [3]. The high stability of the complexes with vic-dioxime ligands has been used extensively for various purposes. vic-Dioximes have also received considerable attention as model compounds which mimic bio-functions such as the reduction of vitamin B12 [4, 5]. Several similar compounds were synthesized by Cukurovali and Tas [6], Bekaro glu et al. [7] and Go¨k et al. [8]. Go¨k et al. [9] have synthesized aza compounds attached to the dioxime group. The transition metal complexes of vic-dioximes have been of particular interest as biological model compounds [10]. Tas et al. have prepared vic-dioxime ligands which have different substituents and their some transition metal complexes [11]. In all studies mentioned above, similar complexation characteristics were observed as the ligand formed mononuclear complexes that contain unsaturated sites. In this paper, we present the synthesis and characterization of three novel dioximes containing cyclohexylamine-p-tolylglyoxime (L1H2), t-butylamine* Author for Correspondence: E-mail: [email protected]

p-tolylglyoxime (L2H2), sec-butylamine-p-tolylglyoxime and we obtained their mononuclear complexes with nickel(II) as well as investigating their electrochemical properties.

Experimental Material and methods Anti-p-tolylchloroglyoxime was synthesized as described in the literature [12]. Cyclohexylamine, t-butylamine and sec-butylamine (Aldrich Chemical Company, Taufkirchen, Germany), and tetra-(n-butyl)ammonium perchlorate (n-Bu4NClO4) (Fluka Chemical Company, Taufkirchen, Germany) were used as received. Elemental analyses were determined in the TUBITAK Laboratory (The Scientific and Technical Research Council of Turkey). I.r. spectra were recorded on a Perkin Elmer Spectrum RXI FT-i.r. Spectrometer as KBr pellets. 13C and 1H-n.m.r. spectra were recorded on a Varian XL-200. Magnetic susceptibilities were determined on a Sherwood Scientific Magnetic Susceptibility Balance (Model MK1) at room temperature (20 C) using Hg[Co(SCN)4] as a calibrant; diamagnetic corrections were calculated from Pascal’s constants [11]. U.v.–vis spectra were recorded on a Schimadzu 1601 PC and thermogravimetric (t.g.a.) curves were recorded on a Seteram Labsys TG-16 thermobalance. An EcoChemie Autolab 12 potentiostat with the electrochemical software package GPES 4.9 (Utrecht, The Netherlands) was used for voltammetric measurements. A three electrode system

759 was used: a Pt wire counter electrode, an Ag/AgCl reference electrode and a 2 mm Pt disc as working electrode. Oxygen-free nitrogen was bubbled through the solution before each experiment. All experiments were carried out at room temperature. Synthesis of the ligands L1H2, L2H2 and L3H2 Anti-p-tolylchloroglyoxime was synthesized as described in the literature [12]. A solution of cyclohexylamine (1.56 g, 15.8 mmol), t-butylamine (1.31 g, 15.8 mmol) and sec-butylamine (1.31 g, 15.8 mmol) in absolute THF (30 cm3) was added to a solution Et3N (1.58 g, 15.8 mmol) in absolute THF (10 cm3). This mixture was cooled to )15 C and kept at this temperature and a solution of anti-p-tolylchloroglyoxime (3.35 g, 15.8 mmol) in absolute THF (50 cm3) was added dropwise under a N2 atmosphere with continuous stirring. The addition of the anti-p-tolylchloroglyoxime solution was carried out during 1.5 h. The mixture was stirred for more than 1 h and the temperature was raised to 20 C. Precipitated Et3NHCl was filtered off and the filtrate was evaporated to remove THF. The oily products were dissolved in CH2Cl2 (10 cm3) and n-hexane (200 cm3) added to precipitate the compound. This process was then repeated several times. The products were filtered and dried in a vacuum. The products are soluble in common solvents such as CH2Cl2, CHCl3, Me2CO, THF, EtOH and DMSO. L1H2 of characteristic 13 C-n.m.r. the chemical shift (CDCl3, TMS, d ppm): C1 (35.16 ve 34.78), C2 (140.00), C3 (126.67), C4 (129.00), C5 (133.93), C6 (149.30), C7 (148.67), C8(52.33), C9 (25.51), C10 (25.07) and C11(21.64 ve 21.77), L3H2 of characteristic 13C-n.m.r. the chemical shift (CDCl3, TMS, d ppm): C1 (31.10 and 31.06), C2 (139.84 and 140.05), C3 (127.47 and 126.73), C4 (129.27 and 128.95), C5 (130.61 and 130.00), C6 (152.30), C7 (149.03 and 148.85), C8(46.27), C9 (50.99 and 50.48) and C10 (21.61–21.75 and 22.51–22.29).

