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Copper(II) complexes of ligands derived from tryptamine
Polyhedron 28 (2009) 1097–1102
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Copper(II) complexes of ligands derived from tryptamine Sang-Tae Lee, Donald C. Craig, Stephen B. Colbran * School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
a r t i c l e
i n f o
Article history: Received 20 November 2008 Accepted 24 January 2009 Available online 6 February 2009 Keywords: Cytochrome c oxidase model Indole Copper Electrochemistry
a b s t r a c t Three new ligands with an indole substituent tethered to a pyridylalkylamine or imidazolylalkylamine metal-binding domain have been prepared from tryptamine. Copper(II) complexes have been prepared and characterized, three by X-ray crystallography. Electrochemistry has been used to ascertain the mutual effects of the copper and indole redox centres upon each other. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Cytochrome c oxidases (CcOs) are membrane-spanning proteins that catalyse the reduction of dioxygen to water at a heme a3–CuB centre coupled to energy conserving proton pumping across the membrane in the terminal steps of aerobic respiration. X-ray crystal structures of CcOs from various sources, bacterial and mammalian, show the CuB ion is coordinated by the imidazole sidechains of three histidines, His 240, His 290 and His 291 (bovine CcO numbering). His 291 is p-stacked with the indole sidechain of tryptophan 236, and it has recently being proposed that the indole of this fully conserved residue is an electron-donor in the catalytic cycle of CcO [1–5]. To date, models for the CuB centre have not included an indolyl group [6–11]. However, spurred by observations of direct p-coordination of the tryptophan sidechain to copper in some prokaryotic copper trafficing proteins, the copper(I) chemistry of several indole-substituted ligands has been reported, including pcoordination of the indole ring at the C2–C3 position to the copper(I) centre [12,13]. Also several X-ray crystal structures of copper(II) complexes of tryptophan-derived ligands reveal p-stacking of the indole pendant with the copper(II)-bound ligand donor groups [14–16]. Herein, we describe copper(II) complexes of three new ligands HL1, HL2 and H2L3, each a derivative of tryptamine. Results are presented that address the following question: does the close proximity of copper(II) and indole centres to each other – such as occurs in CcO’s – lead to a discernable perturbation of the electron-donor properties (e.g. oxidation potentials) of these centres?
* Corresponding author. Tel.: +61 2 93854737; fax: +61 2 93856141. E-mail address: [email protected] (S.B. Colbran). 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2009.01.025
H N
H N
N
N
N
N N
N
HL1
HL2
H N
NH
N
N N
H2L3
2. Experimental 2.1. Materials and instrumentation Elemental analyses for C, H and N were carried out at the Australian National University Microanalytical Laboratory. NMR spectra were recorded on Bruker DPX 300 (300 MHz) spectrometers. Electronic spectra of complexes were recorded between 200 and 1400 nm on a CARY 5 spectrometer in the dual beam mode (1 nm resolution); solution spectra were recorded in sealed 1 cm quartz cuvettes. EPR spectra of frozen solutions (at 77 K; liquid nitrogen dewar) were recorded using a Bruker EMX 10 EPR spectrometer (m 9.5 GHz). Infrared spectra were recorded using a Hitachi 260-10 FTIR spectrometer at 2 cm1 resolution. Electrochemical measurements were performed in a conventional three electrode cell using a computer-controlled Pine Instrument Co.
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AFCBP1 bipotentiostat [17]. Solutions of the compounds were 2 mM in anhydrous acetonitrile (Aldrich, used as received) with 0.1 M tetra-n-butylammonium hexafluorophosphate. Data are reported from cyclic voltammograms recorded at a 0.5 mm Pt disc working electrode at a scan rate of 100 mV s1. Electrochemical potentials are quoted relative to the ferrocene–ferrocenium couple measured under the same experimental conditions (same concentrations, solvent, support electrolyte, electrodes, temperature and scan rate). 2.2. Synthesis of ligands, HL1–H2L3 and their precursors
2.2.1. (3-Indolyl-2-ethyl)[bis(2-pyridylmethyl)]amine (HL1) Tryptamine (1.6 g, 0.01 mol) in tetrahydrofuran (10 mL) was combined with pyridine-2-carboxaldehyde (2.14 g, 0.02 mol) and sodium triacetoxyborohydride (4.87 g, 0.02 mol). After diluting with tetrahydrofuran (50 mL), the reaction mixture was stirred under dinitrogen for 3 days. Then, the reaction mixture was quenched by adding aqueous saturated sodium bicarbonate, and the product extracted with diethylether. The extract was dried over magnesium sulfate and the solvent removed on a rotary evaporator. The residue was purified by column chromatography (silica gel; eluent 1:9 methanol/dichloromethane). The major yellow band afforded a brown oil (2.5 g, 75%). m/z (ESI-MS) 345 (M+, 2%), 252 (100), 248 (4); dH (CDCl3) 8.56 (2H, d, Py), 8.12 (1H, br s, NH), 7.58 (2H, d, Py), 7.52 (2H, d, Py), 7.45 (1H, d, Ind), 7.35 (1H, d, Ind), 7.13 (2H, t, Py), 7.02 (1H, t, Ind), 6.94 (1H, t, Ind), 6.90 (1H, s, Ind), 4.03 (4H, s, CH2-CH2), 3.01 (4H, s, CH2); m/cm1 (KBr disc) 1592s, 1569m, 1474m, 1455m, 1434s, 1360m, 1149w, 1002w. 2.2.2. (3-Indolyl-2-ethyl)[bis(2-pyridyl-2-ethyl)]amine (HL2) 2-Vinylpyridine was purified by passing through a short silica column with diethylether as eluent followed by removal of the diethylether by rotary evaporation. To 2-vinylpyridine (1.64 g, 15.6 mmol) in methanol (20 mL) was added tryptamine (0.5 g, 3.12 mmol) in methanol along with glacial acetic acid (15 mL). The mixture was stirred and heated at reflux under dinitrogen for 5 days. The mixture was then cooled and the solvent removed using a rotary evaporator. The residue was re-dissolved in chloroform. This solution was washed with saturated sodium carbonate and saturated brine solutions, and then dried over magnesium sulfate. The solvent was removed on a rotary evaporator and the oily residue purified by column chromatography (silica support; eluent 2:8 methanol–dichloromethane) to afford the product as a brown oil (0.84 g, 72%). m/z (ESI-MS) 371 (M+, 50%), 332 (100), 266 (90), 252 (30); dH (CDCl3) 9.47 (1H, br s, NH), 8.43 (2H, d, Py), 7.49 (3H, m, Ar), 7.28 (1H, d, Ind), 7.04 (6H, m, Ar), 6.89 (1H, d, Ind), 3.01 (2H, t, CH2), 2.97 (2H, t, CH2), 2.89 (4H, s, CH2). 2.2.3. (3-Indolyl-2-ethyl)(4-imidazolylmethyl)(2pyridylmethyl)amine (H2L3) 2.2.3.1. (3-Indolyl-2-ethyl)(2-pyridylmethyl)amine. To tryptamine (6.5 g, 0.04 mol) dissolved in methanol (10 mL), was combined with pyridine-2-carboxaldehyde (4.3 g, 0.04 mol). After diluting the mixture with methanol (70 mL), the solution was hydrogenated under hydrogen (1 atm) in the presence of 10% Pd–C catalyst (0.6 g) over night. After removal of the catalyst by filtration through a silica plug, the solvent was removed by rotary evaporation to give a pale yellow oil (8.24 g, 82%), which was used without further purification in the synthesis of H2L3. 2.2.3.2. H2L3. (3-Indolyl-2-ethyl)(2-pyridylmethyl)amine (410 mg, 1.63 mmol) dissolved in 1,2-dichloroethane (10 mL) was added
4(5)-imidazolylcarboxaldehyde (156 mg, 1 eq.) and sodium triacetoxyborohydride (800 mg). Additional 1,2-dichloroethane (30 mL) was added and the reaction mixture was stirred under dinitrogen for 3 days. The mixture was quenched by adding aqueous saturated sodium bicarbonate, and the product was extracted with diethylether. The combined extracts were dried over anhydrous magnesium sulfate and the solvents removed on a rotary evaporator; the residue was placed under high vacuum to afford a light brown oil (0.46 g, 85%). m/z (ESI-MS) 334 (M+, 25%), 252 (100); dH (D2O) 8.42 (1H, s, Im), 8.23 (1H, d, Py), 8.03 (1H, d, Py), 7.52 (2H, d, Ind), 7.34 (2H, m, Py & Ind), 7.12 (2H, t, Ind), 7.04 (1H, s, Im), 6.91 (1H, t, Py), 4.17 (2H, s, CH2), 4.11 (2H, s, CH2), 3.09 (2H, d, CH2), 2.84 (2H, d, CH2). 2.3. [Cu(L)Cl2] (L = HL1–H2L3) complexes 2.3.1. [Cu(HL1)Cl2] (1) CuCl2 2H2O (25 mg, 1 eq.) in iso-propanol (10 mL) was layered on HL1 (50 mg, 0.14 mmol) in dichloromethane (2 mL). After several days, a deep green crystalline solid (47.7 mg, 71%) deposited from the mixture. (Found: C, 54.85; H, 4.90; N, 11.49%. C22H22Cl2CuN4 0.5H2O requires C, 54.38; H, 4.77; N, 11.53%); kmax/nm (MeOH) 219 (e/ dm3 mol1 cm1 14833), 256 (12472), 282 (9722), 676 (148); EPR (frozen MeOH solution, 77 K) g|| = 2.23, g ? = 2.04, A|| = 178 G; m/cm1 (KBr disc) 1608s, 1573w, 1480w, 1459m, 1443s, 1471m, 1343m, 1284m, 1092m, 1055m, 1024m. 2.3.2. [Cu(HL2)Cl2] (2) HL2 (94.4 mg, 0.255 mmol) in iso-propanol (2 mL) was layered on CuCl2 2H2O (43 mg, 1 eq.) in iso-propanol (10 mL); upon standing for several days a dark green precipitate formed (92.3 mg, 72%). (Found: C, 49.63; H, 5.81; N, 9.37%. C24H26Cl2CuN4 4H2O requires C, 49.96; H, 5.94; N, 9.71%); kmax/nm (MeOH) 214 (e/ dm3 mol1 cm1 12382), 263 (8470), 674 (158); EPR (frozen MeCN solution, 77 K) g|| = 2.35, g ? = 2.09, A|| = 159 G; m/cm1 (KBr disc) 3335br, 1607s, 1570m, 1484m, 1445s, 1313w, 1108w, 1027w. 2.3.3. [Cu(H2L3)Cl2] (3) H2L3 (100 mg, 0.227 mmol) in methanol (5 mL) was added slowly to a solution of CuCl2 2H2O (38.7 mg, 1 eq.) in methanol (5 mL). After stirring for 10 min, the solution was left to stand under a diethylether atmosphere for a few days; a green crystalline solid (74 mg, 75%) formed. (Found: C, 49.71; H, 4.87; N, 13.87%. C20H21Cl2CuN5 H2O 0.5CH3OH requires C, 49.25; H, 5.04; N, 14.01%); kmax/nm (MeOH) 220 (e/dm3 mol1 cm1 14222), 263 (10444), 280 (911), 653 (100); EPR (frozen MeOH solution, 77 K) g|| = 2.23, g ? = 2.05, A|| = 175 G; m/cm1 (KBr disc) 3248br, 2895s, 1609s, 1572w, 1503w, 1456s, 1445s, 1350m, 1271m, 1104m. 2.4. X-ray crystallography As good quality crystals of complexes 1–3 were not obtained, each complex was separately treated with an excess K[PF6], Na[BF4] or Li(ClO4) in methanol and the resulting green solutions left to stand under an atmosphere of diethylether. Single crystals of [Cu(HL1)Cl](ClO4).0.5Et2O 0.25H2O (4), [Cu(HL2)F]2[BF4]2 (5) and [Cu(H2L3)Cl][PF6] (6) were obtained. X-ray reflection data for 4–5 were measured with an Enraf-Nonius CAD-4 diffractometer in x–2h scan mode at 294 K. Crystal structure and refinement data are collected in Table 1. A summary of key bond lengths and bond angles is given in Table 2. The CIF files have been deposited at the Cambridge Crystallographic Data Centre and allocated the deposition numbers CCDC 710201–710203.
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S.-T. Lee et al. / Polyhedron 28 (2009) 1097–1102 Table 1 X-ray crystal structure collection and refinement data.
