Syntheses, structures, and electrochemical properties of three complexes based on 5-ferrocenylpentanoic acid

Journal of Molecular Structure 933 (2009) 163–168

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Syntheses, structures, and electrochemical properties of three complexes based on 5-ferrocenylpentanoic acid Xiangru Meng, Yun Liu, Hongwei Hou *, Yaoting Fan Department of Chemistry, Zhengzhou University, Henan 450052, PR China

a r t i c l e

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Article history: Received 19 January 2009 Received in revised form 15 May 2009 Accepted 16 June 2009 Available online 23 June 2009 Keywords: 5-Ferrocenylpentanoic acid Complex Electrochemical property

a b s t r a c t Three new complexes, [Co(OOC(CH2)4Fc)2(4,40 -bipy)(H2O)2]n (1), [Cd(OOC(CH2)4Fc)2(4,40 -bipy)(H2O)2]n (2), and {[Co(g2-OOC(CH2)4Fc)2(bbbm)](CH3OH)}n (3) [bbbm = 1,10 -(1,4-butanediyl)bis-1H-benzimidazole, Fc = (g5-C5H4)Fe(g5-C5H4)], were obtained from the corresponding metal salts with the primary ligand 5-ferrocenylpentanoic acid and the subsidiary N-heterocyclic ligands, and their structures were fully characterized. X-ray diffraction analyses reveal that all of the complexes display 1-D chain structure and complexes 1 and 2 are isostructural. The electrochemical properties of complexes 1–3 and 5-ferrocenylpentanoic acid have been investigated in DMF solution. The results show that the half-wave redox potentials of the three complexes are close to the 5-ferrocenylpentanoic acid, which indicates that the coordination of the metal ions does not have significant effects on the redox potential of the 5-ferrocenylpentanoic acid ligand. Further investigations suggest that the redox processes of 5-ferrocenylpentanoic acid and complexes 1–3 are all chemically quasi-reversible processes and controlled by diffusion. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction It is well known that ferrocene is easily oxidized to the ferricenium cation and the ferrocene/ferrocenium (Fc/Fc+) itself represents a strictly reversible redox couple [1]. The redox potentials of the ferrocene/ferrocenium processes in the ferrocenyl derivatives may be significantly changed by the electron-donating or electron-withdrawing abilities of substituents [2]. Up to now, more and more efforts have been devoted into the syntheses of the ferrocenyl derivatives and investigation of their electrochemical properties [3]. Among the ferrocenyl derivatives, ferrocenyl carboxylate derivatives have attracted intense interest for their good electrochemical properties as well as their abilities to bind to many metal ions in various coordination modes such as monodentate mode, symmetric bidentate mode, asymmetric bidentate mode, bridging mode, mix-bridging mode and so on [4]. From the viewpoint of constructing functional complexes, ferrocenyl carboxylates have proved to be good candidates for building organometallic materials with unique physical and chemical properties [5], and coordination complexes with metallocene units in the framework have received more and more attention in the development of new redox-active crystalline solids [6]. However, by far, most of the ferrocenyl carboxylates used as ligands to construct functional complexes are fixed on ferrocenyl unsaturated carboxylates such as ferrob-ferrocenylacrylate (bcenylbenzoate (–OOCC6H4COFc), OOCCH@CHFc), 3-ferrocenyl-2-crotonate (–OOC–CH@(CH3)CFc), * Corresponding author. Tel./fax: +86 371 67761744. E-mail address: [email protected] (H. Hou). 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.06.025