was added dropwise under a N2 atmosphere with continuous stirring. The stirred mixture was then heated to the reflux temperature for 60 min and was maintained at this temperature. The pH of a solution was ca. 1.5–3.0 and was adjusted to 4.5–5.5 by the addition of a 1% NaOH solution in EtOH. After cooling to room temperature, the complexes were filtered, washed with H2O several times, and dried at 100 C for 24 h. Characteristic 13C-n.m.r. chemical shifts of [Ni(L1H)2 (CDCl3, TMS, d ppm)]: C1 (34.90), C2 (140.21), C3 (129.67), C4 (129.99), C5 (126.43), C6 (150.01), C7 (144.21), C8(53.03), C9 (25.37), C10 (24.74) and C11 (21.75) were recorded.

Results and discussion The reaction sequence for the synthesis of L1H2, L2H2 and L3H2 is given in Scheme 1. Cyclohexylamine-p-tolylglyoxime (L1H2), t-butylamine-p-tolylglyoxime (L2H2) and sec-darylbutylamine-p-tolylglyoxime (L3H2) were prepared from anti-p-tolylchloroglyoxime, which was obtained by a procedure published previously [12] and cyclohexylamine, t-butylamine and sec-butylamine in Scheme 1. Excess of Et3N was used to neutralize HCl liberated during the reaction. For the structural characterization of L1H2, L2H2, L3H2 and (L1H)2Ni, (L2H)2Ni, (L3H)2Ni, elemental analyses, ft-i.r., u.v.–vis, 1H-n.m.r and 13C-n.m.r spectra, magnetic susceptibility measurements, thermogravimetric analyses (tga) were used and the data are given in the Experimental section. Additional analytical data are given in Tables 1–4. The ligands L1H2, L2H2 and L3H2,on interaction with Ni(II) salts, yielded complexes corresponding to the general formula [Ni(LxH)2]. The analytical data for complexes are presented in Tables 1–4. The metal-to-ligand ratio of the Ni(II) complexes was found to be 1:2.

Synthesis of Ni(II) complexes

N.m.r. spectra

L1H2(0.27 g, 0.468 mmol), L2H2(0.276 g, 0.468 mmol) and L3H2 (0.276 g, 0.468 mmol) ligands were dissolved in EtOH (30 cm3). A solution of 0.235 mmol of the metal salt NiCl2 . 6H2O (0,057 g) in EtOH (10 cm3)

Although, no chemical shifts were observed for the @NAOH groups of oximes in the 1H-n.m.r spectra of L1H2, L2H2 and L3H2, the proton resonance of ligands supports the structures (Table 4).

Scheme 1.

760 Table 1. Colours, formula, melting points, magnetic usceptibilities, yields and elemental analyses results for the ligands and the complexes Compounds

Colour

M.p. C (dec.)

Yield (%)

leff [B.M]

Elemental analyses Calcd. (Found) % C H N

Ligand, L1H2 [Ni(L1H)2] Ligand, L2H2 [Ni(L2H)2] Ligand, L3H2 [Ni(L2H)2]

pale yellow red pale yellow red pale yellow red

98 247 67 252 75 200

76 63 64 78 69 72

– Dia – Dia – Dia

65.45 (65.2) 59.3 (60.4) 62.65 (62.4) 56.2 (56.0) 62.65 (62.9) 56.2 (56.3)

7.6 6.6 7.6 6.5 7.6 6.5

(8.0) (6.7) (8.15) (6.7) (8.1) (6.8)

15.3 13.8 16.9 15.1 16.9 15.1

(15.1) (13.8) (16.5) (14.9) (16.7) (15.5)

Dia=diamagnetic.