Formula Formula mass Crystal size Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) T (°C) Z Dcalc (g cm3) Radiation k (Å) l (cm1) F (0 0 0) Maximum, minimum transmission factors hmax Number of intensity measurements Rmerge for number of equivalent reflections Number of observed reflections in final refinement [I/r(I) > 3] Number of variables in final refinement R Rw Goodness-of-fit Maximum, minimum peak in final difference map (e Å3)
[Cu(HL1)Cl](ClO4) 0.5C4H10O 0.25H2O (4)
[Cu(HL2)F]2[BF4]2 (5)
[Cu(H2L3)Cl][PF6] (6)
C24H27.5Cl2Cu N4O4.75 582.5 0.26 0.21 0.05 C2/c 23.077(9) 14.427(4) 15.600(7) 90 94.43(4) 90 21 8 1.49 Mo Ka, 0.71073 1.096 2404.0 0.89, 0.77 23 3693 0.037 1460 171 0.093 0.104 2.04 1.13, 1.69
C24H26BCuF5N5 539.8 0.24 0.17 0.10 P1 10.885(6) 11.067(7) 12.225(6) 63.91(3) 64.32(3) 81.29(3) 21 2 1.51 Mo Ka, 0.71073 0.975 554.0 0.87, 0.78 24 3928 0.025 3342 265 0.051 0.077 1.64 1.11, 0.67
C20H21ClCuF6N5P 575.4 irregular P21/c 11.761(6) 15.157(4) 14.866(7) 90 119.70(2) 90 21 4 1.66 Mo Ka, 0.71073 1.205 1164.0 25 4207 0.026 2168 186 0.071 0.082 1.71 1.26, 1.17
Table 2 Selected key bond length (Å) and angle (°) data for complexes 4–6. 4 Cu1–Cl1 Cu1–N1 Cu1–N2 Cu1–N4 Cl1–Cu1–N1 Cl1–Cu1–N2 Cl1–Cu1–N4 N1–Cu1–N2 N1–Cu1–N4 N2–Cu1–N4
5 2.241(5) 1.909(12) 1.907(11) 2.052(13) 97.181(5) 96.472(4) 178.512(4) 165.611(6) 83.211(7) 83.028(7)
Cu1–F1 Cu1–F1 Cu1–N1 Cu1–N2 Cu1–N3 Cu1–Cu1 F1–Cu1–F1 F1–Cu1–N1 F1–Cu1–N2 F1–Cu1–N3 F1–Cu1–N1 F1–Cu1–N2 F1–Cu1–N3 N1–Cu1–N2 N1–Cu1–N3 N2–Cu1–N3 Cu1–F1–Cu1
3. Results and discussion 3.1. Syntheses Two new indole-substituted ligands, HL1 and H2L3, were easily synthesized by reductive amination using an appropriate aldehyde and tryptamine with sodium triacetoxyborohydride in dry 1,2dichloroethane [18]. HL2 was produced by reaction of excess 2-vinyl pyridine with tryptamine. Copper(II) complexes, [Cu(L)Cl2] (1: L = HL1, 2: L = HL2; 3: L = H2L3) were obtained by adding the ligand (L) in dichloromethane to CuCl2 2H2O in iso-propanol. After several days, the green–blue microcrystalline complexes 1–3 precipitated. Unfortunately, none of the complexes gave crystals of sufficient quality for an X-ray crystal structure determination. 3.2. X-ray crystallography As crystals of 1–3 were unavailable, in separate reactions, each of these complexes was treated with excess K[PF6], Na[BF4] or Li(-
6 1.911(2) 2.212(2) 2.064(3) 2.013(3) 2.011(3) 3.125(2) 81.706(8) 88.602(1) 89.141(1) 176.011(1) 101.047(1) 96.576(1) 101.190(1) 161.712(1) 88.146(1) 93.226(1) 98.294(8)
Cu1–Cl1 Cu1–N1 Cu1–N3 Cu1–N5 Cl1–Cu1–N1 Cl1–Cu1–N3 Cl1–Cu1–N5 N1–Cu1–N3 N1–Cu1–N5 N3–Cu1–N5
2.249(2) 1.933(7) 1.940(7) 2.091(7) 95.386(2) 98.691(2) 177.012(2) 164.628(3) 81.723(3) 84.266(3)
ClO4) in methanol and the resulting green solutions were left to stand under an atmosphere of diethylether. Single crystals of [Cu(HL1)Cl](ClO4) 0.5Et2O 0.25H2O (4), [Cu(HL2)F]2[BF4]2 (5) and [Cu(H2L3)Cl][PF6] (6) were obtained that were satisfactory for X-ray structure determinations. Fig. 1 presents views of the structures and selected bond length and angle data are gathered in Table 2. Complex 4 exhibits a distorted square planar copper(II) centre formed by the binding of the two pyridine and amine nitrogen atoms, and a chloro co-ligand, Fig. 1a. The sum of the six lessthan-180° angles about the Cu centre is 704°, which compares with 720° (and 645°) for perfect square planar (and tetrahedral) centre(s) [19,20]. The Cu–Cl1 (2.241(5) Å), Cu–N(amine) (2.052(13) Å), and Cu–N(py) (1.907(11) and 1.909(12) Å) distances are all in the anticipated range for copper(II) complexes. The indole substituent does not bind to the copper(II) centre and lies offset pstacked with the pyridine ring containing N2. The inter-ring separation of the offset p-stacked pyridinyl and indolyl rings is 3.38 Å, which is comparable to the Trp 236 His 291 separation in cyto-
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Fig. 1. Views of the paired cations and closely associated anions in the X-ray crystal structures of: (a) [Cu(HL1)Cl](ClO4), 4; (b) [Cu(HL2)F]2[BF4]2, 5; (c) [Cu(H2L3)Cl][PF6], 6. In all views, 50% thermal ellipsoids are shown and hydrogen atoms bonded to carbon have been omitted for clarity; in each case, atoms labelled with an asterisk are related by a crystallographic inversion to the corresponding atom labelled without the asterisk.
chrome c oxidases (see above) [1]. In the crystal structure, adjacent cation pairs of 4 are connected by weak (3.10 Å) Cu Cl1 interactions. The inter-copper distance between neighbouring cations is 3.75 Å. The indole N–H forms a hydrogen bond to the perchlorate counterion (N3 O 2.97 Å). Fig. 1b shows the centrosymmetric [Cu(HL2)F]22+ dimers and associated [BF4] counterions in the crystal structure of 5. Fluoride abstraction reactions in transition metal complexes containing the tetrafluoroborate anion are well-known [21]. The copper(II) centre in 5 has a typical distorted square pyramidal geometry with a trigonality index [22] s = 0.25 [F1–Cu1–N3 = 176.011(1)° is the largest bond angle and N1–Cu1–N2 = 161.712(1)° the next largest]. Axial Cu–F1 bonds (2.212(2) Å) link to the adjacent Cu to give a dimeric structure. The axial Cu–F1 bond is longer than the in-plane Cu–F1 (1.911(2) Å) and Cu–N bonds (2.029 Å average). The Cu Cu distance in the dimer is 3.125(2) Å. The indole substituent of HL2 is not coordinated to the copper centre and is without any p-stacking interactions within the complex ion. There is an hydrogen bond between the indole N–H and F on tetrafluoroborate (N F 3.04 Å). The molecular structure of 6, Fig. 1c, reveals the copper(II) ion to be distorted square planar bound by the amine, imidazolyl and pyridyl nitrogen atoms and by a chloro co-ligand. The sum of the six less-than-180° bond angles about the copper(II) centre is 701.7° [19,20]. The indolyl ring is offset p-stacked with the pyridyl ring consistent with the larger p-surface area and electron density of pyridyl cf. imidazolyl rings. The separation between the p-stacked indolyl and pyridyl rings is 3.11 Å. The Cu1–Cl1 and Cu1–Cu1 (adjacent cation) distances are 3.14 and 3.72 Å, respectively. The [PF6] counterion forms a F H–N hydrogen bond (2.94 Å) with the coordinated imidazolyl N–H rather than with the pendant indole.
1500 cm1 [23]. The Vis/NIR spectra of 1–3 in methanol solution show a broad band having a maximum at 640–750 nm and a prominent tail to lower energy, consistent with each complex adopting a tetragonally-elongated, square pyramidal (CuCl2N3) structure, Table 3. Likewise EPR spectra of complexes 1–3 in frozen Table 3 Electronic and X-band EPR spectroscopic data for the Cu(II) complexes in methanol.