and ferrocenesuccinate (–OOCC2H4COFc). The complexes containing ferrocenyl saturated carboxylate ligands like 4-ferrocenylbutanoic carboxylate (–OOCC3H6Fc) or 5-ferrocenylpentanoic carboxylate (–OOCC4H8Fc) are limited. In order to enrich the categories and numbers of complexes with ferrocenyl carboxylates and further investigate the influence of the presence of the saturated methylene spacers between the electroactive ferrocenyl and carboxyl group on the redox potentials of ferrocene center, here we select 5-ferrocenylpentanoic carboxylate as the primary ligand. Through the introduction of subsidiary N-heterocyclic ligands like 4,40 -bipy or bbbm, three new complexes [Co(OOC(CH2)4Fc)2(4,40 bipy)(H2O)2]n (1), [Cd(OOC(CH2)4Fc)2(4,40 -bipy)(H2O)2]n (2), and {[Co(g2-OOC(CH2)4Fc)2(bbbm)](CH3OH)}n (3) have been successfully obtained and characterized by single-crystal X-ray diffraction. The results show that all of the complexes display 1-D chain structure and complexes 1 and 2 are isostructural. The systemic electrochemical studies reveal that the half-wave redox potentials of the three complexes are close to the 5-ferrocenylpentanoic acid, which indicates that the coordination of metal ions to the ferrocenyl ligand does not have significant effects on the redox potentials of the 5-ferrocenylpentanoic acid ligand.

2. Experimental 2.1. Materials and general methods All of the reagents required for the syntheses were commercially available and employed without further purification. Carbon, hydro-

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gen, and nitrogen analyses were carried out on a Flash EA 1112 elemental analyzer. IR spectra were recorded on a BRUKER TENSOR 27 spectrophotometer (400–4000 cm1) with samples prepared as KBr pellets. 1,10 -(1,4-butanediyl)bis-1H-benzimidazole) and 5-ferrocenylpentanoic acid (Fc(CH2)4COOH) were prepared according to the literature methods with some modifications [7]. 2.2. Synthesis of complexes 1–3 Synthesis of [Co(OOC(CH2)4Fc)2(4,40 -bipy)(H2O)2]n (1): A methanol solution (2 mL) containing Fc(CH2)4COOH (0.04 mmol) and CH3ONa (0.04 mmol) was added into 2 mL aqueous solution of Co(NO3)26H2O (0.02 mmol), then 2 mL methanol solution of 4,40 bipy (0.02 mmol) was added dropwise to the above mixture. The resulting orange solution was allowed to stand at room temperature in the dark. Red crystals were obtained after one month (60% yield based on Co). Elemental analysis calcd for C40H46CoFe2N2O6: C, 58.49; H, 5.64; N, 3.41. Found: C, 58.20; H, 5.82; N, 3.11%. IR (KBr)/cm1: 3259m, 2924m, 1602s, 1562s, 1410s, 1257w, 1105w, 1048m, 810s, 687s, 482m. Synthesis of [Cd(OOC(CH2)4Fc)2(4,40 -bipy)(H2O)2]n (2): The procedure was similar to that of 1 except that Cd(NO3)24H2O was used instead of Co(NO3)26H2O (yield of 45% based on Zn). Elemental analysis calcd for C40H46CdFe2N2O6: C, 54.91; H, 5.30; N, 3.20. Found: C, 54.44; H, 5.55; N, 3.53%. IR (KBr)/cm1: 3423m, 2933m, 1600s, 1566s, 1414m, 1220w, 1105w, 1044w, 1002m, 808s, 626m, 438w. Synthesis of {[Co(g2-OOC(CH2)4Fc)2(bbbm)](CH3OH)}n (3): 2 mL methanol solution involving Fc(CH2)4COOH (0.04 mmol) and CH3ONa (0.04 mmol) was added into 2 mL aqueous solution of Co(NO3)26H2O (0.02 mmol), then 2 mL methanol solution of bbbm (0.02 mmol) was added dropwise to the above mixture. The resulting orange solution was allowed to stand at room temperature in the dark. Red crystals were obtained after several weeks (55% yield based on Co). Elemental analysis calcd for C49H56CoFe2N4O5: C, 61.85; H, 5.93; N, 5.89; Found: C, 61.52; H, 5.63; N, 5.44%. IR (KBr)/cm1: 3425m, 2930m, 1562s, 1508s, 1462m, 1395s, 1256m, 1104w, 1040w, 1002m, 825m, 747s, 489m. 2.3. X-ray crystallographic analyses A prismatic single crystal was mounted on a glass fiber. The data of the three complexes were collected on a Rigaku Saturn Table 1 Crystal data and structure refinement for complexes 1–3. Complexes