Table 2. Characteristic i.r. bands (cm)1) of the ligands and complexes as KBr pellets Compounds

OAH

NAH

Aliph. CAH

OAH  O

NAO

C@N

L1H2 [Ni(L1H)2] L2H2 [Ni(L2H)2] L3H2 [Ni(L2H)2]

3540–2557 – 3184 – 3351–3243 –

3335 3337 3350 3353 3388 3382

2930–2856 2929–2841 2963–2867 2963–2871 2970–2872 2965–2874

– 1757 – 1759 – 1721

999 966 999 973 994 974

1647 1615 1629 1615 1645 1633

Table 3. Characteristic u.v.–vis bands of the ligands and complexes Compounds

Solvent

Wavelength [k max. (nm)]

L1H2

C2H5OH CHCl3 CHCl3 C2H5OH CHCl3 C2H5OH CHCl3 C2H5OH CHCl3 C2H5OH CHCl3

251 244 246 252 243 241 245 251 247 256 247

[Ni(L1H)2] L2H2 [Ni(L2H)2] L3H2 [Ni(L3H)2] s

257 272 327s 291 250 323 285 257 252 309 326s

311 291 403s 307 260 411 293 291 416 382

307 474s 295 460s

314

310 343 536 416

536

Shoulder.

Table 4. 1H-n.m.r. spectra of the ligands and Ni(L1H)2complexes L1H2

L2H2

L3H2

Ni(L1H)2

(OAH  O) OHa ArACH3 ArACHA

– – 2.35 (3H)s 7.1–7.13 (2H)d 7.26–7.64 (2H)d 7.26–7.64 (2H)d

– – 2.2–2.4 (3H)t 7.1–7.7 (4H)m

15.07 (2H)s – 2.37 (6H)s 7.25–7.32 (8H)s

ACH ACH2 CACH3

2.83 (1H)s 1.02 (4H)s 1.4–1.8 (6H)m –

– – 1.1–1.4 (9H)m

NH–

5.2 (1H)b

5.3 (1H)b

– – 2.33 (3H)s 7.53 and 7.55 ()d 7.64 and 7.66 ()d 7.09 and 7.11 ()d 7.19 and 7.25 ()d 4H 2.91–3.10 (1H)b 1.13–1.31 (2H)m 0.64–0.71 (3H)m 0.88–0.90 ()d 0.96–0.98 ()d 3H 5.27 and 5.05 (1H)b

Funct. Group a

m

multiplet. singlet. d doublet. t triplet. a deuterium exchangeable. b broad bands. s

3.39 (3H)s 1.05–1.77 (20H) –

4.74 (2H)s

m

761 In the 1H-n.m.r spectrum of L1H2, L2H2 and L3H2,the deuterium exchangeable proton of the NH group appears as a singlet at d=5.2, d=5.3 and d=5.27 and d=5.05, respectively (Table 4). In the 13C-n.m.r. spectrum of L1H2 the carbon resonances of the oxime groups are observed at 149.30 (C6) and 148.67 (C7) ppm and, in the 13C-n.m.r. spectrum of L3H2, the carbon resonances of oxime groups, are observed at (152.30) C6 and (149.03 and 148.85) C7 ppm. On the other hand, in the 13C-n.m.r.spectrum of L3H2, (152.30) C6, (149.03 and 148.85) C7 ppm other carbon resonances have a double resonance indicating that ligand L3H2 has cis–trans isomerism [12]. The isomer ratio was found to be 64% cis-isomer and 36% trans-isomer from the 1H-n.m.r and 13C-n.m.r. data (Table 4). The 1H-n.m.r. spectra of the nickel(II) complexes show bands at d = 15.07 ppm supporting the proposed (OAH  O) bridge [11, 13]. This result can be easily identified by deuterium exchange (Table 4). FT-i.r. spectra In the i.r. spectrum of the three new ligands (as KBr pellets), an t(OH) stretching vibration is observed at 3540–2557 cm)1 for L1H2, 3184 cm)1 for L2H2 and 3351–3243 cm)1 for L3H2 as a broad absorption. The other characteristic bands of vic-dioximes t(C@N) and t(NAO) at 1647 and 999 cm)1 for L1H2, 1629 and 999 cm)1 for L2H2, 1645 and 994 cm)1 for L3H2. t(ANH) bands of ligands; 3337, 3350 and 3388 cm)1 is observed, respectively. Stretching absorptions appear at frequencies as expected for the substituted phenylaminoglyoxime [11] (Table 2). The i.r. spectra of the Ni(II) complexes are characterized by the appearance of significant a absorption band at ca. 1759–1716 cm)1 due to t(OAH  O) [14], and a sharp band at ca. 1633–1615 cm)1 due to azomethine groups. Downward shifts (for L1H2, L2H2 and L3H2) of 12–32 cm)1 for the C@N absorption in the Ni(II) complexes indicated coordination through the N atoms [15]. The t(C@N) stretching vibrations are affected upon complexation and are situated at a frequency significantly lower than the free ligands (L1H2, L2H2 and L3H2). A medium, strong band at 999 cm)1 is assignable to NAO stretching vibrations [16]. The Ni(II) complexes are diamagnetic. The alternative chemical environments will give two t(OAH  O) bridge protons in the cis-form, but only one in the trans-form. Observation of the t(OAH  O) in the i.r. spectrum and 1H-n.m.r spectra at two different frequencies in each case, indicate that the Ni(II) complexes are in the cis-form. According to the above results, square-planar geometry, exists for the Ni(II) complexes [17, 18].