1 2 3
UV-NIR (297 K) [kmax/nm] (e/ dm3 mol1 cm1)
EPR (77 K)
676 (148) 674 (158) 653 (100)
g|| = 2.23, g ? = 2.04, A|| = 178 G g|| = 2.35, g ? = 2.09, A|| = 159 G g|| = 2.23, g ? = 2.05, A|| = 175 G
a
b
3.3. Physicochemical data for complexes 1–3 in solution Numerous studies reveal [Cu(N3–L)Cl2] complexes, analogous to 1–3, to contain five-coordinate CuIICl2N3 centres that display characteristic EPR and UV–Vis spectra [17–22,24–26]. Available data are indicative for five-coordinate CuIICl2N3 centres in 1–3. The FTIR of spectrum of each complex (see Supplementary data) shows characteristic pyridyl stretches at 1604 and 1571 cm1 indicative for all pyridines binding to the Cu(II) ion. The fingerprint regions of the IR spectra show characteristic indole peaks between 1400 and
2750
3000 3250 Magnetic Field / G
3500
Fig. 2. X-band EPR spectra of complexes 1 (a) and 3 (b) in frozen methanol solution at 77 K; m/GHz = 9.448.
S.-T. Lee et al. / Polyhedron 28 (2009) 1097–1102
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methanol at 77 K are axial (dx2 y2 ground-state) fully consistent with square pyramidal CuCl2N3 structures, e.g. Fig. 2 and Table 3. N-Superhyperfine coupling is also apparent in the spectra: e.g. Nsuperhyperfine peaks (AN–Cu 12 G) from three Cu(II)-bound Ndonors are clearly discerned in the spectrum of 1, Fig. 2a.
to oxidation of the indole substituent. Notably, although the merging of chloride- and indole-centred oxidation processes complicates the electrochemistry, the potential of the indole oxidation is only slightly, if at all, affected by coordination of the indolesubstituted ligand to the copper(II) centre.
3.4. Electrochemistry of complexes 1–3
4. Conclusions
Cyclic voltammograms (CVs) of 1–3 in acetonitrile were acquired to characterize the redox properties of the indole and copper centres in each complex. The CVs show quasi-reversible Cu(II)/ Cu(I) couples, at 0.57 V for the methylene-bridged amine complexes 1 and 3 and at –0.25 V for the ethylene-linked amine complex 2 (e.g. see Fig. 3). Lower potential Cu(II)/Cu(I) couples are expected for complexes of bis(pyridylmethyl)amine ligands compared to complexes of bis(pyridylethyl)amines [24–26]. On scanning to positive potentials, the CVs of 1–3 all show a broad anodic peak at +0.64 V followed closely by an peak at +0.77 V for an oxidation process that appears to have some reversibility (a broad cathodic peak is discernable ca. at +0.71 V in the return scan: ip,c/ip,a = 1.1). The broad anodic peak at +0.64 V is found in CVs of both bis(triphenylphosphoranylidene)ammonium chloride (PPNCl) and [(Bz-bpa)CuCl2] and, therefore, may be attributed to oxidation of ‘free’ chloride anion (in the complexes, from dissociation of chloro ligand) [27–29]. The second peak at +0.77 V in the CVs of 1–3 is absent in the CVs of PPNCl and [(Bz-bpa)CuCl2] and therefore must be an indole substituent-centred oxidation. For comparison, CVs of indole, Fig. 3a, reveal a broad anodic peak at +0.80 V and CVs of 3-methylindole, Fig. 3b, show a sharp anodic peak at +0.57 V that is followed by a broad peak at +0.80 V; the irreversibility arises from rapid coupling reactions of the indolyl radical [30,31]. Thus, the peaks in this region for 1–3 can be attributed
Three new indole-substituted ligands (HL1–H2L3) were synthesized from tryptamine and copper(II) complexes (1–6) were successfully obtained from them. The results of the electrochemistry for complexes 1–3 show coordination of the ligands to a copper centre and the proximity of the indole and copper(II) centres does not affect the CuII/CuI couples or the indole-centred oxidation processes. This finding contrasts with our previous results for some complexes wherein copper(II) centres closely related to those reported here were substituted with ferrocenyl (Fc) substituents; in these Fc Cu(II) complexes significant mutual perturbations of the electron donor properties of each centre were observed that lead to new reactivity [26]. The crystal structures of 4 and 6 reveal offset p-stacking between the indolyl group and the ring of a pyridinyl or imidazolyl donor bound to the Cu(II) ion. The inter-ring separation is ca. 3.1–3.3 Å, which is analogous to that observed for the Trp 236 His 291 sidechains within CcO. In solution, p-stacking of the indolyl and ligand heterocyclic donor rings in complexes 1–6 is possible, but much less likely due to the disrupting effect(s) of solvent molecules. Thus model systems of considerably more sophistication will be needed if the effects of p-stacking between an indolyl group and a Cu(II)-bound heterocyclic ligand donor are to be explored further. Acknowledgement This research was supported by the Australian Research Council. Appendix A. Supplementary data CCDC 710201, 710202, 710203 contain the supplementary crystallographic data for 4, 5, 6. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.poly.2009.01.025. References
Fig. 3. Cyclic voltammograms of indole (a), 3-methylindole (b), 1 (c), 3 (d) and PPNCl (e) in acetonitrile with 0.1 M [Bu4 N][PF6] at 295 K; 1.0 mm Pt disc working electrode; scan rate = 100 mV s1.
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[12] Y. Shimazaki, T. Nogami, F. Tani, A. Odani, O. Yamauchi, Angew. Chem., Int. Ed. Engl. 40 (2001) 3859. [13] Y. Shimazaki, H. Yokoyama, O. Yamauchi, Angew. Chem., Int. Ed. Engl. 38 (1999) 2401. [14] H. Masuda, T. Sugimori, A. Odani, O. Yamauchi, Inorg. Chim. Acta 180 (1991) 73. [15] À. García-Raso, J.J. Fiol, F. Bádenas, E. Lago, E. Molins, Polyhedron 20 (2001) 2877. [16] T.A. Smith, J.E. Penner-Hahn, M.A. Berding, S. Doniach, K.O. Hodgson, J. Am. Chem. Soc. 107 (1985) 5945. [17] Z. He, S.B. Colbran, D.C. Craig, Chem.-Eur. J. 9 (2003) 116. [18] A.F. Abdel-Magid, K.G. Carson, B.D. Harris, C.A. Maryanoff, R.D. Shah, J. Org. Chem. 61 (1996) 3849. [19] Z. He, P.J. Chaimungkalanont, D.C. Craig, S.B. Colbran, J. Chem. Soc., Dalton Trans. (2000) 1419. [20] S.E. Watkins, D.C. Craig, S.B. Colbran, J. Chem. Soc., Dalton Trans. (2002) 2423.
