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Oxidase studies of some benzimidazole diamide copper(II) complexes
Inorganica Chimica Acta 358 (2005) 1125–1134 www.elsevier.com/locate/ica
Oxidase studies of some benzimidazole diamide copper(II) complexes Farea Afreen a, Pavan Mathur
a,*
, Arnold Rheingold
b
a
b
Department of Chemistry, University of Delhi, Delhi 110 007, India Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, m/c 0358, La Jolla, CA 92093-0358, USA Received 22 September 2003; accepted 15 September 2004 Available online 7 February 2005
Abstract New bis benzimidazole diamide ligands, N,N 0 -bis(benzimidazolyl-2-methyl)-2,2 0 -thiadiethanamide (GBTAA), and N,N 0 bis(benzimidazolyl-2-methyl)-3,3 0 -thiadipropanamide (GBTPA) have been synthesised and utilised to prepare copper(II) complexes with inner sphere ligands like Cl and NO3 . One of the ligands, GBTAA, has been structurally characterised, while the other GBTPA is characterised via an unusual tetrabenzoate bridged dicopper polymeric structure wherein the ligand GBTPA bridges the two dicopper benzoate units. The coordination environment about each copper is five coordinate, while s value is found to be 0.32 indicating a distorted square pyramidal geometry. The copper(II) complexes catalyse the quenching of superoxide radical generated electrochemically. 2004 Elsevier B.V. All rights reserved. Keywords: Dihydrochloride; Tetracyanoethylene; Copper(II) complexes; Covalency; Uncomplexed
1. Introduction Copper containing metalloproteins carry out a variety of diverse redox reactions [1]. One of the forefront reaction is the dismutation of superoxide radical to O2 and H2O2. The copper(II) centre is cyclically reduced and oxidised by superoxide in superoxide dismutase [2a]. Studies have shown that the copper centre is cyclically reduced and oxidised by superoxide. Superoxide first reduces the Cu(II) centre of CuZnSOD to produce dioxygen and then another molecule of superoxide oxidises the Cu(I) centre of CuZnSOD to produce hydrogen peroxide [2a]. These reactions are vital for biological system. In the past decade, several reports have appeared wherein copper complexes have been reported as spectral mimics, while few reports have appeared [2b] which indicate *
Corresponding author. E-mail address: [email protected] (P. Mathur).
0020-1693/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2004.09.062
these complexes to be functional mimics of these enzymes. We have initiated a study of copper(II) complexes with bis benzimidazole diamide ligands that exhibit distorted five coordinate geometry [3]. The present study uses two new diamide ligands with thioether functionalities that have not been reported so far.
2. Experimental 2.1. Materials Glycine benzimidazole dihydrochloride was prepared following the procedure reported by Cescon and Day [4]. Freshly distilled solvents were used for all synthetic purposes. Spectroscopic grade solvents were employed for spectral works. All other chemicals were of AR grade and procured from standard commercial sources and were used as received.
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2.2. Physical measurements Elemental analysis data have been obtained from Micro Analytical Laboratory of RSIC, Punjab University, Chandigarh. UV spectral studies were performed on a Cary-100 UV–Vis and Shimadzu UV-1601 spectrophotometer, cyclic voltammetric data were obtained from BAS CV50W instrument and IR spectral data were obtained from Perkin–Elmer FTIR-2000 Spectrometer, as KBr pellets in solid state at the Department of Chemistry, University of Delhi; all other experimental details 1were as reported earlier [3b]. Electron Paramagnetic Resonance data were recorded on an X-band Varian E-112 spectrometer with a variable temperature liquid Nitrogen Cryostat at RSIC, IIT, Mumbai, India. TCNE (tetracyanoethylene), g = 2.00277, was used as a standard for g factor measurements. X-ray crystallography was carried out at the Department of Chemistry and Biochemistry, University of Delaware, USA. All software and sources of the scattering factors are contained in the SHELXTL program library, provided by Bruker AXS, Madison, WI.
2.3. Synthesis 2.3.1. N,N 0 -bis(benzimidazolyl-2-methyl)-2-2 0 -thiadiethanamide (GBTAA) The ligand was prepared following the procedure reported by Gupta et al. [3a]. A solution of glycine benzimidazole dihydrochloride (5.861 g, 0.02 mol) and 2,2 0 thiodiacetic acid (2 g, 0.01 mol) in 30 ml pyridine was stirred for 5 min till a white precipitate was obtained. Then, triphenyl phosphite (8.3 ml, 0.027 mol) was added to the reaction mixture at a temperature of 50 C and the temperature was slowly raised to 70–80 C. Within an hour, the precipitate redissolved and the stirring was continued for 48 h at 70–80 C. The resulting orange solution was allowed to cool and washed with saturated NaHCO3 solution in a separating funnel, till no effervescence could be seen. Then, it was washed with water 2–3 times. While washing, the oil solidified into a pale solid. Upon further washing with acetone, the solid turned white. This was dried and recrystallised by dissolving in hot methanol; upon cooling a white crystalline product was obtained. This was filtered, washed with cold water, dried in vacuo over P2O5 and was analysed for the composition C20H20N6O2 S Æ 1.5H2O Æ 0.5CH3OH: m.p.: 233 C, yield: 3.2 g (40.7%). Anal. Calc.: C, 54.54; H, 5.54; N, 18.62. Found: C, 54.15; H, 5.60; N, 18.42%. 1H NMR (d6-DMSO): 12.31 (s, 2H), 9.47–9.44 (t, 2H), 7.48–7.37 (q, 4H), 7.15–7.10 (q, 4H), 4.59–4.57 (d, 4H), 3.37 (s, 4H). UV knm max (log e; e in l mol1 cm1) (methanol): 280 (4.22), 274 (4.26), IR:
3375 (m), 3191 (s), 3022 (m), 1654 (s), 1577 (s), 1440 (s), 742 (s). The ligand GBTPA was prepared by a procedure similar to that adopted for GBTAA, except that 2,2 0 thiodiacetic acid was replaced by 3,3 0 -thiodipropionic acid. 2.3.2. N,N 0 -bis(benzimidazolyl-2-methyl) 3,3 0 thiadipropanamide (GBTPA) The crude product obtained was recrystallised by dissolving in hot methanol/water; a white product crystallised on extended cooling. The product was filtered, washed with cold water and dried in vacuo over P2O5. This was then analysed for composition C22H24N6O2S: m.p.: 202 C, yield: 2.9 g (41.3%). Anal. Calc.: C, 60.55; H, 5.50; N, 19.27. Found: C, 60.42; H, 5.40; N, 18.95%. 1H NMR (d6-DMSO): 12.17 (s, 2H), 8.63– 8.60 (t, 2H), 7.52–7.49 (q, 4H), 7.18–7.15 (q, 4H), 4.53–4.52 (d, 4H), 2.79–2.74 (t, 4H), 2.54–2.49 (t, 4H) 1 UV knm cm1) (methanol): 283 max (log e; e in l mol (3.86), 275 (3.88). IR: 3301 (s), 3054 (m), 1649 (s), 1556 (s), 1428 (s), 741 (s). 2.4. Synthesis of complexes 2.4.1. Cu(GBTAA)Cl2 A green solution of copper(II) chloride (0.5 mmol) was added to a suspension of the ligand GBTAA (0.5 mmol) in 25 ml of acetonitrile. The colour of the solution changed to blue upon mixing and the ligand dissolved slowly to give a clear solution. This solution was filtered and then stirred for 2 h when a bluish product started to appear. The solution was further concentrated on rotatory evaporator and the product was centrifuged, washed with acetonitrile 2–3 times and dried over CaCl2. This was then analysed for the composition, [Cu(C20H20N6O2S)Cl2] Æ CH3CN. Yield: 155 mg (54%). Anal. Calc.: C, 45.24; H, 3.94; N, 16.79; Cu, 10.88. Found: C, 45.77; H, 4.12; N, 17.09; Cu, 10.75%. 1 knm cm1); Solvent: (DMSO:methamax (log e; e in l mol nol, 1:9) 271 (4.55), 278 (4.54), 323 (3.53), 662 (2.32). IR (KBr, cm1): 3170 (m), 3011 (m), 1650 (s), 1586 (s), 1458 (s), 758 (s). All other complexes were prepared by a method similar to that adopted for the synthesis of [Cu(GBTAA)Cl2] except that the nitrate bound GBTAA complex was prepared in an ethanolic medium, while the rest of the complexes were prepared using methanol as solvent. 2.4.2. Cu(GBTPA)Cl2 The product was analysed for the composition, [Cu(C22H24 N6O2S)Cl2].3H2O. Yield: 208 mg (69%). Anal. Calc.: C, 42.27; H, 4.80; N, 13.45; Cu, 10.17. Found: C, 41.70; H, 4.67; N, 13.12; Cu, 10.0%. knm max (log e; e in l mol1 cm1); Solvent: (DMSO:methanol,
