DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 1999–2004 1999 Synthesis, crystal structures and properties of copper(II) complexes of SchiV base derivatives containing imidazole and ‚-alanine groups La-Sheng Long, Shi-Ping Yang, Ye-Xiang Tong, Zong-Wan Mao, Xiao-Ming Chen * and Liang-Nian Ji School of Chemistry and Chemical Engineering, Zhongshan University, Guangzhou 510275, China. E-mail: cescxm@zsu.edu.cn Received 22nd December 1998, Accepted 23rd April 1999 Four copper(II) complexes with SchiV base ligands N-[(5-methylimidazol-4-yl)methylene]-b-alanine (HL1) and N-[(1-methylimidazol-2-yl)methylene]-b-alanine (HL2) and their reduced forms N-[(5-methylimidazol-4-yl)methyl]- b-alanine (HL3) and N,N-bis[(5-methylimidazol-4-yl)methyl]-b-alanine (HL4) have been synthesized. The crystal structures of [CuL1(H2O)(ClO4)] 1, [CuL2(H2O)(ClO4)] 2, [CuL3(Py)(ClO4)] 3 and [Cu2(L4)2][ClO4]2?2H2O 4 have been determined.Complexes 1 and 2 are structurally very similar, the copper(II) atom being tridentately chelated by the SchiV base using one carboxy oxygen atom and two nitrogen atoms at the equatorial positions, with the fourth equatorial position being occupied by another carboxy oxygen atom from an adjacent SchiV base; one aqua ligand and one perchlorate oxygen atom ligate at the axial positions, resulting in an elongated octahedral geometry.Each carboxy group in 1 and 2 acts in the syn-anti mode and bridges two adjacent copper(II) atoms via two equatorial positions, resulting in one-dimensional helical (Cu–O–C–O–Cu)n chains.In 3 the copper(II) atom is ligated by two nitrogen atoms, one carboxy oxygen atom from an L3 ligand and another nitrogen atom from pyridine at the equatorial position; the axial positions are occupied by one perchlorato oxygen atom and one carboxy oxygen atom from another L3 ligand, tridentately chelating an adjacent copper(II) atom, resulting in an elongated octahedral geometry.Complex 4 contains a dimeric cation with two very similar square-pyramidally co-ordinated copper(II) atoms. Each L4 ligand chelates a copper(II) atom in a tetradentate mode with the three nitrogen atoms occupying the equatorial positions and the carboxy oxygen atom the apical position. The fourth equatorial position is taken by a carboxy oxygen atom from another L4 ligand chelating another copper(II) atom, resulting in a bis(carboxylato-O)- bridged dimeric structure. The electronic and EPR spectra and redox properties of 1–4 are also discussed.Considerable attention has previously been given to SchiV base adducts formed between pyridoxal or its analogues and amino acids.1,2 Particularly, pyridoxal phosphate (PLP) acts as a cofactor in many enzymes catalysing transformations of amino acids.3–5 Investigations have been conducted on ternary copper(II) complexes of N-salicylideneglycine and its derivatives with imidazole, pyrazole or pyridine by EPR and their redox properties.6–9 Of particular importance are the chelates containing histidine residues; not only do the polydentate SchiV base ligands provide a useful framework to establish relationships between spectral properties of the complexes and the binding mode of the histidine residue,10 but also the histidinelike binding modes of amino acid residues may model structural features in biological systems.The investigation of the structures and spectral properties of metal complexes containing histidine 11 or histidyl residues 10,12 is thus very important for elucidating structure/function relations in histidine-containing biological systems.However, the only reported crystal structure containing a histidine SchiV base appears to be a cobalt complex [Co(SalHis)(Ala)]?2H2O [SalHis = a-N-(o-hydroxybenzyl)- L-histidinate, Ala = L-alaninate].13 We have recently reported the synthesis, structures and properties of a series of metal complexes with imidazole-containing ligands relevant to structure/function relations of some metalloenzymes. 