[21] R.R. Jacobson, Z.T. Tyeklár, K.D. Karlin, J. Zubieta, Inorg. Chem. 30 (1991) 2035. [22] A.W. Addision, T.N. Rao, J. Reedijk, J.V. Rijn, G.C. Verschoor, J. Chem. Soc., Dalton Trans. (1984) 1349. [23] S.V. Tsukerman, A.I. Bugai, Y.N. Surov, V.F. Lavrushin, N.S. Pivenko, J. Appl. Spectr. (1980) 1526. [24] I. Sanyal, M. Mohammad-Tahir, M.S. Nasir, P. Ghosh, B.I. Cohen, Y. Gultneh, R.W. Cruse, A. Farooq, K.D. Karlin, S. Liu, J. Zubieta, Inorg. Chem. 31 (1992) 4322. [25] K.D. Karlin, S.E. Sherman, Inorg. Chim. Acta 64 (1982) L39. [26] A.J. Evans, S.E. Watkins, D.C. Craig, S.B. Colbran, J. Chem. Soc., Dalton Trans. (2002) 983. [27] P. Catellonese, G. Launay, Bull. Soc. Chim. 7-8 (1978) I317. [28] D.M. Novak, B.E. Conway, J. Chem. Soc., Faraday Trans. 77 (1981) 2341. [29] B.E. Conway, Y. Phillips, S.Y. Qian, J. Chem. Soc., Faraday Trans. 91 (1995) 283. [30] P.G. Keech, N.J. Bunce, J. Appl. Electrochem. 33 (2003) 79. [31] G.L. Skiles, J.D. Adams, G.S. Yost, Chem. Res. Toxicol. 2 (1989) 254.
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Copper(II) complexes of ligands derived from tryptamine Sang-Tae Lee, Donald C. Craig, Stephen B. Colbran * School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
a r t i c l e
i n f o
Article history: Received 20 November 2008 Accepted 24 January 2009 Available online 6 February 2009 Keywords: Cytochrome c oxidase model Indole Copper Electrochemistry
a b s t r a c t Three new ligands with an indole substituent tethered to a pyridylalkylamine or imidazolylalkylamine metal-binding domain have been prepared from tryptamine. Copper(II) complexes have been prepared and characterized, three by X-ray crystallography. Electrochemistry has been used to ascertain the mutual effects of the copper and indole redox centres upon each other. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Cytochrome c oxidases (CcOs) are membrane-spanning proteins that catalyse the reduction of dioxygen to water at a heme a3–CuB centre coupled to energy conserving proton pumping across the membrane in the terminal steps of aerobic respiration. X-ray crystal structures of CcOs from various sources, bacterial and mammalian, show the CuB ion is coordinated by the imidazole sidechains of three histidines, His 240, His 290 and His 291 (bovine CcO numbering). His 291 is p-stacked with the indole sidechain of tryptophan 236, and it has recently being proposed that the indole of this fully conserved residue is an electron-donor in the catalytic cycle of CcO [1–5]. To date, models for the CuB centre have not included an indolyl group [6–11]. However, spurred by observations of direct p-coordination of the tryptophan sidechain to copper in some prokaryotic copper trafficing proteins, the copper(I) chemistry of several indole-substituted ligands has been reported, including pcoordination of the indole ring at the C2–C3 position to the copper(I) centre [12,13]. Also several X-ray crystal structures of copper(II) complexes of tryptophan-derived ligands reveal p-stacking of the indole pendant with the copper(II)-bound ligand donor groups [14–16]. Herein, we describe copper(II) complexes of three new ligands HL1, HL2 and H2L3, each a derivative of tryptamine. Results are presented that address the following question: does the close proximity of copper(II) and indole centres to each other – such as occurs in CcO’s – lead to a discernable perturbation of the electron-donor properties (e.g. oxidation potentials) of these centres?
* Corresponding author. Tel.: +61 2 93854737; fax: +61 2 93856141. E-mail address: [email protected] (S.B. Colbran). 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2009.01.025
H N
H N
N
N
N
N N
N
HL1
HL2
H N
NH
N
N N
H2L3
2. Experimental 2.1. Materials and instrumentation Elemental analyses for C, H and N were carried out at the Australian National University Microanalytical Laboratory. NMR spectra were recorded on Bruker DPX 300 (300 MHz) spectrometers. Electronic spectra of complexes were recorded between 200 and 1400 nm on a CARY 5 spectrometer in the dual beam mode (1 nm resolution); solution spectra were recorded in sealed 1 cm quartz cuvettes. EPR spectra of frozen solutions (at 77 K; liquid nitrogen dewar) were recorded using a Bruker EMX 10 EPR spectrometer (m 9.5 GHz). Infrared spectra were recorded using a Hitachi 260-10 FTIR spectrometer at 2 cm1 resolution. Electrochemical measurements were performed in a conventional three electrode cell using a computer-controlled Pine Instrument Co.
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AFCBP1 bipotentiostat [17]. Solutions of the compounds were 2 mM in anhydrous acetonitrile (Aldrich, used as received) with 0.1 M tetra-n-butylammonium hexafluorophosphate. Data are reported from cyclic voltammograms recorded at a 0.5 mm Pt disc working electrode at a scan rate of 100 mV s1. Electrochemical potentials are quoted relative to the ferrocene–ferrocenium couple measured under the same experimental conditions (same concentrations, solvent, support electrolyte, electrodes, temperature and scan rate). 2.2. Synthesis of ligands, HL1–H2L3 and their precursors