1

2

3

Formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dc (g/cm3) l/mm1 Reflns collected Unique reflns Rint Data/restraints/ parameters GOF R1 [I > 2r(I)] wR2[I > 2r(I)] R1 [all data] wR2[all data]

C40H46CoFe2N2O6 821.42 Monoclinic C2/c 28.394(6) 11.514(2) 10.956(2) 90 101.57(3) 90 3509.2(12) 4 1.555 1.335 20794 4022 0.0384 4022/0/241

C40H46CdFe2N2O6 874.89 Monoclinic C2/c 28.263(6) 11.757(2) 11.045(2) 90 100.92(3) 90 3603.6(13) 4 1.613 1.430 21032 4121 0.0622 4121/0/241

C49H56CoFe2N4O5 951.61 Triclinic P-1 8.5125(17) 16.833(3) 17.899(4) 112.76(3) 96.64(3) 103.24(3) 2241.7(8) 2 1.410 1.055 22860 7862 0.0458 7862/43/646

1.082 0.0372 0.0872 0.0396 0.0888

1.067 0.0515 0.1511 0.0548 0.1566

1.087 0.0727 0.1886 0.0942 0.2057

Table 2 Selected bond lengths (Å) and bond angles (deg) for complexes 1–3. Complex 1 Co(1)–O(3) Co(1)–N(1) O(3)–Co(1)–O(3)#1 O(3)#1–Co(1)–O(1)#1 O(3)#1–Co(1)–O(1) O(3)–Co(1)–N(1) O(1)#1–Co(1)–N(1) O(3)–Co(1)–N(2)#2 O(1)#1–Co(1)–N(2)#2 N(1)–Co(1)–N(2)#2

2.093(2) 2.194(2) 173.19(9) 87.73(7) 92.08(7) 86.59(4) 88.48(4) 93.41(4) 91.52(4) 179.999(1)

Co(1)–O(1) Co(1)–N(2)#2 O(3)–Co(1)–O(1)#1 O(3)–Co(1)–O(1) O(1)#1–Co(1)–O(1) O(3)#1–Co(1)–N(1) O(1)–Co(1)–N(1) O(3)#1–Co(1)–N(2)#2 O(1)–Co(1)–N(2)#2

2.104(2) 2.200(2) 92.08(7) 87.74(7) 176.95(7) 86.59(5) 88.48(4) 93.41(5) 91.52(4)

Complex 2 Cd(1)–O(1) Cd(1)–N(2)#2 O(1)#1–Cd(1)–O(1) O(1)–Cd(1)–O(3)#1 O(1)–Cd(1)–O(3) O(1)#1–Cd(1)–N(2)#2 O(3)#1–Cd(1)–N(2)#2 O(1)#1–Cd(1)–N(1) O(3)#1–Cd(1)–N(1) N(2)#2–Cd(1)–N(1)

2.262(3) 2.341(4) 175.76(14) 90.07(13) 89.57(13) 92.12(7) 94.82(10) 87.88(7) 85.18(10) 180.000(1)

Cd(1)–O(3) Cd(1)–N(1) O(1)#1–Cd(1)–O(3)#1 O(1)#1–Cd(1)–O(3) O(3)#1–Cd(1)–O(3) O(1)–Cd(1)–N(2)#2 O(3)–Cd(1)–N(2)#2 O(1)–Cd(1)–N(1) O(3)–Cd(1)–N(1)

2.283(4) 2.353(4) 89.57(13) 90.07(13) 170.37(19) 92.12(7) 94.82(10) 87.88(7) 85.18(10)