F. Afreen et al. / Inorganica Chimica Acta 358 (2005) 1125–1134
1:9): 274 (4.32), 281 (4.28), 312(sh), 761 (2.03). IR (KBr, cm1): 3449 (m), 3246 (m), 3062 (m), 1620 (s), 1579 (w), 1449 (m), 741 (s). 2.4.3. Cu(GBTAA)(NO3)2 This was analysed for the composition, [Cu(C20H20N6O2S)(NO3)2] Æ 1.5EtOH. Yield: 192 mg (59%). Anal. Calc.: C, 41.53; H, 4.36; N, 16.85; Cu, 9.56. Found: C, 41.48; H, 4.09; N, 16.85; Cu, 9.97%. knm max (log e; e in l mol1 cm1); Solvent: (DMSO:methanol, 1:9) 271 (4.55), 278 (4.55), 338 (3.77), 605 (2.64). IR (KBr, cm1): 3386 (m), 3196 (m), 3059 (m), 1652 (s), 1586 (s), 1459 (s), 1384 (s), 843 (w), 749 (s). 2.4.4. Cu(GBTPA)(NO3)2 This was analysed for the composition, [Cu(C22H24N6O2S)(NO3)2] Æ 2H2O. Yield: 195 mg (58%). Anal. Calc.: C, 40.03; H, 4.24; N, 16.98; Cu, 9.63. Found: C, 39.67; H, 3.91; N, 16.67; Cu, 9.81%. knm max (log e; e in l mol1 cm1); Solvent: (DMSO:methanol, 1:9): 273 (4.22), 280 (4.19), 315 (sh), 716 (2.01). IR (KBr, cm1): 3459 (m), 3243 (m), 3066 (m), 1622 (s), 1566 (m), 1451 (m), 1383 (s), 832 (m), 739 (s). 2.4.5. Cu2(GBTPA)(OOCC6H5)4 A green methanolic solution of Cu(OOCC6H5)2 (0.5 mmol) was prepared by dissolving Cu(OH)2 with a dilute solution of benzoic acid in MeOH. This solution was centrifuged and added to an ethanolic solution of GBTPA (0.5 mmol). The colour of the solution changed to light green. This resulting solution was stirred for 2 h and a light green product started to appear. The solution was concentrated and the product obtained was centrifuged, washed with ethanol 2–3 times and dried over P2O5. A small amount of the product was redissolved in warm ethanol and kept aside for a few weeks, when bluish green needle like crystals were obtained, suitable for XRD work. This was then analysed for the composition, [Cu2(C22H24N6O2S)(OOCC6H5)4] Æ 0.5EtOH. Yield: 190 mg (51%). Anal. Calc.: C, 57.19; H, 4.39; N, 7.85; Cu, 11.87. Found: C, 57.81; H, 4.37; N, 7.57; Cu, 12.10. knm max (log e; e in l mol1 cm1); Solvent: (DMSO:methanol, 1:9): 270 (4.27), 278 (4.24), 313 (3.61), 711 (2.61). IR (KBr, cm1): 3396 (m), 3273 (m), 3066 (w), 1630 (s), 1602 (s), 1546 (m), 1449 (m), 720 (s). 2.5. Results and discussion 2.5.1. X-ray crystallography The new ligand GBTAA was characterised by structural determination, while the other GBTPA is characterised via an unusual tetrabenzoate bridged dicopper polymer where ligand GBTPA acts as a bridge between two dicopper units. A crystal of GBTAA and of [Cu2(GBTPA)(PhCO2)4] suitable for
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data collection was selected and mounted with epoxy cement on the tip of a fine glass fibre. A data set for both structures was collected on a Bruker platform diffractometer with an APEX detector at 100 K. The monoclinic space group P21/c (in the case of GBTAA) and C2/c (in case of [Cu2(GBTPA)(PhCO2)4] was chosen on the basis of photographic and intensity data. Solution in the reported space group yielded chemically reasonable and computationally stable results of refinement. Corrections for absorption were performed using SADABS program contained in the Bruker library of programs. The structure was solved by using direct methods, computed by subsequent difference Fourier Syntheses, and refined by full-matrix least-square procedures. All non-hydrogen atoms were refined with anisotropic displacement coefficients and hydrogen atoms were placed in idealised locations. Crystal data are provided in Table 1. An ORTEP plot for the ligand GBTAA is shown in Fig. 1, while that for [Cu2(GBTPA)(PhCO2)4] is shown in Fig. 2(a) and (b). 2.5.2. Comparison of bond lengths for GBTAA and GBTPA in its benzoate complex Bond lengths in the ligands GBTAA and GBTPA are slightly different from each other only with regard to S–C bonds. S–C bond varies in the range 1.789– ˚ for GBTAA, while the value observed for 1.827 A ˚ . Two S–C bond in [Cu2(GBTPA)(PhCO2)4] is 1.80 A carbonyl oxygen C–O bond lengths are similar in ˚ . One of GBTAA and are of the order of 1.23 A ˚, the two C–O bonds in GBTPA complex is 1.23 A ˚ which may while the other gets reduced to 1.216 A be due to complexation. Two benzimidazole N–C bonds are also of the same order in both the cases, ˚. i.e., 1.316 and 1.356 A 2.5.3. Description of crystal structure of [Cu2(GBTPA)(OOCC6H5)4] Crystal structure of the complex [Cu2(GBTPA)(OOCC6 H5)4] reveals that it belongs to the group C2/C of a monoclinic system (Table 2, Fig. 2). The X-ray diffraction analysis evidences a polymeric structure with two tetrabenzoate bridged dicopper units (Cu1)2-GBTPA–(Cu2)-GBTPA-containing two crystallographically independent copper ions Cu1 and Cu2. Each dinuclear core is bridged by four benzoate groups (Cu1–Cu1 0 and Cu2–Cu2 0 distances of 2.78 and 2.69, respectively) with GBTPA acting as a bridge between the copper dimers. The benzoate groups coordinate to copper(II) in a bidentate fashion resulting in square pyramidal coordination environment around each copper atom, which consists of four oxygen donors of benzoate anion in the basal plane and of benzimidazole nitrogen atom in the apical position.
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Table 1 Crystal data and structure refinement for GBTAA and [Cu2(GBTPA)(OOCC6H5)4] Compound
GBTAA
[Cu2(GBTPA)(OOCC6H5)4]
Empirical formula Formula weight Temperature (K) ˚) Wavelength (A Crystal system Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) Volume (A Z Dcalc (mg/m3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm3) h Range for data collection () Index ranges Reflections collected Independent reflections [Rint] Completeness to h = 24.75 Absorption correction Maximum and minimum transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data) ˚ 3) Largest difference peak and hole (e A
C20H20N6O2S 408.48 218(2) 0.71073 monoclinic P21/C
C50H44Cu2N6O10S 1048.05 218(2) 0.71073 monoclinic C2/c
14.2886(12) 8.5582(8) 18.9231(17) 90 108.782(2) 90 2190.8(3) 4 1.238 0.175 856 0.30 · 0.20 · 0.20 2.27–24.75 1.6 6 h 6 16, 10 6 k 6 9, 22 6 l 6 22 12 535 3747 [0.0305] 99.8% none 0.9659 and 0.9494 full-matrix least-squares on F2 3747/0/262 1.039 R1 = 0.0397, wR2 = 0.0930 R1 = 0.0488, wR2 = 0.0964 0.172 and 0.295
27.525(2) 8.5582(8) 28.916(3) 90 107.171(2) 90 9381.1(14) 8 1.484 1.018 4320 0.30 · 0.20 · 0.10 1.550–24.00 31 6 h 6 30, 13 6 k 6 14, 33 6 l 6 32 24 888 7366 [0.0630] 100.0% none 0.9050 and 0.7499 full-matrix least-squares on F2 7366/0/605 1.120 R1 = 0.0681, wR2 = 0.1460 R1 = 0.0995, wR2 = 0.1589 0.629 and 0.456
Fig. 1. ORTEP plot for GBTAA.