2.2.1. (3-Indolyl-2-ethyl)[bis(2-pyridylmethyl)]amine (HL1) Tryptamine (1.6 g, 0.01 mol) in tetrahydrofuran (10 mL) was combined with pyridine-2-carboxaldehyde (2.14 g, 0.02 mol) and sodium triacetoxyborohydride (4.87 g, 0.02 mol). After diluting with tetrahydrofuran (50 mL), the reaction mixture was stirred under dinitrogen for 3 days. Then, the reaction mixture was quenched by adding aqueous saturated sodium bicarbonate, and the product extracted with diethylether. The extract was dried over magnesium sulfate and the solvent removed on a rotary evaporator. The residue was purified by column chromatography (silica gel; eluent 1:9 methanol/dichloromethane). The major yellow band afforded a brown oil (2.5 g, 75%). m/z (ESI-MS) 345 (M+, 2%), 252 (100), 248 (4); dH (CDCl3) 8.56 (2H, d, Py), 8.12 (1H, br s, NH), 7.58 (2H, d, Py), 7.52 (2H, d, Py), 7.45 (1H, d, Ind), 7.35 (1H, d, Ind), 7.13 (2H, t, Py), 7.02 (1H, t, Ind), 6.94 (1H, t, Ind), 6.90 (1H, s, Ind), 4.03 (4H, s, CH2-CH2), 3.01 (4H, s, CH2); m/cm1 (KBr disc) 1592s, 1569m, 1474m, 1455m, 1434s, 1360m, 1149w, 1002w. 2.2.2. (3-Indolyl-2-ethyl)[bis(2-pyridyl-2-ethyl)]amine (HL2) 2-Vinylpyridine was purified by passing through a short silica column with diethylether as eluent followed by removal of the diethylether by rotary evaporation. To 2-vinylpyridine (1.64 g, 15.6 mmol) in methanol (20 mL) was added tryptamine (0.5 g, 3.12 mmol) in methanol along with glacial acetic acid (15 mL). The mixture was stirred and heated at reflux under dinitrogen for 5 days. The mixture was then cooled and the solvent removed using a rotary evaporator. The residue was re-dissolved in chloroform. This solution was washed with saturated sodium carbonate and saturated brine solutions, and then dried over magnesium sulfate. The solvent was removed on a rotary evaporator and the oily residue purified by column chromatography (silica support; eluent 2:8 methanol–dichloromethane) to afford the product as a brown oil (0.84 g, 72%). m/z (ESI-MS) 371 (M+, 50%), 332 (100), 266 (90), 252 (30); dH (CDCl3) 9.47 (1H, br s, NH), 8.43 (2H, d, Py), 7.49 (3H, m, Ar), 7.28 (1H, d, Ind), 7.04 (6H, m, Ar), 6.89 (1H, d, Ind), 3.01 (2H, t, CH2), 2.97 (2H, t, CH2), 2.89 (4H, s, CH2). 2.2.3. (3-Indolyl-2-ethyl)(4-imidazolylmethyl)(2pyridylmethyl)amine (H2L3) 2.2.3.1. (3-Indolyl-2-ethyl)(2-pyridylmethyl)amine. To tryptamine (6.5 g, 0.04 mol) dissolved in methanol (10 mL), was combined with pyridine-2-carboxaldehyde (4.3 g, 0.04 mol). After diluting the mixture with methanol (70 mL), the solution was hydrogenated under hydrogen (1 atm) in the presence of 10% Pd–C catalyst (0.6 g) over night. After removal of the catalyst by filtration through a silica plug, the solvent was removed by rotary evaporation to give a pale yellow oil (8.24 g, 82%), which was used without further purification in the synthesis of H2L3. 2.2.3.2. H2L3. (3-Indolyl-2-ethyl)(2-pyridylmethyl)amine (410 mg, 1.63 mmol) dissolved in 1,2-dichloroethane (10 mL) was added
4(5)-imidazolylcarboxaldehyde (156 mg, 1 eq.) and sodium triacetoxyborohydride (800 mg). Additional 1,2-dichloroethane (30 mL) was added and the reaction mixture was stirred under dinitrogen for 3 days. The mixture was quenched by adding aqueous saturated sodium bicarbonate, and the product was extracted with diethylether. The combined extracts were dried over anhydrous magnesium sulfate and the solvents removed on a rotary evaporator; the residue was placed under high vacuum to afford a light brown oil (0.46 g, 85%). m/z (ESI-MS) 334 (M+, 25%), 252 (100); dH (D2O) 8.42 (1H, s, Im), 8.23 (1H, d, Py), 8.03 (1H, d, Py), 7.52 (2H, d, Ind), 7.34 (2H, m, Py & Ind), 7.12 (2H, t, Ind), 7.04 (1H, s, Im), 6.91 (1H, t, Py), 4.17 (2H, s, CH2), 4.11 (2H, s, CH2), 3.09 (2H, d, CH2), 2.84 (2H, d, CH2). 2.3. [Cu(L)Cl2] (L = HL1–H2L3) complexes 2.3.1. [Cu(HL1)Cl2] (1) CuCl2 2H2O (25 mg, 1 eq.) in iso-propanol (10 mL) was layered on HL1 (50 mg, 0.14 mmol) in dichloromethane (2 mL). After several days, a deep green crystalline solid (47.7 mg, 71%) deposited from the mixture. (Found: C, 54.85; H, 4.90; N, 11.49%. C22H22Cl2CuN4 0.5H2O requires C, 54.38; H, 4.77; N, 11.53%); kmax/nm (MeOH) 219 (e/ dm3 mol1 cm1 14833), 256 (12472), 282 (9722), 676 (148); EPR (frozen MeOH solution, 77 K) g|| = 2.23, g ? = 2.04, A|| = 178 G; m/cm1 (KBr disc) 1608s, 1573w, 1480w, 1459m, 1443s, 1471m, 1343m, 1284m, 1092m, 1055m, 1024m. 2.3.2. [Cu(HL2)Cl2] (2) HL2 (94.4 mg, 0.255 mmol) in iso-propanol (2 mL) was layered on CuCl2 2H2O (43 mg, 1 eq.) in iso-propanol (10 mL); upon standing for several days a dark green precipitate formed (92.3 mg, 72%). (Found: C, 49.63; H, 5.81; N, 9.37%. C24H26Cl2CuN4 4H2O requires C, 49.96; H, 5.94; N, 9.71%); kmax/nm (MeOH) 214 (e/ dm3 mol1 cm1 12382), 263 (8470), 674 (158); EPR (frozen MeCN solution, 77 K) g|| = 2.35, g ? = 2.09, A|| = 159 G; m/cm1 (KBr disc) 3335br, 1607s, 1570m, 1484m, 1445s, 1313w, 1108w, 1027w. 2.3.3. [Cu(H2L3)Cl2] (3) H2L3 (100 mg, 0.227 mmol) in methanol (5 mL) was added slowly to a solution of CuCl2 2H2O (38.7 mg, 1 eq.) in methanol (5 mL). After stirring for 10 min, the solution was left to stand under a diethylether atmosphere for a few days; a green crystalline solid (74 mg, 75%) formed. (Found: C, 49.71; H, 4.87; N, 13.87%. C20H21Cl2CuN5 H2O 0.5CH3OH requires C, 49.25; H, 5.04; N, 14.01%); kmax/nm (MeOH) 220 (e/dm3 mol1 cm1 14222), 263 (10444), 280 (911), 653 (100); EPR (frozen MeOH solution, 77 K) g|| = 2.23, g ? = 2.05, A|| = 175 G; m/cm1 (KBr disc) 3248br, 2895s, 1609s, 1572w, 1503w, 1456s, 1445s, 1350m, 1271m, 1104m. 2.4. X-ray crystallography As good quality crystals of complexes 1–3 were not obtained, each complex was separately treated with an excess K[PF6], Na[BF4] or Li(ClO4) in methanol and the resulting green solutions left to stand under an atmosphere of diethylether. Single crystals of [Cu(HL1)Cl](ClO4).0.5Et2O 0.25H2O (4), [Cu(HL2)F]2[BF4]2 (5) and [Cu(H2L3)Cl][PF6] (6) were obtained. X-ray reflection data for 4–5 were measured with an Enraf-Nonius CAD-4 diffractometer in x–2h scan mode at 294 K. Crystal structure and refinement data are collected in Table 1. A summary of key bond lengths and bond angles is given in Table 2. The CIF files have been deposited at the Cambridge Crystallographic Data Centre and allocated the deposition numbers CCDC 710201–710203.
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S.-T. Lee et al. / Polyhedron 28 (2009) 1097–1102 Table 1 X-ray crystal structure collection and refinement data.