seems to be due to both the p fi p* and n fi p* transitions of C@N, and charge-transfer transition arising from p electron interactions between the metal and ligand which involves either a metal-to-ligand or ligand-to-metal electron transfer [19, 20]. The general character of these spectra in very similar to that of the corresponding complexes of symmetrically disubstituted dioximate ligands. This is probably due to the fact that metal-to-ligand charge transfer and ligandto-metal charge transfer transitions have similar energy differences [21]. The weak d-d transitions of [Ni(L1H)2], [Ni(L2H)2] and [Ni(L2H)2] could be observed at 474, 460 and 536 nm in chloroform or ethanol, respectively. These absorption bands are typical for nickel(II) complexes with a square-planar structure [22, 23]. Thermal study The t.g.a. curves were obtained in a N2 atmosphere between 20 and 900 C. As seen in Figures 1 and 2, the inital decomposition temperatures are approximately the same for L1H2, L2H2 and L3H2 ligands (66.72 C). However, in the second step, decomposition temperatures are 104.85 C for L3H2, 168.39 C for L1H2and 176.34 C for L2H2. The mass loss is ca. 98% at 750 C for all ligands. The stabiliting of ligands are almost the same. Figure 3 shows the t.g.a curves for the Ni(II) complexes. The mass loss is ca. 78% up to 900 C for the Ni(II) complexes. However, the mass loss increases as the temperature rises. Decomposition points are 130.27, 165.22 and 152.51 C for [Ni(L1H)2], [Ni(L2H)2] and [Ni(L3H)2], respectively. Data also reveal that the most stable complex is [Ni(L2H)2] while the least stable is [Ni(L1H)2]. Electrochemistry Electrochemical properties of the Ni(II) complexes were studied by cyclic voltammetry in dimethylsulfoxide containing 0.05 M t-(n-butyl)ammonium perchlorate in the potential range 1 to )1.5 V, and the electrochemical data are summarized in Table 5. The cyclic voltammogram of Ni(L1H)2 exhibits a reduction peak at E 1pc=)1.30 V, with an associated re-oxidation peak at

Electronic spectra In the electronic spectra of the three new ligands and their Ni(II) metal complexes, the 243–416 nm band

Fig. 1. Mononuclear Ni(II) complexes of L1H2, L2H2 and L3H2.

762

Fig. 2. T.g.a. curves of L1H2, L2H2 and L3H2 ligands.

Fig. 3. T.g.a. curves of Ni(L1H)2, Ni(L2H)2 and Ni(L3H)2.

Table 5. Voltammetric data for Ni(II) complexes Complex

E1pa (V)

E1pc (V)

DEp(V)

E2pa (V)

E3pa (V)

E2pc (V)

DEp(V)

Ni(L1H)2 Ni(L2H)2 Ni(L3H)2

)1.21 )1.19 )1.19

)1.30 )1.31 )1.32

0.09 0.12 0.13

0.66 0.68 0.69

)0.65

)1.00

0.35

Supporting electrolyte=0.05 M n-Bu4NClO4, Scan rate=100 mVs)1.

E 1pa=)1.21 V. The peak separation for this couple is DEp=0.09 V. This is typical characteristic behaviour of a quasi-reversible one-electron transfer redox

process corresponding to the Ni(II)/Ni(I) couple as shown in Figure 4. The value of the anodic to cathodic peak current ratio (ipa/ipc) decreased and the

763

Fig. 5. A cyclic voltammogram of Ni(L2H)2 in DMSO containing 0.05 M n-Bu4NClO4 as the supporting electrolyte. Scan rate = 100 mV/s. Fig. 4. A cyclic voltammogram of Ni(L1H)2 in DMSO containing 0.05 M n-Bu4NClO4 as the supporting electrolyte. Scan rate = 100 mV/s.