14 As a sequel, in this paper we report the synthesis of the SchiV base ligands N-[(5-methylimidazol-4-yl)methylene]- b-alanine (HL1), N-[(1-methylimidazol-2-yl)methylene]- b-alanine (HL2) and their reduced forms N-[(5-methylimidazol- 4-yl)methyl]-b-alanine (HL3) and N,N-bis[(5-methylimidazol- 4-yl)methyl]-b-alanine (HL4), and crystal structures, electronic and EPR spectra and redox properties of their copper( II) complexes, [CuL1(H2O)(ClO4)] 1, [CuL2(H2O)(ClO4)] 2, [CuL3(Py)(ClO4)] 3 and [Cu2(L4)2][ClO4]2?2H2O 4.Experimental All reagents were commercially available and used as received. Solvents were dried by conventional procedures prior to use. All samples were thoroughly dried prior to elemental analyses. Physical measurements The C, H and N elemental analyses were performed on a Perkin-Elmer 204 elemental analyser. The IR spectra were recorded on a Nicolet 5DX FT-IR spectrophotometer with KBr discs in the 4000–400 cm21 region, electronic spectra on a Shimadzu MPS-200 spectrometer in DMF solutions and X-band EPR spectra from crystalline samples on a Bruker ER- 420 spectrometer operating at 77 K in DMF solution.Cyclic voltammetry was performed on an electrochemical analyser in DMF. A platinum wire working electrode, a platinum plate auxiliary electrode and a saturated calomel reference electrode (SCE) were employed. CAUTION: perchlorate salts of metal complexes are potentially explosive and should be handled in small quantities with care.Synthesis of ligands HL3. To a solution of b-alanine (0.89 g, 10 mmol) in methanol (10 cm3) containing KOH (0.56 g, 10 mmol) was added2000 J. Chem. Soc., Dalton Trans., 1999, 1999–2004 5-methylimidazole-4-carbaldehyde (1.10 g, 10 mmol) in methanol (10 cm3). The solution was refluxed with stirring for 2 h. The yellowish solution was cooled in an ice-bath, then reduced with an excess of NaBH4 (0.46 g, 12 mmol) in methanol containing a few drops of sodium hydroxide solution.The yellowish colour slowly discharged, and after 10 min the solution was evaporated and extracted with dry methanol, then acidified with HCl gas. The resulting solid was filtered oV, washed with dry methanol and diethyl ether, and dried, which yielded 45.6% (Calc. for C8H13N3O2?2HCl: C, 37.50; H, 5.86; N, 16.41. Found: C, 37.53; H, 6.03; N, 16.34%). IR (KBr, cm21): 3163m, 3100s, 3071s, 2980s, 2790m, 2600m, 1735s, 1602w, 1539w, 1468m, 1398w, 1370m, 1264m, 1208s, 1159m, 1068m, 1004w, 962w, 871w, 800w, 772m, 695w, 603w, 470w and 435w.HL4. To a solution of HL3?2HCl (2.56 g, 10 mmol) in methanol (10 cm3) containing KOH (1.68 g, 30 mmol) was added 5-methylimidazole-4-carbaldehyde (1.10 g, 10 mmol) in methanol (10 cm3). The solution was refluxed with stirring for 2 h. The yellowish solution was cooled in an ice-bath, then reduced with an excess of NaBH4 (0.46 g, 12 mmol) in methanol containing a few drops of sodium hydroxide solution.The yellowish colour slowly discharged, and after 10 min the solution was evaporated and extracted with dry methanol, then acidified with HCl gas. The resulting solid was filtered oV, washed with dry methanol and diethyl ether, and then dried, which yielded 0.98 g of a solid containing