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Fig. 2. (a) ORTEP plot for the complex [Cu2(GBTPA)(OOC6H5)4]. (b) Stereodrawing of the unit cell contents of [Cu2(GBTPA)(OOC6H5)4].
s value for the complex is found to be 0.32, indicating sufficient distortion of square pyramidal geometry [5]. Four oxygen atoms from two benzoate groups occupy the vertices of the square, while nitrogen of benzimidazole acts as the axial ligand. Equatorial bond angles deviate slightly from the expected bond angle of 90, the maximum deviation being 3.32. The axial bond angles deviate in the range 1.53–16.6. The sum of equatorial bond angles at Cu is 357.27 showing that the ligands are accommodated in the plane (Table 2). ˚ [Cu(2)–N(6)] and The Cu–Nbenzim distance of 2.149 A ˚ [Cu(1)–N(1)] is in the range found for similar 2.179 A complexes [6,7]. It is significantly longer than the Cu–O bond distances, confirming it to be in axial position of a square pyramid.
The Cu(1)–O bond length varies in the range of ˚ , while that for Cu(2)–O bond length var1.936–2.054 A ˚ for benzoate O atom ies in the range of 1.956–1.995 A coordination. This is in agreement with other complexes having similarly bridging carboxylate oxygen atoms where Cu–O bond distances are in the range of ˚ [8,9]. 1.9632–2.002 A The structure is unusual as the ligand is coordinating only through imidazole nitrogen but is quite normal considering what is known about such complexes. It is interesting to note that in this copper complex, the benzimidazole rings of the GBTPA ligand acquire a cis conformation in comparison to the uncomplexed ligand GBTAA wherein the benzimidazole rings acquires a trans conformation.
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Table 2 ˚] Selected bond lengths [A [Cu2(GBTPA)(OOCC6H5)4] Cu(1)O(5) Cu(1)–O(6) Cu(1)–O(3) Cu(1)–O(4) Cu(1)–N(1) Cu(1)–Cu(1) #1 C(23)–O(5) C(23)–O(3) #1 C(30)–O(6) C(30–O(4) #1 Cu(2)–O(8) Cu(2)–O(9) Cu(2)–O(7) Cu(2)–O(10) Cu(2)–N(6) Cu(2)–Cu(2) #2 O(7)–C(44) # 2 O(8)–Cu(37) #2 O(9)–C(37) O(10)–C(44)
1.936 (4) 1.955 (4) 1.992 (4) 2.054 (4) 2.179 (4) 2.7821 (14) 1.253 (7) 1.267 (7) 1.250 (7) 1.257 (7) 1.956 (4) 1.960 (4) 1.972 (4) 1.995 (5) 2.149 (4) 2.6975 (12) 1.243 (6) 1.251 (6) 1.253 (6) 1.235 (7)
and
bond
angles
O(5)–Cu(1)–O(6) O(5)–Cu(1)–O(3) O(6)–Cu(1)–O(3) O(5)–Cu(1)–O(4) O(6)–Cu(1)–O(4) O(3)–Cu(1)–O(4) O(5)–Cu(1)–N(1) O(6)–Cu(1)–N(1) O(3)–Cu(1)–N(1) O(4)–Cu(1)–N(1) O(8)–Cu(2)–O(9) O(8)–Cu(2)–O(7) O(9)–Cu(2)–O(7) O(8)–Cu(2)–O(10) O(9)–Cu(2)–O(10) O(7)–Cu(2)–O(10) O(8)–Cu(2)–N(6) O(9)–Cu(2)–N(6) O(7)–Cu(2)–N(6) O(10)–Cu(2)–N(6)
[]
for
173.40 (17) 92.02 (17) 88.97 (18) 89.60 (17) 86.68 (17) 154.10 (16) 94.75 (16) 91.53 (17) 99.02 (17) 106.60 (16) 165.77 (16) 91.4 (2) 88.20 (18) 86.6 (2) 88.32 (19) 165.74 (16) 92.87 (17) 101.133 (16) 96.00 (17) 98.25 (17)
However, this cannot be considered as a representative structure for the group of complexes generated with the ligand GBTPA but it clearly shows that this ligand can bridge two Cu(II) atoms through its benzimidazole imine N-atom, a feature that may be common to other complexes as well. We have therefore tentatively represented the structure of other complexes as a 2:2 ligand bridged structure with an axial carbonyl coordination (Fig. 8). There is sufficient IR evidence in favour of carbonyl coordination. Increase in Amide I band and increase or decrease in amide II band is indicative of carbonyl coordination in complexes, with similar diamide type ligands [3,10]. 2.5.4. Electronic spectroscopy, cyclic voltammetry and EPR studies The electronic spectra of the copper(II) complexes were measured in mixed DMSO/methanol solvent system. Two peaks in the range 270–285 nm are observed for all the complexes, which have been assigned to p– p* transition of benzimidazole moiety in their respective ligands [7]. The bands show enhanced absorptions as indicated by their extinction coefficients. A band is observed in the region 300–400 nm for the complexes generated with GBTAA and GBTPA, which is assigned to imidazole p ! Cu2+ charge transfer transition [7]. All the complexes exhibit a single symmetrical but broad d–d band in the region 600–850 nm (Fig. 3), characteristic of tetragonal geometry. In general, these bands are assigned to the overlapping transitions dxz; yz ! dx2 y 2 and dxy ! dx2 y 2 . It is well established that greater covalency results in lowering of energy of dx2 y 2 orbital and therefore the en-
Fig. 3. Visible spectra for the complexes of GBTAA and GBTPA in DMSO/methanol. (a) [Cu(GBTAA)Cl2], (b) [Cu(GBTAA)(NO3)2], (c) [Cu2(GBTPA)(OOC6H5)], (d) [Cu(GBTPA)Cl2], (e) [Cu(GBTPA)(NO3)].
ergy of dxz; yz =dxy ! dx2 y 2 transition. This is reflected in the kmax of the two series, where with GBTAA the energies of the d–d band are higher than those for the GBTPA series. All the complexes display a quasireversible redox wave due to Cu(II)/Cu(I) process (Table 3, Fig. 4). Anodic shifts in E1/2 values indicate the retention of anion in the coordination sphere of Cu(II). When calculated against NHE, the E1/2 values of the GBTPA series of Cu(II) complexes turn out to be quite positive as compared to the GBTAA, Cu(II) complexes, a feature reflecting the greater covalency of the GBTPA series. The GBTPA E1/2 are also quite positive when compared with other benzimidazole [11,12] and some amide [13] based complexes. X-Band EPR spectra of the copper(II) complexes were recorded in solution state (DMSO and DMF used as solvent) and solid state at liquid nitrogen temperature (Table 3). Solution spectra typically indicate a dx2 y 2 ground state (gi > g^ > 2.0023) and reveal less than four gi lines; the g^ line is quite broad in all
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Table 3 Electrochemical and X-band EPR data for the complexes of GBTAA, GBTPA and GBTBA Compound
[Cu(GBTAA)Cl2] [Cu(GBTAA)(NO3)2] [Cu(GBTPA)Cl2] [Cu(GBTPA)(NO3)2] [Cu2 (GBTPA)(OOCC6H5)4] a b
E1/2 (mV) vs. Ag/AgNO3
vs. NHE
42a 195a 56a 70a 365 (EPc)b
617 380 631 645 225
gi
g^
g1, g2, g3
Ai
gi/Ai · 104
2.33 2.30 2.29 2.32 2.31
2.075 2.075 2.085 2.074 2.073
2.19, 2.14, 2.37, 2.29,
143 157 147 149 164
162 146 156 155 141
2.07 2.06 2.11, 1.93 2.15
Ferrocene 75 mV against Ag/AgNO3. Ferrocene 60 mV against Ag/AgNO3.