difference between anodic and cathodic peak potentials gradually increased at faster scan rates. These data are diagnostic for a simple quasi-reversible one-electron charge transfer [24]. The plot of peak current versus the square root of the scan rate gives a positive slope indicating that the electrochemical process is diffusion controlled [24]. Ni(L1H)2 also shows another quasireversible one-electron transfer process caused by the presence of the cyclic ring moiety of the ligand. The oxidation peak occurs at E 3pa=)0.65 and the reduction peak at E 2pc=)1.00 V. The separation in the peak potential is DEp=0.35 V. However, the Ni(L1H)2 complex also exhibits an irreversible oxidation peak at E 2pa=0.66 V. This single oxidation peak may probably be attributed to the oxime part of the ligand. In the reverse scan, no peaks are observed in the cathodic branch indicating that the oxidation of the oxime functional group is irreversible. The peak potential shifted to more positive potentials at faster scan rates, confirming the irreversibility of the oxime based oxidation. Cyclic voltammograms of Ni(L2H)2 and Ni(L3H)2 are given in Figures 5 and 6, respectively. Electrochemical data obtained for these two complexes are similar. Both voltammograms exhibit a quasireversible one electron charge transfer reaction for Ni(II)/Ni(I) couples at negative potentials along with an irreversible oxidation peak at positive potentials for each complex. Diagnostic tests for Ni(L2H)2 and Ni(L3H)2 show that the electrochemical behaviour of both Ni(II) complexes are diffusion controlled. Data also revealed that the electron transfer rate is higher for Ni(L2H2) than other two metal complexes. The electrochemical properties of the synthesized Ni(II) complexes were also compared with the well-known metal complex, Ni(dimethylglyoximate)2 [Ni(DMG)2]. Voltammetric results showed that Ni(DMG)2 exhibits a quasi-reversible reduction peak at Epc=)1.28 V with

Fig. 6. A cyclic voltammogram of Ni(L3H)2 in DMSO containing 0.05 M n-Bu4NClO4 as the supporting electrolyte. Scan rate = 100 mV/s.

an associated re-oxidation peak at Epa=1.17 V. The separation in the peak potential is DEp=0.11 V. This redox pair is assigned to the quasi reversible one-electron transfer process of Ni(II)/Ni(I) of Ni(DMG)2 complex. However, the formal potential for Ni(DMG)2 complex is E=)1.22 V while it is about )1.25 V in the case of the synthesized Ni(II) complexes. This indicates that the synthesized Ni(II) complexes have higher electron donor affinities than that of Ni(DMG)2. Ni(DMG)2 also exhibits two individual oxidation peaks at Epa=)0.54 V and Epa=0.47 V. These two individual oxidation peaks are attributed to the oxidation of the ligand moiety of Ni(DMG)2. Consequently, electrochemical data have shown that the electrochemical behaviour of all of the Ni(II) complexes shows a quasi-reversible one-electron transfer reduction process in the cathodic region attributed to the Ni(II)/Ni(I) couple along with an irreversible oxidation peak attributed to the oxidation of the oxime moiety of the ligand in the positive region. However,

764 voltammetric experiments clearly indicated that the Ni(L1H)2 complex differs from Ni(L2H)2 and Ni(L3H)2 complexes upon the exhibition of another quasi-reversible oxidation/reduction electron transfer process in the negative region.

Acknowledgement This work was supported by the Scientific Research Council of Harran University (Project No. HUBAK506). References 1. R.C. Mehrotra, in Comprehensive Coordination Chemistry, G. Wilkinson, R.D. Gillard and J.A. McCleverty (Eds), Permagon Press, New York, 1988, vol. 2, p. 269. 2. P.A. Smith, The Chemistry of Open-Chain Organic Nitrogen Compounds, W.A. Benjamin (Ed.), New York, 1966, vol. 2; Y. Go¨k and H. Kantekin, New J. Chem., 19, 461 (1995). 3. B.G. Brown, Prog. Inorg.Chem., 18, 17 (1973). 4. A. Chakravorty, Coord. Chem. Rev., 13, 12 (1974). 5. K. Oguchi, K. Sanui and N. Ogata, Polym. Eng. Sci., 30, 449 (1990). 6. A. Cukurovali and E. Tas, Synth. React. Inorg. Met.-Org. Chem., 28, 449 (1998). 7. V. Ahsen, F. Gokceli and O. Bekaroglu, J. Chem. Soc. Dalton Trans., 2283 (1992).

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