HL4?4HCl (82%) and HL3?2HCl (18%). The percentages of each were calculated according to elemental analysis (Found: C, 36.95; H, 5.76; N, 16.43%).Preparation of metal complexes (a) [CuL1(H2O)(ClO4)] 1. To a solution of b-alanine (0.089 g, 1.0 mmol) in methanol (10 cm3) containing KOH (0.056 g, 1.0 mmol) was added 5-methylimidazole-4-carbaldehyde (0.11 g, 1.0 mmol) in methanol (10 cm3). The solution was refluxed with stirring for 2 h. To the yellowish solution was added a solution of copper(II) nitrate hydrate (0.20 g, 1.0 mmol) and a solution of sodium perchlorate (0.14 g, 1.0 mmol) in aqueous methanol (10 cm3, 1 : 5 v/v).The reaction mixture turned deep blue immediately. After allowing it to stand in air at room temperature for two days, the deposited deep blue crystals of complex 1 were collected and washed with ethanol, yield 67%. A single crystal suitable for X-ray work was obtained by recrystallization in MeCN (Calc. for C8H12ClCuN3O7: C, 26.59; H, 3.32; N, 11.63. Found: C, 26.39; H, 3.35; N, 11.54%). IR (cm21): 3430s, 3128s, 3015s, 2910m, 2853m, 1637s, 1560s, 1518s, 1440m, 1426m, 1398m, 1349w, 1293w, 1236w, 1215w, 1124s, 1089s, 1075s, 976w, 878w, 765w, 695w and 632s.(b) [CuL2(H2O)(ClO4)] 2. Complex 2 was prepared as for 1. Deep blue needle crystals were obtained, yield 65% (Calc. for C8H12ClCuN3O7: C, 26.59; H, 3.32; N, 11.63. Found: C, 26.35; H, 3.28; N, 11.61%). IR (cm21): 3655s, 3325s, 3142s, 3029s, 2910m, 2853m, 1637s, 1567s, 1518s, 1454m, 1426s, 1356w, 1293w, 1250w, 1215w, 1145s, 1110s, 1075s, 962w, 828w, 765w, 674w and 632s.(c) [CuL3(Py)(ClO4)] 3. To an ethanol (10 cm3) solution of L3?2HCl (0.256 g, 1.0 mmol) containing KOH (0.17 g, 3.0 mmol) and pyridine (0.079 g, 1.0 mmol), an aqueous ethanol (10 cm3, 50% v/v) solution of copper(II) nitrate hydrate (0.20 g, 1.0 mmol) and NaClO4?H2O (0.14 g, 1.0 mmol) was added dropwise while stirring. After filtration, the deep blue solution was allowed to stand at room temperature in air, yielding transparent deep blue polyhedral crystals of complex 3, yield 57% (Calc.for C13H17ClCuN4O6: C, 36.80; H, 4.01; N, 13.20. Found: C, 36.39; H, 3.95; N, 13.54%). IR (cm21): 3571s, 3458s, 3148s, 3043s, 2903m, 2714m, 1630s, 1581s, 1525s, 1447m, 1426m, 1384m, 1328w, 1278w, 1243w, 1138s, 1117s, 1082s, 990w, 878w, 821w, 667w and 632s. (d) [Cu2(L4)2][ClO4]2?2H2O 4. To an ethanol (10 cm3) solution of L4?4HCl (0.423 g, 1.0 mmol) containing NaOH (0.20 g, 5.0 mmol), an aqueous ethanol (10 cm3, 50% v/v) solution of copper(II) nitrate hydrate (0.20 g, 1.0 mmol) and NaClO4?H2O (0.14 g, 1.0 mmol) was added dropwise while stirring.After filtration, the blue solution was allowed to stand at room temperature in air, yielding transparent deep blue polyhedral crystals of complex 4, yield 35% (Calc. for C13H20ClCuN5O7: C, 34.14; H, 4.38; N, 15.32. Found: C, 34.39; H, 4.35; N, 15.54%). IR (cm21): 3655s, 3325s, 3142s, 3029s, 2910m, 2853m, 1560s, 1518s, 1454m, 1426s, 1356w, 1293w, 1250w, 1215w, 1145s, 1110s, 1075s, 962w, 828w, 765w, 674w and 632s.X-Ray crystallography DiVraction intensities for the four complexes were collected at 293 K on a Siemens R3m diVractometer. Lorentz-polarization and absorption corrections were applied. The structure solution and full-matrix least-squares refinement based on F2 were performed with the SHELXS 97 and SHELXL 97 program packages, respectively.15,16 Complex 2 crystallizes in a noncentrosymmetric space group; the absolute structure has been determined with a Flack parameter of 0.10(6).17 All the nonhydrogen atoms were refined anisotropically. Hydrogen