the cases, thus indicating a distorted tetragonal geometry. The Ai versus gi values for the complexes do not lie on the Peisach Blumberg Plots [14], typical for a N2O2 type equatorial plane; this also suggests a distortion of the equatorial plane. The values of hyperfine coupling constant Ai are quite low in comparison to the normal range found for other copper(II) complexes [15], reinforcing a marked tetrahedral distortion of the tetragonal site [16,17]. Further, no nitrogen super hyperfine splitting could be observed, due to non planarity of the complexes. For the dimeric copper benzoate complex, EPR spectra
are typical of a monomeric copper(II) complex. It appears that this complex breaks down to monomeric units in DMSO solution. 2.6. Superoxide quenching study The cyclic voltammograms of Cu(II) complexes in DMSO solution exhibit a quasi reversible one electron redox couple in the range 0.3 to 0.1 V, on a glassy carbon electrode. No other redox wave is observed for the complexes beyond this region. Fig. 5 shows a cyclic voltammogram of molecular oxygen saturated in DMSO, obtained by bubbling pure dry O2 in a septum sealed
Fig. 4. Cyclic voltammogram for the complexes of GBTAA and GBTPA in 2:8 DMSO:CH3CN at scan rate, 100 mV/s (a) [Cu(GBTAA)Cl2], (b) [Cu(GBTPA)Cl2], (c) [Cu(GBTAA)(NO3)2], (d) [Cu(GBTPA)(NO3)2].
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Fig. 6. Plot of total peak current (lA) vs. concentration of (a) [Cu(GBTAA)Cl2], (b) [Cu(GBTPA)Cl2].
wave for O2 to O2 is quenched. This irreversibility of the O2 =O2 wave indicates an interaction of the superoxide anion with copper complex. The effect of this interaction is also visible on the Cu(II)/Cu(I) redox couple where the reoxidation of Cu(I) is also severely inhibited. CuI þ O2 ! CuII þ O2 2 Fig. 5. Cyclic voltammogram of DMSO solution containing 0.1 M Bu4N(ClO4). (a) Saturated with oxygen, (b) containing 0.6 mM [Cu(GBTAA)Cl2].
DMSO solution for 20 min, in the absence and presence of Cu(II) complex, recorded at a glassy carbon working electrode. In the absence of Cu(II) complexes, the dioxygen molecule exhibits a reversible couple at E1/2 in the range of 1.10 to 1.19 V due to O2 =O2 redox couple [18]. The reversibility of the redox couple indicates the stability of superoxide anion in the supporting electrolyte, at least on the voltammetric time scale. The slight departure of ia/ic from unity could be due to little moisture leaking into the solvent system. In the presence of Cu(II) complexes, during anodic scan the reoxidation
ð1Þ
As a result of this fast reaction, all superoxide generated gets chemically oxidised and is not available for electrochemical oxidation. Therefore, the reoxidation wave for O2 to O2 is drastically decreased in height. Similarly, all CuI in the solution is chemically oxidised to Cu(II) and hence no electrochemical oxidation wave for the process Cu(I)/Cu(II) is observed in the anodic scan. The quenching of superoxide anion was also studied by a gradual titration with the complexes. The starting concentration of complex was 0.05 M and that of [O2] was 2.0 mM. 0.02 ml of the complex (0.05 M) was added to the reaction solution each time, and the total drop in current (both anodic and cathodic) was measured voltammetrically. Care was taken not to expose the solution to air. A plot of total current versus concen-
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tration of complex displays a gradual decrease in current value with concentration (Fig. 6). The curve exhibits a break point that varies with the nature of complex. The stoichiometry of O2 (consumed):complex turns out to be 5:1 for the complexes of GBTAA and GBTPA with a small inner sphere ligand like chloride. This stoichiometry can be explained in terms of the catalyst turn-
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ing over at least five times. The two reactions occurring in the solution are as follows: 9 2þ electrochemical þ > ½CuL !½CuL > = 2þ þ chemical 2 ½CuL þ O2 !½CuL O2 > Catalytic cycle > ;
Fig. 7. Plot of total peak current (lA) vs. concentration of (a) [Cu(GBTAA)(NO3)2], (b) [Cu(GBTPA)(NO3)2].
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F. Afreen et al. / Inorganica Chimica Acta 358 (2005) 1125–1134 0 2þ electrochemical
½ðCuLÞ
9 > > =
0 þ
!½ðCuLÞ
0 2þ
½ðCuLÞ
þ
chemical
þ
þ O2 ! ½ðCuLÞ00 O2 2
½ðCuLÞ00 O2 2 ! ½Cu
2þ
þ oxidised products of ligands:
At potentials beyond 0.4 V, Cu(II) complex is present in reduced form [(CuL)]+. This Cu(I) reacts with superoxide generated electrochemically to form peroxide O2 2 and oxidised complex [(CuL)]2+. The oxidised complex is similar to the original copper(II) complex and can again be electrochemically reduced to Cu(I) complex [(CuL)]+ which is equally effective in quenching the superoxide ion catalytically, and this forms the catalytic cycle. In the termination step, the cycle yields back the oxidised complex [(CuL) 0 ]2+ and peroxide ion. It seems at this stage that some stereochemical changes have arisen in the oxidised complex [(CuL) 0 ]2+ making it ineffective to carry out the cycle further, i.e., either it cannot be further reduced or if reduced, the reduced product is not capable of further reoxidation by O2 . The termination step may also involve the generation of oxidatively degraded copper(II) complexes. Thus, the cycle does not proceed beyond five steps in the case of the Cu(II) complexes with GBTAA and GBTPA and for each mole of copper complex added, five moles of oxygen is consumed (see Fig. 7). For the nitrate complexes, however the case is different where no such turnover activity was observed. There, however, is a stoichiometric conversion, the con-
X
N
N
N
N
Cu
X
N n
O
O
N n
S n
S
N
O
O
n O
N N
N
Cu X
X N
N
n = 1 for GBTAA n = 2 for GBTPA X = Cl−, NO3−, C6H5COO− Fig. 8. Proposed structure for the copper complexes of GBTAA and GBTPA.
> > ;
Termination step
centration versus total peak current graphs display a break point at [Catalyst] 1.0 mM and the [O2]:[complex] stoichiometry turns out to be 1:1.
Acknowledgements We gratefully acknowledge financial support from the Department of Science and Technology, India for Project No. SR/S1/IC-35/2003. References [1] E.I. Solomon, U.M. Sundaram, T.E. Machonkin, Chem. Rev. 96 (1996) 2563. [2] (a) L.M. Ellerby, D.E. Cabelli, J.A. Graden, J.S. Valentine, J. Am. Chem. Soc. 118 (1996) 6556; (b) B.P Murphy, Coord. Chem. Rev. 124 (1993) 63. [3] (a) M. Gupta, P. Mathur, R.J. Butcher, Inorg. Chem. 40 (2001) 878; (b) M. Gupta, S.K. Das, P. Mathur, A.W. Cordes, Inorg. Chim. Acta 353 (2003) 197. [4] L.A. Cescon, A.R. Day, J. Org. Chem. 27 (1962) 581. [5] P.J.M.W.L. Birker, J. Helder, G. Henkel, B. Krebs, J. Reedijk, Inorg. Chem. 21 (1982) 357. [6] F.J. Reitmeijer, P.J.M.W.L. Birker, S. Gorter, J. Reedijk, J. Chem. Soc., Dalton Trans. (1982) 1191. [7] E. Monzani, L. Quinti, A. Perotti, L. Casella, M. Gullotti, L. Randaccio, S. Geremia, G. Nardin, P. Faleschini, G. Tabbi, Inorg. Chem. 37 (1998) 553. [8] Y.H. Chung, H.H. Wei, Y.H. Liu, G.H. Lee, Y. Wang, Polyhedron 17 (1998) 449. [9] S. Dalai, P.S. Mukherjee, E. Zangrando, F. Lloret, N.R. Chaudhuri, J. Chem. Soc., Dalton Trans. (2002) 822. [10] M. Nonoyama, K. Yamasaki, Inorg. Chim. Acta 3 (1969) 585. [11] (a) Y. Nakao, M. Onoda, T. Sakurai, A. Nakahara, L. Kinoshita, S. Ooi, Inorg. Chim. Acta 55 (1988) 155; (b) A.W. Addison, H.M.J. Henricks, J. Reejidk, L.K. Thompson, Inorg. Chem. 20 (1) (1981) 103. [12] (a) U. Sivagnanam, M. Palaniandavar, J. Chem. Soc., Dalton Trans. (1994) 2277; (b) M. Palaniandavar, T. Pandiyan, M. Laxminarayan, H. Manohar, J. Chem. Soc., Dalton Trans. (1995) 457. [13] A.K. Patra, M. Ray, R. Mukherjee, J. Chem. Soc., Dalton Trans. (1999) 2461. [14] W.E. Blumberg, J. Peisach, J. Biol. Chem. 240 (1965) 870. [15] B.C. Malmstrom, T. Vanngard, J. Mol. Biol. 2 (1960) 118. [16] (a) E.C. Seebauer, E.P. Duliba, R.B. Gennis, R.L. Belford, J. Am. Chem. Soc. 105 (1983) 4926; (b) U. Sakaguchi, A.W. Addison, J. Chem. Soc., Dalton Trans. (1979) 600. [17] G. Batra, P. Mathur, Inorg. Chem. 31 (1992) 1575. [18] Rajesh, S.K. Das, P. Mathur, Polyhedron 16 (20) (1997) 3511.