Complex 3 Co(1)–O(3) Co(1)–N(3) Co(1)–O(2) O(3)–Co(1)–O(1) O(1)–Co(1)–N(3) O(1)–Co(1)–N(1) O(3)–Co(1)–O(2) N(3)–Co(1)–O(2) O(3)–Co(1)–O(4) N(3)–Co(1)–O(4) O(2)–Co(1)–O(4)

2.032(3) 2.077(3) 2.364(4) 146.69(16) 95.18(15) 97.19(15) 93.11(15) 151.24(15) 58.43(13) 88.40(13) 86.51(14)

Co(1)–O(1) Co(1)–N(1) Co(1)–O(4) O(3)–Co(1)–N(3) O(3)–Co(1)–N(1) N(3)–Co(1)–N(1) O(1)–Co(1)–O(2) N(1)–Co(1)–O(2) O(1)–Co(1)–O(4) N(1)–Co(1)–O(4)

2.068(4) 2.077(3) 2.374(4) 108.14(14) 99.98(13) 103.50(13) 58.07(15) 91.22(14) 100.06(15) 158.05(13)

Symmetry transformations used to generate equivalent atoms: Complex 1: #1 x + 2, y, z + 5/2; #2 x, y  1, z; #3 x, y + 1, z; Complex 2: #1 x + 1, y, z + 1/2; #2 x, y + 1, z; #3 x, y1, z.

724 CCD diffractomer (Mo-Ka, k = 0.71073 Å) at temperature of 20 ± 1 °C. The empirical absorption corrections were applied to them. The data were corrected for Lorentz and polarization effects. The structures were solved by direct methods and refined with a full-matrix least-squares technique based on F2 with the SHELXL97 crystallographic software package [8]. All non-hydrogen atoms were refined anisotropically while hydrogen atoms were generated geometrically. Table 1 shows crystallographic crystal data and structure processing parameters for complexes 1–3. Selected bond lengths and bond angles are listed in Table 2. 2.4. Electrochemistry properties determination The electrochemistry properties were determined by a CHI660B electrochemical analyzer utilizing the three-electrode configuration of a Pt working electrode, a Pt auxiliary electrode, and a saturated Ag/AgCl electrode as the reference electrode. The measurements were carried out in DMF solutions with 0.1 mol dm3 tetrabutylammonium perchlorate (TBAP) as a supporting electrolyte. Pure N2 gas was bubbled through the electrolytic solution to remove oxygen. 3. Results and discussion 3.1. Description of crystal structures Single-crystal X-ray structural analysis reveals that 1 crystallizes in a space group C2/c and the asymmetric unit of it contains half a Co(II) center, one coordinated water molecule, one 5-ferrocenylpentanoic carboxylate ligand, and half of a 4,40 -bipy ligand as

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shown in Fig. 1a. Each Co(II) center is six-coordinated and exhibits a slightly distorted octahedral environment supplied by four oxygen atoms from two terminal monodentate carboxylate groups and two coordinated water molecules and two nitrogen atoms from two 4,40 -bipy molecules, as illustrated in Fig. 1a. Around the central Co(II) ion, the bond lengths of Co–O are 2.093(2) (from water molecule) and 2.104(2) Å (from carboxylate group); the bond lengths of Co–N are 2.194(2) and 2.200(2) Å, respectively; the bond angles around the Co(II) ion are close to 90° or 180°; O1A, O1A, O3, O3A, and Co1 atoms are nearly co-planar (the mean deviation from plane is 0.0418 Å). In each ferrocenyl moiety, cyclopentadienyl rings are co-planar (for each cyclopentadienyl ring, the

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mean deviation from plane is 0.0008 or 0.0028 Å) and nearly parallel with a dihedral angle of 3.1°, the intra C–C distances range from 1.389(5) to 1.428(3) Å, which are close to those observed in other reported ferrocenecarboxylate-containing complexes [5b,9]. Cyclopentadienyl rings within the ferrocenyl groups deviate by 25.9°–26.9° from the eclipsed conformation. Furthermore, in complex 1, [Co(OOC(CH2)4Fc)2(H2O)2] units are connected by 4,40 -bipy ligands, leading to the formation of the 1-D polymeric chain. All of the Co(II) ions in one chain are in a straight line, and the intrachain Co  Co separation across the 4,40 -bipy linker is 11.514 Å and the two pyridyl rings of the 4,40 -bipy molecule are twisted by 22.6°. Along the 1-D polymeric chain, all of the 5-ferrocenylpentanoic