atoms of the organic ligands were generated geometrically (C–H 0.96 Å) and those of the aqua ligands located from the diVerence maps; all the hydrogen atoms were assigned the same isotropic thermal parameters and included in the structure-factor calculations.Analytical expressions of neutral-atom scattering factors were employed, and anomalous dispersion corrections incorporated.18 The crystallographic data for 1–4 are summarized in Table 1.Selected bond distances and angles are given in Table 2. CCDC reference number 186/1439. See http://www.rsc.org/suppdata/dt/1999/1999/for crystallographic files in .cif format. Results and discussion Crystal structures (a) [CuL1(H2O)(ClO4)] 1. An ORTEP19 view of the coordination environment of complex 1 is shown in Fig. 1. The surrounding of each copper(II) atom is (412). The L1 ligand acts in a tridentate chelate mode for the copper(II) atom, utilizing the two nitrogen atoms and one carboxy oxygen atom to bind the metal atom at the equatorial positions; the fourth equatorial position is occupied by an oxygen atom O(2a) Fig. 1 An ORTEP view of the co-ordination geometry and hydrogen bonding scheme in complex 1.J. Chem. Soc., Dalton Trans., 1999, 1999–2004 2001 Table 1 Crystal data and structure refinement for complexes 1–4 1 2 3 4 Empirical formula Formula weight Crystal system Space group a/Å b/Å c/Å b/8 ZV /Å3 m(Mo-Ka)/cm21 R1 (I > 2s) R2 (all data) C8H12ClCuN3O7 361.20 Monoclinic C2/c 14.916(7) 7.458(4) 23.687(12) 97.89 4 2610(2) 1.914 0.0728 0.2041 C8H12ClCuN3O7 361.20 Orthorhombic Pna21 12.793(10) 13.167(8) 7.731(3) 4 1302.3(8) 1.918 0.0618 0.1858 C13H17ClCuN4O6 424.29 Monoclinic P21/n 10.314(12) 8.167(4) 19.468(16) 102.09 4 1604(2) 1.569 0.0566 0.1508 C26H40Cl2Cu2N10O14 914.66 Monoclinic P21/c 8.240(3) 18.185(5) 23.445(6) 90.96(3) 4 3512.6(18) 1.445 0.0559 0.1367 belonging to the carboxy group of an adjacent and symmetryrelated fragment.The bond lengths Cu(1)–N(1) and Cu(1)– N(3) are 1.937(6) and 1.967(5) Å, respectively; the Cu–O Table 2 Selected bond lengths (Å) and angles (8) for complexes 1–4 1 Cu(1)–O(1) Cu(1)–O(2a) N(3)–C(6) N(3)–C(5) Cu(1) ? ? ? O(1w) N(2) ? ? ? O(5b) C(5)–N(3)–C(6) C(6)–N(3)–Cu(1) 1.913(5) 1.953(5) 1.443(13) 1.263(8) 2.624(8) 2.976(1) 122.3(6) 123.5(4) Cu(1)–N(1) Cu(1)–N(3) C(4)–C(5) Cu(1) ? ? ? O(5) C(5)–N(3)–Cu(1) 1.937(6) 1.967(5) 1.424(8) 2.701(8) 114.2(5) 2 Cu(1)–O(2a) Cu(1)–O(1) Cu(1) ? ? ? O(1w) N(3)–C(5) O(2)–Cu(1b) C(5)–N(3)–C(6) C(6)–N(3)–Cu(1) 1.933(4) 1.941(5) 2.489(6) 1.278(9) 1.933(4) 123.7(5) 125.2(4) Cu(1)–N(1) Cu(1)–N(3) Cu(1) ? ? ? O(3) N(3)–C(6) C(4)–C(5) C(5)–N(3)–Cu(1) 1.937(6) 2.025(5) 2.749(10) 1.408(10) 1.385(10) 111.2(5) 3 Cu(1)–O(1) Cu(1)–N(3) N(3)–C(5) N(3)–C(6) Cu(1)–O(2a) N(2)–O(2b) C(5)–N(3)–C(6) C(5)–N(3)–Cu(1) 1.916(4) 1.991(5) 1.455(6) 1.453(6) 2.722(4) 2.758(6) 113.8(4) 111.2(3) Cu(1)–N(1) Cu(1)–N(4) C(4)–C(5) Cu(1)–O(5) C(6)–N(3)–Cu(1) 1.952(4) 2.001(5) 1.492(7) 2.795(6) 117.5(3) 4 Cu(1)–N(4) Cu(1)–O(3) Cu(1)–O(1) Cu(2)–N(6) Cu(2)–O(1) Cu(2)–O(3) N(2)–O(11) N(3)–C(5) N(5)–O(2w) N(8)–C(26) N(7)–O(1w) N(10)–O(8) O(2w)–O(4b) C(5)–N(3)–C(6) C(5)–N(3)–Cu(1) 1.935(4) 1.949(4) 2.263(4) 1.931(4) 1.955(3) 2.259(4) 3.054(8) 1.495(7) 2.763(7) 1.492(7) 2.729(6) 2.862(7) 2.786(6) 109.5(5) 106.6(3) Cu(1)–N(1) Cu(1)–N(3) Cu(1)–Cu(2) Cu(2)–N(9) Cu(2)–N(8) N(3)–C(6) N(3)–C(11) C(4)–C(5) N(8)–C(31) N(8)–C(25) O(1w)–O(2a) N(2)–O(12) C(6)–N(3)–Cu(1) C(5)–N(3)–C(11) 1.937(4) 2.081(4) 3.2557(15) 1.940(4) 2.077(4) 1.489(7) 1.497(7) 1.488(8) 1.479(7) 1.493(7) 2.734(6) 2.979(7) 111.2(5) 107.6(4) Symmetry codes: for 1, a, 2x 1 3/2, y 2 1/2, 2z 1 1/2; b, x, y 2 1, z; c, 2x 1 3/2, y 1 1/2, 2z 1 1/2; for 2, a, 2x, 2y 1 1, z 2 1/2; b, 2x, 2y 1 1, z 1 1/2; for 3, a, 2x 1 3/2, y 1 1/2, 2z 1 3/2; b, x 2 1/2, 2y 1 1/2, z 2 1/2; for 4, a, 2x 1 1, y 1 1/2, 2z 1 1/2; b, 2x, 2y, 2z.