Oxidase studies of some benzimidazole diamide copper(II) complexes Farea Afreen a, Pavan Mathur
a,*
, Arnold Rheingold
b
a
b
Department of Chemistry, University of Delhi, Delhi 110 007, India Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, m/c 0358, La Jolla, CA 92093-0358, USA Received 22 September 2003; accepted 15 September 2004 Available online 7 February 2005
Abstract New bis benzimidazole diamide ligands, N,N 0 -bis(benzimidazolyl-2-methyl)-2,2 0 -thiadiethanamide (GBTAA), and N,N 0 bis(benzimidazolyl-2-methyl)-3,3 0 -thiadipropanamide (GBTPA) have been synthesised and utilised to prepare copper(II) complexes with inner sphere ligands like Cl and NO3 . One of the ligands, GBTAA, has been structurally characterised, while the other GBTPA is characterised via an unusual tetrabenzoate bridged dicopper polymeric structure wherein the ligand GBTPA bridges the two dicopper benzoate units. The coordination environment about each copper is five coordinate, while s value is found to be 0.32 indicating a distorted square pyramidal geometry. The copper(II) complexes catalyse the quenching of superoxide radical generated electrochemically. 2004 Elsevier B.V. All rights reserved. Keywords: Dihydrochloride; Tetracyanoethylene; Copper(II) complexes; Covalency; Uncomplexed
1. Introduction Copper containing metalloproteins carry out a variety of diverse redox reactions [1]. One of the forefront reaction is the dismutation of superoxide radical to O2 and H2O2. The copper(II) centre is cyclically reduced and oxidised by superoxide in superoxide dismutase [2a]. Studies have shown that the copper centre is cyclically reduced and oxidised by superoxide. Superoxide first reduces the Cu(II) centre of CuZnSOD to produce dioxygen and then another molecule of superoxide oxidises the Cu(I) centre of CuZnSOD to produce hydrogen peroxide [2a]. These reactions are vital for biological system. In the past decade, several reports have appeared wherein copper complexes have been reported as spectral mimics, while few reports have appeared [2b] which indicate *
Corresponding author. E-mail address: [email protected] (P. Mathur).
0020-1693/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2004.09.062
these complexes to be functional mimics of these enzymes. We have initiated a study of copper(II) complexes with bis benzimidazole diamide ligands that exhibit distorted five coordinate geometry [3]. The present study uses two new diamide ligands with thioether functionalities that have not been reported so far.
2. Experimental 2.1. Materials Glycine benzimidazole dihydrochloride was prepared following the procedure reported by Cescon and Day [4]. Freshly distilled solvents were used for all synthetic purposes. Spectroscopic grade solvents were employed for spectral works. All other chemicals were of AR grade and procured from standard commercial sources and were used as received.
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2.2. Physical measurements Elemental analysis data have been obtained from Micro Analytical Laboratory of RSIC, Punjab University, Chandigarh. UV spectral studies were performed on a Cary-100 UV–Vis and Shimadzu UV-1601 spectrophotometer, cyclic voltammetric data were obtained from BAS CV50W instrument and IR spectral data were obtained from Perkin–Elmer FTIR-2000 Spectrometer, as KBr pellets in solid state at the Department of Chemistry, University of Delhi; all other experimental details 1were as reported earlier [3b]. Electron Paramagnetic Resonance data were recorded on an X-band Varian E-112 spectrometer with a variable temperature liquid Nitrogen Cryostat at RSIC, IIT, Mumbai, India. TCNE (tetracyanoethylene), g = 2.00277, was used as a standard for g factor measurements. X-ray crystallography was carried out at the Department of Chemistry and Biochemistry, University of Delaware, USA. All software and sources of the scattering factors are contained in the SHELXTL program library, provided by Bruker AXS, Madison, WI.
2.3. Synthesis 2.3.1. N,N 0 -bis(benzimidazolyl-2-methyl)-2-2 0 -thiadiethanamide (GBTAA) The ligand was prepared following the procedure reported by Gupta et al. [3a]. A solution of glycine benzimidazole dihydrochloride (5.861 g, 0.02 mol) and 2,2 0 thiodiacetic acid (2 g, 0.01 mol) in 30 ml pyridine was stirred for 5 min till a white precipitate was obtained. Then, triphenyl phosphite (8.3 ml, 0.027 mol) was added to the reaction mixture at a temperature of 50 C and the temperature was slowly raised to 70–80 C. Within an hour, the precipitate redissolved and the stirring was continued for 48 h at 70–80 C. The resulting orange solution was allowed to cool and washed with saturated NaHCO3 solution in a separating funnel, till no effervescence could be seen. Then, it was washed with water 2–3 times. While washing, the oil solidified into a pale solid. Upon further washing with acetone, the solid turned white. This was dried and recrystallised by dissolving in hot methanol; upon cooling a white crystalline product was obtained. This was filtered, washed with cold water, dried in vacuo over P2O5 and was analysed for the composition C20H20N6O2 S Æ 1.5H2O Æ 0.5CH3OH: m.p.: 233 C, yield: 3.2 g (40.7%). Anal. Calc.: C, 54.54; H, 5.54; N, 18.62. Found: C, 54.15; H, 5.60; N, 18.42%. 1H NMR (d6-DMSO): 12.31 (s, 2H), 9.47–9.44 (t, 2H), 7.48–7.37 (q, 4H), 7.15–7.10 (q, 4H), 4.59–4.57 (d, 4H), 3.37 (s, 4H). UV knm max (log e; e in l mol1 cm1) (methanol): 280 (4.22), 274 (4.26), IR:
3375 (m), 3191 (s), 3022 (m), 1654 (s), 1577 (s), 1440 (s), 742 (s). The ligand GBTPA was prepared by a procedure similar to that adopted for GBTAA, except that 2,2 0 thiodiacetic acid was replaced by 3,3 0 -thiodipropionic acid. 2.3.2. N,N 0 -bis(benzimidazolyl-2-methyl) 3,3 0 thiadipropanamide (GBTPA) The crude product obtained was recrystallised by dissolving in hot methanol/water; a white product crystallised on extended cooling. The product was filtered, washed with cold water and dried in vacuo over P2O5. This was then analysed for composition C22H24N6O2S: m.p.: 202 C, yield: 2.9 g (41.3%). Anal. Calc.: C, 60.55; H, 5.50; N, 19.27. Found: C, 60.42; H, 5.40; N, 18.95%. 1H NMR (d6-DMSO): 12.17 (s, 2H), 8.63– 8.60 (t, 2H), 7.52–7.49 (q, 4H), 7.18–7.15 (q, 4H), 4.53–4.52 (d, 4H), 2.79–2.74 (t, 4H), 2.54–2.49 (t, 4H) 1 UV knm cm1) (methanol): 283 max (log e; e in l mol (3.86), 275 (3.88). IR: 3301 (s), 3054 (m), 1649 (s), 1556 (s), 1428 (s), 741 (s). 