Fig. 1. (a) The structure of complex [Co(OOC(CH2)4Fc)2(4,40 -bipy)(H2O)2]n (1) with atom numbering (Hydrogen atoms and solvent molecules are omitted for clarity) and (b) view of 2-D structure of complex [Co(OOC(CH2)4Fc)2(4,40 -bipy)(H2O)2]n (1) showing the interchain hydrogen bonds.

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Fig. 2. (a) The structure of complex {[Co(g2-OOC(CH2)4Fc)2(bbbm)](CH3OH)}n (3) with atom numbering (Hydrogen atoms and solvent molecules are omitted for clarity) and (b) view of 1-D structure of complex {[Co(g2-OOC(CH2)4Fc)2(bbbm)](CH3OH)}n (3) with only one part of the disordered group for clarity.

carboxylate moieties with the monodentate syn-coordination mode of the carboxy groups adopt the same TTG conformation (where T = trans, and G = gauche, the torsion angles of the fragments of methylene chains between ferrocenyl and carboxylate group are 171.61(19), 177.42(19) and 71.8(2)°) and situate on the opposite sides of the central metal ion. As depicted in Fig. 1b, the 1-D polymeric chains are linked orderly by O(3)-H(1W)  O(1) hydrogen bonds leading to 2-D network structure. O(1) is the coordinated oxygen of 5ferrocenylpentanoic carboxylate and O(3) is from the coordinated water molecule of the adjacent chain. The O(3)–O(1) distance is 2.791(2) Å and the O(3)–H(1W)  O(1) angle is 167(3)°. Although these hydrogen bonds are weaker than the Co–O and Co–N coordination bonds, it is also important in the molecular assembly and can stabilize the 2-D network structure. Complexes 1 and 2 are isostructural, and complex 2 also displays a similar 1-D chain structure in which [Cd(OOC(CH2)4Fc)2(H2O)2] units are bridged by 4,40 -bipy ligands (Fig. SI-1a). The Cd(II) ion is in a distorted octahedral geometry, too. The bond lengths of Cd–N [2.341(4), 2.353(4) Å] and Cd–O [2.262(3), 2.283(4) Å] are slightly longer than those of Co–N and Co–O in complex 1, but the Cd–N and Cd–O bond lengths are consistent with those of the reported Cd-complexes such as complex

[Cd2(g2-O2CFcCO2)2(2,20 -bipy)2(H2O)2]2H2O (the av. Cd–N and Cd–O bond lengths are 2.377 and 2.293 Å, respectively) [5b]. In the solid state, the 1-D polymeric chains are linked orderly by hydrogen bonds (O–O distance is 2.713(5) Å, O–H  O angle is 169(8)°) resulting in 2-D network structure (Fig. SI-1b). Single-crystal X-ray diffraction analysis reveals that the crystal structure of 3 is different from 1 and 2. It crystallizes in a space group p-1 and displays an infinite 1-D zigzag chain structure. As shown in Fig. 2a, each Co(II) ion is six-coordinated by four oxygen atoms from two 5-ferrocenylpentanoic carboxylate ligands and two nitrogen atoms from two bbbm units. All of the carboxylate groups in complex 3 are bound in asymmetric bidentate fashion with Co–O bond lengths ranging from 2.032(3) to 2.374(4) Å. The local environment around the Co(II) ion can be described as a distorted octahedron in which atoms O1, O2, O3, N3 occupy the equatorial positions and N1, O4 atoms occupy the axial positions. On the equatorial surface atoms O1, O2, O3 and N3 are nearly co-planar (the mean deviation from plane is 0.0418 Å), the bond lengths of Co1–O1, Co(1)–O(2), Co(1)–O(3) and Co(1)–N(3) are 2.068(4), 2.364(4), 2.032(3) and 2.077(3) Å, respectively, and the bond angles around Co(II) are in the range of 58.07(15)–151.24(15)°. On the axial positions the bond lengths of Co1–N1 and Co1–O4 are 2.077(3) and 2.374(4) Å, and the bond angle of N(1)–Co(1)–O(4)