(carboxy) distances are 1.913(5) and 1.953(5) Å. An aqua ligand and a perchlorate oxygen atom ligate the copper(II) atom at the axial positions with much longer bond distances [Cu(1)–O(1w) 2.624(8), Cu(1)–O(5) 2.701(8) Å], resulting in an elongated octahedral geometry. It is interesting that each carboxylate group acts in the bidentate syn-anti mode and bridges each pair of adjacent copper(II) atoms (C ? ? ? Cu 4.757 Å ) via the two equatorial positions, giving rise to one-dimensional (Cu–O–C–O–Cu)n skeletons running parallel to the crystallographic a axis (Fig. 2), which may best be visualized as helix-like chains, being similar to an example previously documented.20 (b) [CuL2(H2O)(ClO4)] 2. An ORTEP19 view of the coordination environment of complex 2 is shown in Fig. 3. The crystal structure of 2 is very similar to that of 1. The bond lengths Cu(1)–N(1) and Cu(1)–N(3) are 1.937(6) and 2.025(5) Å, and Cu(1)–O(1) and Cu(1)–O(2a) are 1.941(4) and 1.933(4) Å, respectively. The diVerences of the corresponding values of the co-ordination bonds between 1 and 2 may be attributed to the diVerence in the imidazole groups in L1 and L2, where the Fig. 2 An ORTEP view of the helical chain of complex 1 running along the a axis.2002 J. Chem. Soc., Dalton Trans., 1999, 1999–2004 imidazole group in L1 is hydrogen bonded to a perchlorate group and that in L2 is methylated; this may result in diVerent co-ordination abilities for the two organic ligands.21,22 Also similar to that of complex 1, each carboxylate group acts in a bidentate syn-anti mode and bridges each pair of adjacent copper atoms (Cu ? ? ? Cu 4.867 Å) via the two equatorial positions, giving rise to one-dimensional (Cu–O–C– O–Cu)n helix-like chains in the solid.(c) [CuL3(Py)(ClO4)] 3. An ORTEP19 view of the molecular structure of complex 3 is shown in Fig. 4.The crystal structure consists primarily of discrete molecules of 3 with the copper(II) atom in a distorted square-planar geometry. The metal atom is tridentately chelated by an L3 ligand and the co-ordination sphere is completed by a pyridine ligand. One perchlorate oxygen atom and a carboxy oxygen atom from an adjacent molecule have weak interaction with the metal through the axial positions [Cu(1) ? ? ? O(5) 2.795(6), Cu(1) ? ? ? O(2a) 2.722(4) Å]. The significantly weaker axial interaction of the copper(II) atom may be attributed to the stronger donor ability of pyridine in 3, in comparison to that of a carboxy group in 1 and 2.The copper(II)–imidazole Cu(1)–N(1) bond [1.952(4) Å] is slightly shorter than that of the copper(II)–pyridine Cu(1)–N(4) bond [2.001(5) Å] and that of the copper(II)–amine Cu(1)–N(3) Fig. 3 An ORTEP view of the co-ordination geometry and hydrogen bonding scheme in complex 2. Fig. 4 An ORTEP view of the co-ordination geometry and hydrogen bonding scheme in complex 3.bond [1.991(5) Å]. The successful in situ reduction of the imine group is evident from the N(3)–C(5) distance [1.455(6) Å] compared with those [1.263(8) and 1.278(9) Å] for the related ligand (L1 and L2) in 1 and 2. Furthermore, the bond length C(4)–C(5) [1.492(7) Å] is typical of a single bond, which is significantly longer than the corresponding bonds in 1 and 2 [1.424(8) and 1.385(10) Å], and indicates the loss of conjugation. 