2.4. Synthesis of complexes 2.4.1. Cu(GBTAA)Cl2 A green solution of copper(II) chloride (0.5 mmol) was added to a suspension of the ligand GBTAA (0.5 mmol) in 25 ml of acetonitrile. The colour of the solution changed to blue upon mixing and the ligand dissolved slowly to give a clear solution. This solution was filtered and then stirred for 2 h when a bluish product started to appear. The solution was further concentrated on rotatory evaporator and the product was centrifuged, washed with acetonitrile 2–3 times and dried over CaCl2. This was then analysed for the composition, [Cu(C20H20N6O2S)Cl2] Æ CH3CN. Yield: 155 mg (54%). Anal. Calc.: C, 45.24; H, 3.94; N, 16.79; Cu, 10.88. Found: C, 45.77; H, 4.12; N, 17.09; Cu, 10.75%. 1 knm cm1); Solvent: (DMSO:methamax (log e; e in l mol nol, 1:9) 271 (4.55), 278 (4.54), 323 (3.53), 662 (2.32). IR (KBr, cm1): 3170 (m), 3011 (m), 1650 (s), 1586 (s), 1458 (s), 758 (s). All other complexes were prepared by a method similar to that adopted for the synthesis of [Cu(GBTAA)Cl2] except that the nitrate bound GBTAA complex was prepared in an ethanolic medium, while the rest of the complexes were prepared using methanol as solvent. 2.4.2. Cu(GBTPA)Cl2 The product was analysed for the composition, [Cu(C22H24 N6O2S)Cl2].3H2O. Yield: 208 mg (69%). Anal. Calc.: C, 42.27; H, 4.80; N, 13.45; Cu, 10.17. Found: C, 41.70; H, 4.67; N, 13.12; Cu, 10.0%. knm max (log e; e in l mol1 cm1); Solvent: (DMSO:methanol,
F. Afreen et al. / Inorganica Chimica Acta 358 (2005) 1125–1134
1:9): 274 (4.32), 281 (4.28), 312(sh), 761 (2.03). IR (KBr, cm1): 3449 (m), 3246 (m), 3062 (m), 1620 (s), 1579 (w), 1449 (m), 741 (s). 2.4.3. Cu(GBTAA)(NO3)2 This was analysed for the composition, [Cu(C20H20N6O2S)(NO3)2] Æ 1.5EtOH. Yield: 192 mg (59%). Anal. Calc.: C, 41.53; H, 4.36; N, 16.85; Cu, 9.56. Found: C, 41.48; H, 4.09; N, 16.85; Cu, 9.97%. knm max (log e; e in l mol1 cm1); Solvent: (DMSO:methanol, 1:9) 271 (4.55), 278 (4.55), 338 (3.77), 605 (2.64). IR (KBr, cm1): 3386 (m), 3196 (m), 3059 (m), 1652 (s), 1586 (s), 1459 (s), 1384 (s), 843 (w), 749 (s). 2.4.4. Cu(GBTPA)(NO3)2 This was analysed for the composition, [Cu(C22H24N6O2S)(NO3)2] Æ 2H2O. Yield: 195 mg (58%). Anal. Calc.: C, 40.03; H, 4.24; N, 16.98; Cu, 9.63. Found: C, 39.67; H, 3.91; N, 16.67; Cu, 9.81%. knm max (log e; e in l mol1 cm1); Solvent: (DMSO:methanol, 1:9): 273 (4.22), 280 (4.19), 315 (sh), 716 (2.01). IR (KBr, cm1): 3459 (m), 3243 (m), 3066 (m), 1622 (s), 1566 (m), 1451 (m), 1383 (s), 832 (m), 739 (s). 2.4.5. Cu2(GBTPA)(OOCC6H5)4 A green methanolic solution of Cu(OOCC6H5)2 (0.5 mmol) was prepared by dissolving Cu(OH)2 with a dilute solution of benzoic acid in MeOH. This solution was centrifuged and added to an ethanolic solution of GBTPA (0.5 mmol). The colour of the solution changed to light green. This resulting solution was stirred for 2 h and a light green product started to appear. The solution was concentrated and the product obtained was centrifuged, washed with ethanol 2–3 times and dried over P2O5. A small amount of the product was redissolved in warm ethanol and kept aside for a few weeks, when bluish green needle like crystals were obtained, suitable for XRD work. This was then analysed for the composition, [Cu2(C22H24N6O2S)(OOCC6H5)4] Æ 0.5EtOH. Yield: 190 mg (51%). Anal. Calc.: C, 57.19; H, 4.39; N, 7.85; Cu, 11.87. Found: C, 57.81; H, 4.37; N, 7.57; Cu, 12.10. knm max (log e; e in l mol1 cm1); Solvent: (DMSO:methanol, 1:9): 270 (4.27), 278 (4.24), 313 (3.61), 711 (2.61). IR (KBr, cm1): 3396 (m), 3273 (m), 3066 (w), 1630 (s), 1602 (s), 1546 (m), 1449 (m), 720 (s). 2.5. Results and discussion 2.5.1. X-ray crystallography The new ligand GBTAA was characterised by structural determination, while the other GBTPA is characterised via an unusual tetrabenzoate bridged dicopper polymer where ligand GBTPA acts as a bridge between two dicopper units. A crystal of GBTAA and of [Cu2(GBTPA)(PhCO2)4] suitable for
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data collection was selected and mounted with epoxy cement on the tip of a fine glass fibre. A data set for both structures was collected on a Bruker platform diffractometer with an APEX detector at 100 K. The monoclinic space group P21/c (in the case of GBTAA) and C2/c (in case of [Cu2(GBTPA)(PhCO2)4] was chosen on the basis of photographic and intensity data. Solution in the reported space group yielded chemically reasonable and computationally stable results of refinement. Corrections for absorption were performed using SADABS program contained in the Bruker library of programs. The structure was solved by using direct methods, computed by subsequent difference Fourier Syntheses, and refined by full-matrix least-square procedures. All non-hydrogen atoms were refined with anisotropic displacement coefficients and hydrogen atoms were placed in idealised locations. Crystal data are provided in Table 1. An ORTEP plot for the ligand GBTAA is shown in Fig. 1, while that for [Cu2(GBTPA)(PhCO2)4] is shown in Fig. 2(a) and (b). 2.5.2. Comparison of bond lengths for GBTAA and GBTPA in its benzoate complex Bond lengths in the ligands GBTAA and GBTPA are slightly different from each other only with regard to S–C bonds. S–C bond varies in the range 1.789– ˚ for GBTAA, while the value observed for 1.827 A ˚ . Two S–C bond in [Cu2(GBTPA)(PhCO2)4] is 1.80 A carbonyl oxygen C–O bond lengths are similar in ˚ . One of GBTAA and are of the order of 1.23 A ˚, the two C–O bonds in GBTPA complex is 1.23 A ˚ which may while the other gets reduced to 1.216 A be due to complexation. Two benzimidazole N–C bonds are also of the same order in both the cases, ˚. i.e., 1.316 and 1.356 A 2.5.3. Description of crystal structure of [Cu2(GBTPA)(OOCC6H5)4] Crystal structure of the complex [Cu2(GBTPA)(OOCC6 H5)4] reveals that it belongs to the group C2/C of a monoclinic system (Table 2, Fig. 2). The X-ray diffraction analysis evidences a polymeric structure with two tetrabenzoate bridged dicopper units (Cu1)2-GBTPA–(Cu2)-GBTPA-containing two crystallographically independent copper ions Cu1 and Cu2. Each dinuclear core is bridged by four benzoate groups (Cu1–Cu1 0 and Cu2–Cu2 0 distances of 2.78 and 2.69, respectively) with GBTPA acting as a bridge between the copper dimers. The benzoate groups coordinate to copper(II) in a bidentate fashion resulting in square pyramidal coordination environment around each copper atom, which consists of four oxygen donors of benzoate anion in the basal plane and of benzimidazole nitrogen atom in the apical position.