Formula Formula mass Crystal size Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) T (°C) Z Dcalc (g cm3) Radiation k (Å) l (cm1) F (0 0 0) Maximum, minimum transmission factors hmax Number of intensity measurements Rmerge for number of equivalent reflections Number of observed reflections in final refinement [I/r(I) > 3] Number of variables in final refinement R Rw Goodness-of-fit Maximum, minimum peak in final difference map (e Å3)
[Cu(HL1)Cl](ClO4) 0.5C4H10O 0.25H2O (4)
[Cu(HL2)F]2[BF4]2 (5)
[Cu(H2L3)Cl][PF6] (6)
C24H27.5Cl2Cu N4O4.75 582.5 0.26 0.21 0.05 C2/c 23.077(9) 14.427(4) 15.600(7) 90 94.43(4) 90 21 8 1.49 Mo Ka, 0.71073 1.096 2404.0 0.89, 0.77 23 3693 0.037 1460 171 0.093 0.104 2.04 1.13, 1.69
C24H26BCuF5N5 539.8 0.24 0.17 0.10 P1 10.885(6) 11.067(7) 12.225(6) 63.91(3) 64.32(3) 81.29(3) 21 2 1.51 Mo Ka, 0.71073 0.975 554.0 0.87, 0.78 24 3928 0.025 3342 265 0.051 0.077 1.64 1.11, 0.67
C20H21ClCuF6N5P 575.4 irregular P21/c 11.761(6) 15.157(4) 14.866(7) 90 119.70(2) 90 21 4 1.66 Mo Ka, 0.71073 1.205 1164.0 25 4207 0.026 2168 186 0.071 0.082 1.71 1.26, 1.17
Table 2 Selected key bond length (Å) and angle (°) data for complexes 4–6. 4 Cu1–Cl1 Cu1–N1 Cu1–N2 Cu1–N4 Cl1–Cu1–N1 Cl1–Cu1–N2 Cl1–Cu1–N4 N1–Cu1–N2 N1–Cu1–N4 N2–Cu1–N4
5 2.241(5) 1.909(12) 1.907(11) 2.052(13) 97.181(5) 96.472(4) 178.512(4) 165.611(6) 83.211(7) 83.028(7)
Cu1–F1 Cu1–F1 Cu1–N1 Cu1–N2 Cu1–N3 Cu1–Cu1 F1–Cu1–F1 F1–Cu1–N1 F1–Cu1–N2 F1–Cu1–N3 F1–Cu1–N1 F1–Cu1–N2 F1–Cu1–N3 N1–Cu1–N2 N1–Cu1–N3 N2–Cu1–N3 Cu1–F1–Cu1
3. Results and discussion 3.1. Syntheses Two new indole-substituted ligands, HL1 and H2L3, were easily synthesized by reductive amination using an appropriate aldehyde and tryptamine with sodium triacetoxyborohydride in dry 1,2dichloroethane [18]. HL2 was produced by reaction of excess 2-vinyl pyridine with tryptamine. Copper(II) complexes, [Cu(L)Cl2] (1: L = HL1, 2: L = HL2; 3: L = H2L3) were obtained by adding the ligand (L) in dichloromethane to CuCl2 2H2O in iso-propanol. After several days, the green–blue microcrystalline complexes 1–3 precipitated. Unfortunately, none of the complexes gave crystals of sufficient quality for an X-ray crystal structure determination. 3.2. X-ray crystallography As crystals of 1–3 were unavailable, in separate reactions, each of these complexes was treated with excess K[PF6], Na[BF4] or Li(-
6 1.911(2) 2.212(2) 2.064(3) 2.013(3) 2.011(3) 3.125(2) 81.706(8) 88.602(1) 89.141(1) 176.011(1) 101.047(1) 96.576(1) 101.190(1) 161.712(1) 88.146(1) 93.226(1) 98.294(8)
Cu1–Cl1 Cu1–N1 Cu1–N3 Cu1–N5 Cl1–Cu1–N1 Cl1–Cu1–N3 Cl1–Cu1–N5 N1–Cu1–N3 N1–Cu1–N5 N3–Cu1–N5
2.249(2) 1.933(7) 1.940(7) 2.091(7) 95.386(2) 98.691(2) 177.012(2) 164.628(3) 81.723(3) 84.266(3)
ClO4) in methanol and the resulting green solutions were left to stand under an atmosphere of diethylether. Single crystals of [Cu(HL1)Cl](ClO4) 0.5Et2O 0.25H2O (4), [Cu(HL2)F]2[BF4]2 (5) and [Cu(H2L3)Cl][PF6] (6) were obtained that were satisfactory for X-ray structure determinations. Fig. 1 presents views of the structures and selected bond length and angle data are gathered in Table 2. Complex 4 exhibits a distorted square planar copper(II) centre formed by the binding of the two pyridine and amine nitrogen atoms, and a chloro co-ligand, Fig. 1a. The sum of the six lessthan-180° angles about the Cu centre is 704°, which compares with 720° (and 645°) for perfect square planar (and tetrahedral) centre(s) [19,20]. The Cu–Cl1 (2.241(5) Å), Cu–N(amine) (2.052(13) Å), and Cu–N(py) (1.907(11) and 1.909(12) Å) distances are all in the anticipated range for copper(II) complexes. The indole substituent does not bind to the copper(II) centre and lies offset pstacked with the pyridine ring containing N2. The inter-ring separation of the offset p-stacked pyridinyl and indolyl rings is 3.38 Å, which is comparable to the Trp 236 His 291 separation in cyto-
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Fig. 1. Views of the paired cations and closely associated anions in the X-ray crystal structures of: (a) [Cu(HL1)Cl](ClO4), 4; (b) [Cu(HL2)F]2[BF4]2, 5; (c) [Cu(H2L3)Cl][PF6], 6. In all views, 50% thermal ellipsoids are shown and hydrogen atoms bonded to carbon have been omitted for clarity; in each case, atoms labelled with an asterisk are related by a crystallographic inversion to the corresponding atom labelled without the asterisk.