23 The bond angles centred about the N(3) atom are 113.8(4), 117.5(3) and 111.2(3)8 for C(5)–N(3)–C(6), C(6)– N(3)–Cu(1) and C(5)–N(3)–Cu(1), respectively, being consistent with a sp3 tetrahedral configuration, and similar to those for [Co(SalHis)(Ala)].13 (d) [Cu2(L4)2][ClO4]2?2H2O 4. The crystal structure of complex 4 consists of discrete carboxylate-bridged dimeric cations, perchlorate anions and lattice water molecules. An ORTEP19 view of the dimeric cation is shown in Fig. 5, in which the two crystallographically independent copper(II) atoms are in very similar co-ordination environments. The anionic L4 ligand chelates a square-pyramidally co-ordinated copper(II) atom in a tetradentate fashion with the three nitrogen atoms occupying the equatorial positions and the carboxy oxygen atom occupying the apical position. The fourth equatorial position is taken by a carboxy oxygen atom from another L4 ligand chelating another copper(II) atom in the dimeric cation.The bond lengths Cu(1)–N(1), Cu(1)–N(3) and Cu(1)–N(4) are 1.937(4), 2.081(4) and 1.935(4) Å, respectively, while Cu(2)–N(6), Cu(2)–N(8) and Cu(2)–N(9) are 1.931(4), 2.077(4) and 1.940(4) Å, respectively. The two C–O distances of each carboxylate group are markedly diVerent [1.264(6) and 1.216(7); 1.270(6) and 1.213(7) Å], due to the monodentate m-carboxylate-O bridging mode, and similar to those documented for some bis(carboxylato)- dicopper(II) complexes.24 The intradimeric Cu(1) ? ? ? Cu(2) distance [3.256(2) Å], mainly dominated by the nature and coordination mode of the bridging groups,24,25 is shorter than those found in related dinuclear copper(II) complexes (average Cu ? ? ? Cu 3.4 Å).25 The bond lengths Cu(1)–O(1) [2.263(4) Å] and Cu(2)–O(3) [2.259(4) Å] are significantly longer than Cu(2)–O(1) [1.955(3) Å] and Cu(1)–O(3) [1.949(4) Å], respectively, which is due to the Jahn–Teller-distorted copper(II).26 Similar to complex 3, the reduction of the imine group in 4 is also evident from the bond lengths of N(3)–C and N(8)–C [1.479(7) to 1.497(7) Å].The bond angles centred about the N(3) atom are 109.5(5), 111.2(5), 106.6(3) and 107.6(4)8 for C(6)–N(3)–C(5), C(6)–N(3)–Cu(1), C(5)–N(3)–Cu(1) and C(5)–N(3)–C(11), respectively, which are consistent with a sp3 tetrahedral configuration and show smaller deviations from the idealized geometry. Electronic and EPR spectra The electronic spectral data of the four complexes are listed in Table 3.The UV spectra exhibit an intense absorption band Fig. 5 An ORTEP view of the co-ordination geometry and hydrogen bonding scheme in complex 4.J. Chem. Soc., Dalton Trans., 1999, 1999–2004 2003 at 270–300 nm in DMF solution, which can be attributed to a p* �æ p transition of the conjugated imine chromophore. The complexes exhibit additional weaker and approximately symmetrical absorption bands in visible region at 660–690 nm, which are assigned to d–d transitions.27 It is important to note that the violet shift of the p* �æ p transition for 3 and 4 in comparison to those for 1 and 2 indicates that the imine groups have been reduced, and reinforces this assignment; these observations are consistent with those found for copper(II) complexes with unreduced and reduced SchiV bases condensed with salicylaldehyde and glycine.23 In contrast to the copper(II) complex of the SchiV base derived from Fig. 6 Cyclic voltammograms for complexes (a) 1, (b) 2 and (c) 4 in DMF at room temperature with 0.1 mol L21 Bun 4NPF6 as electrolyte at platinum electrode and SCE reference electrode.Conditions: 1.0 × 1024 mol L21, v = 100 mV