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Table 1 Crystal data and structure refinement for GBTAA and [Cu2(GBTPA)(OOCC6H5)4] Compound
GBTAA
[Cu2(GBTPA)(OOCC6H5)4]
Empirical formula Formula weight Temperature (K) ˚) Wavelength (A Crystal system Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) Volume (A Z Dcalc (mg/m3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm3) h Range for data collection () Index ranges Reflections collected Independent reflections [Rint] Completeness to h = 24.75 Absorption correction Maximum and minimum transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data) ˚ 3) Largest difference peak and hole (e A
C20H20N6O2S 408.48 218(2) 0.71073 monoclinic P21/C
C50H44Cu2N6O10S 1048.05 218(2) 0.71073 monoclinic C2/c
14.2886(12) 8.5582(8) 18.9231(17) 90 108.782(2) 90 2190.8(3) 4 1.238 0.175 856 0.30 · 0.20 · 0.20 2.27–24.75 1.6 6 h 6 16, 10 6 k 6 9, 22 6 l 6 22 12 535 3747 [0.0305] 99.8% none 0.9659 and 0.9494 full-matrix least-squares on F2 3747/0/262 1.039 R1 = 0.0397, wR2 = 0.0930 R1 = 0.0488, wR2 = 0.0964 0.172 and 0.295
27.525(2) 8.5582(8) 28.916(3) 90 107.171(2) 90 9381.1(14) 8 1.484 1.018 4320 0.30 · 0.20 · 0.10 1.550–24.00 31 6 h 6 30, 13 6 k 6 14, 33 6 l 6 32 24 888 7366 [0.0630] 100.0% none 0.9050 and 0.7499 full-matrix least-squares on F2 7366/0/605 1.120 R1 = 0.0681, wR2 = 0.1460 R1 = 0.0995, wR2 = 0.1589 0.629 and 0.456
Fig. 1. ORTEP plot for GBTAA.
F. Afreen et al. / Inorganica Chimica Acta 358 (2005) 1125–1134
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Fig. 2. (a) ORTEP plot for the complex [Cu2(GBTPA)(OOC6H5)4]. (b) Stereodrawing of the unit cell contents of [Cu2(GBTPA)(OOC6H5)4].
s value for the complex is found to be 0.32, indicating sufficient distortion of square pyramidal geometry [5]. Four oxygen atoms from two benzoate groups occupy the vertices of the square, while nitrogen of benzimidazole acts as the axial ligand. Equatorial bond angles deviate slightly from the expected bond angle of 90, the maximum deviation being 3.32. The axial bond angles deviate in the range 1.53–16.6. The sum of equatorial bond angles at Cu is 357.27 showing that the ligands are accommodated in the plane (Table 2). ˚ [Cu(2)–N(6)] and The Cu–Nbenzim distance of 2.149 A ˚ [Cu(1)–N(1)] is in the range found for similar 2.179 A complexes [6,7]. It is significantly longer than the Cu–O bond distances, confirming it to be in axial position of a square pyramid.
The Cu(1)–O bond length varies in the range of ˚ , while that for Cu(2)–O bond length var1.936–2.054 A ˚ for benzoate O atom ies in the range of 1.956–1.995 A coordination. This is in agreement with other complexes having similarly bridging carboxylate oxygen atoms where Cu–O bond distances are in the range of ˚ [8,9]. 1.9632–2.002 A The structure is unusual as the ligand is coordinating only through imidazole nitrogen but is quite normal considering what is known about such complexes. It is interesting to note that in this copper complex, the benzimidazole rings of the GBTPA ligand acquire a cis conformation in comparison to the uncomplexed ligand GBTAA wherein the benzimidazole rings acquires a trans conformation.
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Table 2 ˚] Selected bond lengths [A [Cu2(GBTPA)(OOCC6H5)4] Cu(1)O(5) Cu(1)–O(6) Cu(1)–O(3) Cu(1)–O(4) Cu(1)–N(1) Cu(1)–Cu(1) #1 C(23)–O(5) C(23)–O(3) #1 C(30)–O(6) C(30–O(4) #1 Cu(2)–O(8) Cu(2)–O(9) Cu(2)–O(7) Cu(2)–O(10) Cu(2)–N(6) Cu(2)–Cu(2) #2 O(7)–C(44) # 2 O(8)–Cu(37) #2 O(9)–C(37) O(10)–C(44)
1.936 (4) 1.955 (4) 1.992 (4) 2.054 (4) 2.179 (4) 2.7821 (14) 1.253 (7) 1.267 (7) 1.250 (7) 1.257 (7) 1.956 (4) 1.960 (4) 1.972 (4) 1.995 (5) 2.149 (4) 2.6975 (12) 1.243 (6) 1.251 (6) 1.253 (6) 1.235 (7)
and
bond
angles
O(5)–Cu(1)–O(6) O(5)–Cu(1)–O(3) O(6)–Cu(1)–O(3) O(5)–Cu(1)–O(4) O(6)–Cu(1)–O(4) O(3)–Cu(1)–O(4) O(5)–Cu(1)–N(1) O(6)–Cu(1)–N(1) O(3)–Cu(1)–N(1) O(4)–Cu(1)–N(1) O(8)–Cu(2)–O(9) O(8)–Cu(2)–O(7) O(9)–Cu(2)–O(7) O(8)–Cu(2)–O(10) O(9)–Cu(2)–O(10) O(7)–Cu(2)–O(10) O(8)–Cu(2)–N(6) O(9)–Cu(2)–N(6) O(7)–Cu(2)–N(6) O(10)–Cu(2)–N(6)
[]
for
173.40 (17) 92.02 (17) 88.97 (18) 89.60 (17) 86.68 (17) 154.10 (16) 94.75 (16) 91.53 (17) 99.02 (17) 106.60 (16) 165.77 (16) 91.4 (2) 88.20 (18) 86.6 (2) 88.32 (19) 165.74 (16) 92.87 (17) 101.133 (16) 96.00 (17) 98.25 (17)
However, this cannot be considered as a representative structure for the group of complexes generated with the ligand GBTPA but it clearly shows that this ligand can bridge two Cu(II) atoms through its benzimidazole imine N-atom, a feature that may be common to other complexes as well. We have therefore tentatively represented the structure of other complexes as a 2:2 ligand bridged structure with an axial carbonyl coordination (Fig. 8). There is sufficient IR evidence in favour of carbonyl coordination. Increase in Amide I band and increase or decrease in amide II band is indicative of carbonyl coordination in complexes, with similar diamide type ligands [3,10]. 2.5.4. Electronic spectroscopy, cyclic voltammetry and EPR studies The electronic spectra of the copper(II) complexes were measured in mixed DMSO/methanol solvent system. Two peaks in the range 270–285 nm are observed for all the complexes, which have been assigned to p– p* transition of benzimidazole moiety in their respective ligands [7]. The bands show enhanced absorptions as indicated by their extinction coefficients. A band is observed in the region 300–400 nm for the complexes generated with GBTAA and GBTPA, which is assigned to imidazole p ! Cu2+ charge transfer transition [7]. All the complexes exhibit a single symmetrical but broad d–d band in the region 600–850 nm (Fig. 3), characteristic of tetragonal geometry. In general, these bands are assigned to the overlapping transitions dxz; yz ! dx2 y 2 and dxy ! dx2 y 2 . It is well established that greater covalency results in lowering of energy of dx2 y 2 orbital and therefore the en-
Fig. 3. Visible spectra for the complexes of GBTAA and GBTPA in DMSO/methanol. (a) [Cu(GBTAA)Cl2], (b) [Cu(GBTAA)(NO3)2], (c) [Cu2(GBTPA)(OOC6H5)], (d) [Cu(GBTPA)Cl2], (e) [Cu(GBTPA)(NO3)].