chrome c oxidases (see above) [1]. In the crystal structure, adjacent cation pairs of 4 are connected by weak (3.10 Å) Cu Cl1 interactions. The inter-copper distance between neighbouring cations is 3.75 Å. The indole N–H forms a hydrogen bond to the perchlorate counterion (N3 O 2.97 Å). Fig. 1b shows the centrosymmetric [Cu(HL2)F]22+ dimers and associated [BF4] counterions in the crystal structure of 5. Fluoride abstraction reactions in transition metal complexes containing the tetrafluoroborate anion are well-known [21]. The copper(II) centre in 5 has a typical distorted square pyramidal geometry with a trigonality index [22] s = 0.25 [F1–Cu1–N3 = 176.011(1)° is the largest bond angle and N1–Cu1–N2 = 161.712(1)° the next largest]. Axial Cu–F1 bonds (2.212(2) Å) link to the adjacent Cu to give a dimeric structure. The axial Cu–F1 bond is longer than the in-plane Cu–F1 (1.911(2) Å) and Cu–N bonds (2.029 Å average). The Cu Cu distance in the dimer is 3.125(2) Å. The indole substituent of HL2 is not coordinated to the copper centre and is without any p-stacking interactions within the complex ion. There is an hydrogen bond between the indole N–H and F on tetrafluoroborate (N F 3.04 Å). The molecular structure of 6, Fig. 1c, reveals the copper(II) ion to be distorted square planar bound by the amine, imidazolyl and pyridyl nitrogen atoms and by a chloro co-ligand. The sum of the six less-than-180° bond angles about the copper(II) centre is 701.7° [19,20]. The indolyl ring is offset p-stacked with the pyridyl ring consistent with the larger p-surface area and electron density of pyridyl cf. imidazolyl rings. The separation between the p-stacked indolyl and pyridyl rings is 3.11 Å. The Cu1–Cl1 and Cu1–Cu1 (adjacent cation) distances are 3.14 and 3.72 Å, respectively. The [PF6] counterion forms a F H–N hydrogen bond (2.94 Å) with the coordinated imidazolyl N–H rather than with the pendant indole.
1500 cm1 [23]. The Vis/NIR spectra of 1–3 in methanol solution show a broad band having a maximum at 640–750 nm and a prominent tail to lower energy, consistent with each complex adopting a tetragonally-elongated, square pyramidal (CuCl2N3) structure, Table 3. Likewise EPR spectra of complexes 1–3 in frozen Table 3 Electronic and X-band EPR spectroscopic data for the Cu(II) complexes in methanol.
1 2 3
UV-NIR (297 K) [kmax/nm] (e/ dm3 mol1 cm1)
EPR (77 K)
676 (148) 674 (158) 653 (100)
g|| = 2.23, g ? = 2.04, A|| = 178 G g|| = 2.35, g ? = 2.09, A|| = 159 G g|| = 2.23, g ? = 2.05, A|| = 175 G
a
b
3.3. Physicochemical data for complexes 1–3 in solution Numerous studies reveal [Cu(N3–L)Cl2] complexes, analogous to 1–3, to contain five-coordinate CuIICl2N3 centres that display characteristic EPR and UV–Vis spectra [17–22,24–26]. Available data are indicative for five-coordinate CuIICl2N3 centres in 1–3. The FTIR of spectrum of each complex (see Supplementary data) shows characteristic pyridyl stretches at 1604 and 1571 cm1 indicative for all pyridines binding to the Cu(II) ion. The fingerprint regions of the IR spectra show characteristic indole peaks between 1400 and
2750
3000 3250 Magnetic Field / G
3500
Fig. 2. X-band EPR spectra of complexes 1 (a) and 3 (b) in frozen methanol solution at 77 K; m/GHz = 9.448.
S.-T. Lee et al. / Polyhedron 28 (2009) 1097–1102
1101
methanol at 77 K are axial (dx2 y2 ground-state) fully consistent with square pyramidal CuCl2N3 structures, e.g. Fig. 2 and Table 3. N-Superhyperfine coupling is also apparent in the spectra: e.g. Nsuperhyperfine peaks (AN–Cu 12 G) from three Cu(II)-bound Ndonors are clearly discerned in the spectrum of 1, Fig. 2a.
to oxidation of the indole substituent. Notably, although the merging of chloride- and indole-centred oxidation processes complicates the electrochemistry, the potential of the indole oxidation is only slightly, if at all, affected by coordination of the indolesubstituted ligand to the copper(II) centre.
3.4. Electrochemistry of complexes 1–3
4. Conclusions
Cyclic voltammograms (CVs) of 1–3 in acetonitrile were acquired to characterize the redox properties of the indole and copper centres in each complex. The CVs show quasi-reversible Cu(II)/ Cu(I) couples, at 0.57 V for the methylene-bridged amine complexes 1 and 3 and at –0.25 V for the ethylene-linked amine complex 2 (e.g. see Fig. 3). Lower potential Cu(II)/Cu(I) couples are expected for complexes of bis(pyridylmethyl)amine ligands compared to complexes of bis(pyridylethyl)amines [24–26]. On scanning to positive potentials, the CVs of 1–3 all show a broad anodic peak at +0.64 V followed closely by an peak at +0.77 V for an oxidation process that appears to have some reversibility (a broad cathodic peak is discernable ca. at +0.71 V in the return scan: ip,c/ip,a = 1.1). The broad anodic peak at +0.64 V is found in CVs of both bis(triphenylphosphoranylidene)ammonium chloride (PPNCl) and [(Bz-bpa)CuCl2] and, therefore, may be attributed to oxidation of ‘free’ chloride anion (in the complexes, from dissociation of chloro ligand) [27–29]. The second peak at +0.77 V in the CVs of 1–3 is absent in the CVs of PPNCl and [(Bz-bpa)CuCl2] and therefore must be an indole substituent-centred oxidation. For comparison, CVs of indole, Fig. 3a, reveal a broad anodic peak at +0.80 V and CVs of 3-methylindole, Fig. 3b, show a sharp anodic peak at +0.57 V that is followed by a broad peak at +0.80 V; the irreversibility arises from rapid coupling reactions of the indolyl radical [30,31]. Thus, the peaks in this region for 1–3 can be attributed
Three new indole-substituted ligands (HL1–H2L3) were synthesized from tryptamine and copper(II) complexes (1–6) were successfully obtained from them. The results of the electrochemistry for complexes 1–3 show coordination of the ligands to a copper centre and the proximity of the indole and copper(II) centres does not affect the CuII/CuI couples or the indole-centred oxidation processes. This finding contrasts with our previous results for some complexes wherein copper(II) centres closely related to those reported here were substituted with ferrocenyl (Fc) substituents; in these Fc Cu(II) complexes significant mutual perturbations of the electron donor properties of each centre were observed that lead to new reactivity [26]. The crystal structures of 4 and 6 reveal offset p-stacking between the indolyl group and the ring of a pyridinyl or imidazolyl donor bound to the Cu(II) ion. The inter-ring separation is ca. 3.1–3.3 Å, which is analogous to that observed for the Trp 236 His 291 sidechains within CcO. In solution, p-stacking of the indolyl and ligand heterocyclic donor rings in complexes 1–6 is possible, but much less likely due to the disrupting effect(s) of solvent molecules. Thus model systems of considerably more sophistication will be needed if the effects of p-stacking between an indolyl group and a Cu(II)-bound heterocyclic ligand donor are to be explored further. Acknowledgement This research was supported by the Australian Research Council. Appendix A. Supplementary data CCDC 710201, 710202, 710203 contain the supplementary crystallographic data for 4, 5, 6. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.poly.2009.01.025. References
Fig. 3. Cyclic voltammograms of indole (a), 3-methylindole (b), 1 (c), 3 (d) and PPNCl (e) in acetonitrile with 0.1 M [Bu4 N][PF6] at 295 K; 1.0 mm Pt disc working electrode; scan rate = 100 mV s1.
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