s21. Table 3 Electronic and EPR data of complexes 1–4 Electronic spectra (nm) EPR data Complex d–d p* �æ p g|| g^ A||/G 123 4 670.1 688.5 666.5 668.3 299.7 332.9 279.1 272.7 2.307 2.331 2.291 2.301 2.031 2.034 2.042 2.031 180.5 183.5 177.5 g 4.2 2-formylpyridine and histidine, we did not observe decomposition of 1 and 2.6 The EPR spectra recorded on microcrystalline samples at 77 K in DMF solution show axial spectra for complexes 1–3 (Table 3), indicating that 1, 2 and 3 were deaggragated in solution and the copper(II) atom adopts elongated octahedral or squareplanar co-ordination geometry in DMF solution.28 For 4 the EPR spectrum shows an axial spectrum with g|| = 2.301 and g^ = 2.024 and a very weak half-field resonance with g = 4.2 under the same conditions, indicating that 4 remains dimeric in solution. Electrochemistry Complexes 1, 2 and 4 underwent an overall cyclic voltammetric process in DMF containing 0.1 mol L21 of Bun 4NPF6 in the range 2.0 to 22.0 V at room temperature starting with oxidation as shown in Fig. 6. Cyclic voltammetry of 1 displays one reversible wave, one irreversible wave and one quasi-reversible wave. The quasi-reversible redox couple occurs with the oxidation peak at 10.68 V and the corresponding reduction peak at 10.31 V, which may be assigned to the imidazole group of a ligand-based redox couple.The irreversible redox couple occurs with the reduction peak at 20.28 V is assigned to the reaction [CuIIL1]1 1 e æÆ [CuIL1]1 by comparison with the reduction potential value (20.27 V vs. SCE) of CuII–CuI.29 The reversible redox couple (reduction peak at 21.05 V) may be assigned to the imine group of the L1 ligand. Complex 2 exhibits one reversible wave and one irreversible wave [Fig. 6(b)], which supports the assignment of the quasi-reversible redox couple in 1 for the redox of the imidazole group in L1 since 2 does not show a similar quasi-reversible wave in the redox process. Similar to 1, the irreversible and reversible redox couples in 2 were assigned to the reactioIL2]1 1 e æÆ [CuIL2]1 and the imine group of L2, respectively. Cyclic voltammetry of 4 displays two reversible waves, one irreversible and one quasi-reversible. The quasi-reversible redox couple occurs with the oxidation peak at 10.66 V and the corresponding reduction peak at 10.30 V, which is assigned to a ligand-based redox couple.The irreversible redox couple occurs with the reduction peak at 20.28 V and is assigned to the reaction [CuII 2(L4)2]21 1 e æÆ [CuICuII( L4)2]21. The reversible redox couple occurs with the reduction peak at 20.85 V and the corresponding oxidation peak at 20.75 V is a single-electron process, which may be assigned to the reaction [CuICuII(L4)2]21 1 e [CuI 2(L4)2]21.The reversible redox couple occurs at 21.02/20.90 V and may be assigned to a redox process of L4. These observations suggest that ligands L1, L2 and L4 can stabilize the copper(I) ions in the complexes due to the imidazole groups forming p back bonding with the copper(I) atoms. Conclusion The SchiV base ligands containing an imidazole group and balanine and imine-reduced ligands bonding copper(II) have yielded complexes with tri- or tetra-dentate co-ordination modes.The EPR spectra indicate that 1, 2 and 3 were deaggragated in DMF solution, while 4 remains dimeric. The cyclic voltammograms of 1, 2 and 4 indicate that ligands L1, L2 and L4 can stabilize the copper(I) ions in the complexes due to the imidazole groups forming p back bonding with the copper(I) atoms. 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