ergy of dxz; yz =dxy ! dx2 y 2 transition. This is reflected in the kmax of the two series, where with GBTAA the energies of the d–d band are higher than those for the GBTPA series. All the complexes display a quasireversible redox wave due to Cu(II)/Cu(I) process (Table 3, Fig. 4). Anodic shifts in E1/2 values indicate the retention of anion in the coordination sphere of Cu(II). When calculated against NHE, the E1/2 values of the GBTPA series of Cu(II) complexes turn out to be quite positive as compared to the GBTAA, Cu(II) complexes, a feature reflecting the greater covalency of the GBTPA series. The GBTPA E1/2 are also quite positive when compared with other benzimidazole [11,12] and some amide [13] based complexes. X-Band EPR spectra of the copper(II) complexes were recorded in solution state (DMSO and DMF used as solvent) and solid state at liquid nitrogen temperature (Table 3). Solution spectra typically indicate a dx2 y 2 ground state (gi > g^ > 2.0023) and reveal less than four gi lines; the g^ line is quite broad in all
F. Afreen et al. / Inorganica Chimica Acta 358 (2005) 1125–1134
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Table 3 Electrochemical and X-band EPR data for the complexes of GBTAA, GBTPA and GBTBA Compound
[Cu(GBTAA)Cl2] [Cu(GBTAA)(NO3)2] [Cu(GBTPA)Cl2] [Cu(GBTPA)(NO3)2] [Cu2 (GBTPA)(OOCC6H5)4] a b
E1/2 (mV) vs. Ag/AgNO3
vs. NHE
42a 195a 56a 70a 365 (EPc)b
617 380 631 645 225
gi
g^
g1, g2, g3
Ai
gi/Ai · 104
2.33 2.30 2.29 2.32 2.31
2.075 2.075 2.085 2.074 2.073
2.19, 2.14, 2.37, 2.29,
143 157 147 149 164
162 146 156 155 141
2.07 2.06 2.11, 1.93 2.15
Ferrocene 75 mV against Ag/AgNO3. Ferrocene 60 mV against Ag/AgNO3.
the cases, thus indicating a distorted tetragonal geometry. The Ai versus gi values for the complexes do not lie on the Peisach Blumberg Plots [14], typical for a N2O2 type equatorial plane; this also suggests a distortion of the equatorial plane. The values of hyperfine coupling constant Ai are quite low in comparison to the normal range found for other copper(II) complexes [15], reinforcing a marked tetrahedral distortion of the tetragonal site [16,17]. Further, no nitrogen super hyperfine splitting could be observed, due to non planarity of the complexes. For the dimeric copper benzoate complex, EPR spectra
are typical of a monomeric copper(II) complex. It appears that this complex breaks down to monomeric units in DMSO solution. 2.6. Superoxide quenching study The cyclic voltammograms of Cu(II) complexes in DMSO solution exhibit a quasi reversible one electron redox couple in the range 0.3 to 0.1 V, on a glassy carbon electrode. No other redox wave is observed for the complexes beyond this region. Fig. 5 shows a cyclic voltammogram of molecular oxygen saturated in DMSO, obtained by bubbling pure dry O2 in a septum sealed
Fig. 4. Cyclic voltammogram for the complexes of GBTAA and GBTPA in 2:8 DMSO:CH3CN at scan rate, 100 mV/s (a) [Cu(GBTAA)Cl2], (b) [Cu(GBTPA)Cl2], (c) [Cu(GBTAA)(NO3)2], (d) [Cu(GBTPA)(NO3)2].
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F. Afreen et al. / Inorganica Chimica Acta 358 (2005) 1125–1134
Fig. 6. Plot of total peak current (lA) vs. concentration of (a) [Cu(GBTAA)Cl2], (b) [Cu(GBTPA)Cl2].
wave for O2 to O2 is quenched. This irreversibility of the O2 =O2 wave indicates an interaction of the superoxide anion with copper complex. The effect of this interaction is also visible on the Cu(II)/Cu(I) redox couple where the reoxidation of Cu(I) is also severely inhibited. CuI þ O2 ! CuII þ O2 2 Fig. 5. Cyclic voltammogram of DMSO solution containing 0.1 M Bu4N(ClO4). (a) Saturated with oxygen, (b) containing 0.6 mM [Cu(GBTAA)Cl2].
DMSO solution for 20 min, in the absence and presence of Cu(II) complex, recorded at a glassy carbon working electrode. In the absence of Cu(II) complexes, the dioxygen molecule exhibits a reversible couple at E1/2 in the range of 1.10 to 1.19 V due to O2 =O2 redox couple [18]. The reversibility of the redox couple indicates the stability of superoxide anion in the supporting electrolyte, at least on the voltammetric time scale. The slight departure of ia/ic from unity could be due to little moisture leaking into the solvent system. In the presence of Cu(II) complexes, during anodic scan the reoxidation
ð1Þ
As a result of this fast reaction, all superoxide generated gets chemically oxidised and is not available for electrochemical oxidation. Therefore, the reoxidation wave for O2 to O2 is drastically decreased in height. Similarly, all CuI in the solution is chemically oxidised to Cu(II) and hence no electrochemical oxidation wave for the process Cu(I)/Cu(II) is observed in the anodic scan. The quenching of superoxide anion was also studied by a gradual titration with the complexes. The starting concentration of complex was 0.05 M and that of [O2] was 2.0 mM. 0.02 ml of the complex (0.05 M) was added to the reaction solution each time, and the total drop in current (both anodic and cathodic) was measured voltammetrically. Care was taken not to expose the solution to air. A plot of total current versus concen-
F. Afreen et al. / Inorganica Chimica Acta 358 (2005) 1125–1134
tration of complex displays a gradual decrease in current value with concentration (Fig. 6). The curve exhibits a break point that varies with the nature of complex. The stoichiometry of O2 (consumed):complex turns out to be 5:1 for the complexes of GBTAA and GBTPA with a small inner sphere ligand like chloride. This stoichiometry can be explained in terms of the catalyst turn-
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ing over at least five times. The two reactions occurring in the solution are as follows: 9 2þ electrochemical þ > ½CuL !½CuL > = 2þ þ chemical 2 ½CuL þ O2 !½CuL O2 > Catalytic cycle > ;
Fig. 7. Plot of total peak current (lA) vs. concentration of (a) [Cu(GBTAA)(NO3)2], (b) [Cu(GBTPA)(NO3)2].
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F. Afreen et al. / Inorganica Chimica Acta 358 (2005) 1125–1134 0 2þ electrochemical
½ðCuLÞ
9 > > =
0 þ
!½ðCuLÞ
0 2þ
½ðCuLÞ
þ
chemical
þ
þ O2 ! ½ðCuLÞ00 O2 2
½ðCuLÞ00 O2 2 ! ½Cu
2þ
þ oxidised products of ligands:
At potentials beyond 0.4 V, Cu(II) complex is present in reduced form [(CuL)]+. This Cu(I) reacts with superoxide generated electrochemically to form peroxide O2 2 and oxidised complex [(CuL)]2+. The oxidised complex is similar to the original copper(II) complex and can again be electrochemically reduced to Cu(I) complex [(CuL)]+ which is equally effective in quenching the superoxide ion catalytically, and this forms the catalytic cycle. In the termination step, the cycle yields back the oxidised complex [(CuL) 0 ]2+ and peroxide ion. It seems at this stage that some stereochemical changes have arisen in the oxidised complex [(CuL) 0 ]2+ making it ineffective to carry out the cycle further, i.e., either it cannot be further reduced or if reduced, the reduced product is not capable of further reoxidation by O2 . The termination step may also involve the generation of oxidatively degraded copper(II) complexes. Thus, the cycle does not proceed beyond five steps in the case of the Cu(II) complexes with GBTAA and GBTPA and for each mole of copper complex added, five moles of oxygen is consumed (see Fig. 7). For the nitrate complexes, however the case is different where no such turnover activity was observed. There, however, is a stoichiometric conversion, the con-
X
N
N
N
N
Cu
X
N n
O
O
N n
S n
S
N
O
O
n O
N N
N
Cu X
X N
N
n = 1 for GBTAA n = 2 for GBTPA X = Cl−, NO3−, C6H5COO− Fig. 8. Proposed structure for the copper complexes of GBTAA and GBTPA.
> > ;
Termination step
centration versus total peak current graphs display a break point at [Catalyst] 1.0 mM and the [O2]:[complex] stoichiometry turns out to be 1:1.
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