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is 158.05(13)°. In the crystal structure of complex 3, there are two crystallographically independent 5-ferrocenylpentanoic carboxylate moieties. One kind of the 5-ferrocenylpentanoic carboxylate moieties (Fe1) adopts the TTG conformation (the torsion angles of the fragments of methylene chains between ferrocenyl and carboxyl are 171.6(6), 179.4(6) and 70.7(8)°) and the cyclopentadienyl rings within the ferrocenyl groups deviate by 13.9°–16.0° (averaging about 15°) from the eclipsed conformation. The other kind of the 5-ferrocenylpentanoic carboxylate moieties (Fe2) are disordered. Complex 3 also contains two crystallographically independent bbbm units. One kind of the bbbm moieties (bbbm1, N1, N2, N1A, N2A, C19–C24, C19A–C24A) adopts the TTT conformation (the torsion angles of the fragments of methylene chains between two benzimidazole groups are 179.6(5), 180 and 179.6(5)°) and the other kind of the bbbm moieties (bbbm2, N3, N4, N3A, N4A, C25–C33, C25A–C33A) adopts the GTG conformation (the torsion angles of the fragments of methylene chains between two benzimidazole groups are 68.6(6), 180 and 68.6(6)°). Furthermore, As shown in Fig. 2b, the Co(II) ions are linked by two kinds of bbbm units alternately leading to the 1-D zigzag chain. Since the conformations of the bbbm ligands are different, the intrachain Co  Co distances are different. The intrachain Co  Co separation (such as Co1–Co1A, Co1B–Co1C, Co1D–Co1E) across the bbbm1 is 14.206 Å; and the intrachain Co  Co separation (such as Co1A– Co1B, Co1C–Co1D, Co1E–Co1F) across the bbbm2 is 13.001 Å. 3.2. Electrochemical studies The electrochemical properties of complexes 1–3 and the 5ferrocenylpentanoic acid have been investigated in DMF solution (ca. 1.0  103 mol dm3) containing 0.1 mol dm3 tetrabutylammonium perchlorate (TBAP) as the supporting electrolyte at room temperature. In our previous studies, we have determined the number-average molecular weights (Mn) and weight-average molecular weights (Mw) of the coordination polymers based on ferrocenyl carboxylate derivatives and the results indicated that they were not dissociated in DMF solution [10]. Thus, we confirm that the skeletons of the three complexes are intact in DMF solution. So the electrochemical experiments of these complexes in DMF solution can veritably represent the electrochemistry properties of them. The cyclic voltammetric behaviors of the three complexes and the 5-ferrocenylpentanoic acid all exhibit one pair of well-defined and stable reduction and oxidation waves in the potential range of 0.0–1.0 V at the Pt working electrode. The electrochemical data for the 5-ferrocenylpentanoic acid and the three complexes (vs Ag/AgCl) are summarized in Table 3. It is well known that the electron-withdrawing ability of the carboxyl group serves to raise the potential above that of free ferrocene [11]. For instance, under the present experimental conditions, ferrocenecarboxylic acid (FcCOOH) locates at a much higher half-wave redox potential (0.685 V) with a positive shift of 141 mV compared with the redox potential of free ferrocene (0.544 V). In contrast, the ligand 5-ferrocenylpentanoic acid (Fc(CH2)4COOH) has a lower po-

Table 3 Electrochemical data of free 5-ferrocenylpentanoic acid and complexes 1–3.

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tential (0.476 V) with a negative shift of 68 mV comparing with the redox potential of free ferrocene. This result is in accordance with the previous reports [11a,12]. The origin of this behavior can be traced back to the presence of the saturated methylene spacers between the electroactive ferrocenyl and carboxyl group. The electron-donating ability of the methylene spacer group serves to partially or entirely cancel the electron-withdrawing inductive effect of the carboxyl group. Therefore, the redox potential of 5ferrocenylpentanoic acid is lower than the value of the free ferrocene. In addition, it is interesting to find from Table 3 that the formal potentials (E1/2) of complexes 1–3 are all nearly identical to the 5ferrocenylpentanoic acid, which indicates that the coordination of metal ions to the ferrocenyl ligand does not have significant effects on the redox potential of the 5-ferrocenylpentanoic acid. If we keep the compositions of the test solution and change the scan rates (from 50 to 500 mV s1) in the potential range of 0.0– 1.0 V, we can get the cyclic voltammetric diagrams of the 5-ferrocenylpentanoic acid and complexes 1–3 at different scan rates (see Figs. SI-2a–SI-2d). One can notice from the figures that the oxidation wave potentials show gradual increase and the reduction wave potentials show gradual decrease as the scan rates increase; the anodic to cathodic peak current ratios (ipa/ipc) are close to 1. Judging from the above values, it was tentatively assigned that the redox processes of the 5-ferrocenylpentanoic acid and complexes 1–3 are chemically quasi-reversible processes [13]. Additionally, as shown in Fig. SI-3, the dependence of the currents at different scan rates on the square root of the scan rates is linear. So the redox processes of the 5-ferrocenylpentanoic acid and complexes 1–3 are all controlled by diffusion [14]. 4. Conclusion Three new 1-D chain complexes were obtained from the corresponding metal salts with the primary ligand 5-ferrocenylpentanoic acid and the subsidiary N-heterocyclic ligand 4,40 -bipy or bbbm. The cyclic voltammetric studies reveal that 5-ferrocenylpentanoic acid has a lower half-wave redox potential (0.476 V) comparing with the redox potential of free ferrocene (0.544 V) due to the presence of the saturated methylene spacers between the electroactive ferrocenyl and carboxyl group. The half-wave redox potentials of the three complexes are close to the 5-ferrocenylpentanoic acid, which indicates that the coordination of the metal ions to the ferrocenyl ligand does not have significant effects on the redox potential of the 5-ferrocenylpentanoic acid ligand. In addition, the redox processes of the 5-ferrocenylpentanoic acid and complexes 1–3 are all chemically quasi-reversible processes and controlled by diffusion. Acknowledgments We gratefully acknowledge the financial support by the National Natural Science Foundation of China (No. 20671082), NCET and the Outstanding Talents Foundation by the He’nan province, and the Education Department of He’nan Province (2009A150029). Appendix A. Supplementary data

Compound

Epa/V

Epc/V

E1/2/V

DE

ipa/ipc

FcC4H8COOH Complex 1 Complex 2 Complex 3

0.540 0.535 0.533 0.565

0.411 0.414 0.412 0.382

0.476 0.475 0.473 0.473

0.126 0.121 0.121 0.183

0.98 0.97 0.97 1.06

All potentials are referred to Ag/AgCl in DMF solution, v = 100 mV/s; Epa and Epc are the oxidation and reduction wave potentials, respectively; E1/2 is the formal potential and E1/2 = (Epa + Epc)/2; DE = (Epa  Epc); ipa and ipc are the oxidation and reduction wave currents, respectively.

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molstruc.2009.06.025. References [1] (a) E. Peris, Coord. Chem. Rev. 248 (2004) 279; (b) B.X. Ye, Y. Xu, F. Wang, Y. Fu, M.P. Song, Inorg. Chem. Commun. 8 (2005) 44; (c) O. Oms, A. van der Lee, J.L. Bideaub, D. Leclercq, Dalton Trans. (2005) 1903.

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